EP1989706B1 - Device for perceptual weighting in audio encoding/decoding - Google Patents
Device for perceptual weighting in audio encoding/decoding Download PDFInfo
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- EP1989706B1 EP1989706B1 EP07731586A EP07731586A EP1989706B1 EP 1989706 B1 EP1989706 B1 EP 1989706B1 EP 07731586 A EP07731586 A EP 07731586A EP 07731586 A EP07731586 A EP 07731586A EP 1989706 B1 EP1989706 B1 EP 1989706B1
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- perceptual weighting
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- gain compensation
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- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; 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
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- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; 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
- G10L19/0204—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 subband decomposition
- G10L19/0208—Subband vocoders
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- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; 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
- G10L19/08—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters
- G10L19/12—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
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- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; 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 a perceptual weighting device for encoding / decoding an audio signal in a given frequency band. It also relates to a hierarchical audio encoder and decoder comprising a coding / decoding device according to the invention.
- the invention finds a particularly advantageous application in the field of transmission and storage of digital signals, such as audio-frequency signals of speech, music, etc.
- the invention is more particularly directed to coding methods of the "Predictive Transform Coding" type incorporating CELP coding and transform coding techniques.
- the coder generates a fixed rate bit stream.
- This fixed rate constraint simplifies the implementation and use of the encoder and decoder, commonly referred to together as the "codec". Examples of such systems are: ITU-T G.711 coding at 64 kbit / s, ITU-T G.729 coding at 8 kbit / s or GSM-EFR at 12.2 kbit / s.
- the invention is of interest here more particularly to hierarchical coding.
- the bit stream comprises a base layer, or core, and one or more enhancement layers.
- the base layer is generated by a fixed low rate codec, termed a "core codec", guaranteeing the minimum quality of the coding; this layer must be received by the decoder to maintain an acceptable level of quality.
- Improvement layers are used to improve the quality; it may happen that they are not all received by the decoder.
- the main advantage of hierarchical coding is that it allows an adaptation of the bit rate by simple truncation of the bit stream.
- the number of layers namely the number of possible truncations of the bitstream, defines the granularity of the coding: speaks of coding with high granularity if the bit stream comprises few layers (of the order of 2 to 4), while a fine granular coding for example allows a step of the order of 1 kbit / s.
- the invention relates to scalable bandwidth and bandwidth encoding techniques with a CELP heart-coder in a telephone band and one or more band-enhanced enhancement layer with respect to the actual telephone band.
- Examples of such systems are given in the article by H. Taddei et al, Scalable Three Bitrate (8, 14.2 and 24 kbit / s) Audio Coder; 107th Convention AES, 199, with a high granularity of 8, 14.2 and 24 kbit / s, and with fine granularity of 6.4 to 32 kbit / s in the article by B. Kovesi et al supra.
- G.729EV EV for Embedded Variable Bitrate
- the objective of the G.729EV standardization is to obtain a G.729 core hierarchical encoder, producing a signal whose band extends from the narrow band (300-3400 Hz) to the broadband (50-7000 Hz). ) at a rate of 8 to 32 kbit / s for conversational services.
- This encoder is inherently interoperable with Recommendation G.729, which ensures compatibility with existing VoIP devices.
- This is a three-layer coding comprising cascaded CELP coding, full band linear predictive coding (LPC) bandwidth, and transform predictive coding.
- LPC linear predictive coding
- TDAC Time Domain Aliasing Cancellation
- MDCT Modified Discrete Cosine Transform
- the transform predictive coding layer uses a full-band perceptual weighting filter ⁇ WB (z) .
- perceptual weighting filtering noise is explained in WB's work. Kleijn et al supra. In essence, perceptual weighting filtering is used to shape the coding noise by attenuating the signal at frequencies where its intensity is strong and where the noise can be more easily masked.
- the most common perceptual weighting filters used in narrow-band CELP coding are of the form ⁇ (z / ⁇ 1 ) / ⁇ (z / ⁇ 2 ) where 0 ⁇ ⁇ 2 ⁇ ⁇ 1 ⁇ 1 and ⁇ (z) represents the LPC spectrum of a signal segment of length 5 to 30 ms.
- the synthesis analysis in CELP coding thus amounts to minimizing the quadratic error in a signal domain perceptually weighted by this type of filter.
- the patent application WO 01/73759 discloses a method of reducing noise in an audio signal by defining a subband gain factor, wherein the gain factor is determined to obtain a better signal to noise ratio.
- gain factors are however not calculated nor adopted to compensate for any spectral continuity between sub - bumps or between frequency repeats.
- the technical problem to be solved by the object of the present invention is to propose a perceptual weighting device for encoding / decoding an audio signal in a given frequency band, which would make it possible to carry out a full perceptual weighting filtering. band, that is to say over the whole of said given frequency band, in particular the 0-8000 Hz wide band of a hierarchical audio coder, without this operation leading to long and expensive calculations of resources
- the solution to the technical problem posed is, according to the present invention in that, said coding / decoding being performed in a plurality of adjacent subbands in said given frequency band, said device is as defined in claim 1. a gain compensation that ensures spectral continuity over the entire width of the frequency band. The invention therefore makes it possible to obtain a homogeneous band at the output of the perceptual weighting filtering even if the subbands that constitute it have been treated separately from this point of view.
- each subband can be filtered or not by perceptual weighting.
- the spectral continuity can therefore be ensured between a filtered sub-band and another unfiltered, or between two filtered subbands.
- said gain-compensated perceptual weighting filter comprises a perceptual weighting filter and a gain compensation module.
- the gain compensation module is disposed at the output of said perceptual weighting filter.
- the gain compensation module is disposed at the input of said perceptual weighting filter.
- said perceptual weighting filter with gain compensation comprises a perceptual weighting filter incorporating said gain compensation.
- said perceptual weighting filter in the first subband is of the form ((z / ⁇ 1 ) / ((z / ⁇ 2 ) where ((z) represents a linear prediction filter.
- only the first subband is subject to perceptual weighting filtering, the second subband not being filtered.
- said gain-compensated perceptual weighting filter comprises a perceptual weighting filter in the first sub-band
- the invention provides that said perceptual weighting filter in the first sub-band is of the form A 1 (z / ⁇ 1 ) l 1 (z / ⁇ 2 ) where ⁇ 1 (z) represents a linear prediction filter.
- the signal from the perceptual weighting device in the first subband and the original signal in the second sub-bands are respectively applied to transform analysis modules, and said transform analysis modules are connected to a transform encoder in said frequency band.
- said encoder also comprises a perceptual weighting device of the original signal in the second subband, comprising a perceptual weighting filter with gain compensation able to achieve the spectral continuity. between the output signal of said perceptual weighting filter with gain compensation and the output signal of the perceptual weighting device in the first subband.
- said perceptual weighting filter with gain compensation comprises a perceptual weighting filter in the second band
- said perceptual weighting filter in the second subband is of the form ⁇ 2 (z / ⁇ ' 1 ) / ⁇ 2 (z / ⁇ ' 2 ) where ⁇ 2 (z) represents a linear prediction filter.
- the coefficients of said linear prediction filter are provided by a band extension module.
