Patent Publication Number: US-11393479-B2

Title: Apparatus and method for generating an error concealment signal using individual replacement LPC representations for individual codebook information

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending U.S. patent application Ser. No. 16/178,143 filed Nov. 1, 2018, which is a continuation of U.S. patent application Ser. No. 15/267,768 filed Sep. 16, 2016, which is a continuation of International Application No. PCT/EP2015/054488, filed Mar. 4, 2015, which is incorporated herein by reference in its entirety, and additionally claims priority from European Applications Nos. EP EP14160774.7, filed Mar. 19, 2014, EP 14167007.5, filed May 5, 2014, and EP 14178765.5, filed Jul. 28, 2014, all of which are incorporated herein by reference in their entirety. 
     The present invention relates to audio coding and in particular to audio coding based on LPC-like processing in the context of codebooks. 
    
    
     BACKGROUND OF THE INVENTION 
     Perceptual audio coders often utilize linear predictive coding (LPC) in order to model the human vocal tract and in order to reduce the amount of redundancy, which can be modeled by the LPC parameters. The LPC residual, which is obtained by filtering the input signal with the LPC filter, is further modeled and transmitted by representing it by one, two or more codebooks (examples are: adaptive codebook, glottal pulse codebook, innovative codebook, transition codebook, hybrid codebooks consisting of predictive and transform parts). 
     In case of a frame loss, a segment of speech/audio data (typically 10 ms or 20 ms) is lost. To make this loss as less audible as possible, various concealment techniques are applied. These techniques usually consist of extrapolation of the past, received data. This data may be: gains of codebooks, codebook vectors, parameters for modeling the codebooks and LPC coefficients. In all concealment technology known from state-of-the-art, the set of LPC coefficients, which is used for the signal synthesis, is either repeated (based on the last good set) or is extra-/interpolated. 
     ITU G.718 [1]: The LPC parameters (represented in the ISF domain) are extrapolated during concealment. The extrapolation consists of two steps. First, a long term target ISF vector is calculated. This long term target ISF vector is a weighted mean (with the fixed weighting factorbeta) of
     an ISF vector representing the average of the last three known ISF vectors, and   an offline trained ISF vector, which represents a long-term average spectral shape.   

