Patent Publication Number: US-11664035-B2

Title: Spatial transformation of ambisonic audio data

Description:
This application is a continuation of: 
     U.S. application Ser. No. 16/557,650, entitled “SIGNALING LAYERS FOR SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Aug. 30, 2019; which claims the benefit of the following: 
     U.S. application Ser. No. 16/183,063, entitled “SIGNALING LAYERS FOR SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Nov. 7, 2018; 
     U.S. application Ser. No. 14/878,691, entitled “SIGNALING LAYERS FOR SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Oct. 8, 2015; 
     U.S. Provisional Application No. 62/062,584, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Oct. 10, 2014; 
     U.S. Provisional Application No. 62/084,461, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Nov. 25, 2014; 
     U.S. Provisional Application No. 62/087,209, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Dec. 3, 2014; 
     U.S. Provisional Application No. 62/088,445, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Dec. 5, 2014; 
     U.S. Provisional Application No. 62/145,960, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Apr. 10, 2015; 
     U.S. Provisional Application No. 62/175,185, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed Jun. 12, 2015; 
     U.S. Provisional Application No. 62/187,799, entitled “REDUCING CORRELATION BETWEEN HIGHER ORDER AMBISONIC (HOA) BACKGROUND CHANNELS,” filed Jul. 1, 2015, and 
     U.S. Provisional Application No. 62/209,764, entitled “TRANSPORTING CODED SCALABLE AUDIO DATA,” filed Aug. 25, 2015, the entire content of each of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to audio data and, more specifically, scalable coding of higher-order ambisonic audio data. 
     BACKGROUND 
     A higher-order ambisonics (HOA) signal (often represented by a plurality of spherical harmonic coefficients (SHC) or other hierarchical elements) is a three-dimensional representation of a soundfield. The HOA or SHC representation may represent the soundfield in a manner that is independent of the local speaker geometry used to playback a multi-channel audio signal rendered from the SHC signal. The SHC signal may also facilitate backwards compatibility as the SHC signal may be rendered to well-known and highly adopted multi-channel formats, such as a 5.1 audio channel format or a 7.1 audio channel format. The SHC representation may therefore enable a better representation of a soundfield that also accommodates backward compatibility. 
     SUMMARY 
     In one aspect, a device is configured to decode a bitstream. The device may include a memory configured to store a temporally encoded representation of spatial audio signals. The device may be configured to receive the bitstream that includes an indication of a spatial transformation. The device may include a temporal decoding unit, coupled to the memory, configured to decode one or more spatial audio signals represented in a spatial domain, where the one or more spatial audio signals are associated with different angles in the spatial domain. The device may also include an inverse spatial transformation unit, coupled to the temporal decoding unit, is configured to (i) convert the one or more spatial audio signals represented in the spatial domain into at least three ambisonic coefficients that, in part, represent a soundfield in an ambisonics domain, and (ii) perform a spatial transformation of the soundfield based on the indication of the spatial transformation received in the bitstream. In addition, the device may include a rendering unit, that is part of a first layer in a decoder that includes at least two layers, coupled to the inverse spatial transformation unit, configured to render the at least three ambisonic coefficients into a first set of speaker feeds. 
     The details of one or more aspects of the techniques are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating spherical harmonic basis functions of various orders and sub-orders. 
         FIG.  2    is a diagram illustrating a system that may perform various aspects of the techniques described in this disclosure. 
         FIG.  3    is a block diagram illustrating, in more detail, one example of the audio encoding device shown in the example of  FIG.  2    that may perform various aspects of the techniques described in this disclosure. 
         FIG.  4    is a block diagram illustrating the audio decoding device of  FIG.  2    in more detail. 
         FIG.  5    is a diagram illustrating, in more detail, the bitstream generation unit of  FIG.  3    when configured to perform a first one of the potential versions of the scalable audio coding techniques described in this disclosure. 
         FIG.  6    is a diagram illustrating, in more detail, the extraction unit of  FIG.  4    when configured to perform the first one of the potential versions the scalable audio decoding techniques described in this disclosure. 
         FIGS.  7 A- 7 D  are flowcharts illustrating example operation of the audio encoding device in generating an encoded two-layer representation of the higher order ambisonic (HOA) coefficients. 
         FIGS.  8 A and  8 B  are flowcharts illustrating example operation of the audio encoding device in generating an encoded three-layer representation of the HOA coefficients. 
         FIGS.  9 A and  9 B  are flowcharts illustrating example operation of the audio encoding device in generating an encoded four-layer representation of the HOA coefficients. 
         FIG.  10    is a diagram illustrating an example of an HOA configuration object specified in the bitstream in accordance with various aspects of the techniques. 
         FIG.  11    is a diagram illustrating sideband information generated by the bitstream generation unit for the first and second layers. 
         FIGS.  12 A and  12 B  are diagrams illustrating sideband information generated in accordance with the scalable coding aspects of the techniques described in this disclosure. 
         FIGS.  13 A and  13 B  are diagrams illustrating sideband information generated in accordance with the scalable coding aspects of the techniques described in this disclosure. 
         FIGS.  14 A and  14 B  are flowcharts illustrating example operations of audio encoding device in performing various aspects of the techniques described in this disclosure. 
         FIGS.  15 A and  15 B  are flowcharts illustrating example operations of audio decoding device in performing various aspects of the techniques described in this disclosure. 
         FIG.  16    is a diagram illustrating scalable audio coding as performed by the bitstream generation unit shown in the example of  FIG.  16    in accordance with various aspects of the techniques described in this disclosure. 
         FIG.  17    is a conceptual diagram of an example where the syntax elements indicate that there are two layers with four encoded ambient HOA coefficients specified in a base layer and two encoded foreground signals are specified in the enhancement layer. 
         FIG.  18    is a diagram illustrating, in more detail, the bitstream generation unit of  FIG.  3    when configured to perform a second one of the potential versions of the scalable audio coding techniques described in this disclosure. 
         FIG.  19    is a diagram illustrating, in more detail, the extraction unit of  FIG.  3    when configured to perform the second one of the potential versions the scalable audio decoding techniques described in this disclosure. 
         FIG.  20    is a diagram illustrating a second use case by which the bitstream generation unit of  FIG.  18    and the extraction unit of  FIG.  19    may perform the second one of the potential version of the techniques described in this disclosure. 
         FIG.  21    is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded ambient HOA coefficients specified in a base layer, two encoded foreground signals are specified in a first enhancement layer and two encoded foreground signals are specified in a second enhancement layer. 
         FIG.  22    is a diagram illustrating, in more detail, the bitstream generation unit of  FIG.  3    when configured to perform a third one of the potential versions of the scalable audio coding techniques described in this disclosure. 
         FIG.  23    is a diagram illustrating, in more detail, the extraction unit of  FIG.  4    when configured to perform the third one of the potential versions the scalable audio decoding techniques described in this disclosure. 
         FIG.  24    is a diagram illustrating a third use case by which an audio encoding device may specify multiple layers in a multi-layer bitstream in accordance with the techniques described in this disclosure. 
         FIG.  25    is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded foreground signals specified in a base layer, two encoded foreground signals are specified in a first enhancement layer and two encoded foreground signals are specified in a second enhancement layer. 
         FIG.  26    is a diagram illustrating a third use case by which an audio encoding device may specify multiple layers in a multi-layer bitstream in accordance with the techniques described in this disclosure. 
         FIGS.  27  and  28    are block diagrams illustrating a scalable bitstream generation unit and a scalable bitstream extraction unit that may be configured to perform various aspects of the techniques described in this disclosure. 
         FIG.  29    represents a conceptual diagram representing an encoder that may be configured to operate in accordance with various aspects of the techniques described in this disclosure. 
         FIG.  30    is a diagram illustrating the encoder shown in the example of  FIG.  27    in more detail. 
         FIG.  31    is a block diagram illustrating an audio decoder that may be configured to operate in accordance with various aspects of the techniques described in this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The evolution of surround sound has made available many output formats for entertainment nowadays. Examples of such consumer surround sound formats are mostly ‘channel’ based in that they implicitly specify feeds to loudspeakers in certain geometrical coordinates. The consumer surround sound formats include the popular 5.1 format (which includes the following six channels: front left (FL), front right (FR), center or front center, back left or surround left, back right or surround right, and low frequency effects (LFE)), the growing 7.1 format, various formats that includes height speakers such as the 7.1.4 format and the 22.2 format (e.g., for use with the Ultra High Definition Television standard). Non-consumer formats can span any number of speakers (in symmetric and non-symmetric geometries) often termed ‘surround arrays’. One example of such an array includes 32 loudspeakers positioned on coordinates on the corners of a truncated icosahedron. 
     The input to a future MPEG encoder is optionally one of three possible formats: (i) traditional channel-based audio (as discussed above), which is meant to be played through loudspeakers at pre-specified positions; (ii) object-based audio, which involves discrete pulse-code-modulation (PCM) data for single audio objects with associated metadata containing their location coordinates (amongst other information); and (iii) scene-based audio, which involves representing the soundfield using coefficients of spherical harmonic basis functions (also called “spherical harmonic coefficients” or SHC, “Higher-order Ambisonics” or HOA, and “HOA coefficients”). The future MPEG encoder may be described in more detail in a document entitled “Call for Proposals for 3D Audio,” by the International Organization for Standardization/International Electrotechnical Commission (ISO)/(IEC) JTC1/SC29/WG11/N13411, released January 2013 in Geneva, Switzerland, and available at http://mpeg.chiariglione.org/sites/default/files/files/standards/parts/docs/w13411.zip. 
     There are various ‘surround-sound’ channel-based formats in the market. They range, for example, from the 5.1 home theatre system (which has been the most successful in terms of making inroads into living rooms beyond stereo) to the 22.2 system developed by NHK (Nippon Hoso Kyokai or Japan Broadcasting Corporation). Content creators (e.g., Hollywood studios) would like to produce the soundtrack for a movie once, and not spend effort to remix it for each speaker configuration. Recently, Standards Developing Organizations have been considering ways in which to provide an encoding into a standardized bitstream and a subsequent decoding that is adaptable and agnostic to the speaker geometry (and number) and acoustic conditions at the location of the playback (involving a renderer). 
     To provide such flexibility for content creators, a hierarchical set of elements may be used to represent a soundfield. The hierarchical set of elements may refer to a set of elements in which the elements are ordered such that a basic set of lower-ordered elements provides a full representation of the modeled soundfield. As the set is extended to include higher-order elements, the representation becomes more detailed, increasing resolution. 
     One example of a hierarchical set of elements is a set of spherical harmonic coefficients (SHC). The following expression demonstrates a description or representation of a soundfield using SHC: 
     
       
         
           
             
               
                 
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     The expression shows that the pressure p i  at any point {r r , θ r , φ r } of the soundfield, at time t, can be represented uniquely by the SHC, A n   m (k). Here, 
               k   =     ω   c       ,         
c is the speed of sound (˜343 m/s), {r r , θ r , φ r } is a point of reference (or observation point), j n (⋅) is the spherical Bessel function of order n, and Y n   m (θ r , φ r ) are the spherical harmonic basis functions of order n and suborder m. It can be recognized that the term in square brackets is a frequency-domain representation of the signal (i.e., S(ω, r r , θ r , φ r )) which can be approximated by various time-frequency transformations, such as the discrete Fourier transform (DFT), the discrete cosine transform (DCT), or a wavelet transform. Other examples of hierarchical sets include sets of wavelet transform coefficients and other sets of coefficients of multiresolution basis functions.
 
       FIG.  1    is a diagram illustrating spherical harmonic basis functions from the zero order (n=0) to the fourth order (n=4). As can be seen, for each order, there is an expansion of suborders m which are shown but not explicitly noted in the example of  FIG.  1    for ease of illustration purposes. 
     The SHC A n   m (k) can either be physically acquired (e.g., recorded) by various microphone array configurations or, alternatively, they can be derived from channel-based or object-based descriptions of the soundfield. The SHC represent scene-based audio, where the SHC may be input to an audio encoder to obtain encoded SHC that may promote more efficient transmission or storage. For example, a fourth-order representation involving (1+4) 2  (25, and hence fourth order) coefficients may be used. 
     As noted above, the SHC may be derived from a microphone recording using a microphone array. Various examples of how SHC may be derived from microphone arrays are described in Poletti, M., “Three-Dimensional Surround Sound Systems Based on Spherical Harmonics,” J. Audio Eng. Soc., Vol. 53, No. 11, 2005 November, pp. 1004-1025. 
     To illustrate how the SHCs may be derived from an object-based description, consider the following equation. The coefficients A n   m (k) for the soundfield corresponding to an individual audio object may be expressed as:
 
 A   n   m ( k )= g (ω)(−4π ik ) h   n   (2) ( kr   s ) Y   n   m* (θ s ,φ s ),
 
where i is √{square root over (−1)}, h n   (2) (⋅) is the spherical Hankel function (of the second kind) of order n, and {r s , θ s , φ s } is the location of the object. Knowing the object source energy g(ω) as a function of frequency (e.g., using time-frequency analysis techniques, such as performing a fast Fourier transform on the PCM stream) allows us to convert each PCM object and the corresponding location into the SHC A n   m (k). Further, it can be shown (since the above is a linear and orthogonal decomposition) that the A n   m (k) coefficients for each object are additive. In this manner, a multitude of PCM objects can be represented by the A n   m (k) coefficients (e.g., as a sum of the coefficient vectors for the individual objects). Essentially, the coefficients contain information about the soundfield (the pressure as a function of 3D coordinates), and the above represents the transformation from individual objects to a representation of the overall soundfield, in the vicinity of the observation point {r r , θ r , φ r }. The remaining figures are described below in the context of object-based and SHC-based audio coding.
 
       FIG.  2    is a diagram illustrating a system  10  that may perform various aspects of the techniques described in this disclosure. As shown in the example of  FIG.  2   , the system  10  includes a content creator device  12  and a content consumer device  14 . While described in the context of the content creator device  12  and the content consumer device  14 , the techniques may be implemented in any context in which SHCs (which may also be referred to as HOA coefficients) or any other hierarchical representation of a soundfield are encoded to form a bitstream representative of the audio data. Moreover, the content creator device  12  may represent any form of computing device capable of implementing the techniques described in this disclosure, including a handset (or cellular phone), a tablet computer, a smart phone, or a desktop computer to provide a few examples. Likewise, the content consumer device  14  may represent any form of computing device capable of implementing the techniques described in this disclosure, including a handset (or cellular phone), a tablet computer, a smart phone, a set-top box, or a desktop computer to provide a few examples. 
     The content creator device  12  may be operated by a movie studio or other entity that may generate multi-channel audio content for consumption by operators of content consumer devices, such as the content consumer device  14 . In some examples, the content creator device  12  may be operated by an individual user who would like to compress HOA coefficients  11 . Often, the content creator generates audio content in conjunction with video content. The content consumer device  14  may be operated by an individual. The content consumer device  14  may include an audio playback system  16 , which may refer to any form of audio playback system capable of rendering SHC for play back as multi-channel audio content. 
     The content creator device  12  includes an audio editing system  18 . The content creator device  12  obtain live recordings  7  in various formats (including directly as HOA coefficients) and audio objects  9 , which the content creator device  12  may edit using audio editing system  18 . A microphone  5  may capture the live recordings  7 . The content creator may, during the editing process, render HOA coefficients  11  from audio objects  9 , listening to the rendered speaker feeds in an attempt to identify various aspects of the soundfield that require further editing. The content creator device  12  may then edit HOA coefficients  11  (potentially indirectly through manipulation of different ones of the audio objects  9  from which the source HOA coefficients may be derived in the manner described above). The content creator device  12  may employ the audio editing system  18  to generate the HOA coefficients  11 . The audio editing system  18  represents any system capable of editing audio data and outputting the audio data as one or more source spherical harmonic coefficients. 
     When the editing process is complete, the content creator device  12  may generate a bitstream  21  based on the HOA coefficients  11 . That is, the content creator device  12  includes an audio encoding device  20  that represents a device configured to encode or otherwise compress HOA coefficients  11  in accordance with various aspects of the techniques described in this disclosure to generate the bitstream  21 . The audio encoding device  20  may generate the bitstream  21  for transmission, as one example, across a transmission channel, which may be a wired or wireless channel, a data storage device, or the like. The bitstream  21  may represent an encoded version of the HOA coefficients  11  and may include a primary bitstream and another side bitstream, which may be referred to as side channel information. 
     While shown in  FIG.  2    as being directly transmitted to the content consumer device  14 , the content creator device  12  may output the bitstream  21  to an intermediate device positioned between the content creator device  12  and the content consumer device  14 . The intermediate device may store the bitstream  21  for later delivery to the content consumer device  14 , which may request the bitstream. The intermediate device may comprise a file server, a web server, a desktop computer, a laptop computer, a tablet computer, a mobile phone, a smart phone, or any other device capable of storing the bitstream  21  for later retrieval by an audio decoder. The intermediate device may reside in a content delivery network capable of streaming the bitstream  21  (and possibly in conjunction with transmitting a corresponding video data bitstream) to subscribers, such as the content consumer device  14 , requesting the bitstream  21 . 
     Alternatively, the content creator device  12  may store the bitstream  21  to a storage medium, such as a compact disc, a digital video disc, a high definition video disc or other storage media, most of which are capable of being read by a computer and therefore may be referred to as computer-readable storage media or non-transitory computer-readable storage media. In this context, the transmission channel may refer to the channels by which content stored to the mediums are transmitted (and may include retail stores and other store-based delivery mechanism). In any event, the techniques of this disclosure should not therefore be limited in this respect to the example of  FIG.  2   . 
     As further shown in the example of  FIG.  2   , the content consumer device  14  includes the audio playback system  16 . The audio playback system  16  may represent any audio playback system capable of playing back multi-channel audio data. The audio playback system  16  may include a number of different renderers  22 . The renderers  22  may each provide for a different form of rendering, where the different forms of rendering may include one or more of the various ways of performing vector-base amplitude panning (VBAP), and/or one or more of the various ways of performing soundfield synthesis. As used herein, “A and/or B” means “A or B”, or both “A and B”. 
     The audio playback system  16  may further include an audio decoding device  24 . The audio decoding device  24  may represent a device configured to decode HOA coefficients  11 ′ from the bitstream  21 , where the HOA coefficients  11 ′ may be similar to the HOA coefficients  11  but differ due to lossy operations (e.g., quantization) and/or transmission via the transmission channel. The audio playback system  16  may, after decoding the bitstream  21  to obtain the HOA coefficients  11 ′ and render the HOA coefficients  11 ′ to output loudspeaker feeds  25 . The loudspeaker feeds  25  may drive one or more loudspeakers (which are not shown in the example of  FIG.  2    for ease of illustration purposes). 
     To select the appropriate renderer or, in some instances, generate an appropriate renderer, the audio playback system  16  may obtain loudspeaker information  13  indicative of a number of loudspeakers and/or a spatial geometry of the loudspeakers. In some instances, the audio playback system  16  may obtain the loudspeaker information  13  using a reference microphone and driving the loudspeakers in such a manner as to dynamically determine the loudspeaker information  13 . In other instances or in conjunction with the dynamic determination of the loudspeaker information  13 , the audio playback system  16  may prompt a user to interface with the audio playback system  16  and input the loudspeaker information  13 . 
     The audio playback system  16  may then select one of the audio renderers  22  based on the loudspeaker information  13 . In some instances, the audio playback system  16  may, when none of the audio renderers  22  are within some threshold similarity measure (in terms of the loudspeaker geometry) to the loudspeaker geometry specified in the loudspeaker information  13 , generate the one of audio renderers  22  based on the loudspeaker information  13 . The audio playback system  16  may, in some instances, generate one of the audio renderers  22  based on the loudspeaker information  13  without first attempting to select an existing one of the audio renderers  22 . One or more speakers  3  may then playback the rendered loudspeaker feeds  25 . In other words, the speakers  3  may be configured to reproduce a soundfield based on higher order ambisonic audio data. 
       FIG.  3    is a block diagram illustrating, in more detail, one example of the audio encoding device  20  shown in the example of  FIG.  2    that may perform various aspects of the techniques described in this disclosure. The audio encoding device  20  includes a content analysis unit  26 , a vector-based decomposition unit  27  and a directional-based decomposition unit  28 . 
     Although described briefly below, more information regarding the vector-based decomposition unit  27  and the various aspects of compressing HOA coefficients is available in International Patent Application Publication No. WO 2014/194099, entitled “INTERPOLATION FOR DECOMPOSED REPRESENTATIONS OF A SOUND FIELD,” filed 29 May 2014. In addition, more details of various aspects of the compression of the HOA coefficients in accordance with the MPEG-H 3D audio standard, including a discussion of the vector-based decomposition summarized below, can be found in: 
     ISO/IEC DIS 23008-3 document, entitled “Information technology—High efficiency coding and media delivery in heterogeneous environments—Part 3: 3D audio,” by ISO/IEC JTC 1/SC 29/WG 11, dated 2014 Jul. 25 (available at: http://mpeg.chiariglione.org/standards/mpeg-h/3d-audio/dis-mpeg-h-3d-audio, hereinafter referred to as “phase I of the MPEG-H 3D audio standard”); 
     ISO/IEC DIS 23008-3:2015/PDAM 3 document, entitled “Information technology—High efficiency coding and media delivery in heterogeneous environments—Part 3: 3D audio, AMENDMENT 3: MPEG-H 3D Audio Phase 2,” by ISO/IEC JTC 1/SC 29/WG 11, dated 2015 Jul. 25 (available at: http://mpeg.chiariglione.org/standards/mpeg-h/3d-audio/text-isoiec-23008-3201xpdam-3-mpeg-h-3d-audio-phase-2, and hereinafter referred to as “phase II of the MPEG-H 3D audio standard”); and 
     Jürgen Herre, et al., entitled “MPEG-H 3D Audio—The New Standard for Coding of Immersive Spatial Audio,” dated August 2015 and published in Vol. 9, No. 5 of the IEEE Journal of Selected Topics in Signal Processing. 
     The content analysis unit  26  represents a unit configured to analyze the content of the HOA coefficients  11  to identify whether the HOA coefficients  11  represent content generated from a live recording or an audio object. The content analysis unit  26  may determine whether the HOA coefficients  11  were generated from a recording of an actual soundfield or from an artificial audio object. In some instances, when the framed HOA coefficients  11  were generated from a recording, the content analysis unit  26  passes the HOA coefficients  11  to the vector-based decomposition unit  27 . In some instances, when the framed HOA coefficients  11  were generated from a synthetic audio object, the content analysis unit  26  passes the HOA coefficients  11  to the directional-based synthesis unit  28 . The directional-based synthesis unit  28  may represent a unit configured to perform a directional-based synthesis of the HOA coefficients  11  to generate a directional-based bitstream  21 . 
     As shown in the example of  FIG.  3   , the vector-based decomposition unit  27  may include a linear invertible transform (LIT) unit  30 , a parameter calculation unit  32 , a reorder unit  34 , a foreground selection unit  36 , an energy compensation unit  38 , a decorrelation unit  60  (shown as “decorr unit  60 ”), a gain control unit  62 , a psychoacoustic audio coder unit  40 , a bitstream generation unit  42 , a soundfield analysis unit  44 , a coefficient reduction unit  46 , a background (BG) selection unit  48 , a spatio-temporal interpolation unit  50 , and a quantization unit  52 . 
     The linear invertible transform (LIT) unit  30  receives the HOA coefficients  11  in the form of HOA channels, each channel representative of a block or frame of a coefficient associated with a given order, sub-order of the spherical basis functions (which may be denoted as HOA[k], where k may denote the current frame or block of samples). The matrix of HOA coefficients  11  may have dimensions D: M×(N+1) 2 . 
     The LIT unit  30  may represent a unit configured to perform a form of analysis referred to as singular value decomposition. While described with respect to SVD, the techniques described in this disclosure may be performed with respect to any similar transformation or decomposition that provides for sets of linearly uncorrelated, energy compacted output. Also, reference to “sets” in this disclosure is generally intended to refer to non-zero sets unless specifically stated to the contrary and is not intended to refer to the classical mathematical definition of sets that includes the so-called “empty set.” An alternative transformation may comprise a principal component analysis, which is often referred to as “PCA.” Depending on the context, PCA may be referred to by a number of different names, such as discrete Karhunen-Loeve transform, the Hotelling transform, proper orthogonal decomposition (POD), and eigenvalue decomposition (EVD) to name a few examples. Properties of such operations that are conducive to one of the potential underlying goal of compressing audio data may include one or more of ‘energy compaction’ and ‘decorrelation’ of the multichannel audio data. 
     In any event, assuming the LIT unit  30  performs a singular value decomposition (which, again, may be referred to as “SVD”) for purposes of example, the LIT unit  30  may transform the HOA coefficients  11  into two or more sets of transformed HOA coefficients. The “sets” of transformed HOA coefficients may include vectors of transformed HOA coefficients. In the example of  FIG.  3   , the LIT unit  30  may perform the SVD with respect to the HOA coefficients  11  to generate a so-called V matrix, an S matrix, and a U matrix. SVD, in linear algebra, may represent a factorization of a y-by-z real or complex matrix X (where X may represent multi-channel audio data, such as the HOA coefficients  11 ) in the following form:
 
 X=USV*  
 
U may represent a y-by-y real or complex unitary matrix, where the y columns of U are known as the left-singular vectors of the multi-channel audio data. S may represent a y-by-z rectangular diagonal matrix with non-negative real numbers on the diagonal, where the diagonal values of S are known as the singular values of the multi-channel audio data. V* (which may denote a conjugate transpose of V) may represent a z-by-z real or complex unitary matrix, where the z columns of V* are known as the right-singular vectors of the multi-channel audio data.
 