- the signal from the perceptual weighting device in the first subband and the signal from the perceptual weighting device in the second subband are respectively applied to transform analysis modules, and said analysis modules to transformed are connected to a transform encoder in said frequency band.
- the core coder is a linear prediction based coder, for example a CELP coder.
- said inverse perceptual weighting device comprises a perceptual weighting filter with gain compensation, inverse of the perceptual weighting filter with gain compensation of the encoder in the first subband.
- said decoder also comprises an inverse perceptual weighting device of the decoded signal in the second subband, comprising a perceptual weighting filter with gain compensation, inverse of the perceptual weighting filter with gain compensation of the encoder in the second subband.
- said gain-compensated perceptual weighting filter comprises a perceptual weighting filter in the second band
- said gain-compensated inverse perceptual weighting filter comprises an inverse perceptual weighting filter in the second band. subband.
- said inverse perceptual weighting filter in the second subband is of the form ⁇ 2 (z / ⁇ ' 2 ) / ⁇ 2 (z / ⁇ ' 1 ).
- the coefficients of the linear prediction filter ⁇ 2 (z) are provided by a band extension module.
- the invention further relates to a perceptual weighting method for encoding an audio signal in a given frequency band, wherein said encoding is performed in a plurality of adjacent subbands in said given frequency band, said method comprises, in at least one subband, a step of perceptual weighting with gain compensation adapted to achieve the spectral continuity between the signal from said perceptual weighting step with gain compensation and the signals in the subbands adjacent to said sub-band.
- the invention relates to a perceptual weighting method for decoding an audio signal encoded in a given frequency band in accordance with the perceptual weighting method for encoding said signal, which is remarkable in that said method comprises band, a perceptual weighting step with gain compensation, inverse of said perceptual weighting step with gain compensation.
- FIG. 2 On the figure 2 is represented a hierarchical audio coder in subbands at rates ranging from 8 to 32 kbit / s. This figure gives the different steps of the corresponding coding method.
- the input signal in a 50 to 7000 Hz, so-called “expanded" frequency band, sampled at 16 kHz, is first broken down into 2 adjacent subbands by Quadrature Mirror Filter (QMF). .
- the first sub-band, or low band, from 0 to 4000 Hz is obtained by low-pass filtering L 300 and decimation 301, and the second sub-band, or high band, from 4000 to 8000 Hz by high-pass filtering H 302 and decimation 303.
- the filters L 300 and H 302 are of length 64 and conform to those described in the J. Johnston, ICASSP, vol. 5, pp. 291 - 294, 1980 .
- the first sub-band is pre-processed by a high-pass filter 304 eliminating the components below 50 Hz before encoding by a narrow-band CELP 305 core coder.
- the high-pass filtering takes into account that the broadband is defined as covering the range 50-7000 Hz.
- the narrow-band CELP coding corresponds to that described in FIG. figure 1 ; it is a cascaded CELP coding comprising as a first stage a modified G.729 coding (ITU-T G.729 Recommendation, Coding of Speech at 8 kbps using Conjugate Structure Algebraic Code Excited Linear Prediction (CS-ACELP ), March 1996) without a pre-treatment filter, and as a second stage an additional fixed dictionary.
- CS-ACELP Conjugate Structure Algebraic Code Excited Linear Prediction
- the residual signal e related to the error due to the CELP coding is calculated by the stage 306 and then perceptually weighted by a device 307 comprising a perceptual weighting filter to obtain the signal x lo in the time domain.
- This signal is analyzed by Modified Discrete Cosine Transform (MDCT) 308 to obtain the discrete spectrum X lo in the frequency domain.
- MDCT Modified Discrete Cosine Transform
- the device 307 for perceptual weighting is shown in FIG. figure 3 .
- This device W 1 (z) comprises a perceptual weighting filter ⁇ 1 (z / ⁇ 1 ) / ⁇ 1 (z / ⁇ 2 ) comprising the filtering stages 501 and 502 respectively by ⁇ 1 (z / y 1 ) and 1 / ⁇ 1 (z / ⁇ 2 ) .
- the linear prediction filter ⁇ 1 (z) is derived from narrowband CELP coding.
- the second subband, or high band is first unfolded spectrally 309 to compensate for the folding due to high pass filter 302 combined with decimation 303.
- This high band is then pre-processed by a low pass filter 310 eliminating the components between 7000 and 8000 Hz in the original signal.
- the resulting signal x hi in the time domain is transformed by MDCT 311 to obtain the discrete spectrum X hi in the frequency domain.
- a band extension 312 is made from x hi and X hi .
- the MDCT transformation is implemented by means of the algorithm of P. Duhamel, Y. Mahieux, JP Petit, A fast algorithm for the implementation of filter banks on 'time domain aliasing cancellation', ICASSP, vol. 3, pp.2209-2212, 1991 .
- the low band MDCT and high band X lo and X hi spectra are encoded in the transform coding module 313.
- the different bit streams generated by the coding modules 305, 312 and 313 are multiplexed and structured into a hierarchical bit stream in the multiplexer 314.
- the coding is carried out in blocks of samples (or frames) of 20 ms, ie 320 samples.
- the coding rate is 8, 12, 14 to 32 kbit / s.
- the hierarchical audio decoder associated with the encoder which has just been described with regard to the figures 2 , 3 and 4 is represented at the figure 5 .
- This figure illustrates the steps of decoding the signal encoded by said encoder.
- the bits describing each frame of 20 ms are demultiplexed in the demultiplexer 700.
- a decoding operation of 8 to 32 kbit / s is presented, although in practice the bit stream can be truncated to 8, 12, 14 or between 14 and 32 kbit / s.
- the bit stream of the 8 and 12 kbit / s layers is used by the CELP decoder 701 to generate a first synthesis in the first subband, or narrow band, between 0 and 4000 Hz.
- the portion of the bit stream associated with the layer at 14 kbit / s is decoded by the band extension module 702 and the signal obtained in the second subband, or high band, between 4000 and 7000 Hz is transformed by MDCT 703 into a spectrum X hi .
- the MDCT decode 704 generates from the bit stream associated with the bit rates of 14 to 32 kbit / s a reconstructed spectrum X lo in low band and a reconstructed spectrum X hi in high band.
- the extended band output signal is obtained via a bank of QMF synthesis filters which perform the oversampling operations 710 and 712, low-pass filtering 711 and high-pass filtering. 713 and addition 714.
- the coefficients i i are held constant in each 5 ms subframe.
- a variant of the embodiment of the coder of the figure 2 is represented on the figure 6 .
- LPC Linear Prediction
- the gain compensation in low and high bands by the factors fac 1 and fac 2 respectively ensure a continuity of the responses of the filters at 4 kHz. It is this continuity that then makes it possible to code the two discrete spectra X lo and X hi into a single vector X. Again, it is important to note that the value 0 dB used here to define the continuity between low and high bands n is indicative.
- the hierarchical audio decoder corresponding to this variant is described in figure 7 .
- the only difference consists in recovering the quantized LPC coefficients ⁇ 2 (z) used by the band extension module 1002 and applying a perceptual weighting filter.
- the inverse filter W 2 (z) -1 in the high band is of type ⁇ 2 (z / ⁇ ' 2 ) / ⁇ 2 (z / ⁇ ' 1 ) followed by the gain compensation factor 1 / fac 2 where fac 2 has been defined above.
