Method and apparatus for transcoding between different speech encoding/decoding systems and recording medium

• US 20050131682A1
• Filed: 01/24/2005
• Published: 06/16/2005
• Est. Priority Date: 07/24/2002
• Status: Active Grant

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Murashima, Atsushi
• Granted Patent: US 7,231,345 B2
• Time in Patent Office: Days
• US Class Current: 704/219
• CPC Class Codes: G10L 19/083 the excitation function being an excitation gain G10L25/90 takes precedence
• CPC Class Codes: G10L 19/12 the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
• CPC Class Codes: G10L 19/173 Transcoding, i.e. converting between two coded representations avoiding cascaded coding-decoding

Abstract

Disclosed is a code converting apparatus for converting a first code sequence conforming to a first system to a second code sequence conforming to a second system, in which a speech decoding circuit acquires a first linear prediction coefficient and the information on an excitation signal from the first code sequence, and actuates a filter having the aforementioned first linear prediction coefficient with the excitation signal obtained from the information on the excitation signal, to generate a first speech signal. A gain code generating circuit calculates a gain minimizing the distance between a second speech signal, generated from the information, obtained from the second code sequence, and the first speech signal (optimum gain), and corrects the optimum gain and the gain code generating circuit then finds the gain information in the second code sequence, based on the optimum gain as corrected (optimum gain corrected), the above optimum gain and a gain read out from a gain codebook of the second system. The gain is found at this time, in a non-speech segment, based on a speech decision value, using an evaluation function which will reduce time variations of the gain of the second system.

Claims

1. A code converting method for converting a first code sequence conforming to a first system to a second code sequence conforming to a second system, comprising the steps of:

acquiring a first linear prediction coefficient and the information on an excitation signal from said first code sequence and actuating a filter having said first linear prediction coefficient with the excitation signal obtained from said information on the excitation signal to generate a first speech signal;

deriving an optimum gain based on a second speech signal generated by the information obtained from a second code sequence, and on said first speech signal;

correcting said optimum gain; and

finding the gain information in said second code sequence based on an optimum gain corrected (termed ‘

corrected optimum gain’

), said optimum gain and on the gain read out from a gain codebook for said second system.

3. The code converting method according to claim 1, further comprising the steps of:

calculating a first square error from the corrected optimum gain and from the gain read out from said gain codebook;

calculating a second square error from said optimum gain and from the gain read out from said gain codebook; and

selecting a gain minimizing an evaluation function which is based on said first square error and said second square error from said gain codebook to find the gain information in said second code sequence.

5. The code converting method according to claim 1, wherein said corrected optimum gain is based on the long-term average value of said optimum gain.

7. The code converting method according to claim 1, wherein the gain minimizing the distance between the second speech signal generated by the information obtained from said second code sequence and said first speech signal is found as said optimum gain.

8. The code converting method according to claim 3, wherein said evaluation function is composed of said first square error, said second square error and weighting coefficients.

9. The code converting method according to claim 1, further comprising the steps of:

determining a speech decision value, discriminating the speech segment/non-speech segment, based on said first linear prediction coefficient; and

finding the gain information in said second code sequence, using an evaluation function which will decrease a temporal variation of the gain in said second code sequence, when said speech decision value indicates the non-speech segment.

10. The code converting method according to claim 3, further comprising the steps of:

determining a speech decision value, discriminating the speech segment/non-speech segment, based on said first linear prediction coefficient;

said evaluation function being found by taking a weighted average value of said first and second square errors by weighting coefficients; and

setting said weighting coefficients to respective preset values, based on said speech decision value, depending on the speech segment and the non-speech segment, to calculate said evaluation function.

17. The code converting apparatus according to claim 10, wherein the gain minimizing the distance between the second speech signal generated by the information obtained from said second code sequence and said first speech signal is found as said optimum gain.

2. A code converting method for converting a first code sequence conforming to a first system to a second code sequence conforming to a second system, comprising the steps of:

decoding the gain information from said first code sequence;

correcting the gain decoded (termed ‘

decoded gain’

); and

finding the gain information in said code sequence based on the decoding gain corrected (termed ‘

corrected decoded gain’

), said decoded gain and the gain read out from the codebook in said second system.