     In some examples, the V* matrix in the SVD mathematical expression referenced above is denoted as the conjugate transpose of the V matrix to reflect that SVD may be applied to matrices comprising complex numbers. When applied to matrices comprising only real-numbers, the complex conjugate of the V matrix (or, in other words, the V* matrix) may be considered to be the transpose of the V matrix. Below it is assumed, for ease of illustration purposes, that the HOA coefficients  11  comprise real-numbers with the result that the V matrix is output through SVD rather than the V* matrix. Moreover, while denoted as the V matrix in this disclosure, reference to the V matrix should be understood to refer to the transpose of the V matrix where appropriate. While assumed to be the V matrix, the techniques may be applied in a similar fashion to HOA coefficients  11  having complex coefficients, where the output of the SVD is the V* matrix. Accordingly, the techniques should not be limited in this respect to only provide for application of SVD to generate a V matrix, but may include application of SVD to HOA coefficients  11  having complex components to generate a V* matrix. 
     In this way, the LIT unit  30  may perform SVD with respect to the HOA coefficients  11  to output US[k] vectors  33  (which may represent a combined version of the S vectors and the U vectors) having dimensions D: M×(N+1) 2 , and V[k] vectors  35  having dimensions D: (N+1) 2 ×(N+1) 2 . Individual vector elements in the US[k] matrix may also be termed X PS (k) while individual vectors of the V[k] matrix may also be termed v(k). 
     An analysis of the U, S and V matrices may reveal that the matrices carry or represent spatial and temporal characteristics of the underlying soundfield represented above by X. Each of the N vectors in U (of length M samples) may represent normalized separated audio signals as a function of time (for the time period represented by M samples), that are orthogonal to each other and that have been decoupled from any spatial characteristics (which may also be referred to as directional information). The spatial characteristics, representing spatial shape and position (r, theta, phi) may instead be represented by individual i th  vectors, v (i) (k), in the V matrix (each of length (N+1) 2 ). 
     The individual elements of each of v (i) (k) vectors may represent an HOA coefficient describing the shape (including width) and position of the soundfield for an associated audio object. Both the vectors in the U matrix and the V matrix are normalized such that their root-mean-square energies are equal to unity. The energy of the audio signals in U are thus represented by the diagonal elements in S. Multiplying U and S to form US[k] (with individual vector elements X PS (k)), thus represent the audio signal with energies. The ability of the SVD decomposition to decouple the audio time-signals (in U), their energies (in S) and their spatial characteristics (in V) may support various aspects of the techniques described in this disclosure. Further, the model of synthesizing the underlying HOA[k] coefficients, X, by a vector multiplication of US[k] and V[k] gives rise the term “vector-based decomposition,” which is used throughout this document. 
     Although described as being performed directly with respect to the HOA coefficients  11 , the LIT unit  30  may apply the linear invertible transform to derivatives of the HOA coefficients  11 . For example, the LIT unit  30  may apply SVD with respect to a power spectral density matrix derived from the HOA coefficients  11 . By performing SVD with respect to the power spectral density (PSD) of the HOA coefficients rather than the coefficients themselves, the LIT unit  30  may potentially reduce the computational complexity of performing the SVD in terms of one or more of processor cycles and storage space, while achieving the same source audio encoding efficiency as if the SVD were applied directly to the HOA coefficients. 
     The parameter calculation unit  32  represents a unit configured to calculate various parameters, such as a correlation parameter (R), directional properties parameters (θ, φ, r), and an energy property (e). Each of the parameters for the current frame may be denoted as R[k], θ[k], φ[k], r[k] and e[k]. The parameter calculation unit  32  may perform an energy analysis and/or correlation (or so-called cross-correlation) with respect to the US[k] vectors  33  to identify the parameters. The parameter calculation unit  32  may also determine the parameters for the previous frame, where the previous frame parameters may be denoted R[k−1], θ[k−1], φ[k−1], r[k−1] and e[k−1], based on the previous frame of US[k−1] vector and V[k−1] vectors. The parameter calculation unit  32  may output the current parameters  37  and the previous parameters  39  to reorder unit  34 . 
     The parameters calculated by the parameter calculation unit  32  may be used by the reorder unit  34  to re-order the audio objects to represent their natural evaluation or continuity over time. The reorder unit  34  may compare each of the parameters  37  from the first US[k] vectors  33  turn-wise against each of the parameters  39  for the second US[k−1] vectors  33 . The reorder unit  34  may reorder (using, as one example, a Hungarian algorithm) the various vectors within the US[k] matrix  33  and the V[k] matrix  35  based on the current parameters  37  and the previous parameters  39  to output a reordered US[k] matrix  33 ′ (which may be denoted mathematically as  US [k]) and a reordered V[k] matrix  35 ′ (which may be denoted mathematically as  V [k]) to a foreground sound (or predominant sound—PS) selection unit  36  (“foreground selection unit  36 ”) and an energy compensation unit  38 . 
     The soundfield analysis unit  44  may represent a unit configured to perform a soundfield analysis with respect to the HOA coefficients  11  so as to potentially achieve a target bitrate  41 . The soundfield analysis unit  44  may, based on the analysis and/or on a received target bitrate  41 , determine the total number of psychoacoustic coder instantiations (which may be a function of the total number of ambient or background channels (BG TOT ) and the number of foreground channels or, in other words, predominant channels. The total number of psychoacoustic coder instantiations can be denoted as numHOATransportChannels. 
     The soundfield analysis unit  44  may also determine, again to potentially achieve the target bitrate  41 , the total number of foreground channels (nFG)  45 , the minimum order of the background (or, in other words, ambient) soundfield (N BG  or, alternatively, MinAmbHOAorder), the corresponding number of actual channels representative of the minimum order of background soundfield (nBGa=(MinAmbHOAorder+1) 2 ), and indices (i) of additional BG HOA channels to send (which may collectively be denoted as background channel information  43  in the example of  FIG.  3   ). The background channel information  42  may also be referred to as ambient channel information  43 . Each of the channels that remains from numHOATransportChannels−nBGa, may either be an “additional background/ambient channel”, an “active vector-based predominant channel”, an “active directional based predominant signal” or “completely inactive”. In one aspect, the channel types may be indicated (as a “ChannelType”) syntax element by two bits (e.g., 00: directional based signal; 01: vector-based predominant signal; 10: additional ambient signal; 11: inactive signal). The total number of background or ambient signals, nBGa, may be given by (MinAmbHOAorder+1) 2 +the number of times the index 10 (in the above example) appears as a channel type in the bitstream for that frame. 
     The soundfield analysis unit  44  may select the number of background (or, in other words, ambient) channels and the number of foreground (or, in other words, predominant) channels based on the target bitrate  41 , selecting more background and/or foreground channels when the target bitrate  41  is relatively higher (e.g., when the target bitrate  41  equals or is greater than 512 Kbps). In one aspect, the numHOATransportChannels may be set to 8 while the MinAmbHOAorder may be set to 1 in the header section of the bitstream. In this scenario, at every frame, four channels may be dedicated to representing the background or ambient portion of the soundfield while the other 4 channels can, on a frame-by-frame basis vary on the type of channel—e.g., either used as an additional background/ambient channel or a foreground/predominant channel. The foreground/predominant signals can be one of either vector-based or directional based signals, as described above. 
     In some instances, the total number of vector-based predominant signals for a frame, may be given by the number of times the ChannelType index is 01 in the bitstream of that frame. In the above aspect, for every additional background/ambient channel (e.g., corresponding to a ChannelType of 10), corresponding information of which of the possible HOA coefficients (beyond the first four) may be represented in that channel. The information, for fourth order HOA content, may be an index to indicate the HOA coefficients  5 - 25 . The first four ambient HOA coefficients  1 - 4  may be sent all the time when minAmbHOAorder is set to 1, hence the audio encoding device may only need to indicate one of the additional ambient HOA coefficient having an index of 5-25. The information could thus be sent using a 5 bits syntax element (for 4 th  order content), which may be denoted as “CodedAmbCoeffIdx.” In any event, the soundfield analysis unit  44  outputs the background channel information  43  and the HOA coefficients  11  to the background (BG) selection unit  36 , the background channel information  43  to coefficient reduction unit  46  and the bitstream generation unit  42 , and the nFG  45  to a foreground selection unit  36 . 
     The background selection unit  48  may represent a unit configured to determine background or ambient HOA coefficients  47  based on the background channel information (e.g., the background soundfield (N BG ) and the number (nBGa) and the indices (i) of additional BG HOA channels to send). For example, when N BG  equals one, the background selection unit  48  may select the HOA coefficients  11  for each sample of the audio frame having an order equal to or less than one. The background selection unit  48  may, in this example, then select the HOA coefficients  11  having an index identified by one of the indices (i) as additional BG HOA coefficients, where the nBGa is provided to the bitstream generation unit  42  to be specified in the bitstream  21  so as to enable the audio decoding device, such as the audio decoding device  24  shown in the example of  FIGS.  2  and  4   , to parse the background HOA coefficients  47  from the bitstream  21 . The background selection unit  48  may then output the ambient HOA coefficients  47  to the energy compensation unit  38 . The ambient HOA coefficients  47  may have dimensions D: M×[(NBG+1) 2   + nBGa]. The ambient HOA coefficients  47  may also be referred to as “ambient HOA coefficients  47 ,” where each of the ambient HOA coefficients  47  corresponds to a separate ambient HOA channel  47  to be encoded by the psychoacoustic audio coder unit  40 . 
     The foreground selection unit  36  may represent a unit configured to select the reordered US[k] matrix  33 ′ and the reordered V [k] matrix  35 ′ that represent foreground or distinct components of the soundfield based on nFG  45  (which may represent a one or more indices identifying the foreground vectors). The foreground selection unit  36  may output nFG signals  49  (which may be denoted as a reordered US[k] 1, . . . , nFG    49 , FG 1, . . . , nfG [k]  49 , or X PS   (1 . . . nFG) (k)  49 ) to the psychoacoustic audio coder unit  40 , where the nFG signals  49  may have dimensions D: M×nFG and each represent mono-audio objects. The foreground selection unit  36  may also output the reordered V[k] matrix  35 ′ (or v (1 . . . nFG) (k)  35 ′) corresponding to foreground components of the soundfield to the spatio-temporal interpolation unit  50 , where a subset of the reordered V[k] matrix  35 ′ corresponding to the foreground components may be denoted as foreground V[k] matrix  51   k  (which may be mathematically denoted as  V   1, . . . , nFG  [k]) having dimensions D: (N+1) 2 ×nFG. 
     The energy compensation unit  38  may represent a unit configured to perform energy compensation with respect to the ambient HOA coefficients  47  to compensate for energy loss due to removal of various ones of the HOA channels by the background selection unit  48 . The energy compensation unit  38  may perform an energy analysis with respect to one or more of the reordered US[k] matrix  33 ′, the reordered V[k] matrix  35 ′, the nFG signals  49 , the foreground V[k] vectors  51   k  and the ambient HOA coefficients  47  and then perform energy compensation based on the energy analysis to generate energy compensated ambient HOA coefficients  47 ′. The energy compensation unit  38  may output the energy compensated ambient HOA coefficients  47 ′ to the decorrelation unit  60 . 
     The decorrelation unit  60  may represent a unit configured to implement various aspects of the techniques described in this disclosure to reduce or eliminate correlation between the energy compensated ambient HOA coefficients  47 ′ to form one or more decorrelated ambient HOA audio signals  67 . The decorrelation unit  40 ′ may output the decorrelated HOA audio signals  67  to the gain control unit  62 . The gain control unit  62  may represent a unit configured to perform automatic gain control (which may be abbreviated as “AGC”) with respect to the decorrelated ambient HOA audio signals  67  to obtain gain controlled ambient HOA audio signals  67 ′. After applying the gain control, the automatic gain control unit  62  may provide the gain controlled ambient HOA audio signals  67 ′ to the psychoacoustic audio coder unit  40 . 
     The decorrelation unit  60  included within the audio encoding device  20  may represent single or multiple instances of a unit configured to apply one or more decorrelation transforms to the energy compensated ambient HOA coefficients  47 ′, to obtain the decorrelated HOA audio signals  67 . In some examples, the decorrelation unit  40 ′ may apply a UHJ matrix to the energy compensated ambient HOA coefficients  47 ′. At various instances of this disclosure, the UHJ matrix may also be referred to as a “phase-based transform.” Application of the phase-based transform may also be referred to herein as “phaseshift decorrelation.” 
     Ambisonic UHJ format is a development of the Ambisonic surround sound system designed to be compatible with mono and stereo media. The UHJ format includes a hierarchy of systems in which the recorded soundfield will be reproduced with a degree of accuracy that varies according to the available channels. In various instances, UHJ is also referred to as “C-Format”. The initials indicate some of sources incorporated into the system: U from Universal (UD-4); H from Matrix H; and J from System 45J. 
     UHJ is a hierarchical system of encoding and decoding directional sound information within Ambisonics technology. Depending on the number of channels available, a system can carry more or less information. UHJ is fully stereo- and mono-compatible. Up to four channels (L, R, T, Q) may be used. 
     In one form, 2-channel (L, R) UHJ, horizontal (or “planar”) surround information can be carried by normal stereo signal channels—CD, FM or digital radio, etc.—which may be recovered by using a UHJ decoder at the listening end. Summing the two channels may yield a compatible mono signal, which may be a more accurate representation of the two-channel version than summing a conventional “panpotted mono” source. If a third channel (T) is available, the third channel can be used to yield improved localization accuracy to the planar surround effect when decoded via a 3-channel UHJ decoder. The third channel may not be required to have full audio bandwidth for this purpose, leading to the possibility of so-called “2½-channel” systems, where the third channel is bandwidth-limited. In one example, the limit may be 5 kHz. The third channel can be broadcast via FM radio, for example, by means of phase-quadrature modulation. Adding a fourth channel (Q) to the UHJ system may allow the encoding of full surround sound with height, sometimes referred to n as Periphony, with a level of accuracy identical to 4-channel B-Format. 
     2-channel UHJ is a format commonly used for distribution of Ambisonic recordings. 2-channel UHJ recordings can be transmitted via all normal stereo channels and any of the normal 2-channel media can be used with no alteration. UHJ is stereo compatible in that, without decoding, the listener may perceive a stereo image, but one that is significantly wider than conventional stereo (e.g., so-called “Super Stereo”). The left and right channels can also be summed for a very high degree of mono-compatibility. Replayed via a UHJ decoder, the surround capability may be revealed. 
     An example mathematical representation of the decorrelation unit  60  applying the UHJ matrix (or phase-based transform) is as follows: 
     UHJ Encoding:
 
 S =(0.9397* W )+(0.1856* X );
 
 D =imag(hilbert((−0.3420* W )+(0.5099* X )))+(0.6555* Y );
 
 T =imag(hilbert((−0.1432* W )+(0.6512* X )))−(0.7071* Y );
 
 Q= 0.9772* Z;  
 
     conversion of S and D to Left and Right:
 
Left=( S+D )/2
 
Right=( S−D )/2
 
     According to some implementations of the calculations above, assumptions with respect to the calculations above may include the following: HOA Background channel are 1st order Ambisonics, FuMa normalized, in the Ambisonics channel numbering order W (a00), X(a11), Y(a11−), Z(a10). 
     In the calculations listed above, the decorrelation unit  40 ′ may perform a scalar multiplication of various matrices by constant values. For instance, to obtain the S signal, the decorrelation unit  60  may perform scalar multiplication of a W matrix by the constant value of 0.9397 (e.g., by scalar multiplication), and of an X matrix by the constant value of 0.1856. As also illustrated in the calculations listed above, the decorrelation unit  60  may apply a Hilbert transform (denoted by the “Hilbert ( )” function in the above UHJ encoding) in obtaining each of the D and T signals. The “imag( )” function in the above UHJ encoding indicates that the imaginary (in the mathematical sense) of the result of the Hilbert transform is obtained. 
     Another example mathematical representation of the decorrelation unit  60  applying the UHJ matrix (or phase-based transform) is as follows: 
     UHJ Encoding:
 
 S =(0.9396926* W )+(0.151520536509082* X );
 
 D =imag(hilbert((−0.3420201* W )+(0.416299273350443* X )))+(0.535173990363608* Y );
 
 T= 0.940604061228740*(imag(hilbert((−0.1432* W )+(0.531702573500135* X )))−(0.577350269189626* Y ));
 
 Q=Z;  
 
     conversion of S and D to Left and Right:
 
Left=( S+D )/2;
 
Right=( S−D )/2;
 
     In some example implementations of the calculations above, assumptions with respect to the calculations above may include the following: HOA Background channel are 1st order Ambisonics, N3D (or “full three-D”) normalized, in the Ambisonics channel numbering order W (a00), X(a11), Y(a11−), Z(a10). Although described herein with respect to N3D normalization, it will be appreciated that the example calculations may also be applied to HOA background channels that are SN3D normalized (or “Schmidt semi-normalized). N3D and SN3D normalization may differ in terms of the scaling factors used. An example representation of N3D normalization, relative to SN3D normalization, is expressed below:
 
 N   l,m   N3D   =N   l,m   SN3D √{square root over (2 l+ 1)}
 
     An example of weighting coefficients used in SN3D normalization is expressed below: 
     
       
         
           
             
               
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     In the calculations listed above, the decorrelation unit  60  may perform a scalar multiplication of various matrices by constant values. For instance, to obtain the S signal, the decorrelation unit  60  may perform scalar multiplication of a W matrix by the constant value of 0.9396926 (e.g., by scalar multiplication), and of an X matrix by the constant value of 0.151520536509082. As also illustrated in the calculations listed above, the decorrelation unit  60  may apply a Hilbert transform (denoted by the “Hilbert ( )” function in the above UHJ encoding or phaseshift decorrelation) in obtaining each of the D and T signals. The “imag( )” function in the above UHJ encoding indicates that the imaginary (in the mathematical sense) of the result of the Hilbert transform is obtained. 
     The decorrelation unit  60  may perform the calculations listed above, such that the resulting S and D signals represent left and right audio signals (or in other words stereo audio signals). In some such scenarios, the decorrelation unit  60  may output the T and Q signals as part of the decorrelated ambient HOA audio signals  67 , but a decoding device that receives the bitstream  21  may not process the T and Q signals when rendering to a stereo speaker geometry (or, in other words, stereo speaker configuration). In examples, the ambient HOA coefficients  47 ′ may represent a soundfield to be rendered on a mono-audio reproduction system. The decorrelation unit  60  may output the S and D signals as part of the decorrelated ambient HOA audio signals  67 , and a decoding device that receives the bitstream  21  may combine (or “mix”) the S and D signals to form an audio signal to be rendered and/or output in mono-audio format. 
     In these examples, the decoding device and/or the reproduction device may recover the mono-audio signal in various ways. One example is by mixing the left and right signals (represented by the S and D signals). Another example is by applying a UHJ matrix (or phase-based transform) to decode a W signal. By producing a natural left signal and a natural right signal in the form of the S and D signals by applying the UHJ matrix (or phase-based transform), the decorrelation unit  60  may implement techniques of this disclosure to provide potential advantages and/or potential improvements over techniques that apply other decorrelation transforms (such as a mode matrix described in the MPEG-H standard). 
     In various examples, the decorrelation unit  60  may apply different decorrelation transforms, based on a bit rate of the received energy compensated ambient HOA coefficients  47 ′. For example, the decorrelation unit  60  may apply the UHJ matrix (or phase-based transform) described above in scenarios where the energy compensated ambient HOA coefficients  47 ′ represent a four-channel input. More specifically, based on the energy compensated ambient HOA coefficients  47 ′ representing a four-channel input, the decorrelation unit  60  may apply a 4×4 UHJ matrix (or phase-based transform). For instance, the 4×4 matrix may be orthogonal to the four-channel input of the energy compensated ambient HOA coefficients  47 ′. In other words, in instances where the energy compensated ambient HOA coefficients  47 ′ represent a lesser number of channels (e.g., four), the decorrelation unit  60  may apply the UHJ matrix as the selected decorrelation transform, to decorrelate the background signals of the energy compensated ambient HOA signals  47 ′ to obtain the decorrelated ambient HOA audio signals  67 . 
     According to this example, if the energy compensated ambient HOA coefficients  47 ′ represent a greater number of channels (e.g., nine), the decorrelation unit  60  may apply a decorrelation transform different from the UHJ matrix (or phase-based transform). For instance, in a scenario where the energy compensated ambient HOA coefficients  47 ′ represent a nine-channel input, the decorrelation unit  60  may apply a mode matrix (e.g., as described in phase I of the MPEG-H 3D audio standard referenced above), to decorrelate the energy compensated ambient HOA coefficients  47 ′. In examples where the energy compensated ambient HOA coefficients  47 ′ represent a nine-channel input, the decorrelation unit  60  may apply a 9×9 mode matrix to obtain the decorrelated ambient HOA audio signals  67 . 
     In turn, various components of the audio encoding device  20  (such as the psychoacoustic audio coder  40 ) may perceptually code the decorrelated ambient HOA audio signals  67  according to AAC or USAC. The decorrelation unit  60  may apply the phaseshift decorrelation transform (e.g., the UHJ matrix or phase-based transform in case of a four-channel input), to potentially optimize the AAC/USAC coding for HOA. In examples where the energy compensated ambient HOA coefficients  47 ′ (and thereby, the decorrelated ambient HOA audio signals  67 ) represent audio data to be rendered on a stereo reproduction system, the decorrelation unit  60  may apply the techniques of this disclosure to improve or optimize compression, based on AAC and USAC being relatively oriented (or optimized for) stereo audio data. 
     It will be understood that the decorrelation unit  60  may apply the techniques described herein in situations where the energy compensated ambient HOA coefficients  47 ′ include foreground channels, as well in situations where the energy compensated ambient HOA coefficients  47 ′ do not include any foreground channels. As one example, the decorrelation unit  40 ′ may apply the techniques and/or calculations described above, in a scenario where the energy compensated ambient HOA coefficients  47 ′ include zero (0) foreground channels and four (4) background channels (e.g., a scenario of a lower/lesser bit rate). 
     In some examples, the decorrelation unit  60  may cause the bitstream generation unit  42  to signal, as part of the vector-based bitstream  21 , one or more syntax elements that indicate that the decorrelation unit  60  applied a decorrelation transform to the energy compensated ambient HOA coefficients  47 ′. By providing such an indication to a decoding device, the decorrelation unit  60  may enable the decoding device to perform reciprocal decorrelation transforms on audio data in the HOA domain. In some examples, the decorrelation unit  60  may cause the bitstream generation unit  42  to signal syntax elements that indicate which decorrelation transform was applied, such as the UHJ matrix (or other phase based transform) or the mode matrix. 
     The decorrelation unit  60  may apply a phase-based transform to the energy compensated ambient HOA coefficient  47 ′. The phase-based transform for the first O MIN  HOA coefficient sequences of C AMB (k−1) is defined by 
     
       
         
           
             
               
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                           c 
                           
                             
                               A 
                               ⁢ 
                               M 
                               ⁢ 
                               B 
                             
                             , 
                             3 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             k 
                             - 
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
                 ] 
               
             
             , 
           
         
       
     
     with the coefficients d as defined in Table 1, the signal frames S(k−2) and M(k−2) being defined by
 
 S ( k− 2)= A   +90 ( k− 2)+ d (6)· c   AMB,2 ( k− 2)
 
 M ( k− 2)= d (4)· c   AMG,1 ( k− 2)+ d (5)· c   AMB,4 ( k− 2)
 
and A +90 (k−2) and B +90 (k−2) are the frames of +90 degree phase shifted signals A and B defined by
 
 A ( k− 2)= d (0)− c   AMB,Low,1 ( k− 2)+ d (1)− c   AMB,4 ( k− 2)
 
 B ( k− 2)= d (2)− c   AMB,Low,1 ( k− 2)+ d (3)− c   AMB,4 ( k− 2).
 
     The phase-based transform for the first ° M IN  HOA coefficient sequences of C P,AMB (k−1) is defined accordingly. The transform described may introduce a delay of one frame. 
     In the foregoing, the x AMB,LOW,1 (k−2) through x AMB,LOW,4 (k−2) may correspond to decorrelated ambient HOA audio signals  67 . In the foregoing equation, the variable C AMB,1 (k) variable denotes the HOA coefficients for the k th  frame corresponding to the spherical basis functions having an (order:sub-order) of (0:0), which may also be referred to as the ‘W’ channel or component. The variable C AMB,2 (k) variable denotes the HOA coefficients for the k th  frame corresponding to the spherical basis functions having an (order:sub-order) of (1:−1), which may also be referred to as the ‘Y’ channel or component. The variable C AMB,3 (k) variable denotes the HOA coefficients for the k th  frame corresponding to the spherical basis functions having an (order:sub-order) of (1:0), which may also be referred to as the ‘Z’ channel or component. The variable C AMB,4 (k) variable denotes the HOA coefficients for the k th  frame corresponding to the spherical basis functions having an (order:sub-order) of (1:1), which may also be referred to as the ‘X’ channel or component. The C AMB,1 (k) through C AMB,3 (k) may correspond to ambient HOA coefficients  47 ′. 
     Table 1 below illustrates an example of coefficients that the decorrelation unit  40  may use for performing a phase-based transform. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Coefficients for phase-based transform 
               
            
           
           
               
               
            
               
                 n 
                 d(n) 
               
               
                   
               
               
                 0 
                 0.34202009999999999 
               
               
                 1 
                 0.41629927335044281 
               
               
                 2 
                 0.14319999999999999 
               
               
                 3 
                 0.53170257350013528 
               
               
                 4 
                 0.93969259999999999 
               
               
                 5 
                 0.15152053650908184 
               
               
                 6 
                 0.53517399036360758 
               
               
                 7 
                 0.57735026918962584 
               
               
                 8 
                 0.94060406122874030 
               
               
                 9 
                 0.500000000000000 
               
               
                   
               
            
           
         
       
     
