Patent Publication Number: US-8538766-B2

Title: Audio decoder, audio object encoder, method for decoding a multi-audio-object signal, multi-audio-object encoding method, and non-transitory computer-readable medium therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Provisional U.S. Patent Application No. 60/980,571, which was filed on Oct. 17, 2007, and from Provisional U.S. Patent Application No. 60/991,335, which was filed on Nov. 30, 2007, which are both incorporated herein in their entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present application is concerned with audio coding using down-mixing of signals. 
     Many audio encoding algorithms have been proposed in order to effectively encode or compress audio data of one channel, i.e., mono audio signals. Using psychoacoustics, audio samples are appropriately scaled, quantized or even set to zero in order to remove irrelevancy from, for example, the PCM coded audio signal. Redundancy removal is also performed. 
     As a further step, the similarity between the left and right channel of stereo audio signals has been exploited in order to effectively encode/compress stereo audio signals. 
     However, upcoming applications pose further demands on audio coding algorithms. For example, in teleconferencing, computer games, music performance and the like, several audio signals which are partially or even completely uncorrelated have to be transmitted in parallel. In order to keep the bit rate for encoding these audio signals low enough in order to be compatible to low-bit rate transmission applications, recently, audio codecs have been proposed which downmix the multiple input audio signals into a downmix signal, such as a stereo or even mono downmix signal. For example, the MPEG Surround standard downmixes the input channels into the downmix signal in a manner prescribed by the standard. The downmixing is performed by use of so-called OTT −1  and TTT −1  boxes for downmixing two signals into one and three signals into two, respectively. In order to downmix more than three signals, a hierarchic structure of these boxes is used. Each OTT −1  box outputs, besides the mono downmix signal, channel level differences between the two input channels, as well as inter-channel coherence/cross-correlation parameters representing the coherence or cross-correlation between the two input channels. The parameters are output along with the downmix signal of the MPEG Surround coder within the MPEG Surround data stream. Similarly, each TTT −1  box transmits channel prediction coefficients enabling recovering the three input channels from the resulting stereo downmix signal. The channel prediction coefficients are also transmitted as side information within the MPEG Surround data stream. The MPEG Surround decoder upmixes the downmix signal by use of the transmitted side information and recovers, the original channels input into the MPEG Surround encoder. 
     However, MPEG Surround, unfortunately, does not fulfill all requirements posed by many applications. For example, the MPEG Surround decoder is dedicated for upmixing the downmix signal of the MPEG Surround encoder such that the input channels of the MPEG Surround encoder are recovered as they are. In other words, the MPEG Surround data stream is dedicated to be played back by use of the loudspeaker configuration having been used for encoding. 
     However, according to some implications, it would be favorable if the loudspeaker configuration could be changed at the decoder&#39;s side. 
     In order to address the latter needs, the spatial audio object coding (SAOC) standard is currently designed. Each channel is treated as an individual object, and all objects are downmixed into a downmix signal. However, in addition the individual objects may also comprise individual sound sources as e.g. instruments or vocal tracks. However, differing from the MPEG Surround decoder, the SAOC decoder is free to individually upmix the downmix signal to replay the individual objects onto any loudspeaker configuration. In order to enable the SAOC decoder to recover the individual objects having been encoded into the SAOC data stream, object level differences and, for objects forming together a stereo (or multi-channel) signal, inter-object cross correlation parameters are transmitted as side information within the SAOC bitstream. Besides this, the SAOC decoder/transcoder is provided with information revealing how the individual objects have been downmixed into the downmix signal. Thus, on the decoder&#39;s side, it is possible to recover the individual SAOC channels and to render these signals onto any loudspeaker configuration by utilizing user-controlled rendering information. 
     However, although the SAOC codec has been designed for individually handling audio objects, some applications are even more demanding. For example, Karaoke applications necessitate a complete separation of the background audio signal from the foreground audio signal or foreground audio signals. Vice versa, in the solo mode, the foreground objects have to be separated from the background object. However, owing to the equal treatment of the individual audio objects it was not possible to completely remove the background objects or the foreground objects, respectively, from the downmix signal. 
     SUMMARY 
     According to an embodiment, an audio decoder for decoding a multi-audio-object signal having an audio signal of a first type and an audio signal of a second type encoded therein, the multi-audio-object signal having a downmix signal and side information, the side information having level information of the audio signal of the first type and the audio signal of the second type in a first predetermined time/frequency resolution, and a residual signal specifying residual level values in a second predetermined time/frequency resolution, may have a processor for computing prediction coefficients based on the level information; and an up-mixer for up-mixing the downmix signal based on the prediction coefficients and the residual signal to acquire a first up-mix audio signal approximating the audio signal of the first type and/or a second up-mix audio signal approximating the audio signal of the second type. 
     According to another embodiment, an audio object encoder may have: a processor for computing level information of an audio signal of the first type and an audio signal of the second type in a first predetermined time/frequency resolution; a processor for computing prediction coefficients based on the level information; a downmixer for downmixing the audio signal of the first type and the audio signal of the second type to acquire a downmix signal; a setter for setting a residual signal specifying residual level values at a second predetermined time/frequency resolution such that up-mixing the downmix signal based on both the prediction coefficients and the residual signal results in a first up-mix audio signal approximating the audio signal of the first type and a second up-mix audio signal approximating the audio signal of the second type, the approximation being improved compared to the absence of the residual signal, the level information and the residual signal being included by a side information forming, along with the downmix signal, a multi-audio-object signal. 
     According to another embodiment, a method for decoding a multi-audio-object signal having an audio signal of a first type and an audio signal of a second type encoded therein, the multi-audio-object signal having a downmix signal and side information, the side information having level information of the audio signal of the first type and the audio signal of the second type in a first predetermined time/frequency resolution, and a residual signal specifying residual level values in a second predetermined time/frequency resolution, may have the steps of computing prediction coefficients based on the level information; and up-mixing the downmix signal based on the prediction coefficients and the residual signal to acquire a first up-mix audio signal approximating the audio signal of the first type and/or a second up-mix audio signal approximating the audio signal of the second type. 
     According to another embodiment, a multi-audio-object encoding method may have the steps of: computing level information of an audio signal of the first type and an audio signal of the second type in a first predetermined time/frequency resolution; computing prediction coefficients based on the level information; downmixing the audio signal of the first type and the audio signal of the second type to acquire a downmix signal; setting a residual signal specifying residual level values at a second predetermined time/frequency resolution such that up-mixing the downmix signal based on both the prediction coefficients and the residual signal results in a first up-mix audio signal approximating the audio signal of the first type and a second up-mix audio signal approximating the audio signal of the second type, the approximation being improved compared to the absence of the residual signal, the level information and the residual signal being included by a side information forming, along with the downmix signal, a multi-audio-object signal. 
     According to another embodiment, a program may have a program code for executing, when running on a processor, a method for decoding a multi-audio-object signal having an audio signal of a first type and an audio signal of a second type encoded therein, the multi-audio-object signal having a downmix signal and side information, the side information having level information of the audio signal of the first type and the audio signal of the second type in a first predetermined time/frequency resolution, and a residual signal specifying residual level values in a second predetermined time/frequency resolution, wherein the method may have the steps of computing prediction coefficients based on the level information; and up-mixing the downmix signal based on the prediction coefficients and the residual signal to acquire a first up-mix audio signal approximating the audio signal of the first type and/or a second up-mix audio signal approximating the audio signal of the second type. 
     According to another embodiment, a program may have a program code for executing, when running on a processor, a multi-audio-object encoding method, wherein the method may have the steps of: computing level information of an audio signal of the first type and an audio signal of the second type in a first predetermined time/frequency resolution; computing prediction coefficients based on the level information; downmixing the audio signal of the first type and the audio signal of the second type to acquire a downmix signal; setting a residual signal specifying residual level values at a second predetermined time/frequency resolution such that up-mixing the downmix signal based on both the prediction coefficients and the residual signal results in a first up-mix audio signal approximating the audio signal of the first type and a second up-mix audio signal approximating the audio signal of the second type, the approximation being improved compared to the absence of the residual signal, the level information and the residual signal being included by a side information forming, along with the downmix signal, a multi-audio-object signal. 
     According to another embodiment, a multi-audio-object signal may have an audio signal of a first type and an audio signal of a second type encoded therein, the multi-audio-object signal having a downmix signal and side information, the side information having level information of the audio signal of the first type and the audio signal of the second type in a first predetermined time/frequency resolution, and a residual signal specifying residual level values in a second predetermined time/frequency resolution, wherein the residual signal is set such that computing prediction coefficients based on the level information and up-mixing the downmix signal based on the prediction coefficients and the residual signal results in a first up-mix audio signal approximating the audio signal of the first type and a second up-mix audio signal approximating the audio signal of the second type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
         FIG. 1  shows a block diagram of an SAOC encoder/decoder arrangement in which the embodiments of the present invention may be implemented; 
         FIG. 2  shows a schematic and illustrative diagram of a spectral representation of a mono audio signal; 
         FIG. 3  shows a block diagram of an audio decoder according to an embodiment of the present invention; 
         FIG. 4  shows a block diagram of an audio encoder according to an embodiment of the present invention; 
         FIG. 5  shows a block diagram of an audio encoder/decoder arrangement for Karaoke/Solo mode application, as a comparison embodiment; 
         FIG. 6  shows a block diagram of an audio encoder/decoder arrangement for Karaoke/Solo mode application according to an embodiment; 
         FIG. 7   a  shows a block diagram of an audio encoder for a Karaoke/Solo mode application, according to a comparison embodiment; 
         FIG. 7   b  shows a block diagram of an audio encoder for a Karaoke/Solo mode application, according to an embodiment; 
         FIGS. 8   a  and  b  show plots of quality measurement results; 
         FIG. 9  shows a block diagram of an audio encoder/decoder arrangement for Karaoke/Solo mode application, for comparison purposes; 
         FIG. 10  shows a block diagram of an audio encoder/decoder arrangement for Karaoke/Solo mode application according to an embodiment; 
         FIG. 11  shows a block diagram of an audio encoder/decoder arrangement for Karaoke/Solo mode application according to a further embodiment; 
         FIG. 12  shows a block diagram of an audio encoder/decoder arrangement for Karaoke/Solo mode application according to a further embodiment; 
         FIG. 13   a  to  h  show tables reflecting a possible syntax for the SOAC bitstream according to an embodiment of the present invention; 
         FIG. 14  shows a block diagram of an audio decoder for a Karaoke/Solo mode application, according to an embodiment; and 
         FIG. 15  show a table reflecting a possible syntax for signaling the amount of data spent for transferring the residual signal. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before embodiments of the present invention are described in more detail below, the SAOC codec and the SAOC parameters transmitted in an SAOC bitstream are presented in order to ease the understanding of the specific embodiments outlined in further detail below. 
       FIG. 1  shows a general arrangement of an SAOC encoder  10  and an SAOC decoder  12 . The SAOC encoder  10  receives as an input N objects, i.e., audio signals  14   1  to  14   N . In particular, the encoder  10  comprises a downmixer  16  which receives the audio signals  14   1  to  14   N  and downmixes same to a downmix signal  18 . In  FIG. 1 , the downmix signal is exemplarily shown as a stereo downmix signal. However, a mono downmix signal is possible as well. The channels of the stereo downmix signal  18  are denoted L 0  and R 0 , in case of a mono downmix same is simply denoted L 0 . In order to enable the SAOC decoder  12  to recover the individual objects  14   1  to  14   N , downmixer  16  provides the SAOC decoder  12  with side information including SAOC-parameters including object level differences (OLD), inter-object cross correlation parameters (IOC), downmix gain values (DMG) and downmix channel level differences (DCLD). The side information  20  including the SAOC-parameters, along with the downmix signal  18 , forms the SAOC output data stream received by the SAOC decoder  12 . 
     The SAOC decoder  12  comprises an upmixer  22  which receives the downmix signal  18  as well as the side information  20  in order to recover and render the audio signals  14   1  and  14   N  onto any user-selected set of channels  24   1  to  24   M , with the rendering being prescribed by rendering information  26  input into SAOC decoder  12 . 
     The audio signals  14   1  to  14   N  may be input into the downmixer  16  in any coding domain, such as, for example, in time or spectral domain. In case, the audio signals  14   1  to  14   N  are fed into the downmixer  16  in the time domain, such as PCM coded, downmixer  16  uses a filter bank, such as a hybrid QMF bank, i.e., a bank of complex exponentially modulated filters with a Nyquist filter extension for the lowest frequency bands to increase the frequency resolution therein, in order to transfer the signals into spectral domain in which the audio signals are represented in several subbands associated with different spectral portions, at a specific filter bank resolution. If the audio signals  14   1  to  14   N  are already in the representation expected by downmixer  16 , same does not have to perform the spectral decomposition. 
       FIG. 2  shows an audio signal in the just-mentioned spectral domain. As can be seen, the audio signal is represented as a plurality of subband signals. Each subband signal  30   1  to  30   P  consists of a sequence of subband values indicated by the small boxes  32 . As can be seen, the subband values  32  of the subband signals  30   1  to  30   P  are synchronized to each other in time so that for each of consecutive filter bank time slots  34  each subband  30   1  to  30   P  comprises exact one subband value  32 . As illustrated by the frequency axis  36 , the subband signals  30   1  to  30   P  are associated with different frequency regions, and as illustrated by the time axis  38 , the filter bank time slots  34  are consecutively arranged in time. 
     As outlined above, downmixer  16  computes SAOC-parameters from the input audio signals  14   1  to  14   N . Downmixer  16  performs this computation in a time/frequency resolution which may be decreased relative to the original time/frequency resolution as determined by the filter bank time slots  34  and subband decomposition, by a certain amount, with this certain amount being signaled to the decoder side within the side information  20  by respective syntax elements bsFrameLength and bsFreqRes. For example, groups of consecutive filter bank time slots  34  may form a frame  40 . In other words, the audio signal may be divided-up into frames overlapping in time or being immediately adjacent in time, for example. In this case, bsFrameLength may define the number of parameter time slots  41 , i.e. the time unit at which the SAOC parameters such as OLD and IOC, are computed in an SAOC frame  40  and bsFreqRes may define the number of processing frequency bands for which SAOC parameters are computed. By this measure, each frame is divided-up into time/frequency tiles exemplified in  FIG. 2  by dashed lines  42 . 
     The downmixer  16  calculates SAOC parameters according to the following formulas. In particular, downmixer  16  computes object level differences for each object i as 
               OLD   i     =         ∑   n     ⁢       ∑     k   ∈   m       ⁢       x   i     n   ,   k       ⁢     x   i     n   ,     k   *                   max   j     ⁢     (       ∑   n     ⁢       ∑     k   ∈   m       ⁢       x   j     n   ,   k       ⁢     x   j     n   ,     k   *               )               
wherein the sums and the indices n and k, respectively, go through all filter bank time slots  34 , and all filter bank subbands  30  which belong to a certain time/frequency tile  42 . Thereby, the energies of all subband values x i  of an audio signal or object i are summed up and normalized to the highest energy value of that tile among all objects or audio signals.
 
