Abstract:
The purpose of the invention is to bridge the gap between parametric multi-channel audio coding and matrixed-surround multi-channel coding by gradually improving the sound of an up-mix signal while raising the bit-rate consumed by the side-information starting from 0 up to the bit-rates of the parametric methods. More specifically, it provides a method of flexibly choosing an “operating point” somewhere between matrixed-surround (no side-information, limited audio quality) and fully parametric reconstruction (full side-information rate required, good quality). This operating point can be chosen dynamically (i.e. varying over time) and in response to the permissible side-information rate, as it is dictated by the individual application.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 11/323,965 filed on Dec. 29, 2005, which claims the benefit of U.S. application Ser. No. 60/701,001 filed on Jul. 19, 2005, which are incorporated herein by this reference thereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to multi-channel audio coding and transmission, and in particular to techniques to encode multi-channel audio in a manner that is fully backwards compatible with stereo devices and formats, allowing for an efficient coding of multi-channel audio.  
       BACKGROUND OF THE INVENTION AND PRIOR ART  
       [0003]     Parametric coding of multi-channel audio signals is an ongoing topic of research. Generally two approaches to encode multi-channel audio signals can be distinguished. The Moving Pictures Experts Group (MPEG), a subgroup of the International Organization for Standardization (ISO), is currently working on the standardization of technology for the reconstruction of multi-channel audio content from stereo or even mono down-mix signals by adding only a small amount of helper information to the down-mix signals.  
         [0004]     In parallel stereo to multi-channel up-mix methods are being developed which do not need any additional side-information that is not already (implicitly) contained in the down-mix signal in order to reconstruct the spatial image of the original multi-channel audio signal.  
         [0005]     Existing methods for stereo-compatible multi-channel transmission without additional side-information that gained practical relevance can mostly be characterized as matrixed-surround methods, such as Dolby Pro Logic (Dolby Pro Logic II) and Logic-7, as described in more detail in “Dolby Surround Pro Logic II Decoder—Principles of Operation”, http://www.dolby.com/assets/pdf/tech_library/209_Dolby Surround_Pro_Logic_II_Decoder_Principles_of_Operation.pdf and in “Multichannel Matrix Surround Decoders for Two-Eared Listeners”, Griesinger, D., 101st AES Convention, Los Angeles, USA, 1996, Preprint 4402. The common principle of these methods is that they make use of dedicated ways of multi-channel or stereo down-mixing where the encoder applies phase shifts to the surround channels prior to mixing them together with front and centre channels to form a stereo down-mix signal. The generation of the down-mix signal (Lt, Rt) is depicted in the following equation:  
               [         Lt           Rt         ]     =       [         1       0       q         a   ·   j           b   ·   j             0       1       q           -   b     ·   j             -   a     ·   j           ]     ⁡     [         Lf           Rf           C           Ls           Rs         ]               (   1   )             
 
         [0006]     The left down-mix signal (Lt) consists of the left-front signal (Lf), the centre signal (C) multiplied by a factor q, the left-surround signal (Ls) phase rotated by 90 degrees (,j′) and scaled by a factor a, and the right-surround signal (Rs) which is also phase rotated by 90 degrees and scaled by a factor b. The right down-mix signal (Rt) is generated similarly. Typical down-mix factors are 0.707 for q and a, and 0.408 for b. The rationale for the different signs of the surround channels for the right down-mix signal (Rt) and the left down-mix signal (Lt) is, that it is advantageous to mix the surround channels in anti-phase in the down-mix pair (Lt, Rt). This property helps the decoder to discriminate between front and rear channels from the down-mix signal pair. Hence the down-mix matrix allows for a partial reconstruction of a multi-channel output signal out from the stereo down-mix within the decoder by applying a de-matrixing operation. How close the re-created multi-channel signal resembles the original encoder input signal, however, depends on the specific properties of the multi-channel audio content.  
         [0007]     An example for a coding method adding helper information, also called side information, is MPEG Surround audio coding This efficient way for parametric multi-channel audio coding is for example described in “The Reference Model Architecture for MPEG Spatial Audio Coding”, Herre, J., Purnhagen, H., Breebaart, J., Faller, C., Disch, S., Kjoerling, K., Schuijers, E., Hilpert, J., Myburg, F., Proc. 118th AES Convention, Barcelona, Spain, 2005 and in “Text of Working Draft for Spatial Audio Coding (SAC)”, ISO/IEC JTC1/SC29/WG11 (MPEG), Document N7136, Busan, Korea, 2005.  
