Patent Publication Number: US-7716042-B2

Title: Audio coding

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/EP2005/001363, filed Feb. 10, 2005, which designated the United States and was not published in English, and is incorporated herein by reference in its entirety, and which claimed priority to German Patent Application No. 10 2004 007 191.8, filed on Feb. 13, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to audio coder and decoders and audio coding in general and, in particular, to audio codings allowing audio signals to be coded with a short delay time. 
     2. Description of Prior Art 
     The audio compression method best known at present is MPEG-1 Layer III. With this compression method, the sample or audio values of an audio signal are coded into a coded signal in a lossy manner. Put differently, irrelevance and redundancy of the original audio signal are reduced or ideally removed when compressing. In order to achieve this, simultaneous and temporal maskings are recognized by a psycho-acoustic model, i.e. a temporally varying masking threshold depending on the audio signal is calculated or determined indicating from which volume on tones of a certain frequency are perceivable for human hearing. This information in turn is used for coding the signal by quantizing the spectral values of the audio signal in a more precise or less precise manner or not at all, depending on the masking threshold, and integrating same into the coded signal. 
     Audio compression methods, such as, for example, the MP3 format, experience a limit in their applicability when audio data is to be transferred via a bit rate-limited transmission channel in a, on the one hand, compressed manner, but, on the other hand, with as small a delay time as possible. In some applications, the delay time does not play a role, such as, for example, when archiving audio information. Small delay audio coders, which are sometimes referred to as “ultra low delay coders”, however, are necessary where time-critical audio signals are to be transmitted, such as, for example, in teleconferencing, in wireless loudspeakers or microphones. For these fields of application, the article by Schuller G. et al. “Perceptual Audio Coding using Adaptive Pre- and Post-Filters and Lossless Compression”, IEEE Transactions on Speech and Audio Processing, vol. 10, no. 6, September 2002, pp. 379-390, suggests audio coding where the irrelevance reduction and the redundancy reduction are not performed based on a single transform, but on two separate transforms. 
     The principle will be discussed subsequently referring to  FIGS. 12 and 13 . Coding starts with an audio signal  902  which has already been sampled and is thus already present as a sequence  904  of audio or sample values  906 , wherein the temporal order of the audio values  906  is indicated by an arrow  908 . A listening threshold is calculated by means of a psycho-acoustic model for successive blocks of audio values  906  characterized by an ascending numeration by “block#”.  FIG. 13 , for example, shows a diagram where, relative to the frequency f, graph a plots the spectrum of a signal block of 128 audio values  906  and b plots the masking threshold, as has been calculated by a psycho-acoustic model, in logarithmic units. The masking threshold indicates, as has already been mentioned, up to which intensity frequencies remain inaudible for the human ear, namely all tones below the masking threshold b. Based on the listening thresholds calculated for each block, an irrelevance reduction is achieved by controlling a parameterizable filter, followed by a quantizer. For a parameterizable filter, a parameterization is calculated such that the frequency response thereof corresponds to the inverse of the magnitude of the masking threshold. This parameterization is indicated in  FIG. 12  by x # (i). 
     After filtering the audio values  906 , quantization with a constant step size takes place, such as, for example, a rounding operation to the next integer. The quantizing noise caused by this is white noise. On the decoder side, the filtered signal is “retransformed” again by a parameterizable filter, the transfer function of which is set to the magnitude of the masking threshold itself. Not only is the filtered signal decoded again by this, but the quantizing noise on the decoder side is also adjusted to the form or shape of the masking threshold. In order for the quantizing noise to correspond to the masking threshold as precisely as possible, an amplification value a #  applied to the filtered signal before quantizing is calculated on the coder side for each parameter set or each parameterization. In order for the retransform to be performed on the decoder side, the amplification value a and the parameterization x are transferred to the coder as side information  910  apart from the actual main data, namely the quantized filtered audio values  912 . For the redundancy reduction  914 , this data, i.e. the side information  910  and the main data  912 , is subjected to a loss-free compression, namely entropy coding, which is how the coded signal is obtained. 
     The above-mentioned article suggests a size of 128 sample values  906  as a block size. This allows a relatively short delay of 8 ms with a sampling rate of 32 kHz. With reference to the detailed implementation, the article also states that, for increasing the efficiency of the side information coding, the side information, namely the coefficients x #  and a # , will only be transferred if there are sufficient changes compared to a parameter set transferred before, i.e. if the changes exceed a certain threshold value. In addition, it is described that the implementation is preferably performed such that a current parameter set is not directly applied to all the sample values belonging to the respective block, but that a linear interpolation of the filter coefficients x #  is used to avoid audible artifacts. In order to perform the linear interpolation of the filter coefficients, a lattice structure is suggested for the filter to prevent instabilities from occurring. For the case that a coded signal with a controlled bit rate is desired, the article also suggests selectively multiplying or attenuating the filtered signal scaled with the time-depending amplification factor a by a factor unequal to 1 so that audible interferences occur, but the bit rate can be reduced at sites of the audio signal which are complicated to code. 
     Although the audio coding scheme described in the article mentioned above already reduces the delay time for many applications to a sufficient degree, a problem in the above scheme is that, due to the requirement of having to transfer the masking threshold or transfer function of the coder-side filter, subsequently referred to as pre-filter, the transfer channel is loaded to a relatively high degree even though the filter coefficients will only be transferred when a predetermined threshold is exceeded. 
     Another disadvantage of the above coding scheme is that, due to the fact that the masking threshold or inverse thereof has to be made available on the decoder side by the parameter set x #  to be transferred, a compromise has to be made between the lowest possible bit rate or high compression ratio on the one hand and the most precise approximation possible or parameterization of the masking threshold or inverse thereof on the other hand. Thus, it is inevitable for the quantizing noise adjusted to the masking threshold by the above audio coding scheme to exceed the masking threshold in some frequency ranges and thus result in audible audio interferences for the listener.  FIG. 13 , for example, shows the parameterized frequency response of the decoder-side parameterizable filter by graph c. As can be seen, there are regions where the transfer function of the decoder-side filter, subsequently referred to as post-filter, exceeds the masking threshold b. The problem is aggravated by the fact that the parameterization is only transferred intermittently with a sufficient change between parameterizations and interpolated therebetween. An interpolation of the filter coefficients x # , as is suggested in the article, alone results in audible interferences when the amplification value a #  is kept constant from node to node or from new parameterization to new parameterization. Even if the interpolation suggested in the article is also applied to the side information value a # , i.e. the amplification value transferred, audible audio artifacts may remain in the audio signal arriving on the decoder side. 
     Another problem with the audio coding scheme according to  FIGS. 12 and 13  is that the filtered signal may, due to the frequency-selective filtering, take a non-predictable form where, particularly due to a random superposition of many individual harmonic waves, one or several individual audio values of the coded signal add up to very high values which in turn result in a poorer compression ratio in the subsequent redundancy reduction due to their rare occurrence. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a more effective audio coding scheme. 