     Further the SAOC downmixer  16  is able to compute a similarity measure of the corresponding time/frequency tiles of pairs of different input objects  14   1  to  14   N . Although the SAOC downmixer  16  may compute the similarity measure between all the pairs of input objects  14   1  to  14   N , downmixer  16  may also suppress the signaling of the similarity measures or restrict the computation of the similarity measures to audio objects  14   1  to  14   N  which form left or right channels of a common stereo channel. In any case, the similarity measure is called the inter-object cross-correlation parameter IOC i, j . The computation is as follows 
               IOC     i   ,   j       =       IOC     j   ,   i       =     Re   ⁢     {         ∑   n     ⁢       ∑     k   ∈   m       ⁢       x   i     n   ,   k       ⁢     x   j     n   ,     k   *                     ∑   n     ⁢       ∑     k   ∈   m       ⁢       x   i     n   ,   k       ⁢     x   i     n   ,     k   *         ⁢       ∑   n     ⁢       ∑     k   ∈   m       ⁢       x   j     n   ,   k       ⁢     x   j     n   ,     k   *                         }               
with again indexes n and k going through all subband values belonging to a certain time/frequency tile  42 , and i and j denoting a certain pair of audio objects  14   1  to  14   N .
 
     The downmixer  16  downmixes the objects  14   1  to  14   N  by use of gain factors applied to each object  14   1  to  14   N . That is, a gain factor D i  is applied to object i and then all thus weighted objects  14   1  to  14   N  are summed up to obtain a mono downmix signal. In the case of a stereo downmix signal, which case is exemplified in  FIG. 1 , a gain factor D 1, i  is applied to object i and then all such gain amplified objects are summed-up in order to obtain the left downmix channel L 0 , and gain factors D 2, i  are applied to object i and then the thus gain-amplified objects are summed-up in order to obtain the right downmix channel R 0 . 
     This downmix prescription is signaled to the decoder side by means of down mix gains DMG i  and, in case of a stereo downmix signal, downmix channel level differences DCLD i . 
     The downmix gains are calculated according to:
 
DMG i =20 log 10 ( D   i +ε),(mono downmix),
 
DMG i =10 log 10 ( D   1,i   2   +D   2,i   2 +ε),(stereo downmix),
 
where ε is a small number such as 10 −9 .
 
     For the DCLD s  the following formula applies: 
     
       
         
           
             
               DCLD 
               i 
             
             = 
             
               20 
               ⁢ 
               
                 
                   
                     log 
                     10 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         D 
                         
                           1 
                           , 
                           i 
                         
                       
                       
                         
                           D 
                           
                             2 
                             , 
                             i 
                           
                         
                         + 
                         ɛ 
                       
                     
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
     In the normal mode, downmixer  16  generates the downmix signal according to: 
               (     L   ⁢           ⁢   0     )     =       (     D   i     )     ⁢     (           Obj   1             ⋮             Obj   N           )             
for a mono downmix, or
 
               (           L   ⁢           ⁢   0               R   ⁢           ⁢   0           )     =       (           D     1   ,   i                 D     2   ,   i             )     ⁢     (           Obj   1             ⋮             Obj   N           )             
for a stereo downmix, respectively.
 
     Thus, in the abovementioned formulas, parameters OLD and IOC are a function of the audio signals and parameters DMG and DCLD are a function of D. By the way, it is noted that D may be varying in time. 
     Thus, in the normal mode, downmixer  16  mixes all objects  14   1  to  14   N  with no preferences, i.e., with handling all objects  14   1  to  14   N  equally. 
     The upmixer  22  performs the inversion of the downmix procedure and the implementation of the “rendering information” represented by matrix A in one computation step, namely 
                 (           Ch   1             ⋮             Ch   M           )     =           AED     -   1       ⁡     (     DED     -   1       )         -   1       ⁢     (           L   ⁢           ⁢   0               R   ⁢           ⁢   0           )         ,         
where matrix E is a function of the parameters OLD and IOC.
 
     In other words, in the normal mode, no classification of the objects  14   1  to  14   N  into BGO, i.e., background object, or FGO, i.e., foreground object, is performed. The information as to which object shall be presented at the output of the upmixer  22  is to be provided by the rendering matrix A. If, for example, object with index  1  was the left channel of a stereo background object, the object with index  2  was the right channel thereof, and the object with index  3  was the foreground object, then rendering matrix A would be 
                   (           Obj   1               Obj   2               Obj   3           )     ≡     (           B   ⁢           ⁢   G   ⁢           ⁢     O   L                 B   ⁢           ⁢   G   ⁢           ⁢     O   R                 F   ⁢           ⁢   G   ⁢           ⁢   O           )       -&gt;   A     =     (         1       0       0           0       1       0         )           
to produce a Karaoke-type of output signal.
 
