EP0689189A1 - Voice coders - Google Patents
Voice coders Download PDFInfo
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- EP0689189A1 EP0689189A1 EP95108870A EP95108870A EP0689189A1 EP 0689189 A1 EP0689189 A1 EP 0689189A1 EP 95108870 A EP95108870 A EP 95108870A EP 95108870 A EP95108870 A EP 95108870A EP 0689189 A1 EP0689189 A1 EP 0689189A1
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- 230000005284 excitation Effects 0.000 claims abstract description 41
- 238000003786 synthesis reaction Methods 0.000 claims abstract description 22
- 238000000034 method Methods 0.000 claims abstract description 15
- 238000005303 weighing Methods 0.000 claims abstract 2
- 230000015572 biosynthetic process Effects 0.000 description 17
- 230000006870 function Effects 0.000 description 17
- 238000013459 approach Methods 0.000 description 8
- 239000013598 vector Substances 0.000 description 6
- 230000003044 adaptive effect Effects 0.000 description 4
- 238000001914 filtration Methods 0.000 description 3
- 230000003595 spectral effect Effects 0.000 description 3
- RTZKZFJDLAIYFH-UHFFFAOYSA-N Diethyl ether Chemical compound CCOCC RTZKZFJDLAIYFH-UHFFFAOYSA-N 0.000 description 2
- 230000004044 response Effects 0.000 description 2
- 101000622137 Homo sapiens P-selectin Proteins 0.000 description 1
- 102100023472 P-selectin Human genes 0.000 description 1
- 101000873420 Simian virus 40 SV40 early leader protein Proteins 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 210000004704 glottis Anatomy 0.000 description 1
- 230000010355 oscillation Effects 0.000 description 1
- 238000013139 quantization Methods 0.000 description 1
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Classifications
-
- 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
- G10L19/083—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters the excitation function being an excitation gain
-
- 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
Definitions
- Speech coding is of application in several communication fields: from transmission via satellite to radiomobile, store-forward systems, automatic responders, etc.
- the present specification discloses some techniques for improving the features of speech coders based on the above-mentioned techniques.
- the voice coders based on the Linear Prediction (LP) are parametric coders; typically Analysis-by-Synthesis (A-b-S) techniques are used for the correct determination of the parameters of the system.
- Such coders synthesize the voice through the use of a suitable input excitation to a synthesis LP filter.
- the excitation should have the characteristics of the "physical" excitation wave which, coming from the glottis, is then spectrally modified in function of the characteristics of the system that simulates the voice segment (LP filter).
- A-b-S coders make use of an excitation structure which is composed of an Adaptive Codebook and of a Fixed Codebook (eventually structured). Without prejudicing the generality, it can be assumed that the Fixed Codebook is composed of independent vectors of random numbers, as in the case of CELP coders (M.R. Schroeder, B.S. Atal, "Code Excited Linear Prediction (CELP): high-quality speech at very low bit rates", Proc. ICASSP '85, pages 26-29).
- CELP Code Excited Linear Prediction
- Fig. 1 there is represented a block diagram of a typical CELP voice synthesizer; block LPC-IIR denotes the synthesis filter for reconstructing the voice waveform; e a (n) is the adaptive codebook vector (and Ga is the corresponding scaling factor) and e s (n) is the fixed codebook vector (and Gs is the corresponding scaling factor); e(n) is the composite excitation vector.
- e a (n) is the adaptive codebook vector (and Ga is the corresponding scaling factor)
- e s (n) is the fixed codebook vector (and Gs is the corresponding scaling factor)
- e(n) is the composite excitation vector.
- e a (n) and e s (n) are selected from a suitable set of vectors and are determined simultaneously with respective Ga and Gs. The determination occurs in a time interval of about 5 to 10 ms (analysis frame) and is based on the minimization of the objective function according to the well-known criterion of the perceptively weighted minimum-squared error (see M.R. Schroeder, B.S. Atal, "Code Excited Linear Prediction (CELP): high-quality speech at very low bit-rates", Proc.
- CELP Code Excited Linear Prediction
- ICASSP '85 pages 26 to 29), according to the following expression : where N is the length of the time interval for minimization; u i (n) is the zero-state synthesis filter response at the the i-th input of the Codebook (either adaptive or fixed) and G is the corresponding gain; lastly, r s (n) is the reference signal or "objective" signal (i.e. the original voice segment from which the contribution of the reconstruction filter memory deriving from previous synthesis has been subtracted).