- the invention furthermore covers a computer program comprising a sequence of instructions stored on a medium for execution by a computer or a dedicated device, which is remarkable in that, during the execution of these instructions, the latter executes the method of perceptual weighting object of the invention for coding and / or decoding.
- the aforementioned computer program is for example a directly executable program implanted in a perceptual weighting device object of the invention.
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Abstract
Description
La présente invention concerne un dispositif de pondération perceptuelle pour le codage/décodage d'un signal audio dans une bande de fréquence donnée. Elle concerne également un codeur et un décodeur audio hiérarchiques comprenant un dispositif de codage/décodage conforme à l'invention.The present invention relates to a perceptual weighting device for encoding / decoding an audio signal in a given frequency band. It also relates to a hierarchical audio encoder and decoder comprising a coding / decoding device according to the invention.
L'invention trouve une application particulièrement avantageuse dans le domaine de la transmission et du stockage de signaux numériques, tels que les signaux audio-fréquences de parole, de musique, etc.The invention finds a particularly advantageous application in the field of transmission and storage of digital signals, such as audio-frequency signals of speech, music, etc.
Différentes techniques existent aujourd'hui pour convertir sous forme numérique et pour compresser un signal audio-fréquences de parole, de musique, etc. Les méthodes les plus courantes sont :
- les méthodes de « codage de forme d'onde », telles que le codage MIC ou MICDA (PCM ou ADPCM en anglais),
- les méthodes de « codage paramétrique par analyse par synthèse » comme le codage CELP (« Code Excited Linear Prediction »),
- les méthodes de « codage perceptuel en sous-bandes ou par transformée ».
- "waveform coding" methods, such as MIC or ADPCM coding (PCM or ADPCM),
- methods of "parametric coding by synthesis analysis" such as Code Excited Linear Prediction (CELP) coding,
- the methods of "sub-band or transform perceptual coding".
Ces techniques classiques de codage de signaux audio-fréquences sont décrites par exemple dans
Dans ce contexte, l'invention s'adresse plus spécialement aux méthodes de codage du type « codage prédictif par transformée » (ou « Predictive Transform Coding ») intégrant les techniques du codage CELP et du codage par transformée.In this context, the invention is more particularly directed to coding methods of the "Predictive Transform Coding" type incorporating CELP coding and transform coding techniques.
On rappellera d'abord qu'en codage de parole conventionnel, le codeur génère un flux binaire à débit fixe. Cette contrainte de débit fixe simplifie la mise en oeuvre et l'utilisation du codeur et du décodeur, communément désigné ensemble sous le terme de « codec ». Des exemples de tels systèmes sont : le codage UIT-T G.711 à 64 kbit/s, le codage UIT-T G.729 à 8 kbit/s ou le système GSM-EFR à 12,2 kbit/s.It will be recalled first that in conventional speech coding, the coder generates a fixed rate bit stream. This fixed rate constraint simplifies the implementation and use of the encoder and decoder, commonly referred to together as the "codec". Examples of such systems are: ITU-T G.711 coding at 64 kbit / s, ITU-T G.729 coding at 8 kbit / s or GSM-EFR at 12.2 kbit / s.
Cependant, dans certaines applications comme la téléphonie mobile, la voix sur IP ou les communications sur réseaux ad hoc, il est préférable de générer un flux binaire à débit variable, les valeurs du débit étant prises dans un ensemble pré-défini. On peut ainsi distinguer plusieurs techniques de codage multi-débits, plus flexible que le codage à débit fixe :
- le codage multi-modes contrôlé par la source et/ou le canal tel que mis en oeuvre dans les systèmes AMR-NB, AMR-WB, SMV, ou VMR-WB,
- le codage hiérarchique, ou codage "scalable", qui génère un flux binaire dit hiérarchique car il comprend un débit coeur et une ou plusieurs couche(s) d'amélioration. Le système G.722 à 48, 56 et 64 kbit/s est un exemple simple de codage scalable en débit. Le codec MPEG-4 CELP est quant à lui scalable en débit et en largeur de bande ; on trouve d'autres exemples de tels codeurs dans l'article de B. Kovesi, D. Massaloux, A. Sollaud, "A Scalable Speech and Audio Coding Scheme with Continuous Bitrate Flexibility", ICASSP 2004.
- le codage à descriptions multiples.
- multi-mode coding controlled by the source and / or the channel as implemented in the AMR-NB, AMR-WB, SMV, or VMR-WB systems,
- hierarchical coding, or "scalable" coding, which generates a so-called hierarchical bitstream because it comprises a core rate and one or more improvement layer (s). The 48, 56 and 64 kbit / s G.722 system is a simple example of scalable rate scaling. The MPEG-4 CELP codec is scalable in terms of bit rate and bandwidth; other examples of such coders are found in the article by B. Kovesi, D. Massaloux, A. Sollaud, "A Scalable Speech and Audio Coding Scheme with Continuous Bitrate Flexibility", ICASSP 2004.
- multi-description coding.
L'invention s'intéresse ici plus particulièrement au codage hiérarchique.The invention is of interest here more particularly to hierarchical coding.
Le concept de base du codage audio hiérarchique, ou "scalable", est par exemple illustré dans l'article de
Dans ce type de codage, le flux binaire comprend une couche de base, ou de coeur, et une ou plusieurs couches d'amélioration. La couche de base est générée par un codec à bas débit fixe, qualifié de « codec coeur », garantissant la qualité minimale du codage ; cette couche doit être reçue par le décodeur pour maintenir un niveau de qualité acceptable.In this type of coding, the bit stream comprises a base layer, or core, and one or more enhancement layers. The base layer is generated by a fixed low rate codec, termed a "core codec", guaranteeing the minimum quality of the coding; this layer must be received by the decoder to maintain an acceptable level of quality.
Les couches d'amélioration servent à améliorer la qualité ; il peut arriver qu'elles ne soient pas toutes reçues par le décodeur. L'intérêt principal du codage hiérarchique est qu'il permet une adaptation du débit par simple troncature du flux binaire. Le nombre de couches, à savoir le nombre de troncatures possibles du flux binaire, définit la granularité du codage : on parle de codage à granularité forte si le flux binaire comprend peu de couches (de l'ordre de 2 à 4), tandis qu'un codage à granularité fine permet par exemple un pas de l'ordre de 1 kbit/s.Improvement layers are used to improve the quality; it may happen that they are not all received by the decoder. The main advantage of hierarchical coding is that it allows an adaptation of the bit rate by simple truncation of the bit stream. The number of layers, namely the number of possible truncations of the bitstream, defines the granularity of the coding: speaks of coding with high granularity if the bit stream comprises few layers (of the order of 2 to 4), while a fine granular coding for example allows a step of the order of 1 kbit / s.