     This long term target ISF vector is then interpolated with the last correctly received ISF vector once per frame using a time-varying factor alpha to allow a cross-fade from the last received ISF vector to the long term target ISF vector. The resulting ISF vector is subsequently converted back to the LPC domain, in order to generate intermediate steps (ISFs are transmitted every 20 ms, interpolation generates a set of LPCs every 5 ms). The LPCs are then used to synthesize the output signal by filtering the result of the sum of the adaptive and the fixed codebook, which are amplified with the corresponding codebook gains before addition. The fixed codebook contains noise during concealment. In case of consecutive frame loss, the adaptive codebook is fed back without adding the fixed codebook. Alternatively, the sum signal might be fed back, as done in AMR-WB [5]. 
     In [2], a concealment scheme is described which utilizes two sets of LPC coefficients. One set of LPC coefficients is derived based on the last good received frame, the other set of LPC parameters is derived based on the first good received frame, but it is assumed that the signal evolves in reverse direction (towards the past). Then prediction is performed in two directions, one towards the future and one towards the past. Therefore, two representations of the missing frame are generated. Finally, both signals are weighted and averaged before being played out. 
       FIG. 8  shows an error concealment processing in accordance with conventional technology. An adaptive codebook  800  provides an adaptive codebook information to an amplifier  808  which applies a codebook gain g p  to the information from the adaptive codebook  800 . The output of the amplifier  808  is connected to an input of a combiner  810 . Furthermore, a random noise generator  804  together with a fixed codebook  802  provides codebook information to a further amplifier g c . The amplifier g c  indicated at  806  applies the gain factor g c , which is the fixed codebook gain, to the information provided by the fixed codebook  802  together with the random noise generator  804 . The output of the amplifier  806  is then additionally input into the combiner  810 . The combiner  810  adds the result of both codebooks amplified by the corresponding codebook gains to obtain a combination signal which is then input into an LPC synthesis block  814 . The LPC synthesis block  814  is controlled by replacement representation which is generated as discussed before. 
     This conventional-technology procedure has certain drawbacks. 
     In order to cope with changing signal characteristics or in order to converge the LPC envelope towards background noise like-properties, the LPC is changed during concealment by extra/interpolation with some other LPC vectors. There is no possibility to precisely control the energy during concealment. While there is the chance to control the codebook gains of the various codebooks, the LPC will implicitly influence the overall level or energy (even frequency dependent). 
     It might be envisioned to fade out to a distinct energy level (e.g. background noise level) during burst frame loss. This is not possible with state-of-the-art technology, even by controlling the codebook gains. 
     It is not possible to fade the noisy parts of the signal to background noise, while maintaining the possibility to synthesize tonal parts with the same spectral property as before the frame loss. 
     SUMMARY 
     According to an embodiment, an apparatus for generating an error concealment signal may have: an LPC (linear prediction coding) representation generator for generating a first replacement LPC representation and a different second replacement LPC representation; an LPC synthesizer for filtering a first codebook information using the first replacement representation to acquire a first replacement signal and for filtering a different second codebook information using the second replacement LPC representation to acquire a second replacement signal; and a replacement signal combiner for combining the first replacement signal and the second replacement signal by summing-up the first replacement signal and the second replacement signal to acquire the error concealment signal. 
     According to another embodiment, a method of generating an error concealment signal may have the steps of: generating a first replacement LPC representation and a different second replacement LPC representation; filtering a first codebook information using the first replacement representation to acquire a first replacement signal and filtering a different second codebook information using the second replacement LPC representation to acquire a second replacement signal; and combining the first replacement signal and the second replacement signal by summing-up the first replacement signal and the second replacement signal to acquire the error concealment signal. 
     According to another embodiment, a non-transitory digital storage medium may have a computer program stored thereon to perform the method of generating an error concealment signal, which method may have the steps of: generating a first replacement LPC representation and a different second replacement LPC representation; filtering a first codebook information using the first replacement representation to acquire a first replacement signal and filtering a different second codebook information using the second replacement LPC representation to acquire a second replacement signal; and combining the first replacement signal and the second replacement signal by summing-up the first replacement signal and the second replacement signal to acquire the error concealment signal, when said computer program is run by a computer. 
     In an aspect of the present invention, the apparatus for generating an error concealment signal comprises an LPC representation generator for generating a first replacement LPC representation and a different, second replacement LPC representation. Furthermore, an LPC synthesizer is provided for filtering a first codebook information using the first replacement LPC representation to obtain a first replacement signal and for filtering a second different codebook information using the second replacement LPC representation to obtain a second replacement signal. The outputs of the LPC synthesizer are combined by a replacement signal combiner combining the first replacement signal and the second replacement signal to obtain the error concealment signal. 
     The first codebook is advantageously an adaptive codebook for providing the first codebook information and the second codebook as advantageously a fixed codebook for providing the second codebook information. In other words, the first codebook represents the tonal part of the signal and the second or fixed codebook represents the noisy part of the signal and therefore can be considered to be a noise codebook. 
     The first codebook information for the adaptive codebook is generated using a mean value of last good LPC representations, the last good representation and a fading value. Furthermore, the LPC representation for the second or fixed codebook is generated using the last good LPC representation fading value and a noise estimate. Depending on the implementation, the noise estimate can be a fixed value, an offline trained value or it can be adaptively derived from a signal preceding an error concealment situation. 
     Advantageously, an LPC gain calculation for calculating an influence of a replacement LPC representation is performed and this information is then used in order to perform a compensation so that the power or loudness or, generally, an amplitude-related measure of the synthesis signal is similar to the corresponding synthesis signal before the error concealment operation. 
     In a further aspect, an apparatus for generating an error concealment signal comprises an LPC representation generator for generating one or more replacement LPC representations. Furthermore, the gain calculator is provided for calculating the gain information from the LPC representation and a compensator is then additionally provided for compensating a gain influence of the replacement LPC representation and this gain compensation operates using the gain operation provided by the gain calculator. An LPC synthesizer then filters a codebook information using the replacement LPC representation to obtain the error concealment signal, wherein the compensator is configured for weighting the codebook information before being synthesized by the LPC synthesizer or for weighting the LPC synthesis output signal. Thus, any gain or power or amplitude-related perceivable influence at the onset of an error concealment situation is reduced or eliminated. 