     In accordance with a first aspect, the present invention provides a device for coding an audio signal of a sequence of audio values into a coded signal, having: means for applying a psycho-acoustic model to a first block of audio values of the sequence of audio values and a second block of audio values of the sequence of audio values; means for calculating a version of a first parameterization of a parameterizable filter based on a result of applying the psycho-acoustic model to the first block and a version of a second parameterization of the parameterizable filter based on a result of applying the psycho-acoustic model to the second block; means for filtering a predetermined block of audio values of the sequence of audio values with the parameterizable filter using a predetermined parameterization which in a predetermined manner depends on the version of the second parameterization to obtain a block of filtered audio values corresponding to the predetermined block; means for quantizing the filtered audio values to obtain a block of quantized filtered audio values; means for forming a combination of the version of the first parameterization and the version of the second parameterization including at least a difference between the version of the first parameterization and the version of the second parameterization; and means for integrating information from which the quantized filtered audio values and a version of the first parameterization may be derived and which includes the combination into the coded signal. 
     In accordance with a second aspect, the present invention provides a method for coding an audio signal of a sequence of audio values into a coded signal, having the steps of: applying a psycho-acoustic model to a first block of audio values of the sequence of audio values and a second block of audio values of the sequence of audio values; calculating a version of a first parameterization of a parameterizable filter based on a result of applying the psycho-acoustic model to the first block a version of a second parameterization of the parameterizable filter based on a result of applying the psycho-acoustic model to the second block; filtering a predetermined block of audio values of the sequence of audio values with the parameterizable filter using a predetermined parameterization which in a predetermined manner depends on the version of the second parameterization to obtain a block of filtered audio values corresponding to the predetermined block; quantizing the filtered audio values to obtain a block of quantized filtered audio values; forming a combination of the version of the first parameterization and the version of the second parameterization including at least a difference between the version of the first parameterization and the version of the second parameterization; and integrating information from which the quantized filtered audio values may be derived and which includes the combination into the coded signal. 
     In accordance with a third aspect, the present invention provides a device for decoding a coded signal into an audio signal, the coded signal containing information from which a block of quantized filtered audio values and a version of a first parameterization according to which a transfer function of a parameterizable filter corresponds to a first result of applying a psycho-acoustic model may be derived, and which includes a combination between a version of a second parameterization according to which a transfer function of the parameterizable filter corresponds to a second result of applying the psycho-acoustic model and the version of the first parameterization including at least a difference between the version of the first parameterization and the version of the second parameterization, having: means for deriving the version of the first parameterization from the coded signal; means for calculating a sum between the version of the first parameterization and the difference to obtain the version of the second parameterization; and means for filtering the block of quantized filtered audio values with a parameterizable filter using the version of the second parameterization such that the transfer function thereof corresponds to a result of applying the psycho-acoustic model to obtain a block of decoded audio values of the audio signal. 
     In accordance with a fourth aspect, the present invention provides a method for decoding a coded signal into an audio signal, wherein the coded signal contains information from which a block of quantized filtered audio values and a version of a first parameterization according to which a transfer function of a parameterizable filter corresponds to a first result of applying a psycho-acoustic model may be derived, and which includes a combination between a version of a second parameterization according to which a transfer function of the parameterizable filter corresponds to a second result of applying the psycho-acoustic model and the version of the first parameterization which includes at least a difference between the version of the first parameterization and the version of the second parameterization, having the steps of: deriving the version of the first parameterization from the coded signal; calculating a sum between the version of the first parameterization and the difference to obtain the version of the second parameterization; and filtering the block of quantized filtered audio values with a parameterizable filter using the version of the second parameterization such that the transfer function thereof corresponds to a result of applying the psycho-acoustic model to obtain a block of decoded audio values of the audio signal. 
     In accordance with a fifth aspect, the present invention provides a computer program having a program code for performing one of the above mentioned methods when the computer program runs on a computer. 
     Inventive coding of an audio signal of a sequence of audio values into a coded signal includes determining a first listening threshold for a first block of audio values of the sequence of audio values and a second listening threshold for a second block of audio values of the sequence of audio values; calculating a version of a first parameterization of a parameterizable filter such that the transfer function thereof roughly corresponds to the inverse of the magnitude of the first listening threshold and a version of a second parameterization of the parameterizable filter such that the transfer function thereof roughly corresponds to the inverse of the magnitude of the second listening threshold; filtering a predetermined block of audio values of the sequence of audio values with the parameterizable filter using a predetermined parameterization which in a predetermined manner depends on the version of the second parameterization to obtain a block of filtered audio values corresponding to the predetermined block; quantizing the filtered audio values to obtain a block of quantized filtered audio values; forming a combination of the version of the first parameterization and the version of the second parameterization including at least a difference between the version of the first parameterization and the version of the second parameterization; and integrating information from which the quantized filtered audio values and a version of the first parameterization may be derived and which includes the combination into the coded signal. 
     The central idea of the present invention is that a higher compression ratio may be achieved by transferring differences of successive parameterizations. 
     If, additionally, the transfer of parameterizations only takes place when there is a sufficient difference between same, the finding of the present invention will in particular also be that in this case, too, although the parameterization differences do not fall below the minimum difference measure, nevertheless the transfer of differences between two parameterizations provides a compression increase, instead of parameterization, more than compensating for the additional complexity of calculating the difference on the coder side and calculating the sum on the decoder side. 
     According to an embodiment of the present invention, the pure differences between successive parameterizations are transferred, whereas according to another embodiment the minimum threshold starting from which parameterizations of new nodes will be transferred is subtracted from these differences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
         FIG. 1  shows a block circuit diagram of an audio coder according to an embodiment of the present invention; 
         FIG. 2  shows a flow chart for illustrating the mode of functioning of the audio coder of  FIG. 1  at the data input; 
         FIG. 3  shows a flow chart for illustrating the mode of functioning of the audio coder of  FIG. 1  with regard to the evaluation of the incoming audio signal by a psycho-acoustic model; 
         FIG. 4  shows a flow chart for illustrating the mode of functioning of the audio coder of  FIG. 1  with regard to applying the parameters obtained by the psycho-acoustic model to the incoming audio signal; 
         FIG. 5   a  shows a schematic diagram for illustrating the incoming audio signal, the sequence of audio values it consists of, and the operating steps of  FIG. 4  in relation to the audio values; 
         FIG. 5   b  shows a schematic diagram for illustrating the setup of the coded signal; 
         FIG. 6  shows a flow chart for illustrating the mode of functioning of the audio coder of  FIG. 1  with regard to the final processing up to the coded signal; 
         FIG. 7   a  shows a diagram where an embodiment of a quantizing step function is shown; 
         FIG. 7   b  shows a diagram where another embodiment of a quantizing step function is shown; 
         FIG. 8  shows a block circuit diagram of an audio coder which is able to decode an audio signal coded by the audio coder of  FIG. 1  according to an embodiment of the present invention; 
         FIG. 9  shows a flow chart for illustrating the mode of functioning of the decoder of  FIG. 8  at the data input; 
         FIG. 10  shows a flow chart for illustrating the mode of functioning of the decoder of  FIG. 8  with regard to buffering the pre-decoded quantized and filtered audio data and the processing of the audio blocks without corresponding side information; 
         FIG. 11  shows a flow chart for illustrating the mode of functioning of the decoder of  FIG. 8  with regard to the actual reverse-filtering; 
         FIG. 12  shows a schematic diagram for illustrating a conventional audio coding scheme having a short delay time; and 
         FIG. 13  shows a diagram where, exemplarily, a spectrum of an audio signal, a listening threshold thereof and the transfer function of the post-filter in the decoder are shown. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an audio coder according to an embodiment of the present invention. The audio coder, which is generally indicated by  10 , includes a data input  12  where it receives the audio signal to be coded, which, as will be explained in greater detail later referring to  FIG. 5   a , consists of a sequence of audio values or sample values, and a data output where the coded signal is output, the information content of which will be discussed in greater detail referring to  FIG. 5   b.    