     However, as already indicated above, transmitting EGO and FGO by use of this normal mode of the SAOC codec does not achieve acceptable results. 
       FIGS. 3 and 4 , describe an embodiment of the present invention which overcomes the deficiency just described. The decoder and encoder described in these Figs. and their associated functionality may represent an additional mode such as an “enhanced mode” into which the SAOC codec of  FIG. 1  could be switchable. Examples for the latter possibility will be presented thereinafter. 
       FIG. 3  shows a decoder  50 . The decoder  50  comprises means  52  for computing prediction coefficients and means  54  for upmixing a downmix signal. 
     The audio decoder  50  of  FIG. 3  is dedicated for decoding a multi-audio-object signal having an audio signal of a first type and an audio signal of a second type encoded therein. The audio signal of the first type and the audio signal of the second type may be a mono or stereo audio signal, respectively. The audio signal of the first type is, for example, a background object whereas the audio signal of the second type is a foreground object. That is, the embodiment of  FIG. 3  and  FIG. 4  is not necessarily restricted to Karaoke/Solo mode applications. Rather, the decoder of  FIG. 3  and the encoder of  FIG. 4  may be advantageously used elsewhere. 
     The multi-audio-object signal consists of a downmix signal  56  and side information  58 . The side information  58  comprises level information  60  describing, for example, spectral energies of the audio signal of the first type and the audio signal of the second type in a first predetermined time/frequency resolution such as, for example, the time/frequency resolution  42 . In particular, the level information  60  may comprise a normalized spectral energy scalar value per object and time/frequency tile. The normalization may be related to the highest spectral energy value among the audio signals of the first and second type at the respective time/frequency tile. The latter possibility results in OLDs for representing the level information, also called level difference information herein. Although the following embodiments use OLDs, they may, although not explicitly stated there, use an otherwise normalized spectral energy representation. 
     The side information  58  comprises also a residual signal  62  specifying residual level values in a second predetermined time/frequency resolution which may be equal to or different to the first predetermined time/frequency resolution. 
     The means  52  for computing prediction coefficients is configured to compute prediction coefficients based on the level information  60 . Additionally, means  52  may compute the prediction coefficients further based on inter-correlation information also comprised by side information  58 . Even further, means  52  may use time varying downmix prescription information comprised by side information  58  to compute the prediction coefficients. The prediction coefficients computed by means  52  are needed for retrieving or upmixing the original audio objects or audio signals from the downmix signal  56 . 
     Accordingly, means  54  for upmixing is configured to upmix the downmix signal  56  based on the prediction coefficients  64  received from means  52  and the residual signal  62 . By using the residual  62 , decoder  50  is able to better suppress cross talks from the audio signal of one type to the audio signal of the other type. In addition to the residual signal  62 , means  54  may use the time varying downmix prescription to upmix the downmix signal. Further, means  54  for upmixing may use user input  66  in order to decide which of the audio signals recovered from the downmix signal  56  to be actually output at output  68  or to what extent. As a first extreme, the user input  66  may instruct means  54  to merely output the first up-mix signal approximating the audio signal of the first type. The opposite is true for the second extreme according to which means  54  is to output merely the second up-mix signal approximating the audio signal of the second type. Intermediate options are possible as well according to which a mixture of both up-mix signals is rendered an output at output  68 . 
       FIG. 4  shows an embodiment for an audio encoder suitable for generating a multi-audio object signal decoded by the decoder of  FIG. 3 . The encoder of  FIG. 4  which is indicated by reference sign  80 , may comprise means  82  for spectrally decomposing in case the audio signals  84  to be encoded are not within the spectral domain. Among the audio signals  84 , in turn, there is at least one audio signal of a first type and at least one audio signal of a second type. The means  82  for spectrally decomposing is configured to spectrally decompose each of these signals  84  into a representation as shown in  FIG. 2 , for example. That is, the means  82  for spectrally decomposing spectrally decomposes the audio signals  84  at a predetermined time/frequency resolution. Means  82  may comprise a filter bank, such as a hybrid QMF bank. 
     The audio encoder  80  further comprises means  86  for computing level information, means  88  for downmixing, means  90  for computing prediction coefficients and means  92  for setting a residual signal. Additionally, audio encoder  80  may comprise means for computing inter-correlation information, namely means  94 . Means  86  computes level information describing the level of the audio signal of the first type and the audio signal of the second type in the first predetermined time/frequency resolution from the audio signal as optionally output by means  82 . Similarly, means  88  downmixes the audio signals. Means  88  thus outputs the downmix signal  56 . Means  86  also outputs the level information  60 . Means  90  for computing prediction coefficients acts similarly to means  52 . That is, means  90  computes prediction coefficients from the level information  60  and outputs the prediction coefficients  64  to means  92 . Means  92 , in turn, sets the residual signal  62  based on the downmix signal  56 , the predication coefficients  64  and the original audio signals at a second predetermined time/frequency resolution such that up-mixing the downmix signal  56  based on both the prediction coefficients  64  and the residual signal  62  results in a first up-mix audio signal approximating the audio signal of the first type and the second up-mix audio signal approximating the audio signal of the second type, the approximation being approved compared to the absence of the residual signal  62 . 
     The residual signal  62  and the level information  60  are comprised by the side information  58  which forms, along with the downmix signal  56 , the multi-audio-object signal to be decoded by decoder  FIG. 3 . 
     As shown in  FIG. 4 , and analogous to the description of  FIG. 3 , means  90  may additionally use the inter-correlation information output by means  94  and/or time varying downmix prescription output by means  88  to compute the prediction coefficient  64 . Further, by means  92  for setting the residual signal  62  may additionally use the time varying downmix prescription output by means  88  in order to appropriately set the residual signal  62 . 
     Again, it is noted that the audio signal of the first type may be a mono or stereo audio signal. The same applies for the audio signal of the second type. The residual signal  62  may be signaled within the side information in the same time/frequency resolution as the parameter time/frequency resolution used to compute, for example, the level information, or a different time/frequency resolution may be used. Further, it may be possible that the signaling of the residual signal is restricted to a sub-portion of the spectral range occupied by the time/frequency tiles  42  for which level information is signaled. For example, the time/frequency resolution at which the residual signal is signaled, may be indicated within the side information  58  by use of syntax elements bsResidualBands and bsResidualFramesPerSAOCFrame. These two syntax elements may define another sub-division of a frame into time/frequency tiles than the sub-division leading to tiles  42 . 
     By the way, it is noted that the residual signal  62  may or may not reflect information loss resulting from a potentially used core encoder  96  optionally used to encode the downmix signal  56  by audio encoder  80 . As shown in  FIG. 4 , means  92  may perform the setting of the residual signal  62  based on the version of the downmix signal re-constructible from the output of core coder  96  or from the version input into core encoder  96 ′. Similarly, the audio decoder  50  may comprise a core decoder  98  to decode or decompress downmix signal  56 . 
     The ability to set, within the multiple-audio-object signal, the time/frequency resolution used for the residual signal  62  different from the time/frequency resolution used for computing the level information  60  enables to achieve a good compromise between audio quality on the one hand and compression ratio of the multiple-audio-object signal on the other hand. In any case, the residual signal  62  enables to better suppress cross-talk from one audio signal to the other within the first and second up-mix signals to be output at output  68  according to the user input  66 . 
     As will become clear from the following embodiment, more than one residual signal  62  may be transmitted within the side information in case more than one foreground object or audio signal of the second type is encoded. The side information may allow for an individual decision as to whether a residual signal  62  is transmitted for a specific audio signal of a second type or not. Thus, the number of residual signals  62  may vary from one up to the number of audio signals of the second type. 
     In the audio decoder of  FIG. 3 , the means  54  for computing may be configured to compute a prediction coefficient matrix C consisting of the prediction coefficients based on the level information (OLD) and means  56  may be configured to yield the first up-mix signal S 1  and/or the second up-mix signal S 2  from the downmix signal d according to a computation representable by 
                 (           S   1               S   2           )     =       D     -   1       ⁢     {         (         1           C         )     ⁢   d     +   H     }         ,         
where the “1” denotes—depending on the number of channels of d—a scalar, or an identity matrix, and D −1  is a matrix uniquely determined by a downmix prescription according to which the audio signal of the first type and the audio signal of the second type are downmixed into the downmix signal, and which is also comprised by the side information, and H is a term being independent from d but dependent from the residual signal.
 
     As noted above and described further below, the downmix prescription may vary in time and/or may spectrally vary within the side information. If the audio signal of the first type is a stereo audio signal having a first (L) and a second input channel (R), the level information, for example, describes normalized spectral energies of the first input channel (L), the second input channel (R) and the audio signal of the second type, respectively, at the time/frequency resolution  42 . 
     The aforementioned computation according to which the means  56  for up-mixing performs the up-mixing may even be representable by 
                 (           L   ^               R   ^               S   2           )     =       D     -   1       ⁢     {         (         1           C         )     ⁢   d     +   H     }         ,         
wherein {circumflex over (L)} is a first channel of the first up-mix signal, approximating L and {circumflex over (R)} is a second channel of the first up-mix signal, approximating R, and the “1” is a scalar in case d is mono, and a 2×2 identity matrix in case d is stereo. If the downmix signal  56  is a stereo audio signal having a first (L 0 ) and second output channel (R 0 ), and the computation according to which the means  56  for up-mixing performs the up-mixing may be representable by
 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       L 
                       ^ 
                     
                   
                 
                 
                   
                     
                       R 
                       ^ 
                     
                   
                 
                 
                   
                     
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                       2 
                     
                   
                 
               
               ) 
             
             = 
             
               
                 D 
                 
                   - 
                   1 
                 
               
               ⁢ 
               
                 
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                                 L 
                                 ⁢ 
                                 
                                     
                                 
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                         ) 
                       
                     
                     + 
                     H 
                   
                   } 
                 
                 . 
               
             
           
         
       
     
     As far as the term H being dependent on the residual signal res is concerned, the computation according to which the means  56  for up-mixing performs the up-mixing may be representable by 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       S 
                       1 
                     
                   
                 
                 
                   
                     
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                       2 
                     
                   
                 
               
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     The multi-audio-object signal may even comprise a plurality of audio signals of the second type and the side information may comprise one residual signal per audio signal of the second type. A residual resolution parameter may be present in the side information defining a spectral range over which the residual signal is transmitted within the side information. It may even define a lower and an upper limit of the spectral range. 
     Further, the multi-audio-object signal may also comprise spatial rendering information for spatially rendering the audio signal of the first type onto a predetermined loudspeaker configuration. In other words, the audio signal of the first type may be a multi channel (more than two channels) MPEG Surround signal downmixed down to stereo. 
     In the following, embodiments will be described which make use of the above residual signal signaling. However, it is noted that the term “object” is often used in a double sense. Sometimes, an object denotes an individual mono audio signal. Thus, a stereo object may have a mono audio signal forming one channel of a stereo signal. However, at other situations, a stereo object may denote, in fact, two objects, namely an object concerning the right channel and a further object concerning the left channel of the stereo object. The actual sense will become apparent from the context. 
     Before describing the next embodiment, same is motivated by deficiencies realized with the baseline technology of the SAOC standard selected as reference model 0 (RM0) in 2007. The RM0 allowed the individual manipulation of a number of sound objects in terms of their panning position and amplification/attenuation. A special scenario has been presented in the context of a “Karaoke” type application. In this case
         a mono, stereo or surround background scene (in the following called Background Object, BGO) is conveyed from a set of certain SAOC objects, which is reproduced without alteration, i.e. every input channel signal is reproduced through the same output channel at an unaltered level, and   a specific object of interest (in the following called Foreground Object FGO) (typically the lead vocal) which is reproduced with alterations (the FGO is typically positioned in the middle of the sound stage and can be muted, i.e. attenuated heavily to allow sing-along).       

     As it is visible from subjective evaluation procedures, and could be expected from the underlying technology principle, manipulations of the object position lead to high-quality results, while manipulations of the object level are generally more challenging. Typically, the higher the additional signal amplification/attenuation is, the more potential artefacts arise. In this sense, the Karaoke scenario is extremely demanding since an extreme (ideally: total) attenuation of the FGO is necessitated. 
     The dual usage case is the ability to reproduce only the FGO without the background/MBO, and is referred to in the following as the solo mode. 
     It is noted, however, that if a surround background scene is involved, it is referred to as a Multi-Channel Background Object (MBO). The handling of the MBO is the following, which is shown in  FIG. 5 :
         The MBO is encoded using a regular 5-2-5 MPEG Surround tree  102 . This results in a stereo MBO downmix signal  104 , and an MBO MPS side information stream  106 .   The MBO downmix is then encoded by a subsequent SAOC encoder  108  as a stereo object, (i.e. two object level differences, plus an inter-channel correlation), together with the (or several) FGO  110 . This results in a common downmix signal  112 , and a SAOC side information stream  114 .       

