EP2277172B1 - Concealment of transmission error in a digital signal in a hierarchical decoding structure - Google Patents

Concealment of transmission error in a digital signal in a hierarchical decoding structure Download PDF

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Publication number
EP2277172B1
EP2277172B1 EP09730641A EP09730641A EP2277172B1 EP 2277172 B1 EP2277172 B1 EP 2277172B1 EP 09730641 A EP09730641 A EP 09730641A EP 09730641 A EP09730641 A EP 09730641A EP 2277172 B1 EP2277172 B1 EP 2277172B1
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Prior art keywords
frame
signal
erased
samples
missing
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German (de)
French (fr)
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EP2277172A1 (en
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David Virette
Pierrick Philippe
Balazs Kovesi
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Orange SA
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France Telecom SA
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS OR SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/005—Correction of errors induced by the transmission channel, if related to the coding algorithm
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS OR SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/02—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders
    • G10L19/0212—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders using orthogonal transformation
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS OR SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/16—Vocoder architecture
    • G10L19/18—Vocoders using multiple modes
    • G10L19/24—Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding

Definitions

  • the present invention relates to the processing of digital signals in the telecommunications field. These signals may be, for example, speech and music signals.
  • the present invention intervenes in a coding / decoding system adapted for the transmission / reception of such signals. More particularly, the present invention relates to a reception processing for improving the quality of the decoded signals in the presence of data block losses.
  • disturbances may affect the transmitted signal and produce errors on the bitstream received by the decoder. These errors may occur in isolation in the bit stream but occur very frequently in bursts. It is then a packet of bits corresponding to a complete portion of signal which is erroneous or not received. This guy problem occurs for example for transmissions on mobile networks. It is also found in transmission on packet networks and in particular on Internet-type networks.
  • the transmission system or the modules in charge of reception detect that the received data are highly erroneous (for example on mobile networks), or that a block of data has not been received or is corrupted by errors
  • error concealment procedures are implemented.
  • LPC Linear Predictive Coding
  • LTP Long Term Prediction
  • the parameters of the erased frame are conventionally obtained as follows.
  • the LPC parameters of a frame to be reconstructed are obtained from the LPC parameters of the last valid frame, by simple copy of the parameters or with introduction of a certain damping (technique used for example in the standardized encoder G723.1). Then, a voicing or non-voicing in the speech signal is detected to determine a degree of harmonicity of the signal at the erased frame.
  • an excitation signal can be randomly generated (by drawing a codeword of the past excitation, by a slight damping of the gain of the past excitation, by random selection in the past excitement, or still using transmitted codes that may be totally wrong).
  • the pitch period (also called “ LTP delay ”) is generally the one calculated for the previous frame, possibly with a slight “jitter” (increase of the value of the LTP delay for consecutive error frames, the gain LTP being taken very close to 1 or equal to 1).
  • the excitation signal is therefore limited to the long-term prediction made from a past excitation.
  • the figure 1a illustrates the hierarchical coding of the CELP frames C0 to C5 and the transforms M1 to M5 applied to these frames.
  • the line referenced 10 corresponds to the reception of the frames
  • the line referenced 11 corresponds to the CELP synthesis
  • the line referenced 12 corresponds to the total synthesis after the MDCT transform.
  • the decoder when receiving frame 1 (CELP coding C1 and transform coding M1), the decoder synthesizes the CELP frame C1 which will be used to calculate the total synthesis signal of the following frame, and calculates the signal of total synthesis of the current frame O1 (line 12) from the CELP C0 synthesis, the M0 transform and the M1 transform. This additional delay in the total synthesis is well known in the context of transform coding.
  • the decoder in the presence of errors on the bitstream, the decoder operates as follows.
  • the decoder During the first error on the bitstream, the decoder contains in memory the CELP synthesis of the previous frame. So on the figure 1b when the frame 3 (C3 + M3) is erroneous, the decoder uses the decoded CELP synthesis C2 at the previous frame.
  • FEC frame Erasure Concealment
  • a valid frame includes information on the previous frame to improve the concealment of erased frames and resynchronization between erased frames and valid frames.
  • the decoder receives in the bitstream of the frame 5 information on the nature of the previous frame (by example indication of classification, information on the spectral envelope).
  • Classification information means information on voicing, non-voicing, the presence of attacks, etc.
  • the decoder synthesizes the previous erroneous frame (frame 4) by using a technique for concealing erased frames that benefits from the information received with the frame 5, before synthesizing the CELP signal C5.
  • the present invention improves the situation.
  • the use of information present in a valid frame to generate a second set of the missing samples of a previous erased frame makes it possible to increase the quality of the decoded audio signal by optimally adapting the missing samples.
  • the transition step between the first set of missing samples and the second set ensures continuity in the missing samples produced.
  • This transition step may advantageously be a recovery addition step.
  • this transition step can be provided by a linear prediction synthesis filtering step using to generate the second set of missing samples the transition point filter memories stored in the first step of concealment.
  • the memories of the synthesis filter at the transition point are stored in the first concealment step.
  • the excitation is determined according to the information received.
  • the synthesis is performed from the transition point using on the one hand the excitation obtained, on the other hand the memories of the stored synthesis filter.
  • the first set of samples is all the missing samples of the erased frame and the second set of samples is a part of the missing samples of the erased frame.
  • the distribution of the generation of the samples between two different time intervals and the fact of generating only a part of the samples in the second time interval makes it possible to reduce the peak of complexity which can be in the time interval corresponding to the valid frame. Indeed, in this time interval, the decoder must both generate missing samples of the previous frame, perform the transition step and decode the valid frame. It is therefore in this time interval that the peak of complexity of the decoding is located.
  • the information present in a valid frame is for example information on the classification of the signal and / or on the spectral envelope of the signal.
  • the signal classification information makes it possible, for example, for the step of concealing the second set of missing samples to adapt respective gains of a harmonic part of the excitation signal and of a random part of the excitation signal. for the signal corresponding to the erased frame.
  • This information therefore ensures better matching of the missing samples generated by the concealment step.
  • the first time slot being associated with said last erased frame and the second time slot being associated with said valid frame
  • the step of preparing the step of concealing the second set of missing samples is performed in a time interval different from that corresponding to the decoding of the valid frame. This therefore makes it possible to distribute the calculation load of the concealment step of the second set of samples and thus to reduce the peak of complexity in the time interval corresponding to the reception of the first valid frame. As shown above, it is indeed in this time interval corresponding to the valid frame that is the peak complexity or worse case of complexity of the decoding.
  • the distribution of the complexity thus carried out makes it possible to review downward the sizing of the processor of a transmission error concealment device which is dimensioned according to the worst case of complexity.
  • the preparation step comprises a step of generating a harmonic portion of the excitation signal and a step of generating a random portion of the excitation signal for the signal corresponding to the erased frame.
  • This device implements the steps of the concealment method as described above.
  • the invention also relates to a digital signal decoder comprising a transmission error concealment device according to the invention.
  • the invention relates to a computer program intended to be stored in a memory of a transmission error concealment device.
  • This computer program is such that it includes code instructions for carrying out the steps of the error concealment method according to the invention, when executed by a processor of said transmission error concealment device.
  • It relates to a storage medium, readable by a computer or by a processor, integrated or not into the device, storing a computer program as described above.
  • the transmission error concealment method according to a first embodiment of the invention is now described.
  • the frame N received at the decoder is erased.
  • a valid N-1 frame received at the decoder is processed by DEMUX demultiplexing module, normally decoded at 21 by a DE-NO decoding module.
  • the decoded signal is then stored in a memory buffer MEM during a step 22. At least part of this memorized decoded signal is sent to the sound card 30 at the output of the decoder of the frame N-1, the decoded signal remaining in the buffer is retained for sending to the sound card after decoding the next frame.
  • a step is taken to conceal a first set of samples for this missing frame at 23 by means of a DE-DISS error concealment module and by using the decoded signal of a previous frame.
  • the signal thus extrapolated is stored in memory MEM during step 24.
  • this extrapolated signal memorized, together with the decoded signal of the N-1 frame remaining stored, is sent to the sound card 30 at the output of the decoder of the frame N.
  • the extrapolated signal remaining in the buffer memory is retained. to be sent to the sound card after decoding the next frame.
  • a step of concealing a second set of missing samples for the erased N frame is performed at 25 by the DE-MISS error concealment module. This step uses information present in the valid frame N + 1 which is obtained during a step 26 of demultiplexing the N + 1 frame by the DEMUX demultiplexing module.
  • the information present in a valid frame includes information on the previous frame of the bit stream. These include signal classification information (voiced, unvoiced, transient signal) or information on the spectral envelope of the signal.
  • harmonic excitation is meant the excitation calculated from the pitch value (number of samples in a period corresponding to the inverse of the fundamental frequency) of the signal of the preceding frame, the harmonic part of the excitation signal. is thus obtained by copying the excitation passed to the moments corresponding to the delay of the pitch.
  • random excitation is meant the excitation signal obtained from a random signal generator or by random draw of a code word of the past excitation or in a dictionary.
  • a larger gain is calculated for the harmonic part of the excitation and in the case where the classification of the signal indicates an unvoiced frame, a larger gain is calculated for the random part of the excitation.
  • the part of the harmonic excitation is completely erroneous. In this case, several frames may be necessary before the decoder regains normal excitation and therefore an acceptable quality. Thus, a new artificial version of the harmonic excitation can be used to allow the decoder to find normal operation more quickly.
  • the information on the spectral envelope can be a stability information of the LPC linear prediction filter.
  • this information indicates that the filter is stable between the previous frame and the current (valid) frame
  • the step of concealing a second set of missing samples uses the linear prediction filter of the valid frame. Otherwise, the filter from the past is used.
  • a transition step 29 by a TRANS transition module is performed.
  • This module takes into account the first set of samples generated at step 23 not yet played on the sound card and the second set of samples generated in step 25 to obtain a smooth transition between the first set and the second set.
  • this transition step is a step of crossfading or addition-overlap which consists in gradually decreasing the weight of the extrapolated signal in the first set and gradually increasing the weight of the signal extrapolated in the second set to get the missing samples from the erased frame.
  • this fade-in step corresponds to the multiplication of all the samples of the extrapolated signal stored at the frame N with a weighting function decreasing progressively from 1 to 0, and the addition of this weighted signal with the samples of the signal extrapolated to the N + 1 frame multiplied with the complementary weighting function of the weighting function of the memorized signal.
  • complementary weighting function is meant the function obtained by subtracting one by the preceding weighting function.
  • this fade-in step is performed on only a part (at least one sample) of the stored signal.
  • this transition step is provided by the linear prediction synthesis filtering.
  • the memories of the synthesis filter at the transition point are stored in the first concealment step.
  • the excitation is determined according to the information received.
  • the synthesis is performed from the transition point using on the one hand the excitation obtained, on the other hand the memories of the stored synthesis filter.
  • the valid frame is de-multiplexed at 26, decoded normally at 27 and the decoded signal is stored at 28 in the memory buffer MEM.
  • the signal from the transition module TRANS is sent together with the decoded signal of the N + 1 frame to the sound card 30 at the output of the decoder of the N + 1 frame.
  • the signal received by the sound card 30 is intended to be restored by speaker type reproduction means 31.
  • the first set of samples and the second set of samples are the set of samples of the missing frame.
  • a signal corresponding to the erased frame is generated, the crossfade is then performed on the part of the two signals corresponding to the second half of the erased frame (one half-frame) to obtain the samples of the frame missing.
  • the concealment step in the time interval corresponding to the erased frame, the concealment step generates all the samples of the missing frame (these samples will be necessary if the next frame is also erased), while in the time interval corresponding to the decoding of the valid frame, the concealment step generates only a second portion of the samples, for example, the second half of the samples of the missing frame.
  • the overlap addition step is performed to ensure a transition on this second half of the samples of the missing frame.
  • the number of samples generated for the missing frame in the time interval corresponding to the valid frame is smaller than in the case of the first embodiment described above.
  • the decoding complexity in this time interval is therefore reduced.
  • a complexity distribution is performed to further reduce the worst case complexity without increasing the average complexity.
  • the step of concealing the second set of samples is split into two steps.
  • a first preparation step E1 does not produce missing samples and does not use the information from the valid frame, is performed in the previous time interval.
  • a second step E2 generating missing samples and using the information from the valid frame is performed in the time interval corresponding to the valid frame.
  • a preparation step E1 referenced 32 is performed.
  • This preparation step is for example a step of obtaining the harmonic part of the excitation using the value of the LTP delay of the previous frame, and of obtaining the random part of the excitation in a CELP decoding structure.
  • This preparation step uses parameters of the previous frame stored in memory MEM. It is not useful for this step to use the classification information or the spectral envelope information of the erased frame.
  • the concealment step 23 of the first set of samples as described with reference to FIG. figure 2 is also performed.
  • the extrapolated signal derived therefrom is stored at 24 in the memory MEM. At least a part of this extrapolated signal memorized, together with the decoded signal remaining stored in the N-1 frame, is sent to the sound card 30 at the output of the decoder of the frame N. The extrapolated signal remaining in the buffer is retained to be sent to the sound card after decoding the next frame.
  • the concealed step E2 referenced 33 comprising the extrapolation of the second set of missing samples corresponding to the erased N frame, is performed in the time interval corresponding to the N + 1 frame received at the decoder.
  • This step comprises the taking into account of the information contained in the valid frame N + 1 and which concern the frame N.
  • the concealment step corresponds to the calculation of the gains associated with the two parts of the excitation, and possibly to the correction of the phase of the harmonic excitation. Based on the classification information received in the first valid frame, the respective gains of the two portions of the excitation are matched. Thus, for example based on the classification information of the last valid frame received before the erased frames and the classification information received, the concealment step adapts the choice of the excitations and the associated gains to best represent the class of the frame. In this, the quality of the signal generated during the concealment step is improved by benefiting from the information received.
  • the step E2 favors the harmonic excitation obtained at the preparation step E1 rather than the random excitation and vice versa for a signal frame unvoiced.
  • step E2 will generate missing samples according to the precise classification of the transient (voiced to unvoiced or voiceless to voiced).
  • a step 29 addition-overlap or cross-fade as described with reference to the figure 2 is then performed between the first set of samples generated in step 23 and the second set of samples generated in step 33.
  • the N + 1 frame is processed by the DEMUX demultiplexing module, is decoded at 27 and stored at 28 as described previously with reference to FIG. figure 2 .
  • the signal extrapolated obtained by the cross-fading step 29 and the decoded signal of the N + 1 frame are jointly sent to the sound card 30 at the output of the decoder of the N + 1 frame.
  • FIGS. 4a and 4b illustrate the implementation of this method and the synchronization between CELP decoding and transform decoding using low delay windows represented here in the form of windows as described in the patent application. FR 0760258 .
  • the figure 4a illustrates the hierarchical coding of CELP frames C0 to C5 and the low delay transforms M1 to M5 applied to these frames.
  • the figure 4b illustrates the decoding of frames C0 to C5.
  • Line 40 illustrates the signal received at the decoder
  • line 41 illustrates the CELP synthesis in the first decoding stage
  • line 42 illustrates the total synthesis using the low delay transform (MDCT).
  • MDCT low delay transform
  • the time offset between the two decoding stages is less than one frame, it is represented here for the sake of simplicity at a shift of half a frame.
  • part of the CELP synthesis of the previous frame C0 and the transform M0 is used as well as a part of the CELP synthesis of the current frame C1 and the transform M1.
  • the decoder Upon detection of the first erased frame (C3 + M3), the decoder uses the CELP synthesis of the previous frame 2 (C2) to construct the total synthesis signal (03). It is also necessary to generate from an error concealment algorithm, the signal corresponding to the CELP synthesis of the frame 3 (C3).
  • This regenerated signal is named FEC-C3 on the figure 4b .
  • the output signal of the decoder 03 is therefore composed of the last half of the signal C2 and the first half of the extrapolated signal FEC-C3.
  • a concealment step for the frame C4 is performed to generate samples corresponding to the missing frame C4. This gives a first set of samples noted FEC1-C4 for the missing frame C4.
  • the output frame 4 of the decoder is constructed using a portion of extrapolated samples for C3 (FEC-C3) and a portion of the first set of extrapolated samples for C4 (FEC1-C4).
  • a step of concealing a second set of samples for the frame C4 is performed. This step uses the I5 information on the C4 frame that is present in the valid frame C5. This second set of samples is reference FEC2-C4.
  • a transition step between the first set of samples FEC1-C4 and the second set of samples FEC2-C4 is performed by overlapping or cross faded addition to obtain the missing samples FEC-C4 of the second half of the erased frame C4.
  • the output frame 05 of the decoder is constructed using a portion of samples from the cross-fading step (FEC-C4) and a portion of the decoded samples for the valid frame C5.
  • the core decoding is a CELP type decoding.
  • This decoding heart can be of any other type.
  • it can be replaced by a decoder of type ADPCM (as for example the standard encoder / decoder G.722).
  • the continuity between two frames is not necessarily provided by linear prediction synthesis filtering (LPC).
  • LPC linear prediction synthesis filtering
  • the input signal S of the encoder is filtered by an HP high pass filter 50.
  • this filtered signal is downsampled by the module 51 at the frequency of the coder ACELP (for "Algebraic Code Excited Linear Prediction"). "in English) to then be encoded by an ACELP encoding method.
  • the signal from this coding stage is then multiplexed in the multiplexing module 56.
  • Information concerning the previous frame (sub) is also sent to the multiplexing module to form the bit stream T.
  • the signal resulting from the ACELP coding is also oversampled at a sampling frequency corresponding to the original signal, by the module 53.
  • This oversampled signal is subtracted from the filtered signal at 54 to enter a second coding stage where an MDCT transform is performed in the module 55.
  • the signal is then quantized in the module 57 and is multiplexed by the multiplexing module MUX to form the bit stream T.
  • a first ACELP decoding stage 61 is performed.
  • the signal thus decoded is oversampled by the module 62 at the frequency of the signal. It is then processed by an MDCT transform module 63.
  • the transform used here is a weak delay transform as described in the document "Low-Overlap” presented in " Real-Time Implementation of the MPEG-4 Low-Delay Advanced Audio Coding Algorithm (AAC-LD) on Motorola's DSP56300 "by J. Hilpert et al published at the 108th AES Convention in February 2000 or as described in the patent application FR 07 60258 .
  • the time offset between the first decoding stage ACELP and that of the transform is therefore half a frame.
  • the signal is, in a second decoding stage, dequantized in the module 68 and added at 67 to the signal from the transform.
  • An inverse transform is then applied at 64.
  • the signal derived therefrom is then post-processed (PF) 65 using the signal from the module 62 and then filtered at 66 by a high-pass filter which provides the output signal S s of the decoder.
  • the decoder includes a transmission error concealment device 70 which receives from the demultiplexing module erased frame information bfi.
  • This device comprises a concealment module 71 which according to the invention receives when decoding a valid frame, information inf. relating to the concealment of frame loss.
  • This module performs, in a first time interval, the concealment of a first set of samples of an erased frame, then in a time interval corresponding to the decoding of a valid frame, it performs the concealment of a second set of samples of the erased frame.
  • the device 70 also includes a transition module 72 TRANS adapted to make a transition between the first set of samples and the second set of samples to provide at least a portion of the samples of the erased frame.
  • a transition module 72 TRANS adapted to make a transition between the first set of samples and the second set of samples to provide at least a portion of the samples of the erased frame.
  • the output signal of the heart of the hierarchical decoder is either the signal from the ACELP decoder 61 or the signal from the concealment module 70.
  • the continuity between the two signals is ensured by the fact that they share the synthesis memories of the filter LPC linear prediction.
  • the device 70 for concealing a transmission error is for example as illustrated in FIG. figure 7 .
  • this device in the sense of the invention typically comprises a ⁇ P processor cooperating with a memory block BM including a storage and / or working memory, as well as a memory buffer MEM mentioned above as a means for storing the decoded frames. and sent with a time offset.
  • This device receives as input successive frames of the digital signal Se and delivers the synthesized signal Ss comprising the samples of an erased frame.
  • the memory block BM may comprise a computer program comprising the code instructions for implementing the steps of the method according to the invention when these instructions are executed by a ⁇ P processor of the device and in particular a step of concealing a first set of missing samples for the erased frame, implemented in a first time interval, a step of concealing a second set of missing samples for the erased frame taking into account information of said valid frame and implemented in a second time interval; and an overlap adding step between the first set of missing samples and the second set of missing samples to obtain (at least a portion of?) the missing frame.
  • the figures 2 and 3 can illustrate the algorithm of such a computer program.
  • This concealment device according to the invention can be independent or integrated in a digital signal decoder.

