Patent Publication Number: US-9892737-B2

Title: Efficient coding of audio scenes comprising audio objects

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of the filing date of U.S. Provisional Patent Application No: 61/827,246 filed on May 24, 2013, U.S. Provisional Patent Application No: 61/893,770 filed on Oct. 21, 2013 and U.S. Provisional Patent Application No: 61/973,623 filed on Apr. 1, 2014, each of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure herein generally relates to coding of an audio scene comprising audio objects. In particular, it relates to an encoder, a decoder and associated methods for encoding and decoding of audio objects. 
     BACKGROUND 
     An audio scene may generally comprise audio objects and audio channels. An audio object is an audio signal which has an associated spatial position which may vary with time. An audio channel is an audio signal which corresponds directly to a channel of a multichannel speaker configuration, such as a so-called 5.1 speaker configuration with three front speakers, two surround speakers, and a low frequency effects speaker. 
     Since the number of audio objects typically may be very large, for instance in the order of hundreds of audio objects, there is a need for coding methods which allow the audio objects to be efficiently reconstructed at the decoder side. There have been suggestions to combine the audio objects into a multichannel downmix (i.e. into a plurality of audio channels which corresponds to the channels of a certain multichannel speaker configuration such as a 5.1 configuration) on an encoder side, and to reconstruct the audio objects parametrically from the multichannel downmix on a decoder side. 
     An advantage of such an approach is that a legacy decoder which does not support audio object reconstruction may use the multichannel downmix directly for playback on the multichannel speaker configuration. By way of example, a 5.1 downmix may directly be played on the loudspeakers of a 5.1 configuration. 
     A disadvantage with this approach is however that the multichannel downmix may not give a sufficiently good reconstruction of the audio objects at the decoder side. For example, consider two audio objects that have the same horizontal position as the left front speaker of a 5.1. configuration but a different vertical position. These audio objects would typically be combined into the same channel of a 5.1 downmix. This would constitute a challenging situation for the audio object reconstruction at the decoder side which would have to reconstruct approximations of the two audio objects from the same downmix channel, a process that cannot ensure perfect reconstruction and that sometimes even lead to audible artifacts. 
     There is thus a need for encoding/decoding methods which provide an efficient and improved reconstruction of audio objects. 
     Side information or metadata is often employed during reconstruction of audio objects from e.g. a downmix. The form and content of such side information may for example affect the fidelity of the reconstructed audio objects and/or the computational complexity of performing the reconstruction. It would therefore be desirable to provide encoding/decoding methods with a new and alternative side information format which allows for increasing the fidelity of reconstructed audio objects, and/or which allows for reducing the computational complexity of the reconstruction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will now be described with reference to the accompanying drawings, on which: 
         FIG. 1  is a schematic illustration of an encoder according to exemplary embodiments; 
         FIG. 2  is a schematic illustration of a decoder which supports reconstruction of audio objects according to exemplary embodiments; 
         FIG. 3  is a schematic illustration of a low-complexity decoder which does not support reconstruction of audio objects according to exemplary embodiments; 
         FIG. 4  is a schematic illustration of an encoder which comprises a sequentially arranged clustering component for simplification of an audio scene according to exemplary embodiments; 
         FIG. 5  is a schematic illustration of an encoder which comprises a clustering component arranged in parallel for simplification of an audio scene according to exemplary embodiments; 
         FIG. 6  illustrates a typical known process to compute a rendering matrix for a set of metadata instances; 
         FIG. 7  illustrates the derivation of a coefficient curve employed in rendering of audio signals; 
         FIG. 8  illustrates a metadata instance interpolation method, according to an example embodiment; 
         FIGS. 9 and 10  illustrate examples of introduction of additional metadata instances, according to example embodiments; and 
         FIG. 11  illustrates an interpolation method using a sample-and-hold circuit with a low-pass filter, according to an example embodiment. 
     
    
    
     All the figures are schematic and generally only show parts which are necessary in order to elucidate the disclosure, whereas other parts may be omitted or merely suggested. Unless otherwise indicated, like reference numerals refer to like parts in different figures. 
     DETAILED DESCRIPTION 
     In view of the above it is thus an object to provide an encoder, a decoder and associated methods which allow for efficient and improved reconstruction of audio objects, and/or which allows for increasing the fidelity of reconstructed audio objects, and/or which allows for reducing the computational complexity of the reconstruction. 
     I. Overview—Encoder 
     According to a first aspect, there is provided an encoding method, an encoder, and a computer program product for encoding audio objects. 
     According to exemplary embodiments there is provided a method for encoding audio objects into a data stream, comprising: 
     receiving N audio objects, wherein N&gt;1; 
     calculating M downmix signals, wherein M≦N, by forming combinations of the N audio objects according to a criterion which is independent of any loudspeaker configuration; 
     calculating side information including parameters which allow reconstruction of a set of audio objects formed on basis of the N audio objects from the M downmix signals; and 
     including the M downmix signals and the side information in a data stream for transmittal to a decoder. 
     With the above arrangement, the M downmix signals are thus formed from the N audio objects independently of any loudspeaker configuration. This implies that the M downmix signals are not constrained to audio signals which are suitable for playback on the channels of a speaker configuration with M channels. Instead, the M downmix signals may be selected more freely according to a criterion such that they for instance adapt to the dynamics of the N audio objects and improve the reconstruction of the audio objects at the decoder side. 
     Returning to the example with two audio objects that have the same horizontal position as the left front speaker of a 5.1 configuration but a different vertical position, the proposed method allows to put the first audio object in a first downmix signal, and the second audio object in the second downmix signal. This enables perfect reconstruction of the audio objects in the decoder. In general, such perfect reconstruction is possible as long as the number of active audio objects does not exceed the number of downmix signals. If the number of active audio objects is higher, then the proposed method allows for selection of the audio objects that have to be mixed into the same downmix signal such that the possible approximation errors occurring in the reconstructed audio object in the decoder have no or the smallest possible perceptual impact on the reconstructed audio scene. 
     A second advantage of the M downmix signals being adaptive is the ability to keep certain audio objects strictly separate from other audio objects. For example, it can be advantageous to keep any dialog object separate from background objects, to ensure that dialog is rendered accurately in terms of spatial attributes, and allows for object processing in the decoder, such as dialog enhancement or increase of dialog loudness for improved intelligibility. In other applications (e.g. karaoke), it may be advantageous to allow complete muting of one or more objects, which also requires that such objects are not mixed with other objects. Conventional methods using a multichannel downmix corresponding to a specific speaker configuration do not allow for complete muting of audio objects present in a mix of other audio objects. 
     The word downmix signal reflects that a downmix signal is a mix, i.e. a combination, of other signals. The word “down” indicates that the number M of downmix signals typically is lower than the number N of audio objects. 
     According to exemplary embodiments, the method may further comprise associating each downmix signal with a spatial position and including the spatial positions of the downmix signals in the data stream as metadata for the downmix signals. This is advantageous in that it allows for low-complexity decoding to be used in case of a legacy playback system. More precisely, the metadata associated with the downmix signals may be used on a decoder side for rendering the downmix signals to the channels of a legacy playback system. 
     According to exemplary embodiments, the N audio objects are associated with metadata including spatial positions of the N audio objects, and the spatial positions associated with the downmix signals are calculated based on the spatial positions of the N audio objects. Thus, the downmix signals may be interpreted as audio objects having a spatial position which depends on the spatial positions of the N audio objects. 
     Further, the spatial positions of the N audio objects and the spatial positions associated with the M downmix signals may be time-varying, i.e. they may vary between time frames of audio data. In other words, the downmix signals may be interpreted as dynamic audio objects having an associated position which varies between time frames. This is in contrast to prior art systems where the downmix signals correspond to fixed spatial loudspeaker positions. 
     Typically, the side information is also time-varying thereby allowing the parameters governing the reconstruction of the audio objects to vary temporally. 
     The encoder may apply different criteria for the calculation of the downmix signals. According to exemplary embodiments in which the N audio objects are associated with metadata including spatial positions of the N audio objects, the criterion for calculating the M downmix signals may be based on spatial proximity of the N audio objects. For example, audio objects which are close to each other may be combined into the same downmix signal. 
     According to exemplary embodiments in which the metadata associated with N audio objects further comprises importance values indicating the importance of the N audio objects in relation to each other, the criterion for calculating the M downmix signals may further be based on the importance values of the N audio objects. For example, the most important one(s) of the N audio objects may be mapped directly to a downmix signal, while the remaining audio objects are combined to form the remaining downmix signals. 
     In particular, according to exemplary embodiments, the step of calculating M downmix signals comprises a first clustering procedure which includes associating the N audio objects with M clusters based on spatial proximity and importance values, if applicable, of the N audio objects, and calculating a downmix signal for each cluster by forming a combination of audio objects associated with the cluster. In some cases an audio object may form part of at most one cluster. In other cases, an audio object may form part of several clusters. In this way, different groups, i.e. clusters, are formed from the audio objects. Each cluster may in turn be represented by a downmix signal which may be thought of as an audio object. The clustering approach allows associating each downmix signal with a spatial position which is calculated based on the spatial positions of the audio objects associated with the cluster corresponding to the downmix signal. With this interpretation the first clustering procedure thus reduces the dimensionality of the N audio objects to M audio objects in a flexible manner. 
     The spatial position associated with each downmix signal may for example be calculated as a centroid or a weighted centroid of the spatial positions of the audio objects associated with the cluster corresponding to the downmix signal. The weights may for example be based on importance values of the audio objects. 
     According to exemplary embodiments, the N audio objects are associated with the M clusters by applying a K-means algorithm having the spatial positions of the N audio objects as input. 
     Since an audio scene may comprise a vast number of audio objects, the method may take further measures for reducing the dimensionality of the audio scene, thereby reducing the computational complexity at the decoder side when reconstructing the audio objects. In particular, the method may further comprise a second clustering procedure for reducing a first plurality of audio objects to a second plurality of audio objects. 
     According to one embodiment, the second clustering procedure is performed prior to the calculation of the M downmix signals. In that embodiment the first plurality of audio objects hence correspond to the original audio objects of the audio scene, and the second, reduced, plurality of audio objects corresponds to the N audio objects on the basis of which the M downmix signals are calculated. Moreover, in such embodiment, the set of audio objects (to be reconstructed in the decoder) formed on basis of the N audio objects corresponds, i.e. is equal to, to the N audio objects. 