     In the transcoder  116 , the downmix signal  112  is preprocessed and the SAOC and MPS side information streams  106 ,  114  are transcoded into a single MPS output side information stream  118 . This currently happens in a discontinuous way, i.e. either only full suppression of the FGO(s) is supported or full suppression of the MBO. 
     Finally, the resulting downmix  120  and MPS side information  118  are rendered by an MPEG Surround decoder  122 . 
     In  FIG. 5 , both the MBO downmix  104  and the controllable object signal(s)  110  are combined into a single stereo downmix  112 . This “pollution” of the downmix by the controllable object  110  is the reason for the difficulty of recovering a Karaoke version with the controllable object  110  being removed, which is of sufficiently high audio quality. The following proposal aims at circumventing this problem. 
     Assuming one FGO (e.g. one lead vocal), the key observation used by the following embodiment of  FIG. 6  is that the SAOC downmix signal is a combination of the BGO and the FGO signal, i.e. three audio signals are downmixed and transmitted via 2 downmix channels. Ideally, these signals should be separated again in the transcoder in order to produce a clean Karaoke signal (i.e. to remove the FGO signal), or to produce a clean solo signal (i.e. to remove the BGO signal). This is achieved, in accordance with the embodiment of  FIG. 6 , by using a “two-to-three” (TTT) encoder element  124  (TTT −1  as it is known from the MPEG Surround specification) within SAOC encoder  108  to combine the BGO and the FGO into a single SAOC downmix signal in the SAOC encoder. Here, the FGO feeds the “center” signal input of the TTT −1  box  124  while the BGO  104  feeds the “left/right” TTT −1  inputs L.R. The transcoder  116  can then produce approximations of the BGO  104  by using a TTT decoder element  126  (TTT as it is known from MPEG Surround), i.e. the “left/right” TTT outputs L,R carry an approximation of the BGO, whereas the “center” TTT output C carries an approximation of the FGO  110 . 
     When comparing the embodiment of  FIG. 6  with the embodiment of an encoder and decoder of  FIGS. 3 and 4 , reference sign  104  corresponds to the audio signal of the first type among audio signals  84 , means  82  is comprised by MPS encoder  102 , reference sign  110  corresponds to the audio signals of the second type among audio signal  84 , TTT −1  box  124  assumes the responsibility for the functionalities of means  88  to  92 , with the functionalities of means  86  and  94  being implemented in SAOC encoder  108 , reference sign  112  corresponds to reference sign  56 , reference sign  114  corresponds to side information  58  less the residual signal  62 , TTT box  126  assumes responsibility for the functionality of means  52  and  54  with the functionality of the mixing box  128  also being comprised by means  54 . Lastly, signal  120  corresponds to the signal output at output  68 . Further, it is noted that  FIG. 6  also shows a core coder/decoder path  131  for the transport of the down mix  112  from SAOC encoder  108  to SAOC transcoder  116 . This core coder/decoder path  131  corresponds to the optional core coder  96  and core decoder  98 . As indicated in  FIG. 6 , this core coder/decoder path  131  may also encode/compress the side information transported signal from encoder  108  to transcoder  116 . 
     The advantages resulting from the introduction of the TTT box of  FIG. 6  will become clear by the following description. For example, by
         simply feeding the “left/right” TTT outputs L.R. into the MPS downmix  120  (and passing on the transmitted MBO MPS bitstream  106  in stream  118 ), only the MBO is reproduced by the final MPS decoder. This corresponds to the Karaoke mode.   simply feeding the “center” TTT output C. into left and right MPS downmix  120  (and producing a trivial MPS bitstream  118  that renders the FGO  110  to the desired position and level), only the FGO  110  is reproduced by the final MPS decoder  122 . This corresponds to the Solo mode.       

     The handling of the three TTT output signals L.R.C. is performed in the “mixing” box  128  of the SAOC transcoder  116 . 
     The processing structure of  FIG. 6  provides a number of distinct advantages over  FIG. 5 :
         The framework provides a clean structural separation of background (MBO)  100  and FGO signals  110     The structure of the TTT element  126  attempts a best possible reconstruction of the three signals L.R.C. on a waveform basis. Thus, the final MPS output signals  130  are not only formed by energy weighting (and decorrelation) of the downmix signals, but also are closer in terms of waveforms due to the TTT processing.   Along with the MPEG Surround TTT box  126  comes the possibility to enhance the reconstruction precision by using residual coding. In this way, a significant enhancement in reconstruction quality can be achieved as the residual bandwidth and residual bitrate for the residual signal  132  output by TTT −1    124  and used by TTT box for upmixing are increased. Ideally (i.e. for infinitely fine quantization in the residual coding and the coding of the downmix signal), the interference between the background (MBO) and the FGO signal is cancelled.       

     The processing structure of  FIG. 6  possesses a number of characteristics:
         Duality Karaoke/Solo mode: The approach of  FIG. 6  offers both Karaoke and Solo functionality by using the same technical means. That is, SAOC parameters are reused, for example.   Refineability: The quality of the Karaoke/Solo signal can be refined as needed by controlling the amount of residual coding information used in the TTT boxes. For example, parameters bsResidualSamplingFrequencyIndex, bsResidualBands and bsResidualFramesPerSAOCFrame may be used.   Positioning of FGO in downmix: When using a TTT box as specified in the MPEG Surround specification, the FGO would be mixed into the center position between the left and right downmix channels. In order to allow more flexibility in positioning, a generalized TTT encoder box is employed which follows the same principles while allowing non-symmetric positioning of the signal associated to the “center” inputs/outputs.   Multiple FGOs: In the configuration described, the use of only one FGO was described (this may correspond to the most important application case). However, the proposed concept is also able to accommodate several FGOs by using one or a combination of the following measures:
           Grouped FGOs: Like shown in  FIG. 6 , the signal that is connected to the center input/output of the TTT box can actually be the sum of several FGO signals rather than only a single one. These FGOs can be independently positioned/controlled in the multi-channel output signal  130  (maximum quality advantage is achieved, however, when they are scaled &amp; positioned in the same way). They share a common position in the stereo downmix signal  112 , and there is only one residual signal  132 . In any case, the interference between the background (MBO) and the controllable objects is cancelled (although not between the controllable objects).   Cascaded FGOs: The restrictions regarding the common FGO position in the downmix  112  can be overcome by extending the approach of  FIG. 6 . Multiple FGOs can be accommodated by cascading several stages of the described TTT structure, each stage corresponding to one FGO and producing a residual coding stream. In this way, interference ideally would be cancelled also between each FGO. Of course, this option necessitates a higher bitrate than using a grouped FGO approach. An example will be described later.   
           SAOC side information: In MPEG Surround, the side information associated to a TTT box is a pair of Channel Prediction Coefficients (CPCs). In contrast, the SAOC parametrization and the MBO/Karaoke scenario transmit object energies for each object signal, and an inter-signal correlation between the two channels of the MBO downmix (i.e. the parametrization for a “stereo object”). In order to minimize the number of changes in the parametrization relative to the case without the enhanced Karaoke/Solo mode, and thus bitstream format, the CPCs can be calculated from the energies of the downmixed signals (MBO downmix and FGOs) and the inter-signal correlation of the MBO downmix stereo object. Therefore, there is no need to change or augment the transmitted parametrization and the CPCs can be calculated from the transmitted SAOC parametrization in the SAOC transcoder  116 . In this way, a bitstream using the Enhanced Karaoke/Solo mode could also be decoded by a regular mode decoder (without residual coding) when ignoring the residual data.       

     In summary, the embodiment of  FIG. 6  aims at an enhanced reproduction of certain selected objects (or the scene without those objects) and extends the current SAOC encoding approach using a stereo downmix in the following way:
         In the normal mode, each object signal is weighted by its entries in the downmix matrix (for its contribution to the left and to the right downmix channel, respectively). Then, all weighted contributions to the left and right downmix channel are summed to form the left and right downmix channels.   For enhanced Karaoke/Solo performance, i.e. in the enhanced mode, all object contributions are partitioned into a set of object contributions that form a Foreground Object (FGO) and the remaining object contributions (BGO). The FGO contribution is summed into a mono downmix signal, the remaining background contributions are summed into a stereo downmix, and both are summed using a generalized TTT encoder element to form the common SAOC stereo downmix.       

     Thus, a regular summation is replaced by a “TTT summation” (which can be cascaded when desired). 
     In order to emphasize the just-mentioned difference between the normal mode of the SAOC encoder and the enhanced mode, reference is made to  FIGS. 7   a  and  7   b , where  FIG. 7   a  concerns the normal mode, whereas  FIG. 7   b  concerns the enhanced mode. As can be seen, in the normal mode, the SAOC encoder  108  uses the afore-mentioned DMX parameters D ij  for weighting objects j and adding the thus weighed object j to SAOC channel i, i.e. L 0  or R 0 . In case of the enhanced mode of  FIG. 6 , merely a vector of DMX-parameters D i  is needed, namely, DMX-parameters D i  indicating how to form a weighted sum of the FGOs  110 , thereby obtaining the center channel C for the TTT −1  box  124 , and DMX-parameters D i , instructing the TTT −1  box how to distribute the center signal C to the left MBO channel and the right MBO channel respectively, thereby obtaining the L DMX  or R DMX  respectively. 
     Problematically, the processing according to  FIG. 6  does not work very well with non-waveform preserving codecs (HE-AAC/SBR). A solution for that problem may be an energy-based generalized TTT mode for HE-AAC and high frequencies. An embodiment addressing the problem will be described later. 
     A possible bitstream format for the one with cascaded TTTs could be as follows: 
     An addition to the SAOC bitstream that needs to be able to be skipped if to be digested in “regular decode mode”: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 numTTTs  int 
               
               
                   
                 for (ttt=0; ttt&lt;numTTTs; ttt++) 
               
               
                   
                 {  no_TTT_obj[ttt]  int 
               
               
                   
                    TTT_bandwidth[ttt]; 
               
               
                   
                    TTT_residual_stream[ttt] 
               
