EP0694907A2 - Codeur de parole - Google Patents

Codeur de parole Download PDF

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Publication number
EP0694907A2
EP0694907A2 EP95111278A EP95111278A EP0694907A2 EP 0694907 A2 EP0694907 A2 EP 0694907A2 EP 95111278 A EP95111278 A EP 95111278A EP 95111278 A EP95111278 A EP 95111278A EP 0694907 A2 EP0694907 A2 EP 0694907A2
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EP
European Patent Office
Prior art keywords
delay
code
codes
closed
speech
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EP95111278A
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German (de)
English (en)
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EP0694907A3 (fr
Inventor
Keiichi C/O Nec Corporation Funaki
Kazunori C/O Nec Corporation Ozawa
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NEC Corp
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NEC Corp
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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
    • 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
    • G10L2019/0001—Codebooks
    • G10L2019/0003—Backward prediction of 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
    • G10L2019/0001—Codebooks
    • G10L2019/0011—Long term prediction filters, i.e. pitch estimation
    • 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
    • G10L2019/0001—Codebooks
    • G10L2019/0013—Codebook search algorithms

Definitions

  • the present invention relates to a speech coding system and, more particularly, a speech coding system intended for high-quality coding of speech signals at low bit rates, preferably at a bit rate as low as 8 to 4 kb/s.
  • the present invention in particular relates to a speech coder which can realize reduction in the amount of operation to be conducted in a long-period prediction unit and improvement of coding precision.
  • CELP Code Excited LPC Coding
  • each frame into small sections of subframes (of 5 ms, for example).
  • For each subframe determine a delay code indicative of pitch correlation in a manner as described below by using an adaptive code book which stores a sound source signal of subframe length (adaptive code vector) obtained by delaying a previous sound source signal for a period of delay samples corresponding to each delay code (long-period prediction).
  • an adaptive code book which stores a sound source signal of subframe length (adaptive code vector) obtained by delaying a previous sound source signal for a period of delay samples corresponding to each delay code (long-period prediction).
  • a delay code for an adaptive code vector and a quantization code for a sound source code vector are searched by the following procedure.
  • Ns represents a subframe length
  • H represents a matrix which realizes a synthesis filter
  • gej represents a gain of the adaptive code vector ej.
  • the above formula (1) is expanded into the following formula (2).
  • a numerator Cj and a denominator Gj represent a cross-correlation and an auto-correlation which can be calculated according to the following formulas (3) and (4), respectively.
  • the auto-correlation Gj and the cross-correlation Cj are computed after the calculation of Hej[n] by driving the synthesis filter, that is, filtering.
  • filtering processing is conducted (tried) to the amount corresponding to the size of a code book (with a 7-bit sound source, for example, filtering processing will be conducted with respect to 128 codes).
  • the amount of operation (the number of times of sum-of-product operation) for one frame to be processed will be therefore extremely increased.
  • This method is implemented by the procedure described in the following. First, set delay codes to the amount corresponding to the size of an adaptive code book and generate code vectors corresponding to the set delay codes by using an open-loop adaptive code book where previous speech signals weighted with auditory sensation are stored.
  • the foregoing processing is referred to as preliminary selection by open-loop processing.
  • select codes located in the vicinity of the candidate codes selected by the preliminary selection to generate a code vector ad for each code by using a closed-loop adaptive code book where previous sound source signals are stored.
  • the open-loop processing which uses no results of previous coding, involves no filtering processing, it requires an extremely smaller amount of operation than that required for the above-described coding processing in which closed-loop search is conducted with respect to all codes.
  • the open-loop processing since selection of candidates for delay codes in preliminary selection by the open-loop processing allows the number of codes searched in the closed-loop processing to be effectively reduced, the open-loop processing can realize reduction in the amount of operation in long-period prediction though it provides lower precision in delay code search than that obtained by the closed-loop processing.
  • Closed-loop search is a method of generating an adaptive code vector corresponding to a given delay code, generating a synthesis signal by using the generated adaptive code vector and calculating an evaluation function (error power) between the synthesis signal and a speech signal to determine a delay code which makes the calculated error power the smallest.
  • the closed-loop search is filtering processing using previous sound source signals and is different from the open-loop search which requires no filtering processing using previous speech signals.
  • search by the open-loop processing can attain lower search precision in practice compared with that obtained by the closed-loop processing, so that preliminary selection very often causes erroneous selection of delay codes.