     In some examples, various components of the audio encoding device  20  (such as the bitstream generation unit  42 ) may be configured to transmit only first order HOA representations for lower target bitrates (e.g., a target bitrate of 128K or 256K). According to some such examples, the audio encoding device  20  (or components thereof, such as the bitstream generation unit  42 ) may be configured to discard higher order HOA coefficients (e.g., coefficients with a greater order than the first order, or in other words, N&gt;1). However, in examples where the audio encoding device  20  determines that the target bitrate is relatively high, the audio encoding device  20  (e.g., the bitstream generation unit  42 ) may separate the foreground and background channels, and may assign bits (e.g., in greater amounts) to the foreground channels. 
     Although described as being applied to the energy compensated ambient HOA coefficients  47 ′, the audio encoding device  20  may not apply decorrelation to the energy compensated ambient HOA coefficients  47 ′. Instead, energy compensation unit  38  may provide the energy compensated ambient HOA coefficients  47 ′ directly to the gain control unit  62 , which may perform automatic gain control with respect to the energy compensated ambient HOA coefficients  47 ′. As such, the decorrelation unit  60  is shown as a dashed line to indicate that the decorrelation unit may not always perform decorrelation or be included in the audio decoding device  20 . 
     The spatio-temporal interpolation unit  50  may represent a unit configured to receive the foreground V[k] vectors  51   k  for the k th  frame and the foreground V[k−1] vectors  51  for the previous frame (hence the k−1 notation) and perform spatio-temporal interpolation to generate interpolated foreground V[k] vectors. The spatio-temporal interpolation unit  50  may recombine the nFG signals  49  with the foreground V[k] vectors  51   k  to recover reordered foreground HOA coefficients. The spatio-temporal interpolation unit  50  may then divide the reordered foreground HOA coefficients by the interpolated V[k] vectors to generate interpolated nFG signals  49 ′. 
     The spatio-temporal interpolation unit  50  may also output the foreground V[k] vectors  51   k  that were used to generate the interpolated foreground V[k] vectors so that an audio decoding device, such as the audio decoding device  24 , may generate the interpolated foreground V[k] vectors and thereby recover the foreground V[k] vectors  51   k . The foreground V[k] vectors  51   k  used to generate the interpolated foreground V[k] vectors are denoted as the remaining foreground V[k] vectors  53 . In order to ensure that the same V[k] and V[k−1] are used at the encoder and decoder (to create the interpolated vectors V[k]) quantized/dequantized versions of the vectors may be used at the encoder and decoder. The spatio-temporal interpolation unit  50  may output the interpolated nFG signals  49 ′ to the gain control unit  62  and the interpolated foreground V[k] vectors  51   k  to the coefficient reduction unit  46 . 
     The gain control unit  62  may also represent a unit configured to perform automatic gain control (which may be abbreviated as “AGC”) with respect to the interpolated nFG signals  49 ′ to obtain gain controlled nFG signals  49 ″. After applying the gain control, the automatic gain control unit  62  may provide the gain controlled nFG signals  49 ″ to the psychoacoustic audio coder unit  40 . 
     The coefficient reduction unit  46  may represent a unit configured to perform coefficient reduction with respect to the remaining foreground V[k] vectors  53  based on the background channel information  43  to output reduced foreground V[k] vectors  55  to the quantization unit  52 . The reduced foreground V[k] vectors  55  may have dimensions D: [(N+1) 2 −(NBG+1) 2 −BG TOT ]×nFG. The coefficient reduction unit  46  may, in this respect, represent a unit configured to reduce the number of coefficients in the remaining foreground V[k] vectors  53 . In other words, coefficient reduction unit  46  may represent a unit configured to eliminate the coefficients in the foreground V[k] vectors (that form the remaining foreground V[k] vectors  53 ) having little to no directional information. In some examples, the coefficients of the distinct or, in other words, foreground V[k] vectors corresponding to a first and zero order basis functions (which may be denoted as NBG) provide little directional information and therefore can be removed from the foreground V-vectors (through a process that may be referred to as “coefficient reduction”). In this example, greater flexibility may be provided to not only identify the coefficients that correspond N BG  but to identify additional HOA channels (which may be denoted by the variable TotalOfAddAmbHOAChan) from the set of [(N BG +1) 2 +1, (N+1) 2 ]. 
     The quantization unit  52  may represent a unit configured to perform any form of quantization to compress the reduced foreground V[k] vectors  55  to generate coded foreground V[k] vectors  57 , outputting the coded foreground V[k] vectors  57  to the bitstream generation unit  42 . In operation, the quantization unit  52  may represent a unit configured to compress a spatial component of the soundfield, i.e., one or more of the reduced foreground V[k] vectors  55  in this example. The quantization unit  52  may perform any one of the following 12 quantization modes set forth in phase I or phase II of the MPEG-H 3D audio coding standard referenced above. The quantization unit  52  may also perform predicted versions of any of the foregoing types of quantization modes, where a difference is determined between an element of (or a weight when vector quantization is performed) of the V-vector of a previous frame and the element (or weight when vector quantization is performed) of the V-vector of a current frame is determined. The quantization unit  52  may then quantize the difference between the elements or weights of the current frame and previous frame rather than the value of the element of the V-vector of the current frame itself. The quantization unit  52  may provide the coded foreground V[k] vectors  57  to the bitstream generation unit  42 . The quantization unit  52  may also provide the syntax elements indicative of the quantization mode (e.g., the NbitsQ syntax element) and any other syntax elements used to dequantize or otherwise reconstruct the V-vector. 
     The psychoacoustic audio coder unit  40  included within the audio encoding device  20  may represent multiple instances of a psychoacoustic audio coder, each of which is used to encode a different audio object or HOA channel of each of the energy compensated ambient HOA coefficients  47 ′ and the interpolated nFG signals  49 ′ to generate encoded ambient HOA coefficients  59  and encoded nFG signals  61 . The psychoacoustic audio coder unit  40  may output the encoded ambient HOA coefficients  59  and the encoded nFG signals  61  to the bitstream generation unit  42 . 
     The bitstream generation unit  42  included within the audio encoding device  20  represents a unit that formats data to conform to a known format (which may refer to a format known by a decoding device), thereby generating the vector-based bitstream  21 . The bitstream  21  may, in other words, represent encoded audio data, having been encoded in the manner described above. The bitstream generation unit  42  may represent a multiplexer in some examples, which may receive the coded foreground V[k] vectors  57 , the encoded ambient HOA coefficients  59 , the encoded nFG signals  61  and the background channel information  43 . The bitstream generation unit  42  may then generate a bitstream  21  based on the coded foreground V[k] vectors  57 , the encoded ambient HOA coefficients  59 , the encoded nFG signals  61  and the background channel information  43 . In this way, the bitstream generation unit  42  may thereby specify the vectors  57  in the bitstream  21  to obtain the bitstream  21 . The bitstream  21  may include a primary or main bitstream and one or more side channel bitstreams. 
     Although not shown in the example of  FIG.  3   , the audio encoding device  20  may also include a bitstream output unit that switches the bitstream output from the audio encoding device  20  (e.g., between the directional-based bitstream  21  and the vector-based bitstream  21 ) based on whether a current frame is to be encoded using the directional-based synthesis or the vector-based synthesis. The bitstream output unit may perform the switch based on the syntax element output by the content analysis unit  26  indicating whether a directional-based synthesis was performed (as a result of detecting that the HOA coefficients  11  were generated from a synthetic audio object) or a vector-based synthesis was performed (as a result of detecting that the HOA coefficients were recorded). The bitstream output unit may specify the correct header syntax to indicate the switch or current encoding used for the current frame along with the respective one of the bitstreams  21 . 
     Moreover, as noted above, the soundfield analysis unit  44  may identify BG TOT  ambient HOA coefficients  47 , which may change on a frame-by-frame basis (although at times BG TOT  may remain constant or the same across two or more adjacent (in time) frames). The change in BG TOT  may result in changes to the coefficients expressed in the reduced foreground V[k] vectors  55 . The change in BG TOT  may result in background HOA coefficients (which may also be referred to as “ambient HOA coefficients”) that change on a frame-by-frame basis (although, again, at times BG TOT  may remain constant or the same across two or more adjacent (in time) frames). The changes often result in a change of energy for the aspects of the sound field represented by the addition or removal of the additional ambient HOA coefficients and the corresponding removal of coefficients from or addition of coefficients to the reduced foreground V[k] vectors  55 . 
     As a result, the soundfield analysis unit  44  may further determine when the ambient HOA coefficients change from frame to frame and generate a flag or other syntax element indicative of the change to the ambient HOA coefficient in terms of being used to represent the ambient components of the sound field (where the change may also be referred to as a “transition” of the ambient HOA coefficient or as a “transition” of the ambient HOA coefficient). In particular, the coefficient reduction unit  46  may generate the flag (which may be denoted as an AmbCoeffTransition flag or an AmbCoeffldxTransition flag), providing the flag to the bitstream generation unit  42  so that the flag may be included in the bitstream  21  (possibly as part of side channel information). 
     The coefficient reduction unit  46  may, in addition to specifying the ambient coefficient transition flag, also modify how the reduced foreground V[k] vectors  55  are generated. In one example, upon determining that one of the ambient HOA ambient coefficients is in transition during the current frame, the coefficient reduction unit  46  may specify, a vector coefficient (which may also be referred to as a “vector element” or “element”) for each of the V-vectors of the reduced foreground V[k] vectors  55  that corresponds to the ambient HOA coefficient in transition. Again, the ambient HOA coefficient in transition may add or remove from the BG TOT  total number of background coefficients. Therefore, the resulting change in the total number of background coefficients affects whether the ambient HOA coefficient is included or not included in the bitstream, and whether the corresponding element of the V-vectors are included for the V-vectors specified in the bitstream in the second and third configuration modes described above. More information regarding how the coefficient reduction unit  46  may specify the reduced foreground V[k] vectors  55  to overcome the changes in energy is provided in U.S. application Ser. No. 14/594,533, entitled “TRANSITIONING OF AMBIENT HIGHER_ORDER AMBISONIC COEFFICIENTS,” filed Jan. 12, 2015. 
     In this respect, the bitstream generation unit  42  may generate a bitstream  21  in a wide variety of different encoding schemes, which may facilitate flexible bitstream generation to accommodate a large number of different content delivery contexts. One context that appears to be gaining traction within the audio industry is the delivery (or, in other words, “streaming”) of audio data via networks to a growing number of different playback devices. Delivering audio content via bandwidth constricted networks to devices having varying degrees of playback capabilities may be difficult, especially in the context of HOA audio data that permit a high degree of 3D audio fidelity during playback at an expense of large bandwidth consumption (relative to channel- or object-based audio data). 
     In accordance with the techniques described in this disclosure, the bitstream generation unit  42  may utilize one or more scalable layers to allow for various reconstructions of the HOA coefficients  11 . Each of the layers may be hierarchical. For example, a first layer (which may be referred to as a “base layer”) may provide a first reconstruction of the HOA coefficients that permits for stereo loudspeaker feeds to be rendered. A second layer (which may be referred to as a first “enhancement layer”) may, when applied to the first reconstruction of the HOA coefficients, scale the first reconstruction of the HOA coefficient to permit for horizontal surround sound loudspeaker feeds (e.g., 5.1 loudspeaker feeds) to be rendered. A third layer (which may be referred to as a second “enhancement layer”) may provide may, when applied to the second reconstruction of the HOA coefficients, scale the first reconstruction of the HOA coefficient to permit for 3D surround sound loudspeaker feeds (e.g., 22.2 loudspeaker feeds) to be rendered. In this respect, the layers may be considered to hierarchical scale a previous layer. In other words, the layers are hierarchical such that a first layer, when combined with a second layer, provides a higher resolution representation of the higher order ambisonic audio signal. 
     Although described above as allowing for scaling of an immediately preceding layer, any layer above another layer may scale the lower layer. In other words, the third layer described above may be used to scale the first layer, even though the first layer has not been “scaled” by the second layer. The third layer, when applied directly to the first layer, may provide height information and thereby allow for irregular speaker feeds corresponding to irregularly arranged speaker geometries to be rendered. 
     The bitstream generation unit  42  may, in order to permit the layers to be extracted from the bitstream  21 , specify an indication of a number of layers specified in the bitstream. The bitstream generation unit  42  may output the bitstream  21  that includes the indicated number of layers. The bitstream generation unit  42  is described in more detail with respect to  FIG.  5   . Various different examples of generating the scalable HOA audio data are described in the following  FIGS.  7 A- 9 B , with an example of the sideband information for each of the above examples in  FIGS.  10 - 13 B . 
       FIG.  5    is a diagram illustrating, in more detail, the bitstream generation unit  42  of  FIG.  3    when configured to perform a first one of the potential versions of the scalable audio coding techniques described in this disclosure. In the example of  FIG.  5   , the bitstream generation unit  42  includes a scalable bitstream generation unit  1000  and a non-scalable bitstream generation unit  1002 . The scalable bitstream generation unit  1000  represents a unit configured to generate a scalable bitstream  21  comprising two or more layers (although in some instances a scalable bitstream may comprise a single layer for certain audio contexts) having HOAFrames( ) similar to those shown in and described below with respect to the examples of  FIGS.  11 - 13 B . The non-scalable bitstream generation unit  1002  may represent a unit configured to generate a non-scalable bitstream  21  that does not provide for layers or, in other words, scalability. 
     Both the non-scalable bitstream  21  and the scalable bitstream  21  may be referred to as “bitstream  21 ” given that both typically include the same underlying data in terms of the encoded ambient HOA coefficients  59 , the encoded nFG signals  61  and the coded foreground V[k] vectors  57 . One difference, however, between the non-scalable bitstream  21  and the scalable bitstream  21  is that the scalable bitstream  21  includes layers, which may be denoted as layers  21 A,  21 B, etc. The layers  21 A may include subsets of the encoded ambient HOA coefficients  59 , the encoded nFG signals  61  and the coded foreground V[k] vectors  57 , as described in more detail below. 
     Although the scalable and non-scalable bitstreams  21  may effectively be different representations of the same bitstream  21 , the non-scalable bitstream  21  is denoted as non-scalable bitstream  21 ′ to differentiate the scalable bitstream  21  from the non-scalable bitstream  21 ′. Moreover, in some instances, the scalable bitstream  21  may include various layers that conform to the non-scalable bitstream  21 . For example, the scalable bitstream  21  may include a base layer that conforms to non-scalable bitstream  21 . In these instances, the non-scalable bitstream  21 ′ may represent a sub-bitstream of scalable bitstream  21 , where this non-scalable sub-bitstream  21 ′ may be enhanced with additional layers of the scalable bitstream  21  (which are referred to as enhancement layers). 
     The bitstream generation unit  42  may obtain scalability information  1003  indicative of whether to invoke the scalable bitstream generation unit  1000  or the non-scalable bitstream generation unit  1002 . In other words, the scalability information  1003  may indicate whether bitstream generation unit  42  is to output scalable bitstream  21  or non-scalable bitstream  21 ′. For purposes of illustration, the scalability information  1003  is assumed to indicate that the bitstream generation unit  42  is to invoke the scalable bitstream generation unit  1000  to output the scalable bitstream  21 ′. 
     As further shown in the example of  FIG.  5   , the bitstream generation unit  42  may receive the encoded ambient HOA coefficients  59 A- 59 D, the encoded nFG signals  61 A and  61 B, and the coded foreground V[k] vectors  57 A and  57 B. The encoded ambient HOA coefficients  59 A may represent encoded ambient HOA coefficients associated with a spherical basis function having an order of zero and a sub-order of zero. The encoded ambient HOA coefficients  59 B may represent encoded ambient HOA coefficients associated with a spherical basis function having an order of one and a sub-order of zero. The encoded ambient HOA coefficients  59 C may represent encoded ambient HOA coefficients associated with a spherical basis function having an order of one and a sub-order of negative one. The encoded ambient HOA coefficients  59 D may represent encoded ambient HOA coefficients associated with a spherical basis function having an order of one and a sub-order of positive one. The encoded ambient HOA coefficients  59 A- 59 D may represent one example of, and as a result may be referred to collectively as, the encoded ambient HOA coefficients  59  discussed above. 
     The encoded nFG signals  61 A and  61 B may each represent a US audio object representative of, in this example, the two most predominant foreground aspects of the soundfield. The coded foreground V[k] vectors  57 A and  57 B may represent directional information (which may also specify width in addition to direction) for the encoded nFG signals  61 A and  61 B respectively. The encoded nFG signals  61 A and  61 B may represent one example of, and as a result may be referred to collectively as, the encoded nFG signals  61  described above. The coded foreground V[k] vectors  57 A and  57 B may represent one example of, and as a result may be referred to collectively as, the coded foreground V[k] vectors  57  described above. 
     Once invoked, the scalable bitstream generation unit  1000  may generate the scalable bitstream  21  to include the layers  21 A and  21 B in a manner substantially similar to that described below with respect to  FIGS.  7 A- 9 B . The scalable bitstream generation unit  1000  may specify an indication of the number of layers in the scalable bitstream  21  as well as the number of foreground elements and background elements in each of the layers  21 A and  21 B. The scalable bitstream generation unit  1000  may, as one example, specify a NumberOfLayers syntax element that may specify L number of layers, where the variable L may denote the number of layers. The scalable bitstream generation unit  1000  may then specify, for each layer (which may be denoted as the variable i=1 to L), the Bi number of the encoded ambient HOA coefficients  59  and the Fi number of the coded nFG signals  61  sent for each layer (which may also or alternatively indicate the number of corresponding coded foreground V[k] vectors  57 ). 
     In the example of  FIG.  5   , the scalable bitstream generation unit  1000  may specify in the scalable bitstream  21  that scalable coding has been enabled and that two layers are included in the scalable bitstream  21 , that the first layer  21 A includes four encoded ambient HOA coefficients  59  and zero encoded nFG signals  61 , and that the second layer  21 A includes zero encoded ambient HOA coefficients  59  and w encoded nFG signals  61 . The scalable bitstream generation unit  1000  may also generate the first layer  21 A (which may also be referred to as a “base layer  21 A”) to include the encoded ambient HOA coefficients  59 . The scalable bitstream generation unit  1000  may further generate the second layer  21 A (which may be referred to as an “enhancement layer  21 B”) to include the encoded nFG signals  61  and the coded foreground V[k] vectors  57 . The scalable bitstream generation unit  1000  may output the layers  21 A and  21 B as scalable bitstream  21 . In some examples, the scalable bitstream generation unit  1000  may store the scalable bitstream  21 ′ to a memory (either internal to or external from the encoder  20 ). 
     In some instances, the scalable bitstream generation unit  1000  may not specify one or more or any of the indications of the number of layers, the number of foreground components (e.g., number of the encoded nFG signals  61  and coded foreground V[k] vectors  57 ) in the one or more layers, and the number of background components (e.g., the encoded ambient HOA coefficients  59 ) in the one or more layers. The components may also be referred to as channels in this disclosure. Instead, the scalable bitstream generation unit  1000  may compare the number of layers for a current frame to the number of layers for a previous frame (e.g., the most temporally recent previous frame). When the comparison results in no differences (meaning that the number of layers in the current frame is equal to the number of layers in the previous frame, the scalable bitstream generation unit  1000  may compare the number of background and foreground components in each layer in a similar manner. 
     In other words, the scalable bitstream generation unit  1000  may compare the number of background components in the one or more layers for the current frame to the number of background component in the one or more layers for a previous frame. The scalable bitstream generation unit  1000  may further compare the number of foreground components in the one or more layers for the current frame to the number of foreground components in the one or more layers for the previous frame. 
     When both of the component-based comparisons result in no differences (meaning, that the number of foreground and background components in the previous frame is equal to the number of foreground and background components in the current frame), the scalable bitstream generation unit  1000  may specify an indication (e.g., an HOABaseLayerConfigurationFlag syntax element) in the scalable bitstream  21  that the number of layers in the current frame is equal to the number of layers in the previous frame rather than specify one or more or any of the indications of the number of layers, the number of foreground components (e.g., number of the encoded nFG signals  61  and coded foreground V[k] vectors  57 ) in the one or more layers, and the number of background components (e.g., the encoded ambient HOA coefficients  59 ) in the one or more layers. The audio decoding device  24  may then determine that the previous frame indications of the number of layers, background components and foreground components equal the current frame indication of number of the number of layers, background components and foreground components, as described below in more detail. 
     When any of the comparisons noted above result in differences, the scalable bitstream generation unit  1000  may specify an indication (e.g., an HOABaseLayerConfigurationFlag syntax element) in the scalable bitstream  21  that the number of layers in the current frame is not equal to the number of layers in the previous frame. The scalable bitstream generation unit  1000  may then specify the indications of the number of layers, the number of foreground components (e.g., number of the encoded nFG signals  61  and coded foreground V[k] vectors  57 ) in the one or more layers, and the number of background components (e.g., the encoded ambient HOA coefficients  59 ) in the one or more layers, as noted above. In this respect, the scalable bitstream generation unit  1000  may specify, in the bitstream, an indication of whether a number of layers of the bitstream has changed in a current frame when compared to a number of layers of the bitstream in a previous frame and specify the indicated number of layers of the bitstream in the current frame. 
     In some examples, rather than not specify an indication of the number of foreground components and the indication of the number of background components, the scalable bitstream generation unit  1000  may not specify an indication of a number of components (e.g., a “NumChannels” syntax element, which may be an array having [i] entries where i is equal to the number of layers) in the scalable bitstream  21 . The scalable bitstream generation unit  1000  may not specify this indication of the number of components (where these components may also be referred to as “channels”) in place of not specifying the number of foreground and background components given that the number of foreground and background components may be derived from the more general number of channels. The derivation of the indication of the number of foreground components and the indication of the number of background channels may, in some examples, proceed in accordance with the following table: 
                     TABLE                  Syntax of ChannelSideInfoData(i)                                 Mne-       Syntax   No. of bits   monic               ChannelSideInfoData(i)               {                ChannelType[i]   2   uimsbf        switch ChannelType[i]               {                case 0:                 ActiveDirsIds[i];   NumOfBitsPerDirI   uimsbf           dx             break;                case 1:                 if(hoaIndependencyFlag){                  NbitsQ(k)[i]   4   uimsbf          if (NbitsQ(k)[i]== 4) {                   CodebkIdx(k)[i];   3   uimsbf           NumVecIndices(k)[i]++;   NumVVecVqEle   uimsbf           mentsBits              }                  elseif (NbitsQ(k)[i] &gt;= 6) {                   PFlag(k)[i] = 0;                   CbFlag(k)[i];   1   bslbf          }                 }                 else{                  bA;   1   bslbf          bB;   1   bslbf          if ((bA + bB) == 0) {                   NbitsQ(k)[i] = NbitsQ(k−1)[i];                   PFlag(k)[i] = PFlag(k−1)[i];                   CbFlag(k)[i] = CbFlag(k−1)[i];                   CodebkIdx(k)[i] =                   CodebkIdx(k−               1)[i];                   NumVecIndices(k)[i] =               NumVecIndices [k−1][i];                  }                  else{                   NbitsQ(k)[i] =   2   uimsbf       (8*bA)+(4*bB)+uintC;                    if (NbitsQ(k)[i] == 4) {                     CodebkIdx(k)[i];   3   uimsbf            }                    elseif (NbitsQ(k)[i] &gt;= 6) {                    PFlag(k)[i];   1   bslbf            CbFlag(k)[i];   1   bslbf            }                   }                  }                  break;                 case 2:                  AddAmbHoaInfoChannel(i);                  break;                 default:                }               }                    
where the description of the ChannelType is given as follows:
 
     ChannelType: 
     0: Direction-based Signal 
     1: Vector-based Signal (which may represent a foreground signal) 
     2: Additional Ambient HOA Coefficient (which may represent a background or ambient signal) 
     3: Empty 
     As a result of signaling the ChannelType per the above SideChannelInfo syntax table, the number of foreground components per layer may be determined as a function of the number of ChannelType syntax elements set to 1 and the number of background components per layer may be determines as a function of the number of ChannelType syntax elements set to 2. 
     The scalable bitstream generation unit  1000  may, in some examples, specify an HOADecoderConfig on a frame-by-frame basis, which provides the configuration information for extracting the layers from the bitstream  21 . The HOADecoderConfig may be specified as an alternative to or in conjunction with the above table. The following table may define the syntax for the HOADecoderConfig_FrameByFrame( ) object in the bitstream  21 . 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Syntax 
                 No. of bits 
                 Mnemonic 
               
               
                   
               
             
            
               
                 HOADecoderConfig_FrameByFrame(numHO 
                   
                   
               
               
                 ATransportChannels) 
                   
                   
               
               
                 { 
                   
                   
               
               
                  HOABaseLayerPresent; 
                 1 
                 bslbf 
               
               
                  if(HOABaseLayerPresent){ 
                   
                   
               
               
                   HOABaseLayerConfigurationFlag; 
                 1 
                 bslbf 
               
               
                   if(HOABaseLayerConfigurationFlag){ 
                   
                   
               
               
                    NumLayerB its = 
                   
                   
               
               
                 ceil(log2(numHOATransportChannels−2)); 
                   
                   
               
               
                     NumLayers = NumLayers+2; 
                 NumLayerBits 
                 uimsbf 
               
               
                    numAvailableTransportChannels = 
                   
                   
               
               
                 numHOATransportChannels−2; 
                   
                   
               
               
                    numAvailableTransportChannelsBits 
                   
                   
               
               
                 = NumLayerBits; 
                   
                   
               
               
                     for (i=0; i&lt;NumLayers-1; ++i) { 
                   
                   
               
               
                     NumFGchannels[i] = 
                 numAvailableTransport 
                 uimsbf 
               
               
                 NumFGchannels[i]+1; 
                 ChannelsBits 
                   
               
               
                     numAvailableTransportChannels 
                   
                   
               
               
                 = numAvailableTransportChannels − 
                   
                   
               
               
                 NumFGchannels[i] 
                   
                   
               
               
                 numAvailableTransportChannelsBits = 
                   
                   
               
               
                 ceil(log2(numAvailableTransportChannels)); 
                   
                   
               
               
                     NumBGchannels[i] = 
                 numAvailableTransport 
                 uimsbf 
               
               
                 NumBGchannels[i] + 1; 
                 ChannelsBits 
                   
               
               
                     numAvailableTransportChannels 
                   
                   
               
               
                 = numAvailableTransportChannels − 
                   
                   
               
               
                 NumBGchannels[i] 
                   
                   
               
               
                 numAvailableTransportChannelsBits = 
                   
                   
               
               
                 ceil(log2(numAvailableTransportChannels)); 
                   
                   
               
               
                    } 
                   
                   
               
               
                   } else { 
                   
                   
               
               
                    NumLayers=NumLayersPrevFrame; 
                   
                   
               
               
                     for (i=0; i&lt;NumLayers; ++i) { 
                   
                   
               
               
                     NumFGchannels[i] = 
                   
                   
               
               
                 NumFGchannels_PrevFrame[i]; 
                   
                   
               
               
                     NumBGchannels[i] = 
                   
                   
               
               
                 NumBGchannels_PrevFrame[i]; 
                   
                   
               
               
                    } 
                   
                   
               
               
                   } 
                   
                   
               
               
                  } 
                   
                   
               
               
                  MinAmbHoaOrder = escapedValue(3,5,0) 
                 3,8 
                 uimsbf 
               
               
                 alue 
                   
                   
               
               
                  MinNumOfCoeffsForAmbHOA = 
                   
                   
               
               
                 (MinAmbHoaOrder + 1){circumflex over ( )}2; 
                   
                   
               
               
                   . 
                   
                   
               
               
                   . 
                   
                   
               
               
                   . 
                   