L'invention se rapporte plus particulièrement aux techniques de codage scalable en débit et en largeur de bande avec un codeur coeur de type CELP en bande téléphonique et une ou plusieurs couche d'amélioration en bande élargie par rapport à la bande téléphonique proprement dite. Des exemples de tels systèmes sont donnés dans l'article de H. Taddéi et al, A Scalable Three Bitrate (8, 14,2 and 24 kbit/s) Audio Coder; 107th Convention AES, 199, avec une granularité forte 8, 14,2 et 24 kbit/s, et avec granularité fine de 6,4 à 32 kbit/s dans l'article de B. Kovesi et al précité.More particularly, the invention relates to scalable bandwidth and bandwidth encoding techniques with a CELP heart-coder in a telephone band and one or more band-enhanced enhancement layer with respect to the actual telephone band. Examples of such systems are given in the article by H. Taddei et al, Scalable Three Bitrate (8, 14.2 and 24 kbit / s) Audio Coder; 107th Convention AES, 199, with a high granularity of 8, 14.2 and 24 kbit / s, and with fine granularity of 6.4 to 32 kbit / s in the article by B. Kovesi et al supra.
L'UIT-T a lancé en 2004 un projet de codeur hiérarchique à coeur normalisé. Ce codeur, appelé G.729EV (EV pour « Embedded Variable bitrate ») est une annexe du codeur G.729 connu. L'objectif de la normalisation G.729EV est d'obtenir un codeur hiérarchique à coeur G.729, produisant un signal dont la bande s'étend de la bande étroite (300-3400 Hz) à la bande élargie (50-7000 Hz) à un débit de 8 à 32 kbit/s pour les services conversationnels. Ce codeur est par nature inter-opérable avec la recommandation G.729, ce qui assure la compatibilité avec les équipements de voix sur IP existants.In 2004, the ITU-T launched a standardized core hierarchical coder project. This encoder, called G.729EV (EV for Embedded Variable Bitrate) is an appendix of the known G.729 encoder. The objective of the G.729EV standardization is to obtain a G.729 core hierarchical encoder, producing a signal whose band extends from the narrow band (300-3400 Hz) to the broadband (50-7000 Hz). ) at a rate of 8 to 32 kbit / s for conversational services. This encoder is inherently interoperable with Recommendation G.729, which ensures compatibility with existing VoIP devices.
En réponse à ce projet, il a été proposé le codeur audio hiérarchique de 8 à 32 kbit/s représenté sur la
Le concept de mise en forme du bruit de codage par filtrage de pondération perceptuelle est expliqué dans l'ouvrage de WB. Kleijn et al précité. En substance, le filtrage de pondération perceptuelle permet de mettre en forme le bruit de codage en atténuant le signal aux fréquences où son intensité est forte et où le bruit peut être plus facilement masqué.The concept of shaping perceptual weighting filtering noise is explained in WB's work. Kleijn et al supra. In essence, perceptual weighting filtering is used to shape the coding noise by attenuating the signal at frequencies where its intensity is strong and where the noise can be more easily masked.
Les filtres de pondération perceptuelle les plus couramment utilisés en codage CELP en bande étroite sont de la forme Â(z/γ 1)/Â(z/γ 2) où 0 ≤ γ 2 ≤ γ 1 < 1 et Â(z) représente le spectre LPC d'un segment de signal de longueur 5 à 30 ms. L'analyse par synthèse en codage CELP revient ainsi à minimiser l'erreur quadratique dans un domaine de signal pondéré perceptuellement par ce type de filtre.The most common perceptual weighting filters used in narrow-band CELP coding are of the form  (z / γ 1 ) /  (z / γ 2 ) where 0 ≤ γ 2 ≤ γ 1 <1 and  (z) represents the LPC spectrum of a signal segment of length 5 to 30 ms. The synthesis analysis in CELP coding thus amounts to minimizing the quadratic error in a signal domain perceptually weighted by this type of filter.
Cependant, cette technique proposée dans le cadre de la normalisation G.729EV présente l'inconvénient d'utiliser un filtre de pondération perceptuelle pleine bande. Le filtrage associé est en effet relativement complexe en terme de quantité de calcul.However, this technique proposed in the context of G.729EV standardization has the disadvantage of using a full band perceptual weighting filter. The associated filtering is indeed relatively complex in terms of amount of calculation.
D'autre part, la demande de brevet
Aussi, le problème technique à résoudre par l'objet de la présente invention est de proposer un dispositif de pondération perceptuelle pour le codage/décodage d'un signal audio dans une bande de fréquence donnée, qui permettrait de réaliser un filtrage de pondération perceptuelle pleine bande, c'est-à-dire sur la totalité de ladite bande de fréquence donnée, en particulier la bande élargie 0-8000 Hz d'un codeur audio hiérarchique, sans que cette opération n'entraîne des calculs longs et coûteux en ressourcesAlso, the technical problem to be solved by the object of the present invention is to propose a perceptual weighting device for encoding / decoding an audio signal in a given frequency band, which would make it possible to carry out a full perceptual weighting filtering. band, that is to say over the whole of said given frequency band, in particular the 0-8000 Hz wide band of a hierarchical audio coder, without this operation leading to long and expensive calculations of resources
ni de discontinuité spectrale entre sous-bondesde frèquence.nor spectral discontinuity between frequency sub-bumps.
La solution au problème technique posé consiste, selon la présente invention en ce que, ledit codage/décodage étant effectué dans une pluralité de sous-bandes adjacentes dans ladite bande de fréquence donnée, ledit dispositif est tel que défini dans la revendication 1. à une compensation de gain qui assure la continuité spectrale sur toute la largeur de la bande de fréquence. L'invention permet donc d'obtenir une bande homogène en sortie du filtrage de pondération perceptuelle même si les sous-bandes qui la constituent ont été traitées séparément de ce point de vue.The solution to the technical problem posed is, according to the present invention in that, said coding / decoding being performed in a plurality of adjacent subbands in said given frequency band, said device is as defined in
Il en résulte l'avantage particulièrement important qu'un codage par transformée peut être appliqué pleine bande sur des sous-bandes qui autrement seraient inhomogènes du fait de leur filtrage séparé.This results in the particularly important advantage that transform coding can be applied full-band on subbands that otherwise would be inhomogeneous due to their separate filtering.
Bien entendu, chaque sous-bande peut être filtrée ou non par pondération perceptuelle. La continuité spectrale peut donc être assurée entre une sous-bande filtrée et une autre non filtrée, ou entre deux sous-bandes filtrées.Of course, each subband can be filtered or not by perceptual weighting. The spectral continuity can therefore be ensured between a filtered sub-band and another unfiltered, or between two filtered subbands.
Selon un mode de réalisation, ledit filtre de pondération perceptuelle à compensation de gain comprend un filtre de pondération perceptuelle et un module de compensation de gain.According to one embodiment, said gain-compensated perceptual weighting filter comprises a perceptual weighting filter and a gain compensation module.
Dans un mode particulier de réalisation, le module de compensation de gain est disposé à la sortie dudit filtre de pondération perceptuelle.In a particular embodiment, the gain compensation module is disposed at the output of said perceptual weighting filter.
Dans un autre mode particulier de réalisation, le module de compensation de gain est disposé à l'entrée dudit filtre de pondération perceptuelle.In another particular embodiment, the gain compensation module is disposed at the input of said perceptual weighting filter.
Selon un autre mode de réalisation, ledit filtre de pondération perceptuelle avec compensation de gain comprend un filtre de pondération perceptuelle intégrant ladite compensation de gain.According to another embodiment, said perceptual weighting filter with gain compensation comprises a perceptual weighting filter incorporating said gain compensation.