  • Such erroneous selection of a delay code in preliminary selection is one of the factors contributing to deterioration of sound quality.
  • An object of the present invention is to provide a speech coder which solves the above described problems and realizes long-period prediction with a reduced amount of operation and to high precision.
  • Another object of the present invention is to provide a speech coder which can attain improved sound quality by making the number of selection errors in preliminary selection less than that occurring in preliminary selection by conventional open-loop processing using speech signals.
  • a further object of the present invention is to provide a speech coder which allows preliminary selection to be conducted with approximately the same amount of operation as that required for preliminary selection using open-loop processing and also enables more precise selection of delay codes.
  • a speech coder comprising: speech analysis means for determining a short-period prediction code indicative of frequency characteristics of a speech signal for every fixed section of said speech signal; signal storage means for storing a previous sound source signal of a speech coding section; delay code trial means for trying delay codes indicative of a pitch correlation of said speech signal; long-period prediction preliminary selection means for selecting candidates for delay codes from said delay codes by closed-loop processing; long-period prediction main selection means for determining an optimum delay code among said candidates by closed-loop processing; and sound source code book searching means for determining an optimum quantization code for a residual signal determined by said optimum delay code.
  • the delay code trial means for preliminary selection tries delay codes indicative of a pitch correlation of a speech signal while evenly or unevenly thinning them out.
  • the delay code trial means for preliminary selection tries said delay codes while thinning them out at short intervals in a range of small delay values and at long intervals in a range of large delay values.
  • the delay code trial means for preliminary selection selects only the codes in a predetermined section of delay codes indicative of a pitch correlation of a speech signal and tries the selected delay codes.
  • the delay code trial means for preliminary selection selects only the codes in a predetermined section of delay codes indicative of a pitch correlation of a speech signal and tries the selected delay codes while evenly or unevenly thinning them out.
  • the main selection mean further comprises delay code selecting means for selecting and trying delay codes among several candidates for delay codes selected by said long-period prediction preliminary selection means.
  • the preliminary selection means further comprises: means for generating an adaptive code vector corresponding to said delay code set by said preliminary selection delay code trial means from said signal storage means; and means for calculating an evaluation function with said speech signal and said adaptive code vector as inputs; and said long-period prediction main selection means further comprises: means for determining a candidate for said delay code according to said evaluation function; means for generating an adaptive code vector corresponding to a determined delay code from said signal storage means; means for calculating an evaluation function with said speech signal and said adaptive code vector as inputs; and means for determining said delay code which makes said evaluation function take the smallest value as an optimum delay code.
  • the candidate determining means of said long-period prediction main selection means determines a plurality of delay codes having smaller values of said evaluation function as candidates.
  • a speech coder comprising: speech analysis means for determining a short-period prediction code indicative of frequency characteristics of a speech signal for every fixed section of said speech signal; signal storage means for storing a previous sound source signal of a speech coding section; preliminary selection delay code trial means for selecting only the codes in a predetermined section of delay codes indicative of a pitch correlation of said speech signal and trying the selected delay codes while evenly or unevenly thinning them out; long-period prediction preliminary selection means for selecting candidates for delay codes from said delay codes by closed-loop processing; main selection delay code selecting means for selecting delay codes among said candidates for delay codes and trying the selected delay codes; long-period prediction main selection means for determining an optimum delay code among said delay codes by closed-loop processing; and sound source code book search means for determining an optimum quantization code for a residual signal determined by said optimum delay code.
  • the said main selection delay code selecting means determines a delay code with which the evaluation function takes the smallest value among said candidates for delay codes and tries, as a delay code for main selection, a delay code whose delay value has a harmonic relationship with the delay value of the determined delay code.
  • the main selection delay code selecting means selects a plurality of candidates among said candidates for delay codes and tries, as codes for main selection, delay codes whose delay values are approximate to the respective candidates.
  • the preliminary selection means further comprises: means for generating an adaptive code vector corresponding to said delay code set by said preliminary selection delay code trial means from said signal storage means; and means for calculating an evaluation function with said speech signal and said adaptive code vector as inputs; and said long-period prediction main selection means further comprises: means for generating an adaptive code vector corresponding to a determined delay code from said signal storage means; means for calculating an evaluation function with said speech signal and said adaptive code vector as inputs; and means for determining said delay code which makes said evaluation function take the smallest value as an optimum delay code.