                   
               
               
                  NumLayersPrevFrame=NumLayers; 
                   
                   
               
               
                  for (i=0; i&lt;NumLayers; ++i) { 
                   
                   
               
               
                    NumFGchannels_PrevFrame[i] = 
                   
                   
               
               
                 NumFGchannels[i]; 
                   
                   
               
               
                    NumBGchannels_PrevFrame[i] = 
                   
                   
               
               
                 NumBGchannels[i]; 
                   
                   
               
               
                  } 
                   
                   
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     In the foregoing table, the HOABaseLayerPresent syntax element may represent a flag that indicates whether the base layer of the scalable bitstream  21  is present. When present, the scalable bitstream generation unit  1000  specifies an HOABaseLayerConfigurationFlag syntax element, which may represent a syntax element indicating whether configuration information for the base layer is present in the bitstream  21 . When the configuration information for the base layer is present in the bitstream  21 , the scalable bitstream generation unit  1000  specifies a number of layers (i.e., the NumLayers syntax element in the example), a number of foreground channels (i.e., the NumFGchannels syntax element in the example) for each of the layers, and a number of background channels (i.e., the NumBGchannels syntax element in the example) for each of the layers. When the HOABaseLayerPresent flag indicates that the base layer configuration is not present, the scalable bitstream generation unit  1000  may not provide any additional syntax elements and the audio decoding device  24  may determine that the configuration data for the current frame is the same as that for a previous frame. 
     In some examples, the scalable bitstream generation unit  1000  may specify the HOADecoderConfig object in the scalable bitstream  21  but not specify the number of foreground and background channels per layer, where the number of foreground and background channels may be static or determined as described above with respect to the ChannelSideInfo table. The HOADecoderConfig may, in this example, be defined in accordance with the following table. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Syntax 
                 No. of bits 
                 Mnemonic 
               
               
                   
               
             
            
               
                 HOADecoderConfig(numHOATransportCha 
                   
                   
               
               
                 nnels) 
                   
                   
               
               
                 { 
                   
                   
               
               
                  HOABaseLayerPresent; 
                 1 
                 bslbf 
               
               
                  if(HOABaseLayerPresent){ 
                   
                   
               
               
                   HOABaseLayerChBits = 
                   
                   
               
               
                 ceil(log2(numHOATransportChannels)); 
                   
                   
               
               
                   NumHOABaseLayerCh; 
                 HOABaseLayerChBits 
                 uimsbf 
               
               
                   HOABaseLayerConfigurationFlag; 
                 1 
                 bslbf 
               
               
                   if(HOABaseLayerConfigurationFlag){ 
                   
                   
               
               
                    NumLayerBits = 
                   
                   
               
               
                 ceil(log2(numHOATransportChannels)); 
                   
                   
               
               
                    NumLayers; 
                 NumLayerBits 
                 uimsbf 
               
               
                    numAvailableTransportChannels = 
                   
                   
               
               
                 numHOATransportChannels 
                   
                   
               
               
                 numAvailableTransportChannelsBits = 
                   
                   
               
               
                 ceil(log2(numAvailableTransportChannels)); 
                   
                   
               
               
                    for i=1:NumLayers−1 { 
                   
                   
               
               
                     NumChannels [i] 
                 numAvailableTransportC 
                   
               
               
                   
                 hannelsBits 
                   
               
               
                     numAvailableTransportChannels 
                   
                   
               
               
                 = numAvailableTransportChannels − 
                   
                   
               
               
                 NumChannels[i] 
                   
                   
               
               
                 numAvailableTransportChannelsBits = 
                   
                   
               
               
                 ceil(log2(numAvailableTransportChannels)); 
                   
                   
               
               
                    } 
                   
                   
               
               
                   } else { 
                   
                   
               
               
                    NumLayers=NumLayersPrevFrame; 
                   
                   
               
               
                    for i=1:NumLayers { 
                   
                   
               
               
                     NumChannels[i] = 
                   
                   
               
               
                 NumChannels_PrevFrame[i]; 
                   
                   
               
               
                    } 
                   
                   
               
               
                   } 
                   
                   
               
               
                  } 
                   
                   
               
               
                  MinAmbHoaOrder = escapedValue(3,5,0) 
                 3,8 
                 uimsbf 
               
               
                 − 1; 
                   
                   
               
               
                  MinNumOfCoeffsForAmbHOA = 
                   
                   
               
               
                 (MinAmbHoaOrder + 1){circumflex over ( )}2; 
                   
                   
               
               
                  . 
                   
                   
               
               
                  . 
                   
                   
               
               
                  . 
                   
                   
               
               
                  . 
                   
                   
               
               
                  . 
                   
                   
               
               
                  NumLayersPrevFrame=NumLayers; 
                   
                   
               
               
                  for i=1:NumLayers { 
                   
                   
               
               
                   NumChannels_PrevFrame[i] = 
                   
                   
               
               
                 NumChannels[i]; 
                   
                   
               
               
                  } 
                   
                   
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     As yet another alternative, the foregoing syntax tables for HOADecoderConfig may be replaced with the following syntax table for HOADecoderConfig. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Syntax 
                 No. of bits 
                 Mnemonic 
               
               
                   
               
             
            
               
                 HOADecoderConfig(numHOATransportChannels) 
                   
                   
               
               
                 { 
                   
                   
               
               
                  MinAmbHoaOrder = escapedValue(3,5,0) − 1; 
                 3,8 
                 uimsb 
               
               
                   
                   
                 f 
               
               
                  MinNumOfCoeffsForAmbHOA = (MinAmbHoaOrder + 1){circumflex over ( )}2; 
                   
                   
               
               
                  NumOfAdditionalCoders = numHOATransportChannels − 
                   
                   
               
               
                 MinNumOfCoeffsForAmbHOA; 
                   
                   
               
               
                  SingleLayer; 
                 1 
                 bslbf 
               
               
                  if(SingleLayer==0){ 
                   
                   
               
               
                   NumOfAdditionalCoders = escapeValue(5,8,16) +1 + 
                   
                 uimsb 
               
               
                 NumOfAdditionalCoders; 
                   
                 f 
               
               
                   HOALayerChBits = ceil(log2(NumOfAdditionalCoders)); 
                   
                   
               
               
                   NumHOAChannelsLayer[0] = codedLayerCh + 
                 HOALayer 
                 uimsb 
               
               
                 MinNumOfCoeffsForAmbHOA; 
                 ChBits 
                 f 
               
               
                   remainingCh = numHOATransportChannels − 
                   
                   
               
               
                 NumHOACannelsLayer[0]; 
                   
                   
               
               
                    NumLayers = 1; 
                   
                   
               
               
                   while (remainingCh&gt;1) { 
                   
                   
               
               
                    HOALayerChBits = ceil(log2(remainingCh)); 
                   
                   
               
               
                    NumHOAChannelsLayer[NumLayers] = 
                 HOALayer 
                 uimsb 
               
               
                 codedLayerCh + 1; 
                 ChBits 
                 f 
               
               
                    remainingCh = remainingCh − 
                   
                   
               
               
                 NumHOAChannelsLayer[NumLayers]; 
                   
                   
               
               
                    NumLayers++; 
                   
                   
               
               
                   } 
                   
                   
               
               
                   if (remainingCh) { 
                   
                   
               
               
                    NumHOAChannelsLayer[NumLayers] = 1; 
                   
                   
               
               
                    NumLayers++; 
                   
                   
               
               
                   } 
                   
                   
               
               
                  } 
                   
                   
               
               
                  MaxNoOfDirSigsForPrediction = 
                 2 
                 uimsbf 
               
               
                 MaxNoOfDirSigsForPrediction + 1; 
                   
                   
               
               
                  NoOfBitsPerScalefactor = NoOfBitsPerScalefactor + 1; 
                 4 
                 uimsbf 
               
               
                  CodedSpatialInterpolationTime; 
                 3 
                 uimsbf 
               
               
                  SpatialInterpolationMethod; 
                 1 
                 bslbf 
               
               
                  CodedVVecLength; 
                 2 
                 uimsbf 
               
               
                  MaxGainCorrAmpExp; 
                 3 
                 uimsbf 
               
               
                  MaxNumAddActiveAmbCoeffs = NumOfHoaCoeffs − 
                   
                   
               
               
                        MinNumOfCoeffsForAmbHOA; 
                   
                   
               
               
                  AmbAsignmBits = 
                   
                   
               
               
                 ceil( log2( MaxNumAddActiveAmbCoeffs ) ); 
                   
                   
               
               
                  ActivePredIdsBits = ceil( log2( NumOfHoaCoeffs ) ); 
                   
                   
               
               
                  i = 1; 
                   
                   
               
               
                  while( i * ActivePredIdsBits + ceil( log2( i ) ) &lt; 
                   
                   
               
               
                 NumOfHoaCoeffs ){ 
                   
                   
               
               
                   i++; 
                   
                   
               
               
                  } 
                   
                   
               
               
                  NumActivePredIdsBits = ceil( log2( max( 1, i − 1 ) ) ); 
                   
                   
               
               
                  GainCorrPrevAmpExpBits = ceil( log2( ceil( log2( 
                   
                   
               
               
                                1.5 * NumOfHoaCoeffs )) 
                   
                   
               
               
                               + MaxGainCorrAmpExp + 1 ) 
                   
                   
               
               
                 ); 
                   
                   
               
               
                  for (i=0; i&lt;NumOfAdditionalCoders; ++i){ 
                   
                   
               
               
                   AmbCoeffTransitionState[i] = 3; 
                   
                   
               
               
                  } 
                   
                   
               
               
                 } 
               
               
                   
               
               
                 NOTE: 
               
               
                 MinAmbHoaOrder = 30 . . . 37 are reserved. 
               
            
           
         
       
     
     In this respect, the scalable bitstream generation unit  1000  may be configured to, as described above, specify, in the bitstream, an indication of a number of channels specified in one or more layers of the bitstream, and specify the indicated number of the channels in the one or more layers of the bitstream. 
     Moreover, the scalable bitstream generation unit  1000  may be configured to specify a syntax element (e.g., in the form of a NumLayers syntax element or a codedLayerCh syntax element as described below in more detail) indicative of the number of channels. 
     In some examples, the scalable bitstream generation unit  1000  may be configured to specify an indication of a total number of channels specified in the bitstream. The scalable bitstream generation unit  1000  may be configured to, in these instances, specify the indicated total number of the channels in the one or more layers of the bitstream. In these instances, the scalable bitstream generation unit  1000  may be configured to specify a syntax element (e.g., a numHOATransportChannels syntax element as described below in more detail) indicative of the total number of channels. 
     In these and other examples, the scalable bitstream generation unit  1000  may be configured to specify an indication a type of one of the channels specified in the one or more layers in the bitstream. In these instances, the scalable bitstream generation unit  1000  may be configured to specify the indicated number of the indicated type of the one of the channels in the one or more layers of the bitstream. The foreground channel may comprise a US audio object and a corresponding V-vector. 
     In these and other examples, the scalable bitstream generation unit  1000  may be configured to specify an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a foreground channel. In these instances, the scalable bitstream generation unit  1000  may be configured to specify the foreground channel in the one or more layers of the bitstream. 
     In these and other examples, the scalable bitstream generation unit  1000  may be configured to specify an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a background channel. In these instances, the scalable bitstream generation unit  1000  may be configured to specify the background channel in the one or more layers of the bitstream. The background channel may comprise an ambient HOA coefficient. 
     In these and other examples, the scalable bitstream generation unit  1000  may be configured to specify a syntax element (e.g., a ChannelType syntax element) indicative of the type of the one of the channels. 
     In these and other examples, the scalable bitstream generation unit  1000  may be configured to specify the indication of the number of channels based on a number of channels remaining in the bitstream after one of the layers is obtained (as defined for example by a remainingCh syntax element or a numAvailableTransportChannels syntax element as described in more detail below. 
       FIGS.  7 A- 7 D  are flowcharts illustrating example operation of the audio encoding device  20  in generating an encoded two-layer representation of the HOA coefficients  11 . Referring first to the example of  FIG.  7 A , the decorrelation unit  60  may first apply the UHJ decorrelation with respect to the first order ambisonics background (where “ambisonics background” may refer to ambisonic coefficients describing a background component of a soundfield) represented as energy compensated background HOA coefficients  47 A′- 47 D′ ( 300 ). The first order ambisonics background  47 A′- 47 D′ may include the HOA coefficients corresponding to spherical basis functions having the following (order, sub-order): (0, 0), (1, 0), (1, −1), (1, 1). 
     The decorrelation unit  60  may output the decorrelated ambient HOA audio signals  67  as the above noted Q, T, L and R audio signals. The Q audio signal may provide height information. The T audio signal may provide horizontal information (including information for representing channels behind the sweet spot). The L audio signal provides a left stereo channel. The R audio signal provides a right stereo channel. 
     In some examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a left audio channel. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a right audio channel. In still other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a localization channel. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a height channel. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a sideband for automatic gain correction. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a left audio channel, a right audio channel, a localization channel, and a height channel, and a sideband for automatic gain correction. 
     The gain control unit  62  may apply automatic gain control (AGC) to the decorrelated ambient HOA audio signals  67  ( 302 ). The gain control unit  62  may pass the adjusted ambient HOA audio signals  67 ′ to the bitstream generation unit  42 , which may form the base layer based on the adjusted ambient HOA audio signals  67 ′ and at least part of the sideband channel based on the higher order ambisonic gain control data (HOAGCD) ( 304 ). 
     The gain control unit  62  may also apply the automatic gain control with respect to the interpolated nFG audio signals  49 ′ (which may also be referred to as the “vector-based predominant signals”) ( 306 ). The gain control unit  62  may output the adjusted nFG audio signals  49 ″ along with the HOAGCD for the adjusted nFG audio signals  49 ″ to the bitstream generation unit  42 . The bitstream generation unit  42  may form the second layer based on the adjusted nFG audio signals  49 ″ while forming part of the sideband information based on the HOAGCD for the adjusted nFG audio signals  49 ″ and the corresponding coded foreground V[k] vectors  57  ( 308 ). 
     The first layer (i.e., a base layer) of the two or more layers of higher order ambisonic audio data may comprise higher order ambisonic coefficients corresponding to one or more spherical basis functions having an order equal to or less than one. In some examples, the second layer (i.e., an enhancement layer) comprises vector-based predominant audio data. 
     In some examples, the vector-based predominant audio comprises at least a predominant audio data and an encoded V-vector. As described above, the encoded V-vector may be decomposed from the higher order ambisonic audio data through application of a linear invertible transform by the LIT unit  30  of the audio encoding device  20 . In other examples, the vector-based predominant audio data comprises at least an additional higher order ambisonic channel. In still other examples, the vector-based predominant audio data comprises at least an automatic gain correction sideband. In other examples, the vector-based predominant audio data comprises at least a predominant audio data, an encoded V-vector, an additional higher order ambisonic channel, and an automatic gain correction sideband. 
     In forming the first layer and the second layer, the bitstream generation unit  42  may perform error checking processes that provides for error detection, error correction or both error detection and correction. In some examples, the bitstream generation unit  42  may perform an error checking process on the first layer (i.e., the base layer). In another example, the audio coding device may perform an error checking process on the first layer (i.e., the base layer) and refrain from performing an error checking process on the second layer (i.e., the enhancement layer). In yet another example, the bitstream generation unit  42  may perform an error checking process on the first layer (i.e., the base layer) and, in response to determining that the first layer is error free, the audio coding device may perform an error checking process on the second layer (i.e., the enhancement layer). In any of the above examples in which the bitstream generation unit  42  performs the error checking process on the first layer (i.e., the base layer), the first layer may be considered a robust layer that is robust to errors. 
     Referring next to  FIG.  7 B , the gain control unit  62  and the bitstream generation unit  42  perform similar operations to that of the gain control unit  62  and the bitstream generation unit  42  described above with respect to  FIG.  7 A . However, the decorrelation unit  60  may apply a mode matrix decorrelation, rather than the UHJ decorrelation, to the first order ambisonics background  47 A′- 47 D′ ( 301 ). 
     Referring next to  FIG.  7 C , the gain control unit  62  and the bitstream generation unit  42  may perform similar operations to that of the gain control unit  62  and the bitstream unit  42  described above with respect to the examples of  FIGS.  7 A and  7 B . However, in the example of  FIG.  7 C , the decorrelation unit  60  may not apply any transform to the first order ambisonics background  47 A′- 47 D′. In each of the following examples 8A-10B, it is assumed but not illustrated that the decorrelation unit  60  may, as an alternative, not apply decorrelation with respect to one or more of the first order ambisonics background  47 A′- 47 D′. 
     Referring next to  FIG.  7 D , the decorrelation unit  60  and the bitstream generation unit  42  may perform similar operations to that of the gain control unit  52  and the bitstream generation unit  42  described above iwht respect to the examples of  FIGS.  7 A and  7 B . However, in the example of  FIG.  7 D , the gain control unit  62  may not apply any gain control to the decorrelated ambient HOA audio signals  67 . In each of the following examples 8A-10B, it is assumed but not illustrated that the gain control unit  52  may, as an alternative, not apply decorrelation with respect to one or more of the decorrelation ambient HOA audio signals  67 . 
     In each of the examples of  FIGS.  7 A- 7 D , the bitstream generation unit  42  may specify one or more syntax elements in the bitstream  21 .  FIG.  10    is a diagram illustrating an example of an HOA configuration object specified in the bitstream  21 . For each of the examples of  FIGS.  7 A- 7 D , the bitstream generation unit  42  may set the codedVVecLength syntax element  400  to 1 or 2, which indicates that the 1st order background HOA channels contain the 1st order component of all predominant sounds. The bitstream generation unit  42  may also set the ambienceDecorrelationMethod syntax element  402  such that the element  402  signals the use of the UHJ decorrelation (e.g., as described above with respect to  FIG.  7 A ), signals the use of the matrix mode decorrelation (e.g., as described above with respect to  FIG.  7 B ), or signals that no decorrelation was used (e.g., as described above with respect to  FIG.  7 C ). 
       FIG.  11    is a diagram illustrating sideband information  410  generated by the bitstream generation unit  42  for the first and second layers. The sideband information  410  includes sideband base layer information  412  and sideband second layer information  414 A and  414 B. When only the base layer is provided to the audio decoding device  24 , the audio encoding device  20  may provide only the sideband base layer information  412 . The sideband base layer information  412  includes the HOAGCD for the base layer. The sideband second layer information  414 A includes transport channels 1-4 syntax elements and corresponding HOAGCD. The sideband second layer information  414 B includes the corresponding two coded reduced V[k] vectors  57  corresponding to transport channels 1 and 2 (given that transport channels 3 and 4 are empty as denoted by the ChannelType syntax element equaling 11 2  or 3 10 .). 
       FIGS.  8 A and  8 B  are flowcharts illustrating example operation of the audio encoding device  20  in generating an encoded three-layer representation of the HOA coefficients  11 . Referring first to the example of  FIG.  8 A , the decorrelation unit  60  and the gain control unit  62  may perform operations similar to those described above with respect to  FIG.  7 A . However, the bitstream generation unit  42  may form the base layer based on the L audio signal and the R audio signal of the adjusted ambient HOA audio signals  67  rather than all of the adjusted ambient HOA audio signals  67  ( 310 ). The base layer may, in this respect, provide for stereo channels when rendered at the audio decoding device  24 . The bitstream generation unit  42  may also generate sideband information for the base layer that includes the HOAGCD. 
     The operation of the bitstream generation unit  42  may also differ from that described above with respect to  FIG.  7 A  in that the bitstream generation unit  42  may form a second layer based on the Q and T audio signals of the adjusted ambient HOA audio signals  67  ( 312 ). The second layer in the example of  FIG.  8 A  may provide for horizontal channels and 3D audio channels when rendered at the audio decoding device  24 . The bitstream generation unit  42  may also generate sideband information for the second layer that includes the HOAGCD. The bitstream generation unit  42  may also form a third layer in a manner substantially similar to that described above with respect to forming the second layer in the example of  FIG.  7 A . 
     The bitstream generation unit  42  may specify the HOA configuration object for the bitstream  21  similar to that described above with respect to  FIG.  10   . Further, bitstream generation unit  42  of audio encoder  20  sets the MinAmbHoaOrder syntax element  404  to 2 so as to indicate that the 1st order HOA background is transmitted. 
     The bitstream generation unit  42  may also generate sideband information similar to sideband information  412  shown in the example of  FIG.  12 A .  FIG.  12 A  is a diagram illustrating sideband information  412  generated in accordance with the scalable coding aspects of the techniques described in this disclosure. The sideband information  412  includes sideband base layer information  416 , sideband second layer information  418 , and sideband third layer information  420 A and  420 B. The sideband base layer information  416  may provide the HOAGCD for the base layer. The sideband second layer information  418  may provide the HOAGCD for the second layer. The sideband third layer information  420 A and  420 B may be similar to the sideband information  414 A and  414 B described above with respect to  FIG.  11   . 
     Similar to  FIG.  7 A , the bitstream generation device  42  may perform error checking processes. In some examples, bitstream generation device  42  may perform an error checking process on the first layer (i.e., the base layer). In another example, the bitstream generation device  42  may perform an error checking process on the first layer (i.e., the base layer) and refrain from performing an error checking process on the second layer (i.e., the enhancement layer). In yet another example, the bitstream generation device  42  may perform an error checking process on the first layer (i.e., the base layer) and, in response to determining that the first layer is error free, the audio coding device may perform an error checking process on the second layer (i.e., the enhancement layer). In any of the above examples in which the audio coding device performs the error checking process on the first layer (i.e., the base layer), the first layer may be considered a robust layer that is robust to errors. 
     Although described as providing three layers, in some examples, the bitstream generation device  42  may specify an indication in the bitstream that there are only two layers and specify a first one of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide for stereo channel playback, and a second one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for horizontal multi-channel playback by three or more speakers arranged on a single horizontal plane. In other words, while shown as providing three layers, the bitstream generation device  42  may generate only two of the three layers in some instances. It should be understood that any subset of the layers may be generated although not described in detail herein. 
     Referring next to  FIG.  8 B , the gain control unit  62  and the bitstream generation unit  42  perform similar operations to that of the gain control unit  62  and the bitstream generation unit  42  described above with respect to  FIG.  8 A . However, the decorrelation unit  60  may apply a mode matrix decorrelation, rather than the UHJ decorrelation, to the first order ambisonics background  47 A′ ( 316 ). In some examples, the first order ambisonics background  47 A′ may include the zeroth order ambisonic coefficients  47 A′. The gain control unit  62  may apply the automatic gain control to the first order ambisonic coefficients corresponding to the spherical harmonic coefficients having a first order, and the decorrelated ambient HOA audio signal  67 . 
     The bitstream generation unit  42  may form a base layer based on the adjusted ambient HOA audio signal  67  and at least part of the sideband based on the corresponding HOAGCD ( 310 ). The ambient HOA audio signal  67  may provide for a mono channel when rendered at the audio decoding device  24 . The bitstream generation unit  42  may form a second layer based on the adjusted ambient HOA coefficients  47 B″- 47 D″ and at least part of the sideband based on the corresponding HOAGCD ( 318 ). The adjusted ambient HOA coefficients  47 B′- 47 D′ may provide X, Y and Z (or stereo, horizontal and height) channels when rendered at the audio decoding device  24 . The bitstream generation unit  42  may form the third layer and at least part of the sideband information in a manner similar to that described above with respect to  FIG.  8 A . The bitstream generation unit  42  may generate sideband information  412  as described in more detail with respect to  FIG.  12 B  ( 326 ). 
       FIG.  12 B  is a diagram illustrating sideband information  414  generated in accordance with the scalable coding aspects of the techniques described in this disclosure. The sideband information  414  includes sideband base layer information  416 , sideband second layer information  422 , and sideband third layer information  424 A- 424 C. The sideband base layer information  416  may provide the HOAGCD for the base layer. The sideband second layer information  422  may provide the HOAGCD for the second layer. The sideband third layer information  424 A- 424 C may be similar to the sideband information  414 A (except for the sideband information  414 A is specified as sideband third layer information  424 A and  424 B) and  414 B described above with respect to  FIG.  11   . 
       FIGS.  9 A and  9 B  are flowcharts illustrating example operation of the audio encoding device  20  in generating an encoded four-layer representation of the HOA coefficients  11 . Referring first to the example of  FIG.  9 A , the decorrelation unit  60  and the gain control unit  62  may perform operations similar to those described above with respect to  FIG.  8 A . The bitstream generation unit  42  may form the base layer in a manner similar to that described above with respect to the example of  FIG.  8 A , i.e., based on the L audio signal and the R audio signal of the adjusted ambient HOA audio signals  67  rather than all of the adjusted ambient HOA audio signals  67  ( 310 ). The base layer may, in this respect, provide for stereo channels when rendered at the audio decoding device  24  (or, in other words, provide stereo channel playback). The bitstream generation unit  42  may also generate sideband information for the base layer that includes the HOAGCD. 
     The operation of the bitstream generation unit  42  may differ from that described above with respect to  FIG.  8 A  in that the bitstream generation unit  42  may form a second layer based on the T audio signal (and not the Q audio signal) of the adjusted ambient HOA audio signals  67  ( 322 ). The second layer in the example of  FIG.  9 A  may provide for horizontal channels when rendered at the audio decoding device  24  (or, in other words, multi-channel playback by three or more loudspeakers on a single horizontal plane). The bitstream generation unit  42  may also generate sideband information for the second layer that includes the HOAGCD. The bitstream generation unit  42  may also form a third layer based on the Q audio signal of the adjusted ambient HOA audio signals  67  ( 324 ). The third layer may provide for three dimensional playback by three or more speakers arranged on one or more horizontal planes. The bitstream generation unit  42  may form the fourth layer in a manner substantially similar to that described above with respect to forming the third layer in the example of  FIG.  8 A  ( 326 ). 
     The bitstream generation unit  42  may specify the HOA configuration object for the bitstream  21  similar to that described above with respect to  FIG.  10   . Further, bitstream generation unit  42  of audio encoder  20  sets the MinAmbHoaOrder syntax element  404  to 2 so as to indicate that the 1st order HOA background is transmitted. 
     The bitstream generation unit  42  may also generate sideband information similar to sideband information  412  shown in the example of  FIG.  13 A .  FIG.  13 A  is a diagram illustrating sideband information  430  generated in accordance with the scalable coding aspects of the techniques described in this disclosure. The sideband information  430  includes sideband base layer information  416 , sideband second layer information  418 , sideband third layer information  432  and sideband fourth layer information  434 A and  434 B. The sideband base layer information  416  may provide the HOAGCD for the base layer. The sideband second layer information  418  may provide the HOAGCD for the second layer. The sideband third layer information  430  may provide the HOAGCD for the third layer. The sideband fourth layer information  434 A and  434 B may be similar to the sideband information  420 A and  420 B described above with respect to  FIG.  12 A . 
     Similar to  FIG.  7 A , the bitstream generation device  42  may perform error checking processes. In some examples, bitstream generation device  42  may perform an error checking process on the first layer (i.e., the base layer). In another example, the bitstream generation device  42  may perform an error checking process on the first layer (i.e., the base layer) and refrain from performing an error checking process on the remaining layer (i.e., the enhancement layers). In yet another example, the bitstream generation device  42  may perform an error checking process on the first layer (i.e., the base layer) and, in response to determining that the first layer is error free, the audio coding device may perform an error checking process on the second layer (i.e., the enhancement layer). In any of the above examples in which the audio coding device performs the error checking process on the first layer (i.e., the base layer), the first layer may be considered a robust layer that is robust to errors. 
     Referring next to  FIG.  9 B , the gain control unit  62  and the bitstream generation unit  42  perform similar operations to that of the gain control unit  62  and the bitstream generation unit  42  described above with respect to  FIG.  9 A . However, the decorrelation unit  60  may apply a mode matrix decorrelation, rather than the UHJ decorrelation, to the first order ambisonics background  47 A′ ( 316 ). In some examples, the first order ambisonics background  47 A′ may include the zeroth order ambisonic coefficients  47 A′. The gain control unit  62  may apply the automatic gain control to the first order ambisonic coefficients corresponding to the spherical harmonic coefficients having a first order, and the decorrelated ambient HOA audio signal  67  ( 302 ). 
     The bitstream generation unit  42  may form a base layer based on the adjusted ambient HOA audio signal  67  and at least part of the sideband based on the corresponding HOAGCD ( 310 ). The ambient HOA audio signal  67  may provide for a mono channel when rendered at the audio decoding device  24 . The bitstream generation unit  42  may form a second layer based on the adjusted ambient HOA coefficients  47 B″ and  47 C″ and at least part of the sideband based on the corresponding HOAGCD ( 322 ). The adjusted ambient HOA coefficients  47 B″ and  47 C″ may provide X, Y horizontal multi-channel playback by three or more speakers arranged on a single horizontal plane. The bitstream generation unit  42  may form a third layer based on the adjusted ambient HOA coefficients  47 D″ and at least part of the sideband based on the corresponding HOAGCD ( 324 ). The adjusted ambient HOA coefficients  47 D″ may provide for three dimensional playback by three or more speakers arranged in one or more horizontal planes. The bitstream generation unit  42  may form the fourth layer and at least part of the sideband information in a manner similar to that described above with respect to  FIG.  8 A  ( 326 ). The bitstream generation unit  42  may generate sideband information  412  as described in more detail with respect to  FIG.  12 B . 
       FIG.  13 B  is a diagram illustrating sideband information  440  generated in accordance with the scalable coding aspects of the techniques described in this disclosure. The sideband information  440  includes sideband base layer information  416 , sideband second layer information  442 , sideband third layer information  444  and sideband fourth layer information  446 A- 446 C. The sideband base layer information  416  may provide the HOAGCD for the base layer. The sideband second layer information  442  may provide the HOAGCD for the second layer. The sideband third layer information may provide the HOAGCD for the third layer. The sideband fourth layer information  446 A- 446 C may be similar to the sideband information  424 A- 424 C described above with respect to  FIG.  12 B . 
       FIG.  4    is a block diagram illustrating the audio decoding device  24  of  FIG.  2    in more detail. As shown in the example of  FIG.  4    the audio decoding device  24  may include an extraction unit  72 , a directionality-based reconstruction unit  90  and a vector-based reconstruction unit  92 . Although described below, more information regarding the audio decoding device  24  and the various aspects of decompressing or otherwise decoding HOA coefficients is available in International Patent Application Publication No. WO 2014/194099, entitled “INTERPOLATION FOR DECOMPOSED REPRESENTATIONS OF A SOUND FIELD,” filed 29 May 2014. Further information may also be found in the above referenced phase I and phase II of the MPEG-H 3D audio coding standard and the corresponding paper referenced above summarizing phase I of the MPEG-H 3D audio coding standard. 
     The extraction unit  72  may represent a unit configured to receive the bitstream  21  and extract the various encoded versions (e.g., a directional-based encoded version or a vector-based encoded version) of the HOA coefficients  11 . The extraction unit  72  may determine from the above noted syntax element indicative of whether the HOA coefficients  11  were encoded via the various direction-based or vector-based versions. When a directional-based encoding was performed, the extraction unit  72  may extract the directional-based version of the HOA coefficients  11  and the syntax elements associated with the encoded version (which is denoted as directional-based information  91  in the example of  FIG.  4   ), passing the directional based information  91  to the directional-based reconstruction unit  90 . The directional-based reconstruction unit  90  may represent a unit configured to reconstruct the HOA coefficients in the form of HOA coefficients  11 ′ based on the directional-based information  91 . 
     When the syntax element indicates that the HOA coefficients  11  were encoded using a vector-based synthesis, the extraction unit  72  may extract the coded foreground V[k] vectors  57  (which may include coded weights  57  and/or indices  63  or scalar quantized V-vectors), the encoded ambient HOA coefficients  59  and the corresponding audio objects  61  (which may also be referred to as the encoded nFG signals  61 ). The audio objects  61  each correspond to one of the vectors  57 . The extraction unit  72  may pass the coded foreground V[k] vectors  57  to the V-vector reconstruction unit  74  and the encoded ambient HOA coefficients  59  along with the encoded nFG signals  61  to the psychoacoustic decoding unit  80 . The extraction unit  72  is described in more detail with respect to the example of  FIG.  6   . 
       FIG.  6    is a diagram illustrating, in more detail, the extraction unit  72  of  FIG.  4    when configured to perform the first one of the potential versions the scalable audio decoding techniques described in this disclosure. In the example of  FIG.  6   , the extraction unit  72  includes a mode selection unit  1010 , a scalable extraction unit  1012  and a non-scalable extraction unit  1014 . The mode selection unit  1010  represents a unit configured to select whether scalable or non-scalable extraction is to be performed with respect to the bitstream  21 . The mode selection unit  1010  may include a memory to which the bitstream  21  is stored. The mode selection unit  1010  may determine whether scalable or non-scalable extraction is to be performed based on the indication of whether scalable coding has been enabled. A HOABaseLayerPresent syntax element may represent the indication of whether scalable coding was performed when encoding the bitstream  21 . 
     When the HOABaseLayerPresent syntax element indicates that scalable coding has been enabled, the mode selection unit  1010  may identify the bitstream  21  as the scalable bitstream  21  and output the scalable bitstream  21  to the scalable extraction unit  1012 . When the HOABaseLayerPresent syntax element indicates that scalable coding has not been enabled, the mode selection unit  1010  may identify the bitstream  21  as the non-scalable bitstream  21 ′ and output the non-scalable bitstream  21 ′ to the non-scalable extraction unit  1014 . The non-scalable extraction unit  1014  represents a unit configured to operate in accordance with phase I of the MPEG-H 3D audio coding standard. 
     The scalable extraction unit  1012  may represent a unit configured to extract one or more of the ambient HOA coefficients  59 , the encoded nFG signals  61  and the coded foreground V[k] vectors  57  from one or more layers of the scalable bitstream  21  based on various syntax element described below in more detail (and shown above in various HOADecoderConfig tables). In the example of  FIG.  6   , the scalable extraction unit  1012  may extract, as one example, the four encoded ambient HOA coefficients  59 A- 59 D from the base layer  21 A of the scalable bitstream  21 . The scalable extraction unit  1012  may also extract from the enhancement layer  21 B of the scalable bitstream  21 , the two encoded nFG signals  61 A and  61 B (as one example) as well as the two coded foreground V[k] vectors  57 A and  57 B. The scalable extraction unit  1012  may output the ambient HOA coefficients  59 , the encoded nFG signals  61  and the coded foreground V[k] vectors  57  to the vector-based decoding unit  92  shown in the example of  FIG.  4   . 
     More specifically, the extraction unit  72  of the audio decoding device  24  may extract channels of the L layers as set forth in the above HOADecoderCofnig_FrameByFrame syntax table. 
     In accordance with the above HOADecoderCofnig_FrameByFrame syntax table, the mode selection unit  1010  may first obtain the HOABaseLayerPresent syntax element, which may indicate whether scalable audio encoding was performed. When not enabled as specified by, for example, a zero value for the HOABaseLayerPresent syntax element, the mode selection unit  1010  may determine the MinAmbHoaOrder syntax element and provides the non-scalable bitstream to the non-scalable extraction unit  1014 , which performs non-scalable extraction processes similar to those described above. When enabled as specified by, for example, a one value for the HOABaseLayerPresent syntax element, the mode selection unit  1010  sets the MinAmbHOAOrder syntax element value to be negative one (−1) and provides the scalable bitstream  21 ′ to the scalable extraction unit  1012 . 
     The scalable extraction unit  1012  may obtain an indication of whether a number of layers of the bitstream have changed in a current frame when compared to a number of layers of the bitstream in a previous frame. The indication of whether the number of flayers of the bitstream has changed in the current frame when compared to the number of layers of the bitstream in the previous frame may be denoted as an “HOABaseLayerConfigurationFlag” syntax element in the foregoing table. 
     The scalable extraction unit  1012  may obtain an a indication of a number of layers of the bitstream in the current frame based on the indication. When the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, the scalable extraction unit  1012  may determine the number of layers of the bitstream in the current frame as equal to the number of layers of the bitstream in the previous frame in accordance with portion of the above syntax table that states:
     . . .   } else}
       NumLayers=NumLayersPrevFrame;
 