         [0008]     A schematic overview of an encoder used in spatial audio coding is shown in  FIG. 6 . The encoder splits incoming signals  10  (input  1 , . . . input N) in separate time-frequency tiles by means of Quadrature Mirror Filters  12  (QMF). Groups of the resulting frequency tiles (bands) are referred to as “parameter bands”. For every parameter band, a number of spatial parameters  14  are determined by a parameter estimator  16  that describes the properties of the spatial image, e.g. level differences between pairs of channels (CLD), cross correlation between pairs of channels (ICC) or information on signal envelopes (CPC). These parameters are subsequently quantized, encoded and compiled jointly into a bit-stream of spatial data. Depending on the operation mode, this bit-stream can cover a wide range of bit-rates, starting from a few kBit/s for good quality multi-channel audio up to tenths of kBit/s for near-transparent quality.  
         [0009]     Besides the extraction of parameters, the encoder also generates a mono or stereo down-mix from the multi-channel input signal. Moreover, in case of a stereo down-mix, the user has the choice of a conventional (ITU-style) stereo down-mix or of a down-mix that is compatible with matrixed-surround systems. Finally, the stereo down-mix is transferred to the time-domain by means of QMF synthesis banks  18 . The resulting down-mix can be transmitted to a decoder, accompanied by the spatial parameters or the spatial parameter bit-stream  14 . Preferably, the down-mix is also encoded before transmission (using a conventional mono or stereo core coder), while the bit-streams of the core coder and the spatial parameters might additionally be combined (multiplexed) to form a single output bit-stream.  
         [0010]     A decoder, as sketched in  FIG. 7 , in principle performs the reverse process of the encoder. An input-stream is split into a core coder bit-stream and a parameter bit-stream. This is not shown in  FIG. 7 . Subsequently, the decoded down-mix  20  is processed by a QMF analysis bank  22  to derive parameter bands that are the same as those applied in the encoder. A spatial synthesis stage  24  reconstructs the multi-channel signal by means of control data  26  (i.e., the transmitted spatial parameters). Finally, the QMF-domain signals are transferred to the time domain by means of a QMF synthesis bank  27  that derives the final multi-channel output signals  28 .  
         [0011]      FIG. 8  shows a simple example of a QMF analysis, as it is performed within the prior art encoder in  FIG. 6  and the prior art decoder in  FIG. 7 . An audio sample  30 , sampled in the time domain and having four sample values is input into a filter bank  32 . The filter bank  32  derives three output samples  34   a ,  34   b  and  34   c  having four sample values each. In an ideal case, the filter bank  32  derives the output samples  34   a  to  34   c  such that the samples within the output signals do only comprise information on discrete frequency ranges of the underlying audio signal  30 . In the case shown in  FIG. 8 , the sample  34   a  has information on the frequency interval ranging from f 0  to f 1 , the sample  34   b  has information of the frequency interval [f 1 , f 2 ] and the sample  34   c  has information on the frequency interval [f 2 , f 2 ]. Although the frequency intervals in  FIG. 8  do not overlap, in a more general case the frequency intervals of the output samples coming out of a filter bank may very well have a frequency overlap.  
         [0012]     A prior art encoder can, as already described above, deliver either an ITU-style down-mix or a matrixed-surround compatible down-mix, when a two-channel down-mix is desired. In the case of a matrixed-surround compatible down-mix (using for example the matrixing approach given in Equation 1), one possibility would be that the encoder generates a matrixed-surround compatible down-mix directly.  
         [0013]      FIG. 9  shows an alternative approach to generate a matrixed-surround compatible down-mix using a down-mix post processing unit  30  working on a regular stereo down-mix  32 . The matrixed-surround processor  30  (MTX encoder) modifies the regular stereo down-mix  32  to make it matrixed-surround compatible guided by the spatial parameters  14  extracted by the parameter extraction stage  16 . For transmission, a matrixed-surround compatible down-mix  34  is transferred to the time domain by a QMF synthesis using the QMF synthesis bank  18 .  
         [0014]     Deriving the matrixed-surround compatible signal by post-processing a regular stereo down-mix has the advantage that the matrixed-surround compatibility processing can be fully reversed at a decoder side if the spatial parameters are available.  