     According to another embodiment, the second clustering procedure is performed in parallel with the calculation of the M downmix signals. In such embodiment, the N audio objects on the basis of which the M downmix signals are calculated as well as the first plurality of audio objects being input to the second clustering procedure correspond to the original audio objects of the audio scene. Moreover, in such embodiment, the set of audio objects (to be reconstructed in the decoder) formed on basis of the N audio objects corresponds to the second plurality of audio objects. With this approach, the M downmix signals are hence calculated on basis on the original audio objects of the audio scene and not on basis of a reduced number of audio objects. 
     According to exemplary embodiments, the second clustering procedure comprises: 
     receiving the first plurality of audio objects and their associated spatial positions, 
     associating the first plurality of audio objects with at least one cluster based on spatial proximity of the first plurality of audio objects, 
     generating the second plurality of audio objects by representing each of the at least one cluster by an audio object being a combination of the audio objects associated with the cluster, 
     calculating metadata including spatial positions for the second plurality of audio objects, wherein the spatial position of each audio object of the second plurality of audio objects is calculated based on the spatial positions of the audio objects associated with the corresponding cluster; and 
     including the metadata for the second plurality of audio objects in the data stream. 
     In other words, the second clustering procedure exploits spatial redundancy present in the audio scene, such as objects having equal or very similar locations. In addition, importance values of the audio objects may be taken into account when generating the second plurality of audio objects. 
     As mentioned above, the audio scene may also comprise audio channels. Such audio channels may be thought of as an audio object being associated with a static position, viz. the position of the loudspeaker corresponding to the audio channel. In more detail, the second clustering procedure may further comprise: 
     receiving at least one audio channel; 
     converting each of the at least one audio channel to an audio object having a static spatial position corresponding to a loudspeaker position of that audio channel; and 
     including the converted at least one audio channel in the first plurality of audio objects. 
     In this way, the method allows for encoding of an audio scene comprising audio channels as well as audio objects. 
     According to exemplary embodiments, there is provided a computer program product comprising a computer-readable medium with instructions for performing the decoding method according to exemplary embodiments. 
     According to exemplary embodiments, there is provided an encoder for encoding audio objects into a data stream, comprising: 
     a receiving component configured to receive N audio objects, wherein N&gt;1 
     a downmix component configured to calculate M downmix signals, wherein M≦N, by forming combinations of the N audio objects according to a criterion which is independent of any loudspeaker configuration; 
     an analysis component configured to calculate side information including parameters which allow reconstruction of the set of audio objects formed on basis of the N audio objects from the M downmix signals; and 
     a multiplexing component configured to include the M downmix signals and the side information in a data stream for transmittal to a decoder. 
     II. Overview—Decoder 
     According to a second aspect, there is provided a decoding method, a decoder, and a computer program product for decoding multichannel audio content. 
     The second aspect may generally have the same features and advantages as the first aspect. 
     According to exemplary embodiments there is provided a method in a decoder for decoding a data stream including encoded audio objects, comprising: 
     receiving a data stream comprising M downmix signals which are combinations of N audio objects calculated according to a criterion which is independent of any loudspeaker configuration, wherein M≦N, and side information including parameters which allow reconstruction of a set of audio objects formed on basis of the N audio objects from the M downmix signals; and 
     reconstructing the set of audio objects formed on basis of the N audio objects from the M downmix signals and the side information. 
     According to exemplary embodiments, the data stream further comprises metadata for the M downmix signals including spatial positions associated with the M downmix signals, the method further comprising: 
     on a condition that the decoder is configured to support audio object reconstruction, performing the step of reconstructing the set of audio objects formed on basis N audio objects from the M downmix signals and the side information; and 
     on a condition that the decoder is not configured to support audio object reconstruction, using the metadata for the M downmix signals for rendering of the M downmix signals to output channels of a playback system. 
     According to exemplary embodiments, the spatial positions associated with the M downmix signals are time-varying. 
     According to exemplary embodiments, the side information is time-varying. 
     According to exemplary embodiments, the data stream further comprises metadata for the set of audio objects formed on basis of the N audio objects including the spatial positions of the set of audio objects formed on basis of the N audio objects, the method further comprising: 
     using the metadata for the set of audio objects formed on basis of the N audio objects for rendering of the reconstructed set of audio objects formed on basis of the N audio objects to output channels of a playback system. 
     According to exemplary embodiments, the set of audio objects formed on basis of the N audio objects is equal to the N audio objects. 
     According to exemplary embodiments, the set of audio objects formed on basis of the N audio objects comprises a plurality of audio objects which are combinations of the N audio objects, and the number of which is lower than N. 
     According to exemplary embodiments, there is provided a computer program product comprising a computer-readable medium with instructions for performing the decoding method according to exemplary embodiments. 
     According to exemplary embodiments, there is provided a decoder for decoding a data stream including encoded audio objects, comprising: 
     a receiving component configured to receive a data stream comprising M downmix signals which are combinations of N audio objects calculated according to a criterion which is independent of any loudspeaker configuration, wherein M≦N, and side information including parameters which allow reconstruction of a set of audio objects formed on basis of the N audio objects from the M downmix signals; and 
     a reconstructing component configured to reconstruct the set of audio objects formed on basis of the N audio objects from the M downmix signals and the side information. 
     III. Overview—Format for Side Information and Metadata 
     According to a third aspect, there is provided an encoding method, an encoder, and a computer program product for encoding audio objects. 
     The methods, encoders and computer program products according to the third aspect may generally have features and advantages in common with the methods, encoders and computer program products according to the first aspect. 
     According to example embodiments, there is provided a method for encoding audio objects as a data stream. The method comprises: 
     receiving N audio objects, wherein N&gt;1; 
     calculating M downmix signals, wherein M≦N, by forming combinations of the N audio objects; 
     calculating time-variable side information including parameters which allow reconstruction of a set of audio objects formed on the basis of the N audio objects from the M downmix signals; and 
     including the M downmix signals and the side information in a data stream for transmittal to a decoder. 
     In the present example embodiments, the method further comprises including, in the data stream: 
     a plurality of side information instances specifying respective desired reconstruction settings for reconstructing the set of audio objects formed on the basis of the N audio objects; and 
     for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to the desired reconstruction setting specified by the side information instance, and a point in time to complete the transition. 
     In the present example embodiment, the side information is time-variable, e.g. time-varying, allowing for the parameters governing the reconstruction of the audio objects to vary with respect to time, which is reflected by the presence of the side information instances. By employing a side information format which includes transition data defining points in time to begin and points in time to complete transitions from current reconstruction settings to respective desired reconstruction settings, the side information instances are made more independent of each other in the sense that interpolation may be performed based on a current reconstruction setting and a single desired reconstruction setting specified by a single side information instance, i.e. without knowledge of any other side information instances. The provided side information format therefore facilitates calculation/introduction of additional side information instances between existing side information instances. In particular, the provided side information format allows for calculation/introduction of additional side information instances without affecting the playback quality. In this disclosure, the process of calculating/introducing new side information instances between existing side information instances is referred to as “resampling” of the side information. Resampling of side information is often required during certain audio processing tasks. For example, when audio content is edited, by e.g. cutting/merging/mixing, such edits may occur in between side information instances. In this case, resampling of the side information may be required. Another such case is when audio signals and associated side information are encoded with a frame-based audio codec. In this case, it is desirable to have at least one side information instance for each audio codec frame, preferably with a time stamp at the start of that codec frame, to improve resilience of frame losses during transmission. For example, the audio signals/objects may be part of an audio-visual signal or multimedia signal which includes video content. In such applications, it may be desirable to modify the frame rate of the audio content to match a frame rate of the video content, whereby a corresponding resampling of side information may be desirable. 
     The data stream in which the downmix signal and the side information is included may for example be a bitstream, in particular a stored or transmitted bitstream. 
     It is to be understood that calculating the M downmix signals by forming combinations of the N audio objects means that each of the M downmix signals is obtained by forming a combination, e.g. a linear combination, of the audio content of one or more of the N audio objects. In other words, each of the N audio objects need not necessarily contribute to each of the M downmix signals. 
     The word downmix signal reflects that a downmix signal is a mix, i.e. a combination, of other signals. The downmix signal may for example be an additive mix of other signals. The word “down” indicates that the number M of downmix signals typically is lower than the number N of audio objects. 
     The downmix signals may for example be calculated by forming combinations of the N audio signals according to a criterion which is independent of any loudspeaker configuration, according to any of the example embodiments within the first aspect. Alternatively, the downmix signals may for example be calculated by forming combinations of the N audio signals such that the downmix signals are suitable for playback on the channels of a speaker configuration with M channels, referred to herein as a backwards compatible downmix. 
     By the transition data including two independently assignable portions is meant that the two portions are mutually independently assignable, i.e. may be assigned independently of each other. However, it is to be understood that the portions of the transition data may for example coincide with portions of transition data for other types of side information of metadata. 
     In the present example embodiment, the two independently assignable portions of the transition data, in combination, define the point in time to begin the transition and the point in time to complete the transition, i.e. these two points in time are derivable from the two independently assignable portions of the transition data. 
     According to an example embodiment, the method may further comprise a clustering procedure for reducing a first plurality of audio objects to a second plurality of audio objects, wherein the N audio objects constitute either the first plurality of audio objects or the second plurality of audio objects, and wherein the set of audio objects formed on the basis of the N audio objects coincides with the second plurality of audio objects. In the present example embodiment, the clustering procedure may comprise: 
     calculating time-variable cluster metadata including spatial positions for the second plurality of audio objects; and 
     further including, in the data stream, for transmittal to the decoder: 
     a plurality of cluster metadata instances specifying respective desired rendering settings for rendering the second set of audio objects; and 
     for each cluster metadata instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current rendering setting to the desired rendering setting specified by the cluster metadata instance, and a point in time to complete the transition to the desired rendering setting specified by the cluster metadata instance. 
     Since an audio scene may comprise a vast number of audio objects, the method according to the present example embodiment, takes further measures for reducing the dimensionality of the audio scene by reducing the first plurality of audio objects to a second plurality of audio objects. In the present example embodiment, the set of audio objects, which is formed on the basis of the N audio objects and which is to be reconstructed on a decoder side based on the downmix signals and the side information, coincides with the second plurality of audio objects, which corresponds to a simplification and/or lower-dimensional representation of the audio scene represented by the first plurality of audio signals, and the computational complexity for reconstruction on a decoder side is reduced. 
     The inclusion of the cluster metadata in the data stream allows for rendering of the second set of audio signals on a decoder side, e.g. after the second set of audio signals has been reconstructed based on the downmix signals and the side information. 