where the “NumLayers” may represent a syntax element representing the number of layers of the bitstream in the current frame and the “NumLayersPrevFrame” may represent a syntax element representing the number of layers of the bitstream in the previous frame.
   
       

     According to the above HOADecoderConfig_FrameByFrame syntax table, the scalable extraction unit  1012  may, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, determine a current foreground indication of a current number of foreground components in one or more of the layers for the current frame to be equal to a previous foreground indication for a previous number of foreground components in one or more of the layers of the previous frame. In other words, the scalable extraction unit  1012  may, when the HOABaseLayerConfigurationFlag is equal to zero, determine the NumFGchannels[i] syntax element representative of the current foreground indication of the current number of foreground component in one or more of the layers of the current frame to be equal to the NumFGchannels_PrevFrame[i] syntax element that is representative of the previous foreground indication of the previous number of foreground components in the one or more layers of the previous frame. The scalable extraction unit  1012  may further obtain the foreground components from the one or more layers in the current frame based on the current foreground indication. 
     The scalable extraction unit  1012  may also, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, determine a current background indication of a current number of background components in one or more of the layers for the current frame to be equal to a previous background indication for a previous number of background components in one or more of the layers of the previous frame. In other words, the scalable extraction unit  1012  may, when the HOABaseLayerConfigurationFlag is equal to zero, determine the NumBGchannels[i] syntax element representative of the current background indication of the current number of background component in one or more of the layers of the current frame to be equal to the NumBGchannels_PrevFrame[i] syntax element that is representative of the previous background indication of the previous number of background components in the one or more layers of the previous frame. The scalable extraction unit  1012  may further obtain the background components from the one or more layers in the current frame based on the current background indication. 
     To enable the foregoing techniques that may potentially reduce signaling of various indications of the number of layers, foreground components and background components, the scalable extraction unit  1012  may set the NumFGchannels_PrevFrame[i] syntax element and the NumBGchannel_PrevFrame[i] syntax element to the indications for the current frame (e.g., the NumFGchannels[i] syntax element and the NumBGchannels[i]), iterating through all i layers. This is represented in the following syntax: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                     
                 NumLayersPrevFrame=NumLayers; 
               
               
                   
                 for i=1:NumLayers 1 
               
               
                   
                  NumFGchannels_PrevFrame[i] = NumFGchannels[i]; 
               
               
                   
                  NumBGchannels_PrevFrame[i] = NumBGchannels[i]; 
               
               
                   
                 } 
               
               
                   
               
            
           
         
       
     
     When the indication indicates that the number of layers of the bitstream has changed in the current frame when compared to the number of layers of the bitstream in the previous frame (e.g., when the HOABaseLayerConfigurationFlag is equal to one), the scalable extraction unit  1012  obtains the NumLayerBits syntax element as a function of the numHOATransportChannels, which is passed into the syntax table having been obtained in accordance with other syntax tables not described in this disclosure. 
     The scalable extraction unit  1012  may obtain an indication of the number of layers specified in the bitstream (e.g., the NumLayers syntax element), where the indication may have a number of bits indicated by the NumLayerBits syntax element. The NumLayers syntax element may specify the number of layers specified in the bitstream, where the number of layers may be denoted as L above. The scalable extraction unit  1012  may next determining the numAvailableTransportChannels as a function of the numHOATransportChannels and the numAvailable TransportChannelBits as a function of the numAvailableTransportChannels. 
     The scalable extraction unit  1012  may then iterate through the NumLayers from 1 to NumLayers−1 to determine the number of background HOA channels (Be) and the number of foreground HOA channels (Fe) specified for the i-th layer. The scalable extraction unit  1012  may not iterate through the number of last layer (NumLayer) and only through the NumLayer−1 as the last layer B L  may be determined when the total number of foreground and background HOA channels sent in the bitstream are known by the scalable extraction unit  1012  (e.g., when the total number of foreground and background HOA channels are signaled as syntax elements). 
     In this respect, the scalable extraction unit  1012  may obtain the layers of the bitstream based on the indication of the number of layers. The scalable extraction unit  1012  may, as described above, obtain an indication of a number of channels specified in the bitstream  21  (e.g., numHOATransportChannels), and obtain the layers, by at least in part, obtain the layers of the bitstream  21  based on the indication of the number of layers and the indication of the number of channels. 
     When iterating through each layer, the scalable extraction unit  1012  may first determine the number of foreground channels for the i-th layer by obtaining the NumFGchannels[i] syntax element. The scalable extraction unit  1012  may then subtract the NumFGchannels[i] from the numAvailableTransportChannels to update the NumAvailableTransportChannels and reflect that NumFGchannels [i] of the foreground HOA channels  61  (which may also be referred to as the “encoded nFG signals  61 ”) have been extracted from the bitstream. In this way, the scalable extraction unit  1012  may obtain an indication of a number of foreground channels specified in the bitstream  21  for at least one of the layers (e.g., NumFGchannels) and obtain the foreground channels for the at least one of the layers of the bitstream based on the indication of the number of foreground channels. 
     Likewise, the scalable extraction unit  1012  may determine the number of background channels for the i-th layer by obtaining the NumBGchannels[i] syntax element. The scalable extraction unit  1012  may then subtract the NumBGchannels[i] from the numAvailableTransportChannels to reflect that NumBGchannels[i] of the background HOA channels  59  (which may also be referred to as the “encoded ambient HOA coefficients  59 ”) have been extracted from the bitstream. In this way, the scalable extraction unit  1012  may obtain an indication of a number of background channels (e.g., NumBGChannels) specified in the bitstream  21  for at least one of the layers and obtain the background channels for the at least one of the layers of the bitstream based on the indication of the number of background channels. 
     The scalable extraction unit  1012  may continue by obtaining the numAvailableTransportChannelsBits as a function of the numAvailableTransports. Per the above syntax table, the scalable extraction unit  1012  may parse the number of bits specified by the numAvailableTransportChannelsBits to determine the NumFGchannels[i] and the NumBGchannels [i]. Given that the numAvailableTransportChannelBits changes (e.g., becomes smaller after each iteration), the number of bits used to represent the NumFGchannels[i] syntax element and the NumBGchannels [i] syntax element reduces, thereby provides a form of variable length coding that potentially reduces overhead in signaling the NumFGchannels[i] syntax element and the NumBGchannels [i] syntax element. 
     As noted above, the scalable bitstream generation unit  1000  may specify the NumChannels syntax element in place of the NumFGchannels and NumBGchannels syntax elements. In this instance, the scalable extraction unit  1012  may be configured to operate in accordance with the second HOADecoderConfig syntax table shown above. 
     In this respect, the scalable extraction unit  1012  may, when the indication indicates that the number of layers of the bitstream has changed in the current frame when compared to the number of layers of the bitstream in the previous frame, obtain an indication of a number of components in one or more of the layers for the current frame based on the a number of components in one or more of the layers of the previous frame. The scalable extraction unit  1012  may further obtain an indication of a number of background components in the one or more layers for the current frame based on the indication of the number of components. The scalable extraction unit  1012  may also obtain an indication of a number of foreground components in the one or more layers for the current frame based on the indication of the number of components. 
     Given that the number of layers may change from frame to frame that the indication of the number of foreground and background channels may change from frame to frame, the indication that the number of layers has changed may effectively also indicate that the number of channels has changed. As a result, the indication that the number of layers has changed may result in the scalable extraction unit  1012  obtaining an indication of whether the number of channels specified in one or more layers in the bitstream  21  has changed in a current frame when compared to a number of channels specified in one or more layers in the bitstream of the previous frame. As such, the scalable extraction unit  1012  may obtain the one of the channels based on the indication of whether the number of channels specified in one or more layers in the bitstream has changed in the current frame. 
     Moreover, the scalable extraction unit  1012  may determine the number of channels specified in the one or more layers of the bitstream  21  in the current frame as the same as the number of channels specified in the one or more layers of the bitstream  21  in the previous frame when the indication indicates that the number of channels specified in the one or more layers of the bitstream  21  has not changed in the current frame when compared to the number of channels specified in the one or more layers of the bitstream in the previous frame. 
     In addition, the scalable extraction unit  1012  may, when the indication indicates that the number of channels specified in the one or more layers of the bitstream  21  has not changed in the current frame when compared to the number of channels specified in the one or more layers of the bitstream in the previous frame, obtain an indication of a current number of channels in one or more of the layers for the current frame to be the same as a previous number of channels in one or more of the layers of the previous frame. 
     To enable the foregoing techniques that may potentially reduce signaling of various indications of the number of layers and components (which may also be referred to as “channels” in this disclosure), the scalable extraction unit  1012  may set the NumChannels_PrevFrame[i] syntax element to the indications for the current frame (e.g., the NumChannels[i] syntax element), iterating through all i layers. This is represented in the following syntax: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                     
                 NumLayersPrevFrame=NumLayers; 
               
               
                   
                 for i=1:NumLayers { 
               
               
                   
                  NumChannels_PrevFrame[i] = NumChannels[i]; 
               
               
                   
                 } 
               
               
                   
               
            
           
         
       
     
     Alternatively, the foregoing syntax (NumLayersPrevFrame=NumLayers etc.) may be omitted and the syntax table HOADecoderConfig(numHOATransportChannels) listed above may be updated as set forth in the following table: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Syntax 
                 No. of bits 
                 Mnemonic 
               
               
                   
               
             
            
               
                 HOADecoderConfig(numHOATranspor 
                   
                   
               
               
                 tChannels) 
                   
                   
               
               
                 { 
                   
                   
               
               
                  HOALayerPresent; 
                 1 
                 bslbf 
               
               
                  if(HOALayerPresent){ 
                   
                   
               
               
                   NumLayerB its = 
                   
                   
               
               
                 ceil(log2(numHOATransportChannels− 
                   
                   
               
               
                 2)); 
                   
                   
               
               
                   NumLayers = NumLayers+2; 
                 NumLayerBits 
                 uimsbf 
               
               
                   numAvailableTransportChannels = 
                   
                   
               
               
                 numHOATransportChannels−2; 
                   
                   
               
               
                 numAvailableTransportChannelsBits = 
                   
                   
               
               
                 NumLayerBits; 
                   
                   
               
               
                   for (i=0; i&lt;NumLayers−1; ++i) { 
                   
                   
               
               
                    NumChannels[i] = 
                 numAvailableTransportChan 
                 uimsbf 
               
               
                 NumChannels[i]+1; 
                 nelsBits 
                   
               
               
                 numAvailableTransportChannels = 
                   
                   
               
               
                 numAvailableTransportChannels − 
                   
                   
               
               
                 NumChannels[i]; 
                   
                   
               
               
                 numAvailableTransportChannelsBits = 
                   
                   
               
               
                 ceil(log2(numAvailableTransportChann 
                   
                   
               
               
                 els)); 
                   
                   
               
               
                   } 
                   
                   
               
               
                  } 
                   
                   
               
               
                  MinAmbHoaOrder = 
                 3,8 
                 uimsbf 
               
               
                 escapedValue(3,5,0) − 1; 
                   
                   
               
               
                  MinNumOfCoeffsForAmbHOA = 
                   
                   
               
               
                 (MinAmbHoaOrder + 1){circumflex over ( )}2; 
                   
                   
               
               
                  . 
                   
                   
               
               
                  . 
                   
                   
               
               
                  . 
                   