  • the preliminary selection delay code trial means tries said delay codes while thinning them out at short intervals in a range of small delay values and at long intervals in a range of large delay values.
  • a speech coder including long-period prediction means for outputting an optimum delay code of delay codes indicative of a pitch correlation of a speech signal, said long-period prediction means further comprising: means for extracting delay codes while selecting a section of delay codes and/or thinning out the delay codes; means for conducting preliminary selection of delay codes by executing closed-loop search with respect to the extracted delay codes; and means for determining an optimum delay code by executing closed-loop search with respect to the preliminarily selected delay codes.
  • the speech processing device includes a coding processing unit 10 and a decoding processing unit 20.
  • the speech coding processing unit 10 includes a buffer 110, an LPC analysis circuit 120, a parameter quantization circuit 130, a weighting circuit 140, an adaptive code book 150, a long-period prediction circuit 160, a sound source code book 170, a sound source code book search circuit 180, a gain code book 190, a gain code book search circuit 200 and a multiplexer 210.
  • the 100 represents a speech input terminal of the encoder.
  • the buffer circuit 110 stores a speech signal input through the speech input terminal 100.
  • the LPC analysis circuit 120 is for extracting an LPC coefficient, a spectrum parameter of speech.
  • the parameter quantization circuit 130 is for quantizing LPC coefficients.
  • the weighting circuit 140 is for weighting a speech signal with auditory sensation.
  • the adaptive code book 150 is where previous sound source signals are stored.
  • the long-period prediction circuit 160 is for searching for a delay code (adaptive code vector) indicative of pitch correlation.
  • the sound source code book 170 is where sound source code vectors of subframe length indicative of long-period prediction residues are stored.
  • the sound source code book search circuit 180 is for determining an optimum quantization code (sound source code vector) by using the sound source code book.
  • the gain code book 190 is where parameters indicative of gain terms of adaptive code vectors and sound source code vectors are stored.
  • the gain code book search circuit 200 is for determining quantization gains of adaptive code vectors and sound source code vectors by using the gain code book.
  • the multiplexer 210 is a circuit for combining a series of codes and outputting them.
  • the speech decoding processing unit 20 includes a demultiplexer 220, an adaptive code book 150a, a sound source code book 170a, a gain code book 190a and a synthesis filter 230.
  • the demultiplexer 220 is a circuit for decoding encoded codes into a series of codes.
  • the adaptive code book 150a, the sound source code book 170a and the gain code book 190a are the same as their counterparts in the speech coding processing unit 10.
  • the synthesis filter 230 is a circuit for reproducing a speech signal by using a generated sound source and a speech synthesis filter.
  • a speech signal reproduced by the synthesis filter 230 is output from a speech output terminal 240.
  • Fig. 2 is a block diagram showing a first embodiment of a first long-term prediction circuit 160-1, which is a characteristic component of the present invention provided in the speech coding processing unit 10.
  • the first long-period prediction circuit 160-1 includes a closed-loop delay code preliminary selection unit 300 and a closed-loop delay code main selection unit 400.
  • the closed-loop delay code preliminary selection unit 300 includes a first closed-loop preliminary selection delay code trial circuit 310, a closed-loop adaptive code vector generating circuit 320 and a closed-loop evaluation function calculating circuit 330
  • the closed-loop delay code main selection unit 400 includes a first closed-loop main selection delay code trial circuit 410, a closed-loop adaptive code vector generating circuit 420, a closed-loop evaluation function calculating circuit 430 and an optimum delay code determining circuit 440.
  • the closed-loop adaptive code vector generating circuits 320 and 420 generate a closed-loop adaptive code vector ad with respect to a set delay code d according to a previous sound source signal stored in the closed-loop adaptive code book 150.
  • the closed-loop evaluation function calculating circuits 330 and 430 calculate an auto-correlation ⁇ H ad, H ad ⁇ and a cross-correlation ⁇ z, H ad ⁇ of a zero-state synthesis signal H ad to calculate ⁇ z, H ad ⁇ 2/ ⁇ H ad, H ad ⁇ as an evaluation function.
  • ⁇ , ⁇ denotes correlation.
  • the closed-loop adaptive code vector generating circuits 320 and 420 provide the same function, either can be shared by the closed-loop delay code preliminary selection unit 300 and the closed-loop delay code main selection unit 400.