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     As to complexity and memory requirements, the following can be stated. As can be seen from the previous explanations, the enhanced Karaoke/Solo mode of  FIG. 6  is implemented by adding stages of one conceptual element in the encoder and decoder/transcoder each, i.e. the generalized TTT-1/TTT encoder element. Both elements are identical in their complexity to the regular “centered” TTT counterparts (the change in coefficient values does not influence complexity). For the envisaged main application (one FGO as lead vocals), a single TTT is sufficient. 
     The relation of this additional structure to the complexity of an MPEG Surround system can be appreciated by looking at the structure of an entire MPEG Surround decoder which for the relevant stereo downmix case (5-2-5 configuration) consists of one TTT element and 2 OTT elements. This already shows that the added functionality comes at a moderate price in terms of computational complexity and memory consumption (note that conceptual elements using residual coding are on average no more complex than their counterparts which include decorrelators instead). 
     This extension of  FIG. 6  of the MPEG SAOC reference model provides an audio quality improvement for special solo or mute/Karaoke type of applications. Again it is noted, that the description corresponding to  FIGS. 5 ,  6  and  7  refer to a MBO as background scene or BGO, which in general is not limited to this type of object and can rather be a mono or stereo object, too. 
     A subjective evaluation procedure reveals the improvement in terms of audio quality of the output signal for a Karaoke or solo application. The conditions evaluated are:
         RM0   Enhanced mode (res 0) (=without residual coding)   Enhanced mode (res 6) (=with residual coding in the lowest 6 hybrid QMF bands)   Enhanced mode (res 12) (=with residual coding in the lowest 12 hybrid QMF bands)   Enhanced mode (res 24) (=with residual coding in the lowest 24 hybrid QMF bands)   Hidden Reference   Lower anchor (3.5 kHz band limited version of reference)       

     The bitrate for the proposed enhanced mode is similar to RM0 if used without residual coding. All other enhanced modes necessitate about 10 kbit/s for every 6 bands of residual coding. 
       FIG. 8   a  shows the results for the mute/Karaoke test with 10 listening subjects. The proposed solution has an average MUSHRA score which is higher than RM0 and increases with each step of additional residual coding. A statistically significant improvement over the performance of RM0 can be clearly observed for modes with 6 and more bands of residual coding. 
     The results for the solo test with 9 subjects in  FIG. 8   b  show similar advantages for the proposed solution. The average MUSHRA score is clearly increased when adding more and more residual coding. The gain between enhanced mode without and enhanced mode with 24 bands of residual coding is almost 50 MUSHRA points. 
     Overall, for a Karaoke application good quality is achieved at the cost of a ca. 10 kbit/s higher bitrate than RM0. Excellent quality is possible when adding ca. 40 kbit/s on top of the bitrate of RM0. In a realistic application scenario where a maximum fixed bitrate is given, the proposed enhanced mode nicely allows to spend “unused bitrate” for residual coding until the permissible maximum rate is reached. Therefore, the best possible overall audio quality is achieved. A further improvement over the presented experimental results is possible due to a more intelligent usage of residual bitrate: While the presented setup was using residual coding from DC to a certain upper border frequency, an enhanced implementation would spend only bits for the frequency range that is relevant for separating FGO and background objects. 
     In the foregoing description, an enhancement of the SAOC technology for the Karaoke-type applications has been described. Additional detailed embodiments of an application of the enhanced Karaoke/solo mode for multi-channel FGO audio scene processing for MPEG SAOC are presented. 
     In contrast to the FGOs, which are reproduced with alterations, the MBO signals have to be reproduced without alteration, i.e. every input channel signal is reproduced through the same output channel at an unchanged level. Consequently, the preprocessing of the MBO signals by an MPEG Surround encoder had been proposed yielding a stereo downmix signal that serves as a (stereo) background object (BGO) to be input to the subsequent Karaoke/solo mode processing stages comprising an SAOC encoder, an MBO transcoder and an MPS decoder.  FIG. 9  shows a diagram of the overall structure, again. 
     As can be seen, according to the Karaoke/solo mode coder structure, the input objects are classified into a stereo background object (EGO)  104  and foreground objects (FGO)  110 . 
     While in RM0 the handling of these application scenarios is performed by an SAOC encoder/transcoder system, the enhancement of  FIG. 6  additionally exploits an elementary building block of the MPEG Surround structure. Incorporating the three-to-two (TTT −1 ) block at the encoder and the corresponding two-to-three (TTT) complement at the transcoder improves the performance when strong boost/attenuation of the particular audio object is necessitated. The two primary characteristics of the extended structure are:
         better signal separation due to exploitation of the residual signal (compared to RM0),   flexible positioning of the signal that is denoted as the center input (i.e. the FGO) of the TTT −1  box by generalizing its mixing specification.       

     Since the straightforward implementation of the TTT building block involves three input signals at encoder side,  FIG. 6  was focused on the processing of FGOs as a (downmixed) mono signal as depicted in  FIG. 10 . The treatment of multi-channel FGO signals has been stated, too, but will be explained in more detail in the subsequent chapter. 
     As can be seen from  FIG. 10 , in the enhanced mode of  FIG. 6 , a combination of all FGOs is fed into the center channel of the TTT −1  box. 
     In case of an FGO mono downmix as is the case with  FIG. 6  and  FIG. 10 , the configuration of the TTT −1  box at the encoder comprises the FGO that is fed to the center input and the BGO providing the left and right input. The underlying symmetric matrix is given by: 
               D   =     (         1       0         m   1             0       1         m   2               m   1           m   2           -   1           )       ,         
which provides the downmix (L 0  R 0 ) T  and a signal F 0 :
 
     
       
         
           
             
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                       L 
                       ⁢ 
                       
                           
                       
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                         R 
                       
                     
                     
                       
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               . 
             
           
         
       
     
     The 3 rd  signal obtained through this linear system is discarded, but can be reconstructed at transcoder side incorporating two prediction coefficients c 1  and c 2  (CPC) according to:
 
 {circumflex over (F)} 0= c   1   L 0+ c   2   R 0.
 
     The inverse process at the transcoder is given by: 
     
       
         
           
             
               
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     The parameters m 1  and m 2  correspond to:
 
 m   1 =cos(μ) and  m   2 =sin(μ)
 
and μ is responsible for panning the FGO in the common TTT downmix (L 0  R 0 ) T . The prediction coefficients c 1  and c 2  necessitated by the TTT upmix unit at transcoder side can be estimated using the transmitted SAOC parameters, i.e. the object level differences (OLDs) for all input audio objects and inter-object correlation (IOC) for BGO downmix (MBO) signals. Assuming statistical independence of FGO and BGO signals the following relationship holds for the CPC estimation:
 
     
       
         
           
             
               
                 c 
                 1 
               
               = 
               
                 
                   
                     
                       P 
                       LoFo 
                     
                     ⁢ 
                     
                       P 
                       Ro 
                     
                   
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                       P 
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                     ⁢ 
                     
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     The variables P Lo , P Ro , P LoRo , P LoFo  and P RoFo  can be estimated as follows, where the parameters OLD L , OLD R  and IOC LR  correspond to the BGO, and OLD F  is an FGO parameter:
 
 P   Lo =OLD L   +m   1   2 OLD F ,
 
 P   Ro =OLD R   +m   2   2 OLD F ,
 
 P   LoRo =IOC LR   +m   1   m   2 OLD F ,
 
 P   LoFo   =m   1 (OLD L −OLD F )+ m   2 IOC LR ,
 
 P   RoFo   =m   2 (OLD R −OLD F )+ m   1 IOC LR .
 
     Additionally, the error introduced by the implication of the CPCs is represented by the residual signal  132  that can be transmitted within the bitstream, such that:
 
res= F 0− {circumflex over (F)} 0.
 
     In some application scenarios the restriction of a single mono downmix of all FGOs is inappropriate, hence needs to be overcome. For example, the FGOs can be divided into two or more independent groups with different positions in the transmitted stereo downmix and/or individual attenuation. Therefore, the cascaded structure shown in  FIG. 11  implies two or more consecutive TTT −1  elements  124   a ,  124   b , yielding a step-by-step downmixing of all FGO groups F 1 , F 2  at encoder side until the desired stereo downmix  112  is obtained. Each—or at least some—of the TTT −1  boxes  124   a,b  (in  FIG. 11  each) sets a residual signal  132   a ,  132   b  corresponding to the respective stage or TTT −1  box  124   a,b  respectively. Conversely, the transcoder performs sequential upmixing by use of respective sequentially applied TTT boxes  126   a,b , incorporating the corresponding CPCs and residual signals, where available. The order of the FGO processing is encoder-specified and must be considered at transcoder side. 
     The detailed mathematics involved with the two-stage cascade shown in  FIG. 11  is described in the following. 
     Without loss in generality, but for a simplified illustration the following explanation is based on a cascade consisting of two TTT elements as shown in  FIG. 11 . The two symmetric matrices are similar to the FGO mono downmix, but have to be applied adequately to the respective signals: 
     
       
         
           
             
               D 
               1 
             
             = 
             
               ( 
               
                 
                   
                     1 
                   
                   
                     0 
                   
                   
                     
                       m 
                       11 
                     
                   
                 
                 
                   
                     0 
                   
                   
                     1 
                   
                   
                     
                       m 
                       21 
                     
                   
                 
                 
                   
                     
                       m 
                       11 
                     
                   
                   
                     
                       m 
                       21 
                     
                   
                   
                     
                       - 
                       1 
                     
                   
                 
               
               ) 
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               D 
               2 
             
             = 
             
               
                 ( 
                 
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       
                         m 
                         12 
                       
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       
                         m 
                         22 
                       
                     
                   
                   
                     
                       
                         m 
                         12 
                       
                     
                     
                       
                         m 
                         22 
                       
                     
                     
                       
                         - 
                         1 
                       
                     
                   
                 
                 ) 
               
               . 
             
           
         
       
     
     Here, the two sets of CPCs result in the following signal reconstruction:
 
 {circumflex over (F)} 0 1   =c   11   L 0 1   +c   12   R 0 1  and  {circumflex over (F)} 0 2   =c   21   L 0 2   +c   22   R 0 2 .
 