         [0015]     Although both of the approaches are suited to transmit a multi-channel signal, there are specific drawbacks of state of the art systems. Matrixed-surround methods are very efficient (since no additional parameters are required) at the price of a very limited multi-channel reconstruction quality.  
         [0016]     Parametric multi-channel approaches on the other hand require a higher bit-rate due to the side information, which becomes a problem when a limit is set as a maximum acceptable bit-rate for the parametric representation. When the encoded parameters require a comparatively high amount of bit-rate, the only possible way to stay within such a bit-rate limit is to decrease the quality of an encoded down-mix channel by increasing the compression of the channel. Hence, the result is a general loss in audio quality, which may be unacceptably high. In other words, for parametric multi-channel approaches, there is often a hard limit of the minimum bit-rate that is required for the spatial parameter layer, which may in some cases be unacceptably high.  
         [0017]     Although principle backwards compatibility between matrixed-surround methods and spatial audio methods can be achieved by a prior art encoder as illustrated in  FIG. 9 , no additional bit-rate can be saved with this approach when only matrix-based decoding is required. Even then the full set of spatial parameters has to be transmitted, wasting transmission bandwidth.  
         [0018]     Whereas the bit-rate that has to be spent when applying the parametric method may be too high in case of certain application scenarios, the audio quality delivered by the methods without transmission of side-information might not be sufficient.  
         [0019]     The US Patent Application 2005157883 is showing an apparatus for constructing a multi-channel audio signal using an input signal and parametric side information, the input signal including the first input channel and the second input channel derived from an original multi-channel signal, and the parametric side information describing interrelations between channels of the multi-channel original signal.  
       SUMMARY OF THE INVENTION  
       [0020]     It is the object of the present invention to provide a concept for more efficient coding of multi-channel audio signals while being backwards compatible to matrix-based coding solutions.  
         [0021]     In accordance with a first aspect of the present invention, this object is achieved by a multi-channel audio decoder for processing an audio signal and for processing first parametric data describing a first portion of a multi-channel signal, wherein for a second portion of the multi-channel signal no parametric data or second parametric data is processed, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal, comprising: a processor for deriving an intermediate signal from the audio signal, using a first deriving rule for deriving a first portion of the intermediate signal, the first portion of the intermediate signal corresponding to the first portion of the multi-channel audio signal, wherein the first deriving rule is depending on the first parametric data; and using a second deriving rule for deriving a second portion of the intermediate signal, the second deriving rule using no parametric data or the second parametric data.  
         [0022]     In accordance with a second aspect of the present invention, this object is achieved by a multi-channel encoder for generating a parametric representation describing spatial properties of a multi-channel audio signal, the multi-channel encoder comprising: a parameter generator for generating spatial parameters; an output interface for generating the parametric representation, wherein the parameter generator or the output interface is adapted to generate the parametric representation such that the parametric representation includes first parametric data for a first portion of the multi-channel signal and wherein for a second portion of the multi-channel signal no parametric data or second parametric data is included in the parametric representation, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal.  
         [0023]     In accordance with a third aspect of the present invention, this object is achieved by a method for processing an audio signal and for processing first parametric data describing a first portion of a multi-channel signal, wherein for a second portion of the multi-channel signal no parametric data or second parametric data is processed, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal, the method comprising: deriving an intermediate signal from the down-mix signal using a first deriving rule depending on the first parametric data for deriving a first portion of the intermediate signal, the first portion of the intermediate signal corresponding to the first portion of the multi-channel audio signal; and deriving a second portion of the intermediate signal using a second deriving rule, the second deriving rule using the second parametric data or no parametric data.  
         [0024]     In accordance with a fourth aspect of the present invention, this object is achieved by a method for generating a parametric representation describing spatial properties of a multi-channel audio signal, the method comprising: generating spatial parameters; and generating the parametric representation such that the parametric representation includes first parametric data for a first portion of the multi-channel signal and wherein for a second portion of the multi-channel signal no parametric data or second parametric data is included in the parametric representation, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal.  
         [0025]     In accordance with a fifth aspect of the present invention, this object is achieved by a parametric representation describing spatial properties of a multi-channel audio signal, the parametric representation including first parametric data for a first portion of the multi-channel signal and wherein the parametric representation is including no parametric data or second parametric data for a second portion of the multi-channel signal, the second parametric data requiring less information units than the first parametric data for an identical portion of the multi-channel signal.  