Abstract

The invention relates to a method of concealing a transmission error in a digital signal chopped into a plurality of successive frames associated with different time intervals in which, on reception, the signal is liable to comprise erased frames and valid frames, the valid frames comprising information (inf.) relating to the concealment of frame loss. The method is implemented during a hierarchical decoding using a core decoding and a transform-based decoding using windows with small delay introducing a time delay of less than a frame with respect to the core decoding. To replace at least the last frame erased before a valid frame, the method comprises a step (23) of concealing a first set of missing samples for the erased frame, implemented in a first time interval; a step (25) of concealing a second set of missing samples taking into account information of said valid frame and implemented in a second time interval; and a step (29) of transition between the first and the second set of missing samples to obtain at least part of the missing frame.

Description

La présente invention concerne le traitement de signaux numériques dans le domaine des télécommunications. Ces signaux peuvent être par exemple des signaux de parole, de musique.The present invention relates to the processing of digital signals in the telecommunications field. These signals may be, for example, speech and music signals.

La présente invention intervient dans un système de codage/décodage adapté pour la transmission/réception de tels signaux. Plus particulièrement, la présente invention porte sur un traitement à la réception permettant d'améliorer la qualité des signaux décodés en présence de pertes de blocs de données.The present invention intervenes in a coding / decoding system adapted for the transmission / reception of such signals. More particularly, the present invention relates to a reception processing for improving the quality of the decoded signals in the presence of data block losses.

Différentes techniques existent pour convertir sous forme numérique et compresser un signal audionumérique. Les techniques les plus courantes sont :

  • les méthodes de codage de forme d'onde, telles que le codage MIC (pour "Modulation par Impulsions Codées") et MICDA (pour "Modulation par Impulsion et Codage Différentiel Adaptatif'), dits aussi "PCM" et "ADPCM" en anglais,
  • les méthodes de codage paramétrique par analyse par synthèse comme le codage CELP (pour "Code Excited Linear Prediction"), et
  • les méthodes de codage perceptuel en sous-bandes ou par transformée.
Different techniques exist for converting into digital form and compressing a digital audio signal. The most common techniques are:
  • waveform coding methods, such as MIC (for "Coded Pulse Modulation") and ADPCM (for "Pulse Modulation and Adaptive Differential Coding"), also known as "PCM" and "ADPCM" in English ,
  • parametric coding methods using synthetic analysis such as Code Excited Linear Prediction (CELP), and
  • perceptual encoding methods in subbands or by transform.