     The audio coder  10  of  FIG. 1  is divided into an irrelevance reduction part  16  and a redundancy reduction part  18 . The irrelevance reduction part  16  includes means  20  for determining a listening threshold, means  22  for calculating an amplification value, means  24  for calculating a parameterization, node comparing means  26 , a quantizer  28  and a parameterizable pre-filter  30  and an input FIFO (first in first out) buffer  32 , a buffer or memory  38  and a multiplier or multiplying means  40 . The redundancy reduction part  18  includes a compressor  34  and a bit rate controller  36 . 
     The irrelevance reduction part  16 , the redundancy reduction part  18  and an integrator  15   r  integrating the redundancy and irrelevancy reduced signal into a coded signal, the functionality of which is described in more detail below with respect to  FIG. 5   b , are connected in series in this order between the data input  12  and the data output  14 . In particular, the data input  12  is connected to a data input of the means  20  for determining a listening threshold and to a data input of the input buffer  32 . A data output of the means  20  for determining a listening threshold is connected to an input of the means  24  for calculating a parameterization and to a data input of the means  22  for calculating an amplification value to pass on a listening threshold determined to same. The means  22  and  24  calculate a parameterization or amplification value based on the listening threshold and are connected to the node comparing means  26  to pass on these results to same. Depending on the result of the comparison, the node comparing means  26 , as will be discussed subsequently, passes on the results calculated by the means  22  and  24  as input parameter or parameterization to the parameterizable pre-filter  30 . The parameterizable pre-filter  30  is connected between a data output of the input buffer  32  and a data input of the buffer  38 . The multiplier  40  is connected between a data output of the buffer  38  and the quantizer  28 . The quantizer  28  passes on filtered audio values which may be multiplied or scaled, but always quantized, to the redundancy reduction part  18 , more precisely to a data input of the compressor  34 . The node comparing means  26  passes on information from which the input parameters passed to the parameterizable pre-filter  30  may be derived to the redundancy reduction part  18 , more precisely to another data input of the compressor  34 . The bit rate controller is connected to a control input of the multiplier  40  via a control connection to provide for the quantized filtered audio values, as received from the pre-filter  30 , to be multiplied by the multiplier  40  by a suitable multiplicand, as will be discussed in greater detail below. The bit rate controller  36  is connected between a data output of the compressor  34  and the data output  14  of the audio coder  10  in order to determine the multiplicand for the multiplier  40  in a suitable manner. When each audio value passes the quantizer  40  for the first time, the multiplicand is at first set to a suitable scaling factor, such as, for example, 1. The buffer  38 , however, continues storing each filtered audio value to give the bit rate controller  36 , as will be described subsequently, a possibility of changing the multiplicand for another pass of a block of audio values. If such a change is not indicated by the bit rate controller  36 , the buffer  38  may release the memory taken up by this block. 
     After the setup of the audio coder of  FIG. 1  has been described above, the mode of functioning thereof will subsequently be described referring to  FIGS. 2 to 7   b.    
     As can be seen from  FIG. 2 , the audio signal, when having reached the audio input  12 , has already been obtained by audio signal sampling  50  from an analog audio signal. The audio signal sampling is performed with a predetermined sampling frequency, which is usually between 32 and 48 kHz. Consequently, at the data input  12  there is an audio signal consisting of a sequence of sample or audio values. Although the coding of the audio signal does not take place in a block-based manner, as will become obvious from the subsequent description, the audio values at the data input  12  are at first combined to form audio blocks in step  52 . The combination to form audio blocks takes place only for the purpose of determining the listening threshold, as will become obvious from the following description, and takes place in an input stage of the means  20  for determining a listening threshold. In the present embodiment, it is exemplarily assumed that 128 successive audio values each are combined to form audio blocks and that the combination takes place such that, one the one hand, successive audio blocks do not overlap and, on the other hand, are direct neighbors of one another. This will exemplarily be discussed shortly referring to  FIG. 5   a.    
       FIG. 5   a  at  54  indicates the sequence of sample values, each sample value being illustrated by a rectangle  56 . The sample values are numbered for illustration purposes, wherein for reasons of clarity in turn only some sample values of the sequence  54  are shown. As is indicated by braces above the sequence  54 , 128 successive sample values each are combined to form a block according to the present embodiment, wherein the directly successive 128 sample values form the next block. Only as a precautionary measure, it is to be pointed out that the combination to form blocks could also be performed differently, exemplarily by overlapping blocks or spaced-apart blocks and blocks having another block size, although the block size of 128 in turn is preferred since it provides a good tradeoff between high audio quality on the one hand and the smallest possible delay time on the other hand. 
     Whereas the audio blocks combined in the means  20  in step  52  are processed in the means  20  for determining a listening threshold block by block, the incoming audio values will be buffered  54  in the input buffer  32  until the parameterizable pre-filter  30  has obtained input parameters from the node comparing means  26  to perform pre-filtering, as will be described subsequently. 
     As can be seen from  FIG. 3 , the means  20  for determining a listening threshold starts its processing directly after sufficient audio values have been received at the data input  12  to form an audio block or to form the next audio block, which the means  20  monitors by an inspection in step  60 . If there is no complete processable audio block, the means  20  will wait. If a complete audio block to be processed is present, the means  20  for determining a listening threshold will calculate a listening threshold in step  62  on the basis of a suitable psycho-acoustic model in step  62 . For illustrating the listening threshold, reference is again made to  FIG. 12  and, in particular, to graph b having been obtained on the basis of a psycho-acoustic model, exemplarily with regard to a current audio block with a spectrum a. The masking threshold which is determined in step  62  is a frequency-dependent function which may vary for successive audio blocks and may also vary considerably from audio signal to audio signal, such as, for example, from rock music to classical music pieces. The listening threshold indicates for each frequency a threshold value below which the human hearing cannot perceive interferences. 