     The inverse process is represented by: 
     
       
         
           
             
               
                 D 
                 1 
                 
                   - 
                   1 
                 
               
               = 
               
                 
                   1 
                   
                     1 
                     + 
                     
                       m 
                       11 
                       2 
                     
                     + 
                     
                       m 
                       21 
                       2 
                     
                   
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       
                         
                           1 
                           + 
                           
                             m 
                             21 
                             2 
                           
                           + 
                           
                             
                               c 
                               11 
                             
                             ⁢ 
                             
                               m 
                               11 
                             
                           
                         
                       
                       
                         
                           
                             
                               - 
                               
                                 m 
                                 11 
                               
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                           + 
                           
                             
                               c 
                               12 
                             
                             ⁢ 
                             
                               m 
                               11 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               - 
                               
                                 m 
                                 11 
                               
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                           + 
                           
                             
                               c 
                               11 
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                         
                       
                       
                         
                           1 
                           + 
                           
                             m 
                             11 
                             2 
                           
                           + 
                           
                             
                               c 
                               12 
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             m 
                             11 
                           
                           - 
                           
                             c 
                             11 
                           
                         
                       
                       
                         
                           
                             m 
                             21 
                           
                           - 
                           
                             c 
                             12 
                           
                         
                       
                     
                   
                   ) 
                 
               
             
             , 
             
               
 
             
             ⁢ 
             and 
           
         
       
       
         
           
             
               D 
               2 
               
                 - 
                 1 
               
             
             = 
             
               
                 1 
                 
                   1 
                   + 
                   
                     m 
                     12 
                     2 
                   
                   + 
                   
                     m 
                     22 
                     2 
                   
                 
               
               ⁢ 
               
                 
                   ( 
                   
                     
                       
                         
                           1 
                           + 
                           
                             m 
                             22 
                             2 
                           
                           + 
                           
                             
                               c 
                               21 
                             
                             ⁢ 
                             
                               m 
                               12 
                             
                           
                         
                       
                       
                         
                           
                             
                               - 
                               
                                 m 
                                 12 
                               
                             
                             ⁢ 
                             
                               m 
                               22 
                             
                           
                           + 
                           
                             
                               c 
                               22 
                             
                             ⁢ 
                             
                               m 
                               12 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               - 
                               
                                 m 
                                 12 
                               
                             
                             ⁢ 
                             
                               m 
                               22 
                             
                           
                           + 
                           
                             
                               c 
                               21 
                             
                             ⁢ 
                             
                               m 
                               22 
                             
                           
                         
                       
                       
                         
                           1 
                           + 
                           
                             m 
                             12 
                             2 
                           
                           + 
                           
                             
                               c 
                               22 
                             
                             ⁢ 
                             
                               m 
                               22 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             m 
                             12 
                           
                           - 
                           
                             c 
                             21 
                           
                         
                       
                       
                         
                           
                             m 
                             22 
                           
                           - 
                           
                             c 
                             22 
                           
                         
                       
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     A special case of the two-stage cascade comprises one stereo FGO with its left and right channel being summed properly to the corresponding channels of the BGO, yielding μ=0 and 
     
       
         
           
             
               μ 
               2 
             
             = 
             
               
                 π 
                 2 
               
               ⁢ 
               
                 : 
               
             
           
         
       
     
     
       
         
           
             
               
                 D 
                 L 
               
               = 
               
                 ( 
                 
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       1 
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       0 
                     
                   
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       
                         - 
                         1 
                       
                     
                   
                 
                 ) 
               
             
             , 
             
               
 
             
             ⁢ 
             and 
           
         
       
       
         
           
             
               D 
               R 
             
             = 
             
               
                 ( 
                 
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       1 
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       
                         - 
                         1 
                       
                     
                   
                 
                 ) 
               
               . 
             
           
         
       
     
     For this particular panning style and by neglecting the inter-object correlation, OLD LR =0 the estimation of two sets of CPCs reduce to: 
                 c     L   ⁢           ⁢   1       =         O   ⁢           ⁢   L   ⁢           ⁢     D   L       -     O   ⁢           ⁢   L   ⁢           ⁢     D   FL             O   ⁢           ⁢   L   ⁢           ⁢     D   L       +     O   ⁢           ⁢   L   ⁢           ⁢     D   FL             ,     
     ⁢       c     L   ⁢           ⁢   2       =   0     ,     
     ⁢       c     R   ⁢           ⁢   1       =   0     ,     
     ⁢       c     R   ⁢           ⁢   2       =         O   ⁢           ⁢   L   ⁢           ⁢     D   R       -     O   ⁢           ⁢   L   ⁢           ⁢     D   FR             O   ⁢           ⁢   L   ⁢           ⁢     D   R       +     O   ⁢           ⁢   L   ⁢           ⁢     D   FR             ,         
with OLD FL  and OLD FR  denoting the OLDs of the left and right FGO signal, respectively.
 
     The general N-stage cascade case refers to a multi-channel FGO downmix according to: 
                 D   1     =     (         1       0         m   11             0       1         m   21               m   11           m   21           -   1           )       ,     
     ⁢       D   2     =     (         1       0         m   12             0       1         m   22               m   12           m   22           -   1           )       ,   …   ⁢           ,     
     ⁢       D   N     =       (         1       0         m     1   ⁢           ⁢   N               0       1         m     2   ⁢   N                 m     1   ⁢   N             m     2   ⁢   N             -   1           )     .             
where each stage features its own CPCs and residual signal.
 
     At the transcoder side, the inverse cascading steps are given by: 
     
       
         
           
             
               
                 D 
                 1 
                 
                   - 
                   1 
                 
               
               = 
               
                 
                   1 
                   
                     1 
                     + 
                     
                       m 
                       11 
                       2 
                     
                     + 
                     
                       m 
                       21 
                       2 
                     
                   
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       
                         
                           1 
                           + 
                           
                             m 
                             21 
                             2 
                           
                           + 
                           
                             
                               c 
                               11 
                             
                             ⁢ 
                             
                               m 
                               11 
                             
                           
                         
                       
                       
                         
                           
                             
                               - 
                               
                                 m 
                                 11 
                               
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                           + 
                           
                             
                               c 
                               12 
                             
                             ⁢ 
                             
                               m 
                               11 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               - 
                               
                                 m 
                                 11 
                               
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                           + 
                           
                             
                               c 
                               11 
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                         
                       
                       
                         
                           1 
                           + 
                           
                             m 
                             11 
                             2 
                           
                           + 
                           
                             
                               c 
                               12 
                             
                             ⁢ 
                             
                               m 
                               21 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             m 
                             11 
                           
                           - 
                           
                             c 
                             11 
                           
                         
                       
                       
                         
                           
                             m 
                             21 
                           
                           - 
                           
                             c 
                             12 
                           
                         
                       
                     
                   
                   ) 
                 
               
             
             , 
             … 
             ⁢ 
             
                 
             
             , 
             
               
 
             
             ⁢ 
             
               
                 D 
                 N 
                 
                   - 
                   1 
                 
               
               = 
               
                 
                   1 
                   
                     1 
                     + 
                     
                       m 
                       
                         1 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         N 
                       
                       2 
                     
                     + 
                     
                       m 
                       
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         N 
                       
                       2 
                     
                   
                 
                 ⁢ 
                 
                   
                     ( 
                     
                       
                         
                           
                             1 
                             + 
                             
                               m 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 N 
                               
                               2 
                             
                             + 
                             
                               
                                 c 
                                 
                                   N 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 m 
                                 
                                   1 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   N 
                                 
                               
                             
                           
                         
                         
                           
                             
                               
                                 - 
                                 
                                   m 
                                   
                                     1 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     N 
                                   
                                 
                               
                               ⁢ 
                               
                                 m 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   N 
                                 
                               
                             
                             + 
                             
                               
                                 c 
                                 
                                   N 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               ⁢ 
                               
                                 m 
                                 
                                   1 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   N 
                                 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               
                                 - 
                                 
                                   m 
                                   
                                     1 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     N 
                                   
                                 
                               
                               ⁢ 
                               
                                 m 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   N 
                                 
                               
                             
                             + 
                             
                               
                                 c 
                                 
                                   N 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 m 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   N 
                                 
                               
                             
                           
                         
                         
                           
                             1 
                             + 
                             
                               m 
                               
                                 1 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 N 
                               
                               2 
                             
                             + 
                             
                               
                                 c 
                                 
                                   N 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               ⁢ 
                               
                                 m 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   N 
                                 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               m 
                               
                                 1 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 N 
                               
                             
                             - 
                             
                               c 
                               
                                 N 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                         
                         
                           
                             
                               m 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 N 
                               
                             
                             - 
                             
                               c 
                               
                                 N 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     To abolish the necessity of preserving the order of the TTT elements, the cascaded structure can easily be converted into an equivalent parallel by rearranging the N matrices into one single symmetric TTN matrix, thus yielding a general TTN style: 
                 D   N     =     (         1       0         m   11         …         m     1   ⁢   N               0       1         m   21         …         m     2   ⁢   N                 m   11           m   21           -   1         …       0           …       …       …       ⋱       ⋮             m     1   ⁢   N             m     2   ⁢   N           0       …         -   1           )       ,         
where the first two lines of the matrix denote the stereo downmix to be transmitted. On the other hand, the term TTN—two-to-N—refers to the upmixing process at transcoder side.
 
     Using this description the special case of the particularly panned stereo FGO reduces the matrix to: 
     
       
         
           
             D 
             = 
             
               
                 ( 
                 
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       1 
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       0 
                     
                     
                       1 
                     
                   
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       
                         - 
                         1 
                       
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       0 
                     
                     
                       
                         - 
                         1 
                       
                     
                   
                 
                 ) 
               
               . 
             
           
         
       
     
     Accordingly this unit can be termed two-to-four element or TTF. 
     It is also possible to yield a TTF structure reusing the SAOC stereo preprocessor module. 
     For the limitation of N=4 an implementation of the two-to-four (TTF) structure which reuses parts of the existing SAOC system becomes feasible. The processing is described in the following paragraphs. 
     The SAOC standard text describes the stereo downmix preprocessing for the “stereo-to-stereo transcoding mode”. Precisely the output stereo signal Y is calculated from the input stereo signal X together with a decorrelated signal X d  as follows:
 
 Y=G   mod   X+P   2   X   d  
 
     The decorrelated component X d  is a synthetic representation of parts of the original rendered signal which have already been discarded in the encoding process. According to  FIG. 12 , the decorrelated signal is replaced with a suitable encoder generated residual signal  132  for a certain frequency range. 
     The nomenclature is defined as:
         D is a 2×N downmix matrix   A is a 2×N rendering matrix   E is a model of the N×N covariance of the input objects S   G Mod  (corresponding to G in  FIG. 12 ) is the predictive 2×2 upmix matrix       

     Note that G Mod  is a function of D, A and E. 
     To calculate the residual signal X Res  the decoder processing may be mimicked in the encoder, i.e. to determine G Mod . In general scenarios A is not known, but in the special case of a Karaoke scenario (e.g. with one stereo background and one stereo foreground object, N=4) it is assumed that 
             A   =     (         0       0       1       0           0       0       0       1         )           
which means that only the BGO is rendered.
 
     For an estimation of the foreground object the reconstructed background object is subtracted from the downmix signal X. This and the final rendering is performed in the “Mix” processing block. Details are presented in the following. 
     The rendering matrix A is set to 
               A   BGO     =     (         0       0       1       0           0       0       0       1         )           
where it is assumed that the first 2 columns represent the 2 channels of the FGO and the second 2 columns represent the 2 channels of the BGO.
 
     The BGO and FGO stereo output is calculated according to the following formulas.
 