Ces techniques traitent le signal d'entrée de façon séquentielle échantillon par échantillon (MIC ou MICDA) ou par blocs d'échantillons dits "trames" (CELP et codage par transformée). Pour tous ces codeurs, les valeurs codées sont ensuite transformées en un train binaire qui est transmis sur un canal de transmission.These techniques treat the input signal sequentially sample by sample (MIC or ADPCM) or sample blocks called "frames" (CELP and transform coding). For all these encoders, the encoded values are then transformed into a bit stream which is transmitted over a transmission channel.

Selon la qualité de ce canal et le type de transport, des perturbations peuvent affecter le signal transmis et produire des erreurs sur le train binaire reçu par le décodeur. Ces erreurs peuvent intervenir de manière isolée dans le train binaire mais se produisent très fréquemment par rafales. C'est alors un paquet de bits correspondant à une portion complète de signal qui est erroné ou non reçue. Ce type de problème se rencontre par exemple pour les transmissions sur les réseaux mobiles. Il se rencontre aussi dans les transmissions sur les réseaux par paquets et en particulier sur les réseaux de type internet.Depending on the quality of this channel and the type of transport, disturbances may affect the transmitted signal and produce errors on the bitstream received by the decoder. These errors may occur in isolation in the bit stream but occur very frequently in bursts. It is then a packet of bits corresponding to a complete portion of signal which is erroneous or not received. This guy problem occurs for example for transmissions on mobile networks. It is also found in transmission on packet networks and in particular on Internet-type networks.

Lorsque le système de transmission ou les modules chargés de la réception permettent de détecter que les données reçues sont fortement erronées (par exemple sur les réseaux mobiles), ou qu'un bloc de données n'a pas été reçu ou est corrompu par des erreurs binaires (cas de systèmes à transmission par paquets par exemple), des procédures de dissimulation des erreurs sont alors mises en oeuvre.When the transmission system or the modules in charge of reception detect that the received data are highly erroneous (for example on mobile networks), or that a block of data has not been received or is corrupted by errors For example, in the case of packet transmission systems, for example, error concealment procedures are implemented.

La trame courante à décoder est alors déclarée effacée ("bad frame" en anglais). Ces procédures permettent d'extrapoler au décodeur les échantillons du signal manquant à partir des signaux et données issus des trames précédentes.The current frame to be decoded is then declared erased (" bad frame " in English). These procedures make it possible to extrapolate to the decoder the samples of the missing signal from the signals and data from the previous frames.

De telles techniques ont été mises en oeuvre principalement dans le cas des codeurs paramétriques et prédictifs (techniques de récupération/dissimulation des trames effacées). Elles permettent de limiter fortement la dégradation subjective du signal perçue au décodeur en présence de trames effacées. Ces algorithmes reposent sur la technique utilisée pour le codeur et le décodeur, et constituent en fait une extension du décodeur. Les dispositifs de dissimulation de trames effacées ont pour objectif d'extrapoler les paramètres de la trame effacée à partir de la (ou des) dernières trames précédentes considérées comme valides.Such techniques have been implemented mainly in the case of parametric and predictive coders (techniques for recovery / concealment of erased frames). They make it possible to strongly limit the subjective degradation of the signal perceived at the decoder in the presence of erased frames. These algorithms rely on the technique used for the encoder and decoder, and are in fact an extension of the decoder. The purpose of the hiding devices of erased frames is to extrapolate the parameters of the erased frame from the last (or more) previous frames considered valid.

Certains paramètres manipulés ou codés par les codeurs prédictifs présentent une forte corrélation inter-trames (cas des paramètres LPC (pour « Linear Prédictive Coding » en anglais) qui représentent l'enveloppe spectrale, et des paramètres LTP (pour « Long Term Prédiction » en anglais) de prédiction à long terme qui représente la périodicité du signal (pour les sons voisés, par exemple). Du fait de cette corrélation, il est beaucoup plus avantageux de réutiliser les paramètres de la dernière trame valide pour synthétiser la trame effacée que d'utiliser des paramètres erronés ou aléatoires.Some parameters manipulated or coded by predictive coders have a strong inter-frame correlation (in the case of Linear Predictive Coding (LPC) parameters which represent the spectral envelope, and LTP parameters (for Long Term Prediction). English) long-term prediction that represents the periodicity of the signal (for voiced sounds, for example) .This correlation makes it much more advantageous to reuse the parameters of the last valid frame to synthesize the erased frame than use erroneous or random parameters.

Dans le contexte d'un décodage CELP, les paramètres de la trame effacée sont classiquement obtenus comme suit.In the context of a CELP decoding, the parameters of the erased frame are conventionally obtained as follows.

Les paramètres LPC d'une trame à reconstruire sont obtenus à partir des paramètres LPC de la dernière trame valide, par simple recopie des paramètres ou encore avec introduction d'un certain amortissement (technique utilisée par exemple dans le codeur normalisé G723.1). Ensuite, on détecte un voisement ou un non voisement dans le signal de parole pour déterminer un degré d'harmonicité du signal au niveau de la trame effacée.The LPC parameters of a frame to be reconstructed are obtained from the LPC parameters of the last valid frame, by simple copy of the parameters or with introduction of a certain damping (technique used for example in the standardized encoder G723.1). Then, a voicing or non-voicing in the speech signal is detected to determine a degree of harmonicity of the signal at the erased frame.

Si le signal est non voisé, un signal d'excitation peut être généré de manière aléatoire (par tirage d'un mot de code de l'excitation passée, par léger amortissement du gain de l'excitation passée, par sélection aléatoire dans l'excitation passée, ou en utilisant encore des codes transmis qui peuvent être totalement erronés).If the signal is unvoiced, an excitation signal can be randomly generated (by drawing a codeword of the past excitation, by a slight damping of the gain of the past excitation, by random selection in the past excitement, or still using transmitted codes that may be totally wrong).

Si le signal est voisé, la période de pitch (appelée aussi "délai LTP") est généralement celle calculée pour la trame précédente, éventuellement avec une légère "gigue" (augmentation de la valeur du délai LTP pour les trames d'erreur consécutive, le gain LTP étant pris très voisin de 1 ou égal à 1). Le signal d'excitation est donc limité à la prédiction à long terme effectuée à partir d'une excitation passée.If the signal is voiced, the pitch period (also called " LTP delay ") is generally the one calculated for the previous frame, possibly with a slight "jitter" (increase of the value of the LTP delay for consecutive error frames, the gain LTP being taken very close to 1 or equal to 1). The excitation signal is therefore limited to the long-term prediction made from a past excitation.

La complexité de calcul de ce type d'extrapolation de trames effacées est généralement comparable à celle d'un décodage d'une trame valide (ou "good frame" en anglais) : à la place du décodage et de la quantification inverse des paramètres on utilise les paramètres estimés à partir du passé, éventuellement légèrement modifiés, puis on synthétise le signal reconstruit de la même manière que pour une trame valide en utilisant les paramètres ainsi obtenus.The computation complexity of this type of extrapolation of erased frames is generally comparable to that of a decoding of a valid frame (or " good frame " in English): instead of decoding and inverse quantization of the parameters, uses the parameters estimated from the past, possibly slightly modified, and then synthesizes the reconstructed signal in the same way as for a valid frame using the parameters thus obtained.

Dans une structure de codage hiérarchique, utilisant en codage coeur une technique de type CELP et en codage du signal d'erreur, un codage par transformée, il peut être intéressant d'utiliser le décalage temporel généré par ce système de décodage hiérarchique pour la dissimulation de trame effacée.In a hierarchical coding structure, using in core coding a CELP type technique and in coding the error signal, a transform coding, it may be advantageous to use the temporal offset generated by this hierarchical decoding system for concealment. of erased frame.

La figure 1a illustre le codage hiérarchique des trames CELP C0 à C5 et les transformées M1 à M5 appliquées à ces trames.The figure 1a illustrates the hierarchical coding of the CELP frames C0 to C5 and the transforms M1 to M5 applied to these frames.

Lors de la transmission de ces trames à un décodeur correspondant, les trames C3 et C4 grisées et les transformées M3 et M4 sont effacées.When transmitting these frames to a corresponding decoder, the grayed C3 and C4 frames and the M3 and M4 transforms are erased.

Ainsi, au décodeur, en référence à la figure 1b, la ligne référencée 10 correspond à la réception des trames, la ligne référencée 11 correspond à la synthèse CELP et la ligne référencée 12 correspond à la synthèse totale après transformée MDCT.Thus, in the decoder, with reference to FIG. 1b , the line referenced 10 corresponds to the reception of the frames, the line referenced 11 corresponds to the CELP synthesis and the line referenced 12 corresponds to the total synthesis after the MDCT transform.

On peut noter que lors de la réception de la trame 1 (codage CELP C1 et codage par transformée M1), le décodeur synthétise la trame CELP C1 qui sera utilisée pour calculer le signal de synthèse total de la trame suivante, et calcule le signal de synthèse total de la trame courante O1 (ligne 12) à partir de la synthèse CELP C0, de la transformée M0 et de la transformée M1. Ce retard supplémentaire dans la synthèse totale est bien connu dans le contexte de codage par transformée.It may be noted that when receiving frame 1 (CELP coding C1 and transform coding M1), the decoder synthesizes the CELP frame C1 which will be used to calculate the total synthesis signal of the following frame, and calculates the signal of total synthesis of the current frame O1 (line 12) from the CELP C0 synthesis, the M0 transform and the M1 transform. This additional delay in the total synthesis is well known in the context of transform coding.

Dans ce cas, en présence d'erreurs sur le train binaire, le décodeur fonctionne comme suit.In this case, in the presence of errors on the bitstream, the decoder operates as follows.

Lors de la première erreur sur le train binaire, le décodeur contient en mémoire la synthèse CELP de la trame précédente. Ainsi sur la figure 1b, lorsque la trame 3 (C3 + M3) est erronée, le décodeur utilise la synthèse CELP C2 décodée à la trame précédente.During the first error on the bitstream, the decoder contains in memory the CELP synthesis of the previous frame. So on the figure 1b when the frame 3 (C3 + M3) is erroneous, the decoder uses the decoded CELP synthesis C2 at the previous frame.

Le remplacement de la trame erronée (C3) est nécessaire pour générer la sortie suivante (04), pour faire cela une technique de dissimulation de trames effacées encore appelée FEC (pour "frame Erasure Concealment" en anglais) est utilisée, comme par exemple décrit dans le document intitulé "Method of packet errors cancellation suitable for any speech and sound compression scheme" de B. KOVESI et D. Massaloux, ISIVC-2004.The replacement of the erroneous frame (C3) is necessary to generate the next output (04), to do this a technique for concealing erased frames still called FEC (for "frame Erasure Concealment" in English) is used, as for example described in the document "B. KOVESI and D. Massaloux, Method of packet errors cancellation for a speech and sound compression scheme", ISIVC-2004.