On peut alors prévoir que ledit filtre de pondération perceptuelle dans la première sous-bande est de la forme Â(z/γ 1)/Â(z/γ 2) où Â(z) représente un filtre de prédiction linéaire. Dans ce cas, l'invention propose que ladite compensation de gain effectue une multiplication par un facteur fac égal à :
où les âi sont les coefficients du filtre Â(z) de prédiction linéaire.It can then be provided that said perceptual weighting filter in the first subband is of the form ((z / γ 1 ) / ((z / γ 2 ) where ((z) represents a linear prediction filter. In this case, the invention proposes that said gain compensation multiplies by a fac factor equal to:
where ? i are the coefficients of the linear prediction filter ? (z) .
On rappelle ici qu'un filtre Â(z) de prédiction linéaire d'ordre p et de coefficients âi est donné par :
L'invention concerne également un codeur audio hiérarchique dans une bande de fréquence décomposée en une première et une deuxième sous-bandes adjacentes, ledit codeur comprenant :
- un codeur coeur destiné à coder un signal original dans la première sous-bande de ladite bande de fréquence,
- un étage de calcul d'un signal résiduel à partir dudit signal original et du signal issu dudit codeur coeur,
- un dispositif de pondération perceptuelle dudit signal résiduel, conforme à la
revendication 1.
- a core encoder for encoding an original signal in the first subband of said frequency band,
- a stage for calculating a residual signal from said original signal and the signal from said core coder,
- a perceptual weighting device of said residual signal according to
claim 1.
Dans ce mode de réalisation, seule la première sous-bande est soumise à un filtrage de pondération perceptuelle, la deuxième sous-bande n'étant pas filtrée.In this embodiment, only the first subband is subject to perceptual weighting filtering, the second subband not being filtered.
Par ailleurs, lorsque ledit filtre de pondération perceptuelle à compensation de gain comprend un filtre de pondération perceptuelle dans la première sous-bande, l'invention prévoit que ledit filtre de pondération perceptuelle dans la première sous-bande est de la forme Â1(z/γ 1)lÂ1(z/γ 2) où Â1(z) représente un filtre de prédiction linéaire. Dans ce cas, ladite compensation de gain dans la première sous-bande effectue une multiplication par un facteur fac1 égal à :
où les âi sont les coefficients du filtre Â1(z) de prédiction linéaire.On the other hand, when said gain-compensated perceptual weighting filter comprises a perceptual weighting filter in the first sub-band, the invention provides that said perceptual weighting filter in the first sub-band is of the form A 1 (z / γ 1 ) l 1 (z / γ 2 ) where λ 1 (z) represents a linear prediction filter. In this case, said gain compensation in the first subband performs a multiplication by a fac 1 factor equal to:
where ? i are the coefficients of the linear prediction filter ? 1 (z) .
Avantageusement, le signal issu du dispositif de pondération perceptuelle dans la première sous-bande et le signal original dans la deuxième sous-bande sont appliqués respectivement à des modules d'analyse par transformée, et lesdits modules d'analyse par transformée sont reliés à un codeur par transformée dans ladite bande de fréquence.Advantageously, the signal from the perceptual weighting device in the first subband and the original signal in the second sub-bands are respectively applied to transform analysis modules, and said transform analysis modules are connected to a transform encoder in said frequency band.
Selon une variante de réalisation du codeur audio hiérarchique conforme à l'invention, ledit codeur comprend également un dispositif de pondération perceptuelle du signal original dans la deuxième sous-bande, comprenant un filtre de pondération perceptuelle avec compensation de gain apte à réaliser la continuité spectrale entre le signal en sortie dudit filtre de pondération perceptuelle avec compensation de gain et le signal en sortie du dispositif de pondération perceptuelle dans la première sous-bande.According to an alternative embodiment of the hierarchical audio encoder according to the invention, said encoder also comprises a perceptual weighting device of the original signal in the second subband, comprising a perceptual weighting filter with gain compensation able to achieve the spectral continuity. between the output signal of said perceptual weighting filter with gain compensation and the output signal of the perceptual weighting device in the first subband.
Il s'agit donc là d'un codeur pour lequel un filtrage de pondération perceptuelle est effectué séparément dans les deux sous-bandes.This is therefore an encoder for which perceptual weighting filtering is performed separately in the two subbands.
Il est alors prévu que, lorsque ledit filtre de pondération perceptuelle avec compensation de gain comprend un filtre de pondération perceptuelle dans la deuxième bande, ledit filtre de pondération perceptuelle dans la deuxième sous-bande est de la forme Â2(z/γ'1)/Â2(z/γ'2) où Â2(z) représente un filtre de prédiction linéaire. Dans ce cas, ladite compensation de gain dans la deuxième sous-bande effectue une multiplication par un facteur fac2 égal à :
où les â'i sont les coefficients du filtre Â2(z) de prédiction linéaire:It is then expected that when said perceptual weighting filter with gain compensation comprises a perceptual weighting filter in the second band, said perceptual weighting filter in the second subband is of the form λ 2 (z / γ ' 1 ) / Λ 2 (z / γ ' 2 ) where λ 2 (z) represents a linear prediction filter. In this case, said gain compensation in the second subband performs a multiplication by a factor fac 2 equal to:
where ? i are the coefficients of the linear prediction filter ? 2 (z) :
Avantageusement, les coefficients dudit filtre de prédiction linéaire sont fournis par un module d'extension de bande.Advantageously, the coefficients of said linear prediction filter are provided by a band extension module.
Avantageusement, le signal issu du dispositif de pondération perceptuelle dans la première sous-bande et le signal issu du dispositif de pondération perceptuelle dans la deuxième sous-bande sont appliqués respectivement à des modules d'analyse par transformée, et lesdits modules d'analyse par transformée sont reliés à un codeur par transformée dans ladite bande de fréquence.Advantageously, the signal from the perceptual weighting device in the first subband and the signal from the perceptual weighting device in the second subband are respectively applied to transform analysis modules, and said analysis modules to transformed are connected to a transform encoder in said frequency band.
Dans un mode particulier de réalisation, le codeur coeur est un codeur à base de prédiction linéaire, par exemple un codeur CELP.In a particular embodiment, the core coder is a linear prediction based coder, for example a CELP coder.
L'invention concerne en outre un décodeur audio hiérarchique dans une bande de fréquence décomposée en une première et une deuxième sous-bandes adjacentes, ledit décodeur comprenant :
- un décodeur coeur destiné à décoder dans la première sous-bande de ladite bande de fréquence un signal reçu codé par le codeur selon l'invention,
- un dispositif de pondération perceptuelle inverse d'un signal représentatif du signal résiduel pondéré dans la première sous-bande par le dispositif de pondération perceptuelle dudit codeur,
- a heart decoder for decoding in the first sub-band of said frequency band a received signal coded by the coder according to the invention,
- an inverse perceptual weighting device of a signal representative of the weighted residual signal in the first sub-band by the perceptual weighting device of said encoder,
remarquable en ce que ledit dispositif de pondération perceptuelle inverse comprend un filtre de pondération perceptuelle avec compensation de gain, inverse du filtre de pondération perceptuelle avec compensation de gain du codeur dans la première sous-bande.characterized in that said inverse perceptual weighting device comprises a perceptual weighting filter with gain compensation, inverse of the perceptual weighting filter with gain compensation of the encoder in the first subband.