     This compensation is not only useful for individual LPC representations as outlined in the above aspect, but is also useful in the case of using only a single LPC replacement representation together with a single LPC synthesizer. 
     The gain values are determined by calculating impulse responses of the last good LPC representation and a replacement LPC representation and by particularly calculating an rms value over the impulse response of the corresponding LPC representation over a certain time which is between 3 and 8 ms and is advantageously 5 ms. 
     In an implementation, the actual gain value is determined by dividing a new rms value, i.e. an rms value for a replacement LPC representation by an rms value of good LPC representation. 
     Advantageously, the single or several replacement LPC representations is/are calculated using a background noise estimate which is advantageously a background noise estimate derived from the currently decoded signals in contrast to an offline trained vector simply predetermined noise estimate. 
     In a further aspect, an apparatus for generating a signal comprises an LPC representation generator for generating one or more replacement LPC representations, and an LPC synthesizer for filtering a codebook information using the replacement LPC representation. Additionally, a noise estimator for estimating a noise estimate during a reception of good audio frames is provided, and this noise estimate depends on the good audio frames. The representation generator is configured to use the noise estimate estimated by the noise estimator in generating the replacement LPC representation. 
     Spectral representation of a past decoded signal is process to provide a noise spectral representation or target representation. The noise spectral representation is converted into a noise LPC representation and the noise LPC representation is advantageously the same kind of LPC representation as the replacement LPC representation. ISF vectors are advantageous for the specific LPC-related processing procedures. 
     Estimate is derived using a minimum statistics approach with optimal smoothing to a past decoded signal. This spectral noise estimate is then converted into a time domain representation. Then, a Levinson-Durbin recursion is performed using a first number of samples of the time domain representation, where the number of samples is equal to an LPC order. Then, the LPC coefficients are derived from the result of the Levinson-Durbin recursion and this result is finally transformed in a vector. The aspect of using individual LPC representations for individual codebooks, the aspect of using one or more LPC representations with a gain compensation and the aspect of using a noise estimate in generating one or more LPC representations, which estimate is not an offline-trained vector but is a noise estimate derived from the past decoded signal are individually useable for obtaining an improvement with respect to conventional technology. 
     Additionally, these individual aspects can also be combined with each other so that, for example, the first aspect and the second aspect can be combined or the first aspect or the third aspect can be combined or the second aspect and the third aspect can be combined to each other to provide an even improved performance with respect to conventional technology. Even more advantageously, all three aspects can be combined with each other to obtain improvements over conventional technology. Thus, even though the aspects are described by separate figures all aspects can be applied in combination with each other, as can be seen by referring to the enclosed figures and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
         FIG. 1 a    illustrates an embodiment of the first aspect; 
         FIG. 1 b    illustrates a usage of an adaptive codebook; 
         FIG. 1 c    illustrates a usage of a fixed codebook in the case of a normal mode or a concealment mode; 
         FIG. 1 d    illustrates a flowchart for calculating the first LPC replacement representation; 
         FIG. 1 e    illustrates a flowchart for calculating the second LPC replacement representation; 
         FIG. 2  illustrates an overview over a decoder with error concealment controller and noise estimator; 
         FIG. 3  illustrates a detailed representation of the synthesis filters; 
         FIG. 4  illustrates a advantageous embodiment combining the first aspect and the second aspect; 
         FIG. 5  illustrates a further embodiment combining the first and second aspects; 
         FIG. 6  illustrates the embodiment combining the first and second aspects; 
         FIG. 7 a    illustrates an embodiment for performing a gain compensation. 
         FIG. 7 b    illustrates a flowchart for performing a gain compensation; 
         FIG. 8  illustrates a conventional-technology error concealment signal generator; 
         FIG. 9  illustrates an embodiment in accordance with the second aspect with gain compensation; 
         FIG. 10  illustrates a further implementation of the embodiment of  FIG. 9 ; 
         FIG. 11  illustrates an embodiment of the third aspect using the noise estimator; 
         FIG. 12 a    illustrates a advantageous implementation for calculating the noise estimate; 
         FIG. 12 b    illustrates a further advantageous implementation for calculating the noise estimate; and 
         FIG. 13  illustrates the calculation of a single LPC replacement representation or individual LPC replacement representations for individual codebooks using a noise estimate and applying a fading operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Advantageous embodiments of the present invention relate to controlling the level of the output signal by means of the codebook gains independently of any gain change caused by an extrapolated LPC and to control the LPC modeled spectral shape separately for each codebook. For this purpose, separate LPCs are applied for each codebook and compensation means are applied to compensate for any change of the LPC gain during concealment. 
     Embodiments of the present invention as defined in the different aspects or in combined aspects have the advantage of providing a high subjective quality of speech/audio in case of one or more data packets not being correctly or not being received at all at the decoder side. 
     Furthermore, the advantageous embodiments compensate the gain differences between subsequent LPCs during concealment, which might result from the LPC coefficients being changed over time, and therefore unwanted level changes are avoided. 
     Furthermore, embodiments are advantageous in that during concealment two or more sets of LPC coefficients are used to independently influence the spectral behavior of voiced and unvoiced speech parts and also tonal and noise-like audio parts. 
     All aspects of the present invention provide an improved subjective audio quality. 
     According to one aspect of this invention, the energy is precisely controlled during the interpolation. Any gain that is introduced by changing the LPC is compensated. 
     According to another aspect of this invention, individual LPC coefficient sets are utilized for each of the codebook vectors. Each codebook vector is filtered by its corresponding LPC and the individual filtered signals are just afterwards summed up to obtain the synthesized output. In contrast, state-of-the-art technology first adds up all excitation vectors (being generated from different codebooks) and just then feeds the sum to a single LPC filter. 
     According to another aspect, a noise estimate is not used, for example as an offline-trained vector, but is actually derived from the past decoded frames so that, after a certain amount of erroneous or missing packets/frames, a fade-out to the actual background noise rather than any predetermined noise spectrum is obtained. This particularly results in a feeling of acceptance at a user side, but to the fact that even when an error situation occurs, the signal provided by the decoder after a certain number of frames is related to the preceding signal. However, the signal provided by a decoder in the case of a certain number of lost or erroneous frames is a signal completely unrelated to the signal provided by the decoder before an error situation. 