Ce décalage temporel entre la détection de trame erronée et la nécessité de synthétiser le signal correspondant permet d'utiliser des techniques de transmission d'information de correction d'erreurs pour la trame CELP précédente comme décrit dans " Efficient frame erasure concealment in predictive speech codecs using glotal pulse resynchronisation" T. Vaillancourt et al publié à ICASSP 2007 .This time shift between the erroneous frame detection and the need to synthesize the corresponding signal makes it possible to use error correction information transmission techniques for the previous CELP frame as described in " Efficient frame erasure concealment in predictive speech codecs using glotal pulse resynchronization "T. Vaillancourt et al published at ICASSP 2007 .

Dans ce document, une trame valide comporte des informations sur la trame précédente pour améliorer la dissimulation des trames effacées et la resynchronisation entre les trames effacées et les trames valides.In this document, a valid frame includes information on the previous frame to improve the concealment of erased frames and resynchronization between erased frames and valid frames.

Ainsi, sur la figure 1b, lors de la réception de la trame 5 (C5 + M5) après la détection de deux trames erronées (trame 3 et 4), le décodeur reçoit dans le train binaire de la trame 5 des informations sur la nature de la trame précédente (par exemple indication de classification, information sur l'enveloppe spectrale). Par information de classification, on entend une information sur le voisement, le non voisement, la présence d'attaques, etc...So, on the figure 1b at the reception of the frame 5 (C5 + M5) after the detection of two errored frames (frame 3 and 4), the decoder receives in the bitstream of the frame 5 information on the nature of the previous frame (by example indication of classification, information on the spectral envelope). Classification information means information on voicing, non-voicing, the presence of attacks, etc.

Ce type d'informations dans le train binaire est par exemple décrit dans le document " Wideband Speech Coding Advances in VMR-WV Standard" de M. Jelinek et R. Salami publié dans IEEE Transactions on audio, speech and language processing Mai 2007 .This type of information in the bit stream is for example described in the document " Wideband Speech Coding Advances in VMR-WV Standard "by M. Jelinek and R. Salami published in IEEE Transactions on audio, speech and language processing May 2007 .

Ainsi, le décodeur synthétise la trame erronée précédente (trame 4) en utilisant une technique de dissimulation de trames effacées qui bénéficie de l'information reçue avec la trame 5, avant de synthétiser le signal CELP C5.Thus, the decoder synthesizes the previous erroneous frame (frame 4) by using a technique for concealing erased frames that benefits from the information received with the frame 5, before synthesizing the CELP signal C5.

D'autre part, des techniques de codage hiérarchique ont été développées pour diminuer le décalage temporel entre les deux étages de codage. Ainsi, il existe des transformées à faible retard qui diminue le décalage temporel à une demi-trame. C'est par exemple le cas de l'utilisation d'une fenêtre appelée "Low-Overlap" présentée dans " Real-Time Implementation of the MPEG-4 Low-Delay Advanced Audio Coding Algorithm (AAC-LD) on Motorola's DSP56300" de J. Hilpert et al publié à la 108ème convention AES en février 2000 .On the other hand, hierarchical coding techniques have been developed to reduce the time difference between the two coding stages. Thus, there are low delay transforms that decrease the time offset to half a frame. This is for example the case of the use of a window called "Low-Overlap" presented in " Real-Time Implementation of the MPEG-4 Low-Delay Advanced Audio Coding Algorithm (AAC-LD) on Motorola's DSP56300 "by J. Hilpert et al published at the 108th AES Convention in February 2000 .

Dans ces techniques de transformée à faible retard, il n'est alors plus possible de bénéficier de l'information de la trame courante valide pour générer les échantillons manquants d'une trame effacée comme pour les techniques décrites précédemment, le décalage temporel étant inférieur à une trame. La qualité du signal en cas de trames erronées est donc plus faible.In these low-delay transform techniques, it is no longer possible to benefit from the information of the valid current frame to generate the missing samples of an erased frame as for the techniques described above, the time offset being less than a frame. The quality of the signal in case of erroneous frames is therefore lower.

Il existe donc un besoin d'améliorer la qualité de la dissimulation de trames effacées dans un système de décodage hiérarchique à faible retard sans pour autant introduire de retard temporel supplémentaire.There is therefore a need to improve the quality of hiding erased frames in a low delay hierarchical decoding system without introducing additional time delay.

La présente invention vient améliorer la situation.The present invention improves the situation.

Elle propose à cet effet un procédé de dissimulation d'erreur de transmission dans un signal numérique découpé en une pluralité de trames successives associées à des intervalles temporels différents dans lequel, à la réception, le signal est susceptible de comporter des trames effacées et des trames valides, les trames valides comportant des informations (inf.) relatives à la dissimulation de perte de trame. Le procédé est tel qu'il est mis en oeuvre lors d'un décodage hiérarchique utilisant un décodage coeur et un décodage par transformée utilisant des fenêtres à faible retard introduisant un retard temporel inférieur à une trame par rapport au décodage coeur, et que pour remplacer au moins la dernière trame effacée avant une trame valide, il comporte:

  • une étape de dissimulation d'un premier ensemble d'échantillons manquants pour la trame effacée, mise en oeuvre dans un premier intervalle temporel;
  • une étape de dissimulation d'un deuxième ensemble d'échantillons manquants pour la trame effacée prenant en compte des informations de ladite trame valide et mise en oeuvre dans un second intervalle temporel; et
  • une étape de transition entre le premier ensemble d'échantillons manquants et le deuxième ensemble d'échantillons manquants pour obtenir au moins une partie de la trame manquante.
To this end, it proposes a transmission error concealment method in a digital signal divided into a plurality of successive frames associated with different time intervals in which, on reception, the signal may comprise erased frames and frames. valid, valid frames with information (inf.) relating to the loss of frame concealment. The method is such that it is implemented during a hierarchical decoding using a core decoding and a transform decoding using low delay windows introducing a time delay less than one frame with respect to the decoding heart, and that to replace at least the last frame erased before a valid frame, it comprises:
  • a step of concealing a first set of missing samples for the erased frame, implemented in a first time interval;
  • a step of concealing a second set of missing samples for the erased frame taking into account information of said valid frame and implemented in a second time slot; and
  • a transition step between the first set of missing samples and the second set of missing samples to obtain at least a portion of the missing frame.

Ainsi, l'utilisation d'informations présentes dans une trame valide pour générer un deuxième ensemble des échantillons manquants d'une trame effacée précédente, permet d'augmenter la qualité du signal audio décodée en adaptant au mieux les échantillons manquants. L'étape de transition entre le premier ensemble d'échantillons manquants et le deuxième ensemble permet d'assurer la continuité dans les échantillons manquants produits.Thus, the use of information present in a valid frame to generate a second set of the missing samples of a previous erased frame makes it possible to increase the quality of the decoded audio signal by optimally adapting the missing samples. The transition step between the first set of missing samples and the second set ensures continuity in the missing samples produced.

Cette étape de transition peut être avantageusement une étape d'addition recouvrement.This transition step may advantageously be a recovery addition step.

Dans un second mode de réalisation, cette étape de transition peut être assurée par une étape de filtrage de synthèse de prédiction linéaire utilisant pour générer le deuxième ensemble d'échantillons manquants les mémoires de filtre au point de transition, stockées lors de la première étape de dissimulation.In a second embodiment, this transition step can be provided by a linear prediction synthesis filtering step using to generate the second set of missing samples the transition point filter memories stored in the first step of concealment.

Dans ce cas, les mémoires du filtre de synthèse au point de transition sont stockées dans la première étape de dissimulation. Lors de la deuxième étape de dissimulation on détermine l'excitation en fonction des informations reçues. La synthèse est effectuée à partir du point de transition en utilisant d'une part l'excitation obtenue, d'autre part les mémoires du filtre de synthèse stockées.In this case, the memories of the synthesis filter at the transition point are stored in the first concealment step. During the second concealment step the excitation is determined according to the information received. The synthesis is performed from the transition point using on the one hand the excitation obtained, on the other hand the memories of the stored synthesis filter.

Dans un mode particulier de réalisation le premier ensemble d'échantillons est la totalité des échantillons manquants de la trame effacée et le deuxième ensemble d'échantillons est une partie des échantillons manquants de la trame effacée.In a particular embodiment, the first set of samples is all the missing samples of the erased frame and the second set of samples is a part of the missing samples of the erased frame.

Ainsi, la répartition de la génération des échantillons entre deux intervalles temporels différents et le fait de ne générer qu'une partie des échantillons dans le deuxième intervalle temporel, permet de réduire le pic de complexité qui peut se trouver dans l'intervalle temporel correspondant à la trame valide. En effet, dans cet intervalle temporel, le décodeur doit à la fois générer des échantillons manquants de la trame précédente, effectuer l'étape de transition et décoder la trame valide. C'est donc dans cet intervalle temporel que se situe le pic de complexité du décodage.Thus, the distribution of the generation of the samples between two different time intervals and the fact of generating only a part of the samples in the second time interval, makes it possible to reduce the peak of complexity which can be in the time interval corresponding to the valid frame. Indeed, in this time interval, the decoder must both generate missing samples of the previous frame, perform the transition step and decode the valid frame. It is therefore in this time interval that the peak of complexity of the decoding is located.

Les informations présentes dans une trame valide sont par exemple des informations sur la classification du signal et/ou sur l'enveloppe spectrale du signal.The information present in a valid frame is for example information on the classification of the signal and / or on the spectral envelope of the signal.

L'information de classement du signal permet par exemple à l'étape de dissimulation du deuxième ensemble d'échantillons manquants d'adapter des gains respectifs d'une partie harmonique du signal d'excitation et d'une partie aléatoire du signal d'excitation pour le signal correspondant à la trame effacée.The signal classification information makes it possible, for example, for the step of concealing the second set of missing samples to adapt respective gains of a harmonic part of the excitation signal and of a random part of the excitation signal. for the signal corresponding to the erased frame.

Ces informations assurent donc une meilleure adaptation des échantillons manquants générés par l'étape de dissimulation.This information therefore ensures better matching of the missing samples generated by the concealment step.

Dans un mode particulier de réalisation, le premier intervalle temporel étant associé à ladite dernière trame effacée et le deuxième intervalle temporel étant associé à la dite trame valide, une étape de préparation de l'étape de dissimulation du deuxième ensemble d'échantillons manquants, ne produisant aucun échantillon manquant, est mise en oeuvre dans le premier intervalle temporel.In a particular embodiment, the first time slot being associated with said last erased frame and the second time slot being associated with said valid frame, a step of preparing the step of concealing the second set of missing samples, does not producing no missing sample, is carried out in the first time interval.

Ainsi, l'étape de préparation de l'étape de dissimulation du deuxième ensemble d'échantillons manquants s'effectue dans un intervalle temporel différent de celui correspondant au décodage de la trame valide. Ceci permet donc de répartir la charge de calcul de l'étape de dissimulation du deuxième ensemble d'échantillons et ainsi de réduire le pic de complexité dans l'intervalle temporel correspondant à la réception de la première trame valide. Comme présenté ci-dessus, c'est en effet dans cet intervalle temporel correspondant à la trame valide que se situe le pic de complexité ou pire cas de complexité du décodage.Thus, the step of preparing the step of concealing the second set of missing samples is performed in a time interval different from that corresponding to the decoding of the valid frame. This therefore makes it possible to distribute the calculation load of the concealment step of the second set of samples and thus to reduce the peak of complexity in the time interval corresponding to the reception of the first valid frame. As shown above, it is indeed in this time interval corresponding to the valid frame that is the peak complexity or worse case of complexity of the decoding.