  • the closed-loop evaluation function calculating circuits 330 and 430 also have the same function, so that they can be shared by the units 300 and 400.
  • the optimum delay code determining circuit 440 determines an optimum delay code with which an evaluation function takes the smallest value.
  • the speech buffer 110 stores a speech signal of a section to be subjected to coding processing. Such a delay code is searched for that makes an error power with respect to the speech signal the smallest.
  • the closed-loop adaptive code book 150 stores previous sound source signals as mentioned above.
  • the first closed-loop preliminary selection delay code trial circuit 310 is for changing (trying) delay codes to the amount corresponding to the size of the code book.
  • the first closed-loop main selection delay code trial circuit 410 is for preliminarily selecting a delay code according to an evaluation function calculated by the closed-loop evaluation function calculating circuit 330 and changing (trying) the selected delay code.
  • a speech signal is applied through the speech input terminal 100 and stored in the speech buffer 110.
  • the LPC analysis circuit 120 conducts short-period analysis by using a fixed number of samples of speech signals stored in the speech buffer 110 to calculation an LPC coefficient indicative of spectrum characteristics of the speech signal.
  • the spectrum parameter obtained by the LPC analysis is quantized by the parameter quantization circuit 130 and a quantization code of the LPC coefficient is sent to the multiplexer 210, while the parameter is inversely quantized for use in the following coding processing.
  • Speech signals stored in the buffer 110 are weighted with auditory sensation using quantized/inversely quantized LPC coefficients by the weighting circuit 140 and used for the following code book search.
  • Code book search is conducted with respect to the adaptive code book, the sound source code book and the gain code book.
  • the long-period prediction circuit 160 conducts long-period prediction to determine an optimum delay code indicative of a pitch correlation and transfer the code (delay code) to the multiplexer 210, as well as generating an adaptive code vector.
  • the sound source code book search circuit 180 searches the sound source code book to determine a quantization code and generate a sound source code vector, as well as transferring the code (quantization code) to the multiplexer 210.
  • the gain code book search circuit 200 calculates gains of two sound sources and transfers their codes to the multiplexer 210.
  • the multiplexer 210 combines the respective codes and converts them into transmission codes for output.
  • the demultiplexer resolves applied transmission codes into respective codes.
  • the demultiplexer decodes codes indicative of LPC coefficients into filter factors and transfers the same to the synthesis filter 230.
  • An adaptive code vector is generated from a delay code by using the adaptive code book 150a.
  • a sound source code vector is generated from a quantization code indicative of a code of a sound source by using the sound source code book 170a.
  • Gains of an adaptive code vector and a sound source code vector are calculated based on gain codes and each sound source is multiplied by a gain term to generate an input signal to the synthesis filter. Lastly, a speech signal is synthesized by the synthesis filter 230 in response to the input signal.
  • the first closed-loop preliminary selection delay code trial circuit 310 tries a delay code (Step 301) and the closed-loop adaptive vector generating circuit 320 and the closed-loop evaluation function calculating circuit 330 execute processing for each delay code.
  • the closed-loop adaptive vector generating circuit 320 generates a closed-loop adaptive code vector ad corresponding to a delay code d set by the first closed-loop preliminary selection delay code trial circuit 310 by using the closed-loop adaptive code book 150 (Step 302).
  • the closed-loop evaluation function calculating circuit 330 calculates an evaluation function (Step 303).
  • the first closed-loop main selection delay code trial circuit 410 determines candidates for delay codes for final main selection (Step 304).
  • the candidates should be delay codes with small evaluation function values. More specifically, delay codes to the number of M will be selected, from a delay code with the smallest value of the evaluation function to a delay code with a M-th smallest evaluation function value.
  • the closed-loop adaptive vector generating circuit 420 With respect to the delay code set by the first closed-loop main selection delay code trial circuit 410, the closed-loop adaptive vector generating circuit 420, the closed-loop evaluation function calculating circuit 430 and the optimum delay code determining circuit 440 execute common closed-loop adaptive code book search.
  • the closed-loop adaptive code vector generating circuit 420 generates a closed-loop adaptive code vector ad with respect to the set delay code according to previous sound source signals stored in the closed-loop adaptive code book 150 (Step 305).