     Similar to the side information, the cluster metadata in the present example embodiment is time-variable, e.g. time-varying, allowing for the parameters governing the rendering of the second plurality of audio objects to vary with respect to time. The format for the downmix metadata may be analogous to that of the side formation and may have the same or corresponding advantages. In particular, the form of the cluster metadata provided in the present example embodiment, facilitates resampling of the cluster metadata. Resampling of the cluster metadata may e.g. be employed to provide common points in time to start and complete respective transitions associated with the cluster metadata and the side information, and/or for adjusting the cluster metadata to a frame rate of the associated audio signals. 
     According to an example embodiment, the clustering procedure may further comprise: 
     receiving the first plurality of audio objects and their associated spatial positions; 
     associating the first plurality of audio objects with at least one cluster based on spatial proximity of the first plurality of audio objects; 
     generating the second plurality of audio objects by representing each of the at least one cluster by an audio object being a combination of the audio objects associated with the cluster; and 
     calculating the spatial position of each audio object of the second plurality of audio objects based on the spatial positions of the audio objects associated with the respective cluster, i.e. with the cluster which the audio object represent. 
     In other words, the clustering procedure exploits spatial redundancy present in the audio scene, such as objects having equal or very similar locations. In addition, importance values of the audio objects may be taken into account when generating the second plurality of audio objects, as described with respect to example embodiments within the first aspect. 
     Associating the first plurality of audio objects with at least one cluster includes associating each of the first plurality of audio objects with one or more of the at least one cluster. In some cases, an audio object may form part of at most one cluster, while in other cases, an audio object may form part of several clusters. In other words, in some cases, an audio object may be split between several clusters as part of the clustering procedure. 
     Spatial proximity of the first plurality of audio objects may be related to distances between, and/or relative positions of, the respective audio objects in the first plurality of audio objects. For example, audio objects which are close to each other may be associated with the same cluster. 
     By an audio object being a combination of the audio objects associated with the cluster is meant that the audio content/signal associated with the audio object may be formed as a combination of the audio contents/signals associated with the respective audio objects associated with the cluster. 
     According to an example embodiment, the respective points in time defined by the transition data for the respective cluster metadata instances may coincide with the respective points in time defined by the transition data for corresponding side information instances. 
     By employing the same points in time to begin and to complete transitions associated with the side information and the cluster metadata, joint processing of the side information and the cluster metadata, such as joint resampling, is facilitated. 
     Moreover, the use of common points in time to begin and to complete transitions associated with the side information and the cluster metadata facilitates joint reconstruction and rendering at a decoder side. If for example, reconstruction and rendering is performed as a joint operation on a decoder side, joint settings for reconstruction and rendering may be determined for each side information instance and metadata instance and/or interpolation between joint settings for reconstruction and rendering may be employed instead of performing interpolation separately for the respective settings. Such joint interpolation may reduce computational complexity at the decoder side as fewer coefficients/parameters need to be interpolated. 
     According to an example embodiment, the clustering procedure may be performed prior to the calculation of the M downmix signals. In the present example embodiment, the first plurality of audio objects corresponds to the original audio objects of the audio scene, and the N audio objects on the basis of which the M downmix signals are calculated constitute the second, reduced, plurality of audio objects. Hence, in the present example embodiment, the set of audio objects (to be reconstructed on a decoder side) formed on the basis of the N audio objects coincides with the N audio objects. 
     Alternatively, the clustering procedure may be performed in parallel with the calculation of the M downmix signals. According to the present alternative, the N audio objects on the basis of which the M downmix signals are calculated constitute the first plurality of audio objects which correspond to the original audio objects of the audio scene. With this approach, the M downmix signals are hence calculated on basis of the original audio objects of the audio scene and not on basis of a reduced number of audio objects. 
     According to an example embodiment, the method may further comprise: 
     associating each downmix signal with a time-variable spatial position for rendering the downmix signals, and 
     further including, in the data stream, downmix metadata including the spatial positions of the downmix signals, 
     wherein the method further comprises including, in the data stream: 
     a plurality of downmix metadata instances specifying respective desired downmix rendering settings for rendering the downmix signals; and 
     for each downmix metadata instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current downmix rendering setting to the desired downmix rendering setting specified by the downmix metadata instance, and a point in time to complete the transition to the desired downmix rendering setting specified by the downmix metadata instance. 
     Including downmix metadata in the data stream is advantageous in that it allows for low-complexity decoding to be used in case of legacy playback equipment. More precisely, the downmix metadata may be used on a decoder side for rendering the downmix signals to the channels of a legacy playback system, i.e. without reconstructing the plurality of audio objects formed on the basis of the N objects, which typically is a computationally more complex operation. 
     According to the present example embodiment, the spatial positions associated with the M downmix signals may be time-variable, e.g. time-varying, and the downmix signals may be interpreted as dynamic audio objects having an associated position which may change between time frames or downmix metadata instances. This is in contrast to prior art systems where the downmix signals correspond to fixed spatial loudspeaker positions. It is recalled that the same data stream may be played in an object oriented fashion in a decoding system with more evolved capabilities. 
     In some example embodiments, the N audio objects may be associated with metadata including spatial positions of the N audio objects, and the spatial positions associated with the downmix signals may for example be calculated based on the spatial positions of the N audio objects. Thus, the downmix signals may be interpreted as audio objects having spatial positions which depend on the spatial positions of the N audio objects. 
     According to an example embodiment, the respective points in time defined by the transition data for the respective downmix metadata instances may coincide with the respective points in time defined by the transition data for corresponding side information instances. Employing the same points in time for beginning and for completing transitions associated with the side information and the downmix metadata facilitates joint processing, e.g. resampling, of the side information and the downmix metadata. 
     According to an example embodiment, the respective points in time defined by the transition data for the respective downmix metadata instances may coincide with the respective points in time defined by the transition data for corresponding cluster metadata instances. Employing the same points in time for beginning and ending transitions associated with the cluster metadata and the downmix metadata facilitates joint processing, e.g. resampling, of the cluster metadata and the downmix metadata. 
     According to example embodiments, there is provided an encoder for encoding N audio objects as a data stream, wherein N&gt;1. The encoder comprises: 
     a downmix component configured to calculate M downmix signals, wherein M≦N, by forming combinations of the N audio objects; 
     an analysis component configured to calculate time-variable side information including parameters which allow reconstruction of a set of audio objects formed on the basis of the N audio objects from the M downmix signals; and 
     a multiplexing component configured to include the M downmix signals and the side information in a data stream for transmittal to a decoder, 
     wherein the multiplexing component is further configured to include, in the data stream, for transmittal to the decoder: 
     a plurality of side information instances specifying respective desired reconstruction settings for reconstructing the set of audio objects formed on the basis of the N audio objects; and 
     for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to the desired reconstruction setting specified by the side information instance, and a point in time to complete the transition. 
     According to a fourth aspect, there is provided a decoding method, a decoder, and a computer program product for decoding multichannel audio content. 
     The methods, decoders and computer program products according to the fourth aspect are intended for cooperation with the methods, encoders and computer program products according to the third aspect, and may have corresponding features and advantages. 
     The methods, decoders and computer program products according to the fourth aspect, may generally have features and advantages in common with the methods, decoders and computer program products according to the second aspect. 
     According to example embodiments, there is provided a method for reconstructing audio objects based on a data stream. The method comprises: 
     receiving a data stream comprising M downmix signals which are combinations of N audio objects, wherein N&gt;1 and M≦N, and time-variable side information including parameters which allow reconstruction of a set of audio objects formed on the basis of the N audio objects from the M downmix signals; and 
     reconstructing, based on the M downmix signals and the side information, the set of audio objects formed on the basis of the N audio objects, 
     wherein the data stream comprises a plurality of side information instances, wherein the data stream further comprises, for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to a desired reconstruction setting specified by the side information instance, and a point in time to complete the transition, and wherein reconstructing the set of audio objects formed on the basis of the N audio objects comprises: 
     performing reconstruction according to a current reconstruction setting; 
     beginning, at a point in time defined by the transition data for a side information instance, a transition from the current reconstruction setting to a desired reconstruction setting specified by the side information instance; and 
     completing the transition at a point in time defined by the transition data for the side information instance. 
     As described above, employing a side information format which includes transition data defining points in time to begin and points in time to complete transitions from current reconstruction settings to respective desired reconstruction settings e.g. facilitates resampling of the side information. 
     The data stream may for example be received in the form of a bitstream, e.g. generated on an encoder side. 
     Reconstructing, based on the M downmix signals and the side information, the set of audio objects formed on the basis of the N audio objects, may for example include forming at least one linear combination of the downmix signals employing coefficients determined based on the side information. Reconstructing, based on the M downmix signals and the side information, the set of audio objects formed on the basis of the N audio objects, may for example include forming linear combinations of the downmix signals, and, optionally one or more additional (e.g. decorrelated) signal derived from the downmix signals, employing coefficients determined based on the side information. 
     According to an example embodiment, the data stream may further comprise time-variable cluster metadata for the set of audio objects formed on the basis of the N audio objects, the cluster metadata including spatial positions for the set of audio objects formed on the basis of the N audio objects. The data stream may comprise a plurality of cluster metadata instances, and the data stream may further comprise, for each cluster metadata instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current rendering setting to a desired rendering setting specified by the cluster metadata instance, and a point in time to complete the transition to the desired rendering setting specified by the cluster metadata instance. The method may further comprise: 
     using the cluster metadata for rendering of the reconstructed set of audio objects formed on the basis of the N audio objects to output channels of a predefined channel configuration, the rendering comprising: 
     performing rendering according to a current rendering setting; 
     beginning, at a point in time defined by the transition data for a cluster metadata instance, a transition from the current rendering setting to a desired rendering setting specified by the cluster metadata instance; and 
     completing the transition to the desired rendering setting at a point in time defined by the transition data for the cluster metadata instance. 
     The predefined channel configuration may for example correspond to a configuration of the output channels compatible with a particular playback system, i.e. suitable for playback on a particular playback system. 
     Rendering of the reconstructed set of audio objects formed on the basis of the N audio objects to output channels of a predefined channel configuration may for example include mapping, in a renderer, the reconstructed set of audio signals formed on the basis of the N audio objects to (a predefined configuration of) output channels of the renderer under control of the cluster metadata. 
     Rendering of the reconstructed set of audio objects formed on the basis of the N audio objects to output channels of a predefined channel configuration may for example include forming linear combinations of the reconstructed set of audio objects formed on the basis of the N audio objects, employing coefficients determined based on the cluster metadata. 
     According to an example embodiment, the respective points in time defined by the transition data for the respective cluster metadata instances may coincide with the respective points in time defined by the transition data for corresponding side information instances. 