La répartition de la complexité ainsi effectuée permet de revoir à la baisse le dimensionnement du processeur d'un dispositif de dissimulation d'erreur de transmission qui est dimensionné en fonction du pire cas de complexité.The distribution of the complexity thus carried out makes it possible to review downward the sizing of the processor of a transmission error concealment device which is dimensioned according to the worst case of complexity.

Dans un mode de réalisation particulier l'étape de préparation comprend une étape de génération d'une partie harmonique du signal d'excitation et une étape de génération d'une partie aléatoire du signal d'excitation pour le signal correspondant à la trame effacéeIn a particular embodiment, the preparation step comprises a step of generating a harmonic portion of the excitation signal and a step of generating a random portion of the excitation signal for the signal corresponding to the erased frame.

La présente invention vise également un dispositif de dissimulation d'erreur de transmission dans un signal numérique découpé en une pluralité de trames successives associées à des intervalles temporels différents dans lequel, à la réception, le signal est susceptible de comporter des trames effacées et des trames valides, les trames valides comportant des informations (inf.) relatives à la dissimulation de perte de trame. Le dispositif est tel qu'il intervient lors d'un décodage hiérarchique utilisant un décodage coeur et un décodage par transformée utilisant des fenêtres à faible retard introduisant un retard temporel inférieur à une trame par rapport au décodage coeur, et qu'il comprend:

  • un module de dissimulation apte à générer, dans un premier intervalle temporel, un premier ensemble d'échantillons manquants pour au moins la dernière trame effacée avant une trame valide et apte à générer, dans un second intervalle temporel, un deuxième ensemble d'échantillons manquants pour la trame effacée prenant en compte des informations de ladite trame valide ;et
  • un module de transition apte à effectuer une transition entre le premier ensemble d'échantillons manquants et le deuxième ensemble d'échantillons manquants pour obtenir au moins une partie de la trame manquante.
The present invention also relates to a transmission error concealment device in a digital signal divided into a plurality of successive frames associated with different time intervals in which, on reception, the signal may comprise erased frames and frames. valid, valid frames with information (inf.) relating to the loss of frame concealment. The device is such that it intervenes during a hierarchical decoding using a core decoding and a transform decoding using low delay windows introducing a time delay less than one frame with respect to the core decoding, and that it comprises:
  • a concealment module able to generate, in a first time interval, a first set of missing samples for at least the last frame erased before a valid frame and able to generate, in a second time slot, a second set of missing samples for the erased frame taking into account information of said valid frame, and
  • a transition module adapted to transition between the first set of missing samples and the second set of missing samples to obtain at least a portion of the missing frame.

Ce dispositif met en oeuvre les étapes du procédé de dissimulation tel que décrit ci-dessus.This device implements the steps of the concealment method as described above.

L'invention vise aussi un décodeur de signal numérique comportant un dispositif de dissimulation d'erreur de transmission selon l'invention.The invention also relates to a digital signal decoder comprising a transmission error concealment device according to the invention.

Enfin, l'invention se rapporte à un programme informatique destiné à être stocké dans une mémoire d'un dispositif de dissimulation d'erreur de transmission. Ce programme informatique est tel qu'il comporte des instructions de code pour la mise en oeuvre des étapes du procédé de dissimulation d'erreur selon l'invention, lorsqu'il est exécuté par un processeur dudit dispositif de dissimulation d'erreur de transmission.Finally, the invention relates to a computer program intended to be stored in a memory of a transmission error concealment device. This computer program is such that it includes code instructions for carrying out the steps of the error concealment method according to the invention, when executed by a processor of said transmission error concealment device.

Elle se rapporte à un support de stockage, lisible par un ordinateur ou par un processeur, intégré ou non au dispositif, mémorisant un programme informatique tel que décrit ci-dessus.It relates to a storage medium, readable by a computer or by a processor, integrated or not into the device, storing a computer program as described above.

D'autres avantages et caractéristiques de l'invention apparaîtront à l'examen de la description détaillée, donnée à titre d'exemple ci-après, et des dessins annexés sur lesquels :

  • les figures 1a et 1b illustrent la technique de l'art antérieur de dissimulation de trames erronées dans le contexte de codage hiérarchique;
  • la figure 2 illustre le procédé de dissimulation selon l'invention dans un premier mode de réalisation;
  • la figure 3 illustre le procédé de dissimulation selon l'invention dans un second mode de réalisation;
  • les figures 4a et 4b illustrent la synchronisation de la reconstruction en utilisant le procédé de dissimulation selon l'invention;
  • la figure 5 illustre un exemple de codeur hiérarchique qui peut être utilisé dans le cadre de l'invention;
  • la figure 6 illustre un décodeur hiérarchique selon l'invention;
  • la figure 7 illustre un dispositif de dissimulation selon l'invention.
Other advantages and characteristics of the invention will appear on examining the detailed description, given by way of example below, and the appended drawings in which:
  • the Figures 1a and 1b illustrate the prior art technique of concealing erroneous frames in the context of hierarchical coding;
  • the figure 2 illustrates the concealment method according to the invention in a first embodiment;
  • the figure 3 illustrates the concealment method according to the invention in a second embodiment;
  • the Figures 4a and 4b illustrate the synchronization of the reconstruction using the concealment method according to the invention;
  • the figure 5 illustrates an exemplary hierarchical coder that can be used in the context of the invention;
  • the figure 6 illustrates a hierarchical decoder according to the invention;
  • the figure 7 illustrates a concealment device according to the invention.

En référence à la figure 2 , le procédé de dissimulation d'erreur de transmission selon un premier mode de réalisation de l'invention est maintenant décrit. Dans cet exemple, la trame N reçue au décodeur est effacée.With reference to the figure 2 , the transmission error concealment method according to a first embodiment of the invention is now described. In this example, the frame N received at the decoder is erased.

Une trame valide N-1 reçue au décodeur, est traitée en 20 par un module de démultiplexage DEMUX, est décodée normalement en 21 par un module de décodage DE-NO. Le signal décodé est ensuite mémorisé dans une mémoire tampon MEM lors d'une étape 22. Au moins une partie de ce signal décodé mémorisé est envoyé à la carte son 30 en sortie du décodeur de la trame N-1, le signal décodé restant dans la mémoire tampon est conservé pour être envoyé à la carte son 30 après décodage de la trame suivante.A valid N-1 frame received at the decoder is processed by DEMUX demultiplexing module, normally decoded at 21 by a DE-NO decoding module. The decoded signal is then stored in a memory buffer MEM during a step 22. At least part of this memorized decoded signal is sent to the sound card 30 at the output of the decoder of the frame N-1, the decoded signal remaining in the buffer is retained for sending to the sound card after decoding the next frame.

Ainsi, à la détection de la trame effacée N, on effectue une étape de dissimulation d'un premier ensemble d'échantillons pour cette trame manquante en 23 à l'aide d'un module de dissimulation d'erreurs DE-DISS et en utilisant le signal décodé d'une trame précédente. Le signal ainsi extrapolé est mémorisé en mémoire MEM lors de l'étape 24.Thus, when the erased frame N is detected, a step is taken to conceal a first set of samples for this missing frame at 23 by means of a DE-DISS error concealment module and by using the decoded signal of a previous frame. The signal thus extrapolated is stored in memory MEM during step 24.

Au moins une partie de ce signal extrapolé mémorisé, conjointement avec le signal décodé de la trame N-1 restant mémorisé, est envoyé à la carte son 30 en sortie du décodeur de la trame N. Le signal extrapolé restant dans la mémoire tampon est conservé pour être envoyé à la carte son après décodage de la trame suivante.At least a part of this extrapolated signal memorized, together with the decoded signal of the N-1 frame remaining stored, is sent to the sound card 30 at the output of the decoder of the frame N. The extrapolated signal remaining in the buffer memory is retained. to be sent to the sound card after decoding the next frame.

A la réception de la trame valide N+1, une étape de dissimulation d'un deuxième ensemble d'échantillons manquants pour la trame N effacée est effectuée en 25 par le module de dissimulation d'erreurs DE-MISS. Cette étape utilise des informations présentes dans la trame valide N+1 qui sont obtenues lors d'une étape 26 de démultiplexage de la trame N+1 par le module de démultiplexage DEMUX.Upon receipt of the valid N + 1 frame, a step of concealing a second set of missing samples for the erased N frame is performed at 25 by the DE-MISS error concealment module. This step uses information present in the valid frame N + 1 which is obtained during a step 26 of demultiplexing the N + 1 frame by the DEMUX demultiplexing module.

Les informations présentes dans une trame valide comportent des informations sur la trame précédente du train binaire. Ce sont notamment des informations de classement du signal (signal voisé, non-voisé, transitoire) ou encore des informations sur l'enveloppe spectrale du signal.The information present in a valid frame includes information on the previous frame of the bit stream. These include signal classification information (voiced, unvoiced, transient signal) or information on the spectral envelope of the signal.

Ces informations vont permettre d'adapter au mieux l'étape de dissimulation des erreurs en calculant par exemple des gains respectifs pour partie harmonique de l'excitation et la partie aléatoire de l'excitation. Par excitation harmonique, on entend l'excitation calculée à partir de la valeur de pitch (nombre d'échantillons dans une période correspondant à l'inverse de la fréquence fondamentale) du signal de la trame précédente, la partie harmonique du signal d'excitation est donc obtenu par recopie de l'excitation passée aux instants correspondant au retard du pitch. Par excitation aléatoire, on entend le signal d'excitation obtenu à partir d'un générateur de signal aléatoire ou par tirage aléatoire d'un mot de code de l'excitation passée ou dans un dictionnaire.This information will make it possible to better adapt the error concealment step by calculating, for example, respective gains for the harmonic part of the excitation and the random part of the excitation. By harmonic excitation is meant the excitation calculated from the pitch value (number of samples in a period corresponding to the inverse of the fundamental frequency) of the signal of the preceding frame, the harmonic part of the excitation signal. is thus obtained by copying the excitation passed to the moments corresponding to the delay of the pitch. By random excitation is meant the excitation signal obtained from a random signal generator or by random draw of a code word of the past excitation or in a dictionary.

Ainsi, dans le cas où le classement du signal indique une trame voisée, un gain plus important est calculé pour la partie harmonique de l'excitation et dans le cas où le classement du signal indique une trame non-voisée, un gain plus important est calculé pour la partie aléatoire de l'excitation.Thus, in the case where the classification of the signal indicates a voiced frame, a larger gain is calculated for the harmonic part of the excitation and in the case where the classification of the signal indicates an unvoiced frame, a larger gain is calculated for the random part of the excitation.

D'autre part, dans le cas d'une transition entre non voisée vers voisée, la partie de l'excitation harmonique est complètement erronée. Dans ce cas plusieurs trames peuvent être nécessaires avant que le décodeur retrouve une excitation normale et donc une qualité acceptable. Ainsi, une nouvelle version artificielle de l'excitation harmonique peut être utilisée pour permettre au décodeur de retrouver plus rapidement un fonctionnement normal.On the other hand, in the case of a transition from voiceless to voiced, the part of the harmonic excitation is completely erroneous. In this case, several frames may be necessary before the decoder regains normal excitation and therefore an acceptable quality. Thus, a new artificial version of the harmonic excitation can be used to allow the decoder to find normal operation more quickly.

L'information sur l'enveloppe spectrale peut être une information de stabilité du filtre de prédiction linéaire LPC. Ainsi si cette information indique que le filtre est stable entre la trame précédente et la trame courante (valide), l'étape de dissimulation d'un deuxième ensemble d'échantillons manquants utilise le filtre de prédiction linéaire de la trame valide. Dans le cas contraire, le filtre issu du passé est utilisé.The information on the spectral envelope can be a stability information of the LPC linear prediction filter. Thus if this information indicates that the filter is stable between the previous frame and the current (valid) frame, the step of concealing a second set of missing samples uses the linear prediction filter of the valid frame. Otherwise, the filter from the past is used.