     In a subsequent step  64 , the means  24  and the means  22  calculate from the listening threshold M(f) calculated (f indicating the frequency) an amplification value a or parameter set of filter coefficients a k . The parameterization x(i) which the means  24  calculates in step  64  is provided for the parameterizable pre-filter  30  which is, for example, embodied in an adaptive filter structure, as is used in LPC coding (LPC=linear predictive coding). For example, s(n), n=0, . . . , 127, be the 128 audio values of the current audio block and s′(n) be the resulting filtered 128 audio values, then the filter is exemplarily embodied such that the following equation applies: 
                   s   ′     ⁡     (   n   )       =       s   ⁡     (   n   )       -       ∑     k   =   1     K     ⁢       a   k   t     ⁢     s   ⁡     (     n   -   k     )               ,         
K being the filter order and a k   t , k=1, . . . , K, being the filter coefficients, and the index t is to illustrate that the filter coefficients change in successive audio blocks. The means  24  then calculates the parameterization a k   t  such that the transfer function H(f) of the parameterizable pre-filter  30  roughly equals the inverse of the magnitude of the masking threshold M(f), i.e. such that the following applies:
 
               H   ⁡     (     f   ,   t     )       ≈     1          M   ⁡     (     f   ,   t     )                    
wherein the dependence of t in turn is to illustrate that the masking threshold M(f) changes for different audio blocks. When implementing the pre-filter  30  as the adaptive filter mentioned above, the filter coefficients a k   t  will be obtained as follows: the inverse discrete Fourier transform of |M(f, t)| 2  over the frequency for the block at the time t results in the target auto-correlation function r mm   t (i). Then, the a k   t  are obtained by solving the linear equation system:
 
     
       
         
           
             
               
                 
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     In order for no instabilities to arise between the parameterizations in the linear interpolation described in greater detail below, a lattice structure is preferably used for the filter  30 , wherein the filter coefficients for the lattice structure are re-parameterized to form reflection coefficients. With regard to further details as to the design of the pre-filter, the calculation of the coefficients and the re-parameterization, reference is made to the article by Schuller etc. mentioned in the introduction to the description and, in particular, to page 381, division III, which is incorporated herein by reference. 
     Whereas consequently the means  24  calculates a parameterization for the parameterizable pre-filter  30  such that the transfer function thereof equals the inverse of the masking threshold, the means  22  calculates a noise power limit based on the listening threshold, namely a limit indicating which noise power the quantizer  28  is allowed to introduce into the audio signal filtered by the pre-filter  30  in order for the quantizing noise on the decoder side to be below the listening threshold M(f) or exactly equal it after post- or reverse-filtering. The means  22  calculates this noise power limit as the area below the square of the magnitude of the listening threshold M, i.e. as Σ|M(f)| 2 . The means  22  calculates the amplification value a from the noise power limit by calculating the root of the fraction of the quantizing noise power divided by the noise power limit. The quantizing noise is the noise caused by the quantizer  28 . The noise caused by the quantizer  28  is, as will be described below, white noise and thus frequency-independent. The quantizing noise power is the power of the quantizing noise. 
     As has become evident from the above description, the means  22  also calculates the noise power limit apart from the amplification value a. Although it is possible for the node comparing means  26  to again calculate the noise power limit from the amplification value a obtained from the means  22 , it is also possible for the means  22  to also transmit the noise power limit determined to the node comparing means  26  apart from the amplification value a. 
     After calculating the amplification value and the parameterization, the node comparing means  26  checks in step  66  whether the parameterization just calculated differs by more than a predetermined threshold from the current last parameterization passed on to the parameterizable pre-filter. If the check in step  66  has the result that the parameterization just calculated differs from the current one by more than the predetermined threshold, the filter coefficients just calculated and the amplification value just calculated or noise power limit are buffered in the node comparing means  26  for an interpolation to be discussed and the node comparing means  26  hands over to the pre-filter  30  the filter coefficients just calculated in step  68  and the amplification value just calculated in step  70 . If, however, this is not the case and the parameterization just calculated does not differ from the current one by more than the predetermined threshold, the node comparing means ( 26 ) will hand over to the pre-filter  30  in step  72 , instead of the parameterization just calculated, only the current node parameterization, i.e. that parameterization which last resulted in a positive result in step  66 , i.e. differed from a previous node parameterization by more than a predetermined threshold. After steps  70  and  72 , the process of  FIG. 3  returns to processing the next audio block, i.e. to a query  60 . 
     In the case that the parameterization just calculated does not differ from the current node parameterization and consequently the pre-filter  30  in step  72  again obtains the node parameterization already obtained for at least the last audio block, the pre-filter  30  will apply this node parameterization to all the sample values of this audio block in the FIFO  32 , as will be described in greater detail below, which is how this current block is taken out of the FIFO  32  and the quantizer  28  receives a resulting audio block of pre-filtered audio values. 
       FIG. 4  illustrates the mode of functioning of the parameterizable pre-filter  30  for the case it receives the parameterization just calculated and the amplification value just calculated, because they differ sufficiently from the current node parameterization in greater detail. As has been described referring to  FIG. 3 , there is no processing according to  FIG. 4  for each of the successive audio blocks, but only for audio blocks where the respective parameterization differed sufficiently from the current node parameterization. The other audio blocks are, as has just been described, pre-filtered by applying the respective current node parameterization and the pertaining respective current amplification value to all the sample values of these audio blocks. 
     In step  80 , the parameterizable pre-filter  30  checks whether a handover of filter coefficients just calculated from the node comparing means  26  has taken place, or of older node parameterizations. The pre-filter  30  performs the check  80  until such a handover has taken place. 
     As soon as such a handover has taken place, the parameterizable pre-filter  30  starts processing the current audio block of audio values just in the buffer  32 , i.e. that one for which the parameterization has just been calculated. In  FIG. 5   a , it is for example illustrated that all the audio values  56  in front of the audio value with number 0 have already been processed and have thus already passed the memory  32 . The processing of the block of audio values in front of the audio value with number 0 was triggered because the parameterization calculated for the audio block in front of block 0, namely x 0 (i), differed from the node parameterization passed on before to the pre-filter  30  by more than the predetermined threshold. The parameterization x 0 (i) thus is a node parameterization as is described in the present invention. The processing of the audio values in the audio block in front of the audio value 0 was performed on the basis of the parameter set a 0 , x 0 (i). 
     It is assumed in  FIG. 5   a  that the parameterization having been calculated for block 0 with the audio values  0 - 127  differed by less than the predetermined threshold from the parameterization x 0 (i) which referred to the block in front. This block 0 was thus also taken out of the FIFO  32  by the pre-filter  30 , equally processed with regard to all its sample values  0 - 127  by means of the parameterization x 0 (i) supplied in step  72 , as is indicated by the arrow  81  described by “direct application”, and then passed on to the quantizer  28 . 
     The parameterization calculated for block 1 still located in the FIFO  32 , however, in contrast differed, according to the illustrative example of  FIG. 5   a , by more than the predetermined threshold from the parameterization x 0 (i) and was thus passed on in step  68  to the pre-filter  30  as a parameterization x 1 (i), together with the amplification value a 1  (step  70 ) and, if applicable, the pertaining noise power limit, wherein the indices of a and x in  FIG. 5   a  are to be an index for the nodes, as are used in the interpolation to be discussed below, which is performed with regard to the sample values  128 - 255  in block 1, symbolized by an arrow  82  and realized by the steps following step  80  in  FIG. 4 . The processing at step  80  would thus start with the occurrence of the audio block with number 1. 
     At the time when the parameter set a 1 , x 1  is passed on, only the audio values  128 - 255 , i.e. the current audio block after the last audio block 0 processed by the pre-filter  30 , are in the memory  32 . After determining the handover of node parameters x 1 (i) in step  80 , the pre-filter  30  determines the noise power limit q 1  corresponding to the amplification value a 1  in step  84 . This may take place by the node comparing means  26  passing on this value to the pre-filter  30  or by the pre-filter  30  again calculating this value, as has been described above referring to step  64 . 