- a first approach consists in using a signal r s el (n) longer than N samples as a reference signal of the objective function (i.e. signal r s (n) of eq. (1) ).
- the voice can always be considered as obtained from an ideal excitation that constitutes the input of an all-pole synthesis filter (the filter denoted by LPC-IIR in Fig. 1).
- Such ideal excitation is nothing else than the prediction residue, obtained by filtering the voice through the "inverse filter", i.e. the all-zero filter derived from LPC-IIR.
- the synthetic excitation has spectral and time location (e.g. the pitch pulse) characteristics similar to those of the ideal excitation. Therefore, it is evident that by including, in the objective function, the contributions of the free evolutions due to both the ideal excitation and to the synthetic excitation, it is possible to carry out a more correct choice of the latter. In fact, depending on the spectral/time characteristics of the signal, the difference between the ideal free evolution and the synthetic one may have a preponderant weight in the modified objective function.
- the above-mentioned concepts may be expressed according to the revised objective function: in which where u i (n) is the (zero state) synthesis filter response at the i-th input and el i (n) is the corresponding "synthetic" free evolution.
- the excitation parameters i.e. the i-th index and the corresponding gain G are then choosen in such a way as to minimize the modified objective function (4).
- the synthetic excitation has spectral and time location (e.g. pitch pulse) characteristics, similar to the ones present in the ideal excitation. From this it derives that it may be important to obtain not only a good similarity between the original voice and the synthetic voice, but also a good similarity between ideal excitation and synthetic excitation.
- the parameters of the reconstructed excitation allow the achievement of a synthetic voice which "averagely" is similar to the original voice.
- e s (n) is the prediction residue obtained from the reference signal r s (n)
- e i (n) is the codebook excitation generating the synthetic signal u i (n).
- the prediction residue e s (n) must be calculated starting from r s (n) through inverse filtering (with all-zero filter) with null initial state.
- the reference has been obtained from the voice signal by subtracting its reconstruction filter memory deriving from the previous synthesis. The reference signal is then "free" from every contribution due to the filter memory and can be considered as obtained from a suitable ideal excitation e s (n) coming into the synthesis filter with null initial state.
- excitation parameters i.e. the i-th index and the corresponding gain G
- Parameter ⁇ can be either fixed or even made adaptive (i.e. varying with time), for instance in function of certain characteristics of the signal that can be estimated a priori (e.g.: estimate of voiced/unvoiced, estimate of transients, estimate of the pitch period or of the synthesis filter, etc.).
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- Engineering & Computer Science (AREA)
- Computational Linguistics (AREA)
- Signal Processing (AREA)
- Health & Medical Sciences (AREA)
- Audiology, Speech & Language Pathology (AREA)
- Human Computer Interaction (AREA)
- Physics & Mathematics (AREA)
- Acoustics & Sound (AREA)
- Multimedia (AREA)
- Compression, Expansion, Code Conversion, And Decoders (AREA)
- Reduction Or Emphasis Of Bandwidth Of Signals (AREA)
Abstract
Description
- Speech coding is of application in several communication fields: from transmission via satellite to radiomobile, store-forward systems, automatic responders, etc.
- In particular there is a strong need of techniques effective for voice signal coding where there are remarkable band limitations (consider the "limited" availability of band in the ether); therefore, it is important to be able to reduce drastically the bit-rate to be transmitted, still maintaining a high quality of the received signal.
- Various voice signal coding techniques are used for this purpose; the most usual (assuring a high quality of the received signal at various bit-rates) are based upon the LP (Linear Prediction) and A-b-S (Analysis-by-Synthesis) principles (P. Kroon, E.F. Deprettere "A class of analysis-by-synthesis predictive coders for high quality speech coding at rates between 4.8 and 16 kbits/s", IEEE Journal on Selected Areas in Communications, Vol. 6, No. 2, pages 353-363, Feb. 1988).
- The present specification discloses some techniques for improving the features of speech coders based on the above-mentioned techniques.
- The voice coders based on the Linear Prediction (LP) are parametric coders; typically Analysis-by-Synthesis (A-b-S) techniques are used for the correct determination of the parameters of the system. Such coders synthesize the voice through the use of a suitable input excitation to a synthesis LP filter.