Une étape 29 de transition par un module de transition TRANS est effectuée. Ce module prend en compte le premier ensemble d'échantillons générés à l'étape 23 pas encore joué sur la carte son et le deuxième ensemble d'échantillons générés à l'étape 25 pour obtenir une transition douce entre le premier ensemble et le deuxième ensemble. Dans un mode de réalisation, cette étape de transition est une étape de fondu enchainé ou d'addition-recouvrement qui consiste à diminuer progressivement le poids du signal extrapolé dans le premier ensemble et à augmenter progressivement le poids du signal extrapolé dans le deuxième ensemble pour obtenir les échantillons manquants de la trame effacée.A transition step 29 by a TRANS transition module is performed. This module takes into account the first set of samples generated at step 23 not yet played on the sound card and the second set of samples generated in step 25 to obtain a smooth transition between the first set and the second set. In one embodiment, this transition step is a step of crossfading or addition-overlap which consists in gradually decreasing the weight of the extrapolated signal in the first set and gradually increasing the weight of the signal extrapolated in the second set to get the missing samples from the erased frame.

Par exemple, cette étape de fondu enchainé correspond à la multiplication de tous les échantillons du signal extrapolé mémorisé à la trame N avec une fonction de pondération décroissante progressivement de 1 à 0, et l'addition de ce signal pondéré avec les échantillons du signal extrapolé à la trame N+1 multiplié avec la fonction de pondération complémentaire de la fonction de pondération du signal mémorisé. Par fonction de pondération complémentaire, on entend la fonction obtenu en effectuant la soustraction de un par la fonction de pondération précédente.For example, this fade-in step corresponds to the multiplication of all the samples of the extrapolated signal stored at the frame N with a weighting function decreasing progressively from 1 to 0, and the addition of this weighted signal with the samples of the signal extrapolated to the N + 1 frame multiplied with the complementary weighting function of the weighting function of the memorized signal. By complementary weighting function is meant the function obtained by subtracting one by the preceding weighting function.

Dans une variante de ce mode de réalisation, cette étape de fondu enchainé est effectuée sur une partie seulement (au moins un échantillon) du signal mémorisé.In a variant of this embodiment, this fade-in step is performed on only a part (at least one sample) of the stored signal.

Dans un autre mode de réalisation, cette étape de transition est assurée par le filtrage de synthèse de prédiction linéaire. Dans ce cas, les mémoires du filtre de synthèse au point de transition sont stockées dans la première étape de dissimulation. Lors de la deuxième étape de dissimulation on détermine l'excitation en fonction des informations reçues. La synthèse est effectuée à partir du point de transition en utilisant d'une part l'excitation obtenue, d'autre part les mémoires du filtre de synthèse stockées.In another embodiment, this transition step is provided by the linear prediction synthesis filtering. In this case, the memories of the synthesis filter at the transition point are stored in the first concealment step. During the second concealment step the excitation is determined according to the information received. The synthesis is performed from the transition point using on the one hand the excitation obtained, on the other hand the memories of the stored synthesis filter.

Dans le même intervalle temporel, la trame valide est donc dé-multiplexée en 26, décodée normalement en 27 et le signal décodé est mémorisé en 28 en mémoire tampon MEM. Le signal issu du module de transition TRANS est envoyé conjointement avec le signal décodé de la trame N+1 à la carte son 30 en sortie du décodeur de la trame N+1.In the same time interval, the valid frame is de-multiplexed at 26, decoded normally at 27 and the decoded signal is stored at 28 in the memory buffer MEM. The signal from the transition module TRANS is sent together with the decoded signal of the N + 1 frame to the sound card 30 at the output of the decoder of the N + 1 frame.

Le signal reçu par la carte son 30 est destiné à être restitué par des moyens de restitution de type haut-parleur 31.The signal received by the sound card 30 is intended to be restored by speaker type reproduction means 31.

Dans un mode de réalisation du procédé selon l'invention, le premier ensemble d'échantillons et le deuxième ensemble d'échantillons sont l'ensemble des échantillons de la trame manquante. A chaque intervalle temporel, un signal correspondant à la trame effacée est généré, le fondu enchainé s'effectue alors sur la partie des deux signaux correspondants à la deuxième moitié de la trame effacée (une demi-trame) pour obtenir les échantillons de la trame manquante. Ce mode de réalisation a l'avantage d'utiliser plus facilement les structures de dissimulation d'erreur habituelles qui fonctionnent sur une trame entière.In one embodiment of the method according to the invention, the first set of samples and the second set of samples are the set of samples of the missing frame. At each time interval, a signal corresponding to the erased frame is generated, the crossfade is then performed on the part of the two signals corresponding to the second half of the erased frame (one half-frame) to obtain the samples of the frame missing. This embodiment has the advantage of more easily using the usual error concealment structures that operate over an entire frame.

Dans une variante de réalisation, dans l'intervalle temporel correspondant à la trame effacée, l'étape de dissimulation génère la totalité des échantillons de la trame manquante (ces échantillons seront nécessaire si la trame suivante est aussi effacée), alors que dans l'intervalle temporel correspondant au décodage de la trame valide, l'étape de dissimulation génère seulement une seconde partie des échantillons, par exemple, la deuxième moitié des échantillons de la trame manquante. L'étape d'addition recouvrement est effectuée pour assurer une transition sur cette deuxième moitié des échantillons de la trame manquante.In an alternative embodiment, in the time interval corresponding to the erased frame, the concealment step generates all the samples of the missing frame (these samples will be necessary if the next frame is also erased), while in the time interval corresponding to the decoding of the valid frame, the concealment step generates only a second portion of the samples, for example, the second half of the samples of the missing frame. The overlap addition step is performed to ensure a transition on this second half of the samples of the missing frame.

Dans cette variante de réalisation, le nombre d'échantillons générés pour la trame manquante dans l'intervalle temporel correspondant à la trame valide, est moins important que dans le cas du premier mode de réalisation décrit ci-dessus. La complexité de décodage dans cet intervalle temporel est donc réduite.In this variant embodiment, the number of samples generated for the missing frame in the time interval corresponding to the valid frame is smaller than in the case of the first embodiment described above. The decoding complexity in this time interval is therefore reduced.

C'est en effet dans cet intervalle temporel que le pire cas de complexité se situe. En effet, dans cet intervalle temporel, à la fois le décodage de la trame valide s'effectue mais également l'étape de dissimulation du deuxième ensemble d'échantillons. En réduisant le nombre d'échantillons à générer, on réduit le pire cas de complexité et donc le dimensionnement d'un processeur de type DSP (pour "Digital Signal Processor" en anglais).It is indeed in this time interval that the worst case of complexity lies. Indeed, in this time interval, both the decoding of the valid frame is performed but also the concealment step of the second set of samples. By reducing the number of samples to be generated, it reduces the worst case of complexity and therefore the sizing of a DSP type processor (for "Digital Signal Processor").

Dans un second mode de réalisation de l'invention, une distribution de la complexité est effectuée permettant de réduire encore plus le pire cas de complexité sans augmenter pour autant la complexité moyenne.In a second embodiment of the invention, a complexity distribution is performed to further reduce the worst case complexity without increasing the average complexity.

Ainsi, en référence à la figure 3 , un deuxième mode de réalisation du procédé selon l'invention est illustré dans le cas où la trame N reçue au décodeur est effacée.So, with reference to the figure 3 a second embodiment of the method according to the invention is illustrated in the case where the frame N received at the decoder is erased.

Dans cet exemple, l'étape de dissimulation du deuxième ensemble d'échantillons est scindée en deux étapes. Une première étape E1 de préparation ne produisant pas d'échantillons manquants et n'utilisant pas l'information issu de la trame valide, est effectuée dans l'intervalle temporel précédent. Une deuxième étape E2 générant des échantillons manquants et utilisant les informations issues de la trame valide est effectuée dans l'intervalle temporel correspondant à la trame valide.In this example, the step of concealing the second set of samples is split into two steps. A first preparation step E1 does not produce missing samples and does not use the information from the valid frame, is performed in the previous time interval. A second step E2 generating missing samples and using the information from the valid frame is performed in the time interval corresponding to the valid frame.

Ainsi, les mêmes opérations que celles décrites en référence à la figure 2, pour la trame N-1 reçue au décodeur, sont effectuées, c'est-à-dire démultiplexage 20, décodage normal 21 et mémorisation 22.Thus, the same operations as those described with reference to the figure 2 , for the N-1 frame received at the decoder, are performed, that is, demultiplexing 20, normal decoding 21 and storage 22.

Dans l'intervalle temporel correspondant à la trame N effacée, une étape de préparation E1 référencée 32 est effectuée. Cette étape de préparation est par exemple une étape d'obtention de la partie harmonique de l'excitation utilisant la valeur du retard LTP de la trame précédente, et d'obtention de la partie aléatoire de l'excitation dans une structure de décodage CELP.In the time interval corresponding to the erased N frame, a preparation step E1 referenced 32 is performed. This preparation step is for example a step of obtaining the harmonic part of the excitation using the value of the LTP delay of the previous frame, and of obtaining the random part of the excitation in a CELP decoding structure.

Cette étape de préparation utilise des paramètres de la trame précédente mémorisée en mémoire MEM. Il n'est pas utile pour cette étape d'utiliser les informations de classement ou les informations sur l'enveloppe spectrale de la trame effacée.This preparation step uses parameters of the previous frame stored in memory MEM. It is not useful for this step to use the classification information or the spectral envelope information of the erased frame.

Dans ce même intervalle temporel correspondant à la trame effacée, l'étape de dissimulation 23 du premier ensemble d'échantillons tel que décrit en référence à la figure 2 est également effectuée. Le signal extrapolé qui en est issu est mémorisé en 24 dans la mémoire MEM. Au moins une partie de ce signal extrapolé mémorisé, conjointement avec le signal décodé restant mémorisé de la trame N-1, est envoyée à la carte son 30 en sortie du décodeur de la trame N. Le signal extrapolé restant dans la mémoire tampon est conservé pour être envoyé à la carte son après décodage de la trame suivante.In the same time interval corresponding to the erased frame, the concealment step 23 of the first set of samples as described with reference to FIG. figure 2 is also performed. The extrapolated signal derived therefrom is stored at 24 in the memory MEM. At least a part of this extrapolated signal memorized, together with the decoded signal remaining stored in the N-1 frame, is sent to the sound card 30 at the output of the decoder of the frame N. The extrapolated signal remaining in the buffer is retained to be sent to the sound card after decoding the next frame.

L'étape E2 référencé 33 de dissimulation comprenant l'extrapolation du deuxième ensemble d'échantillons manquants correspondant à la trame N effacée, est réalisée dans l'intervalle temporel correspondant à la trame N+1 reçue au décodeur. Cette étape comprend la prise en compte des informations contenue dans la trame valide N+1 et qui concernent la trame N.The concealed step E2 referenced 33 comprising the extrapolation of the second set of missing samples corresponding to the erased N frame, is performed in the time interval corresponding to the N + 1 frame received at the decoder. This step comprises the taking into account of the information contained in the valid frame N + 1 and which concern the frame N.