En variante, l'invention propose que ledit décodeur comprend également un dispositif de pondération perceptuelle inverse du signal décodé dans la deuxième sous-bande, comprenant un filtre de pondération perceptuelle avec compensation de gain, inverse du filtre de pondération perceptuelle avec compensation de gain du codeur dans la deuxième sous-bande.Alternatively, the invention proposes that said decoder also comprises an inverse perceptual weighting device of the decoded signal in the second subband, comprising a perceptual weighting filter with gain compensation, inverse of the perceptual weighting filter with gain compensation of the encoder in the second subband.
Dans ce dernier cas, on prévoit que, lorsque ledit filtre de pondération perceptuelle avec compensation de gain comprend un filtre de pondération perceptuelle dans la deuxième bande, ledit filtre de pondération perceptuelle inverse avec compensation de gain comprend un filtre de pondération perceptuelle inverse dans la deuxième sous-bande. En particulier, ledit filtre de pondération perceptuelle inverse dans la deuxième sous-bande est de la forme Â2(z/γ'2)/Â2(z/γ'1). Dans ce cas, les coefficients du filtre Â2(z) de prédiction linéaire sont fournis par un module d'extension de bande.In the latter case, it is expected that, when said gain-compensated perceptual weighting filter comprises a perceptual weighting filter in the second band, said gain-compensated inverse perceptual weighting filter comprises an inverse perceptual weighting filter in the second band. subband. In particular, said inverse perceptual weighting filter in the second subband is of the form λ 2 (z / γ ' 2 ) / λ 2 (z / γ ' 1 ). In this case, the coefficients of the linear prediction filter λ 2 (z) are provided by a band extension module.
L'invention concerne par ailleurs un procédé de pondération perceptuelle pour le codage d'un signal audio dans une bande de fréquence donnée, remarquable en ce que, ledit codage étant effectué dans une pluralité de sous-bandes adjacentes dans ladite bande de fréquence donnée, ledit procédé comprend, dans au moins une sous-bande, une étape de pondération perceptuelle avec compensation de gain apte à réaliser la continuité spectrale entre le signal issu de ladite étape de pondération perceptuelle avec compensation de gain et les signaux dans les sous-bandes adjacentes à ladite sous-bande.The invention further relates to a perceptual weighting method for encoding an audio signal in a given frequency band, wherein said encoding is performed in a plurality of adjacent subbands in said given frequency band, said method comprises, in at least one subband, a step of perceptual weighting with gain compensation adapted to achieve the spectral continuity between the signal from said perceptual weighting step with gain compensation and the signals in the subbands adjacent to said sub-band.
Enfin, l'invention concerne un procédé de pondération perceptuelle pour le décodage d'un signal audio codé dans une bande de fréquence donnée conformément au procédé de pondération perceptuelle pour le codage dudit signal, remarquable en ce que, ledit procédé comprend, dans ladite sous-bande, une étape de pondération perceptuelle avec compensation de gain, inverse de ladite étape de pondération perceptuelle avec compensation de gain.Finally, the invention relates to a perceptual weighting method for decoding an audio signal encoded in a given frequency band in accordance with the perceptual weighting method for encoding said signal, which is remarkable in that said method comprises band, a perceptual weighting step with gain compensation, inverse of said perceptual weighting step with gain compensation.
La description qui va suivre en regard des dessins annexés, donnés à titre d'exemples non limitatifs, fera bien comprendre en quoi consiste l'invention et comment elle peut être réalisée.
- La
figure 1 est un schéma d'un codeur audio hiérarchique de l'art antérieur, comprenant un filtrage de pondération perceptuelle pleine bande avant codage par transformée. - La
figure 2 est un schéma de haut niveau d'un codeur audio hiérarchique selon l'invention. - La
figure 3 est un schéma du dispositif de pondération perceptuelle du codeur de lafigure 2 . - La
figure 4 est un spectre donnant l'amplitude d'un signal filtré puis compensé en gain selon l'invention dans une première sous-bande et l'amplitude d'un signal non filtré dans une deuxième sous-bande. - La
figure 5 est un schéma de haut niveau d'un décodeur audio hiérarchique selon l'invention. - La
figure 6 est un schéma d'une variante du codeur audio hiérarchique de lafigure 2 . - La
figure 7 est un schéma d'une variante du décodeur audio hiérarchique de lafigure 5 . - La
figure 8 est un spectre donnant l'amplitude d'un signal filtré puis compensé en gain selon l'invention dans une première sous-bande et l'amplitude d'un signal filtré puis égalisé selon l'invention dans une deuxième sous-bande.
- The
figure 1 is a schematic diagram of a hierarchical audio coder of the prior art, comprising a full-band perceptual weighting filter before transform coding. - The
figure 2 is a high-level diagram of a hierarchical audio coder according to the invention. - The
figure 3 is a diagram of the perceptual weighting device of the coder of thefigure 2 . - The
figure 4 is a spectrum giving the amplitude of a filtered signal then gain-compensated according to the invention in a first sub-band and the amplitude of an unfiltered signal in a second sub-band. - The
figure 5 is a high level diagram of a hierarchical audio decoder according to the invention. - The
figure 6 is a diagram of a variant of the hierarchical audio coder of thefigure 2 . - The
figure 7 is a diagram of a variant of the hierarchical audio decoder of thefigure 5 . - The
figure 8 is a spectrum giving the amplitude of a filtered signal then compensated in gain according to the invention in a first subband and the amplitude of a filtered signal then equalized according to the invention in a second subband.
Sur la
Le signal d'entrée dans une bande de fréquence utile 50 à 7000 Hz, dite « élargie », échantillonné à 16 kHz, est d'abord décomposé en 2 sous-bandes adjacentes par filtrage miroir en quadrature QMF (« Quadrature Mirror Filter »). La première sous-bande, ou bande basse, de 0 à 4000 Hz est obtenue par filtrage passe-bas L 300 et décimation 301, et la deuxième sous-bande, ou bande haute, de 4000 à 8000 Hz par filtrage passe-haut H 302 et décimation 303. Dans un mode de réalisation préféré, les filtres L 300 et H 302 sont de longueur 64 et conformes à ceux décrits dans l'article de
La première sous-bande est pré-traitée par un filtre passe-haut 304 éliminant les composantes en dessous de 50 Hz avant codage par un codeur coeur CELP 305 en bande étroite. Le filtrage passe-haut tient compte du fait que la bande élargie est définie comme couvrant l'intervalle 50-7000 Hz. Dans ce mode de réalisation, le codage CELP en bande étroite correspond à celui décrit à la
Le dispositif 307 de pondération perceptuelle est montré à la
où les âi sont les coefficients du filtre Â1(z) :
where ? i are the coefficients of the filter ? 1 (z) :
Dans un mode de réalisation préféré, les coefficients âi sont mis à jour à chaque sous-trame de 5 ms, et γ 1 = 0,96 et γ 2 = 0,6.In a preferred embodiment, the coefficients a i are updated at each 5 ms subframe, and γ 1 = 0.96 and γ 2 = 0.6.