                   
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     As yet another alternative, the extraction unit  72  may operate in accordance with the third HOADecoder Config listed above. In accordance with the third HOADecoderConfig syntax table listed above, the scalable extraction unit  1012  may be configured to obtain, from the scalable bitstream  21 , an indication of a number of channels specified in one or more layers in the bitstream, and obtain the channels specified in the one or more layers in the bitstream based on the indication of the number of channels (which may refer to a background component or a foreground component of the soundfield). In these and other instances, the scalable extraction unit  1012  may be configured to obtain a syntax element (e.g., the codedLayerCh in the above referenced table) indicative of the number of channels. 
     In these and other instances, the scalable extraction unit  1012  may be configured to obtain an indication of a total number of channels specified in the bitstream. The scalable extraction unit  1012  may also be configured to obtain the channels specified in the one or more layers based on the indication of the number of channels specified in the one or more layers and the indication of the total number of channels. In these and other instances, the scalable extraction unit  1012  may be configured to obtain a syntax element (e.g., the above noted NumHOATransportChannels syntax element) indicative of the total number of channels. 
     In these and other instances, the scalable extraction unit  1012  may be configured to obtain an indication a type of one of the channels specified in the one or more layers in the bitstream. The scalable extraction unit  1012  may also be configured to obtain the one of the channels based on the indication of the number of layers and the indication of the type of the one of the channels. 
     In these and other instances, the scalable extraction unit  1012  may be configured to obtain an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a foreground channel. The scalable extraction unit  1012  may be configured to obtain the one of the channels based on the indication of the number of layers and the indication that the type of the one of the channels is the foreground channel. In these instances, the one of the channels comprises a US audio object and a corresponding V-vector. 
     In these and other instances, the scalable extraction unit  1012  may be configured to obtain an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a background channel. In these instances, the scalable extraction unit  1012  may also be configured to obtain the one of the channels based on the indication of the number of layers and the indication that the type of the one of the channels is the background channel. In these instances, the one of the channels comprises a background higher order ambisonic coefficient. 
     In these and other instances, the scalable extraction unit  1012  may be configured to obtain a syntax element (e.g., the ChannelType syntax element described above with respect to  FIG.  30   ) indicative of the type of the one of the channels. 
     In these and other instances, the scalable extraction unit  1012  may be configured to obtain the indication of the number of channels based on a number of channels remaining in the bitstream after one of the layers is obtained. That is, the value of the HOALayerChBits syntax element varies as a function of the remainingCh syntax element as set forth in the above syntax table throughout the course of the while loop. The scalable extraction unit  1012  may then parse the codedLayerCh syntax element based on the changing HOALayerChBits syntax element. 
     Returning to the example of the four background channels and the two foreground channels, the scalable extraction unit  1012  may receive an indication that the number of layers is two, i.e., the base layer  21 A and the enhancement layer  21 B in the example of  FIG.  6   . The scalable extraction unit  1012  may obtain an indication that the number of foreground channels is zero for the base layer  21 A (e.g., from NumFGchannels[ 0 ]) and two for the enhancement layer  21 B (e.g., from NumFGchannels[ 1 ]). The scalable extraction unit  1012  may, in this example, also obtain an indication that the number of background channels is four for the base layer  21 A (e.g., from NumBGchannels[ 0 ]) and zero for the enhancement layer  21 B (e.g., from NumBGchannels[ 1 ]). Although described with respect to a particular example, any different combination of background and foreground channels may be indicated. The scalable extraction unit  1012  may then extract the specified four background channels  59 A- 59 D from the base layer  21 A and the two foreground channels  61 A and  61 B from the enhancement layer  21 B (along with the corresponding V-vector information  57 A and  57 B from the sideband information). 
     Although described above with respect to the NumFGchannels and the NumBGchannels syntax element, the techniques may also be performed using the ChannelType syntax element from the ChannelSideInfo syntax table above. In this respect, the NumFGchannels and the NumBG channels may also represent an indication of a type of one of the channels. In other words, the NumBGchannels may represent an indication that a type of one of the channels is a background channel. The NumFG channels may represent an indication that a type of one of the channels is a foreground channel. 
     As such, whether the ChannelType syntax element or the NumFGchannels syntax element with the NumBGchannels syntax element are used (or potentially both or some subset of either), the scalable bitstream extraction unit  1012  may obtain an indication of a type of one of the channels specified in the one or more layers in the bitstream. The scalable bitstream extraction unit  1012  may, when the indication of the type indicates that the one of the channels is a background channel, obtain the one of the channels based on the indication of the number of layers and the indication that the type of the one of the channels is the background channel. The scalable bitstream extraction unit  1012  may, when the indication of the type indicates that the one of the channels is a foreground channel, obtain the one of the channels based on the indication of the number of layers and the indication that the type of the one of the channels is the foreground channel. 
     The V-vector reconstruction unit  74  may represent a unit configured to reconstruct the V-vectors from the encoded foreground V[k] vectors  57 . The V-vector reconstruction unit  74  may operate in a manner reciprocal to that of the quantization unit  52 . 
     The psychoacoustic decoding unit  80  may operate in a manner reciprocal to the psychoacoustic audio coder unit  40  shown in the example of  FIG.  3    so as to decode the encoded ambient HOA coefficients  59  and the encoded nFG signals  61  and thereby generate adjusted ambient HOA audio signals  67 ′ and the adjusted interpolated nFG signals  49 ″ (which may also be referred to as adjusted interpolated nFG audio objects  49 ′). The psychoacoustic decoding unit  80  may pass the adjusted ambient HOA audio signals  67 ′ and the adjusted interpolated nFG signals  49 ″ to the inverse gain control unit  86 . 
     The inverse gain control unit  86  may represent a unit configured to perform an inverse gain control with respect to each of the adjusted ambient HOA audio signals  67 ′ and the adjusted interpolated nFG signals  49 ″, where this inverse gain control is reciprocal to the gain control performed by the gain control unit  62 . The inverse gain control unit  86  may perform the inverse gain control in accordance with the corresponding HOAGCD specified in the sideband information discussed above with respect to the examples of  FIGS.  11 - 13 B . The inverse gain control unit  86  may output decorrelated ambient HOA audio signals  67  to the recorrelation unit  88  (shown as “recorr unit  88 ” in the example of  FIG.  4   ) and the interpolated nFG audio signals  49 ″ to the foreground formulation unit  78 . 
     The recorrelation unit  88  may implement techniques of this disclosure to reduce correlation between background channels of the decorrelated ambient HOA audio signals  67  to reduce or mitigate noise unmasking. In examples where the recorrelation unit  88  applies a UHJ matrix (e.g., an inverse UHJ matrix) as the selected recorrelation transform, the recorrelation unit  81  may improve compression rates and conserve computing resources by reducing data processing operations. 
     In some examples, the scalable bitstream  21  may include one or more syntax elements that indicate that a decorrelation transform was applied during encoding. The inclusion of such syntax elements in the vector-based bitstream  21  may enable recorrelation unit  88  to perform reciprocal decorrelation (e.g., correlation or recorrelation) transforms on the decorrelated ambient HOA audio signals  67 . In some examples, the signal syntax elements may indicate which decorrelation transform was applied, such as the UHJ matrix or the mode matrix, thereby enabling the recorrelation unit  88  to select the appropriate recorrelation transform to apply to the decorrelated HOA audio signals  67 . 
     The recorrelation unit  88  may perform the recorrelation with respect to the decorrelated ambient HOA audio signals  67  to obtain energy compensated ambient HOA coefficients  47 ′. The recorrelation unit  88  may output the energy compensated ambient HOA coefficients  47 ′ to the fade unit  770 . Although described as performing the decorrelation, in some examples no decorrelation may have been performed. As such, the vector-based reconstruction unit  92  may not perform or in some examples include a recorrelation unit  88 . The absence of the recorrelation unit  88  in some examples is denoted by the dashed line of the recorrelation unit  88 . 
     The spatio-temporal interpolation unit  76  may operate in a manner similar to that described above with respect to the spatio-temporal interpolation unit  50 . The spatio-temporal interpolation unit  76  may receive the reduced foreground V[k] vectors  55   k  and perform the spatio-temporal interpolation with respect to the foreground V[k] vectors  55   k  and the reduced foreground V[k−1] vectors  55   k −1 to generate interpolated foreground V[k] vectors  55   k ″. The spatio-temporal interpolation unit  76  may forward the interpolated foreground V[k] vectors  55   k ″ to the fade unit  770 . 
     The extraction unit  72  may also output a signal  757  indicative of when one of the ambient HOA coefficients is in transition to fade unit  770 , which may then determine which of the SHC BG    47 ′ (where the SHC BG    47 ′ may also be denoted as “ambient HOA channels  47 ′” or “ambient HOA coefficients  47 ′”) and the elements of the interpolated foreground V[k] vectors  55   k ″ are to be either faded-in or faded-out. In some examples, the fade unit  770  may operate opposite with respect to each of the ambient HOA coefficients  47 ′ and the elements of the interpolated foreground V[k] vectors  55   k ″. That is, the fade unit  770  may perform a fade-in or fade-out, or both a fade-in or fade-out with respect to corresponding one of the ambient HOA coefficients  47 ′, while performing a fade-in or fade-out or both a fade-in and a fade-out, with respect to the corresponding one of the elements of the interpolated foreground V[k] vectors  55   k ″. The fade unit  770  may output adjusted ambient HOA coefficients  47 ″ to the HOA coefficient formulation unit  82  and adjusted foreground V[k] vectors  55   k ′″ to the foreground formulation unit  78 . In this respect, the fade unit  770  represents a unit configured to perform a fade operation with respect to various aspects of the HOA coefficients or derivatives thereof, e.g., in the form of the ambient HOA coefficients  47 ′ and the elements of the interpolated foreground V[k] vectors  55   k″.    
     The foreground formulation unit  78  may represent a unit configured to perform matrix multiplication with respect to the adjusted foreground V[k] vectors  55   k ′″ and the interpolated nFG signals  49 ′ to generate the foreground HOA coefficients  65 . In this respect, the foreground formulation unit  78  may combine the audio objects  49 ′ (which is another way by which to denote the interpolated nFG signals  49 ′) with the vectors  55   k ′″ to reconstruct the foreground or, in other words, predominant aspects of the HOA coefficients  11 ′. The foreground formulation unit  78  may perform a matrix multiplication of the interpolated nFG signals  49 ′ by the adjusted foreground V[k] vectors  55   k′″.    
     The HOA coefficient formulation unit  82  may represent a unit configured to combine the foreground HOA coefficients  65  to the adjusted ambient HOA coefficients  47 ″ so as to obtain the HOA coefficients  11 ′. The prime notation reflects that the HOA coefficients  11 ′ may be similar to but not the same as the HOA coefficients  11 . The differences between the HOA coefficients  11  and  11 ′ may result from loss due to transmission over a lossy transmission medium, quantization or other lossy operations. 
       FIGS.  14 A and  14 B  are flowcharts illustrating example operations of audio encoding device  20  in performing various aspects of the techniques described in this disclosure. Referring first to the example of  FIG.  14 A , the audio encoding device  20  may obtain channels for a current frame of HOA coefficients  11  in the manner described above (e.g., a linear decomposition, interpolation, etc.) ( 500 ). The channels may comprise encoded ambient HOA coefficients  59 , encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ) or both encoded ambient HOA coefficient  59  and encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ). 
     The bitstream generation unit  42  of the audio encoding device  20  may then specify an indication of a number of layers in the scalable bitstream  21  in the manner described above ( 502 ). The bitstream generation unit  42  may specify a subset of the channels in the current layer of the scalable bitstream  21  ( 504 ). The bitstream generation unit  42  may maintain a counter for the current layer, where the counter provides an indication of the current layer. After specifying the channels in the current layer, the bitstream generation unit  42  may increment the counter. 
     The bitstream generation unit  42  may then determine whether the current layer (e.g., the counter) is greater than the number of layers specified in the bitstream ( 506 ). When the current layer is not greater than the number of layers (“NO”  506 ), the bitstream generation unit  42  may specify a different subset of the channels in the current layer (which changed when the counter was incremented) ( 504 ). The bitstream generation unit  42  may continue in this manner until the current layer is greater than the number of layers (“YES”  506 ). When the current layer is greater than the number of layers (“YES”  506 ), the bitstream generation unit may proceed to the next frame with the current frame becoming the previous frame and obtain the channels for the now current frame of the scalable bitstream  21  ( 500 ). The process may continue until reaching the last frame of the HOA coefficients  11  ( 500 - 506 ). As noted above, in some examples, the indication of the number of layers may not be explicitly indicated but implicitly specified in the scalable bitstream  21  (e.g., when the number of layers has not changed from the previous frame to the current frame). 
     Referring next to the example of  FIG.  14 B , the audio encoding device  20  may obtain channels for a current frame of HOA coefficients  11  in the manner described above (e.g., a linear decomposition, interpolation, etc.) ( 510 ). The channels may comprise encoded ambient HOA coefficients  59 , encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ) or both encoded ambient HOA coefficient  59  and encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ). 
     The bitstream generation unit  42  of the audio encoding device  20  may then specify an indication of a number of channels in a layer of the scalable bitstream  21  in the manner described above ( 512 ). The bitstream generation unit  42  may specify the corresponding channels in the current layer of the scalable bitstream  21  ( 514 ). 
     The bitstream generation unit  42  may then determine whether the current layer (e.g., the counter) is greater than a number of layers ( 516 ). That is, in the example of  FIG.  14 B , the number of layers may be static or fixed (rather than specified in the scalable bitstream  21 ), while the number of channels per layer may be specified, unlike the example of  FIG.  14 A  where the number of channels may be static or fixed and not signaled. The bitstream generation unit  42  may still maintain the counter indicative of the current layer. 
     When the current layer (as indicated by the counter) is not greater than the number of layers (“NO”  516 ), the bitstream generation unit  42  may specify another indication of the number of channels in another layer of the scalable bitstream  21  for the now current layer (which changed due to incrementing the counter) ( 512 ). The bitstream generation unit  42  may also specify the corresponding number of channels in the additional layer of the bitstream  21  ( 514 ). The bitstream generation unit  42  may continue in this manner until the current layer is greater than the number of layers (“YES”  516 ). When the current layer is greater than the number of layers (“YES”  516 ), the bitstream generation unit may proceed to the next frame with the current frame becoming the previous frame and obtain the channels for the now current frame of the scalable bitstream  21  ( 510 ). The process may continue until reaching the last frame of the HOA coefficients  11  ( 510 - 516 ). 
     As noted above, in some examples, the indication of the number of channels may not be explicitly indicated but implicitly specified in the scalable bitstream  21  (e.g., when the number of layers has not changed from the previous frame to the current frame). Moreover, although described as separate processes, the techniques described with respect to  FIGS.  14 A and  14 B  may be performed in combination in the manner described above. 
       FIGS.  15 A and  15 B  are flowcharts illustrating example operations of audio decoding device  24  in performing various aspects of the techniques described in this disclosure. Referring first to the example of  FIG.  15 A , the audio decoding device  24  may obtain a current frame from the scalable bitstream  21  ( 520 ). The current frame may include one or more layers, each of which may include one or more channels. The channels may comprise encoded ambient HOA coefficients  59 , encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ) or both encoded ambient HOA coefficient  59  and encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ). 
     The extraction unit  72  of the audio decoding device  24  may then obtain an indication of a number of layers in the current frame of the scalable bitstream  21  in the manner described above ( 522 ). The extraction unit  72  may obtain a subset of the channels in the current layer of the scalable bitstream  21  ( 524 ). The extraction unit  72  may maintain a counter for the current layer, where the counter provides an indication of the current layer. After specifying the channels in the current layer, the extraction unit  72  may increment the counter. 
     The extraction unit  72  may then determine whether the current layer (e.g., the counter) is greater than the number of layers specified in the bitstream ( 526 ). When the current layer is not greater than the number of layers (“NO”  526 ), the extraction unit  72  may obtain a different subset of the channels in the current layer (which changed when the counter was incremented) ( 524 ). The extraction unit  72  may continue in this manner until the current layer is greater than the number of layers (“YES”  526 ). When the current layer is greater than the number of layers (“YES”  526 ), the extraction unit  72  may proceed to the next frame with the current frame becoming the previous frame and obtain the now current frame of the scalable bitstream  21  ( 520 ). The process may continue until reaching the last frame of the scalable bitstream  21  ( 520 - 526 ). As noted above, in some examples, the indication of the number of layers may not be explicitly indicated but implicitly specified in the scalable bitstream  21  (e.g., when the number of layers has not changed from the previous frame to the current frame). 
     Referring next to the example of  FIG.  15 B , the audio decoding device  24  may obtain a current frame from the scalable bitstream  21  ( 530 ). The current frame may include one or more layers, each of which may include one or more channels. The channels may comprise encoded ambient HOA coefficients  59 , encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ) or both encoded ambient HOA coefficient  59  and encoded nFG signals  61  (and corresponding sideband in the form of coded foreground V-vectors  57 ). 
     The extraction unit  72  of the audio decoding device  24  may then obtain an indication of a number of channels in a layer of the scalable bitstream  21  in the manner described above ( 532 ). The bitstream generation unit  42  may obtain the corresponding number of channels from the current layer of the scalable bitstream  21  ( 534 ). 
     The extraction unit  72  may then determine whether the current layer (e.g., the counter) is greater than a number of layers ( 536 ). That is, in the example of  FIG.  15 B , the number of layers may be static or fixed (rather than specified in the scalable bitstream  21 ), while the number of channels per layer may be specified, unlike the example of  FIG.  15 A  where the number of channels may be static or fixed and not signaled. The extraction unit  72  may still maintain the counter indicative of the current layer. 
     When the current layer (as indicated by the counter) is not greater than the number of layers (“NO”  536 ), the extraction unit  72  may obtain another indication of the number of channels in another layer of the scalable bitstream  21  for the now current layer (which changed due to incrementing the counter) ( 532 ). The extraction unit  72  may also specify the corresponding number of channels in the additional layer of the bitstream  21  ( 514 ). The extraction unit  72  may continue in this manner until the current layer is greater than the number of layers (“YES”  516 ). When the current layer is greater than the number of layers (“YES”  516 ), the bitstream generation unit may proceed to the next frame with the current frame becoming the previous frame and obtain the channels for the now current frame of the scalable bitstream  21  ( 510 ). The process may continue until reaching the last frame of the HOA coefficients  11  ( 510 - 516 ). 
     As noted above, in some examples, the indication of the number of channels may not be explicitly indicated but implicitly specified in the scalable bitstream  21  (e.g., when the number of layers has not changed from the previous frame to the current frame). Moreover, although described as separate processes, the techniques described with respect to  FIGS.  15 A and  15 B  may be performed in combination in the manner described above. 
       FIG.  16    is a diagram illustrating scalable audio coding as performed by the bitstream generation unit  42  shown in the example of  FIG.  16    in accordance with various aspects of the techniques described in this disclosure. In the example of  FIG.  16   , an HOA audio encoder, such as the audio encoding device  20  shown in the examples of  FIGS.  2  and  3   , may encode HOA coefficients  11  (which may also be referred to as an “HOA signal  11 ”). The HOA signal  11  may comprise 24 channels, each channel having 1024 samples. As noted above, each channel includes 1024 samples, which may refer to 1024 HOA coefficients corresponding to one of the spherical basis functions. The audio encoding device  20  may, as described above with respect to the bitstream generation unit  42  shown in the example of  FIG.  5   , perform various operations to obtain the encoded ambient HOA coefficients  59  (which may also be referred to as the “background HOA channels  59 ”) from the HOA signal  11 . 
     As further shown in the example of  FIG.  16   , the audio encoding device  20  obtains the background HOA channels  59  as the first four channels of the HOA signal  11 . The background HOA channels  59  are denoted as H 1:4   BG , where the 1:4 reflects that the first four channels of the HOA signal  11  was selected to represent the background components of the soundfield. This channel selection may be signaled as B=4 in a syntax element. The scalable bitstream generation unit  1000  of the audio encoding device  20  may then specify the HOA background channels  59  in the base layer  21 A (which may be referred to as a first layer of the two or more layers). 
     The scalable bitstream generation unit  1000  may generate the base layer  21 A to include the background channels  59  and gain information as specified in accordance with the following equation:
 
 H   1   BG (1st  BG  channel audio signal)+ G   1   BG (1st  BG  gain)
 
 H   2   BG (2nd  BG  channel audio signal)+ G   2   BG (2nd  BG  gain)
 
 H   3   BG (3rd  BG  channel audio signal)+ G   3   BG (3rd  BG  gain)
 
.
 
.
 
.
 
     As further shown in the example of  FIG.  16   , the audio encoding device  20  may obtain F foreground HOA channels, which may be expressed as the US audio objects and the corresponding V-vector. It is assumed for purposes of illustration that F=2. The audio encoding device  20  may therefore select the first and second US audio objects  61  (which may also be referred to the “encoded nFG signals  61 ”) and the first and second V-vectors  57  (which may also be referred to as the “coded foreground V[k] vectors  57 ”), where the selection is denoted in the example of  FIG.  5    as US 1:2  and V 1:2  respectively. The scalable bitstream generation unit  1000  may then generate the second layer  21 B of the scalable bitstream  21  to include the first and second US audio objects  61  and the first and second V-vectors  57 . 
     The scalable bitstream generation unit  1000  may also generate the enhancement layer  21 B to include the foreground HOA channels  61  and gain information along with the V-vectors  57  as specified in accordance with the following equation:
 
 US   1   FG (1st  FG  channel audio signal)+ G   1   FG (1st  FG  gain)+ V   1   FG (1st  V -vector)
 
 US   2   FG (2nd  FG  channel audio signal)+ G   2   FG (2nd  FG  gain)+ V   2   FG (2nd  V -vector)
 
 US   3   FG (3rd  FG  channel audio signal)+ GD   3   FG (3rd  FG  gain)+ V   3   FG (3rd  V -vector)
 
.
 
.
 
.
 
     To obtain the HOA coefficients  11 ′ from the scalable bitstream  21 ′, the audio decoding device  24  shown in the examples of  FIGS.  2  and  3    may invoke extraction unit  72  shown in more detail in the example of  FIG.  6   . The extraction unit  72  which may extract the encoded ambient HOA coefficients  59 A- 59 D, the encoded nFG signals  61 A and  61 B, and the coded foreground V[k] vectors  57 A and  57 B in the manner described above with respect to  FIG.  6   . The extraction unit  72  may then output the encoded ambient HOA coefficients  59 A- 59 D, the encoded nFG signals  61 A and  61 B, and the coded foreground V[k] vectors  57 A and  57 B to the vector-based decoding unit  92 . 
     The vector-based decoding unit  92  may then multiply the US audio objects  61  by the V-vectors  57  in accordance with the following equations: 
     
       
         
           
             
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     The first equation provides the mathematical expression of the generic operation with respect to F. The second equation provides the mathematical expression in the example where F is assumed to equal two. The result of this multiplication is denoted as the foreground HOA signal  1020 . The vector-based decoding unit  92  then selects the higher channels (given that the lowest four coefficients were already selected as the HOA background channels  59 ), where these higher channels are denoted as H 5:25   FG,1:2 . The vector-based decoding unit  92  in other words obtains the HOA foreground channels  65  from the foreground HOA signal  1020 . 
     As a result, the techniques may facilitate variable layering (as opposed to requiring a static number of layers) to accommodate a large number of coding contexts and potentially provide for much more flexibility in specifying the background and foreground components of the soundfield. The techniques may provide for many other use cases, as described with respect to  FIGS.  17 - 26   . These various use cases may be performed separately or together within a given audio stream. Moreover, the flexibility in specifying these components within the scalable audio encoding techniques may allow for many more use cases. In other words, the techniques should not be limited to the use cases described below but may include any way by which background and foreground components can be signaled in one or more layers of a scalable bitstream. 
       FIG.  17    is a conceptual diagram of an example where the syntax elements indicate that there are two layers with four encoded ambient HOA coefficients specified in a base layer and two encoded nFG signals are specified in the enhancement layer. The example of  FIG.  17    shows the HOA frame as the scalable bitstream generation unit  1000  shown in the example of  FIG.  5    may segment the frame to form the base layer including sideband HOA gain correction data for the encoded ambient HOA coefficients  59 A- 59 D. The scalable bitstream generation unit  1000  may also segment the HOA frame form an enhancement layer  21  that includes the two coded foreground V[k] vectors  57  and the HOA gain correction data for the encoded ambient nFG signals  61 . 
     As further shown in the example of  FIG.  17   , the psychoacoustic audio encoding unit  40  is shown as divided into separate instantiations of psychoacoustic audio encoder  40 A, which may be referred to as base layer temporal encoders  40 A, and psychoacoustic audio encoders  40 B, which may be referred to as enhancement layer temporal encoders  40 B. The base layer temporal encoders  40 A represent four instantiations of psychoacoustic audio encoders that process the four components of the base layer. The enhancement layer temporal encoders  40 B represent two instantiations of psychoacoustic audio encoders that process the two components of the enhancement layer. 
       FIG.  18    is a diagram illustrating, in more detail, the bitstream generation unit  42  of  FIG.  3    when configured to perform a second one of the potential versions of the scalable audio coding techniques described in this disclosure. In this example, the bitstream generation unit  42  is substantially similar to the bitstream generation unit  42  described above with respect to the example of  FIG.  5   . However, the bitstream generation unit  42  performs the second version of the scalable coding techniques to specify three layers  21 A- 21 C rather than two layers  21 A and  21 B. The scalable bitstream generation unit  1000  may specify indications that two encoded ambient HOA coefficients and zero encoded nFG signals are specified in the base layer  21 A, indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a first enhancement layer  21 B, and indications that zero encoded ambient HOA coefficients and two encoded nFG signals  61  are specified in a second enhancement layer  21 C. The scalable bitstream generation unit  1000  may then specify the two encoded ambient HOA coefficients  59 A and  59 B in the base layer  21 A, the two encoded nFG signals  61 A and  61 B with the corresponding two coded foreground V[k] vectors  57 A and  57 B in the first enhancement layer  21 B, and the two encoded nFG signals  61 C and  61 D with the corresponding two coded foreground V[k] vectors  57 C and  57 D in the second enhancement layer  21 C. The scalable bitstream generation unit  1000  may then output these layers as scalable bitstream  21 . 
       FIG.  19    is a diagram illustrating, in more detail, the extraction unit  72  of  FIG.  3    when configured to perform the second one of the potential versions the scalable audio decoding techniques described in this disclosure. In this example, the bitstream extraction unit  72  is substantially similar to the bitstream extraction unit  72  described above with respect to the example of  FIG.  6   . However, the bitstream extraction unit  72  performs the second version of the scalable coding techniques with respect to three layers  21 A- 21 C rather than two layers  21 A and  21 B. The scalable bitstream extraction unit  1012  may obtain indications that two encoded ambient HOA coefficients and zero encoded nFG signals are specified in the base layer  21 A, indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a first enhancement layer  21 B, and indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a second enhancement layer  21 C. The scalable bitstream extraction unit  1012  may then obtain the two encoded ambient HOA coefficients  59 A and  59 B from the base layer  21 A, the two encoded nFG signals  61 A and  61 B with the corresponding two coded foreground V[k] vectors  57 A and  57 B from the first enhancement layer  21 B, and the two encoded nFG signals  61 C and  61 D with the corresponding two coded foreground V[k] vectors  57 C and  57 D from the second enhancement layer  21 C. The scalable bitstream extraction unit  1012  may output the encoded ambient HOA coefficients  59 , the encoded nFG signals  61  and the coded foreground V[k] vectors  57  to the vector-based decoding unit  92 . 
       FIG.  20    is a diagram illustrating a second use case by which the bitstream generation unit of  FIG.  18    and the extraction unit of  FIG.  19    may perform the second one of the potential version of the techniques described in this disclosure. For example, the bitstream generation unit  42  shown in the example of  FIG.  18    may specify the NumLayer (which is shown as “NumberOfLayers” for ease of understanding) syntax element to indicate the number of layers specified in the scalable bitstream  21  is three. The bitstream generation unit  42  may further specify that the number of background channels specified in the first layer  21 A (which is also referred to as the “base layer”) is two while the number of foreground channels specified in the first layer  21 B is zero (i.e., B 1 =2, F 1 =0 in the example of  FIG.  20   ). The bitstream generation unit  42  may further specify that the number of background channels specified in the second layer  21 B (which is also referred to as the “enhancement layer”) is zero while the number of foreground channels specified in the second layer  21 B is two (i.e., B 2 =0, F 2 =2 in the example of  FIG.  20   ). The bitstream generation unit  42  may further specify that the number of background channels specified in the second layer  21 C (which is also referred to as the “enhancement layer”) is zero while the number of foreground channels specified in the second layer  21 C is two (i.e., B 3 =0, F 3 =2 in the example of  FIG.  20   ). However, the audio encoding device  20  may not necessarily signal the third layer background and foreground channel information when the total number of foreground and background channels are already known at the decoder (e.g., by way of additional syntax elements, such as totalNumBGchannels and totalNumFGchannels). 
     The bitstream generation unit  42  may specify these B i  and F i  values as NumBGchannels[i] and NumFGchannels[i]. For the above example, the audio encoding device  20  may specify the NumBGchannels syntax element as {2, 0, 0} and the NumFGchannels syntax element as 0, 2, 21. The bitstream generation unit  42  may also specify the background HOA audio channels  59 , the foreground HOA channels  61  and the V-vectors  57  in the scalable bitstream  21 . 
     The audio decoding device  24  shown in the examples of  FIGS.  2  and  4    may operate in a manner reciprocal to that of the audio encoding device  20  to parse these syntax elements from the bitstream (e.g., as set forth in the above HOADecoderConfig syntax table), as described above with respect to the bitstream extraction unit  72  of the  FIG.  19   . The audio decoding device  24  may also parse the corresponding background HOA audio channels  1002  and the foreground HOA channels  1010  from the bitstream  21  in accordance with the parsed syntax elements, again as described above with respect to the bitstream extraction unit  72  of the  FIG.  19   . 
       FIG.  21    is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded ambient HOA coefficients specified in a base layer, two encoded nFG signals are specified in a first enhancement layer and two encoded nFG signals are specified in a second enhancement layer. The example of  FIG.  21    shows the HOA frame as the scalable bitstream generation unit  1000  shown in the example of  FIG.  18    may segment the frame to form the base layer including sideband HOA gain correction data for the encoded ambient HOA coefficients  59 A and  59 B. The scalable bitstream generation unit  1000  may also segment the HOA frame form an enhancement layer  21 B that includes the two coded foreground V[k] vectors  57  and the HOA gain correction data for the encoded ambient nFG signals  61  and an enhancement layer  21 C that includes the two additional coded foreground V[k] vectors  57  and the HOA gain correction data for the encoded ambient nFG signals  61 . 
     As further shown in the example of  FIG.  21   , the psychoacoustic audio encoding unit  40  is shown as divided into separate instantiations of psychoacoustic audio encoder  40 A, which may be referred to as base layer temporal encoders  40 A, and psychoacoustic audio encoders  40 B, which may be referred to as enhancement layer temporal encoders  40 B. The base layer temporal encoders  40 A represent two instantiations of psychoacoustic audio encoders that process the four components of the base layer. The enhancement layer temporal encoders  40 B represent four instantiations of psychoacoustic audio encoders that process the two components of the enhancement layer. 
       FIG.  22    is a diagram illustrating, in more detail, the bitstream generation unit  42  of  FIG.  3    when configured to perform a third one of the potential versions of the scalable audio coding techniques described in this disclosure. In this example, the bitstream generation unit  42  is substantially similar to the bitstream generation unit  42  described above with respect to the example of  FIG.  18   . However, the bitstream generation unit  42  performs the third version of the scalable coding techniques to specify three layers  21 A- 21 C rather than two layers  21 A and  21 B. Moreover, the scalable bitstream generation unit  1000  may specify indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in the base layer  21 A, indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a first enhancement layer  21 B, and indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a second enhancement layer  21 C. The scalable bitstream generation unit  1000  may then specify the two encoded nFG signals  61 A and  61 B with the corresponding two coded foreground V[k] vectors  57 A and  57 B in the base layer  21 A, the two encoded nFG signals  61 C and  61 D with the corresponding two coded foreground V[k] vectors  57 C and  57 D in the first enhancement layer  21 B, and the two encoded nFG signals  61 E and  61 F with the corresponding two coded foreground V[k] vectors  57 E and  57 F in the second enhancement layer  21 C. The scalable bitstream generation unit  1000  may then output these layers as scalable bitstream  21 . 
       FIG.  23    is a diagram illustrating, in more detail, the extraction unit  72  of  FIG.  4    when configured to perform the third one of the potential versions the scalable audio decoding techniques described in this disclosure. In this example, the bitstream extraction unit  72  is substantially similar to the bitstream extraction unit  72  described above with respect to the example of  FIG.  19   . However, the bitstream extraction unit  72  performs the third version of the scalable coding techniques with respect to three layers  21 A- 21 C rather than two layers  21 A and  21 B. Moreover, the scalable bitstream extraction unit  1012  may obtain indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in the base layer  21 A, indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a first enhancement layer  21 B, and indications that zero encoded ambient HOA coefficients and two encoded nFG signals are specified in a second enhancement layer  21 C. The scalable bitstream extraction unit  1012  may then obtain the two encoded nFG signals  61 A and  61 B with the corresponding two coded foreground V[k] vectors  57 A and  57 B from the base layer  21 A, the two encoded nFG signals  61 C and  61 D with the corresponding two coded foreground V[k] vectors  57 C and  57 D from the first enhancement layer  21 B, and the two encoded nFG signals  61 E and  61 F with the corresponding two coded foreground V[k] vectors  57 E and  57 F from the second enhancement layer  21 C. The scalable bitstream extraction unit  1012  may output the encoded nFG signals  61  and the coded foreground V[k] vectors  57  to the vector-based decoding unit  92 . 
       FIG.  24    is a diagram illustrating a third use case by which an audio encoding device may specify multiple layers in a multi-layer bitstream in accordance with the techniques described in this disclosure. For example, the bitstream generation unit  42  of  FIG.  22    may specify the NumLayer (which is shown as “NumberOfLayers” for ease of understanding) syntax element to indicate the number of layers specified in the bitstream  21  is three. The bitstream generation unit  42  may further specify that the number of background channels specified in the first layer (which is also referred to as the “base layer”) is zero while the number of foreground channels specified in the first layer is two (i.e., B 1 =0, F 1 =2 in the example of  FIG.  24   ). In other words, the base layer does not always provide only for transport of ambient HOA coefficients but may allow for specification of predominant or in other words foreground HOA audio signals. 
     These two foreground audio channels are denoted as the encoded nFG signals  61 A/B and the coded foreground V[k] vectors  57 A/B and may be mathematically represented by the following equation: 
               H     1   :   25       FG   ,     1   :   2         =       ∑     i   =   1     2     ⁢     U   ⁢     S   i     ⁢       V   i   T     .               
The H 1:25   FG,1:2  denotes the two foreground audio channels, which may be represented by the first and second audio objects (US 1  and US 2 ) along with the corresponding V-vectors (V 1  and V 2 ).
 