Dans ce mode de réalisation particulier, l'étape de dissimulation correspond alors au calcul des gains associés aux deux parties de l'excitation, et éventuellement à la correction de la phase de l'excitation harmonique. En fonction de l'information de classification reçue dans la première trame valide, les gains respectifs des deux parties de l'excitation sont adaptés. Ainsi, par exemple en fonction de l'information de classification de la dernière trame valide reçue avant les trames effacées et de l'information de classification reçue, l'étape de dissimulation adapte le choix des excitations et les gains associés pour représenter au mieux la classe de la trame. En cela, la qualité du signal généré lors de l'étape de dissimulation est améliorée en bénéficiant de l'information reçue.In this particular embodiment, the concealment step then corresponds to the calculation of the gains associated with the two parts of the excitation, and possibly to the correction of the phase of the harmonic excitation. Based on the classification information received in the first valid frame, the respective gains of the two portions of the excitation are matched. Thus, for example based on the classification information of the last valid frame received before the erased frames and the classification information received, the concealment step adapts the choice of the excitations and the associated gains to best represent the class of the frame. In this, the quality of the signal generated during the concealment step is improved by benefiting from the information received.

Par exemple, si l'information est que la trame N est une trame de signal voisé, l'étape E2 privilégie l'excitation harmonique obtenue à l'étape de préparation E1 plutôt que l'excitation aléatoire et vice versa pour une trame de signal non-voisée.For example, if the information is that the frame N is a voiced signal frame, the step E2 favors the harmonic excitation obtained at the preparation step E1 rather than the random excitation and vice versa for a signal frame unvoiced.

Dans le cas où l'information décrit une trame N transitoire, l'étape E2 va générer des échantillons manquants en fonction de la classification précise du transitoire (voisée vers non-voisée ou non-voisée vers voisée).In the case where the information describes a transient N frame, step E2 will generate missing samples according to the precise classification of the transient (voiced to unvoiced or voiceless to voiced).

Une étape 29 d'addition-recouvrement ou de fondu enchainé comme celle décrite en référence à la figure 2 est ensuite effectuée entre le premier ensemble d'échantillons généré à l'étape 23 et le deuxième ensemble d'échantillons généré à l'étape 33.A step 29 addition-overlap or cross-fade as described with reference to the figure 2 is then performed between the first set of samples generated in step 23 and the second set of samples generated in step 33.

Pendant l'intervalle temporel correspondant à la trame valide N+1, la trame N+1 est traitée par le module de démultiplexage DEMUX, est décodée en 27 et mémorisée en 28 comme décrit précédemment en référence à la figure 2. Le signal extrapolé obtenu par l'étape de fondu enchainé 29 et le signal décodé de la trame N+1 sont conjointement envoyés à la carte son 30 en sortie du décodeur de la trame N+1.During the time interval corresponding to the valid N + 1 frame, the N + 1 frame is processed by the DEMUX demultiplexing module, is decoded at 27 and stored at 28 as described previously with reference to FIG. figure 2 . The signal extrapolated obtained by the cross-fading step 29 and the decoded signal of the N + 1 frame are jointly sent to the sound card 30 at the output of the decoder of the N + 1 frame.

Les figures 4a et 4b illustrent la mise en oeuvre de ce procédé et la synchronisation entre le décodage de type CELP et le décodage par transformée qui utilise des fenêtres à faible retard représentée ici sous la forme de fenêtres telles que décrites dans la demande de brevet FR 0760258 .The Figures 4a and 4b illustrate the implementation of this method and the synchronization between CELP decoding and transform decoding using low delay windows represented here in the form of windows as described in the patent application. FR 0760258 .

Dans ce contexte de décodage hiérarchique, la figure 4a illustre le codage hiérarchique des trames CELP C0 à C5 et les transformées à faible retard M1 à M5 appliquées à ces trames.In this context of hierarchical decoding, the figure 4a illustrates the hierarchical coding of CELP frames C0 to C5 and the low delay transforms M1 to M5 applied to these frames.

Lors de la transmission de ces trames à un décodeur correspondant, les trames C3 et C4 grisées sont effacées.When transmitting these frames to a corresponding decoder, the grayed C3 and C4 frames are erased.

La figure 4b illustre le décodage des trames C0 à C5. La ligne 40 illustre le signal reçu au décodeur, la ligne 41 illustre la synthèse CELP dans le premier étage de décodage, la ligne 42 illustre la synthèse totale utilisant la transformée (MDCT) à faible retard.The figure 4b illustrates the decoding of frames C0 to C5. Line 40 illustrates the signal received at the decoder, line 41 illustrates the CELP synthesis in the first decoding stage, line 42 illustrates the total synthesis using the low delay transform (MDCT).

On voit bien que dans cet exemple, le décalage temporel entre les deux étages de décodage est inférieur à une trame, il est représenté ici dans un souci de simplicité à un décalage d'une demi-trame.It is clear that in this example, the time offset between the two decoding stages is less than one frame, it is represented here for the sake of simplicity at a shift of half a frame.

Ainsi, pour décoder la trame O1 (ligne 42) du décodeur, une partie de la synthèse CELP de la trame précédente C0 et la transformée M0 est utilisée ainsi qu'une partie de la synthèse CELP de la trame courante C1 et la transformée M1.Thus, to decode the frame O1 (line 42) of the decoder, part of the CELP synthesis of the previous frame C0 and the transform M0 is used as well as a part of the CELP synthesis of the current frame C1 and the transform M1.

Il en est de même pour la trame 02 qui utilise une partie de la synthèse CELP de la trame 1 (C1) et la transformée M1 et une partie de la synthèse CELP de la trame 2 (C2) et la transformée M2.It is the same for the frame 02 which uses a part of the CELP synthesis of the frame 1 (C1) and the transform M1 and a part of the synthesis CELP of the frame 2 (C2) and the transform M2.

Lors de la détection de la première trame effacée (C3+M3), le décodeur utilise la synthèse CELP de la trame précédente 2 (C2) pour construire le signal de synthèse total (03). Il est également nécessaire de générer à partir d'un algorithme de dissimulation d'erreur, le signal correspondant à la synthèse CELP de la trame 3 (C3).Upon detection of the first erased frame (C3 + M3), the decoder uses the CELP synthesis of the previous frame 2 (C2) to construct the total synthesis signal (03). It is also necessary to generate from an error concealment algorithm, the signal corresponding to the CELP synthesis of the frame 3 (C3).

Ce signal régénéré est nommé FEC-C3 sur la figure 4b. Le signal de sortie du décodeur 03 est donc composé de la dernière moitié du signal C2 et de la première moitié du signal extrapolé FEC-C3.This regenerated signal is named FEC-C3 on the figure 4b . The output signal of the decoder 03 is therefore composed of the last half of the signal C2 and the first half of the extrapolated signal FEC-C3.

Lors de la seconde trame erronée C4, une étape de dissimulation pour la trame C4 est effectuée pour générer des échantillons correspondants à la trame manquante C4. On obtient ainsi un premier ensemble d'échantillons noté FEC1-C4 pour la trame manquante C4.During the second erroneous frame C4, a concealment step for the frame C4 is performed to generate samples corresponding to the missing frame C4. This gives a first set of samples noted FEC1-C4 for the missing frame C4.

Ainsi, la trame 4 de sortie 04 du décodeur est construite en utilisant une partie d'échantillons extrapolés pour C3 (FEC-C3) et une partie du premier ensemble d'échantillons extrapolés pour C4 (FEC1-C4).Thus, the output frame 4 of the decoder is constructed using a portion of extrapolated samples for C3 (FEC-C3) and a portion of the first set of extrapolated samples for C4 (FEC1-C4).

Lors de la réception de la première trame valide (C5+M5), une étape de dissimulation d'un deuxième ensemble d'échantillons pour la trame C4 est effectuée. Cette étape utilise les informations I5 sur la trame C4 qui sont présentes dans la trame valide C5. Ce second ensemble d'échantillons est référence FEC2-C4.Upon receipt of the first valid frame (C5 + M5), a step of concealing a second set of samples for the frame C4 is performed. This step uses the I5 information on the C4 frame that is present in the valid frame C5. This second set of samples is reference FEC2-C4.

Une étape de transition entre le premier ensemble d'échantillons FEC1-C4 et le deuxième ensemble d'échantillons FEC2-C4 est effectuée par addition recouvrement ou fondu enchainé pour obtenir les échantillons manquants FEC-C4 de la seconde moitié de la trame effacée C4.A transition step between the first set of samples FEC1-C4 and the second set of samples FEC2-C4 is performed by overlapping or cross faded addition to obtain the missing samples FEC-C4 of the second half of the erased frame C4.

La trame 5 de sortie 05 du décodeur est construite en utilisant une partie d'échantillons issus de l'étape de fondu enchainé (FEC-C4) et une partie des échantillons décodés pour la trame valide C5.The output frame 05 of the decoder is constructed using a portion of samples from the cross-fading step (FEC-C4) and a portion of the decoded samples for the valid frame C5.

Dans une variante de ce mode de réalisation, lors de l'étape de dissimulation d'un deuxième ensemble d'échantillons pour la trame C4, seule la deuxième moitié des échantillons manquants FEC2-C4 est généré pour réduire la complexité. L'étape de fondu enchainé est réalisée sur cette deuxième moitié.In a variant of this embodiment, in the step of concealing a second set of samples for the C4 frame, only the second half of the missing FEC2-C4 samples is generated to reduce the complexity. The fade-in step is performed on this second half.

L'invention a été décrite ici avec un exemple de réalisation où le décodage coeur est un décodage de type CELP. Ce décodage coeur peut être de tout autre type. Par exemple, il peut être remplacé par un décodeur de type ADPCM (comme par exemple le codeur/décodeur normalisé G.722). Dans ce mode de réalisation, à la différence du décodage CELP, la continuité entre deux trames n'est pas obligatoirement assurée par le filtrage de synthèse de prédiction linéaire (LPC). Ainsi, à la réception de la première trame valide après une ou des trames effacées, le procédé comprend en plus une étape de prolongement du signal d'extrapolation des trames effacées et une étape d'addition recouvrement entre le signal d'au moins une partie de la première trame valide et de ce prolongement du signal d'extrapolation.The invention has been described here with an exemplary embodiment where the core decoding is a CELP type decoding. This decoding heart can be of any other type. For example, it can be replaced by a decoder of type ADPCM (as for example the standard encoder / decoder G.722). In this embodiment, unlike CELP decoding, the continuity between two frames is not necessarily provided by linear prediction synthesis filtering (LPC). Thus, upon receipt of the first valid frame after one or more erased frames, the method further comprises a step of extending the extrapolation signal of the erased frames and a step of adding overlap between the signal of at least a part the first valid frame and this extension of the extrapolation signal.

En référence à la figure 5 , un exemple de codeur hiérarchique avec un étage de codage par transformée est décrit.With reference to the figure 5 an example of a hierarchical coder with a transform coding stage is described.

Le signal d'entrée S du codeur est filtré par un filtre passe-haut HP 50. Dans un premier étage de codage ce signal filtré est sous-échantillonné par le module 51 à la fréquence du codeur ACELP (pour "Algebraic Code Excited Linear Prediction" en anglais) pour ensuite être codé par une méthode de codage ACELP. Le signal issu de cet étage de codage est ensuite multiplexé dans le module de multiplexage 56. Une information concernant la trame précédente (inf.) est également envoyé au module de multiplexage pour former le train binaire T.The input signal S of the encoder is filtered by an HP high pass filter 50. In a first coding stage, this filtered signal is downsampled by the module 51 at the frequency of the coder ACELP (for "Algebraic Code Excited Linear Prediction"). "in English) to then be encoded by an ACELP encoding method. The signal from this coding stage is then multiplexed in the multiplexing module 56. Information concerning the previous frame (sub) is also sent to the multiplexing module to form the bit stream T.