Une définition équivalente du facteur fac1 correspond à l'inverse du gain du filtre Â1(z/γ 1)/Â1(z/γ 2) pris à la fréquence de Nyquist (4 kHz) soit :
pour z = -1.An equivalent definition of the factor fac 1 corresponds to the inverse of the gain of the filter λ 1 (z / γ 1 ) / λ 1 (z / γ 2 ) taken at the Nyquist frequency (4 kHz), ie:
for z = -1.
La deuxième sous-bande, ou bande haute, est d'abord dépliée spectralement 309 pour compenser le repliement dû au filtre passe-haut 302 combiné avec la décimation 303. Cette bande haute est ensuite pré-traitée par un filtre passe-bas 310 éliminant les composantes entre 7000 et 8000 Hz dans le signal original. Le signal résultant xhi dans le domaine temporel est transformé par MDCT 311 pour obtenir le spectre discret Xhi dans le domaine fréquentiel. Une extension de bande 312 est réalisée à partir de xhi et Xhi. The second subband, or high band, is first unfolded spectrally 309 to compensate for the folding due to
Les signaux xlo et xhi sont découpés en trames de N échantillons et la transformation MDCT de longueur L=2N analyse les trames courante et future. Dans un mode de réalisation préféré, xlo et xhi sont des signaux en bande étroite échantillonné à 8 kHz et N = 160 (soit 20 ms). Les transformées MDCT Xlo et Xhi comprennent donc N = 160 coefficients, chaque coefficient représentant une bande fréquentielle de 4000/160 = 25 Hz. Dans un mode de réalisation préféré, la transformation MDCT est implantée au moyen de l'algorithme de
Les spectres MDCT bande basse et bande haute Xlo et Xhi sont codés dans le module 313 de codage par transformée.The low band MDCT and high band X lo and X hi spectra are encoded in the
Les différents flux binaires générés par les modules de codage 305, 312 et 313 sont multiplexés et structurés en un train binaire hiérarchique dans le multiplexeur 314.The different bit streams generated by the
Le codage est réalisé par blocs d'échantillons (ou trames) de 20 ms, soit 320 échantillons. Le débit de codage est de 8, 12, 14 à 32 kbit/s.The coding is carried out in blocks of samples (or frames) of 20 ms, ie 320 samples. The coding rate is 8, 12, 14 to 32 kbit / s.
L'intérêt de l'étape de pondération perceptuelle avec compensation de gain par le facteur fac1 est maintenant expliqué à l'aide de la
Cette figure montre la décomposition de la bande de fréquence totale en une première sous-bande, la bande basse entre 0 et 4 kHz, et une deuxième sous-bande, la bande haute entre 4 et 8 kHz. Dans un mode de réalisation préféré, le codeur MDCT 313 s'applique à ces deux sous-bandes avec :
- un filtrage W1(z) de pondération perceptuelle et compensation en gain avant transformation MDCT en bande basse,
- une transformation MDCT directe en bande haute sans filtrage de pondération perceptuelle.
- a filtering W 1 (z) of perceptual weighting and gain compensation before MDCT transformation in a low band,
- a direct high-band MDCT transformation without perceptual weighting filtering.
Ces deux opérations en sous-bandes sont schématisées respectivement sur la
Il est important de noter que la valeur 0 dB utilisée ici pour définir la continuité entre bandes basse et haute n'est qu'indicative.It is important to note that the 0 dB value used here to define the continuity between low and high bands is only indicative.
Le décodeur audio hiérarchique associé au codeur qui vient d'être décrit en regard des
Les bits décrivant chaque trame de 20 ms sont démultiplexés dans le démultiplexeur 700. Dans la suite, on présente un décodage fonctionnant de 8 à 32 kbit/s, bien qu'en pratique le flux binaire peut être tronqué à 8, 12, 14 ou entre 14 et 32 kbit/s.The bits describing each frame of 20 ms are demultiplexed in the
Le flux binaire des couches à 8 et 12 kbit/s est utilisé par le décodeur CELP 701 pour générer une première synthèse dans la première sous-bande, ou bande étroite, entre 0 et 4000 Hz. La portion du flux binaire associé à la couche à 14 kbit/s est décodée par le module 702 d'extension de bande et le signal obtenu dans la deuxième sous-bande, ou bande haute, entre 4000 et 7000 Hz est transformé par MDCT 703 en un spectre X̃hi . Le décodage MDCT 704 génère à partir du flux binaire associé aux débits de 14 à 32 kbit/s un spectre reconstruit X̂lo en bande basse et un spectre reconstruit X̂hi en bande haute. Ces deux spectres sont ramenés à des signaux temporels x̂lo et x̂hi par MDCT inverse dans les blocs 705 et 706. Le signal x̂lo est ajouté à la synthèse CELP par l'additionneur 708 après filtrage par un dispositif 707 de pondération perceptuelle inverse. Le résultat est ensuite post-filtré en 709.The bit stream of the 8 and 12 kbit / s layers is used by the
Le signal de sortie en bande élargie, échantillonné à 16 kHz, est obtenu par l'intermédiaire d'un banc de filtres QMF de synthèse qui effectuent les opérations de sur-échantillonnage 710 et 712, de filtrage passe-bas 711 et passe-haut 713 et addition 714.The extended band output signal, sampled at 16 kHz, is obtained via a bank of QMF synthesis filters which perform the
Une étape de décodage perceptuelle avec compensation de gain est effectuée par le dispositif 707 de pondération perceptuelle inverse W1(z) -1 comprenant un filtre de pondération perceptuelle inverse Â1(z/γ 2)/Â1(z/γ 1) et un module de compensation de gain destiné à multiplier le signal issu dudit filtre de pondération perceptuelle inverse par le facteur 1/fac1 avec :
Une variante du mode de réalisation du codeur de la
Sur cette figure, on retrouve le banc de filtres 900 à 903 d'analyse, le traitement de la bande basse par les blocs 904 à 908, le pré-traitement de la bande haute par les blocs 909 à 910, le codeur MDCT 913 et le multiplexeur 915.In this figure, there is the bank of
La différence principale entre cette variante et le mode de réalisation de la
Dans cette variante, le dispositif 912 de pondération perceptuelle avec compensation de gain W2(z) en bande haute prend la même forme que le filtre W1(z) dans la bande basse. Il s'agit donc d'un filtre de type Â2(z/γ' 1)/Â2(z/γ'2) suivi d'un facteur de compensation de gain fac2 défini comme :
où les â'i sont les coefficients du filtre Â2(z) :
et γ'1 = 0,96 et γ'2 = 0,6.In this variant, the
where ? i are the coefficients of the filter ? 2 (z) :
and γ ' 1 = 0.96 and γ ' 2 = 0.6.
Ce facteur correspond à :
pour z = 1, soit la fréquence 0 Hz ou composante continue dans le bande haute qui correspond en fait à 4 kHz une fois que cette fréquence est ramenée au signal d'entrée avant filtrage QMF.This factor corresponds to:
for z = 1, ie the
L'intérêt de l'étape de pondération perceptuelle avec compensation de gain dans les deux sous-bandes est expliqué à la
- un filtrage W1(z) avant MDCT en bande basse,
- un filtrage W2(z) avant MDCT en bande haute.
- a filter W 1 (z) before MDCT in low band,
- a filter W 2 (z) before MDCT in high band.