4. The code converting method according to claim 2, further comprising the steps of:

calculating a first square error from the corrected decoded gain and from the gain read out from said gain codebook;

calculating a second square error from said decoded gain and from the gain read out from said gain codebook; and

selecting a gain minimizing an evaluation function which is based on said first square error and said second square error from said gain codebook to find the gain information in said second code sequence.

6. The code converting method according to claim 2, wherein said corrected decoded gain is based on the long-term average value of said decoded gain.

11. A code converting apparatus for converting a first code sequence conforming to a first system to a second code sequence conforming to a second system, said apparatus comprising:

a speech decoding circuit for acquiring a first linear prediction coefficient and the information on an excitation signal from said first code sequence and actuating a filter having said first linear prediction coefficient with the excitation signal obtained from said information on the excitation signal to generate a first speech signal;

an optimum gain calculating circuit for calculating an optimum gain based on a second speech signal generated by the information obtained from a second code sequence, and on said first speech signal;

an optimum gain correcting circuit for correcting said optimum gain; and

a gain encoding circuit for finding the gain information in said second code sequence based on an optimum gain corrected (termed ‘

corrected optimum gain’

), said optimum gain and on the gain read out from a gain codebook for said second system.

13. The code converting apparatus according to claim 11, wherein said gain encoding circuit includes a unit for finding the gain information in said second code sequence by calculating a first square error from the corrected optimum gain and from the gain read out from said gain codebook;

calculating a second square error from said optimum gain and from the gain read out from said gain codebook; and

selecting a gain minimizing an evaluation function which is based on said first square error and said second square error from said gain codebook to find the gain information in said second code sequence.

15. The code converting apparatus according to claim 11, wherein said corrected optimum gain is based on the long-term average value of said optimum gain.

18. The code converting apparatus according to claim 13, wherein said evaluation function is composed of said first square error, said second square error and weighting coefficients.

19. The code converting apparatus according to claim 11, further comprising:

a speech/non-speech discriminating circuit for discriminating a speech decision value, discriminating the speech segment/non-speech segment, based on said first linear prediction coefficient;

said gain encoding circuit finding the gain information in said second code sequence, using an evaluation function which will decrease temporal variations of the gain in said second code sequence, when said speech decision value indicates the non-speech segment.

20. The code converting apparatus according to claim 13, further comprising;

a speech/non-speech discriminating circuit for outputting a speech decision value, discriminating the speech segment/non-speech segment, based on said first linear prediction coefficient;

said gain encoding circuit finding said evaluation function by taking a weighted average value of said first and second square errors by weighting coefficients; and

setting said weighting coefficients to respective preset values, based on said speech decision value, depending on the speech segment and the non-speech segment, to calculate said evaluation function.

12. A code converting apparatus for converting a first code sequence conforming to a first system to a second code sequence conforming to a second system, said apparatus comprising:

a gain decoding circuit for decoding the gain information from said first code sequence;

a decoded gain correcting circuit for correcting the gain decoded (termed ‘

decoded gain’

); and

a gain encoding circuit for finding the gain information in said code sequence based on the decoding gain corrected (‘

corrected decoded gain’

), said decoded gain and the gain read out from the codebook in said second system.

14. The code converting apparatus according to claim 12, wherein said gain encoding circuit includes a unit for calculating a first square error from the corrected decoded gain and from the gain read out from said gain codebook, calculating a second square error from said decoded gain and from the gain read out from said gain codebook, and selecting a gain minimizing an evaluation function which is based on said first square error and said second square error from said gain codebook to find the gain information in said second code sequence.

16. The code converting apparatus according to claim 12, wherein said corrected decoded gain is based on the long-term average value of said decoded gain.