  • the closed-loop evaluation function calculating circuit 430 calculates an auto-correlation ⁇ H ⁇ ad, H ⁇ ad ⁇ and a cross-correlation ⁇ z, H ⁇ ad ⁇ of a zero-state synthesis signal H ad to calculate ⁇ z, H ⁇ ad ⁇ 2/ ⁇ H ⁇ ad, H ⁇ ad ⁇ as an evaluation function (Step 306).
  • the optimum delay code determining circuit 440 determines an optimum delay code with which the evaluation function takes the smallest value and outputs the same as an optimum delay code d_opt (step 307).
  • FIG. 4 is a diagram showing the structure of the second long-period prediction circuit 160-2 according to the second embodiment of the present invention.
  • component modules identified by the same reference numerals as those in Fig. 2 have the same structure and therefore no description thereof will be given here.
  • Fig. 5 is a diagram showing a specific example of selection of delay codes in the second closed-loop preliminary selection delay code trial circuit 311.
  • the second closed-loop preliminary selection delay code trial circuit 311 changes (try) delay codes while thinning them out evenly (at fixed intervals) or unevenly (at unfixed intervals).
  • the delay codes are not all selected but are selected at certain intervals so that the number of delay codes to be selected is reduced.
  • the second closed-loop preliminary selection delay code trial circuit 311 tries delay codes while thinning them out evenly or unevenly as mentioned above. Then, the closed-loop adaptive vector generating circuit 320 and the closed-loop evaluation function calculating circuit 330 conduct processing for each of the set delay codes.
  • the delay codes denoted by the sign of a circle in the section of "in case of selection" is adopted, for example.
  • delay codes with small delay values are selected every two codes, while those with large delay values are selected every three codes.
  • Such selection prevents degradation of coding precision. It is as a matter of course possible to select the codes at fixed intervals (e.g. every two codes) from the beginning to the end.
  • Thinning-out of delay codes as shown in Fig. 5 leads to reduction of the number of delay codes from 128 down to 40.
  • the closed-loop adaptive code vector generating circuit 320 generates a closed-loop adaptive code vector ad corresponding to a delay code d set in the second closed-loop preliminary selection delay code trial circuit 311 by using the closed-loop adaptive code book 150. Then, the closed-loop evaluation function calculating circuit 330 calculates an evaluation function. According to the evaluation function calculated in the closed-loop evaluation function calculating circuit 330, the first closed-loop main selection delay code trial circuit 410 determines a candidate for a delay code for main selection. At this time, a delay code with a small evaluation function value is selected as a candidate. More specifically, delay codes are selected to the number of M, from a delay code with the smallest evaluation function value to a delay code with a M-th smallest evaluation function value.
  • the closed-loop adaptive code vector generating circuit 420, the closed-loop evaluation function calculating circuit 430 and the optimum delay code determining circuit 440 execute the same closed-loop adaptive code book searching processing as that described in the first embodiment.
  • FIG. 6 is a diagram showing the structure of the third long-period prediction circuit 160-3 according to the third embodiment of the present invention.
  • component modules identified by the same reference numerals as those in the first embodiment shown in Fig. 2 have the same structure and therefore no description thereof will be given here.
  • Fig. 7 is a diagram showing a specific example of selection of delay codes by the third closed-loop preliminary selection delay code trial circuit 312.
  • the third closed-loop preliminary selection delay code trial circuit 312 the characteristic component of the present embodiment, selects only the codes included in a certain section of delay codes and changes (try) them. In other words, not all the delay codes are selected but those included in a specific range set are selected so that the number of delay codes to be selected is reduced.
  • the third closed-loop preliminary selection delay code trial circuit 312 selects a part of the sections of delay codes and tries the selected codes. Then, the closed-loop adaptive code vector generating circuit 320 and the closed-loop evaluation function calculating circuit 330 execute processing for each of the tried delay codes.
  • Adopted as a manner of selecting delay codes by the third closed-loop preliminary selection delay code trial circuit 312 is, for example, to select a range of delay codes indicated by the sign of circle in the section of "in case of selection" in Fig. 7. More specifically, only the delay codes having delay values from "40" to "59.5" are selected in Fig. 7.
  • the closed-loop adaptive code vector generating circuit 320 generates a closed-loop adaptive code vector ad corresponding to a delay code d set in the third closed-loop preliminary selection delay code trial circuit 312 by using the closed-loop adaptive code book 150.