- In particular, the excitation should have the characteristics of the "physical" excitation wave which, coming from the glottis, is then spectrally modified in function of the characteristics of the system that simulates the voice segment (LP filter).
- The most recent A-b-S coders make use of an excitation structure which is composed of an Adaptive Codebook and of a Fixed Codebook (eventually structured). Without prejudicing the generality, it can be assumed that the Fixed Codebook is composed of independent vectors of random numbers, as in the case of CELP coders (M.R. Schroeder, B.S. Atal, "Code Excited Linear Prediction (CELP): high-quality speech at very low bit rates", Proc. ICASSP '85, pages 26-29).
- In Fig. 1 there is represented a block diagram of a typical CELP voice synthesizer; block LPC-IIR denotes the synthesis filter for reconstructing the voice waveform; ea(n) is the adaptive codebook vector (and Ga is the corresponding scaling factor) and es(n) is the fixed codebook vector (and Gs is the corresponding scaling factor); e(n) is the composite excitation vector. For a detailed description of the synthesizer, reference can be made to W.B. Kleijn, D.J. Krasinski, R.H. Ketchum "Improved Speech Quality and Efficient Vector Quantization in SELP", Proc. ICASSP '88, pages 155-158.
- In general, ea(n) and es(n) are selected from a suitable set of vectors and are determined simultaneously with respective Ga and Gs. The determination occurs in a time interval of about 5 to 10 ms (analysis frame) and is based on the minimization of the objective function according to the well-known criterion of the perceptively weighted minimum-squared error (see M.R. Schroeder, B.S. Atal, "Code Excited Linear Prediction (CELP): high-quality speech at very low bit-rates", Proc. ICASSP '85, pages 26 to 29), according to the following expression :
where N is the length of the time interval for minimization; ui(n) is the zero-state synthesis filter response at the the i-th input of the Codebook (either adaptive or fixed) and G is the corresponding gain; lastly, rs(n) is the reference signal or "objective" signal (i.e. the original voice segment from which the contribution of the reconstruction filter memory deriving from previous synthesis has been subtracted). - The objective function described at (1), even if usually used, cannot be optimal for the choice of the parameters. In particular, it must be kept in mind that the system is random: this entails that the contribution to the synthetic signal made by the excitation samples in the vicinity of n = 0, in general is greater than the contribution made by the excitation samples in the vicinity of
- This phenomenon becomes more apparent during signal transients, i.e. in the passages from unvoiced segments to voiced segments and within the voice portions in the segments in which the ideal excitation changes its shape (still maintaining the "quasi-period ic" characteristic) because of prediction filter variations.
- In the following, two possible approaches are described for overcoming the problems described above; these approaches can be used both separately and jointly and allow the characteristics of the A-b-S coders operating at various bit-rates to be improved.
- A first approach consists in using a signal rs el(n) longer than N samples as a reference signal of the objective function (i.e. signal rs(n) of eq. (1) ). Such a signal is obtained from the time linkage of the signal rs(n) (for n = o...N - 1) and from the free evolution of such a signal, said free evolution el(n) being obtained by charging the last p samples of rs(n) in the synthesis filter memory LPC-IIR (p being the order of the filter) and letting the filter "discharge" (i.e. calculating its output corresponding to a null input).
-
- Such approach can be justified in the following manner: the voice can always be considered as obtained from an ideal excitation that constitutes the input of an all-pole synthesis filter (the filter denoted by LPC-IIR in Fig. 1). Such ideal excitation is nothing else than the prediction residue, obtained by filtering the voice through the "inverse filter", i.e. the all-zero filter derived from LPC-IIR.
- Assume to carry out a dashed stationary analysis of the voice signal: then, within the analysis interval, the ideal excitation constitutes the forcing term of the synthesis filter. But, if at the end of the analysis interval, the input of the filter is "turned off" (i.e. the ideal excitation is set to zero), the synthesis filter is discharged according to a waveform depending on its poles and on the samples of the ideal excitation (especially those right preceding the time instant
- Therefore, it is evident that in the case in which the last samples of the ideal excitation are significant (e.g. a pitch pulse is present) and the filter is near to instability (e.g. during segments of voiced signal), the free evolution of the filter due to the ideal excitation, typically will exhibit sinusoidal oscillations which will damp rather slowly and therefore the term el(n) of equation (3) will contribute significantly.