21. A computer program product in a medium used by a computer that composes a code converting apparatus for converting a first code sequence, conforming to a first system, into a second code sequence conforming to a second system, comprising a program for causing said computer to execute:

(a) the processing of acquiring first linear prediction coefficient and the information on an excitation signal from said first code sequence and actuating a filter having said first linear prediction coefficient with the excitation signal obtained from said information on the excitation signal to generate a first speech signal;

(b) the processing of calculating an optimum gain based on a second speech signal generated by the information obtained from a second code sequence, and on said first speech signal;

(c) the processing of correcting said optimum gain; and

(d) the processing of finding the gain information in said second code sequence based on an optimum gain corrected (termed ‘

corrected optimum gain’

), said optimum gain and on the gain read out from a gain codebook for said second system.

23. The computer program product according to claim 21, further comprising a program to cause said computer to execute the processing of finding the gain information in said second code sequence by calculating a first square error from the corrected optimum gain and from the gain read out from said gain codebook:

calculating a second square error from said optimum gain and from the gain read out from said gain codebook; and

selecting a gain minimizing an evaluation function which is based on said first square error and said second square error from said gain codebook to find the gain information in said second code sequence.

25. The computer program product according to claim 21, wherein said corrected optimum gain is based on a long-term average value of said optimum gain.

28. The computer program product according to claim 21, wherein said evaluation function is composed of said first square error, said second square error and weighting coefficients.

29. The computer program product according to claim 21, further comprising a program to cause the computer to execute:

the processing of outputting a speech decision value, discriminating the speech segment/non-speech segment, based on said first linear prediction coefficient; and

the processing of finding the gain information in said second code sequence, using an evaluation function which will decrease temporal variations of the gain in said second code sequence, when said speech decision value indicates the non-speech segment.

30. The computer program product according to claim 23, further comprising a program to cause the computer to execute:

the processing of outputting a speech decision value, discriminating the speech segment/non-speech segment, based on said first linear prediction coefficient; and

the processing of finding said evaluation function by taking a weighted average value of said first and second square errors by weighting coefficients, and setting said weighting coefficients to respective preset values, based on said speech decision value, depending on the speech segment and the non-speech segment, to calculate said evaluation function.

31. A recording medium that may be read out by a computer, said recording medium having recorded thereon said program as defined in claim 21.

22. A computer program product in a medium used by a computer, that composes a code converting apparatus for converting a first code sequence, into a second code sequence conforming to a second system, comprising a program to cause said computer to execute:

(a) the processing of decoding the gain information from said first code sequence;

(b) the processing of correcting the gain decoded (decoded gain); and

(c) the processing of finding the gain information in said code sequence based on the decoding gain corrected (corrected decoded gain), said decoded gain and the gain read out from the codebook in said second system.

24. The computer program product according to claim 22, further comprising a program to cause said computer to execute the processing of calculating a first square error from the corrected decoded gain and from the gain read out from said gain codebook, calculating a second square error from said decoded gain and from the gain read out from said gain codebook, and selecting a gain minimizing an evaluation function which is based on said first square error and said second square error from said gain codebook to find the gain information in said second code sequence.

26. The computer program product according to claim 22, wherein said corrected decoded gain is based on a long-term average value of said decoded gain.

27. The computer program product according to claim 22, wherein the gain minimizing the distance between the second speech signal generated by the information obtained from said second code sequence and said first speech signal is found as said optimum gain.

32. A code converting apparatus in which code sequence data, obtained on multiplexing codes obtained on encoding a speech signal by a first system, is input to a code demultiplexing circuit,

a code, separated by said code demultiplexing circuit, is converted to a code conforming to a second system distinct from said first system, said code conforming to said second system is sent to a code multiplexing circuit, and in which said code conforming to said second system and multiplexed by said code multiplexing circuit is output from said code multiplexing circuit;

said code converting apparatus comprising;

a circuit for generating first and second linear prediction coefficients, obtained on decoding a linear prediction coefficient, separated by said code demultiplexing circuit, in accordance with said first system and said second system;

an adaptive codebook code converting circuit (termed ‘

ACB code converting circuit’