 Y   BGO   =G   Mod   X+X   Res  
 
     As the downmix weight matrix D is defined as 
             D   =     (       D   FGO     |     D   BGO       )               with               D   BGO     =     (           d   11           d   12               d   21           d   22           )               and               Y   BGO     =     (           y   BGO   1               y   BGO   r           )           
the FGO object can be set to
 
     
       
         
           
             
               Y 
               FGO 
             
             = 
             
               
                 D 
                 BGO 
                 
                   - 
                   1 
                 
               
               · 
               
                 [ 
                 
                   X 
                   - 
                   
                     ( 
                     
                       
                         
                           
                             
                               
                                 d 
                                 11 
                               
                               · 
                               
                                 y 
                                 BGO 
                                 1 
                               
                             
                             + 
                             
                               
                                 d 
                                 12 
                               
                               · 
                               
                                 y 
                                 BGO 
                                 r 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               
                                 d 
                                 21 
                               
                               · 
                               
                                 y 
                                 BGO 
                                 1 
                               
                             
                             + 
                             
                               
                                 d 
                                 22 
                               
                               · 
                               
                                 y 
                                 BGO 
                                 r 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                 
                 ] 
               
             
           
         
       
     
     As an example, this reduces to
 
 Y   FGO   =X−Y   BGO  
 
for a downmix matrix of
 
     
       
         
           
             D 
             = 
             
               ( 
               
                 
                   
                     1 
                   
                   
                     0 
                   
                   
                     1 
                   
                   
                     0 
                   
                 
                 
                   
                     0 
                   
                   
                     1 
                   
                   
                     0 
                   
                   
                     1 
                   
                 
               
               ) 
             
           
         
       
     
     X Res  are the residual signals obtained as described above. Please note that no decorrelated signals are added. 
     The final output Y is given by 
     
       
         
           
             Y 
             = 
             
               A 
               · 
               
                 ( 
                 
                   
                     
                       
                         Y 
                         FGO 
                       
                     
                   
                   
                     
                       
                         Y 
                         BGO 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The above embodiments can also be applied if a mono FGO instead of a stereo FGO is used. The processing is then altered according to the following. 
     The rendering matrix A is set to 
               A   FGO     =     (         1       0       0           0       0       0         )           
where it is assumed that the first column represents the mono FGO and the subsequent columns represent the 2 channels of the BGO.
 
     The BGO and FGO stereo output is calculated according to the following formulas.
 
 Y   FGO   =G   Mod   X+X   Res  
 
     As the downmix weight matrix D is defined as 
             D   =     (       D   FGO     |     D   BGO       )               with               D   FGO     =     (           d   FGO   1               d   FGO   r           )               and               Y   FGO     =     (           y   FGO             0         )           
the BGO object can be set to
 
     
       
         
           
             
               Y 
               BGO 
             
             = 
             
               
                 D 
                 BGO 
                 
                   - 
                   1 
                 
               
               · 
               
                 [ 
                 
                   X 
                   - 
                   
                     ( 
                     
                       
                         
                           
                             
                               d 
                               FGO 
                               1 
                             
                             · 
                             
                               y 
                               FGO 
                             
                           
                         
                       
                       
                         
                           
                             
                               d 
                               FGO 
                               r 
                             
                             · 
                             
                               y 
                               FGO 
                             
                           
                         
                       
                     
                     ) 
                   
                 
                 ] 
               
             
           
         
       
     
     As an example, this reduces to 
               Y   BGO     =     X   -     (           y   FGO               y   FGO           )             
for a downmix matrix of
 
     
       
         
           
             D 
             = 
             
               ( 
               
                 
                   
                     1 
                   
                   
                     1 
                   
                   
                     0 
                   
                 
                 
                   
                     1 
                   
                   
                     0 
                   
                   
                     1 
                   
                 
               
               ) 
             
           
         
       
     
     X Res  are the residual signals obtained as described above. Please note that no decorrelated signals are added. 
     The final output Y is given by 
     
       
         
           
             Y 
             = 
             
               A 
               · 
               
                 ( 
                 
                   
                     
                       
                         Y 
                         FGO 
                       
                     
                   
                   
                     
                       
                         Y 
                         BGO 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
     For the handling of more than 4 FGO objects, the above embodiments can be extended by assembling parallel stages of the processing steps just described. 
     The above just-described embodiments provided the detailed description of the enhanced Karaoke/solo mode for the cases of multi-channel FGO audio scene. This generalization aims to enlarge the class of Karaoke application scenarios, for which the sound quality of the MPEG SAOC reference model can be further improved by application of the enhanced Karaoke/solo mode. The improvement is achieved by introducing a general NTT structure into the downmix part of the SAOC encoder and the corresponding counterparts into the SAOCtoMPS transcoder. The use of residual signals enhanced the quality result. 
       FIGS. 13   a  to  13   h  show a possible syntax of the SAOC side information bit stream according to an embodiment of the present invention. 
     After having described some embodiments concerning an enhanced mode for the SAOC codec, it should be noted that some of the embodiments concern application scenarios where the audio input to the SAOC encoder contains not only regular mono or stereo sound sources but multi-channel objects. This was explicitly described with respect to  FIGS. 5 to 7   b . Such multi-channel background object MBO can be considered as a complex sound scene involving a large and often unknown number of sound sources, for which no controllable rendering functionality is necessitated. Individually, these audio sources cannot be handled efficiently by the SAOC encoder/decoder architecture. The concept of the SAOC architecture may, therefore, be thought of being extended in order to deal with these complex input signals, i.e., MBO channels, together with the typical SAOC audio objects. Therefore, in the just-mentioned embodiments of  FIGS. 5 to 7   b , the MPEG Surround encoder is thought of being incorporated into the SAOC encoder as indicated by the dotted line surrounding SAOC encoder  108  and MPS encoder  100 . The resulting downmix  104  serves as a stereo input object to the SAOC encoder  108  together with a controllable SAOC object  110  producing a combined stereo downmix  112  transmitted to the transcoder side. In the parameter domain, both the MPS bit stream  106  and the SAOC bit stream  114  are fed into the SAOC transcoder  116  which, depending on the particular MBO applications scenario, provides the appropriate MPS bit stream  118  for the MPEG Surround decoder  122 . This task is performed using the rendering information or rendering matrix and employing some downmix pre-processing in order to transform the downmix signal  112  into a downmix signal  120  for the MPS decoder  122 . 
     A further embodiment for an enhanced Karaoke/Solo mode is described below. It allows the individual manipulation of a number of audio objects in terms of their level amplification/attenuation without significant decrease in the resulting sound quality. A special “Karaoke-type” application scenario necessitates a total suppression of the specific objects, typically the lead vocal, (in the following called ForeGround Object FGO) keeping the perceptual quality of the background sound scene unharmed. It also entails the ability to reproduce the specific FGO signals individually without the static background audio scene (in the following called BackGround Object BGO), which does not necessitate user controllability in terms of panning. This scenario is referred to as a “Solo” mode. A typical application case contains a stereo BGO and up to four FGO signals, which can, for example, represent two independent stereo objects. 
     According to this embodiment and  FIG. 14 , the enhanced Karaoke/Solo transcoder  150  incorporates either a “two-to-N” (TTN) or “one-to-N” (OTN) element  152 , both representing a generalized and enhanced modification of the TTT box known from the MPEG Surround specification. The choice of the appropriate element depends on the number of downmix channels transmitted, i.e. the TTN box is dedicated to the stereo downmix signal while for a mono downmix signal the OTN box is applied. The corresponding TTN −1  or OTN −1  box in the SAOC encoder combines the BGO and FGO signals into a common SAOC stereo or mono downmix  112  and generates the bitstream  114 . The arbitrary pre-defined positioning of all individual FGOs in the downmix signal  112  is supported by either element, i.e. TTN or OTN  152 . At transcoder side, the BGO  154  or any combination of FGO signals  156  (depending on the operating mode  158  externally applied) is recovered from the downmix  112  by the TTN or OTN box  152  using only the SAOC side information  114  and optionally incorporated residual signals. The recovered audio objects  154 / 156  and rendering information  160  are used to produce the MPEG Surround bitstream  162  and the corresponding preprocessed downmix signal  164 . Mixing unit  166  performs the processing of the downmix signal  112  to obtain the MPS input downmix  164 , and MPS transcoder  168  is responsible for the transcoding of the SAOC parameters  114  to MPS parameters  162 . TTN/OTN box  152  and mixing unit  166  together perform the enhanced Karaoke/solo mode processing  170  corresponding to means  52  and  54  in  FIG. 3  with the function of the mixing unit being comprised by means  54 . 
     An MBO can be treated the same way as explained above, i.e. it is preprocessed by an MPEG Surround encoder yielding a mono or stereo downmix signal that serves as BGO to be input to the subsequent enhanced SAOC encoder. In this case the transcoder has to be provided with an additional MPEG Surround bitstream next to the SAOC bitstream. 
     Next, the calculation performed by the TTN (OTN) element is explained. The TTN/OTN matrix expressed in a first predetermined time/frequency resolution  42 , M, is the product of two matrices
 
 M=D   −1   C,  
 
where D −1  comprises the downmix information and C implies the channel prediction coefficients (CPCs) for each FGO channel. C is computed by means  52  and box  152 , respectively, and D −1  is computed and applied, along with C, to the SAOC downmix by means and box  152 , respectively. The computation is performed according to
 
             C   =     (         1       0       0       …       0           0       1       0       …       0             c   11           c   12         1       …       0           ⋮       ⋮       ⋮       ⋱       ⋮             c     N   ⁢           ⁢   1             c     N   ⁢           ⁢   2           0       …       1         )           
for the TTN element, i.e. a stereo downmix and
 
             C   =     (         1       0       …       0             c   1         1       …       0           ⋮       ⋮       ⋱       ⋮             c   N         0       …       1         )           
for the OTN element, i.e. a mono downmix.
 
     The CPCs are derived from the transmitted SAOC parameters, i.e. the OLDs, IOCs, DMGs and DCLDs. For one specific FGO channel j the CPCs can be estimated by 
     
       
         
           
             
                 
             
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     The parameters OLD L , OLD R  and IOC LR  correspond to the BGO, the remainder are FGO values. 
     The coefficients m j  and n j  denote the downmix values for every FGO j for the right and left downmix channel, and are derived from the downmix gains DMG and downmix channel level differences DCLD 
     
       
         
           
             
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     With respect to the OTN element, the computation of the second CPC values c j2  becomes redundant. 
     To reconstruct the two object groups BGO and FGO, the downmix information is exploited by the inverse of the downmix matrix D that is extended to further prescribe the linear combination for signals F 0   1  to F 0   N , i.e. 
     
       
         
           
             
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     In the following, the downmix at encoder&#39;s side is recited: Within the TTN −1  element, the extended downmix matrix is 
             D   =     (         1       0         m   1         …         m   N             0       1         n   1         …         n   N               m   1           n   1           -   1         …       0           ⋮       ⋮       0       ⋱       ⋮             m   N           n   N         0       …         -   1           )           
for a stereo BGO,
 
             D   =     (         1         m   1         …         m   N             1         n   1         …         n   N                 m   1     +     n   1             -   1         …       0           ⋮       0       ⋱       ⋮               m   N     +     n   N           0       …         -   1           )           
for a mono BGO,
 
and for the OTN −1  element it is
 
             D   =     (         1       1         m   1         …         m   N                 m   1     /   2             m   1     /   2           -   1         …       0           ⋮       ⋮       0       ⋱       ⋮               m   N     /   2             m   N     /   2         0       …         -   1           )           
for a stereo BGO,
 
             D   =     (         1         m   1         …         m   N               m   1           -   1         …       0           ⋮       0       ⋱       ⋮             m   N         0       …         -   1           )           
for a mono BGO.
 