     After that, an index j is initialized to a sample value in step  86  to point to the oldest sample value remaining in the FIFO memory  32  or the first sample value of the current audio block “block 1”, i.e. in the present example of  FIG. 5   a  the sample value  128 . In step  88 , the parameterizable pre-filter performs an interpolation between the filter coefficients x 0  and x 1 , wherein here the parameterization x 0  acts as a node at the node having the audio value number  127  of the previous block 0 and the parameterization x 1  acts as a node at the node having the audio value number  255  of the current block 1. These audio value positions  127  and  255  will subsequently be referred to as node 0 and node 1, wherein the node parameterizations referring to the nodes in  FIG. 5   a  are indicated by the arrows  90  and  92 . 
     In step  88 , the parameterizable pre-filter  30  performs the interpolation of the filter coefficients x 0 , x 1  between the two nodes in the form of a linear interpolation to obtain the interpolated filter coefficients at the sample position j, i.e. x(t j )(i), i=1 . . . N. 
     After that, namely in step  90 , the parameterizable pre-filter  30  performs an interpolation between the noise power limit q 1  and q 0  to obtain an interpolated noise power limit at the sample position j, i.e. q(t j ). 
     In step  92 , the parameterizable pre-filter  30  subsequently calculates the amplification value for the sample position j on the basis of the interpolated noise power limit and the quantizing noise power, and preferably also the interpolated filter coefficients, namely for example depending on the root of 
                 quantizing   ⁢           ⁢   noise   ⁢           ⁢   power       q   ⁡     (     t   j     )         ,         
wherein for this reference is made to the explanations of step  64  of  FIG. 3 .
 
     In step  94 , the parameterizable pre-filter  30  then applies the amplification value calculated and the interpolated filter coefficients to the sample value at the sample position j to obtain a filtered sample value for this sample position, namely s′(t j ). 
     In step  96 , the parameterizable pre-filter  30  then checks whether the sample position j has reached the current node, i.e. node 1, in the case of  FIG. 5   a  the sample position  255 , i.e. the sample value for which the parameterization transferred to the parameterizable pre-filter  30  plus amplification value is to be valid directly, i.e. without interpolation. If this is not the case, the parameterizable pre-filter  30  will increase or increment the index j by 1, wherein steps  88 - 96  will be repeated. If the check in step  96 , however, is positive, the parameterizable pre-filter will apply, in step  100 , the last amplification value transmitted from the node comparing means  26  and the last filter coefficients transmitted from the node comparing means  26  directly without an interpolation to the sample value at the new node, whereupon the current block, i.e. in the present case block 1, has been processed, and the process is performed again at step  80  relative to the subsequent block to be processed which, depending on whether the parameterization of the next audio block 2 differs sufficiently from the parameterization x 1 (i), may be this next audio block 2 or else a later audio block. 
     Before the further procedure when processing the filtered sample values s′ will be described referring to  FIGS. 5   a  and  b , the purpose and background of the procedure of  FIGS. 3 and 4  will be described below. The purpose of filtering is filtering the audio signal at the input  12  with an adaptive filter, the transfer function of which is continually adjusted to the inverse of the listening threshold to the best degree possible, which also changes overtime. The reason for this is that, on the decoder side, the reverse-filtering the transfer function of which is correspondingly continuously adjusted to the listening threshold shapes the white quantizing noise introduced by quantizing the filtered audio signal, i.e. the frequency-constant quantizing noise, by an adaptive filter, namely adjusts same to the form of the listening threshold. 
     The application of the amplification value in steps  94  and  100  in the pre-filter  30  is a multiplication of the audio signal or the filtered audio signal, i.e. the sample values s or the filtered sample values s′, by the amplification factor. The purpose is to set by this the quantizing noise introduced into the filtered audio signal by the quantization described in greater detail below, and which is adjusted by the reverse-filtering on the decoder side to the form of the listening threshold, as high as possible without exceeding the listening threshold. This can be exemplified by Parsevals formula according to which the square of the magnitude of a function equals the square of the magnitude of the Fourier transform. When on the decoder side the multiplication of the audio signal in the pre-filter by the amplification value is reversed again by dividing the filtered audio signal by the amplification value, the quantizing noise power is also reduced, namely by the factor a −2 , a being the amplification value. Consequently, the quantizing noise power can be set to an optimally high degree by applying the amplification value in the pre-filter  30 , which is synonymous to the quantizing step size being increased and thus the number of quantizing steps to be coded being reduced, which in turn increases the compression in the subsequent redundancy reduction part. 
     Put differently, the effect of the pre-filter could be considered as a normalization of the signal to its masking threshold, so that the level of the quantizing interferences or quantizing noise can be kept constant in both time and frequency. Since the audio signal is in the time domain, the quantization may thus be performed step by step with a uniform constant quantization, as will be described subsequently. In this way, ideally any possible irrelevance is removed from the audio signal and a lossless compression scheme may be used to also remove the remaining redundancy in the pre-filtered and quantized audio signal, as will be described below. 
     Referring to  FIG. 5   a , it is again to be pointed out explicitly that of course the filter coefficients and amplification values a 0 , a 1 , x 0 , x 1  used must be available on the decoder side as side information, that the transfer complexity of this, however, is decreased by not simply using new filter coefficients and new amplification values for each block. Rather, a threshold value check  66  takes place to only transfer the parameterizations as side information with a sufficient parameterization change and to otherwise not transfer the side information or parameterizations. An interpolation from the old to the new parameterization takes place at the audio blocks for which the parameterizations have been transferred. The interpolation of the filter coefficients takes place in the manner described above referring to step  88 . The interpolation with regard to the amplification takes place by a detour, namely via a linear interpolation  90  of the noise power limit q 0 , q 1 . Compared to a direct interpolation via the amplification value, the linear interpolation results in a better listening result or fewer audible artifacts with regard to the noise power limit. 
     Subsequently, the further processing of the pre-filtered signal will be described referring to  FIG. 6 , which basically includes quantization and redundancy reduction. First, the filtered sample values output by the parameterizable pre-filter  30  are stored in the buffer  38  and at the same time let pass from the buffer  38  to the multiplier  40  where there are, since it is their first pass, at first passed on unchanged, namely with a scaling factor of one, by the multiplier  40  to the quantizer  28 . There, the filtered audio values above an upper limit are cut in step  110  and then quantized in step  112 . The two steps  110  and  112  are executed by the quantizer  28 . In particular, the two steps  110  and  112  are preferably executed by the quantizer  28  in one step by quantizing the filtered audio values s′ by a quantizing step function which maps the filtered sample values s′ exemplarily present in a floating point illustration to a plurality of integer quantizing step values or indices and which has a flat course for the filtered sample values from a certain threshold value on so that filtered sample values greater than the threshold value are quantized to one and the same quantizing step. An example of such a quantizing step function is illustrated in  FIG. 7   a.    