- For a high-quality of the reconstructed signal it is very important that the synthetic excitation has spectral and time location (e.g. the pitch pulse) characteristics similar to those of the ideal excitation. Therefore, it is evident that by including, in the objective function, the contributions of the free evolutions due to both the ideal excitation and to the synthetic excitation, it is possible to carry out a more correct choice of the latter. In fact, depending on the spectral/time characteristics of the signal, the difference between the ideal free evolution and the synthetic one may have a preponderant weight in the modified objective function.
-
- The excitation parameters (i.e. the i-th index and the corresponding gain G) are then choosen in such a way as to minimize the modified objective function (4).
- For instance, to obtain the "original" free evolution e1(n) one could proceed in the following way:
- Inverse filtering (through an all-zero filter) of the voice signal along the interval 0...N - 1, thus obtaining the ideal excitation (prediction residue), limited to the time interval 0...N - 1.
- Providing at the input of the synthesis filter LPC-IIR the ideal excitation thus attained, obtaining again at the output the original voice signal within the time interval 0... N - 1.
- Starting from the final status of the synthesis filter thus attained, provide at the input of the synthesis filter a null input and let the filter "discharge" for a number M of samples equal to the length of the free evolution to be obtained.
- From the procedure described above it can be noted at once that there is no need of computing the prediction residue. In order to obtain the desired free evolution it is sufficient to force into the state of the synthesis filter the last p samples (p being the order of the filter) of the original voice signal (i.e. the samples N - 1, N - 2, ..., N - p) and letting the null-input filter discharge. Evidently one can proceed in a similar fashion for computing the synthetic free evolution.
- To be noted, lastly, that this approach does not entail an increase in the coding delay since, in the objective function, the voice samples beyond the time interval 0... N - 1 are not used.
- In the previous paragraph it has been pointed out that to obtain a high-quality of the reconstructed signal it is very important that the synthetic excitation has spectral and time location (e.g. pitch pulse) characteristics, similar to the ones present in the ideal excitation. From this it derives that it may be important to obtain not only a good similarity between the original voice and the synthetic voice, but also a good similarity between ideal excitation and synthetic excitation.
- In fact, by using an approach to the minimum squares in the classical objective function, the parameters of the reconstructed excitation allow the achievement of a synthetic voice which "averagely" is similar to the original voice.
- Actually, from the perceptive point of view it is often more important that the synthetic voice is similar to the original voice only locally (for instance it is very important to reconstruct the connection from an unvoiced segment to a voiced segment with the correct time alignment and with the correct dynamics. It is not rare to find connection transients whose time duration is much shorter than the duration of the synthesis frame).
- For a fair local reconstruction it is then important to maintain a certain similarity degree, also with the ideal excitation.
- The objective function may than be composed of two contributes, in function of the original voice and of the ideal excitation, respectively, and it assumes the following expression:
In equation (9) es(n) is the prediction residue obtained from the reference signal rs(n) and ei(n) is the codebook excitation generating the synthetic signal ui(n). To be noted that the prediction residue es(n) must be calculated starting from rs(n) through inverse filtering (with all-zero filter) with null initial state. In fact, as it is known, the reference has been obtained from the voice signal by subtracting its reconstruction filter memory deriving from the previous synthesis. The reference signal is then "free" from every contribution due to the filter memory and can be considered as obtained from a suitable ideal excitation es(n) coming into the synthesis filter with null initial state. - In equation (7), α is a parameter whose value is comprised between 0 and 1 and controls the importance to be attached to the minimization with respect to the reference signal. Letting α = 1 the original objective function is found again.
- The excitation parameters (i.e. the i-th index and the corresponding gain G) are then chosen in such a way as to minimize the objective function described by equations (7), (8), (9). Parameter α can be either fixed or even made adaptive (i.e. varying with time), for instance in function of certain characteristics of the signal that can be estimated a priori (e.g.: estimate of voiced/unvoiced, estimate of transients, estimate of the pitch period or of the synthesis filter, etc.).
- Finally, notice that the contribution due to the free evolution described in the preceding paragraph can be included in the objective function described by equations (7), (8), (9). In this case, term (8) of the objective function is modified as described in the preceding paragraph.