     The bitstream generation device  42  may further specify that the number of background channels specified in the second layer (which is also referred to as the “enhancement layer”) is zero while the number of foreground channels specified in the second layer is two (i.e., B 2 =0, F 2 =2 in the example of  FIG.  24   ). These two foreground audio channels are denoted as the encoded nFG signals  61 C/D and the coded foreground V[k] vectors  57 C/D and may be mathematically represented by the following equation: 
               H     1   :   25       FG   ,     3   :   4         =       ∑     i   =   3     4     ⁢     U   ⁢     S   i     ⁢       V   i   T     .               
The H 1:25   FG,3:4  denotes the two foreground audio channels, which may be represented by the third and fourth audio objects (US 3  and US 4 ) along with the corresponding V-vectors (V 3  and V 4 ).
 
     Furthermore, the bitstream generation unit  42  may specify that the number of background channels specified in the third layer (which is also referred to as the “enhancement layer”) is zero while the number of foreground channels specified in the third layer is two (i.e., B 3 =0, F 3 =2 in the example of  FIG.  24   ). These two foreground audio channels are denoted as foreground audio channels  1024  and may be mathematically represented by the following equation: 
     
       
         
           
             
               H 
               
                 1 
                 : 
                 25 
               
               
                 FG 
                 , 
                 
                   5 
                   : 
                   6 
                 
               
             
             = 
             
               
                 ∑ 
                 
                   i 
                   = 
                   5 
                 
                 6 
               
               ⁢ 
               
                 U 
                 ⁢ 
                 
                   S 
                   i 
                 
                 ⁢ 
                 
                   
                     V 
                     i 
                     T 
                   
                   . 
                 
               
             
           
         
       
     
     The H 1:25   FG,5:6  denotes the two foreground audio channels  1024 , which may be represented by the fifth and sixth audio objects (US 5  and US 6 ) along with the corresponding V-vectors (V 5  and V 6 ). However, the bitstream generation unit  42  may not necessarily signal this third layer background and foreground channel information when the total number of foreground and background channels are already known at the decoder (e.g., by way of additional syntax elements, such as totalNumBGchannels and totalNumFGchannels). The bitstream generation unit  42  may, however, not signal the third layer background and foreground channel information when the total number of foreground and background channels are already known at the decoder (e.g., by way of additional syntax elements, such as totalNumBGchannels and totalNumFGchannels). 
     The bitstream generation unit  42  may specify these B i  and F i  values as NumBGchannels[i] and NumFGchannels[i]. For the above example, the audio encoding device  20  may specify the NumBGchannels syntax element as {0, 0, 0} and the NumFGchannels syntax element as {2, 2, 2}. The audio encoding device  20  may also specify the foreground HOA channels  1020 - 1024  in the bitstream  21 . 
     The audio decoding device  24  shown in the examples of  FIGS.  2  and  4    may operate in a manner reciprocal to that of the audio encoding device  20  to parse, as described above with respect to the bitstream extraction unit  72  of  FIG.  23   , these syntax elements from the bitstream (e.g., as set forth in the above HOADecoderConfig syntax table). The audio decoding device  24  may also parse, again as described above with respect to the bitstream extraction unit  72  of  FIG.  23   , the corresponding foreground HOA audio channels  1020 - 1024  from the bitstream  21  in accordance with the parsed syntax elements and reconstruct HOA coefficients  1026  through summation of the foreground HOA audio channels  1020 - 1024 . 
       FIG.  25    is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded nFG signals specified in a base layer, two encoded nFG signals are specified in a first enhancement layer and two encoded nFG signals are specified in a second enhancement layer. The example of  FIG.  25    shows the HOA frame as the scalable bitstream generation unit  1000  shown in the example of  FIG.  22    may segment the frame to form the base layer including sideband HOA gain correction data for the encoded nFG signals  61 A and  61 B and two coded foreground V[k] vectors  57 . The scalable bitstream generation unit  1000  may also segment the HOA frame to form an enhancement layer  21 B that includes the two coded foreground V[k] vectors  57  and the HOA gain correction data for the encoded ambient nFG signals  61  and an enhancement layer  21 C that includes the two additional coded foreground V[k] vectors  57  and the HOA gain correction data for the encoded ambient nFG signals  61 . 
     As further shown in the example of  FIG.  25   , the psychoacoustic audio encoding unit  40  is shown as divided into separate instantiations of psychoacoustic audio encoder  40 A, which may be referred to as base layer temporal encoders  40 A, and psychoacoustic audio encoders  40 B, which may be referred to as enhancement layer temporal encoders  40 B. The base layer temporal encoders  40 A represent two instantiations of psychoacoustic audio encoders that process the four components of the base layer. The enhancement layer temporal encoders  40 B represent four instantiations of psychoacoustic audio encoders that process the two components of the enhancement layer. 
       FIG.  26    is a diagram illustrating a third use case by which an audio encoding device may specify multiple layers in a multi-layer bitstream in accordance with the techniques described in this disclosure. For example, the audio encoding device  20  shown in the example of  FIGS.  2  and  3    may specify the NumLayer (which is shown as “NumberOfLayers” for ease of understanding) syntax element to indicate the number of layers specified in the bitstream  21  is four. The audio encoding device  20  may further specify that the number of background channels specified in the first layer (which is also referred to as the “base layer”) is one while the number of foreground channels specified in the first layer is zero (i.e., B 1 =1, F 1 =0 in the example of  FIG.  26   ). 
     The audio encoding device  20  may further specify that the number of background channels specified in the second layer (which is also referred to as a “first enhancement layer”) is one while the number of foreground channels specified in the second layer is zero (i.e., B 2 =1, F 2 =0 in the example of  FIG.  26   ). The audio encoding device  20  may also specify that the number of background channels specified in the third layer (which is also referred to as a “second enhancement layer”) is one while the number of foreground channels specified in the third layer is zero (i.e., B 3 =1, F 3 =0 in the example of  FIG.  26   ). In addition, the audio encoding device  20  may specify that the number of background channels specified in the fourth layer (which is also referred to as the “enhancement layer”) is one while the number of foreground channels specified in the third layer is zero (i.e., B 4 =1, F 4 =0 in the example of  FIG.  26   ). However, the audio encoding device  20  may not necessarily signal the fourth layer background and foreground channel information when the total number of foreground and background channels are already known at the decoder (e.g., by way of additional syntax elements, such as totalNumBGchannels and totalNumFGchannels). 
     The audio encoding device  20  may specify these B i  and F i  values as NumBGchannels[i] and NumFGchannels[i]. For the above example, the audio encoding device  20  may specify the NumBGchannels syntax element as {1, 1, 1, 1} and the NumFGchannels syntax element as {0, 0, 0, 0}. The audio encoding device  20  may also specify the background HOA audio channels  1030  in the bitstream  21 . In this respect, the techniques may allow for enhancement layers to specify ambient or, in other words, background HOA channels  1030 , which may have been decorrelated prior to being specified in the base and enhancement layers of the bitstream  21  as described above with respect to the examples of  FIGS.  7 A- 9 B . However, again, the techniques set forth in this disclosure are not necessarily limited to decorrelation and may not provide for syntax elements or any other indications in the bitstream relevant to decorrelation as described above. 
     The audio decoding device  24  shown in the examples of  FIGS.  2  and  4    may operate in a manner reciprocal to that of the audio encoding device  20  to parse these syntax elements from the bitstream (e.g., as set forth in the above HOADecoderConfig syntax table). The audio decoding device  24  may also parse the corresponding background HOA audio channels  1030  from the bitstream  21  in accordance with the parsed syntax elements. 
     As noted above, in some instances, the scalable bitstream  21  may include various layers that conform to the non-scalable bitstream  21 . For example, the scalable bitstream  21  may include a base layer that conforms to non-scalable bitstream  21 . In these instances, the non-scalable bitstream  21  may represent a sub-bitstream of scalable bitstream  21 , where this non-scalable sub-bitstream  21  may be enhanced with additional layers of the scalable bitstream  21  (which are referred to as enhancement layers). 
       FIGS.  27  and  28    are block diagrams illustrating a scalable bitstream generation unit  42  and a scalable bitstream extraction unit  72  that may be configured to perform various aspects of the techniques described in this disclosure. In the example of  FIG.  27   , the scalable bitstream generation unit  42  may represent an example of the bitstream generation unit  42  described above with respect to the example of  FIG.  3   . The scalable bitstream generation unit  42  may output a base layer  21  that conforms (in terms of syntax and ability to be decoded by audio decoders that do not support scalable coding) to a non-scalable bitstream  21 . The scalable bitstream generation unit  42  may operate in ways described above with respect to any of the foregoing bitstream generation units  42  except that the scalable bitstream generation unit  42  does not include a non-scalable bitstream generation unit  1002 . Instead, the scalable bitstream generation unit  42  outputs a base layer  21  that conforms to a non-scalable bitstream and as such does not require a separate non-scalable bitstream generation unit  1000 . In the example of  FIG.  28   , the scalable bitstream extraction unit  72  may operate reciprocally to the scalable bitstream generation unit  42 . 
       FIG.  29    represents a conceptual diagram representing an encoder  900  that may be configured to operate in accordance with various aspects of the techniques described in this disclosure. The encoder  900  may represent another example of the audio encoding device  20 . The encoder  900  may include a spatial decomposition unit  902 , a decorrelation unit  904  and a temporal encoding unit  906 . The spatial decomposition unit  902  may represent a unit configured to output the vector-based predominant sounds (in the form of the audio objects noted above), the corresponding V-vectors associated with these vector-based predominant sounds and horizontal ambient HOA coefficients  903 . The spatial decomposition unit  902  may differ from a directional based decomposition in that the V-vectors describe both the direction and the width of the corresponding one of the audio objects as each audio object moves over time within the soundfield. 
     The spatial decomposition unit  902  may include units  30 - 38  and  44 - 52  of the vector-based synthesis unit  27  shown in the example of  FIG.  3    and generally operate in the manner described above with respect to unit  30 - 38  and  44 - 52 . The spatial decomposition unit  902  may differ from the vector-based synthesis unit  27  in that the spatial decomposition unit  902  may not perform psychoacoustic encoding or otherwise include psychoacoustic coder unit  40  and may not include a bitstream generation unit  42 . Moreover, in the scalable audio encoding context, the spatial decomposition unit  902  may pass through the horizontal ambient HOA coefficients  903  (meaning, in some examples, that these horizontal HOA coefficients may not be modified or otherwise adjusted and are parsed from HOA coefficients  901 ). 
     The horizontal ambient HOA coefficients  903  may refer to any of the HOA coefficients  901  (which may also be referred to as HOA audio data  901 ) that describe a horizontal component of the soundfield. For example, the horizontal ambient HOA coefficients  903  may include HOA coefficients associated with a spherical basis function having an order of zero and a sub-order of zero, higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a sub-order of negative one, and third higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a sub-order of one. 
     The decorrelation unit  904  represents a unit configured to perform decorrelation with respect to a first layer of two or more layers of the higher order ambisonic audio data  903  (where the ambient HOA coefficients  903  are one example of this HOA audio data) to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. Base layer  903  may be similar to any of the first layers, base layers or base sub-layers described above with respect to  FIGS.  21 - 26   . The decorrelation unit  904  may perform decorrelation using the above noted UHJ matrix or the mode matrix. The decorrelation unit  904  may also perform decorrelation using a transformation, such as rotation, in a manner similar to that described in U.S. application Ser. No. 14/192,829, entitled “TRANSFORMING SPHERICAL HARMONIC COEFFICIENTS,” filed Feb. 27, 2014, except that the rotation is performed to obtain a decorrelated representation of the first layer rather than reduce the number of coefficients. 
     In other words, the decorrelation unit  904  may perform a rotation of the soundfield to align energy of the ambient HOA coefficients  903  along three different horizontal axes separated by 120 degrees (such as 0 azimuthal degrees/0 elevational degrees, 120 azimuthal degrees/0 elevational degrees, and 240 azimuthal degrees/0 elevational degrees). By aligning these energies with the three horizontal axes, the decorrelation unit  904  may attempt to decorrelate the energies from one another such that the decorrelation unit  904  may utilize a spatial transformation to effectively render three decorrelation audio channels  905 . The decorrelation unit  904  may apply this spatial transformation so as to compute the spatial audio signals  905  at the azimuth angles of 0 degrees, 120 degrees and 240 degrees. 
     Although described with respect to azimuth angles of 0 degrees, 120 degrees and 240 degrees, the techniques may be applied with respect to any three azimuthal angles that evenly or nearly evenly divide the 360 azimuth degrees of the circle. For example, the techniques may also be performed with respect to a transformation that computes the spatial audio signals  905  at the azimuth angles of 60 degrees, 180 degrees, and 300 degrees. Moreover, although described with respect to three ambient HOA coefficients  901 , the techniques may be performed more generally with respect to any horizontal HOA coefficients, including those as described above and any other horizontal HOA coefficients, such as those associated with a spherical basis function having an order of two and sub-order of two, a spherical basis function having an order of two and a sub-order of negative two, . . . , a spherical basis function having an order of X and a sub-order of X, and a spherical basis function having an order of X and a sub-order of negative X, where X may represent any number including 3, 4, 5, 6, etc. 
     As the number of horizontal HOA coefficients increases, the number of even or nearly even portions of the 360 degree circle may increase. For example, when the number of horizontal HOA coefficients increases to five, the decorrelation unit  904  may segment the circle into five even partitions (e.g., of approximately 72 degrees each). The number of horizontal HOA coefficients of X may, as another example, result in X even partitions with each partition having 360 degrees/X degrees. 
     The decorrelation unit  904  may, to identify the rotation information indicative of the amount by which to rotate the soundfield represented by the horizontal ambient HOA coefficients  903 , perform a soundfield analysis, content-characteristics analysis, and/or spatial analysis. Based on one or more of these analyses, the decorrelation unit  904  may identify the rotation information (or other transformation information of which the rotation information is one example) as a number of degrees by which to horizontally rotate the soundfield, and rotate the soundfield, effectively obtaining a rotated representation (which is one example of the more general transformed representation) of the base layer of the higher order ambisonic audio data. 
     The decorrelation unit  904  may then apply a spatial transform to the rotated representation of the base layer  903  (which may also be referred to as a first layer  903  of two or more layers) of the higher order ambisonic audio data. The spatial transform may convert the rotated representation of the base layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data. The decorrelation representation of the first layer may include spatial audio signals  905  rendered at the three corresponding azimuth angles of 0 degrees, 120 degrees and 240 degrees, as noted above. The decorrelation unit  904  may then pass the horizontal ambient spatial audio signals  905  to the temporal encoding unit  906 . 
     The temporal encoding unit  906  may represent a unit configured to perform psychoacoustic audio coding. The temporal encoding unit  906  may represent an AAC encoder or a unified speech and audio coder (USAC) to provide two examples. Temporal audio encoding units, such as the temporal encoding unit  906 , may normally operate with respect to decorrelated audio data, such as the 6 channels of a 5.1 speaker setup, these 6 channels having been rendered to decorrelated channels. However, the horizontal ambient HOA coefficients  903  are additive in nature and thereby correlate in certain respect. Providing these horizontal ambient HOA coefficients  903  directly to the temporal encoding unit  906  without first performing some form of decorrelation may result in spatial noise unmasking in which sounds appear in locations that were not intended. These perceptual artifacts, such as the spatial noise unmasking, may be reduced by performing the transformation-based (or, more specifically, rotation-based in the example of  FIG.  29   ) decorrelation described above. 
       FIG.  30    is a diagram illustrating the encoder  900  shown in the example of  FIG.  27    in more detail. In the example of  FIG.  30   , encoder  900  may represent a base layer encoder  900  that encodes the HOA first order horizontal-only base layer  903  and does not show spatial decomposition unit  902  as this unit  902  does not perform, in this pass through example, meaningful operations other than provide the base layer  903  to a soundfield analysis unit  910  and a two-dimensional (2D) rotation unit  912  of the decorrelation unit  904 . 
     That is, the decorrelation unit  904  includes the soundfield analysis unit  910  and the 2D rotation unit  912 . The soundfield analysis unit  910  represents a unit configured to perform the soundfield analysis described above in more detail to obtain a rotation angle parameter  911 . The rotation angle parameter  911  represents one example of transformation information in the form of rotation information. The 2D rotation unit  912  represents a unit configured to perform a horizontal rotation around the Z-axis of the soundfield based on the rotation angle parameter  911 . This rotation is two-dimensional in that the rotation only involves a single axis of rotation and does not include any, in this example, elevational rotation. The 2D rotation unit  912  may obtain inverse rotation information  913  (by inverting, as one example, the rotation angle parameter  911  to obtain the inverse rotation angle parameter  913 ), which may be an example of more general inverse transformation information. The 2D rotation unit  912  may provide the inverse rotation angle parameter  913  such that the encoder  900  may specify the inverse rotation angle parameter  913  in the bitstream. 
     In other words, the 2D rotation unit  912  may, based on the soundfield analysis, rotate the 2D soundfield so that the predominant energy is potentially arriving from one of the spatial sampling points used in the 2D spatial transform module (0°, 120°, 240°). The 2D rotation unit  912  may, as one example, apply the following rotation matrix: 
                   [         1       0       0             cos   ⁢           ⁢     (   ϕ   )           0         sin   ⁢           ⁢     (   ϕ   )                   -   sin     ⁢           ⁢     (   ϕ   )           0         cos   ⁢           ⁢     (   ϕ   )             ]           
In some examples, the 2D rotation unit  912  may, to avoid frame artifacts, apply a smoothing (interpolation) function to ensure a smooth transition of the time-varying rotation angle. This smoothing function may comprise a linear smoothing function. However, other smoothing functions, including non-linear smoothing functions may be used. The 2D rotation unit  912  may, for example, use a spline smoothing function.
 
     To illustrate, when the soundfield analysis unit  910  module indicates that the soundfield&#39;s dominant direction is at 70° azimuth within one analysis frame, the 2D rotation unit  912  may smoothly rotate the soundfield by ϕ=−70° so that the dominant direction is now 0°. As another possibility, the 2D rotation unit  912  may rotate the soundfield by ϕ=50°, so that the dominant direction is now 120°. The 2D rotation unit  912  may then signal the applied rotation angle  913  as an additional sideband parameter within the bitstream, so that a decoder can apply the correct inverse rotation operation. 
     As further shown in the example of  FIG.  30   , the decorrelation unit  904  also includes a 2D spatial transformation unit  914 . The 2D spatial transformation unit  914  represents a unit configured to convert the rotated representation of the base layer from the spherical harmonic domain to the spatial domain, effectively rendering the rotated base layer  915  to the three azimuth angles (e.g., 0, 120 and 240). The 2D spatial transformation unit  914  may multiply the coefficients of the rotated base layer  915  with the following transformation matrix, which assumes the HOA coefficient order ‘00+’, ‘11−’, ‘11+’ and N3D normalization: 
                   [           1   ⁢     /     ⁢   3         0       0.384900179459750             1   ⁢     /     ⁢   3           1   ⁢     /     ⁢   3           -   0.192450089729875               1   ⁢     /     ⁢   3             -   1     ⁢     /     ⁢   31           -   0.192450089729875           ]           
The foregoing matrix computes the spatial audio signals  905  at the azimuth angles 0°, 120° and 240°, so that the circle of 360° is evenly divided in 3 portions. As noted above, other separations are possible, as long as each portion covers 120 degrees, e.g., computing the spatial signals at 60°, 180°, and 300°.
 
     In this way, the techniques may provide for a device  900  configured to perform scalable higher order ambisonic audio data encoding. The device  900  may be configured to perform decorrelation with respect to a first layer  903  of two or more layers of the higher order ambisonic audio data to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the first layer  903  of the two or more layers of the higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding to one or more spherical basis functions having an order equal to or less than one. In these and other instances, the first layer  903  of the two or more layers of the higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of horizontal aspects of the soundfield. In these and other instances, the ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of the horizontal aspects of the soundfield may comprise first ambient higher order ambisonic coefficients corresponding to a spherical basis function having an order of zero and a sub-order of zero, second higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a sub-order of negative one, and third higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a sub-order of one. 
     In these and other instances, the device  900  may be configured to perform a transformation (e.g., by way of the 2D rotation unit  912 ) with respect to the first layer  903  of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to perform a rotation (e.g., by way of the 2D rotation unit  912 ) with respect to the first layer  903  of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to apply a transformation (e.g., by way of the 2D rotation unit  912 ) with respect to the first layer  903  of the two or more layers of the higher order ambisonic audio data to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data (e.g., by way of the 2D spatial transformation unit  914 ) from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to apply a rotation with respect to the first layer  903  of the two or more layers of the higher order ambisonic audio data to obtain a rotated representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the rotated representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to obtain transformation information  911 , apply a transformation with respect to the first layer  903  of the two or more layers of the higher order ambisonic audio data based on the transformation information  911  to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to obtain rotation information  911 , and apply a rotation with respect to the first layer  903  of the two or more layers of the higher order ambisonic audio data based on the rotation information  911  to obtain a rotated representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and converting the rotated representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to apply a transformation with respect to the first layer  903  of the two or more layers of the higher order ambisonic audio data using at least in part a smoothing function to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to apply a rotation with respect to the first layer  903  of the two or more layers of the higher order ambisonic audio data using at least in part a smoothing function to obtain a rotated representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the rotated representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be configured to specify an indication of the smoothing function to be used when applying an inverse transformation or an inverse rotation. 
     In these and other instances, the device  900  may be further configured to apply a linear invertible transform to the higher order ambisonic audio data to obtain a V-vector and specify the V-vector as a second layer of the two or more layers of the higher order ambisonic audio data, as described above with respect to  FIG.  3   . 
     In these and other instances, the device  900  may be further configured to obtain higher order ambisonic coefficients associated with a spherical basis function having an order of one and a sub-order of zero and specify the higher order ambisonic coefficients as a second layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  900  may be further configured to perform a temporal encoding with respect to the decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data. 
       FIG.  31    is a block diagram illustrating an audio decoder  920  that may be configured to operate in accordance with various aspects of the techniques described in this disclosure. The decoder  920  may represent another example of the audio decoding device  24  shown in the example of  FIG.  2    in terms of reconstructing the HOA coefficients, reconstructing V-vectors of the enhancement layers, performing temporal audio decoding (as performed by a temporal audio decoding unit  922 ), etc. However, decoder  920  differs in that the decoder  920  operates with respect to scalable coded higher order ambisonic audio data as specified in the bitstream. 
     As shown in the example of  FIG.  31   , the audio decoder  920  includes a temporal decoding unit  922 , an inverse 2D spatial transformation unit  924 , a base layer rendering unit  928  and an enhancement layer processing unit  930 . The temporal decoding unit  922  may be configured to operate in a manner reciprocal to that of the temporal encoding unit  906 . The inverse 2D spatial transformation unit  924  may represent a unit configured to operate in a manner reciprocal to that of the 2D spatial transformation unit  914 . 
     In other words, the inverse 2D spatial transformation unit  924  may be configured to apply the below matrix to the spatial audio signals  905  to obtain the rotated horizontal ambient HOA coefficients  915  (which may also be referred to as “the rotated base layer  915 ”). The inverse 2D spatial transformation unit  924  may transform the 3 transmitted audio signals  905  back into the HOA domain using the following transformation matrix, which like the matrix above assumes the HOA coefficient order ‘00+’, ‘11−’, ‘11+’ and N3D normalization: 
                   [         1.0       1.0       1.0           0.0       1.5         -   1.5             1.732050807568878         -   0.866025403784438           -   0.866025403784440           ]           
The foregoing matrix is the inverse of the transformation matrix used in the decoder.
 
     The inverse 2D rotation unit  926  may be configured to operate in a manner reciprocal to that described above with respect to the 2D rotation unit  912 . In this respect, the 2D rotation unit  912  may perform a rotation in accordance with the rotation matrix noted above based on the inverse rotation angle parameter  913  instead of the rotation angle parameter  911 . In other words, the inverse rotation unit  926  may, based on the signaled rotation ϕ, applied the following matrix, which again assumes the HOA coefficient order ‘00+’, ‘11−’, ‘11+’ and N3D normalization: 
                   [         1       0       0             cos   ⁢           ⁢     (   ϕ   )           0         sin   ⁢           ⁢     (   ϕ   )                   -   sin     ⁢           ⁢     (   ϕ   )           0         cos   ⁢           ⁢     (   ϕ   )             ]           
The inverse 2D rotation unit  926  may use the same smoothing (interpolation) function used in the decoder to ensure a smooth transition for the time varying rotation angle, which may be signaled in the bitstream or configured a priori.
 