     Applying gain compensation for the time-varying gain of the LPC allows the following advantages: 
     It compensates any gain that is introduced by changing the LPC. 
     Hence, the level of the output signal can be controlled by the codebook gains of the various codebooks. This allows for a pre-determined fade-out by eliminating any unwanted influence by the interpolated LPC. 
     Using a separate set of LPC coefficients for each codebook used during concealment allows the following advantages: 
     It creates the possibility to influence the spectral shape of tonal and noise like parts of the signal separately. 
     It gives the chance to play out the voiced signal part almost unchanged (e.g. desired for vowels), while the noise part may quickly be converging to background noise. 
     It gives the chance to conceal voiced parts, and fade out the voiced part with arbitrary fading speed (e.g. fade out speed dependent from signal characteristics), while simultaneously maintaining the background noise during concealment. State-of-the-art codecs usually suffer from a very clean voiced concealment sound. 
     It provides means to fade to background noise during concealment smoothly, by fading out the tonal parts without changing the spectral properties, and fading the noise like parts to the background spectral envelope. 
       FIG. 1 a    illustrates an apparatus for generating an error concealment signal  111 . The apparatus comprises an LPC representation generator  100  for generating a first replacement representation and additionally for generating a second replacement LPC representation. As outlined in  FIG. 1 a   , the first replacement representation is input into an LPC synthesizer  106  for filtering a first codebook information output by a first codebook  102  such as an adaptive codebook  102  to obtain a first replacement signal at the output of block  106 . Furthermore, the second replacement representation generated by the LPC representation generator  100  is input into the LPC synthesizer for filtering a second different codebook information provided by a second codebook  104  which is, for example, a fixed codebook, to obtain a second replacement signal at the output of block  108 . Both replacement signals are then input into a replacement signal combiner  110  for combining the first replacement signal and the second replacement signal to obtain the error concealment signal  111 . Both LPC synthesizers  106 ,  108  can be implemented in a single LPC synthesizer block or can be implemented as separate LPC synthesizer filters. In other implementations, both LPC synthesizer procedures can be implemented by two LPC filters actually being implemented and operating in parallel. However, the LPC synthesis can also be an LPC synthesis filter and a certain control so that the LPC synthesis filter provides an output signal for the first codebook information and the first replacement representation and then, subsequent to this first operation, the control provides the second codebook information and the second replacement representation to the synthesis filter to obtain the second replacement signal in a serial way. Other implementations for the LPC synthesizer apart from a single or several synthesis blocks are clear for those skilled in the art. 
     Typically, the LPC synthesis output signals are time domain signals and the replacement signal combiner  110  performs a synthesis output signal combination by performing a synchronized sample-by-sample addition. However, other combinations, such as a weighted sample-by-sample addition or a frequency domain addition or any other signal combination can be performed by the replacement signal combiner  110  as well. 
     Furthermore, the first codebook  102  is indicated as comprising an adaptive codebook and the second codebook  104  is indicated as comprising a fixed codebook. However, the first codebook and the second codebook can be any codebooks such as a predictive codebook as the first codebook and a noise codebook as the second codebook. However, other codebooks can be glottal pulse codebooks, innovative codebooks, transition codebooks, hybrid codebooks consisting of predictive and transform parts, codebooks for individual voice generators such as males/females/children or codebooks for different sounds such as for animal sounds, etc. 
       FIG. 1 b    illustrates a representation of an adaptive codebook. The adaptive codebook is provided with a feedback loop  120  and receives, as an input, a pitch lag  118 . The pitch lag can be a decoded pitch lag in the case of a good received frame/packet. However, if an error situation is detected indicating an erroneous or missing frame/packet, then an error concealment pitch lag  118  is provided by the decoder and input into the adaptive codebook. The adaptive codebook  102  can be implemented as a memory storing the fed back output values provided via the feedback line  120  and, depending on the applied pitch lag  118 , a certain amount of sampling values is output by the adaptive codebook. 
     Furthermore,  FIG. 1 c    illustrates a fixed codebook  104 . In the case of the normal mode, the fixed codebook  104  receives a codebook index and, in response to the codebook index, a certain codebook entry  114  is provided by the fixed codebook as codebook information. However, if a concealment mode is determined, a codebook index is not available. Then, a noise generator  112  provided within the fixed codebook  104  is activated which provides a noise signal as the codebook information  116 . Depending on the implementation, the noise generator may provide a random codebook index. However, it is advantageous that a noise generator actually provides a noise signal rather than a random codebook index. The noise generator  112  may be implemented as a certain hardware or software noise generator or can be implemented as noise tables or a certain “additional” entry in the fixed codebook which has a noise shape. 
     Furthermore, combinations of the above procedures are possible, i.e. a noise codebook entry together with a certain post-processing. 
       FIG. 1 d    illustrates a advantageous procedure for calculating a first replacement LPC representation in the case of an error. Step  130  illustrates the calculation of a mean value of LPC representations of two or more last good frames. Three last good frames are advantageous. Thus, a mean value over the three last good frames is calculated in block  130  and provided to block  136 . Furthermore, a stored last good frame LPC information is provided in step  132  and additionally provided to the block  136 . Furthermore, a fading factor  134  is determined in block  134 . Then, depending on the last good LPC information, depending on the mean value of the LPC information of the last good frame and depending on the fading factor of block  134 , the first replacement representation  138  is calculated. 
     For the state-of-the-art just one LPC is applied. For the newly proposed method, each excitation vector, which is generated by either the adaptive or the fixed codebook, is filtered by its own set of LPC coefficients. The derivation of the individual ISF vectors is as follows: 
     Coefficient set A (for filtering the adaptive codebook) is determined by this formula: 
               isf   ′     =           isf     -   2       +     isf     -   3       +     isf     -   4         3     ⁢           ⁢     (     block   ⁢           ⁢   136     )                     isf   A     -   1       =         alpha   A     ·     isf     -   2         +         (     1   -   alpha     )     ·     isf   ′       ⁢           ⁢     (     block   ⁢           ⁢   136     )               
where alpha A  is a time varying adaptive fading factor which may depend on signal stability, signal class, etc. isf −x  are the ISF coefficients, where x denotes the frame number, relative to the end of the current frame: x=−1 denotes the first lost ISF, x=−2 the last good, x=−3 second last good and so on. This leads to fading the LPC which is used for filtering the tonal part, starting from the last correctly received frame towards the average LPC (averaged over three of the last good 20 ms frames). The more frames get lost, the closer the ISF, which is used during concealment, will be to this short term average ISF vector (isf′).
 