), including a unit for replacing an adaptive codebook (ACB) code of the first system, input from said code demultiplexing circuit, using the relationship of correspondence between a code of the first system and a code of the second system, to generate an adaptive codebook (ACB) code of the second system, outputting the ACB code of the second system to said code multiplexing circuit, and for outputting an ACB delay associated with said second ACB code as a second ACB delay to a target signal computing circuit;

a speech decoding circuit for synthesizing a decoded speech signal and for outputting the synthesized signal, by actuating a synthesis filter by an excitation signal obtained from the information on the excitation signal, said synthesis filter receiving, as an input, the excitation signal information containing an ACB code, a fixed codebook (FCB) code and the gain code in the first system, separated by said code demultiplexing circuit, and decoding said excitation signal information, said synthesis filter having a first linear prediction coefficient obtained on decoding the linear prediction coefficient code separated by said code separation circuit in accordance with a first system;

a fixed codebook code generating circuit (termed ‘

FCB code generating circuit’

), supplied with the FCB code of the first system output from said code separation circuit, and converting said FCB code into a code decodable by said second system, said fixed codebook code generating circuit outputting the converted FCB code as the second FCB code to said code multiplexing circuit to output a second FCB signal corresponding to said second FCB code;

an impulse response calculating circuit for outputting an impulse response signal of an auditory perceptual weighting synthesis filter made up by said first linear prediction coefficient and said second linear prediction coefficient;

said target signal calculating circuit; and

a gain code generating circuit;

wherein said target signal calculating circuit includes;

a weighting signal calculating circuit supplied with a decoded speech output from a synthesis filter of said speech decoding circuit, and actuating an auditory perceptual weighting filter, formed using said first linear prediction coefficient, with said decoded speech, to generate an auditory perceptually weighted speech signal, said weighting signal calculating circuit generating a first target signal by subtracting a zero-input response of the auditory perceptual weighting synthesis filter, made up by said first linear prediction coefficient and said second linear prediction coefficients, from said auditory perceptually weighted speech signal;

an ACB signal generating circuit supplied with said first target signal, output from said weighting signal calculating circuit, said second ACB delay output from said ACB code converting circuit, said impulse response signal output from said impulse response calculating circuit and a past second excitation signal output from a second excitation signal storage circuit, storing and holding the past second excitation signal;

said ACB signal generating circuit calculating the past filter-processed excitation signal with a delay k, from said past second excitation signal, by convolution of a signal extracted with a delay k, where k is said second ACB delay, and said impulse response signal, to output the so calculated past excitation signal as second ACB signal; and

an optimum ACB gain calculating circuit supplied with said first target signal, output from said weighting signal calculating circuit, and with the past filter-processed excitation signal with the delay k, output from said ACB signal generating circuit, said optimum ACB gain calculating circuit deriving an optimum ACB gain from said first target signal and from the past filter-processed excitation signal with the delay k to output the so derived optimum ACB gain; and

wherein said gain code generating circuit includes;

a unit supplied with said first target signal, output from said target signal calculating circuit, said second ACB signal, said optimum ACB gain, said second FCB signal, output from said FCB code generating circuit, said impulse response signal output from said impulse response calculating circuit and with said first linear prediction coefficient, said unit calculating a second target signal from said first target signal, second ACB signal, said optimum ACB gain and the impulse response signal and also calculating an optimum FCB gain from said second target signal, said second FCB signal and the impulse response signal;

a unit for finding a corrected ACB gain from said optimum ACB gain;

a unit supplied with the optimum FCB gain calculated to calculate a corrected FCB gain from said optimum FCB gain;

a unit for determining a speech decision value from said first linear prediction coefficient;

a unit for calculating a first square error from the ACB gain sequentially read from an ACB gain codebook and said optimum ACB gain and for calculating a second square error from said ACB gain and said corrected ACB gain;

a unit for selecting a weighting coefficient, calculated from said speech decision value, the ACB gain minimizing the first evaluation function calculated from said first square error and the second square error and the corresponding ACB gain code;

a unit for calculating a third square error from the FCB gain sequentially read from an FCB gain codebook and said optimum FCB gain and for calculating a fourth square error from said FCB gain and said corrected FCB gain;

a unit for selecting an FCB gain minimizing a second evaluation function as calculated from the weighting coefficient, calculated in turn from said speech decision value, said third square error and the fourth square error, and a corresponding FCB gain code; and

a unit for outputting a second gain code, made up by the ACB gain code and the FCB gain code as selected, to said code multiplexing circuit, as a code decodable by the gain decoding method in the second system.