Le signal issu du codage ACELP est également sur-échantillonné à une fréquence d'échantillonnage correspondant au signal d'origine, par le module 53. Ce signal sur-échantillonné est soustrait du signal filtré en 54 pour entrer dans un second étage de codage où une transformée MDCT est effectuée dans le module 55. Le signal est ensuite quantifié dans le module 57 et est multiplexé par le module de multiplexage MUX pour former le train binaire T.The signal resulting from the ACELP coding is also oversampled at a sampling frequency corresponding to the original signal, by the module 53. This oversampled signal is subtracted from the filtered signal at 54 to enter a second coding stage where an MDCT transform is performed in the module 55. The signal is then quantized in the module 57 and is multiplexed by the multiplexing module MUX to form the bit stream T.

En référence à la figure 6 , un décodeur selon l'invention est décrit.With reference to the figure 6 , a decoder according to the invention is described.

Celui-ci comporte un module de démultiplexage 60 apte à traiter le train binaire T entrant. Un premier étage de décodage ACELP 61 est effectué. Le signal ainsi décodé est sur-échantillonné par le module 62 à la fréquence du signal. Il est ensuite traité par un module de transformée MDCT 63. La transformée utilisée ici est une transformée faible retard tel que décrit dans le document "Low-Overlap" présentée dans " Real-Time Implementation of the MPEG-4 Low-Delay Advanced Audio Coding Algorithm (AAC-LD) on Motorola's DSP56300" de J. Hilpert et al publié à la 108ème convention AES en février 2000 ou encore tel que décrit dans la demande de brevet FR 07 60258 .This includes a demultiplexing module 60 able to process the incoming bit stream T. A first ACELP decoding stage 61 is performed. The signal thus decoded is oversampled by the module 62 at the frequency of the signal. It is then processed by an MDCT transform module 63. The transform used here is a weak delay transform as described in the document "Low-Overlap" presented in " Real-Time Implementation of the MPEG-4 Low-Delay Advanced Audio Coding Algorithm (AAC-LD) on Motorola's DSP56300 "by J. Hilpert et al published at the 108th AES Convention in February 2000 or as described in the patent application FR 07 60258 .

Le décalage temporel entre le premier étage de décodage ACELP et celui de la transformée est donc d'une demi trame.The time offset between the first decoding stage ACELP and that of the transform is therefore half a frame.

A la sortie du module de démultiplexage, le signal est, dans un deuxième étage de décodage, déquantifié dans le module 68 et additionné en 67 au signal issu de la transformée. Une transformée inverse est ensuite appliqué en 64. Le signal qui en est issu est ensuite post-traité (PF) 65 en utilisant le signal issu du module 62 puis filtré en 66 par un filtre passe-haut qui fournit le signal de sortie Ss du décodeur.At the output of the demultiplexing module, the signal is, in a second decoding stage, dequantized in the module 68 and added at 67 to the signal from the transform. An inverse transform is then applied at 64. The signal derived therefrom is then post-processed (PF) 65 using the signal from the module 62 and then filtered at 66 by a high-pass filter which provides the output signal S s of the decoder.

Le décodeur comporte un dispositif 70 de dissimulation d'erreur de transmission qui reçoit du module de démultiplexage une information de trame effacée bfi. Ce dispositif comporte un module de dissimulation 71 qui selon l'invention reçoit lors du décodage d'une trame valide, des informations inf. relative à la dissimulation de perte de trame.The decoder includes a transmission error concealment device 70 which receives from the demultiplexing module erased frame information bfi. This device comprises a concealment module 71 which according to the invention receives when decoding a valid frame, information inf. relating to the concealment of frame loss.

Ce module effectue dans un premier intervalle temporel la dissimulation d'un premier ensemble d'échantillons d'une trame effacée puis dans un intervalle temporel correspondant au décodage d'une trame valide, il effectue la dissimulation d'un second ensemble d'échantillons de la trame effacée.This module performs, in a first time interval, the concealment of a first set of samples of an erased frame, then in a time interval corresponding to the decoding of a valid frame, it performs the concealment of a second set of samples of the erased frame.

Le dispositif 70 comporte également un module 72 TRANS de transition apte à effectuer une transition entre le premier ensemble d'échantillons et le deuxième ensemble d'échantillons pour fournir au moins une partie des échantillons de la trame effacée.The device 70 also includes a transition module 72 TRANS adapted to make a transition between the first set of samples and the second set of samples to provide at least a portion of the samples of the erased frame.

Le signal de sortie du coeur du décodeur hiérarchique est soit le signal issu du décodeur ACELP 61, soit le signal issu du module de dissimulation 70. La continuité entre les deux signaux est assurée par le fait qu'ils partagent les mémoires de synthèse du filtre de prédiction linéaire LPC.The output signal of the heart of the hierarchical decoder is either the signal from the ACELP decoder 61 or the signal from the concealment module 70. The continuity between the two signals is ensured by the fact that they share the synthesis memories of the filter LPC linear prediction.

Le dispositif 70 de dissimulation d'erreur de transmission selon l'invention est par exemple tel qu'illustré en figure 7 . Matériellement, ce dispositif au sens de l'invention comporte typiquement, un processeur µP coopérant avec un bloc mémoire BM incluant une mémoire de stockage et/ou de travail, ainsi qu'une mémoire tampon MEM précitée en tant que moyen pour mémoriser les trames décodées et envoyées avec un décalage temporel. Ce dispositif reçoit en entrée des trames successives du signal numérique Se et délivre le signal synthétisé Ss comportant les échantillons d'une trame effacée.The device 70 for concealing a transmission error according to the invention is for example as illustrated in FIG. figure 7 . Materially, this device in the sense of the invention typically comprises a μP processor cooperating with a memory block BM including a storage and / or working memory, as well as a memory buffer MEM mentioned above as a means for storing the decoded frames. and sent with a time offset. This device receives as input successive frames of the digital signal Se and delivers the synthesized signal Ss comprising the samples of an erased frame.

Le bloc mémoire BM peut comporter un programme informatique comportant les instructions de code pour la mise en oeuvre des étapes du procédé selon l'invention lorsque ces instructions sont exécutées par un processeur µP du dispositif et notamment une étape de dissimulation d'un premier ensemble d'échantillons manquants pour la trame effacée, mise en oeuvre dans un premier intervalle temporel, une étape de dissimulation d'un deuxième ensemble d'échantillons manquants pour la trame effacée prenant en compte des informations de ladite trame valide et mise en oeuvre dans un second intervalle temporel; et une étape d'addition recouvrement entre le premier ensemble d'échantillons manquants et le deuxième ensemble d'échantillons manquants pour obtenir (au moins une partie de?) la trame manquante.The memory block BM may comprise a computer program comprising the code instructions for implementing the steps of the method according to the invention when these instructions are executed by a μP processor of the device and in particular a step of concealing a first set of missing samples for the erased frame, implemented in a first time interval, a step of concealing a second set of missing samples for the erased frame taking into account information of said valid frame and implemented in a second time interval; and an overlap adding step between the first set of missing samples and the second set of missing samples to obtain (at least a portion of?) the missing frame.

Les figures 2 et 3 peuvent illustrer l'algorithme d'un tel programme informatique.The figures 2 and 3 can illustrate the algorithm of such a computer program.

Ce dispositif de dissimulation selon l'invention peut être indépendant ou intégré dans un décodeur de signal numérique.This concealment device according to the invention can be independent or integrated in a digital signal decoder.

Claims (11)

  1. Method of concealing transmission error in a digital signal chopped up into a plurality of successive frames associated with different time intervals in which, on reception, the signal may comprise erased frames and valid frames, the valid frames comprising information (inf.) relating to the concealment of frame loss, the method is characterized in that it is implemented during a hierarchical decoding using a core decoding and a transform-based decoding using low-delay windows introducing a time delay of less than a frame with respect to the core decoding, and in that to replace at least the last frame erased before a valid frame, it comprises:
    - a step (23) of concealing a first set of missing samples for the erased frame, implemented in a first time interval;
    - a step (25) of concealing a second set of missing samples for the erased frame taking into account information of said valid frame and implemented in a second time interval; and
    - a step (29) of transition between the first set of missing samples and the second set of missing samples so as to obtain at least a part of the missing frame.
  2. Method according to Claim 1, characterized in that the step of transition between the first set of missing samples and the second set of missing samples is ensured by an overlap addition step.
  3. Method according to Claim 1, characterized in that the step of transition between the first set of missing samples and the second set of missing samples is ensured by a linear prediction synthesis filtering step using to generate the second set of missing samples the filter memories at the transition point, which memories are stored during the first concealment step.
  4. Method according to Claim 1, characterized in that the first set of samples is the entirety of the missing samples of the erased frame and the second set of samples is a part of the missing samples of the erased frame.
  5. Method according to Claim 1, characterized in that the information of a valid frame relating to the concealment of frame loss is information about the classification of the signal and/or about the spectral envelope of the signal.
  6. Method according to Claim 1, characterized in that the step of concealing the second set of missing samples uses an information item regarding the classing of the signal to adapt respective gains of a harmonic part of the excitation signal and of a random part of the excitation signal for the signal corresponding to the erased frame.
  7. Method according to Claim 1, characterized in that the first time interval being associated with said last erased frame and the second time interval being associated with said valid frame, a step of preparing the step of concealing the second set of missing samples, not producing any missing sample, is implemented in the first time interval.
  8. Method according to Claim 7, characterized in that the preparation step comprises a step of generating a harmonic part of the excitation signal and a step of generating a random part of the excitation signal for the signal corresponding to the erased frame
  9. Device for concealing transmission error in a digital signal chopped up into a plurality of successive frames associated with different time intervals in which, on reception, the signal may comprise erased frames and valid frames, the valid frames comprising information (inf.) relating to the concealment of frame loss, the device is characterized in that it intervenes during a hierarchical decoding using a core decoding and a transform-based decoding using low-delay windows introducing a time delay of less than a frame with respect to the core decoding, and in that it comprises:
    - a concealment module (DE-DISS) able to generate, in a first time interval, a first set of missing samples for at least the last frame erased before a valid frame and able to generate, in a second time interval, a second set of missing samples for the erased frame taking into account information of said valid frame; and
    - a transition module (TRANS) able to perform a transition between the first set of missing samples and the second set of missing samples so as to obtain at least a part of the missing frame.
  10. Digital signal decoder characterized in that it comprises a transmission error concealment device according to Claim 9.
  11. Computer program intended to be stored in a memory of a transmission error concealment device, characterized in that it comprises code instructions for the implementation of the steps of the method according to one of Claims 1 to 8, when it is executed by a processor of said transmission error concealment device.
EP09730641A 2008-03-28 2009-03-20 Concealment of transmission error in a digital signal in a hierarchical decoding structure Active EP2277172B1 (en)

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FR0852043A FR2929466A1 (en) 2008-03-28 2008-03-28 DISSIMULATION OF TRANSMISSION ERROR IN A DIGITAL SIGNAL IN A HIERARCHICAL DECODING STRUCTURE
PCT/FR2009/050489 WO2009125114A1 (en) 2008-03-28 2009-03-20 Concealment of transmission error in a digital signal in a hierarchical decoding structure

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