       FIG. 1 e    illustrates a advantageous procedure for calculating the second replacement representation. In block  140 , a noise estimate is determined. Then, in block  142 , a fading factor is determined. Additionally, in block  144 , the last good frame is LPC information which has been stored before is provided. Then, in block  146 , a second replacement representation is calculated. Advantageously, a coefficient set B (for filtering the fixed codebook) is determined by this formula:
   isf   B   −1 =alpha B   ·isf   −2 +(1−beta)· isf   cng   (block 146)
 
where isf cng  is the ISF coefficient set derived from a background noise estimate and alpha B  is the time-varying fading speed factor which advantageously is signal dependent. The target spectral shape is derived by tracing the past decoded signal in the FFT domain (power spectrum), using a minimum statistics approach with optimal smoothing, similar to [3]. This FFT estimate is converted to the LPC representation by calculating the auto-correlation by doing inverse FFT and then using Levinson-Durbin recursion to calculate LPC coefficients using the first N samples of the inverse FFT, where N is the LPC order. This LPC is then converted into the ISF domain to retrieve isf cng . Alternatively—if such tracing of the background spectral shape is not available—the target spectral shape might also be derived based on any combination of an offline trained vector and the short-term spectral mean, as it is done in G.718 for the common target spectral shape.
 
     Advantageously, the fading factors A and α B  are determined depending on the decoded audio signal, i.e., depending on the decoded audio signal before the occurrence of an error. The fading factor may depend on signal stability, signal class, etc. Thus, is the signal is determined to be a quite noisy signal, then the fading factor is determined in such a way that the fading factor decreases, from time to time, more quickly than compared to a situation where a signal is quite tonal. In this situation, the fading factor decreases from one time frame to next time frame by a reduced amount. This makes sure that the fading out from the last good frame to the mean value of the last three good frames takes place more quickly in the case of noisy signals compared to non-noisy or tonal signals, where the fading out speed is reduced. Similar procedures can be performed for signal classes. For voiced signals, a fading out can be performed slower than for unvoiced signals or for music signals a certain fading speed can be reduced compared to further signal characteristics and corresponding determinations of the fading factor can be applied. 
     As discussed in the context of  FIG. 1 e   , a different fading factor α B  can be calculated for the second codebook information. Thus, the different codebook entries can be provided with a different fading speed. Thus, a fading out to the noise estimate as f cng  can be set differently from the fading speed from the last good frame ISF representation to the mean ISF representation as outlined in block  136  of  FIG. 1   d.    
       FIG. 2  illustrates an overview of a advantageous implementation. An input line receives, for example, from a wireless input interface or a cable interface packets or frames of an audio signal. The data on the input line  202  is provided to a decoder  204  and at the same time to an error concealment controller  200 . The error concealment controller determines whether received packet or frames are erroneous or missing. If this is determined, the error concealment controller inputs a control message to the decoder  204 . In the  FIG. 2  implementation, a “1” message on the control line CTRL signals that the decoder  204  is to operate in the concealment mode. However, if the error concealment controller does not find an error situation, then the control line CTRL carries a “0” message indicating a normal decoding mode as indicated in table  210  of  FIG. 2 . The decoder  204  is additionally connected to a noise estimator  206 . During the normal decoding mode, the noise estimator  206  receives the decoded audio signal via a feedback line  208  and determines a noise estimate from the decoded signal. However, when the error concealment controller indicates a change from the normal decoding mode to the concealment mode, the noise estimator  206  provides the noise estimate to the decoder  204  so that the decoder  204  can perform an error concealment as discussed in the preceding and the next figures. Thus, the noise estimator  206  is additionally controlled by the control line CTRL from the error concealment controller to switch, from the normal noise estimation mode in the normal decoding mode to the noise estimate provision operation in the concealment mode. 
       FIG. 4  illustrates a advantageous embodiment of the present invention in the context of a decoder, such as the decoder  204  of  FIG. 2 , having an adaptive codebook  102  and additionally having a fixed codebook  104 . In the normal decoding mode indicated by a control line data “0” as discussed in the context of the table  210  in  FIG. 2 , the decoder operates as illustrated in  FIG. 8 , when item  804  is neglected. Thus, the correctly received packet comprises a fixed codebook index for controlling the fixed codebook  802 , a fixed codebook gain g c  for controlling amplifier  806  and an adaptive codebook g p  in order to control the amplifier  808 . Furthermore, the adaptive codebook  800  is controlled by the transmitted pitch lag and the switch  812  is connected so that the adaptive codebook output is fed back into the input of the adaptive codebook. Furthermore, the coefficients for the LPC synthesis filter  804  are derived from the transmitted data. 
     However, if an error concealment situation is detected by the error concealment controller  202  of  FIG. 2 , the error concealment procedure is initiated in which, in contrast to the normal procedure, two synthesis filters  106 ,  108  are provided. Furthermore, the pitch lag for the adaptive codebook  102  is generated by an error concealment device. Additionally, the adaptive codebook gain g p  and the fixed codebook gain g c  are also synthesized by an error concealment procedure as known in the art in order to correctly control the amplifiers  402 ,  404 . 
     Furthermore, depending on the signal class, a controller  409  controls the switch  405  in order to either feedback a combination of both codebook outputs (subsequent to the application of the corresponding codebook gain) or to only feedback the adaptive codebook output. 
     In accordance with an embodiment, the data for the LPC synthesis filter A  106  and the data for the LPC synthesis filter B  108  is generated by the LPC representation generator  100  of  FIG. 1 a    and additionally a gain correction is performed by the amplifiers  406 ,  408 . To this end, the gain compensation factors g A  and g B  are calculated in order to correctly drive the amplifiers  408 ,  406  so that any gain influence generated by the LPC representation is stopped. Finally, the output of the LPC synthesis filters A, B indicated by  106  and  108  are combined by the combiner  110 , so that the error concealment signal is obtained. 
     Subsequently, the switching from the normal mode to the concealment mode on one hand and from the concealment mode back to the normal mode is discussed. 
     The transition from one common to several separate LPCs when switching from clean channel decoding to concealment does not cause any discontinuities, as the memory state of the last good LPC may be used to initialize each AR or MA memory of the separate LPCs. When doing so, a smooth transition from the last good to the first lost frame is ensured. 
     When switching from concealment to clean channel decoding (recovery phase), the approach of the separate LPCs introduces the challenge to correctly update the internal memory state of the single LPC filter during clean-channel decoding (usually AR (auto-regressive) models are used). Just using the AR memory of one LPC or an averaged AR memory would lead to discontinuities at the frame border between the last lost and the first good frame. In the following a method is described to overcome deal with this challenge: 