33. The code converting apparatus according to claim 32, further comprising:

a second excitation signal calculating circuit supplied with a second ACB signal, output from said target signal calculating circuit, a second FCB signal, output from said FCB code generating circuit, and second ACB and FCB signals output from said gain code generating circuit;

said second excitation signal calculating circuit summing a signal obtained on multiplying said second ACB signal with the second ACB gain to a signal obtained on multiplying said second FCB signal with the second FCB gain to generate a second excitation signal to output said second excitation signal to said second excitation signal storage circuit;

said second excitation signal storage circuit being supplied with said second excitation signal output from said second excitation signal calculating circuit to store and hold said second excitation signal to output the second excitation signal input in the past and stored therein to said target signal calculating circuit.

34. The code converting apparatus according to claim 32 wherein said gain code generating circuit includes:

a second target signal calculating circuit supplied with said second ACB signal, output from said ACB generating circuit, said first target signal, output from said weighting signal calculating circuit, said impulse response signal, output from said impulse response calculating circuit, and with said second ACB gain, output from said ACB gain encoding circuit, said second target signal calculating circuit calculating the filter-processed second ACB signal by convolution of said second ACB signal and said impulse response signal, and subtracting a signal obtained on multiplying the filter-processed second ACB signal with said second ACB gain from said first target signal to derive a second target signal for outputting said second target signal;

an optimum FCB gain calculating circuit supplied with said second FCB signal output from said FCB signal generating circuit, said impulse response signal, output from said impulse response calculating circuit, and with said second target signal, output from said second target signal calculating circuit, to calculate the filter-processed second FCB signal by convolution of said second FCB signal and the impulse response signal and to calculate an optimum FCB gain minimizing the distance between the second target signal and the second FCB signal;

a speech/non-speech discriminating circuit for calculating the variation of the linear prediction coefficient from said first linear prediction coefficient and a long-term average thereof to determine a speech decision value;

an optimum ACB gain correction circuit supplied with said optimum ACB gain output from said ACB gain generating circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said optimum ACB gain correction circuit calculating, in a non-speech segment, a long-term average of said optimum ACB gain in a non-speech segment, with the long-term average of said optimum ACB gain as a corrected ACB gain, when said speech decision value indicates the non-speech segment, said optimum ACB gain correction circuit outputting said optimum ACB gain itself, as a corrected ACB gain, when said speech decision value indicates the non-speech segment;

an ACB code gain encoding circuit supplied with said optimum ACB gain output from said ACB gain generating circuit, said corrected ACB gain output from said optimum ACB gain correction circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said ACB code gain encoding circuit calculating a first square error from the ACB gain sequentially read from said ACB gain codebook and said optimum ACB gain, calculating a second square error from said ACB gain and said corrected ACB gain, finding an evaluation function from the weighting coefficient calculated from said speech decision value, said first square error and the second square error, selecting an ACB gain minimizing said evaluation function, outputting the selected ACB gain as the second ACB gain to said second target signal calculating circuit and to said second excitation signal calculating circuit, and outputting a code corresponding to said second ACB gain as ACB gain code to a gain code multiplexing circuit;

an optimum FCB gain correction circuit supplied with said optimum FCB gain output from said FCB gain calculating circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said optimum FCB gain correction circuit calculating, in a non-speech segment, a long-term average of said optimum FCB gain as a corrected FCB gain, when said speech decision value indicates the non-speech segment, said optimum FCB gain correction circuit outputting, when said speech decision value indicates a speech segment, said optimum FCB gain itself as a corrected FCB gain to an FCB gain encoding circuit;