     The quantized filtered sample values are referred to by σ′ in  FIG. 7   a . The quantizing step function preferably is a quantizing step function with a step size which is constant below the threshold value, i.e. the jump to the next quantizing step will always take place after a constant interval along the input values S′. In the implementation, the step size to the threshold value is adjusted such that the number of quantizing steps preferably corresponds to a power of 2. Compared to the floating point illustration of the incoming filtered sample values s′, the threshold value is smaller so that a maximum value of the illustratable region of the floating point illustration exceeds the threshold value. 
     The reason for this threshold value is that it has been observed that the filtered audio signal output by the pre-filter  30  occasionally comprises audio values adding up to very large values due to an unfavorable accumulation of harmonic waves. Furthermore, it has been observed that cutting these values, as is achieved by the quantizing step function shown in  FIG. 7   a , results in a high data reduction, but only in a minor impairment of the audio quality. Rather, these occasional locations in the filtered audio signal are formed artificially by a frequency-selective filtering in the parameterizable filter  30  so that cutting them impairs audio quality only to a minor extent. 
     A somewhat more specific example of the quantizing step function shown in  FIG. 7   a  would be one which rounds all the filtered sample values s′ to the next integer up to the threshold value, wherein all filtered sample values that would lead to an integer value higher than the threshold value would be quantized to the threshold value, i.e., the highest quantizing step, such as, for example, 256. This case is illustrated in  FIG. 7   a.    
     Another example of a possible quantizing step function would be the one shown in  FIG. 7   b . Up to the threshold value, the quantizing step function of  FIG. 7   b  corresponds to that of  FIG. 7   a . Instead of having an abruptly flat course for sample values s′ above the threshold value, however, the quantizing step function continues with a steepness smaller than the steepness in the region below the threshold value. Put differently, the quantizing step size is greater above the threshold value. By this, a similar effect is achieved like by the quantizing function of  FIG. 7   a , but, on the one hand, with more complexity due to the different step sizes of the quantizing step function above and below the threshold value and, on the other hand, improved audio quality, since very high filtered audio values s′ are not cut off completely but only quantized with greater a quantizing step size. 
     As has already been described before, on the decoder side not only the quantized and filtered audio values σ′ must be available, but also the input parameters for the pre-filter  30  being the basis of filtering these values, namely the node parameterization including a hint to the pertaining amplification value. In step  114 , the compressor  34  thus performs a first compression trial and thus compresses side information containing the amplification values a 0  and a 1  at the nodes, such as, for example, 127 and 255, and the filter coefficients x 0  and x 1  at the nodes and the quantized filtered sample values σ′ to a temporally filtered signal. The compressor  34  thus is a losslessly operating coder, such as, for example, a Huffman or arithmetic coder with or without prediction and/or adaptation. 
     The memory  38  which the sampled audio values σ′ pass through serves as a buffer for a suitable block size with which the compressor  34  processes the quantized, filtered and also scaled, as will be described before, audio values σ′ output by the quantizer  28 . The block size may differ from the block size of the audio blocks as are used by the means  20 . 
     As has already been mentioned, the bit rate controller  36  has controlled the multiplexer  40  by a multiplicand of 1 for the first compression trial so that the filtered audio values go unchanged from the pre-filter  30  to the quantizer  28  and from there as quantized filtered audio values to the compressor  34 . The compressor  34  monitors in step  116  whether a certain compression block size, i.e. a certain number of quantized sampled audio values, has been coded into the temporary coded signal, or whether further quantized filtered audio values σ′ are to be coded into the current temporary coded signal. If the compression block size has not been reached, the compressor  34  will continue performing the current compression  114 . If the compression block size, however, has been reached, the bit rate controller  36  will check in step  118  whether the bit quantity required for the compression is greater than a bit quantity dictated by a desired bit rate. If this is not the case, the bit rate controller  36  will check in step  120  whether the bit quantity required is smaller than the bit quantity dictated by the desired bit rate. If this is the case, the bit rate controller  36  will fill up the coded signal in step  122  with filler bits until the bit quantity dictated by the desired bit rate has been reached. Subsequently, the coded signal is output in step  124 . As an alternative to step  122 , the bit rate controller  36  could pass on the compression block of filtered audio values σ′ still stored in the memory  38  on which the last compression has been based in a form multiplied by a multiplicand greater than 1 by the multiplier  40  to the quantizer  28  for again passing steps  110 - 118 , until the bit quantity dictated by the desired bit rate has been reached, as is indicated by a step  125  illustrated in broken lines. 
     If, however, the check in step  118  results in that the required bit quantity is greater than the one dictated by the desired bit rate, the bit rate controller  36  will change the multiplicand for the multiplier  40  to a factor between 0 and 1 exclusive. This is performed in step  126 . After step  126 , the bit rate controller  36  provides for the memory  38  to again output the last compression block of filtered audio values σ′ on which the compression has been based, wherein they are subsequently multiplied by the factor set in step  126  and again supplied to the quantizer  28 , whereupon steps  110 - 118  are performed again and the up to then temporarily coded signal is disposed of. 
     It is to be pointed out that when performing steps  110 - 116  again, in step  114  of course the factor used in step  126  (or step  125 ) is also integrated into the coded signal. 
     The purpose of the procedure after step  126  is increasing the effective step size of the quantizer  28  by the factor. This means that the resulting quantizing noise is uniformly above the masking threshold, which results in audible interferences or audible noise, but results in a reduced bit rate. If, after passing steps  110 - 116  again, it is again determined in step  118  that the required bit quantity is greater than the one dictated by the desired bit rate, the factor will be reduced again in step  126 , etc. 
     If the data is finally output at step  124  as a coded signal, the next compression block will be performed from the subsequent quantized filtered audio values σ′. 
     It is also to be pointed out that another pre-initialized value than 1 could be used as the multiplication factor, namely, for example, a value or greater than or smaller than 1. Then, scaling would take place in any case at first, i.e. at the very top of  FIG. 6 . 
       FIG. 5   b  illustrates again the resulting coded signal which is generally indicated by  130 , and inherently, the functionality of integrator  15  of  FIG. 1  which, in turn, integrates the information outlined hereinafter into the coded signal. The coded signal includes side information and main data therebetween. The side information includes, as has already been mentioned, information from which for special audio blocks, namely audio blocks where a significant change in the filter coefficients has resulted in the sequence of audio blocks, the value of the amplification value and the value of the filter coefficients can be derived. If necessary, the side information will include further information relating to the amplification value used for the bit controller. Due to the mutual dependence of the amplification value and the noise power limit q, the side information may optionally, apart from the amplification value a #  to a node #, also include the noise power limit q # , or only the latter. The side information is preferably arranged within the coded signal such that the side information to filter coefficients and pertaining amplification value or pertaining noise power limit is arranged in front of the main data to the audio block of quantized filtered audio values σ′, from which these filter coefficients with pertaining amplification values or pertaining noise power limit have been derived, i.e. the side information a 0 , x 0 (i) after block −1 and the side information a 1 , x 1 (i) after block 1. Put differently, the main data, i.e. the quantized filtered audio values σ′, starting from, excluding, an audio block of the kind where a significant change in the sequence of audio blocks has resulted in the filter coefficients, up to, including, the next audio block of this kind, in  FIG. 5 , for example, the audio values σ′(t 0 ) to σ′(t 255 ), will always be arranged between the side information block  132  to the first one of these two audio blocks (block −1) and the other side information block  134  to the second one of the two audio blocks (block 1). The audio values σ′(t 0 ) to σ′(t 127 ) are decodable or have been, as has been mentioned before referring to  FIG. 5   a , obtained only by means of the side information  132 , whereas the audio values σ′(t 128 ) to σ′(t 255 ) have been obtained by interpolation by means of the side information  132  as support values at the node with the sample value number  127  and by means of the side information  134  as support values at the node with the sample value number  255  and are thus decodable only by means of both side information. 