Ces deux opérations en sous-bandes sont schématisées respectivement par la réponse en amplitude de Â1(z/γ 1)/Â1(z/γ 2) en bande basse et la réponse en amplitude de Â2(z/γ'1)/Â2(z/γ'2) en bande haute.These two operations in sub-bands are schematized respectively by the amplitude response of λ 1 (z / γ 1 ) / λ 1 (z / γ 2 ) in the low band and the amplitude response of λ 2 (z / γ ' 1 ) / Λ 2 (z / γ ' 2 ) in high band.
Les compensations de gain en bandes basse et haute par les facteurs fac1 et fac2 respectivement assurent une continuité des réponses des filtres à 4 kHz. C'est cette continuité qui permet ensuite de coder les deux spectres discrets Xlo et Xhi en un seul vecteur X. Là encore, il est important de noter que la valeur 0 dB utilisée ici pour définir la continuité entre bandes basse et haute n'est qu'indicative.The gain compensation in low and high bands by the factors fac 1 and fac 2 respectively ensure a continuity of the responses of the filters at 4 kHz. It is this continuity that then makes it possible to code the two discrete spectra X lo and X hi into a single vector X. Again, it is important to note that the
Le décodeur audio hiérarchique correspondant à cette variante est décrit à la
L'invention couvre en outre un programme d'ordinateur comportant une suite d'instructions mémorisées sur un support pour exécution par un ordinateur ou un dispositif dédié, remarquable en ce que lors de l'exécution de ces instructions, ce dernier exécute le procédé de pondération perceptuelle objet de l'invention pour le codage et/ou le décodage.The invention furthermore covers a computer program comprising a sequence of instructions stored on a medium for execution by a computer or a dedicated device, which is remarkable in that, during the execution of these instructions, the latter executes the method of perceptual weighting object of the invention for coding and / or decoding.
Le programme d'ordinateur précité est par exemple un programme directement exécutable implanté dans un dispositif de pondération perceptuelle objet de l'invention.The aforementioned computer program is for example a directly executable program implanted in a perceptual weighting device object of the invention.
Il est bien entendu que l'invention n'est pas limitée aux seuls modes de réalisation qui viennent d'être décrits. En particulier, on notera que
- les valeurs numériques des paramètres ajustables γ 1, γ 2, γ' 1 et γ' 2 peuvent être différentes de celles choisies ci-dessus,
- le facteur de compensation fac1 peut être appliqué avant filtrage par Â(z/γ 1)/Â(z/γ 2) ou entre les filtres Â(z/γ 1) et Â(z/γ 2) ou encore intégré dans l'un des filtres Â(z/γ 1) ou Â(z/γ 2). Il en est de même pour le facteur fac2 et les filtres inverses correspondants,
- le filtre de pondération perceptuelle n'est pas nécessairement de la forme Â(z/γ 1)/Â(z/γ 2),
- le nombre de sous-bandes définies dans la bande de fréquence totale peut être supérieur à 2.
- the numerical values of the adjustable parameters γ 1 , γ 2 , γ ' 1 and γ' 2 may be different from those chosen above,
- the fac 1 compensation factor can be applied before filtering by  (z / γ 1 ) /  (z / γ 2 ) or between the filters  (z / γ 1 ) and  (z / γ 2 ) or else integrated into one of the filters  (z / γ 1 ) or Å (z / γ 2 ). It is the same for the fac 2 factor and the corresponding inverse filters,
- the perceptual weighting filter is not necessarily of the form  (z / γ 1 ) /  (z / γ 2 ),
- the number of sub-bands defined in the total frequency band may be greater than 2.
Claims (13)
- Perceptual weighting device for the coding/decoding of an audio signal in a given frequency band, said coding/decoding being performed in a plurality of adjacent sub-bands in said given frequency band, characterized in that said device comprises, in at least one sub-band, a filter (307) for perceptual weighting with gain compensation of the form fac Â(z/γ 1)/Â(z/γ 2) where Â(z) represents a linear prediction filter, with 0 ≤ γ 2 ≤ 1 and 0 ≤ γ 1 ≤ 1 and fac the gain compensation factor dependent on the coefficients of said filter Â(z).
- Device according to Claim 1, characterized in that said filter (307) for perceptual weighting with gain compensation comprises a perceptual weighting filter (501, 502) and a gain compensation module (503).
- Device according to Claim 1, characterized in that said filter for perceptual weighting with gain compensation comprises a perceptual weighting filter incorporating said gain compensation.
- Hierarchical audio coder in a frequency band decomposed into a first and a second adjacent sub-band, said coder comprising:- a core coder (305; 905) intended to code an original signal in the first sub-band of said frequency band,- a stage (306; 906) for the calculation of a residual signal (e) on the basis of said original signal and of the signal arising from said core coder,- a device for the perceptual weighting of said residual signal (e), in accordance with Claim 1.
- Coder according to Claim 5, characterized in that said filter (307) for perceptual weighting having gain compensation comprises a filter (501, 502) for perceptual weighting in the first sub-band.
- Coder according to one of Claims 5 to 7, characterized in that the coefficients of said linear prediction filter are provided by said core coder (305).
- Coder according to any one of Claims 5 to 8, characterized in that the signal arising from the device (307) for perceptual weighting in the first subband and the original signal in the second sub-band are applied respectively to transfonrm-based analysis modules (308, 311), and in that said transform-based analysis modules are linked to a transform-based coder (313) in said frequency band.
- Hierarchical audio decoder in a frequency band decomposed into a first and a second adjacent sub-band, said decoder comprising:- a core decoder (701; 1001) intended to decode in the first sub-band of said frequency band a received signal coded by the coder according to any one of Claims 5 to 9,- a device for the inverse perceptual weighting of a signal representative of the residual signal (e) weighted in the first sub-band by the perceptual weighting device (307; 907)of said coder,
characterized in that said inverse perceptual weighting device (707; 1008) comprises a filter for perceptual weighting with gain compensation, the inverse of the filter (307) for perceptual weighting with gain compensation of the coder in the first sub-band. - Method of perceptual weighting for the coding of an audio signal in a given frequency band, said coding being performed in a plurality of adjacent sub-bands in said given frequency band, characterized in that said method comprises, in at least one sub-band, a step of perceptual weighting with gain compensation of the form fac Â(z/γ 1)/Â(z/γ 2) where Â(z) represents a linear prediction filter, with 0 ≤ γ 2 ≤ 1 and 0 ≤ γ 1 ≤ 1 and fac the gain compensation factor dependent on the coefficients of said filter Â(z).
- Method of perceptual weighting for the decoding of an audio signal coded in a given frequency band in accordance with the method according to Claim 11, characterized in that said method comprises, in said sub-band, a step of perceptual weighting with gain compensation, the inverse of said step of perceptual weighting with gain compensation.
- Computer program comprising a series of instructions stored on a medium for execution by a computer or a dedicated device, characterized in that during the execution of said instructions, it implements the perceptual weighting method according to Claim 11 and/or Claim 12.
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PCT/FR2007/050760 WO2007093726A2 (en) | 2006-02-14 | 2007-02-07 | Device for perceptual weighting in audio encoding/decoding |
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WO2007093726A3 (en) | 2007-10-18 |
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