     A small portion of all excitation vectors (suggestion: 5 ms) is added at the end of any concealed frame. This summed excitation vector may then be fed to the LPC which would be used for recovery. This is shown in  FIG. 5 . Depending on the implementation it is also possible to sum up the excitation vectors after the LPC gain compensation. 
     It is advisable to start at frame end minus 5 ms, setting the LPC AR memory to zero, derive the LPC synthesis by using any of the individual LPC coefficient sets and save the memory state at the very end of the concealed frame. If the next frame is correctly received, this memory state may then be used for recovery (meaning: used for initializing the start-of-frame LPC memory), otherwise it is discarded. This memory has to be additionally introduced; it is to be handled separately from any of the used LPC AR memories of the concealment used during concealment. 
     Another solution for recovery is to use the method LPCO, known from USAC [4]. 
     Subsequently,  FIG. 5  is discussed in more detail. Generally, the adaptive codebook  102  can be termed to be a predictive codebook as indicated in  FIG. 5  or can be replaced by a predictive codebook. Furthermore, the fixed codebook  104  can be replaced or implemented as the noise codebook  104 . The codebook gains g p  and g c , in order to correctly drive the amplifiers  402 ,  404  are transmitted, in the normal mode, in the input data or can be synthesized by an error concealment procedure in the error concealment case. Furthermore, a third codebook  412 , which can be any other codebook, is used which additionally has an associated codebook gain gr as indicated by amplifier  414 . In an embodiment, an additional LPC synthesis by a separate filter controlled by an LPC replacement representation for the other codebook is implemented in block  416 . Furthermore, a gain correction g c  is performed in a similar way as discussed in the context of g A  and g B , as outlined. 
     Furthermore, the additional recovery LPC synthesizer X indicated at  418  is shown which receives, as an input, a sum of at least a small portion of all excitation vectors such as 5 ms. 
     This excitation vector is input into the LPC synthesizer X  418  memory states of the LPC synthesis filter X. 
     Then, when a switchback from the concealment mode to the normal mode occurs, the single LPC synthesis filter is controlled by copying the internal memory states of the LPC synthesis filter X into this single normal operating filter and additionally the coefficients of the filter are set by the correctly transmitted LPC representation. 
       FIG. 3  illustrates a further, more detailed implementation of the LPC synthesizer having two LPC synthesis filters  106 ,  108 . Each filter is, for example, an FIR filter or an IIR filter having filter taps  304 ,  306  and filter-internal memories  304 ,  308 . The filter taps  302 ,  306  are controlled by the corresponding LPC representation correctly transmitted or the corresponding replacement LPC representation generated by the LPC representation generator such as  100  of  FIG. 1 a   . Furthermore, a memory initializer  320  is provided. The memory initializer  320  receives the last good LPC representation and, when switch over to the error concealment mode is performed, the memory initializer  320  provides the memory states of the single LPC synthesis filter to the filter-internal memories  304 ,  308 . In particular, the memory initializer receives, instead of the last good LPC representation or in addition to the last good LPC representation, the last good memory states, i.e. the internal memory states of the single LPC filter in the processing, and particularly after the processing of the last good frame/packet. 
     Additionally, as already discussed in the context of  FIG. 5 , the memory initializer  320  can also be configured to perform the memory initialization procedure for a recovery from an error concealment situation to the normal non-erroneous operating mode. To this end, the memory initializer  320  or a separate future LPC memory initializer is configured for initializing a single LPC filter in the case of a recovery from an erroneous or lost frame to a good frame. The LPC memory initializer is configured for feeding at least a portion of a combined first codebook information and second codebook information or at least a portion of a combined weighted first codebook information or a weighted second codebook information into a separate LPC filter such as LPC filter  418  of  FIG. 5 . Additionally, the LPC memory initializer is configured for saving memory states obtained by processing the fed in values. Then, when a subsequent frame or packet is a good frame or packet, the single LPC filter  814  of  FIG. 8  for the normal mode is initialized using the saved memory states, i.e. the states from filter  418 . Furthermore, as outlined in  FIG. 5 , the filter coefficients for the filter can be either the coefficient for LPC synthesis filter  106  or LPC synthesis filter  108  or LPC synthesis filter  416  or a weighted or unweighted combination of those coefficients. 
       FIG. 6  illustrates a further implementation with gain compensation. To this end, the apparatus for generating an error concealment signal comprises a gain calculator  600  and a compensator  406 ,  408 , which has already been discussed in the context of  FIG. 4  ( 406 ,  408 ) and  FIG. 5  ( 406 ,  408 ,  409 ). In particular, the LPC representation calculator  100  outputs the first replacement LPC representation and the second replacement LPC representation to a gain calculator  600 . The gain calculator then calculates a first gain information for the first replacement LPC representation and the second gain information for the second LPC replacement representation and provides this data to the compensator  406 ,  408 , which receives, in addition to the first and second codebook information, as outlined in  FIG. 4  or  FIG. 5 , the LPC of the last good frame/packet/block. Then, the compensator outputs the compensated signal. The input into the compensator can either be an output of amplifiers  402 ,  404 , an output of the codebooks  102 ,  104  or an output of the synthesis blocks  106 ,  108  in the embodiment of  FIG. 4 . 
     Compensator  406 ,  408  partly or fully compensates a gain influence of the first replacement LPC in the first gain information and compensates a gain influence of the second replacement LPC representation using the second gain information. 
     In an embodiment, the calculator  600  is configured to calculate a last good power information related to a last good LPC representation before a start of the error concealment. Furthermore, the gain calculator  600  calculates a first power information for the first replacement LPC representation, a second power information for the second LPC representation, the first gain value using the last good power information and the first power information, and a second gain value using the last good power information and the second power information. Then, the compensation is performed in the compensator  406 ,  408  using the first gain value and using the second gain value. Depending on the information, however, the calculation of the last good power information can also be performed, as illustrated in the  FIG. 6  embodiment, by the compensator directly. However, due to the fact that the calculation of the last good power information is basically performed in the same way as the first gain value for the first replacement representation and the second gain value for the second replacement LPC representation, it is advantageous to perform the calculation of all gain values in the gain calculator  600  as illustrated by the input  601 . 
     In particular, the gain calculator  600  is configured to calculate from the last good LPC representation or the first and second LPC replacement representations an impulse response and to then calculate an rms (root mean square) value from the impulse response to obtain the correspondent power information in the gain compensation, each excitation vector is—after being gained by the corresponding codebook gain—again amplified by the gains: g A  or g B . These gains are determined by calculating the impulse response of the currently used LPC and then calculating the rms: 
     