an FCB gain encoding circuit supplied with said optimum FCB gain output from said optimum FCB gain calculating circuit, said corrected FCB gain output from said optimum FCB gain correction circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said FCB gain encoding circuit calculating a third square error from the FCB gain sequentially read from said FCB gain codebook and said optimum FCB gain, calculating a fourth square error from said FCB gain and said corrected FCB gain, calculating an evaluation function from the weighting coefficient calculated from said speech decision value, said third square error and the fourth square error, selecting an FCB gain minimizing said evaluation function, outputting the selected FCB gain as the second FCB gain to said second excitation signal calculating circuit, and outputting a code corresponding to said second FCB gain as FCB gain code to a gain code multiplexing circuit; and

a gain code multiplexing circuit supplied with an ACB gain code output from said ACB gain encoding circuit and with an FCB gain code output from said FCB gain encoding circuit to output a second gain code obtained on multiplexing the ACB gain code and the FCB gain code to said code multiplexing circuit as a code decodable by a gain decoding method of the second system.

35. A code converting apparatus in which a code obtained on encoding a speech signal in accordance with a first system is input to a code demultiplexing circuit, a code obtained on separation by said code demultiplexing circuit is converted into a code conforming to a second system distinct from said first system, the so converted code is sent to a code multiplexing circuit and code sequence data obtained on multiplexing the so converted code is output from said code multiplexing circuit, characterized in that said apparatus comprises:

a circuit for generating first and second linear prediction coefficients, decoded in accordance with the first and second systems, from linear prediction coefficients, separated by said code demultiplexing circuit;

an ACB code conversion circuit supplied with a first ACB code, output from said code demultiplexing circuit, to convert said first ACB code into a code decodable by said second system, to output the so converted ACB code as second ACB code to said code multiplexing circuit;

an FCB code conversion circuit supplied with a first FCB code, output from said code demultiplexing circuit, to convert said first FCB code into a code decodable by said second system, to output the so converted FCB code as second FCB code to said code multiplexing circuit; and

a gain code conversion circuit supplied with a first gain code output from said code separation circuit to convert said first gain code into a code decodable by said second system to output the so converted gain code as second gain code to said code multiplexing circuit;

said gain code conversion circuit including a unit supplied with a first gain output from said code demultiplexing circuit and with said first linear prediction coefficient to calculate a corrected ACB gain and corrected FCB gain from a first adaptive codebook (ACB) gain and from a first fixed codebook (FCB) gain, obtained on decoding said first gain code in accordance with the gain decoding method of the first system;

a unit for determining a speech decision value from said first linear prediction coefficient;

a unit for calculating a first square error from the ACB gain sequentially read from an ACB gain codebook, and said first ACB gain, and calculating a second square error from said ACB gain and said corrected ACB gain, to select an ACB gain minimizing a first evaluation function as calculated from the weighting coefficients calculated from said speech decision value and said first and second square errors;

a unit for calculating a third square error from the FCB gain sequentially read from an FCB gain codebook and said first FCB gain to calculate a fourth square error from said FCB gain and said corrected FCB gain and for selecting an FCB gain minimizing a second evaluation function as calculated from the weighting coefficient calculated from said speech decision value, said third square error and the fourth square error, and a corresponding FCB gain code; and

a unit for outputting a second gain code, made up by the ACB gain code and the FCB gain code as selected, as a code decodable by the gain decoding method in the second system, to said code multiplexing circuit.

36. The code converting apparatus according to claim 35, wherein said gain code converting circuit includes

a speech/non-speech discriminating circuit for calculating the variation of the linear prediction coefficients from said first linear prediction coefficient and a long-term average thereof to determine a speech decision value;

a gain code separation circuit supplied with a first gain code output from said code separation circuit to separate a first ACB gain code and a first FCB gain code, associated with the ACB gain and the FCB gain, respectively, from said first gain code, to output said first ACB gain code and the first FCB gain code to an ACB gain decoding circuit and to an FCB gain decoding circuit, respectively;

an ACB gain decoding circuit including an ACB gain codebook, having plural sets of the ACB gain stored therein, said ACB gain decoding circuit being supplied with a first ACB gain code output from said gain code separation circuit to read out an ACB gain associated with said first ACB gain code from a first ACB gain codebook to output the so read out ACB gain as a first ACB gain to an ACB gain correction circuit and to an ACB gain encoding circuit, said ACB gain decoding circuit decoding the ACB gain from the ACB gain code in accordance with an ACB gain decoding method of the first system, using an ACB gain codebook of the first system;