     According to an example embodiment, the method may further comprise: 
     performing at least part of the reconstruction and at least part of the rendering as a combined operation corresponding to a first matrix formed as a matrix product of a reconstruction matrix and a rendering matrix associated with a current reconstruction setting and a current rendering setting, respectively; 
     beginning, at a point in time defined by the transition data for a side information instance and a cluster metadata instance, a combined transition from the current reconstruction and rendering settings to desired reconstruction and rendering settings specified by the side information instance and the cluster metadata instance, respectively; and 
     completing the combined transition at a point in time defined by the transition data for the side information instance and the cluster metadata instance, wherein the combined transition includes interpolating between matrix elements of the first matrix and matrix elements of a second matrix formed as a matrix product of a reconstruction matrix and a rendering matrix associated with the desired reconstruction setting and the desired rendering setting, respectively. 
     By performing a combined transition in the above sense, instead of separate transitions of reconstruction settings and rendering settings, fewer parameters/coefficients need to be interpolated, which allows for a reduction of computational complexity. 
     It is to be understood that a matrix, such as reconstruction matrix or a rendering matrix, as referenced in the present example embodiment, may for example consist of a single row or a single column, and may therefore correspond to a vector. 
     Reconstruction of audio objects from downmix signals is often performed by employing different reconstruction matrices in different frequency bands, while rendering is often performed by employing the same rendering matrix for all frequencies. In such cases, a matrix corresponding to a combined operation of reconstruction and rendering, e.g. the first and second matrices referenced in the present example embodiment, may typically be frequency-dependent, i.e. different values for the matrix elements may typically be employed for different frequency bands. 
     According to an example embodiment, the set of audio objects formed on the basis of the N audio objects may coincide with the N audio objects, i.e. the method may comprise reconstructing the N audio objects based on the M downmix signals and the side information. 
     Alternatively, the set of audio objects formed on the basis of the N audio objects may comprise a plurality of audio objects which are combinations of the N audio objects, and whose number is less than N, i.e. the method may comprise reconstructing these combinations of the N audio objects based on the M downmix signals and the side information. 
     According to an example embodiment, the data stream may further comprise downmix metadata for the M downmix signals including time-variable spatial positions associated with the M downmix signals. The data stream may comprise a plurality of downmix metadata instances, and the data stream may further comprise, for each downmix metadata instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current downmix rendering setting to a desired downmix rendering setting specified by the downmix metadata instance, and a point in time to complete the transition to the desired downmix rendering setting specified by the downmix metadata instance. The method may further comprise: 
     on a condition that the decoder is operable (or configured) to support audio object reconstruction, performing the step of reconstructing, based on the M downmix signals and the side information, the set of audio objects formed on the basis of the N audio objects; and 
     on a condition that the decoder is not operable (or configured) to support audio object reconstruction, outputting the downmix metadata and the M downmix signals for rendering of the M downmix signals. 
     In case the decoder is operable to support audio object reconstruction and the data stream further comprises cluster metadata associated with the set of audio objects formed on the basis of the N audio objects, the decoder may e.g. output the reconstructed set of audio objects the cluster metadata for rendering of the reconstructed set of audio objects. 
     In case the decoder is not operable to support audio object reconstruction, it may for example discard the side information and, if applicable, the cluster metadata, and provide the downmix metadata and the M downmix signals as output. Then, the output may be employed by a renderer for rendering the M downmix signals to output channels of the renderer. 
     Optionally, the method may further comprise rendering the M downmix signals to output channels of a predefined output configuration, e.g. to output channels of a renderer, or to output channels of the decoder (in case the decoder has rendering capabilities), based on the downmix metadata. 
     According to example embodiments, there is provided a decoder for reconstructing audio objects based on a data stream. The decoder comprises: 
     a receiving component configured to receive a data stream comprising M downmix signals which are combinations of N audio objects, wherein N&gt;1 and M≦N, and time-variable side information including parameters which allow reconstruction of a set of audio objects formed on the basis of the N audio objects from the M downmix signals; and 
     a reconstructing component configured to reconstruct, based on the M downmix signals and the side information, the set of audio objects formed on the basis of the N audio objects, 
     wherein the data stream comprises a plurality of side information instances associated, and wherein the data stream further comprises, for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to a desired reconstruction setting specified by the side information instance, and a point in time to complete the transition. The reconstructing component is configured to reconstruct the set of audio objects formed on the basis of the N audio objects by at least: 
     performing reconstruction according to a current reconstruction setting; 
     beginning, at a point in time defined by the transition data for a side information instance, a transition from the current reconstruction setting to a desired reconstruction setting specified by the side information instance; and 
     completing the transition at a point in time defined by the transition data for the side information instance. 
     According to an example embodiment, the method within the third or fourth aspect may further comprise generating one or more additional side information instances specifying substantially the same reconstruction setting as a side information instance directly preceding or directly succeeding the one or more additional side information instances. Example embodiments are also envisaged in which additional cluster metadata instances and/or downmix metadata instances are generated in an analogous fashion. 
     As described above, resampling of the side information by generating more side information instances may be advantageous in several situations, such as when audio signals/objects and associated side information are encoded using a frame-based audio codec, since then it is desirable to have at least one side information instance for each audio codec frame. At an encoder side, the side information instances provided by an analysis component may e.g. be distributed in time in such a way that they do not match a frame rate of the downmix signals provided by a downmix component, and the side information may therefore advantageously be resampled by introducing new side information instances such that there is at least one side information instance for each frame of the downmix signals. Similarly, at a decoder side, the received side information instances may e.g. be distributed in time in such a way that they do not match a frame rate of the received downmix signals, and the side information may therefore advantageously be resampled by introducing new side information instances such that there is at least one side information instance for each frame of the downmix signals. 
     An additional side information instance may for example be generated for a selected point in time by: copying the side information instance directly succeeding the additional side information instance and determining transition data for the additional side information instance based on the selected point in time and the points in time defined by the transition data for the succeeding side information instance. 
     According to a fifth aspect, there is provided a method, a device, and a computer program product for transcoding side information encoded together with M audio signals in a data stream. 
     The methods, devices and computer program products according to the fifth aspect are intended for cooperation with the methods, encoders, decoder and computer program products according to the third and fourth aspect, and may have corresponding features and advantages. 
     According to example embodiments, there is provided a method for transcoding side information encoded together with M audio signals in a data stream. The method comprises: 
     receiving a data stream; 
     extracting, from the data stream, M audio signals and associated time-variable side information including parameters which allow reconstruction of a set of audio objects from the M audio signals, wherein M≧1, and wherein the extracted side information includes:
         a plurality of side information instances specifying respective desired reconstruction settings for reconstructing the audio objects, and   for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to the desired reconstruction setting specified by the side information instance, and a point in time to complete the transition;       

     generating one or more additional side information instances specifying substantially the same reconstruction setting as a side information instance directly preceding or directly succeeding the one or more additional side information instances; and 
     including the M audio signals and the side information in a data stream. 
     In the present example embodiment, the one or more additional side information instances may be generated after the side information has been extracted from the received data stream, and the generated one or more additional side information instances may then be included in a data stream together with the M audio signals and the other side information instances. 
     As described above in relation to the third aspect, resampling of the side information by generating more side information instances may be advantageous in several situations, such as when audio signals/objects and associated side information are encoded using a frame-based audio codec, since then it is desirable to have at least one side information instance for each audio codec frame. 
     Embodiments are also envisaged in which the data stream further comprises cluster metadata and/or downmix metadata, as described in relation to the third and fourth aspect, and wherein the method further comprises generating additional downmix metadata instances and/or cluster metadata instances, analogously to how the additional side information instances are generated. 
     According to an example embodiment, the M audio signals may be coded in the received data stream according to a first frame rate, and the method may further comprise: 
     processing the M audio signals to change the frame rate according to which the M downmix signals are coded to a second frame rate different than the first frame rate; and 
     resampling the side information to match, and/or to be compatible with, the second frame rate by at least generating the one or more additional side information instances. 
     As described above in relation to the third aspect, it may be advantageous in several situations to process audio signals so as to change the frame rate employed for coding them, e.g. so that the modified frame rate matches the frame rate of video content of an audio-visual signal to which the audio signals belong. The presence of the transition data for each side information instance facilitates resampling of the side information, as described above in relation to the third aspect. The side information may be resampled to match the new frame rate e.g. by generating additional side information instances such that there is at least one side information instance for each frame of the processed audio signals. 
     According to example embodiments, there is provided a device for transcoding side information encoded together with M audio signals in a data stream. The device comprises: 
     a receiving component configured to receive a data stream and to extract, from the data stream, M audio signals and associated time-variable side information including parameters which allow reconstruction of a set of audio objects from the M audio signals, wherein M≧1, and wherein the extracted side information includes:
         a plurality of side information instances specifying respective desired reconstruction settings for reconstructing the audio objects, and   for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to the desired reconstruction setting specified by the side information instance, and a point in time to complete the transition.       

     The device further comprises: 
     a resampling component configured to generate one or more additional side information instances specifying substantially the same reconstruction setting as a side information instance directly preceding or directly succeeding the one or more additional side information instances; and 
     a multiplexing component configured to include the M audio signals and the side information in a data stream. 
     According to an example embodiment, the method within the third, fourth or fifth aspect may further comprise: computing a difference between a first desired reconstruction setting specified by a first side information instance and one or more desired reconstruction settings specified by one or more side information instances directly succeeding the first side information instance; and removing the one or more side information instances in response to the computed difference being below a predefined threshold. Example embodiments are also envisaged in which cluster metadata instances and/or downmix metadata instances are removed in an analogous fashion. 
     By removing side information instances according to the present example embodiment, unnecessary computations based on these side information instances may be avoided, e.g. during reconstruction at a decoder side. By setting the predefined threshold at an appropriate (e.g. low enough) level, side information instances may be removed while the playback quality and/or the fidelity of the reconstructed audio signals is at least approximately maintained. 
     The difference between the desired reconstruction settings may for example be computed based on differences between respective values for a set of coefficients employed as part of the reconstruction. 
     According to example embodiments within the third, fourth or fifth aspect, the two independently assignable portions of the transition data for each side information instance may be: 
     a time stamp indicating the point in time to begin the transition to the desired reconstruction setting and a time stamp indicating the point in time to complete the transition to the desired reconstruction setting; 
     a time stamp indicating the point in time to begin the transition to the desired reconstruction setting and an interpolation duration parameter indicating a duration for reaching the desired reconstruction setting from the point in time to begin the transition to the desired reconstruction setting; or 
     a time stamp indicating the point in time to complete the transition to the desired reconstruction setting and an interpolation duration parameter indicating a duration for reaching the desired reconstruction setting from the point in time to begin the transition to the desired reconstruction setting. 