     In addition, the side information regarding the amplification value or the noise power limit and the filter coefficients in each side information block  132  and  134  are not always integrated independently of each other. Rather, this side information is transferred in differences to the previous side information block. In  FIG. 5   b  for example, the side information block  132  contains the amplification value a 0  and filter coefficients x 0  with regard to the node at the time t −1 . In the side information block  132 , these values may be derived from the block itself. From the side information block  134 , however, the side information regarding the node at the time t 255  may no longer be derived from this block alone. Rather, the side information block  134  only includes information on differences of the amplification value a 1  of the node at the time t 255  and the amplification value of the node at the time t 0  and the differences of the filter coefficients x 1  and the filter coefficients x 0 . The side information block  134  consequently only contains the information on a 1 -a 0  and x 1 (i)-x 0 (i). At intermitting times, however, the filter coefficients and the amplification value or the noise power limit should be transferred completely and not only as a difference to the previous node, such as, for example, each second to allow a receiver or decoder latching into a running stream of coding data, as will be discussed below. 
     This kind of integrating the side information into the side information blocks  132  and  134  offers the advantage of the possibility of a higher compression rate. The reason for this is that, although the side information will, if possible, only be transferred if a sufficient change of the filter coefficients to the filter coefficients of a previous node has resulted, the complexity of calculating the difference on the coder side or calculating the sum on the decoder side pays off since the resulting differences are small in spite of the query of step  66  to thus allow advantages in entropy coding. 
     After an embodiment of an audio coder has been described before, an embodiment of an audio decoder which is suitable for decoding the coded signal generated by the audio coder  10  of  FIG. 1  to a decoded playable or processable audio signal will be described subsequently. 
     The setup of this decoder is shown in  FIG. 8 . The decoder generally indicated by  210  includes a decompressor  212 , a FIFO memory  214 , a multiplier  216  and a parameterizable post-filter  218 . The decompressor  212 , the FIFO memory  214 , the multiplier  216  and the parameterizable post-filter  218  are connected in this order between a data input  220  and a data output  222  of the decoder  210 , wherein the coded signal is received at the data input  220  and the decoded audio signal only differing from the original audio signal at the data input  12  of the audio coder  10  by the quantizing noise generated by the quantizer  28  in the audio coder  10  is output at the data output  222 . The decompressor  212  is connected to a control input of the multiplier  216  at another data output to pass on a multiplicand to same, and to a parameterization input of the parameterizable post-filter  218  via another data output. 
     As is shown in  FIG. 9 , the decompressor  212  at first decompresses in step  224  the compressed signal at the data input  220  to obtain the quantized filtered audio data, namely the sample values σ′, and the pertaining side information in the side information blocks  132 ,  134 , which, as is known, indicate the filter coefficients and amplification values or, instead of the amplification values, the noise power limits at the nodes. 
     As is shown in  FIG. 10 , the decompressor  212  checks the decompressed signal in the order of appearance in step  226  whether side information with filter coefficients is contained therein, in a self-contained form without a difference reference to a previous side information block. Put differently, the decompressor  212  looks for the first side information block  132 . As soon as the decompressor  212  has found something, the quantized filtered audio values σ′ are buffered in the FIFO memory  214  in step  228 . If a complete audio block of quantized filtered audio values σ′ has been stored during step  228  without a directly following side information block, it will at first be post-filtered in step  228  by means of the information contained in the side information received in step  226  on parameterization and amplification value in a post-filter and amplified in the multiplier  216 , which is how it is decoded and thus the pertaining decoded audio block is achieved. 
     In step  230 , the decompressor  212  monitors the decompressed signal for the occurrence of any kind of side information block, namely with absolute filter coefficients or filter coefficients differences to a previous side information block. In the example of  FIG. 5   b , the decompressor  212  would, for example, recognize the occurrence of the side information block  134  in step  230  upon recognizing the side information block  132  in step  226 . Thus, the block of quantized filtered audio values σ′(t 0 )-σ′(t 127 ) would have been decoded in step  228 , using the side information  132 . As long as the side information block  134  in the decompressed signal has not yet occurred, the buffering and, maybe, decoding of blocks is continued in step  228  by means of the side information of step  226 , as has been described before. 
     As soon as the side information block  132  has occurred, the decompressor  212  will calculate the parameter values at the node 1, i.e. a 1 , x 1 (i), in step  232  by adding up the difference values in the side information block  134  and the parameter values in the side information block  132 . Step  232  is of course omitted if the current side information block is a self-contained side information block without differences, which, as has been described before, may exemplarily occur every second. In order for the waiting time for the decoder  210  not to be too long, side information blocks  132  where the parameter values may be derived absolutely, i.e. with no relation to another side information block, are arranged in sufficiently small distances so that the turn-on time or down time when switching on the audio coder  210  in the case of, for example, a radio transmission or broadcast transmission is not too large. Preferably, the number of side information blocks  132  arranged therebetween with the difference values are arranged in a fixed predetermined number between the side information blocks  132  so that the decoder knows when a side information block of type  132  is again to be expected in the coded signal. Alternatively, the different side information block types are indicated by corresponding flags. 
     As is shown in  FIG. 11 , after a side information block for a new node has been reached, in particular after step  226  or  232 , a sample value index j is at first initialized to 0 in step  234 . This value corresponds to the sample position of the first sample value in the audio block currently remaining in the FIFO  214  to which the current side information relates. Step  234  is performed by the parameterizable post-filter  218 . The post-filter  218  then calculates the noise power limit at the new node in step  236 , wherein this step corresponds to step  84  of  FIG. 4  and may be omitted when, for example, the noise power limit at the nodes is transmitted in addition to the amplification values. In subsequent steps  238  and  240 , the post-filter  218  performs interpolations with regard to the filter coefficients and the noise power limit corresponding to the interpolations  88  and  90  of  FIG. 4 . The subsequent calculation of the amplification value for the sample position j on the basis of the interpolated noise power limit and the interpolated filter coefficients of steps  238  and  240  in step  242  corresponds to step  92  of  FIG. 4 . In step  244 , the post-filter  218  applies the amplification value calculated in step  242  and the interpolated filter coefficients to the sample value at the sample position j. This step differs from step  94  of  FIG. 4  by the fact that the interpolated filter coefficients are applied to the quantized filtered sample values σ′ such that the transfer function of the parameterizable post-filter does not correspond to the inverse of the listening threshold, but to the listening threshold itself. In addition, the post-filter does not perform a multiplication by the amplification value, but a division by the amplification value at the quantized filtered sample values σ′ or the already reverse-filtered, quantized filtered sample value at the position j. 