       
         
           
             
               rms 
               new 
             
             = 
             
               
                 
                   ∑ 
                   
                     t 
                     = 
                     
                       0 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ms 
                     
                   
                   
                     5 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     ms 
                   
                 
                 ⁢ 
                 
                   
                     imp_resp 
                     2 
                   
                   ⁢ 
                   
                     ( 
                     t 
                     ) 
                   
                 
               
             
           
         
       
     
     The result is then compared to the rms of the last correctly received LPC and the quotient is used as gain factor in order to compensate for energy increase/loss of LPC interpolation: 
     
       
         
           
             g 
             = 
             
               
                 rms 
                 old 
               
               
                 rms 
                 new 
               
             
           
         
       
     
     This procedure can be seen as a kind of normalization. It compensates the gain, which is caused by LPC interpolation. 
     Subsequently,  FIGS. 7 a  and 7 b    are discussed in more detail to illustrate the apparatus for generating an error concealment signal or the gain calculator  600  or the compensator  406 ,  408  calculates the last good power information as indicated at  700  in  FIG. 7 a   . Furthermore, the gain calculator  600  calculates the first and second power information for the first and second LPC replacement representation as indicated at  702 . Then, as illustrated by  704 , the first and the second gain values are calculated advantageously by the gain calculator  600 . Then, the codebook information or the weighted codebook information or the LPC synthesis output is compensated using these gain values as illustrated at  706 . This compensation is advantageously done by the amplifiers  406 ,  408 . 
     To this end, several steps are performed in an advantageous embodiment as illustrated in  FIG. 7 b   . In step  710 , an LPC representation, such as the first or second replacement LPC representation or the last good LPC representation is provided. In step  712  the codebook gains are applied to the codebook information/output as indicated by block  402 ,  404 . Furthermore, in step  716 , impulse responses are calculated from the corresponding LPC representations. 
     Then, in step  718 , an rms value is calculated for each impulse response and in block  720  the corresponding gain is calculated using an old rms value and a new rms value and this calculation is advantageously done by dividing the old rms value by the new rms value. Finally, the result of block  720  is used to compensate the result of step  712  in order to finally obtained the compensated results as indicated at step  714 . 
     Subsequently, a further aspect is discussed, i.e. an implementation for an apparatus for generating an error concealment signal which ha the LPC representation generator  100  generating only a single replacement LPC representation, such as for the situation illustrated in  FIG. 8 . In contrast to  FIG. 8 , however, the embodiment illustrating a further aspect in  FIG. 9  comprises the gain calculator  600  and the compensator  406 ,  408 . Thus, any gain influence by the replacement LPC representation generated by the LPC representation generator is compensated for. In particular, this gain compensation can be performed on the input side of the LPC synthesizer as illustrated in  FIG. 9  by compensator  406 ,  408   n  or can be alternatively performed to the output of the LPC synthesizer as illustrated by the compensator  900  in order to finally obtain the error concealment signal. Thus, the compensator  406 ,  408 ,  900  is configured for weighting the codebook information or an LPC synthesis output signal provided by the LPC synthesizer  106 ,  108 . 
     The other procedures for the LPC representation generator, the gain calculator, the compensator and the LPC synthesizer can be performed in the same way as discussed in the context of  FIGS. 1 a    to  8 . 
     As has been outlined in the context of  FIG. 4 , the amplifier  402  and the amplifier  406  perform two weighting operations in series to each other, particularly in the case where not the sum of the multiplier output  402 ,  404  is fed back into the adaptive codebook, but where only the adaptive codebook output is fed back, i.e. when the switch  405  is in the illustrated position or the amplifier  404  and the amplifier  408  perform two weighting operations in series. In an embodiment, illustrated in  FIG. 10 , these two weighting operations can be performed in a single operation. To this end, the gain calculator  600  provides its output g p  or g c  to a single value calculator  1002 . Furthermore, a codebook gain generator  1000  is implemented in order to generate a concealment codebook gain as known in the art. The single value calculator  1002  then advantageously calculators a product between g p  and g A  in order to obtain the single value. Furthermore, for the second branch, the single value calculator  1002  calculates a product between g A  or g B  in order to provide the single value for the lower branch in  FIG. 4 . A further procedure can be performed for the third branch having amplifiers  414 ,  409  of  FIG. 5 . 
     Then a manipulator  1004  is provided which together performs the operations of for example amplifiers  402 ,  406  to the codebook information of a single codebook or to the codebook information of two or more codebooks in order to finally obtain a manipulated signal such as a codebook signal or a concealment signal, depending on whether the manipulator  1004  is located before the LPC synthesizer in  FIG. 9  or subsequent to the LPC synthesizer of  FIG. 9 .  FIG. 11  illustrates a third aspect, in which the LPC representation generator  100 , the LPC synthesizer  106 ,  108  and the additional noise estimator  206 , which has already been discussed in the context of  FIG. 2 , are provided. The LPC synthesizer  106 ,  108  receives codebook information and a replacement LPC representation. The LPC representation is generated by the LPC representation generator using the noise estimate from the noise estimator  206 , and the noise estimator  206  operates by determining the noise estimate from the last good frames. Thus, the noise estimate depends on the last good audio frames and the noise estimate is estimated during a reception of good audio frames, i.e. in the normal decoding mode indicated by “0” on the control line of  FIG. 2  and this noise estimate generated during the normal decoding mode is then applied in the concealment mode as illustrated by the connection of blocks  206  and  204  in  FIG. 2 . 