     In other words, the points in time to start and to end a transition may be defined in the transition data either by two time stamps indicating the respective points in time, or a combination of one of the time stamps and an interpolation duration parameter indicating a duration of the transition. 
     The respective time stamps may for example indicate the respective points in time by referring to a time base employed for representing the M downmix signals and/or the N audio objects. 
     According to example embodiments within the third, fourth or fifth aspect, the two independently assignable portions of the transition data for each cluster metadata instance may be: 
     a time stamp indicating the point in time to begin the transition to the desired rendering setting and a time stamp indicating the point in time to complete the transition to the desired rendering setting; 
     a time stamp indicating the point in time to begin the transition to the desired rendering setting and an interpolation duration parameter indicating a duration for reaching the desired rendering setting from the point in time to begin the transition to the desired rendering setting; or 
     a time stamp indicating the point in time to complete the transition to the desired rendering setting and an interpolation duration parameter indicating a duration for reaching the desired rendering setting from the point in time to begin the transition to the desired rendering setting. 
     According to example embodiments within the third, fourth or fifth aspect, the two independently assignable portions of the transition data for each downmix metadata instance may be: 
     a time stamp indicating the point in time to begin the transition to the desired downmix rendering setting and a time stamp indicating the point in time to complete the transition to the desired downmix rendering setting; 
     a time stamp indicating the point in time to begin the transition to the desired downmix rendering setting and an interpolation duration parameter indicating a duration for reaching the desired downmix rendering setting from the point in time to begin the transition to the desired downmix rendering setting; or 
     a time stamp indicating the point in time to complete the transition to the desired downmix rendering setting and an interpolation duration parameter indicating a duration for reaching the desired downmix rendering setting from the point in time to begin the transition to the desired downmix rendering setting. 
     According to example embodiments, there is provided a computer program product comprising a computer-readable medium with instructions for performing the method of any of the methods within the third, fourth or fifth aspect. 
     IV. Example Embodiments 
       FIG. 1  illustrates an encoder  100  for encoding audio objects  120  into a data stream  140  according to an exemplary embodiment. The encoder  100  comprises a receiving component (not shown), a downmix component  102 , an encoder component  104 , an analysis component  106 , and a multiplexing component  108 . The operation of the encoder  100  for encoding one time frame of audio data is described in the following. However, it is understood that the below method is repeated on a time frame basis. The same also applies to the description of  FIGS. 2-5 . 
     The receiving component receives a plurality of audio objects (N audio objects)  120  and metadata  122  associated with the audio objects  120 . An audio object as used herein refers to an audio signal having an associated spatial position which typically is varying with time (between time frames), i.e. the spatial position is dynamic. The metadata  122  associated with the audio objects  120  typically comprises information which describes how the audio objects  120  are to be rendered for playback on the decoder side. In particular, the metadata  122  associated with the audio objects  120  includes information about the spatial position of the audio objects  120  in the three-dimensional space of the audio scene. The spatial positions can be represented in Cartesian coordinates or by means of direction angles, such as azimuth and elevation, optionally augmented with distance. The metadata  122  associated with the audio objects  120  may further comprise object size, object loudness, object importance, object content type, specific rendering instructions such as application of dialog enhancement or exclusion of certain loudspeakers from rendering (so-called zone masks) and/or other object properties. 
     As will be described with reference to  FIG. 4 , the audio objects  120  may correspond to a simplified representation of an audio scene. 
     The N audio objects  120  are input to the downmix component  102 . The downmix component  102  calculates a number M of downmix signals  124  by forming combinations, typically linear combinations, of the N audio objects  120 . In most cases, the number of downmix signals  124  is lower than the number of audio objects  120 , i.e. M&lt;N, such that the amount of data that is included in the data stream  140  is reduced. However, for applications where the target bit rate of the data stream  140  is high, the number of downmix signals  124  may be equal to the number of objects  120 , i.e. M=N. 
     The downmix component  102  may further calculate one or more auxiliary audio signals  127 , here labeled by L auxiliary audio signals  127 . The role of the auxiliary audio signals  127  is to improve the reconstruction of the N audio objects  120  at the decoder side. The auxiliary audio signals  127  may correspond to one or more of the N audio objects  120 , either directly or as a combination of these. For example, the auxiliary audio signals  127  may correspond to particularly important ones of the N audio objects  120 , such as an audio object  120  corresponding to a dialogue. The importance may be reflected by or derived from the metadata  122  associated with the N audio objects  120 . 
     The M downmix signals  124 , and the L auxiliary signals  127  if present, may subsequently be encoded by the encoder component  104 , here labeled core encoder, to generate M encoded downmix signals  126  and L encoded auxiliary signals  129 . The encoder component  104  may be a perceptual audio codec as known in the art. Examples of known perceptual audio codecs include Dolby Digital and MPEG AAC. 
     In some embodiments, the downmix component  102  may further associate the M downmix signals  124  with metadata  125 . In particular, downmix component  102  may associate each downmix signal  124  with a spatial position and include the spatial position in the metadata  125 . Similar to the metadata  122  associated with the audio objects  120 , the metadata  125  associated with the downmix signals  124  may also comprise parameters related to size, loudness, importance, and/or other properties. 
     In particular, the spatial positions associated with the downmix signals  124  may be calculated based on the spatial positions of the N audio objects  120 . Since the spatial positions of the N audio objects  120  may be dynamic, i.e. time-varying, also the spatial positions associated with the M downmix signals  124  may be dynamic. In other words, the M downmix signals  124  may themselves be interpreted as audio objects. 
     The analysis component  106  calculates side information  128  including parameters which allow reconstruction of the N audio objects  120  (or a perceptually suitable approximation of the N audio objects  120 ) from the M downmix signals  124  and the L auxiliary signals  129  if present. Also the side information  128  may be time-variable. For example, the analysis component  106  may calculate the side information  128  by analyzing the M downmix signals  124 , the L auxiliary signals  127  if present, and the N audio objects  120  according to any known technique for parametric encoding. Alternatively, the analysis component  106  may calculate the side information  128  by analyzing the N audio objects, and information on how the M downmix signals were created from the N audio objects, for example by providing a (time-varying) downmix matrix. In that case, the M downmix signals  124  are not strictly required as an input to the analysis component  106 . 
     The M encoded downmix signals  126 , the L encoded auxiliary signals  129 , the side information  128 , the metadata  122  associated with the N audio objects, and the metadata  125  associated with the downmix signals are then input to the multiplexing component  108  which includes its input data in a single data stream  140  using multiplexing techniques. The data stream  140  may thus include four types of data: 
     a) M downmix signals  126  (and optionally L auxiliary signals  129 ) 
     b) metadata  125  associated with the M downmix signals, 
     c) side information  128  for reconstruction of the N audio objects from the M downmix signals, and 
     d) metadata  122  associated with the N audio objects. 
     As mentioned above, some prior art systems for coding of audio objects requires that the M downmix signals are chosen such that they are suitable for playback on the channels of a speaker configuration with M channels, referred to herein as a backwards compatible downmix. Such a prior art requirement constrains the calculation of the downmix signals in that the audio objects may only be combined in a predefined manner. Accordingly, according to prior art, the downmix signals are not selected from the point of view of optimizing the reconstruction of the audio objects at a decoder side. 
     As opposed to prior art systems, the downmix component  102  calculates the M downmix signals  124  in a signal adaptive manner with respect to the N audio objects. In particular, the downmix component  102  may, for each time frame, calculate the M downmix signals  124  as the combination of the audio objects  120  that currently optimizes some criterion. The criterion is typically defined such that it is independent with respect to a any loudspeaker configuration, such as a 5.1 or other loudspeaker configuration. This implies that the M downmix signals  124 , or at least one of them, are not constrained to audio signals which are suitable for playback on the channels of a speaker configuration with M channels. Accordingly, the downmix component  102  may adapt the M downmix signals  124  to the temporal variation of the N audio objects  120  (including temporal variation of the metadata  122  including spatial positions of the N audio objects), in order to e.g. improve the reconstruction of the audio objects  120  at the decoder side. 
     The downmix component  102  may apply different criteria in order to calculate the M downmix signals. According to one example, the M downmix signals may be calculated such that the reconstruction of the N audio objects based on the M downmix signals is optimized. For example, the downmix component  102  may minimize a reconstruction error formed from the N audio objects  120  and a reconstruction of the N audio objects based on the M downmix signals  124 . 
     According to another example, the criterion is based on the spatial positions, and in particular spatial proximity, of the N audio objects  120 . As discussed above, the N audio objects  120  have associated metadata  122  which includes the spatial positions of the N audio objects  120 . Based on the metadata  122 , spatial proximity of the N audio objects  120  may be derived. 
     In more detail, the downmix component  102  may apply a first clustering procedure in order to determine the M downmix signals  124 . The first clustering procedure may comprise associating the N audio objects  120  with M clusters based on spatial proximity. Further properties of the N audio objects  120  as represented by the associated metadata  122 , including object size, object loudness, object importance, may also be taken into account during the association of the audio objects  120  with the M clusters. 
     According to one example, the well-known K-means algorithm, with the metadata  122  (spatial positions) of the N audio objects as input, may be used for associating the N audio objects  120  with the M clusters based on spatial proximity. The further properties of the N audio objects  120  may be used as weighting factors in the K-means algorithm. 
     According to another example, the first clustering procedure may be based on a selection procedure which uses the importance of the audio objects, as given by the metadata  122 , as a selection criterion. In more detail, the downmix component  102  may pass through the most important audio objects  120  such that one or more of the M downmix signals correspond to one or more of the N audio objects  120 . The remaining, less important, audio objects may be associated with clusters based on spatial proximity as discussed above. 
     Further examples of clustering of audio objects are given in US provisional application with No. 61/865,072 or subsequent applications claiming the priority of that application. 
     According to yet another example, the first clustering procedure may associate an audio object  120  with more than one of the M clusters. For example an audio object  120  may be distributed over the M clusters, wherein the distribution e.g. depends on the spatial position of the audio object  120  and optionally also further properties of the audio object including object size, object loudness, object importance, etc. The distribution may be reflected by percentages, such that an audio object for instance is distributed over three clusters according to the percentages 20%, 30%, 50%. 
     Once the N audio objects  120  have been associated with the M clusters, the downmix component  102  calculates a downmix signal  124  for each cluster by forming a combination, typically a linear combination, of the audio objects  120  associated with the cluster. Typically, the downmix component  102  may use parameters comprised in the metadata  122  associated with audio objects  120  as weights when forming the combination. By way of example, the audio objects  120  being associated with a cluster may be weighted according to object size, object loudness, object importance, object position, distance from an object with respect to a spatial position associated with the cluster (see details in the following) etc. In the case where the audio objects  120  are distributed over the M clusters, the percentages reflecting the distribution may be used as weights when forming the combination. 