  • the closed-loop evaluation function calculating circuit 330 calculates an evaluation function.
  • the first closed-loop main selection delay code trial circuit 410 determines a candidate for a delay code for main selection. At this time, a delay code with a small evaluation function value is selected as a candidate. More specifically, delay codes are selected to the number of M, from a delay code with the smallest evaluation function value to a delay code with a M-th smallest evaluation function value.
  • the closed-loop adaptive code vector generating circuit 420, the closed-loop evaluation function calculating circuit 430 and the optimum delay code determining circuit 440 execute the same closed-loop adaptive code book searching processing as that described in the first embodiment.
  • FIG. 8 is a diagram showing the structure of the fourth long-period prediction circuit 160-4 according to the fourth embodiment of the present invention.
  • component modules identified by the same reference numerals as those in the first embodiment shown in Fig. 2 have the same structure and therefore no description thereof will be given here.
  • Fig. 9 is a diagram showing a specific example of selection of delay codes by the fourth closed-loop preliminary selection delay code trial circuit 313.
  • the fourth closed-loop preliminary selection delay code trial circuit 313 selects a part of the sections of delay codes and tries the selected codes while evenly or unevenly thinning them out. Then, the closed-loop adaptive code vector generating circuit 320 and the closed-loop evaluation function calculating circuit 330 conduct processing with respect to each of the tried delay codes.
  • Adopted as a manner of selecting delay codes by the fourth closed-loop preliminary selection delay code trial circuit 313 is, for example, to select delay codes indicated by the sign of circle in the section of "in case of selection" in Fig. 9. More specifically, illustrated in Fig. 9 is selecting the range of delay values between "40" and "59.5" and selecting delay codes every other code in that range.
  • the closed-loop adaptive code vector generating circuit 320 generates a closed-loop adaptive code vector ad corresponding to a delay code d set in the third closed-loop preliminary selection delay code trial circuit 312 by using the closed-loop adaptive code book 150.
  • the closed-loop evaluation function calculating circuit 330 calculates an evaluation function.
  • the first closed-loop main selection delay code trial circuit 410 determines a candidate for a delay code for main selection.
  • delay codes with small evaluation function values are selected as candidates. More specifically, delay codes are selected to the number of M, from a delay code with the smallest evaluation function value to a delay code with a M-th smallest evaluation function value.
  • the closed-loop adaptive code vector generating circuit 420, the closed-loop evaluation function calculating circuit 430 and the optimum delay code determining circuit 430 execute the same closed-loop adaptive code book searching processing as that described in the first embodiment.
  • Fig. 10 is a diagram showing the structure of the fifth long-period prediction circuit 160-5 according to the fifth embodiment of the present invention.
  • component modules identified by the same reference numerals as those in the first embodiment shown in Fig. 2 have the same structure and therefore no description thereof will be given here.
  • Fig. 11 is a diagram showing a specific example of selection of delay codes by the fifth closed-loop preliminary selection delay code trial circuit 314.
  • the second closed-loop main selection delay code trial circuit 411 determines a delay code with which the evaluation function calculated in the closed-loop evaluation function calculating circuit 330 takes the smallest value and tries, as a code for main selection, a delay code which has a harmonic relationship with the determined delay code, that is, which has a relationship of a multiple of an integer to a fraction of an integral number, or a delay code which has a relationship approximate to the former relationship.
  • the fifth closed-loop preliminary selection delay code trial circuit 314 selects a part of the sections of delay codes and tries delay codes in the selected range while evenly or unevenly thinning them out. Then, the closed-loop adaptive code vector generating circuit 320 and the closed-loop evaluation function calculating circuit 330 conduct processing with respect to each of the tried delay codes.
  • Adopted as a manner of selecting delay codes in the fifth closed-loop preliminary selection delay code trial circuit 314 is to select delay codes indicated by the sign of circle in the section of "in case of selection" in Fig. 11, for example. More specifically, illustrated in Fig. 11 is selecting the range of delay values between "39" and "63" and selecting delay codes having small delay values while thinning them out every other code and delay codes having large delay values while thinning them out every two codes.
  • the closed-loop adaptive code vector generating circuit 320 generates a closed-loop adaptive code vector ad corresponding to a delay code d set in the fifth closed-loop preliminary selection delay code trial circuit 314 by using the closed-loop adaptive code book 150.