     The noise estimator is configured to process a spectral representation of a past decoded signal to provide a noise spectral representation and to convert the noise spectral representation into a noise LPC representation, where the noise LPC representation is the same kind of an LPC representation as the replacement LPC representation. Thus, when the replacement LPC representation is in the ISF-domain representation or an ISF vector, then the noise LPC representation additionally is an ISF vector or ISF representation. 
     Furthermore, the noise estimator  206  is configured to apply a minimum statistics approach with optimal smoothing to a past decoded signal to derive the noise estimate. For this procedure, it is advantageous to perform the procedure illustrated in [3]. However, other noise estimation procedures relying on, for example, suppression of tonal parts compared to non-tonal parts in a spectrum in order to filter out the background noise or noise in an audio signal can be applied as well for obtaining the target spectral shape or noise spectral estimate. 
     Thus, in one embodiment, a spectral noise estimate is derived from a past decoded signal and the spectral noise estimate is then converted into an LPC representation and then into an ISF domain to obtain the final noise estimate or target spectral shape. 
       FIG. 12 a    illustrates a advantageous embodiment. In step  1200 , the past decoded signal is obtained, as for example illustrated in  FIG. 2  by the feedback loop  208 . In step  1202 , a spectral representation, such as a Fast Fourier transform (FFT) representation is calculated. Then, in step  1204  a target spectral shape is derived such as by the minimum statistics approach with optimal smoothing or by any other noise estimator processing. Then, the target spectral shape is converted into an LPC representation as indicated by block  1206  and finally the LPC representation is converted to an ISF factor as outlined by block  1208  in order to finally obtain the target spectral shape in the ISF domain which can then be directly used by the LPC representation generator for generating a replacement LPC representation. In the equations of this application, the target spectral shape in the ISF domain is indicated as “ISF cng ”. 
     In a advantageous embodiment illustrated in  FIG. 12 b   , the target spectral shape is derived for example by a minimum statistics approach and optimal smoothing. Then, in step  1212 , a time domain representation is calculated by applying an inverse FFT, for example, to the target spectral shape. Then, LPC coefficients are calculated by using Levinson-Durbin recursion. However, the LPC coefficients calculation of block  1214  can also be performed by any other procedure apart from the mentioned Levinson-Durbin recursion. Then, in step  1216 , the final 
     ISF factor is calculated to obtain the noise estimate ISF cng  to be used by the LPC representation generator  100 . 
     Subsequently,  FIG. 13  is discussed for illustrating the usage of the noise estimate in the context of the calculation of a single LPC replacement representation  1308  for the procedure, for example, illustrated in  FIG. 8  or for calculating individual LPC representations for individual codebooks as indicated by block  1310  for the embodiment illustrated in  FIG. 1 . 
     In step  1300 , a mean value of two or three last good frames is calculated. In step  1302 , the last good frame LPC representation is provided. Furthermore, in step  1304 , a fading factor is provided which can be controlled, for example, by a separate signal analyzer which can be, for example, included in the error concealment controller  200  of  FIG. 2 . Then, in step  1306 , a noise estimate is calculated and the procedure in step  1306  can be performed by any of the procedures illustrated in  FIGS. 12 a   ,  12   b.    
     In the context of calculating a single LPC replacement representation, the outputs of blocks  1300 ,  1304 ,  1306  are provided to the calculator  1308 . Then, a single replacement LPC representation is calculated in such a way that subsequent to a certain number of lost or missing or erroneous frames/packets, the fading over to the noise estimate LPC representation is obtained. 
     However, individual LPC representations for an individual codebook, such as for the adaptive codebook and the fixed codebook, are calculated as indicated at block  1310 , then the procedure as discussed before for calculating ISF A   −1  (LPC A) on the hand and the calculation of ISF B   −1  (LPC B) is performed. 
     Although the present invention has been described in the context of block diagrams where the blocks represent actual or logical hardware components, the present invention can also be implemented by a computer-implemented method. In the latter case, the blocks represent corresponding method steps where these steps stand for the functionalities performed by corresponding logical or physical hardware blocks. 
     Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus. 
     Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disc, a DVD, a Blu-Ray, a CD, a ROM, a PROM, and EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable. 
     Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed. 
     Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may, for example, be stored on a machine readable carrier. 
     Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. 
     In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer. 
     A further embodiment of the inventive method is, therefore, a data carrier (or a non-transitory storage medium such as a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitory. 
     A further embodiment of the invention method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may, for example, be configured to be transferred via a data communication connection, for example, via the internet. 
     A further embodiment comprises a processing means, for example, a computer or a programmable logic device, configured to, or adapted to, perform one of the methods described herein. 
     A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein. 
     A further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver. The receiver may, for example, be a computer, a mobile device, a memory device or the like. The apparatus or system may, for example, comprise a file server for transferring the computer program to the receiver. 
     In some embodiments, a programmable logic device (for example, a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are advantageously performed by any hardware apparatus. 
     While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention. 
     REFERENCES 
     
         
         [1] ITU-T G.718 Recommendation, 2006 
         [2] Kazuhiro Kondo, Kiyoshi Nakagawa, “A Packet Loss Concealment Method Using Recursive Linear Prediction” Department of Electrical Engineering, Yamagata University, Japan. 
         [3] R. Martin, Noise Power Spectral Density Estimation Based on Optimal Smoothing and Minimum Statistics, IEEE Transactions on speech and audio processing, vol. 9, no. 5, July 2001 
         [4] Ralf Geiger et. al., Patent application US20110173011 A1, Audio Encoder and Decoder for Encoding and Decoding Frames of a Sampled Audio Signal 
         [5] 3GPP TS 26.190; Transcoding functions;—3GPP technical specification