     The first clustering procedure is advantageous in that it easily allows association of each of the M downmix signals  124  with a spatial position. For example, the downmix component  120  may calculate a spatial position of a downmix signal  124  corresponding to a cluster based on the spatial positions of the audio objects  120  associated with the cluster. The centroid or a weighted centroid of the spatial positions of the audio objects being associated with the cluster may be used for this purpose. In case of a weighted centroid, the same weights may be used as when forming the combination of the audio objects  120  associated with the cluster. 
       FIG. 2  illustrates a decoder  200  corresponding to the encoder  100  of  FIG. 1 . The decoder  200  is of the type that supports audio object reconstruction. The decoder  200  comprises a receiving component  208 , a decoder component  204 , and a reconstruction component  206 . The decoder  200  may further comprise a renderer  210 . Alternatively, the decoder  200  may be coupled to a renderer  210  which forms part of a playback system. 
     The receiving component  208  is configured to receive a data stream  240  from the encoder  100 . The receiving component  208  comprises a demultiplexing component configured to demultiplex the received data stream  240  into its components, in this case M encoded downmix signals  226 , optionally L encoded auxiliary signals  229 , side information  228  for reconstruction of N audio objects from the M downmix signals and the L auxiliary signals, and metadata  222  associated with the N audio objects. 
     The decoder component  204  processes the M encoded downmix signals  226  to generate M downmix signals  224 , and optionally L auxiliary signals  227 . As further discussed above, the M downmix signals  224  were formed adaptively on the encoder side from the N audio objects, i.e. by forming combinations of the N audio objects according to a criterion which is independent of any loudspeaker configuration. 
     The object reconstruction component  206  then reconstructs the N audio objects  220  (or a perceptually suitable approximation of these audio objects) based on the M downmix signals  224  and optionally the L auxiliary signals  227  guided by the side information  228  derived on the encoder side. The object reconstruction component  206  may apply any known technique for such parametric reconstruction of the audio objects. 
     The reconstructed N audio objects  220  are then processed by the renderer  210  using the metadata  222  associated with the audio objects  222  and knowledge about the channel configuration of the playback system in order to generate an multichannel output signal  230  suitable for playback. Typical speaker playback configurations include 22.2 and 11.1. Playback on soundbar speaker systems or headphones (binaural presentation) is also possible with dedicated renderers for such playback systems. 
       FIG. 3  illustrates a low-complexity decoder  300  corresponding to the encoder  100  of  FIG. 1 . The decoder  300  does not support audio object reconstruction. The decoder  300  comprises a receiving component  308 , and a decoding component  304 . The decoder  300  may further comprise a renderer  310 . Alternatively, the decoder is coupled to a renderer  310  which forms part of a playback system. 
     As discussed above, prior art systems which use a backwards compatible downmix (such as a 5.1 downmix), i.e. a downmix comprising M downmix signals which are suitable for direct playback on a playback system with M channels, easily enable low complexity decoding for legacy playback systems (that e.g. only support a 5.1 multichannel loudspeaker setup). Such prior art systems typically decodes the backwards compatible downmix signals themselves and discards additional parts of the data stream such as side information (cf. item  228  of  FIG. 2 ) and metadata associated with the audio objects (cf. item  222  of  FIG. 2 ). However, when the downmix signals are formed adaptively as described above, the downmix signals are generally not suitable for direct playback on a legacy system. 
     The decoder  300  is an example of a decoder which allows low-complexity decoding of M downmix signals which are adaptively formed for playback on a legacy playback system which only supports a particular playback configuration. 
     The receiving component  308  receives a bit stream  340  from an encoder, such as encoder  100  of  FIG. 1 . The receiving component  308  demultiplexes the bit stream  340  into its components. In this case, the receiving component  308  will only keep the encoded M downmix signals  326  and the metadata  325  associated with the M downmix signals. The other components of the data stream  340 , such as the L auxiliary signals (cf. item  229  of  FIG. 2 ) metadata associated with the N audio objects (cf. item  222  of  FIG. 2 ) and the side information (cf. item  228  of  FIG. 2 ) are discarded. 
     The decoding component  304  decodes the M encoded downmix signals  326  to generate M downmix signals  324 . The M downmix signals are then, together with the downmix metadata, input to the renderer  310  which renders the M downmix signals to a multichannel output  330  corresponding to a legacy playback format (which typically has M channels). Since the downmix metadata  325  comprises spatial positions of the M downmix signals  324 , the renderer  310  may typically be similar to the renderer  210  of  FIG. 2 , with the only difference that the renderer  310  now takes the M downmix signals  324  and the metadata  325  associated with the M downmix signals  324  as input instead of audio objects  220  and their associated metadata  222 . 
     As mentioned above in connection to  FIG. 1 , the N audio objects  120  may correspond to a simplified representation of an audio scene. 
     Generally, an audio scene may comprise audio objects and audio channels. By an audio channel is here meant an audio signal which corresponds to a channel of a multichannel speaker configuration. Examples of such multichannel speaker configurations include a 22.2 configuration, a 11.1 configuration etc. An audio channel may be interpreted as a static audio object having a spatial position corresponding to the speaker position of the channel. 
     In some cases the number of audio objects and audio channels in the audio scene may be vast, such as more than 100 audio objects and 1-24 audio channels. If all of these audio objects/channels are to be reconstructed on the decoder side, a lot of computational power is required. Furthermore, the resulting data rate associated with object metadata and side information will generally be very high if many objects are provided as input. For this reason it is advantageous to simplify the audio scene in order to reduce the number of audio objects to be reconstructed on the decoder side. For this purpose, the encoder may comprise a clustering component which reduces the number of audio objects in the audio scene based on a second clustering procedure. The second clustering procedure aims at exploiting the spatial redundancy present in the audio scene, such as audio objects having equal or very similar locations. Additionally, perceptual importance of audio objects may be taken into account. Generally, such a clustering component may be arranged in sequence or in parallel with the downmix component  102  of  FIG. 1 . The sequential arrangement will be described with reference to  FIG. 4  and the parallel arrangement will be described with reference to  FIG. 5 . 
       FIG. 4  illustrates an encoder  400 . In addition to the components described with reference to  FIG. 1 , the encoder  400  comprises a clustering component  409 . The clustering component  409  is arranged in sequence with the downmix component  102 , meaning that the output of the clustering component  409  is input to the downmix component  102 . 
     The clustering component  409  takes audio objects  421   a  and/or audio channels  421   b  as input together with associated metadata  423  including spatial positions of the audio objects  421   a . The clustering component  409  converts the audio channels  421   b  to static audio objects by associating each audio channel  421   b  with the spatial position of the speaker position corresponding to the audio channel  421   b . The audio objects  421   a  and the static audio objects formed from the audio channels  421   b  may be seen as a first plurality of audio objects  421 . 
     The clustering component  409  generally reduces the first plurality of audio objects  421  to a second plurality of audio objects, here corresponding to the N audio objects  120  of  FIG. 1 . For this purpose the clustering component  409  may apply a second clustering procedure. 
     The second clustering procedure is generally similar to the first clustering procedure described above with respect to the downmix component  102 . The description of the first clustering procedure therefore also applies to the second clustering procedure. 
     In particular, the second clustering procedure involves associating the first plurality of audio objects  121  with at least one cluster, here N clusters, based on spatial proximity of the first plurality of audio objects  121 . As further described above, the association with clusters may also be based on other properties of the audio objects as represented by the metadata  423 . Each cluster is then represented by an object which is a (linear) combination of the audio objects associated with that cluster. In the illustrated example, there are N clusters and hence N audio objects  120  are generated. The clustering component  409  further calculates metadata  122  for the so generated N audio objects  120 . The metadata  122  includes spatial positions of the N audio objects  120 . The spatial position of each of the N audio objects  120  may be calculated based on the spatial positions of the audio objects associated with the corresponding cluster. By way of example the spatial position may be calculated as a centroid or a weighted centroid of the spatial positions of the audio objects associated with the cluster as further explained above with reference to  FIG. 1 . 
     The N audio objects  120  generated by the clustering component  409  are then input to the downmix component  120  as further described with reference to  FIG. 1 . 
       FIG. 5  illustrates an encoder  500 . In addition to the components described with reference to  FIG. 1 , the encoder  500  comprises a clustering component  509 . The clustering component  509  is arranged in parallel with the downmix component  102 , meaning that the downmix component  102  and the clustering component  509  have the same input. 
     The input comprises a first plurality of audio objects, corresponding to the N audio objects  120  of  FIG. 1 , together with associated metadata  122  including spatial positions of the first plurality of audio objects. The first plurality of audio objects  120  may, similar to the first plurality of audio objects  121  of  FIG. 4 , comprise audio objects and audio channels being converted into static audio objects. In contrast to the sequential arrangement of  FIG. 4  where the downmix component  102  operates on a reduced number of audio objects corresponding to a simplified version of the audio scene, the downmix component  102  of  FIG. 5  operates on the full audio content of the audio scene in order to generate M downmix signals  124 . 
     The clustering component  509  is similar in functionality to the clustering component  409  described with reference to  FIG. 4 . In particular, the clustering component  509  reduces the first plurality of audio objects  120  to a second plurality of audio objects  521 , here illustrated by K audio objects where typically M&lt;K&lt;N (for high bit applications M≦K≦N), by applying the second clustering procedure described above. The second plurality of audio objects  521  is thus a set of audio objects formed on basis of the N audio objects  126 . Moreover the clustering component  509  calculates metadata  522  for the second plurality of audio objects  521  (the K audio objects) including spatial positions of the second plurality of audio objects  521 . The metadata  522  is included in the data stream  540  by the demultiplexing component  108 . The analysis component  106  calculates side information  528  which enables reconstruction of second plurality of audio objects  521 , i.e. the set of audio objects formed on basis of the N audio objects (here the K audio objects), from the M downmix signals  124 . The side information  528  is included in the data stream  540  by the multiplexing component  108 . As further discussed above, the analysis component  106  may for example derive the side information  528  by analyzing the second plurality of audio objects  521  and the M downmix signals  124 . 
     The data stream  540  generated by the encoder  500  may generally be decoded by the decoder  200  of  FIG. 2  or the decoder  300  of  FIG. 3 . However, the reconstructed audio objects  220  of  FIG. 2  (labeled N audio objects) now correspond to the second plurality of audio objects  521  (labeled K audio objects) of  FIG. 5 , and the metadata  222  associated with the audio objects (labeled metadata of N audio objects) now correspond to the metadata  522  of the second plurality of audio objects (labeled metadata of K audio objects) of  FIG. 5 . 