an FCB gain decoding circuit including an FCB gain codebook, having plural sets of the FCB gain stored therein, said FCB gain decoding circuit being supplied with a first FCB gain code output from said gain code separation circuit to read out an FCB gain associated with said first FCB gain code from a first FCB gain codebook to output the so read out FCB gain as a first FCB gain to an FCB gain correction circuit and to an FCB gain encoding circuit, said FCB gain decoding circuit decoding the FCB gain from the FCB gain code in accordance with an FCB gain decoding method of the first system, using an FCB gain codebook of the first system;

an ACB gain correction circuit supplied with said first ACB gain, output from said ACB gain decoding circuit, and with said speech decision value output from said speech/non-speech discriminating circuit, to set a long-term average of said first ACB gain as corrected ACB gain if said speech decision value indicates a non-speech segment, as well as to set said first ACB gain itself as a corrected ACB gain if said speech decision value indicates a speech segment, said ACB gain correction circuit outputting said corrected ACB gain to an ACB gain encoding circuit;

an FCB gain correction circuit supplied with said first FCB gain, output from said FCB gain decoding circuit, and with said speech decision value output from said speech/non-speech discriminating circuit, to set a long-term average of said first FCB gain as corrected FCB gain if said speech decision value indicates a non-speech segment, as well as to set said first FCB gain itself as a corrected FCB gain if said speech decision value indicates a speech segment, said FCB gain correction circuit outputting said corrected FCB gain to an FCB gain encoding circuit;

an ACB gain encoding circuit supplied with said first ACB gain output from said ACB gain decoding circuit, said corrected ACB gain output from said ACB gain correction circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said ACB gain encoding circuit calculating a first square error from the ACB gain sequentially read from said ACB gain codebook and said first ACB gain, calculating a second square error from said ACB gain and said corrected ACB gain, calculating a first evaluation function from the weighting coefficient calculated from said speech decision value, said first square error and the second square error, selecting an ACB gain minimizing said evaluation function, outputting the code corresponding to said second ACB code as second ACB gain code to a gain code multiplication circuit and outputting a code corresponding to said second ACB gain as second ACB gain code to said gain code multiplication circuit;

an ACB gain encoding circuit supplied with said first ACB gain output from said ACB gain decoding circuit, said corrected ACB gain output from said ACB gain correction circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said ACB gain encoding circuit calculating a first square error from the ACB gain sequentially read from said ACB gain codebook and said first ACB gain, calculating a second square error from said ACB gain and said corrected ACB gain, calculating a first evaluation function from the weighting coefficient calculated from said speech decision value, said first square error and the second square error, selecting an ACB gain minimizing said evaluation function, outputting the code corresponding to said second ACB code as second ACB gain code to a gain code multiplication circuit and outputting a code corresponding to said second ACB gain as second ACB gain code to said gain code multiplication circuit;

an FCB gain encoding circuit supplied with said first FCB gain output from said FCB gain decoding circuit, said corrected FCB gain output from said FCB gain correction circuit and with said speech decision value output from said speech/non-speech discriminating circuit, said FCB gain encoding circuit calculating a third square error from the FCB gain sequentially read from said FCB gain codebook and said first FCB gain, calculating a fourth square error from said FCB gain and said corrected FCB gain, calculating a second evaluation function from the weighting coefficient calculated from said speech decision value, said first square error and the second square error, selecting an FCB gain minimizing said evaluation function, outputting the code corresponding to said second FCB code as second FCB gain code to a gain code multiplication circuit and outputting a code corresponding to said second FCB gain as second FCB gain code to said gain code multiplication circuit; and

a gain code multiplexing circuit supplied with an ACB gain code output from said ACB gain encoding circuit and with an FCB gain code output from said FCB gain encoding circuit, to output a second gain code, obtained on multiplexing the ACB gain code and the FCB gain code, to said code multiplexing circuit as a code decodable by the gain decoding method in the second system.