  • the closed-loop evaluation function calculating circuit 330 calculates an evaluation function. According to the evaluation function calculated in the closed-loop evaluation function calculating circuit 330, the second closed-loop main selection delay code trial circuit 411 determines a candidate for a delay code for main selection.
  • Selection of delay codes by the second closed-loop main selection delay code trial circuit 411 is carried out by first determining one delay code with which the evaluation function takes the smallest value and then selecting, as candidates to the number of N, delay codes which take delay values having a harmonic relationship (a multiple of an integer to a fraction of an integral number) with the delay value of the determined delay code, or delay codes which take delay values approximate to the delay values having the harmonic relationship (a multiple of an integer to a fraction of an integral number) with the delay value of the determined delay code.
  • the closed-loop adaptive code vector generating circuit 420, the closed-loop evaluation function calculating circuit 430 and the optimum delay code determining circuit 440 execute the same closed-loop adaptive code book searching processing as that described in the first embodiment.
  • FIG. 12 is a diagram showing the structure of the sixth long-period prediction circuit 160-6 according to the sixth embodiment of the present invention.
  • component modules identified by the same reference numerals as those in the fifth embodiment shown in Fig. 10 have the same structure and therefore no description thereof will be given here.
  • the sixth closed-loop preliminary selection delay code trial circuit 315 is structured to selectively adopt the delay code selecting methods used by the closed-loop preliminary selection delay code trial circuits 310, 311, 312 and 313 as described in the above first to fourth embodiments.
  • the remaining part is the same as that of the above-described fifth embodiment.
  • the sixth closed-loop preliminary selection delay code trial circuit 315 first selects and tries delay codes by any of the methods described in the above first to fourth embodiments. Then, the closed-loop adaptive code vector generating circuit 320 and the closed-loop evaluation function calculating circuit 330 conduct processing with respect to each of the tried delay codes.
  • delay codes are tried without thinning them out as is described in the first embodiment.
  • the closed-loop adaptive code vector generating circuit 320 generates a closed-loop adaptive code vector ad corresponding to a delay code d set in the sixth closed-loop preliminary selection delay code trial circuit 315 by using the closed-loop adaptive code book 150. Then, the closed-loop evaluation function calculating circuit 330 calculates an evaluation function.
  • the second closed-loop main selection delay code trial circuit 411 determines a candidate for a delay code for main selection.
  • Selection of delay codes by the second closed-loop main selection delay code trial circuit 411 is carried out by first determining one delay code with which the evaluation function takes the smallest value and then selecting, as candidates to the number of N, delay codes which take delay values having a harmonic relationship (a multiple of an integer to a fraction of an integral number) with the delay value of the determined delay code, or delay codes which take delay values approximate to the delay values having the harmonic relationship (a multiple of an integer to a fraction of an integral number) with the delay value of the determined delay code.
  • the closed-loop adaptive code vector generating circuit 420 With respect to the candidates for delay codes set by the second closed-loop main selection delay code trial circuit 411, the closed-loop adaptive code vector generating circuit 420, the closed-loop evaluation function calculating circuit 430 and the optimum delay code determining circuit 440 conduct the same closed-loop adaptive code book searching processing as that shown in the first embodiment.
  • closed-loop preliminary selection can realize reduction of the amount of operation, without decreasing precision, through selection of only a part of sections of delay codes and selection of delay codes at fixed intervals while evenly or unevenly thinning them out.
  • an obtained SNR signal-noise ratio
  • (cross-correlation)2/(auto-correlation) is used as an evaluation function in the above-described embodiments, (cross-correlation)2 or energy of an error signal may be used to obtain the same effect.
  • a signal stored in the speech buffer is a zero-state subtraction signal z
  • the same effect can be attained by using an input speech signal, a residual signal or a signal weighted with auditory sensation.
  • the embodiments can produce the same effect by selecting a plurality of candidates and conducting main selection in the following steps (search of a sound source code book or a gain code book) or by conducting optimum delay code search simultaneously with the following code book search.
  • delay codes have been described with respect to decimal fractional delay in the above embodiments, the same effect can be obtained in a case of integral number delay.
  • a noise code book or a learning code book obtained by leaning with the vector quantization (VQ) algorithm may be used to produce the same effect.
  • the Literature 1 may be referred to for the noise code book.
  • sound source code book search circuit of one-stage structure in the above embodiments may be of multistage structure to produce the same effect.