     In object-based audio encoding/decoding systems, side information or metadata associated with the objects is typically updated relatively infrequently (sparsely) in time to limit the associated data rate. Typical update intervals for object positions can range between 10 and 500 milliseconds, depending on the speed of the object, the required position accuracy, the available bandwidth to store or transmit metadata, etc. Such sparse, or even irregular metadata updates require interpolation of metadata and/or rendering matrices (i.e. matrices employed in rendering) for audio samples in-between two subsequent metadata instances. Without interpolation, the consequential step-wise changes in the rendering matrix may cause undesirable switching artifacts, clicking sounds, zipper noises, or other undesirable artifacts as a result of spectral splatter introduced by step-wise matrix updates. 
       FIG. 6  illustrates a typical known process to compute rendering matrices for rendering of audio signals or audio objects, based on a set of metadata instances. As shown in  FIG. 6 , a set of metadata instances (m 1  to m 4 )  610  correspond to a set of points in time (t 1  to t 4 ) which are indicated by their position along the time axis  620 . Subsequently, each metadata instance is converted to a respective rendering matrix (c 1  to c 4 )  630 , or rendering setting, which is valid at the same time point as the metadata instance. Thus, as shown, metadata instance m 1  creates rendering matrix c 1  at time t 1 , metadata instance m 2  creates rendering matrix c 2  at time t 2 , and so on. For simplicity,  FIG. 6  shows only one rendering matrix for each metadata instance m 1  to m 4 . In practical systems, however, a rendering matrix c 1  may comprise a set of rendering matrix coefficients or gain coefficients c 1,i,j  to be applied to respective audio signals x i (t) to create output signals y j (t):
 
 y   j ( t )=Σ i   x   i ( t ) c   1,i,j .
 
The rendering matrices  630  generally comprise coefficients that represent gain values at different points in time. Metadata instances are defined at certain discrete points in time, and for audio samples in-between the metadata time points, the rendering matrix is interpolated, as indicated by the dashed line  640  connecting the rendering matrices  630 . Such interpolation can be performed linearly, but also other interpolation methods can be used (such as band-limited interpolation, sine/cosine interpolation, and etc.). The time interval between the metadata instances (and corresponding rendering matrices) is referred to as an “interpolation duration,” and such intervals may be uniform or they may be different, such as the longer interpolation duration between times t 3  and t 4  as compared to the interpolation duration between times t 2  and t 3 .
 
     In many cases, the calculation of rendering matrix coefficients from metadata instances is well-defined, but the reverse process of calculating metadata instances given a (interpolated) rendering matrix, is often difficult, or even impossible. In this respect, the process of generating a rendering matrix from metadata can sometimes be regarded as a cryptographic one-way function. The process of calculating new metadata instances between existing metadata instances is referred to as “resampling” of the metadata. Resampling of metadata is often required during certain audio processing tasks. For example, when audio content is edited, by cutting/merging/mixing and so on, such edits may occur in between metadata instances. In this case, resampling of the metadata is required. Another such case is when audio and associated metadata are encoded with a frame-based audio codec. In this case, it is desirable to have at least one metadata instance for each audio codec frame, preferably with a time stamp at the start of that codec frame, to improve resilience of frame losses during transmission. Moreover, interpolation of metadata is also ineffective for certain types of metadata, such as binary-valued metadata, where standard techniques would derive the incorrect value more or less every second time. For example, if binary flags such as zone exclusion masks are used to exclude certain objects from the rendering at certain points in time, it is virtually impossible to estimate a valid set of metadata from the rendering matrix coefficients or from neighboring instances of metadata. This is shown in  FIG. 6  as a failed attempt to extrapolate or derive a metadata instance m 3   a  from the rendering matrix coefficients in the interpolation duration between times t 3  and t 4 . As shown in  FIG. 6 , metadata instances m x  are only definitely defined at certain discrete points in time t x , which in turn produces the associated set of matrix coefficients c x . In between these discrete times t x , the sets of matrix coefficients must be interpolated based on past or future metadata instances. However, as described above, present metadata interpolation schemes suffer from loss of spatial audio quality due to unavoidable inaccuracies in metadata interpolation processes. Alternative interpolation schemes, according to example embodiments, will be described below with reference to  FIGS. 7-11 . 
     In the exemplary embodiments described with reference to  FIGS. 1-5 , the metadata  122 ,  222  associated with the N audio objects  120 ,  220  and the metadata  522  associated with the K objects  522  originate, at least in some example embodiments, from clustering components  409  and  509 , and may be referred to as cluster metadata. Further, the metadata  125 ,  325  associated with the downmix signals  124 ,  324  may be referred to as downmix metadata. 
     As described with reference to  FIGS. 1, 4 and 5 , the downmix component  102  may calculate the M downmix signals  124  by forming combinations of the N audio objects  120  in a signal-adaptive manner, i.e. according to a criterion which is independent of any loudspeaker configuration. Such operation of the downmix component  102  is characteristic of example embodiments within a first aspect. According to example embodiments within other aspects, the downmix component  102  may e.g. calculate the M downmix signals  124  by forming combinations of the N audio objects  120  in a signal-adaptive manner, or, alternatively, such that the M downmix signals are suitable for playback on the channels of a speaker configuration with M channels, i.e. as a backwards compatible downmix. 
     In an example embodiment, the encoder  400  described with reference to  FIG. 4  employs a metadata and side information format particularly suitable for resampling, i.e. for generating additional metadata and side information instances. In the present example embodiment, the analysis component  106  calculates the side information  128  in a form which includes a plurality of side information instances specifying respective desired reconstruction settings for reconstructing the N audio objects  120 , and, for each side information instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current reconstruction setting to the desired reconstruction setting specified by the side information instance, and a point in time to complete the transition. In the present example embodiment, the two independently assignable portions of the transition data for each side information instance are: a time stamp indicating the point in time to begin the transition to the desired reconstruction setting and an interpolation duration parameter indicating a duration for reaching the desired reconstruction setting from the point in time to begin the transition to the desired reconstruction setting. The interval during which a transition is to take place is in the present example embodiment uniquely defined by the time at which the transition is to begin and the duration of the transition interval. This particular form of the side information  128  will be described below with reference to  FIGS. 7-11 . It is to be understood that there are several other ways to uniquely define this transition interval. For example, a reference point in the form of a start, end or middle point of the interval, accompanied by the duration of the interval, may be employed in the transition data to uniquely define the interval. Alternatively, the start and end points of the interval may be employed in the transition data to uniquely define the interval. 
     In the present example embodiment, the clustering component  409  reduces the first plurality of audio objects  421  to a second plurality of audio objects, here corresponding to the N audio objects  120  of  FIG. 1 . The clustering component  409  calculates the cluster metadata  122  for the generated N audio objects  120  which enables rendering of the N audio objects  122  in a renderer  210  at a decoder side. The clustering component  409  provides the cluster metadata  122  in a form which includes a plurality of cluster metadata instances specifying respective desired rendering settings for rendering the N audio objects  120 , and, for each cluster metadata instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current rendering setting to the desired rendering setting specified by the cluster metadata instance, and a point in time to complete the transition to the desired rendering setting. In the present example embodiment, the two independently assignable portions of the transition data for each cluster metadata instance are: a time stamp indicating the point in time to begin the transition to the desired rendering setting and an interpolation duration parameter indicating a duration for reaching the desired rendering setting from the point in time to begin the transition to the desired rendering setting. This particular form of the cluster metadata  122  will be described below with reference to  FIGS. 7-11 . 
     In the present example embodiment, the downmix component  102  associates each downmix signal  124  with a spatial position and includes the spatial position in the downmix metadata  125  which allows rendering of the M downmix signals in a renderer  310  at a decoder side. The downmix component  102  provides the downmix metadata  125  in a form which includes a plurality of downmix metadata instances specifying respective desired downmix rendering settings for rendering the downmix signals, and, for each downmix metadata instance, transition data including two independently assignable portions which in combination define a point in time to begin a transition from a current downmix rendering setting to the desired downmix rendering setting specified by the downmix metadata instance, and a point in time to complete the transition to the desired downmix rendering setting. In the present example embodiment, the two independently assignable portions of the transition data for each downmix metadata instance are: a time stamp indicating the point in time to begin the transition to the desired downmix rendering setting and an interpolation duration parameter indicating a duration for reaching the desired downmix rendering setting from the point in time to begin the transition to the desired downmix rendering setting. 
     In the present example embodiment, the same format is employed for the side information  128 , the cluster metadata  122  and the downmix metadata  125 . This format will now be described with reference to  FIGS. 7-11  in terms of metadata for rendering of audio signals. However, it is to be understood that in the following examples described with reference to  FIGS. 7-11 , terms or expressions like “metadata for rendering of audio signals” may just as well be replaced by corresponding terms or expressions like “side information for reconstruction of audio objects”, “cluster metadata for rendering of audio objects” or “downmix metadata for rendering of downmix signals”. 
       FIG. 7  illustrates the derivation, based on metadata, of coefficient curves employed in rendering of audio signals, according to an example embodiment. As shown in  FIG. 7 , a set of metadata instances m x  generated at different points in time t x , e.g. associated with unique time stamps, are converted by a converter  710  into corresponding sets of matrix coefficient values c x . These sets of coefficients represent gain values, also referred to as gain factors, to be employed for rendering of the audio signals to various speakers and drivers in a playback system to which the audio content is to be rendered. An interpolator  720  then interpolates the gain factors c x  to produce a coefficient curve between the discrete times t x . In an embodiment, the time stamps t x  associated with each metadata instance m x  may correspond to random points in time, synchronous points in time generated by a clock circuit, time events related to the audio content, such as frame boundaries, or any other appropriate timed event. Note that, as described above, the description provided with reference to  FIG. 7  applies analogously to side information for reconstruction of audio objects. 