         [0026]     In accordance with a sixth aspect of the present invention, this object is achieved by a computer program having a program code for performing, when running on a computer, a method for processing an audio signal and for processing first parametric data describing a first portion of a multi-channel signal, wherein for a second portion of the multi-channel signal no parametric data or second parametric data is processed, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal, the method comprising: deriving an intermediate signal from the down-mix signal using a first deriving rule depending on the first parametric data for deriving a first portion of the intermediate signal, the first portion of the intermediate signal corresponding to the first portion of the multi-channel audio signal; and deriving a second portion of the intermediate signal using a second deriving rule, the second deriving rule using the second parametric data or no parametric data.  
         [0027]     In accordance with a seventh aspect of the present invention, this object is achieved by a computer program having a program code for performing, when running on a computer, a method for generating a parametric representation describing spatial properties of a multi-channel audio signal, the method comprising: generating spatial parameters; and generating the parametric representation such that the parametric representation includes first parametric data for a first portion of the multi-channel signal and wherein for a second portion of the multi-channel signal no parametric data or second parametric data is included in the parametric representation, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal.  
         [0028]     In accordance with an eighth aspect of the present invention, this object is achieved by a transcoder for generating a parametric representation of a multi-channel audio signal using spatial parameters describing the spatial properties of the multi-channel audio signal, comprising: a parameter generator to generate the parametric representation such that the parametric representation includes first parametric data being derived from the spatial parameters for a first portion of the multi-channel signal and wherein for a second portion of the multi-channel signal no parametric data or second parametric data is included in the parametric representation, the second parametric data requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal.  
         [0029]     The present invention is based on the finding that a multi-channel audio signal can be efficiently represented by a parametric representation, when a first deriving rule is used for deriving first parametric data of the parametric representation describing a first portion of the multi-channel signal, and when for a second portion of the multi-channel signal second parametric data or no parametric data is included in the parametric representation, whereas the second parametric data is requiring less information units than the first parametric data when describing an identical portion of the multi-channel signal.  
         [0030]     Thus, a first portion of the multi channel signal is represented by first parameters allowing for a reconstruction of the multi channel signal with higher quality and a second portion can be represented by second parameters allowing for a reconstruction with slightly lower quality. The bit-rate consumed by the first parametric data is consequently higher than the bit rate consumed by the second parametric data when both parametric data is to describe the same portion of a multi-channel signal. In other words, the first parameters require more bit rate per signal portion than the second parameters.  
         [0031]     The purpose of the invention is to bridge the gap between both prior art worlds by gradually improving the sound of the up-mix signal while raising the bit-rate consumed by the side-information starting from 0 up to the bit-rates of the parametric methods. That is, the present invention aims at bridging the gap in bit-rates and perceptual quality between fully parametric methods and matrixed-surround methods. More specifically, it provides a method of flexibly choosing an “operating point” somewhere between matrixed-surround (no side-information, limited audio quality) and fully parametric reconstruction (full side-information rate required, good quality). This operating point can be chosen dynamically (i.e. varying in time) and in response to the permissible side-information rate, as it is dictated by the individual application.  
         [0032]     By dynamically choosing the size of the first portion of the multi-channel audio signal which is the part of the multi-channel audio signal that is represented by the spatial audio parameters, the demanded bit-rate can be varied within a broad range. Representing major parts of a multi-channel signal by the spatial audio parameters will consume a comparatively high bit-rate at the benefit of a good perceptual quality. Since for the second portion of the multi-channel audio signal a parameter deriving rule is chosen that results in parameters consuming less bit-rate, the resulting total bit-rate can be decreased by increasing the size of the second portion of the multi-channel signal. In a preferred embodiment of the present invention, no parametric data at all is transmitted for the second portion of the multi-channel signal, which is of course most bit-saving. Therefore, by dynamically shifting the size of the first portion with respect to the size of the second portion, the bit-rate (or the perceptual quality) can be dynamically adjusted to the needs.  
         [0033]     In a preferred embodiment of the present invention, a down-mix signal is derived in a matrix compatible way. Therefore, the first portion of the multi-channel audio signal can be reproduced with high perceptual quality using the spatial audio parameters and the second portion of the multi-channel signal can be reproduced using matrix-based solutions. This allows for a high-quality reproduction of parts of the signals requiring higher quality. At the same time, the overall bit-rate is decreased by relying on a matrix-based reproduction for signal parts less vital for the quality of a reproduced signal.  