  • the present invention realizes long-period prediction by two-stage selection including the closed-loop preliminary selection and the closed-loop main selection.
  • Preliminary selection by closed-loop processing enables preliminary selection with higher precision than conventional open-loop preliminary selection.
  • the present invention can realize reduction of the amount of operation, without decreasing precision, by selecting only delay codes of a part of sections (for example, extracting delay codes with medium delay values out of all the delay codes) at the time of closed-loop preliminary selection or evenly or unevenly thinning out selected codes (for example, thinning out delay codes every other code in a range of small delay values and every two codes in a range of large delay values).
  • the present invention realizes long-period prediction by two-stage selection including preliminary selection employing closed-loop processing and main selection employing closed-loop processing and realizes preliminary selection of delay codes by closed-loop processing using sound source signals, the preliminary selection involves less selection error than that occurs in preliminary selection by conventional open-loop processing using speech signals, whereby sound quality is improved.
  • the present invention enables approximately the same extent of reduction in the amount of operation and pitch estimation with higher precision, thereby effectively preventing deterioration of sound quality.
  • the present invention realizes reduction of the amount of operation, without decreasing precision in estimation, by selecting a part of sections of delay codes or evenly or unevenly thinning out the delay codes.

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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)
EP95111278A 1994-07-19 1995-07-18 Codeur de parole Withdrawn EP0694907A3 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP187903/94 1994-07-19
JP6187903A JPH0830299A (ja) 1994-07-19 1994-07-19 音声符号化装置

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EP0694907A2 true EP0694907A2 (fr) 1996-01-31
EP0694907A3 EP0694907A3 (fr) 1997-10-15

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WO2000025302A1 (fr) * 1998-10-27 2000-05-04 Matsushita Electric Industrial Co., Ltd. Codeur vocal plec
AU725922B2 (en) * 1995-04-24 2000-10-26 Nec Corporation Speech reproducing device capable of reproducing long-time speech with reduced memory
EP1098298A2 (fr) * 1999-11-08 2001-05-09 Mitsubishi Denki Kabushiki Kaisha Codage de la parole utilisant des candidats multiples pour le prédicteur à long-terme
USRE43209E1 (en) 1999-11-08 2012-02-21 Mitsubishi Denki Kabushiki Kaisha Speech coding apparatus and speech decoding apparatus

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JPWO2008072732A1 (ja) * 2006-12-14 2010-04-02 パナソニック株式会社 音声符号化装置および音声符号化方法
JP4525694B2 (ja) * 2007-03-27 2010-08-18 パナソニック株式会社 音声符号化装置

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JPH0398099A (ja) 1989-08-31 1991-04-23 Codex Corp デジタル音声コーダおよびそのコーダに用いられるパラメータを求める方法

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JP3254696B2 (ja) * 1991-09-25 2002-02-12 三菱電機株式会社 音声符号化装置、音声復号化装置および音源生成方法
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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AU725922B2 (en) * 1995-04-24 2000-10-26 Nec Corporation Speech reproducing device capable of reproducing long-time speech with reduced memory
WO2000025302A1 (fr) * 1998-10-27 2000-05-04 Matsushita Electric Industrial Co., Ltd. Codeur vocal plec
US6804639B1 (en) 1998-10-27 2004-10-12 Matsushita Electric Industrial Co., Ltd Celp voice encoder
EP1098298A2 (fr) * 1999-11-08 2001-05-09 Mitsubishi Denki Kabushiki Kaisha Codage de la parole utilisant des candidats multiples pour le prédicteur à long-terme
EP1098298A3 (fr) * 1999-11-08 2002-12-11 Mitsubishi Denki Kabushiki Kaisha Codage de la parole utilisant des candidats multiples pour le prédicteur à long-terme
US7047184B1 (en) 1999-11-08 2006-05-16 Mitsubishi Denki Kabushiki Kaisha Speech coding apparatus and speech decoding apparatus
USRE43190E1 (en) 1999-11-08 2012-02-14 Mitsubishi Denki Kabushiki Kaisha Speech coding apparatus and speech decoding apparatus
USRE43209E1 (en) 1999-11-08 2012-02-21 Mitsubishi Denki Kabushiki Kaisha Speech coding apparatus and speech decoding apparatus

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EP0694907A3 (fr) 1997-10-15
CA2154192A1 (fr) 1996-01-20

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