       FIG. 8  illustrates a metadata format according to an embodiment (and as described above, the following description applies analogously to a corresponding side information format), which addresses at least some of the interpolation problems associated with present methods, as described above, by defining a time stamp as the start time of a transition or an interpolation, and augmenting each metadata instance with an interpolation duration parameter that represents the transition duration or interpolation duration (also referred to as “ramp size”). As shown in  FIG. 8 , a set of metadata instances m 2  to m 4  ( 810 ) specifies a set of rendering matrices c 2  to c 4  ( 830 ). Each metadata instance is generated at a particular point in time t x , and each metadata instance is defined with respect to its time stamp, m 2  to t 2 , m 3  to t 3 , and so on. The associated rendering matrices  830  are generated after performing transitions during respective interpolation durations d 2 , d 3 , d 4  ( 830 ), from the associated time stamp (t 1  to t 4 ) of each metadata instance  810 . An interpolation duration parameter indicating the interpolation duration (or ramp size) is included with each metadata instance, i.e., metadata instance m 2  includes d 2 , m 3  includes d 3 , and so on. Schematically this can be represented as follows: m x =(metadata(t x ), d x )→c x . In this manner, the metadata essentially provides a schematic of how to proceed from a current rendering setting (e.g., the current rendering matrix resulting from previous metadata) to a new rendering setting (e.g., the new rendering matrix resulting from the current metadata). Each metadata instance is meant to take effect at a specified point in time in the future relative to the moment the metadata instance was received and the coefficient curve is derived from the previous state of the coefficient. Thus, in  FIG. 8 , m 2  generates c 2  after a duration d 2 , m 3  generates c 3  after a duration d 3  and m 4  generates c 4  after a duration d 4 . In this scheme for interpolation, the previous metadata need not be known, only the previous rendering matrix or rendering state is required. The interpolation employed may be linear or non-linear depending on system constraints and configurations. 
     The metadata format of  FIG. 8  allows for lossless resampling of metadata, as shown in  FIG. 9 .  FIG. 9  illustrates a first example of lossless processing of metadata, according to an example embodiment (and as described above, the following description applies analogously to a corresponding side information format).  FIG. 9  shows metadata instances m 2  to m 4  that refer to the future rendering matrices c 2  to c 4 , respectively, including interpolation durations d 2  to d 4 . The time stamps of the metadata instances m 2  to m 4  are given as t 2  to t 4 . In the example of  FIG. 9 , a metadata instance m 4   a , at time t 4   a , is added. Such metadata may be added for several reasons, such as to improve error resilience of the system or to synchronize metadata instances with the start/end of an audio frame. For example, time t 4   a  may represent the time that an audio codec employed for coding audio content associated with the metadata starts a new frame. For lossless operation, the metadata values of m 4   a  are identical to those of m 4  (i.e. they both describe a target rendering matrix c 4 ), but the time d 4   a  to reach that point has been reduced by d 4 -d 4   a . In other words, metadata instance m 4   a  is identical to that of the previous metadata instance m 4  so that the interpolation curve between c 3  and c 4  is not changed. However, the new interpolation duration d 4   a , is shorter than the original duration d 4 . This effectively increases the data rate of the metadata instances, which can be beneficial in certain circumstances, such as error correction. 
     A second example of lossless metadata interpolation is shown in  FIG. 10  (and as described above, the following description applies analogously to a corresponding side information format). In this example, the goal is to include a new set of metadata m 3   a  in between two metadata instances m 3  and m 4 .  FIG. 10  illustrates a case where the rendering matrix remains unchanged for a period of time. Therefore, in this situation, the values of the new set of metadata m 3   a  are identical to those of the prior metadata m 3 , except for the interpolation duration d 3   a . The value of the interpolation duration d 3   a  should be set to the value corresponding to t 4 -t 3   a , i.e. to the difference between time t 4  associated with the next metadata instance m 4  and the time t 3   a  associated with the new set of metadata m 3   a . The case illustrated in  FIG. 10  may for example occur when an audio object is static and an authoring tool stops sending new metadata for the object due to this static nature. In such a case, it may be desirable to insert new metadata instances m 3   a , e.g. to synchronize the metadata with codec frames. 
     In the examples illustrated in  FIGS. 8 to 10 , the interpolation from a current to a desired rendering matrix or rendering state was performed by linear interpolation. In other example embodiments, different interpolation schemes may also be used. One such alternative interpolation scheme uses a sample-and-hold circuit combined with a subsequent low-pass filter.  FIG. 11  illustrates an interpolation scheme using a sample-and-hold circuit with a low-pass filter, according to an example embodiment (and as described above, the following description applies analogously to a corresponding side information format). As shown in  FIG. 11 , the metadata instances m 2  to m 4  are converted to sample-and-hold rendering matrix coefficients c 2  and c 3 . The sample-and-hold process causes the coefficient states to jump immediately to the desired state, which results in a step-wise curve  1110 , as shown. This curve  1110  is then subsequently low-pass filtered to obtain a smooth, interpolated curve  1120 . The interpolation filter parameters (e.g., cut-off frequency or time constant) can be signaled as part of the metadata, in addition to the time stamps and the interpolation duration parameters. It is to be understood that different parameters may be used depending on the requirements of the system and the characteristics of the audio signal. 
     In an example embodiment, the interpolation duration or ramp size can have any practical value, including a value of or substantially close to zero. Such small interpolation duration is especially helpful for cases such as initialization in order to enable setting the rendering matrix immediately at the first sample of a file, or allowing for edits, splicing, or concatenation of streams. With this type of destructive edits, having the possibility to instantaneously change the rendering matrix can be beneficial to maintain the spatial properties of the content after editing. 
     In an example embodiment, the interpolation scheme described herein is compatible with the removal of metadata instances (and analogously with the removal of side information instances, as described above), such as in a decimation scheme that reduces metadata bitrates. Removal of metadata instances allows the system to resample at a frame rate that is lower than an initial frame rate. In this case, metadata instances and their associated interpolation duration data that are provided by an encoder may be removed based on certain characteristics. For example, an analysis component in an encoder may analyze the audio signal to determine if there is a period of significant stasis of the signal, and in such a case remove certain metadata instances already generated to reduce bandwidth requirements for the transmittal of data to a decoder side. The removal of metadata instances may alternatively or additionally be performed in a component separate from the encoder, such as in a decoder or in a transcoder. A transcoder may remove metadata instances that have been generated or added by the encoder, and may be employed in a data rate converter that re-samples an audio signal from a first rate to a second rate, where the second rate may or may not be an integer multiple of the first rate. Alternatively to analyzing the audio signal in order to determine which metadata instances to remove, the encoder, decoder or transcoder may analyze the metadata. For example, with reference to  FIG. 10 , a difference may be computed between a first desired reconstruction setting c 3  (or reconstruction matrix), specified by a first metadata instance m 3 , and desired reconstruction settings c 3   a  and c 4  (or reconstruction matrices) specified by metadata instances m 3   a  and m 4  directly succeeding the first metadata instance m 3 . The difference may for example be computed by employing a matrix norm to the respective rendering matrices. If the difference is below a predefined threshold, e.g. corresponding to a tolerated distortion of the reconstructed audio signals, the metadata instances m 3   a  and m 4  succeeding the first metadata instance m 2  may be removed. In the example illustrated in  FIG. 10 , the metadata instance m 3   a  directly succeeding the first metadata instance m 3  specifies the same rendering settings c 3 =c 3   a  as the first metadata instance m 3  and will therefore be removed, while the next metadata setting m 4  specifies a different rendering setting c 4  and may, depending on the threshold employed, be kept as metadata. 
     In the decoder  200  described with reference to  FIG. 2 , the object reconstruction component  206  may employ interpolation as part of reconstructing the 
     N audio objects  220  based on the M downmix signals  224  and the side information  228 . In analogy with the interpolation scheme described with reference to  FIGS. 7-11 , reconstructing the N audio objects  220  may for example include: performing reconstruction according to a current reconstruction setting; beginning, at a point in time defined by the transition data for a side information instance, a transition from the current reconstruction setting to a desired reconstruction setting specified by the side information instance; and completing the transition to the desired reconstruction setting at a point in time defined by the transition data for the side information instance. 
     Similarly, the renderer  210  may employ interpolation as part of rendering the reconstructed N audio objects  220  in order to generate the multichannel output signal  230  suitable for playback. In analogy with the interpolation scheme described with reference to  FIGS. 7-11 , the rendering may include: performing rendering according to a current rendering setting; beginning, at a point in time defined by the transition data for a cluster metadata instance, a transition from the current rendering setting to a desired rendering setting specified by the cluster metadata instance; and completing the transition to the desired rendering setting at a point in time defined by the transition data for the cluster metadata instance. 
     In some example embodiments, the object reconstruction section  206  and the renderer  210  may be separate units, and/or may correspond to operations performed as separate processes. In other example embodiments, the object reconstruction section  206  and the renderer  210  may be embodied as a single unit or process in which reconstruction and rendering is performed as a combined operation. In such example embodiments, matrices employed for reconstruction and rendering may be combined into a single matrix which may be interpolated, instead of performing interpolation on a rendering matrix and a reconstruction matrix, separately. 
     In the low-complexity decoder  300 , described with reference to  FIG. 3 , the renderer  310  may perform interpolation as part of rendering the M downmix signals  324  to the multichannel output  330 . In analogy with the interpolation scheme described with reference to  FIGS. 7-11 , the rendering may include: performing rendering according to a current downmix rendering setting; beginning, at a point in time defined by the transition data for a downmix metadata instance, a transition from the current downmix rendering setting to a desired downmix rendering setting specified by the downmix metadata instance; and completing the transition to the desired downmix rendering setting at a point in time defined by the transition data for the downmix metadata instance. As previously described, the renderer  310  may be comprised in the decoder  300  or may be a separate device/unit. In example embodiments where the renderer  310  is separate from the decoder  300 , the decoder may output the downmix metadata  325  and the M downmix signals  324  for rendering of the M downmix signals in the renderer  310 . 
     EQUIVALENTS, EXTENSIONS, ALTERNATIVES AND MISCELLANEOUS 
     Further embodiments of the present disclosure will become apparent to a person skilled in the art after studying the description above. Even though the present description and drawings disclose embodiments and examples, the disclosure is not restricted to these specific examples. Numerous modifications and variations can be made without departing from the scope of the present disclosure, which is defined by the accompanying claims. Any reference signs appearing in the claims are not to be understood as limiting their scope. 
     Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the disclosure, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. 
     The systems and methods disclosed hereinabove may be implemented as software, firmware, hardware or a combination thereof. In a hardware implementation, the division of tasks between functional units referred to in the above description does not necessarily correspond to the division into physical units; to the contrary, one physical component may have multiple functionalities, and one task may be carried out by several physical components in cooperation. Certain components or all components may be implemented as software executed by a digital signal processor or microprocessor, or be implemented as hardware or as an application-specific integrated circuit. Such software may be distributed on computer readable media, which may comprise computer storage media (or non-transitory media) and communication media (or transitory media). As is well known to a person skilled in the art, the term computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by a computer. Further, it is well known to the skilled person that communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. 
     All the figures are schematic and generally only show parts which are necessary in order to elucidate the disclosure, whereas other parts may be omitted or merely suggested. Unless otherwise indicated, like reference numerals refer to like parts in different figures.