         [0034]     In a further preferred embodiment of the present invention, the inventive concept is applied on the decoder side within a QMF representation of a received down-mix signal. The up-mixing process can principally be sub-divided into three steps: 
        Pre-processing of the input signals (down-mix signals received in the QMF domain) by application of a pre-de-correlator matrix;     de-correlation of part of the pre-processed signals; and mixing the thus derived signals (pre-processed signals and de-correlated signals) within a mix matrix, the output of the mixing being the channels of the up-mix signal.        
 
         [0037]     Both, the pre-de-correlator matrix as well as the mixed-matrix are two-dimensional matrices with the dimensions “number of time slots” on the one hand and “number of parameter bands” on the other hand. Within a decoding process, the elements of these matrices are filled up with values that are derived from the parameters read from the spatial bit-stream, i.e. by the first parametric data. When the first parametric data is only received for a first portion or the multi-channel signal, only that portion of a reconstruction or a multi-channel signal can be derived using the first parametric data submitted. The matrix elements for deriving the second part of the reconstruction of the multi-channel signal are, according to the present invention, derived using matrix compatible coding schemes. These matrix elements can therefore either be derived based only on knowledge achieved from the down-mix signal or be replaced by pre-defined values.  
         [0038]     In a preferred embodiment, a multi-channel audio decoder according to the present invention recognizes by the amount of the transmitted first parametric data, which part of the matrix or which part of the multi-channel audio signal is to be processed by the rule depending on the spatial parameters and which part is to be processed by the matrix based solution.  
         [0039]     In another embodiment of the present invention, an audio encoder creates window information, indicating which parts of a multi-channel signal are being processed by the matrix based solution or by the spatial audio compatible approach. The window information is included in the parametric representation of a multi-channel signal.  
         [0040]     An inventive decoder, therefore, is able to receive and to process the window information created to apply the appropriate up-mixing rules on the portions of the multi-channel audio signal indicated by the window information.  
         [0041]     In a preferred embodiment of the present invention, the inventive concept is applied in the QMF domain during the signal processing, i.e. in a domain where the signals are represented by multiple representations each representation holding information on a certain frequency band.  
         [0042]     In a further preferred embodiment of the present invention, the side-information free method (matrix based approach) is applied only to the higher frequency parts while applying (explicit) parametric information (i.e. the first encoding and decoding rule) for a proper reproduction of the low-frequency parts. This is advantageous due to the property of the human hearing to notice small deviations of two similar signals (e.g. phase deviations) a lot easier for low frequencies than for high frequencies.  
         [0043]     A great benefit of the present invention is that a backwards compatibility of a spatial audio encoding and decoding scheme with matrix based solutions is achieved without having to introduce additional hard- or software when the encoding and decoding rules of the spatial audio coders are chosen appropriately.  
         [0044]     Furthermore, the compatibility is achieved without having to transmit additional data, as it is the case in other prior art attempts. The coding scheme according to the present invention is furthermore extremely flexible, as it allows a seamless adjustment of the bit-rate or the quality, i.e. a smooth transition between full matrix based coding to full spatial audio coding of a given signal. That is, the coding scheme applied can be adjusted to the actual needs, either with respect to the required bit-rate or with respect to the desired quality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]     Preferred embodiments of the present invention are subsequently described by referring to the enclosed drawings, wherein:  
         [0046]      FIG. 1  shows an inventive encoder;  
         [0047]      FIG. 2  shows an example of a parameter bit-stream created by the inventive concept;  
         [0048]      FIG. 2   a  shows an inventive transcoder;  
         [0049]      FIG. 3  shows an inventive decoder;  
         [0050]      FIG. 4  shows an example of a spatial audio decoder implementing the inventive concept;  
         [0051]      FIG. 5  illustrates the use of the different coding schemes on a decoder side;  
         [0052]      FIG. 6  shows a prior art encoder;  
         [0053]      FIG. 7  shows a prior art decoder;  
         [0054]      FIG. 8  shows a block diagram of a filterbank; and  
         [0055]      FIG. 9  shows a further example of a prior art encoder.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0056]      FIG. 1  shows an inventive multi-channel encoder. The multi-channel encoder  100  is having a parameter generator  102  and an output interface  104 .  
         [0057]     A multi-channel audio signal  106  is input into the encoder  100 , where a first portion  108  and a second portion  110  of the multi-channel signal  106  are processed. The parameter generator  102  receives the first portion  108  and the second portion  110  and derives spatial parameters describing spatial properties of the multi-channel signal  106 .  