     The base layer rendering unit  928  may represent a unit configured to renderer the horizontal-only ambient HOA coefficients of the base layer to loudspeaker feeds. The enhancement layer processing unit  930  may represent a unit configured to perform further processing of the base layer with any received enhancement layers (decoded via a separate enhancement layer decoding path that involves much of the decoding described above with respect to additional ambient HOA coefficients and the V-vectors along with the audio objects corresponding to the V-vectors) to render speaker feeds. The enhancement layer processing unit  930  may effectively augment the base layer to provide a higher resolution representation of the soundfield that may provide for a more immersive audio experience having sounds that potentially move realistically within the soundfield. The base layer may be similar to any of the first layers, base layers or base sub-layers described above with respect to  FIGS.  11 - 13 B . The enhancement layers may be similar to any of the second layers, enhancement layers, or enhancement sub-layers described above with respect to  FIGS.  11 - 13 B . 
     In this respect, the techniques provide for a device  920  configured to perform scalable higher order ambisonic audio data decoding. The device may be configured to obtain a decorrelated representation of a first layer of two or more layers of the higher order ambisonic audio data (e.g., spatial audio signals  905 ), the higher order ambisonic audio data descriptive of a soundfield. The decorrelated representation of the first layer is decorrelated by performing decorrelation with respect to the first layer of the higher order ambisonic audio data. 
     In some instances, the first layer of the two or more layers of the higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding to one or more spherical basis functions having an order equal to or less than one. In these and other instances, the first layer of the two or more layers of the higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of horizontal aspects of the soundfield. In these and other instances, the ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of the horizontal aspects of the soundfield comprises first ambient higher order ambisonic coefficients corresponding to a spherical basis function having an order of zero and a sub-order of zero, second higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a sub-order of negative one, and third higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a sub-order of one. 
     In these and other instances, the decorrelated representation of the first layer is decorrelated by performing a transformation with respect to the first layer of the higher order ambisonic audio data, as described above with respect to the encoder  900 . 
     In these and other instances, the device  920  may be configured to perform a rotation (e.g., by inverse 2D rotation unit  926 ) with respect to the first layer of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be configured to recorrelate the decorrelated representation of the first layer of two or more layers of the higher order ambisonic audio data to obtain the first layer of the two or more layers of the higher order ambisonic audio data as described above for example with respect to inverse 2D spatial transformation unit  924  and inverse 2D rotation unit  926 . 
     In these and other instances, the device  920  may be configured to convert the decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and apply an inverse transformation (e.g., as described above with respect to the inverse 2D rotation unit  926 ) with respect to the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data to obtain the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be configured to convert the decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and apply an inverse rotation with respect to the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data to obtain the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be configured to convert the decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, obtain transformation information  913 , and apply an inverse transformation with respect to the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data based on the transformation information  913  to obtain the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be configured to convert the decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, obtain rotation information  913 , and apply an inverse rotation with respect to the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data based on the rotation information  913  to obtain the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be configured to convert the decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and apply an inverse transformation with respect to the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data using, at least in part, a smoothing function to obtain the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be configured to convert the decorrelated representation  905  of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data, and apply an inverse rotation with respect to the transformed representation  915  of the first layer of the two or more layers of the higher order ambisonic audio data using, at least in part, a smoothing function to obtain the first layer of the two or more layers of the higher order ambisonic audio data. 
     In these and other instances, the device  920  may be further configured to obtain an indication of the smoothing function to be used when applying the inverse transformation or the inverse rotation. 
     In these and other instances, the device  920  may be further configured to obtain a representation of a second layer of the two or more layers of the higher order ambisonic audio data, where the representation of the second layer comprises vector-based predominant audio data, the vector-based predominant audio data comprises at least a predominant audio data and an encoded V-vector, and the encoded V-vector is decomposed from the higher order ambisonic audio data through application of a linear invertible transform, as described above with respect to the example of  FIG.  3   . 
     In these and other instances, the device  920  may be further configured to obtain a representation of a second layer of the two or more layers of the higher order ambisonic audio data, where the representation of the second layer comprises higher order ambisonic coefficients associated with a spherical basis function having an order of one and a sub-order of zero. 
     In this way, the techniques may enable a device to be configured to, or provide for an apparatus comprising means for performing, or a non-transitory computer-readable medium having stored thereon instructions that, when executed, cause one or more processors to perform the method set forth in the following clauses. 
     Clause 1A. A method of encoding a higher order ambisonic audio signal to generate a bitstream, the method comprising specifying an indication of a number of layers in the bitstream and outputting the bitstream that includes the indicated number of the layers. 
     Clause 2A. The method of clause 1A, further comprising specifying an indication of a number of channels included in the bitstream. 
     Clause 3A. The method of clause 1A, wherein the indication of the number of layers comprises an indication of a number of layers in the bitstream for a previous frame, and wherein the method further comprises specifying, in the bitstream, an indication of whether a number of layers of the bitstream has changed for a current frame when compared to the number of layers of the bitstream for the previous frame and specifying the indicated number of layers of the bitstream in the current frame. 
     Clause 4A. The device of clause 3A, wherein specifying the indicated number of layers comprises, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, specifying the indicated number of layers without specifying, in the bitstream, an indication of a current number of background components in one or more of the layers for the current frame to be equal to a previous number of background components in one or more of the layers of the previous frame. 
     Clause 5A. The method of clause 1A, wherein the layers are hierarchical such that a first layer, when combined with a second layer, provides a higher resolution representation of the higher order ambisonic audio signal. 
     Clause 6A. The method of clause 1A, wherein the layers of the bitstream comprise a base layer and an enhancement layer, and wherein the method further comprises applying a decorrelation transform with respect to one or more channels of the base layer to obtain a decorrelated representation of background components of the higher order ambisonic audio signal. 
     Clause 7A. The method of clause 6A, wherein the decorrelation transform comprises a UHJ transform. 
     Clause 8A. The method of clause 6A, wherein the decorrelation transform comprises a mode matrix transform. 
     Moreover, the techniques may enable a device to be configured to, or provide for an apparatus comprising means for performing, or a non-transitory computer-readable medium having stored thereon instructions that, when executed, cause one or more processors to perform the method set forth in the following clauses. 
     Clause 1B. A method of encoding a higher order ambisonic audio signal to generate a bitstream, the method comprising specifying, in the bitstream, an indication of a number of channels specified in one or more layers of the bitstream and specifying the indicated number of the channels in the one or more layers of the bitstream. 
     Clause 2B. The method of clause 1B, further comprising specifying an indication of a total number of channels specified in the bitstream, wherein specifying the indicated number of channels comprises specifying the indicated total number of the channels in the one or more layers of the bitstream. 
     Clause 3B. The method of clause 1B, further comprising specifying an indication a type of one of the channels specified in the one or more layers in the bitstream and specifying the indicated number of channels comprises specifying the indicated number of the indicated type of the one of the channels in the one or more layers of the bitstream. 
     Clause 4B. The method of clause 1B, further comprising specifying an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a foreground channel, and wherein specifying the indicated number of channels comprises specifying the foreground channel in the one or more layers of the bitstream. 
     Clause 5B. The method of clause 1B, further comprising specifying an indication, in the bitstream, of a number of layers specified in the bitstream. 
     Clause 6B. The method of clause 1B, further comprising specifying an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a background channel, wherein specifying the indicated number of the channels comprises specifying the background channel in the one or more layers of the bitstream. 
     Clause 7B. The method of clause 6B, wherein the one of the channels comprises a background higher order ambisonic coefficient. 
     Clause 1B. The method of clause 1B, wherein specifying the indication of the number of channels comprises specifying the indication of the number of channels based on a number of channels remaining in the bitstream after one of the layers is specified. 
     In this way, the techniques may enable a device to be configured to, or provide for an apparatus comprising means for performing, or a non-transitory computer-readable medium having stored thereon instructions that, when executed, cause one or more processors to perform the method set forth in the following clauses. 
     Clause 1C. A method of decoding a bitstream representative of a higher order ambisonic audio signal, the method comprising obtaining, from the bitstream, an indication of a number of layers specified in the bitstream and obtaining the layers of the bitstream based on the indication of the number of layers. 
     Clause 2C. The method of clause 1C, further comprising obtaining an indication of a number of channels specified in the bitstream, and wherein obtaining the layers comprises obtaining the layers of the bitstream based on the indication of the number of layers and the indication of the number of channels. 
     Clause 3C. The method of clause 1C, further comprising obtaining an indication of a number of foreground channels specified in the bitstream for at least one of the layers, and wherein obtaining the layers comprises obtaining the foreground channels for the at least one of the layers of the bitstream based on the indication of the number of foreground channels. 
     Clause 4C. The method of clause 1C, further comprising obtaining an indication of a number of background channels specified in the bitstream for at least one of the layers, and wherein obtaining the layers comprises obtaining the background channels for the at least one of the layers of the bitstream based on the indication of the number of background channels. 
     Clause 5C. The method of clause 1C, wherein the indication of the number of the layers indicates that the number of layer is two, wherein the two layers comprise a base layer and an enhancement layer, and wherein obtaining the layers comprises obtaining an indication that a number of foreground channels is zero for the base layer and two for the enhancement layer. 
     Clause 6C. The method of clause 1C or 5C, wherein the indication of the number of the layers indicates that the number of layer is two, wherein the two layers comprise a base layer and an enhancement layer, and wherein the method further comprises obtaining an indication that a number of background channels is four for the base layer and zero for the enhancement layer. 
     Clause 7. The method of clause 1C, wherein the indication of the number of the layers indicates that the number of layer is three, wherein the three layers comprise a base layer, a first enhancement layer and a second enhancement layer, and wherein the method further comprises obtaining an indication that a number of foreground channels is zero for the base layer, two for the first enhancement layer and two for the third enhancement layer. 
     Clause 8C. The method of clause 1C or 7C, wherein the indication of the number of the layers indicates that the number of layer is three, wherein the three layers comprise a base layer, a first enhancement layer and a second enhancement layer, and wherein the method further comprises obtaining an indication that a number of background channels is two for the base layer, zero for the first enhancement layer and zero for the third enhancement layer. 
     Clause 9C. The method of clause 1C, wherein the indication of the number of the layers indicates that the number of layer is three, wherein the three layers comprise a base layer, a first enhancement layer and a second enhancement layer, and wherein the method further comprises obtaining an indication that a number of foreground channels is two for the base layer, two for a first enhancement layer and two for a third enhancement layer. 
     Clause 10C. The method of clause 1C or 9C, wherein the indication of the number of the layers indicates that the number of layer is three, wherein the three layers comprise a base layer, a first enhancement layer and a second enhancement layer, and wherein the method further comprises obtaining a background syntax element indicating that the number of background channels is zero for the base layer, zero for the first enhancement layer and zero for the third enhancement layer. 
     Clause 11C. The method of clause 1C, wherein the indication of the number of layers comprises an indication of a number of layers in a previous frame of the bitstream, and wherein the method further comprises obtaining an indication of whether a number of layers of the bitstream has changed in a current frame when compared to the number of layers of the bitstream in the previous frame, and obtaining the number of layers of the bitstream in the current frame based on the indication of whether the number of layers of the bitstream has changed in the current frame. 
     Clause 12C. The method of clause 11C, further comprising determining the number of layers of the bitstream in the current frame as the same as the number of layers of the bitstream in the previous frame when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame. 
     Clause 13C. The method of clause 11C, wherein method further comprises, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, obtain an indication of a current number of components in one or more of the layers for the current frame to be the same as a previous number of components in one or more of the layers of the previous frame. 
     Clause 14C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first one of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide for stereo channel playback, obtaining a second one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for three dimensional playback by three or more speakers arranged on one or more horizontal planes, and obtaining a third one of the layers of the bitstream indicative of foreground components of the higher order ambisonic audio signal. 
     Clause 15C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first one of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide for mono channel playback, obtaining a second one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for three dimensional playback by three or more speakers arranged on one or more horizontal planes, and obtaining a third one of the layers of the bitstream indicative of foreground components of the higher order ambisonic audio signal. 
     Clause 16C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first one of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide for stereo channel playback, obtaining a second one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for multi-channel playback by three or more speakers arranged on a single horizontal plane, obtaining a third one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for three dimensional playback by three or more speakers arranged on two or more horizontal planes, and obtaining a fourth one of the layers of the bitstream indicative of foreground components of the higher order ambisonic audio signal. 
     Clause 17C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first one of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide for mono channel playback, obtaining a second one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for multi-channel playback by three or more speakers arranged on a single horizontal plane, and obtaining a third one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for three dimensional playback by three or more speakers arranged on two or more horizontal planes, and obtaining a fourth one of the layers of the bitstream indicative of foreground components of the higher order ambisonic audio signal. 
     Clause 18C. The method of clause 1C, wherein the indication of the number of layers indicates that two layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first one of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide for stereo channel playback, and obtaining a second one of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide for horizontal multi-channel playback by three or more speakers arranged on a single horizontal plane. 
     Clause 19C. The method of clause 1C, further comprising obtaining an indication of a number of channels specified in the bitstream, wherein obtaining the layers comprises obtaining the layers of the bitstream based on the indication of the number of layers and the indication of the number of channels. 
     Clause 20C. The method of clause 1C, further comprising obtaining an indication of a number of foreground channels specified in the bitstream for at least one of the layers, wherein obtaining the layers comprises obtaining the foreground channels for the at least one of the layers of the bitstream based on the indication of the number of foreground channels. 
     Clause 21C. The method of clause 1C, further comprising obtaining an indication of a number of background channels specified in the bitstream for at least one of the layers, wherein obtaining the layers comprises obtaining the background channels for the at least one of the layers of the bitstream based on the indication of the number of background channels. 
     Clause 22C. The method of clause 1C, further comprising parsing an indication of a number of foreground channels specified in the bitstream for at least one of the layers based on a number of channels remaining in the bitstream after the at least one of the layers is obtained, wherein obtaining the layers comprises obtaining the foreground channels of the at least one of the layers based on the indication of the number of foreground channels. 
     Clause 23C. The method of clause 22C, wherein the number of channels remaining in the bitstream after the at least one of the layers is obtained is represented by a syntax element. 
     Clause 24C. The method of clause 1C, further comprising parsing an indication of a number of background channels specified in the bitstream for at least one of the layers based on a number of channels after the at least one of the layers is obtained, wherein obtaining the background channels comprises obtaining the background channels for the at least one of the layers from the bitstream based on the indication of the number of background channels. 
     Clause 25C. The method of clause 24C, wherein the number of channels remaining in the bitstream after the at least one of the layers is obtained is represented by a syntax element. 
     Clause 26C. The method of clause 1C, wherein the layers of the bitstream comprise a base layer and an enhancement layer, and wherein the method further comprises applying a correlation transform with respect to one or more channels of the base layer to obtain a correlated representation of background components of the higher order ambisonic audio signal. 
     Clause 27C. The method of clause 26C, wherein the correlation transform comprises an inverse UHJ transform. 
     Clause 28C. The method of clause 26C, wherein the correlation transform comprises an inverse mode matrix transform. 
     Clause 29C. The method of clause 1C, wherein a number of channels for each of the layers of the bitstream is fixed. 
     Moreover, the techniques may enable a device to be configured to, or provide for an apparatus comprising means for performing, or a non-transitory computer-readable medium having stored thereon instructions that, when executed, cause one or more processors to perform the method set forth in the following clauses. 
     Clause 1D. A method of decoding a bitstream representative of a higher order ambisonic audio signal, the method comprising obtaining, from the bitstream, an indication of a number of channels specified in one or more layers in the bitstream, and obtaining the channels specified in the one or more layers in the bitstream based on the indication of the number of channels. 
     Clause 2D. The method of clause 1D, further comprising obtaining an indication of a total number of channels specified in the bitstream, and wherein obtaining the channels comprises obtaining the channels specified in the one or more layers based on the indication of the number of channels specified in the one or more layers and the indication of the total number of channels. 
     Clause 3D. The method of clause 1D, further comprising obtaining an indication of a type of one of the channels specified in the one or more layers in the bitstream, and wherein obtaining the channels comprises obtaining the one of the channels based on the indication of the number of channels and the indication of the type of the one of the channels. 
     Clause 4D. The method of clause 1D, further comprising obtaining an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a foreground channel, and wherein obtaining the channels comprises obtaining the one of the channels based on the indication of the number of channels and the indication that the type of the one of the channels is the foreground channel. 
     Clause 5D. The method of clause 1D, further comprising obtaining an indication of a number of layers specified in the bitstream, and wherein obtaining the channels comprises obtaining the one of the channels based on the indication of the number of channels and the indication of the number of layers. 
     Clause 6D. The method of clause 5D, wherein the indication of the number of layers comprises an indication of a number of layers in a previous frame of the bitstream, wherein the method further comprises obtaining an indication of whether the number of channels specified in one or more layers in the bitstream has changed in a current frame when compared to a number of channels specified in one or more layers in the bitstream of the previous frame, and wherein obtaining the channels comprises obtaining the one of the channels based on the indication of whether the number of channels specified in one or more layers in the bitstream has changed in the current frame. 
     Clause 7D. The method of clause 5D, further comprising determining the number of channels specified in the one or more layers of the bitstream in the current frame as the same as the number of channels specified in the one or more layers of the bitstream in the previous frame when the indication indicates that the number of channels specified in the one or more layers of the bitstream has not changed in the current frame when compared to the number of channels specified in the one or more layers of the bitstream in the previous frame. 
     Clause 8D. The method of clause 5D, wherein the one or more processors are further configured to, when the indication indicates that the number of channels specified in the one or more layers of the bitstream has not changed in the current frame when compared to the number of channels specified in the one or more layers of the bitstream in the previous frame, obtain an indication of a current number of channels in one or more of the layers for the current frame to be the same as a previous number of channels in one or more of the layers of the previous frame. 
     Clause 9D. The method of clause 1D, further comprising obtaining an indication of a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a background channel, wherein obtaining the channels comprises obtaining the one of the channels based on the indication of the number of layers and the indication that the type of the one of the channels is the background channel. 
     Clause 10D. The method of clause 9D, further comprising obtaining an indication a type of one of the channels specified in the one or more layers in the bitstream, the indication of the type of the one of the channels indicating that the one of the channels is a background channel, wherein obtaining the channels comprises obtaining the one of the channels based on the indication of the number of layers and the indication that the type of the one of the channels is the background channel. 
     Clause 11D. The method of clause 9D, wherein the one of the channels comprises a background higher order ambisonic coefficient. 
     Clause 12D. The method of clause 9D, wherein obtaining the indication of the type of the one of the channels comprises obtaining a syntax element indicative of the type of the one of the channels. 
     Clause 13D. The method of clause 1D, wherein obtaining the indication of the number of channels comprises obtaining the indication of the number of channels based on a number of channels remaining in the bitstream after one of the layers is obtained. 
     Clause 14D. The method of clause 1D, wherein the layers comprise a base layer. 
     Clause 15D. The method of clause 1D, wherein the layers comprises a base layer and one or more enhancement layers. 
     Clause 16D. The method of clause 1D, wherein a number of the one or more layers is fixed. 
     The foregoing techniques may be performed with respect to any number of different contexts and audio ecosystems. A number of example contexts are described below, although the techniques should be limited to the example contexts. One example audio ecosystem may include audio content, movie studios, music studios, gaming audio studios, channel based audio content, coding engines, game audio stems, game audio coding/rendering engines, and delivery systems. 
     The movie studios, the music studios, and the gaming audio studios may receive audio content. In some examples, the audio content may represent the output of an acquisition. The movie studios may output channel based audio content (e.g., in 2.0, 5.1, and 7.1) such as by using a digital audio workstation (DAW). The music studios may output channel based audio content (e.g., in 2.0, and 5.1) such as by using a DAW. In either case, the coding engines may receive and encode the channel based audio content based one or more codecs (e.g., AAC, AC3, Dolby True HD, Dolby Digital Plus, and DTS Master Audio) for output by the delivery systems. The gaming audio studios may output one or more game audio stems, such as by using a DAW. The game audio coding/rendering engines may code and or render the audio stems into channel based audio content for output by the delivery systems. Another example context in which the techniques may be performed comprises an audio ecosystem that may include broadcast recording audio objects, professional audio systems, consumer on-device capture, HOA audio format, on-device rendering, consumer audio, TV, and accessories, and car audio systems. 
     The broadcast recording audio objects, the professional audio systems, and the consumer on-device capture may all code their output using HOA audio format. In this way, the audio content may be coded using the HOA audio format into a single representation that may be played back using the on-device rendering, the consumer audio, TV, and accessories, and the car audio systems. In other words, the single representation of the audio content may be played back at a generic audio playback system (i.e., as opposed to requiring a particular configuration such as 5.1, 7.1, etc.), such as audio playback system  16 . 
     Other examples of context in which the techniques may be performed include an audio ecosystem that may include acquisition elements, and playback elements. The acquisition elements may include wired and/or wireless acquisition devices (e.g., Eigen microphones), on-device surround sound capture, and mobile devices (e.g., smartphones and tablets). In some examples, wired and/or wireless acquisition devices may be coupled to mobile device via wired and/or wireless communication channel(s). 
     In accordance with one or more techniques of this disclosure, the mobile device may be used to acquire a soundfield. For instance, the mobile device may acquire a soundfield via the wired and/or wireless acquisition devices and/or the on-device surround sound capture (e.g., a plurality of microphones integrated into the mobile device). The mobile device may then code the acquired soundfield into the HOA coefficients for playback by one or more of the playback elements. For instance, a user of the mobile device may record (acquire a soundfield of) a live event (e.g., a meeting, a conference, a play, a concert, etc.), and code the recording into HOA coefficients. 
     The mobile device may also utilize one or more of the playback elements to playback the HOA coded soundfield. For instance, the mobile device may decode the HOA coded soundfield and output a signal to one or more of the playback elements that causes the one or more of the playback elements to recreate the soundfield. As one example, the mobile device may utilize the wireless and/or wireless communication channels to output the signal to one or more speakers (e.g., speaker arrays, sound bars, etc.). As another example, the mobile device may utilize docking solutions to output the signal to one or more docking stations and/or one or more docked speakers (e.g., sound systems in smart cars and/or homes). As another example, the mobile device may utilize headphone rendering to output the signal to a set of headphones, e.g., to create realistic binaural sound. 
     In some examples, a particular mobile device may both acquire a 3D soundfield and playback the same 3D soundfield at a later time. In some examples, the mobile device may acquire a 3D soundfield, encode the 3D soundfield into HOA, and transmit the encoded 3D soundfield to one or more other devices (e.g., other mobile devices and/or other non-mobile devices) for playback. 
     Yet another context in which the techniques may be performed includes an audio ecosystem that may include audio content, game studios, coded audio content, rendering engines, and delivery systems. In some examples, the game studios may include one or more DAWs which may support editing of HOA signals. For instance, the one or more DAWs may include HOA plugins and/or tools which may be configured to operate with (e.g., work with) one or more game audio systems. In some examples, the game studios may output new stem formats that support HOA. In any case, the game studios may output coded audio content to the rendering engines which may render a soundfield for playback by the delivery systems. 
     The techniques may also be performed with respect to exemplary audio acquisition devices. For example, the techniques may be performed with respect to an Eigen microphone which may include a plurality of microphones that are collectively configured to record a 3D soundfield. In some examples, the plurality of microphones of Eigen microphone may be located on the surface of a substantially spherical ball with a radius of approximately 4 cm. In some examples, the audio encoding device  20  may be integrated into the Eigen microphone so as to output a bitstream  21  directly from the microphone. 
     Another exemplary audio acquisition context may include a production truck which may be configured to receive a signal from one or more microphones, such as one or more Eigen microphones. The production truck may also include an audio encoder, such as audio encoder  20  of  FIG.  3   . 
     The mobile device may also, in some instances, include a plurality of microphones that are collectively configured to record a 3D soundfield. In other words, the plurality of microphone may have X, Y, Z diversity. In some examples, the mobile device may include a microphone which may be rotated to provide X, Y, Z diversity with respect to one or more other microphones of the mobile device. The mobile device may also include an audio encoder, such as audio encoder  20  of  FIG.  3   . 
     A ruggedized video capture device may further be configured to record a 3D soundfield. In some examples, the ruggedized video capture device may be attached to a helmet of a user engaged in an activity. For instance, the ruggedized video capture device may be attached to a helmet of a user whitewater rafting. In this way, the ruggedized video capture device may capture a 3D soundfield that represents the action all around the user (e.g., water crashing behind the user, another rafter speaking in front of the user, etc. . . . ). 
     The techniques may also be performed with respect to an accessory enhanced mobile device, which may be configured to record a 3D soundfield. In some examples, the mobile device may be similar to the mobile devices discussed above, with the addition of one or more accessories. For instance, an Eigen microphone may be attached to the above noted mobile device to form an accessory enhanced mobile device. In this way, the accessory enhanced mobile device may capture a higher quality version of the 3D soundfield than just using sound capture components integral to the accessory enhanced mobile device. 
     Example audio playback devices that may perform various aspects of the techniques described in this disclosure are further discussed below. In accordance with one or more techniques of this disclosure, speakers and/or sound bars may be arranged in any arbitrary configuration while still playing back a 3D soundfield. Moreover, in some examples, headphone playback devices may be coupled to a decoder  24  via either a wired or a wireless connection. In accordance with one or more techniques of this disclosure, a single generic representation of a soundfield may be utilized to render the soundfield on any combination of the speakers, the sound bars, and the headphone playback devices. 
     A number of different example audio playback environments may also be suitable for performing various aspects of the techniques described in this disclosure. For instance, a 5.1 speaker playback environment, a 2.0 (e.g., stereo) speaker playback environment, a 9.1 speaker playback environment with full height front loudspeakers, a 22.2 speaker playback environment, a 16.0 speaker playback environment, an automotive speaker playback environment, and a mobile device with ear bud playback environment may be suitable environments for performing various aspects of the techniques described in this disclosure. 
     In accordance with one or more techniques of this disclosure, a single generic representation of a soundfield may be utilized to render the soundfield on any of the foregoing playback environments. Additionally, the techniques of this disclosure enable a rendered to render a soundfield from a generic representation for playback on the playback environments other than that described above. For instance, if design considerations prohibit proper placement of speakers according to a 7.1 speaker playback environment (e.g., if it is not possible to place a right surround speaker), the techniques of this disclosure enable a render to compensate with the other 6 speakers such that playback may be achieved on a 6.1 speaker playback environment. 
     Moreover, a user may watch a sports game while wearing headphones. In accordance with one or more techniques of this disclosure, the 3D soundfield of the sports game may be acquired (e.g., one or more Eigen microphones may be placed in and/or around the baseball stadium), HOA coefficients corresponding to the 3D soundfield may be obtained and transmitted to a decoder, the decoder may reconstruct the 3D soundfield based on the HOA coefficients and output the reconstructed 3D soundfield to a renderer, the renderer may obtain an indication as to the type of playback environment (e.g., headphones), and render the reconstructed 3D soundfield into signals that cause the headphones to output a representation of the 3D soundfield of the sports game. 
     In each of the various instances described above, it should be understood that the audio encoding device  20  may perform a method or otherwise comprise means to perform each step of the method for which the audio encoding device  20  is configured to perform In some instances, the means may comprise one or more processors. In some instances, the one or more processors may represent a special purpose processor configured by way of instructions stored to a non-transitory computer-readable storage medium. In other words, various aspects of the techniques in each of the sets of encoding examples may provide for a non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause the one or more processors to perform the method for which the audio encoding device  20  has been configured to perform. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     Likewise, in each of the various instances described above, it should be understood that the audio decoding device  24  may perform a method or otherwise comprise means to perform each step of the method for which the audio decoding device  24  is configured to perform. In some instances, the means may comprise one or more processors. In some instances, the one or more processors may represent a special purpose processor configured by way of instructions stored to a non-transitory computer-readable storage medium. In other words, various aspects of the techniques in each of the sets of encoding examples may provide for a non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause the one or more processors to perform the method for which the audio decoding device  24  has been configured to perform. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 
     Various aspects of the techniques have been described. These and other aspects of the techniques are within the scope of the following claims.