     If the post-filter  218  has not yet reached the current node with the sample position j, which it checks in step  246 , it will increment the sample position index j in step  248  and start steps  238 - 246  again. Only when the node has been reached, it will apply the amplification value and the filter coefficients of the new node to the sample value at the node, namely in step  250 . The application in turn includes, like in step  218 , a division by means of the amplification value and filtering with a transfer function equaling the listening threshold and not the inverse of the latter, instead of a multiplication. After step  250 , the current audio block is decoded by an interpolation between two node parameterizations. 
     As has already been mentioned, the noise introduced by the quantization when coding in step  110  or  112  is adjusted in both shape and magnitude to the listening threshold by the filtering and the application of an amplification value in steps  218  and  224 . 
     It is also to be pointed out that in the case that the quantized filtered audio values have been subjected to another multiplication in step  126  due to the bit rate controller before being coded into the coded signal, this factor may also be considered in steps  218  and  224 . Alternatively, the audio values obtained by the process of  FIG. 11  could of course be subjected to another multiplication to correspondingly amplify again the audio values weakened by a lower bit rate. 
     With regard to  FIGS. 3 ,  4 ,  6  and  9 - 11 , it is pointed out that same show flow charts illustrating the mode of functioning of the coder of  FIG. 1  or the decoder of  FIG. 8  and that each of the steps illustrated in the flow chart by a block, as described, is implemented in corresponding means, as has been described before. The implementation of the individual steps may be realized in hardware, as an ASIC circuit part, or in software, as subroutines. In particular, the explanations written into the blocks in these figures roughly indicate to which process the respective step corresponding to the respective block refers, whereas the arrows between the blocks illustrate the order of the steps when operating the coder and decoder, respectively. 
     Referring to the previous description, it is pointed out again that the coding scheme illustrated above may be varied in many regards. Exemplarily, it is not necessary for a parameterization and an amplification value or a noise power limit, as were determined for a certain audio block, to be considered as directly valid for a certain audio value, like in the previous embodiment the last respective audio value of each audio block, i.e. the 128th value in this audio block so that interpolation for this audio value may be omitted. Rather, it is possible to relate these node parameter values to a node which is temporally between the sample times t n , n=0, . . . , 127, of the audio values of this audio block so that an interpolation would be necessary for each audio value. In particular, the parameterization determined for an audio block or the amplification value determined for this audio block may also be applied indirectly to another value, such as, for example, the audio value in the middle of the audio block, such as, for example, the 64 th  audio value in the case of the above block size of 128 audio values. 
     Additionally, it is pointed out that the above embodiment referred to an audio coding scheme designed for generating a coded signal with a controlled bit rate. Controlling the bit rate, however, is not necessary for every case of application. This is why the corresponding steps  116  to  122  and  126  may also be omitted. 
     With reference to the compression scheme mentioned referring to step  114 , for reasons of completeness, reference is made to the document by Schuller et al. described in the introduction to the description and, in particular, to division IV, the contents of which with regard to the redundancy reduction by means of lossless coding is incorporated herein by reference. 
     In addition, the following is to be pointed out referring to the previous embodiment. Although it has been described before that the threshold value always remains constant when quantizing or even the quantizing step function always remains constant, i.e. the artifacts generated in the filtered audio signal are always quantized or cut off by rougher a quantization, which may impair the audio quality to an audible extent, it is also possible to only use these measures if the complexity of the audio signal requires this, namely if the bit rate required for coding exceeds a desired bit rate. In this case, in addition to the quantizing step functions shown in  FIGS. 7   a  and  7   b , for example one with a quantizing step size constant over the entire range of values possible at the output of the pre-filter might be used and the quantizer would, for example, respond to a signal to use either the quantizing step function with an always constant quantizing step size or one of the quantizing step functions according to  FIG. 7   a  or  7   b  so that the quantizer could be told by the signal to perform, with little audio quality impairment, the quantizing step decrease above the threshold value or cutting off above the threshold value. Alternatively, the threshold value could also be reduced gradually. In this case, the threshold value reduction could be performed instead of the factor reduction of step  126 . After a first compression trial without step  110 , the temporarily compressed signal could only be subjected to a selective threshold value quantization in a modified step  126  if the bit rate were still too high ( 118 ). In another pass, the filtered audio values would then be quantized with the quantizing step function having a flatter course above the audio threshold. Further bit rate reductions could be performed in the modified step  126  by reducing the threshold value and thus by another modification of the quantization step function. 
     Furthermore, some aspects of the above embodiment are of advantage, but not necessary. Exemplarily, interpolation may be omitted in the above audio coding scheme. In addition, it would be possible to transfer the parameterizations and the amplification value or the parameterizations and the noise power limit with regard to each audio block with regard to which they were calculated, and not to leave out a single one when the successive parameterizations differ by less than the predetermined measure already mentioned. 
     In addition, it would be possible to only apply the difference coding to the parameterizations, but not to the amplification value or the noise power limit. 
     In addition, it is conceivable in the above coding scheme to transfer the filter coefficients in the difference side blocks  134  in a different manner, namely, for example, in the form of the current filter coefficients minus the previously transferred filter coefficients minus the minimum threshold of step  66 . 
     The above-described audio coding scheme consequently relates to, among other things, effectively transferring side information in an audio coder with a very small delay time. The side information having to be transferred for the decoder in order for the audio signal to be reconstructed suitably has the feature of usually changing only slowly. This is why only differences are transferred, which decreases the bit rate. In addition, they will only be transferred when there are sufficient changes. From time to time, the absolute value will be transferred in case past values were lost. Put differently, the side information from the prefilter or the coefficients are transferred such that the post-filter in the decoder has the inverse transfer function so that the audio signal may again be reconstructed suitably. The bit rate required for this is reduced by transferring differences, but only if they have a sufficient size. These differences have smaller values and occur more frequency, which is why they require fewer bits when coding. The difference coding thus particularly pays off since the differences will also only change steadily with continually changing audio signals. 
     In particular, it is pointed out that, depending on the circumstances, the inventive audio coding scheme may also be implemented in software. The implementation may be on a digital storage medium, in particular on a disc or a CD having control signals which may be readout electronically, which can cooperate with a programmable computer system such that the corresponding method will be executed. In general, the invention also is in a computer program product having a program code stored on a machine-readable carrier for performing the inventive method when the computer program product runs on a computer. Put differently, the invention may also be realized as a computer program having a program code for performing the method when the computer program runs on a computer. 
     In particular, above method steps in the blocks of the flow chart may be implemented individually or in groups of several ones together in subprogram routines. Alternatively, an implementation of an inventive device in the form of an integrated circuit is, of course, also possible where these blocks are, for example, implemented as individual circuit parts of an ASIC. 
     In particular, it is pointed out that, depending on the circumstances, the inventive scheme may also be implemented in software. The implementation may be on a digital storage medium, in particular on a disc or a CD having control signals which may be read out electronically, which can cooperate with a programmable computer system such that the corresponding method will be executed. In general, the invention thus also is in a computer program product having a program code stored on a machine-readable carrier for performing the inventive method when the computer program runs on a computer. Put differently, the invention may also be realized as a computer program having a program code for performing the method when the computer program runs on a computer. 
     While this invention has been described in terms of several preferred 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.