         [0058]     The spatial parameters are transferred to the output interface  104  that derives a parametric representation  112  of the multi-channel signal  106  such that the parametric representation  112  includes first parametric data for a first portion  108  of the multi-channel signal and wherein for a second portion  110  or the multi-channel signal  106  second parametric data requiring less information than the first parametric data or no parametric data is included in the parametric representation  112 .  
         [0059]     Several variations of the multi-channel encoder  100  are possible to achieve the same goal. For example, the parameter generator  102  can apply two different parameter deriving rules on the first portion  108  and on the second portion  110  that result in different parameter sets that are then transferred to the output interface  104  that combines the different parameter sets into the parametric representation  112 . A special and preferred case is that for the second portion  110  no parameters are included in the parametric representation (and therefore not derived by the parameter generator  102 ) since on a decoder side the decoder derives the required decoding parameters by some heuristic rules.  
         [0060]     Another possibility is that the parameter generator  102  derives a full set of spatial audio parameters as well for the first portion  108  as for the second portion  110 . Hence, the output interface  104  would have to process the spatial parameters such that the second parametric data require less 1 bits than the first parametric data.  
         [0061]     Furthermore, the output interface  104  could add an additional window signal to the parametric representation  112  that shall signal to a decoder, how the multi-channel signal  106  was split into the first portion  108  and into the second portion  110  during the encoding. In a modification of the preferred embodiment of a multi-channel encoder  100 , the multi-channel encoder  100  may additionally have a portion decider for deciding, which part of the multi-channel signal  106  is used as the first portion  108  and which part is used as the second portion  110 , the decision being based on a quality criterion.  
         [0062]     The quality criterion can be derived with respect to a resulting total bit-rate of the parametric representation  112  or with respect to quality aspects, taking into account the perceptual quality of a reproduction of the multi-channel signal  106  based on the parametric representation  112 .  
         [0063]     A major advantage is that the bit-rate consumed by the parametric representation can thus be varied in time, assuring that the quality criterion is met at any time during the encoding while allowing for an overall reduction of the required bit-rate compared to prior art methods.  
         [0064]      FIG. 2  shows an example of a parametric representation  112  created by an inventive encoder.  
         [0065]     As mentioned above, the processing of the audio signals is done block-wise, i.e. a number of subsequent samples of the multi-channel signal in the time domain, building a so-called frame, is processed in one step.  FIG. 2  shows a parameter bit-stream, i.e. a parametric representation for two consecutive frames. The parameter bit-stream is having a representation of a high-quality frame  120  and a representation of a lower quality frame  122 . During the encoding of the high-quality frame  120 , the decision was taken that the first portion  108 , which is being represented by parametric data has to be big compared to the second portion, which may for example be the case if the audio scene to encode is rather complex. The parameter bit-stream of  FIG. 2  is furthermore created under the assumption that a preferred embodiment of an inventive encoder is used that does not derive any parametric data for the second portion  110  of the multi-channel signal  106 . As can be seen in  FIG. 2, 28  spatial parameters ICC and ICLD are included in the parametric representation to describe the high-quality frame  120 . For example, the 28 spatial parameters describe the lower frequency bands of a QMF representation of the multi-channel signal.  
         [0066]     The lower quality frame  122  comprises only 21 spatial parameter sets having ICC and ICLD parameters as this was found to be sufficient for the desired perceptual quality.  
         [0067]      FIG. 2   a  shows an inventive transcoder  150 . The inventive transcoder receives as an input an input bit stream  152  having a full set of spatial parameters describing a first frame  154  and a second frame  156  of a multi-channel audio signal.  
         [0068]     The transcoder  150  generates a bit stream  158  holding a parametric representation representing the spatial properties of the multi-channel audio signal. In the example shown in  FIG. 2   a , the transcoder  150  derives the parametric representation such that for the first frame the number of parameters  160  is only slightly decreased. The number of parameters  162  describing the second frame corresponding to the input parameters  156  are strongly decreased, which reduces the amount of bit rate needed by the resulting parametric representation significantly. Such an inventive transcoder  150  can therefore be used to post-process an already existing bit stream of spatial parameters to derive an inventive parametric representation requiring less bit rate during transmission or less storage space when stored on a computer-readable medium. It should be noted here that it is of course also possible to implement a transcoder for transcoding in the other direction, i.e. using the parametric representation to generate spatial parameters.  