     The output of the TTN/OTN element yields 
               (           L   ^               R   ^                 F     ^   _       1             ⋮               F   ^     N           )     =     M   ⁡     (           L   ⁢           ⁢   0               R   ⁢           ⁢   0                 res   1     _             ⋮             res   N           )             
for a stereo BGO and a stereo downmix. In case the BGO and/or downmix is a mono signal, the linear system changes accordingly.
 
     The residual signal res i  corresponds to the FGO object and if not transferred by SAOC stream—because, for example, it lies outside the residual frequency range, or it is signalled that for FGO object i no residual signal is transferred at all—res i  is inferred to be zero. {circumflex over (F)} i  is the reconstructed/up-mixed signal approximating FGO object i. After computation, it may be passed through an synthesis filter bank to obtain the time domain such as PCM coded version of FGO object i. It is recalled that L 0  and R 0  denote the channels of the SAOC downmix signal and are available/signalled in an increased time/frequency resolution compared to the parameter resolution underlying indices (n,k). {circumflex over (L)} and {circumflex over (R)} are the reconstructed/up-mixed signals approximating the left and right channels of the BGO object. Along with the MPS side bitstream, it may be rendered onto the original number of channels. 
     According to an embodiment, the following TTN matrix is used in an energy mode. 
     The energy based encoding/decoding procedure is designed for non-waveform preserving coding of the downmix signal. Thus the TTN upmix matrix for the corresponding energy mode does not rely on specific waveforms, but only describe the relative energy distribution of the input audio objects. The elements of this matrix M Energy  are obtained from the corresponding OLDs according to 
               M   Energy     =       (             O   ⁢           ⁢   L   ⁢           ⁢     D   L           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                 0           0           O   ⁢           ⁢   L   ⁢           ⁢     D   R           O   ⁢           ⁢   L   ⁢           ⁢     D   R       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                           m   1   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   1           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                       n   1   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   1           O   ⁢           ⁢   L   ⁢           ⁢     D   R       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                     ⋮       ⋮                 m   N   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   N           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                       n   N   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   N           O   ⁢           ⁢   L   ⁢           ⁢     D   R       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                   )       1   2             
for a stereo BGO,
 
and
 
               M   Energy     =       (             O   ⁢           ⁢   L   ⁢           ⁢     D   L           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                     O   ⁢           ⁢   L   ⁢           ⁢     D   L           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                           m   1   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   1           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                       n   1   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   1           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                     ⋮       ⋮                 m   N   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   N           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                       n   N   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   N           O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                   )       1   2             
for a mono BGO,
 
so that the output of the TTN element yields
 
                 (           L   ^               R   ^                 F     ^   _       1             ⋮               F   ^     n           )     =       M   Energy     ⁡     (           L   ⁢           ⁢   0               R   ⁢           ⁢   0           )         ,         
or respectively
 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       L 
                       ^ 
                     
                   
                 
                 
                   
                     
                       
                         F 
                         
                           ^ 
                           _ 
                         
                       
                       1 
                     
                   
                 
                 
                   
                     ⋮ 
                   
                 
                 
                   
                     
                       
                         F 
                         ^ 
                       
                       N 
                     
                   
                 
               
               ) 
             
             = 
             
               
                 
                   M 
                   Energy 
                 
                 ⁡ 
                 
                   ( 
                   
                     
                       
                         
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           0 
                         
                       
                     
                     
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           0 
                         
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     Accordingly, for a mono downmix the energy-based upmix matrix M Energy  becomes 
               M   Energy     =       (             O   ⁢           ⁢   L   ⁢           ⁢     D   L                     O   ⁢           ⁢   L   ⁢           ⁢     D   R                         m   1   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   1         +         n   1   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   1                   ⋮                   m   N   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   N         +         n   N   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   N                 )     ⁢     (       1         O   ⁢           ⁢   L   ⁢           ⁢     D   L       +       ∑   i     ⁢       m   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i               +     1         O   ⁢           ⁢   L   ⁢           ⁢     D   R       +       ∑   i     ⁢       n   i   2     ⁢   O   ⁢           ⁢   L   ⁢           ⁢     D   i                 )             
for a stereo BGO, and
 
               M   Energy     =       (             OLD   L                     m   1   2     ⁢     OLD   1                 ⋮                 m   N   2     ⁢     OLD   N               )     ⁢     (     1         OLD   L     +       ∑   i             ⁢       m   i   2     ⁢     OLD   i               )             
for a mono BGO,
 
so that the output of the OTN element results in.
 
                 (           L   ^                 R   ^         F   ^     1               ⋮               F   ^     N           )     =       M   Energy     ⁡     (     L   ⁢           ⁢   0     )         ,         
or respectively
 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       
                         L 
                         ^ 
                       
                       
                         
                           F 
                           ^ 
                         
                         1 
                       
                     
                   
                 
                 
                   
                     ⋮ 
                   
                 
                 
                   
                     
                       
                         F 
                         ^ 
                       
                       N 
                     
                   
                 
               
               ) 
             
             = 
             
               
                 
                   M 
                   Energy 
                 
                 ⁡ 
                 
                   ( 
                   
                     L 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     Thus, according to the just mentioned embodiment, the classification of all objects (Obj 1  . . . Obj N ) into BGO and FGO, respectively, is done at encoder&#39;s side. The BGO may be a mono (L) or stereo 
                   (         L           R         )           
object. The downmix of the BGO into the downmix signal is fixed. As far as the FGOs are concerned, the number thereof is theoretically not limited. However, for most applications a total of four FGO objects seems adequate. Any combinations of mono and stereo objects are feasible. By way of parameters m i  (weighting in left/mono downmix signal) and n i  (weighting in right downmix signal), the FGO downmix is variable both in time and frequency. As a consequence, the downmix signal may be mono (L 0 ) or stereo
 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                   
                 
                 
                   
                     
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                   
                 
               
               ) 
             
             . 
           
         
       
     
     Again, the signals (F 0   1  . . . F 0   N ) T  are not transmitted to the decoder/transcoder. Rather, same are predicted at decoder&#39;s side by means of the aforementioned CPCs. 
     In this regard, it is again noted that the residual signals res may even be disregarded by a decoder. In this case, a decoder—means  52 , for example—predicts the virtual signals merely based in the CPCs, according to: 
     Stereo Downmix: 
               (           L   ⁢           ⁢   0               R   ⁢           ⁢   0                 F   ^     ⁢     0   1               ⋮               F   ^     ⁢     0   N             )     =       C   ⁡     (           L   ⁢           ⁢   0               R   ⁢           ⁢   0           )       =       (         1       0           0       1             c   11           c   12             ⋮       ⋮             c     N   ⁢           ⁢   1             c     N   ⁢           ⁢   2             )     ⁢     (           L   ⁢           ⁢   0               R   ⁢           ⁢   0           )               
Mono Downmix:
 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                   
                 
                 
                   
                     
                       
                         F 
                         ^ 
                       
                       ⁢ 
                       
                         0 
                         1 
                       
                     
                   
                 
                 
                   
                     ⋮ 
                   
                 
                 
                   
                     
                       
                         F 
                         ^ 
                       
                       ⁢ 
                       
                         0 
                         N 
                       
                     
                   
                 
               
               ) 
             
             = 
             
               
                 C 
                 ⁡ 
                 
                   ( 
                   
                     L 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   ) 
                 
               
               = 
               
                 
                   ( 
                   
                     
                       
                         1 
                       
                     
                     
                       
                         
                           c 
                           11 
                         
                       
                     
                     
                       
                         ⋮ 
                       
                     
                     
                       
                         
                           c 
                           
                             N 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                     
                   
                   ) 
                 
                 ⁢ 
                 
                   
                     ( 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     Then, BGO and/or FGO are obtained by—by, for example, means  54 —inversion of one of the four possible linear combinations of the encoder, 
     for example, 
                 (           L   ^               R   ^                 F   ^     1             ⋮               F   ^     N           )     =       D     -   1       ⁡     (           L   ⁢           ⁢   0               R   ⁢           ⁢   0                 F   ^     ⁢     0   1               ⋮               F   ^     ⁢     0   N             )         ,         
where again D −1  is a function of the parameters DMG and DCLD.
 
     Thus, in total, a residual neglecting TTN (OTN) Box  152  computes both just-mentioned computation steps 
     for example: 
     
       
         
           
             
               ( 
               
                 
                   
                     
                       L 
                       ^ 
                     
                   
                 
                 
                   
                     
                       R 
                       ^ 
                     
                   
                 
                 
                   
                     
                       
                         F 
                         ^ 
                       
                       1 
                     
                   
                 
                 
                   
                     ⋮ 
                   
                 
                 
                   
                     
                       
                         F 
                         ^ 
                       
                       N 
                     
                   
                 
               
               ) 
             
             = 
             
               
                 D 
                 
                   - 
                   1 
                 
               
               ⁢ 
               
                 
                   C 
                   ⁡ 
                   
                     ( 
                     
                       
                         
                           
                             L 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                       
                       
                         
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                       
                     
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
     It is noted, that the inverse of D can be obtained straightforwardly in case D is quadratic. In case of a non-quadratic matrix D, the inverse of D shall be the pseudo-inverse, i.e. pinv(D)=D*(DD*) −1  or pinv(D)=(D*D) −1  D*. In either case, an inverse for D exists. 
     Finally,  FIG. 15  shows a further possibility how to set, within the side information, the amount of data spent for transferring residual data. According to this syntax, the side information comprises bsResidualSamplingFrequencyIndex, i.e. an index to a table associating, for example, a frequency resolution to the index. Alternatively, the resolution may be inferred to be a predetermined resolution such as the resolution of the filter bank or the parameter resolution. Further, the side information comprises bsResidualFramesPerSAOCFrame defining the time resolution at which the residual signal is transferred. BsNumGroupsFGO also comprised by the side information, indicates the number of FGOs. For each FGO, a syntax element bsResidualPresent is transmitted, indicating as to whether for the respective FGO a residual signal is transmitted or not. If present, bsResidualBands indicates the number of spectral bands for which residual values are transmitted. 
     Depending on an actual implementation, the inventive encoding/decoding methods can be implemented in hardware or in software. Therefore, the present invention also relates to a computer program, which can be stored on a computer-readable medium such as a CD, a disk or any other data carrier. The present invention is, therefore, also a computer program having a program code which, when executed on a computer, performs the inventive method of encoding or the inventive method of decoding described in connection with the above figures. 
     While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.