Claims (6)
- Method of computing the excitation parameters in voice coders based on linear prediction and analysis-by-synthesis techniques using an objective function to be minimized, characterized in that said objective function comprises, jointly or alternately the free evolution of the objective signal and of the synthetic signal and a weighing with respect to the error between prediction residue and synthetic excitation.
- Method according to claim 1, characterized by using the objective function:
- Method according to claim 2, characterized in that said weight factor is allowed to vary with time.
- Audio coder characterized by using the method according to one of the preceding claims.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
ITMI941283 | 1994-06-20 | ||
ITMI941283A IT1271182B (en) | 1994-06-20 | 1994-06-20 | METHOD TO IMPROVE THE PERFORMANCE OF VOICE CODERS |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0689189A1 true EP0689189A1 (en) | 1995-12-27 |
EP0689189B1 EP0689189B1 (en) | 1999-09-22 |
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ID=11369140
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
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EP95108870A Expired - Lifetime EP0689189B1 (en) | 1994-06-20 | 1995-06-08 | Method of improving the features of voice coders |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP0689189B1 (en) |
AU (1) | AU698340B2 (en) |
DE (1) | DE69512323T2 (en) |
IT (1) | IT1271182B (en) |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP3273455B2 (en) * | 1994-10-07 | 2002-04-08 | 日本電信電話株式会社 | Vector quantization method and its decoder |
DE10047172C1 (en) * | 2000-09-22 | 2001-11-29 | Siemens Ag | Speech processing involves comparing output parameters generated and to be generated and deriving change instruction using reduced weight of input parameters with little influence |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0465057A1 (en) * | 1990-06-29 | 1992-01-08 | AT&T Corp. | Low-delay code-excited linear predictive coding of wideband speech at 32kbits/sec |
EP0515138A2 (en) * | 1991-05-20 | 1992-11-25 | Nokia Mobile Phones Ltd. | Digital speech coder |
EP0516439A2 (en) * | 1991-05-31 | 1992-12-02 | Motorola, Inc. | Efficient CELP vocoder and method |
-
1994
- 1994-06-20 IT ITMI941283A patent/IT1271182B/en active IP Right Grant
-
1995
- 1995-06-08 DE DE69512323T patent/DE69512323T2/en not_active Expired - Lifetime
- 1995-06-08 EP EP95108870A patent/EP0689189B1/en not_active Expired - Lifetime
- 1995-06-20 AU AU21753/95A patent/AU698340B2/en not_active Ceased
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0465057A1 (en) * | 1990-06-29 | 1992-01-08 | AT&T Corp. | Low-delay code-excited linear predictive coding of wideband speech at 32kbits/sec |
EP0515138A2 (en) * | 1991-05-20 | 1992-11-25 | Nokia Mobile Phones Ltd. | Digital speech coder |
EP0516439A2 (en) * | 1991-05-31 | 1992-12-02 | Motorola, Inc. | Efficient CELP vocoder and method |
Non-Patent Citations (3)
Title |
---|
"Proc. ICASSP '85 (International Conference on Acoustics, Speech and Signal Processing)", article M.R. SCHROEDER, B.S. ATAL: "Code Excited Linear Prediction (CELP): high-quality speech at very low bit rates", pages: 26 - 29 |
"Proc. ICASSP '88 (International Conferences on Acoustics, Speech and Signal Processing)", article W.B. KLEIJN, D.J. KRASINSKI, R.H. KETCHUM: "Improved Speech Quality and Efficient Vector Quantization in SELP", pages: 155 - 158 |
P. KROON, E.F. DEPRETTERE: "A class of analysis-by-synthesis predictive coders for high quality speech coding at rates between 4.8 and 16 Kbit/s", IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, vol. 6, no. 2, February 1988 (1988-02-01), pages 353 - 363 |
Also Published As
Publication number | Publication date |
---|---|
DE69512323D1 (en) | 1999-10-28 |
AU698340B2 (en) | 1998-10-29 |
ITMI941283A0 (en) | 1994-06-20 |
DE69512323T2 (en) | 2000-07-06 |
IT1271182B (en) | 1997-05-27 |
EP0689189B1 (en) | 1999-09-22 |
ITMI941283A1 (en) | 1995-12-20 |
AU2175395A (en) | 1996-01-04 |
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