         [0069]     The inventive transcoder  150  can be implemented in various different ways, as for example by reducing the amount of parameters with a given rule or by additionally receiving the multi-channel audio signal to analyze the reduction of bit rate possible without disturbing the perceptual quality beyond an acceptable limit.  
         [0070]      FIG. 3  shows an inventive multi-channel audio decoder  200  having a processor  202 .  
         [0071]     The processor is receiving as an input a down-mix signal  204  derived from a multi-channel audio signal, first parametric data  206  describing a first portion of the multi-channel signal and, for a second portion of the multi-channel signal, optional second parametric data  208  requiring less bits than the first parametric data  206 . The processor  202  is deriving an intermediate signal  210  from the down-mix signal  204  using a first deriving rule for deriving a high-quality portion  212  of the intermediate signal, wherein the high-quality portion  212  of the intermediate signal  212  is corresponding to the first portion of the multi-channel audio signal The processor  202  is using a second deriving rule for a second portion  214  of the intermediate signal  210 , wherein the second deriving rule is using the second parametric data or no parametric data and wherein the first deriving rule is depending on the first parametric data  206 .  
         [0072]     The intermediate signal  210  derived by the processor  202  is built from a combination of the high-quality portion  212  and of the second portion  214 .  
         [0073]     The multi-channel audio decoder  200  may derive by itself, which portions of the down-mix signal  204  are to be processed with the first parametric data  206  by applying some appropriate rules, for example counting the number of spatial parameters included in the first parametric data  206 . Alternatively, the processor  202  may be signalled the fractions of the high-quality portion  212  and of the second portion  214  within the down-mix signal  204  by some additional window information which is derived on an encoder side and that is additionally transmitted to the multi-channel audio decoder  200 .  
         [0074]     In a preferred embodiment, the second parametric data  208  is omitted and the processor  202  derives the second deriving rule from information already contained in the down-mix signal  204 .  
         [0075]      FIG. 4  shows a further embodiment of the present invention that combines the inventive feature of matrix compatibility in a spatial audio decoder. The multi-channel audio decoder  600  comprises a pre-de-correlator  601 , a de-correlator  602  and a mix-matrix  603 .  
         [0076]     The multi-channel audio decoder  600  is a flexible device allowing to operate in different modi depending on the configuration of input signals  605  input into the pre-de-correlator  601 . Generally, the pre-de-correlator  601  derives intermediate signals  607  that serve as input for the de-correlator  600  and that are partially transmitted unaltered to form, together with decorrelated signals calculated by the de-correlator  602 , input signals  608 . The input signals  608  are the signals input into the mix-matrix  603  that derives output channel configurations  610   a  or  610   b , depending on the input channel configuration  605 .  
         [0077]     In a 1-to-5 configuration, a down-mix signal and an optional residual signal is supplied to the pre-de-correlator  601 , that derives four intermediate signals (e 1  to e 4 ) that are used as an input of the de-correlator, which derives four de-correlated signals (d 1  to d 4 ) that form the input parameters  608  together with a directly transmitted signal m derived from the input signal.  
         [0078]     It may be noted, that in the case where an additional residual signal is supplied as input, the de-correlator  602  that is generally working in a sub-band domain, may be operative to simply forward the residual signal instead of deriving a de-correlated signal. This may also be done in a frequency selective manner for certain frequency bands only.  
         [0079]     In the 2-to-S configuration the input signals  605  comprise a left channel, a right channel and optionally a residual signal. In that configuration, the pre-de-correlator matrix  601  derives a left, a right and a center channel and in addition two intermediate channels (e 1 , e 2 ). Hence, the input signals to the mix-matrix  603  are formed by the left channel, the right channel, the centre channel, and two de-correlated signals (d 1  and d 2 ).  
         [0080]     In a further modification, the pre-de-correlator matrix may derive an additional intermediate signal (e 5 ) that is used as an input for a de-correlator (D 5 ) whose output is a combination of the de-correlated signal (d 5 ) derived from the signal (e 5 ) and the de-correlated signals (d 1  and d 2 ). In this case, an additional de-correlation can be guaranteed between the centre channel and the left and the right channel.  
         [0081]     The inventive audio decoder  600  implements the inventive concept in the 2-to-5 configuration. The transmitted parametric representation is used in the pre-de-correlation matrix  601  and in the mix-matrix  603 . There, the inventive concept can be implemented in different ways as shown in more detail in  FIG. 5 .  
         [0082]      FIG. 5  shows the pre-de-correlator, implemented as pre-decorrelator-matrix  601  and the mix-matrix  603  in a principle sketch, wherein the other components of the multi-channel audio decoder  600  are omitted.  
         [0083]     The matrix used to perform the pre-de-correlation and the mixing has columns that represent time slots, i.e. the individual time samples of a signal and rows that represent the different parameter bands, i.e. each row is associated with one parameter band of an audio signal.  
         [0084]     According to the concept of the present invention, the matrix elements of the matrices  601  and  603  are only partly derived from transmitted parametric data, wherein the remaining matrix elements are derived by the decoder, based for example on knowledge of the down-mix signal.  FIG. 5  shows one example where below a given frequency border line  622  the elements of the pre-de-correlator matrix  601  and the mix-matrix  603  are derived from parameters  620  that are read from the bit-stream, i.e. based on information transmitted from the encoder. Above the frequency borderline  622  the matrix elements are derived in the decoder based on knowledge of the down-mix signal only.  
         [0085]     The border frequency (or in general: the amount of matrix elements derived from transmitted data) can be freely adapted according to the quality and/or bit-rate constraints that have to be met for the particular application scenario.  
         [0086]     It is preferred for the novel coding method outlined here that a side-information free up-mix process may be performed with the same structure that has been outlined in the MPEG Spatial Audio Coding Reference Model 0. This invention may consist in describing a method for side-information free up-mix, but preferably provides a method for seamless and advantageous combination of such concepts with methods for side-information assisted up-mix.  
         [0087]     In contrary to the MPEG Spatial Audio Coding Reference Model 0, in the side-information free up-mix process the elements of the matrices M 1  ( 601 ) and M 2  ( 603 ) are preferably not derived from data transmitted in a bit-stream but by different means without the help of side-information, e.g. by applying heuristic rules based only on knowledge achieved from the down-mix signal.  
         [0088]     In this way it is possible to achieve a gradual scaling between both techniques—in terms of bit-rate as well as in terms of sound quality—by acquiring only parts of the matrices based on the transmitted parameters and applying the rules of the method without side-information to fill up the remaining parts. Conceptually speaking, this corresponds to transmitting for certain parts of the matrices the spatial parameters and for other parts generating them at the decoder.  
         [0089]     The determination of the parts of matrices that are to be derived from either the one ore the other method can be done in a lot of different ways, such as 
        deriving the parts of the matrices below a given horizontal border line by one method and above this border line by the other method;     deriving the parts of the matrices left of a given vertical border line by one method and right from this border line by the other method;     determining arbitrary time-frequency tiles within both matrices the elements of which are derived by one method and deriving the elements of the remaining time-frequency tiles by means of the other method.        
 
         [0093]     It has been detailed in the above paragraphs that it is advantageous to describe all frequency parts of a multi-channel signal up to a certain border frequency by spatial parameters whereas the remaining frequency parts of the multi-channel signal are not represented by spatial parameters. This takes into account the characteristics of the human ear that has a better perception of lower frequencies than of higher frequencies. Of course, the present invention is by no means limited to this splitting of the multi-channel signal into a first portion and a second portion as it may also be advantageous or appropriate to describe higher frequency parts of the signal with better accuracy. This may especially be the case when in the lower frequency region only little energy is contained in the signal since most of the energy is contained in a high-frequency domain of the audio signal. Due to masking effects the low-frequency part will be mostly dominated by the high frequency parts then and it may be advantageous to provide the possibility for a high-quality reproduction of the high-frequency part of the signal.  
         [0094]     Depending on certain implementation requirements of the inventive methods, the inventive methods can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, in particular a disk, DVD or a CD having electronically readable control signals stored thereon, which cooperate with a programmable computer system such that the inventive methods are performed. Generally, the present invention is, therefore, a computer program product with a program code stored on a machine readable carrier, the program code being operative for performing the inventive methods when the computer program product runs on a computer. In other words, the inventive methods are, therefore, a computer program having a program code for performing at least one of the inventive methods when the computer program runs on a computer.  
         [0095]     While the foregoing has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope thereof. It is to be understood that various changes may be made in adapting to different embodiments without departing from the broader concepts disclosed herein and comprehended by the claims that follow.