Coded speech communication system having code books for synthesizing small-amplitude components

• US 4,975,958 A
• Filed: 05/22/1989
• Issued: 12/04/1990
• Est. Priority Date: 05/20/1988
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Hanada, Eisuke, Ozawa, Kazunori
• Primary Examiner(s): KEMENY, EMANUEL
• Application Number:
  US07/354,662
• Time in Patent Office: 561 Days
• Field of Search: 381/35, 381/36, 381/38
• US Class Current: 704/223
• CPC Class Codes: G10L 19/04 using predictive techniques
• CPC Class Codes: G10L 19/06 Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients
• CPC Class Codes: G10L 19/083 the excitation function being an excitation gain G10L25/90 takes precedence
• CPC Class Codes: G10L 19/10 the excitation function being a multipulse excitation
• CPC Class Codes: G10L 2019/0003 Backward prediction of gain
• CPC Class Codes: G10L 2019/0005 Multi-stage vector quantisation
• CPC Class Codes: G10L 2019/0011 Long term prediction filters, i.e. pitch estimation
• CPC Class Codes: G10L 25/06 the extracted parameters being correlation coefficients
• CPC Class Codes: G10L 25/93 Discriminating between voiced and unvoiced parts of speech signals G10L25/90 takes precedence

Abstract

In coded speech communication, discrete speech samples are analyzed to generate a first signal indicating the fine pitch structure of the speech samples and a second signal indicating their spectral characteristic. The amplitudes and locations of main excitation pulses are determined from the fine pitch structure and spectral characteristic and a third signal indicating the determined pulse amplitudes and locations is generated. The difference between the speech samples and the main excitation pulses is detected and used in auxiliary excitation pulse calculation to determine gain and index values of auxiliary excitation pulses by retrieving stored auxiliary excitation pulses from a code book so that the retrieved auxiliary excitation pulses approximate the difference. The first, second and third coded signals and the gain and index values are transmitted through a communication channel to a distant end where a replica of the main excitation pulses is recovered from the received first and third signals and a replica of the auxiliary excitation pulses is recovered from a code book in response to the received fourth signal. These replicas are modified with the second signal to recover a replica of the original speech samples.

Claims

1. A speech encoder comprising:

means for analyzing a series of discrete speech samples and generating a first coded signal representative of a fine structure of the pitch of said speech samples and a second coded signal representative of a spectral characteristic of said speech samples;

means for determining amplitudes and locations of main excitation pulses from said first and second signals and generating a third coded signal representative of said determined pulse amplitudes and locations;

means for detecting a difference between said speech samples and said main excitation pulses;

a code book for storing auxiliary excitation pulses in locations addressable as a function of an index signal;

means for deriving said index signal from said difference and retrieving auxiliary excitation pulses from said code book with said index signal and deriving a gain signal and controlling the amplitude of the retrieved auxiliary excitation pulses with the gain signal so that the amplitude-controlled auxiliary excitation pulses approximate said difference; and

means for transmitting said first, second and third coded signals, and said index and gain signals through a communication channel to a distant end.

2. A speech encoder as claimed in claim 1, wherein said amplitudes and locations determining means sequentially determines amplitudes and locations of excitation pulses so that said difference reduces to a minimum.

3. A speech encoder as claimed in claim 1, further comprising means for detecting a voiced sound component from said speech samples and disabling the transmission of said index signal and said gain signal upon detection of said voiced sound component.

4. A speech encoder as claimed in claim 3, wherein said index and gain signals deriving means comprises a pitch synthesis filter having a pitch characteristic variable in accordance with said first coded signal for modifying the auxiliary excitation pulses retrieved from said code book with said pitch characteristic.

5. A speech encoder as claimed in claim 4, wherein said index and gain signals deriving means further comprises a spectral envelope filter having a spectral envelope characteristic variable in accordance with said second coded signal for modifying the auxiliary excitation pulses retrieved from said code book with said spectral envelope characteristic.

6. A speech encoder as claimed in claim 1, further comprising:

means for detecting whether said speech samples contain a vowel component or a consonant component and disabling the transmission of said index signal and said gain signal upon the detection of said vowel component;

means responsive to the detection of said consonant component for analyzing consonant components of said speech samples and generating a select signal representative of different constituents of said consonant components;

a second code book for storing auxiliary excitation pulses of different characteristic from those stored in the first-mentioned code book; and

means for selecting one of said first and second code books in accordance with said select signal,

wherein said transmitting means transmits said select signal through said communication channel.

7. A speech encoder as claimed in claim 1, further comprising:

means for recovering said auxiliary excitation pulses from said index signal and said gain signal; and

means for determining when the recovered auxiliary excitation pulses are ineffective and disabling the transmission of said index signal and said gain signal.

8. A speech encoder as claimed in claim 1, wherein said index and gain signals deriving means comprises:

a spectral envelope filter having a spectral envelope characteristic variable in accordance with said second coded signal for modifying the auxiliary excitation pulses retrieved from said code book with said spectral envelope characteristic;

a first weighting filter having a perceptual weighting function variable with said second coded signal for modifying said difference with said perceptual weighting function;

a second weighting filter having a perceptual weighting function variable with said second coded signal for modifying said auxiliary excitation pulses retrieved from said code book with said perceptual weighting function;

wherein said gain signal is given by "g" which satisfies the following relation;

##EQU5## where, e_w (n)=e(n)*w(n)=n(n)*h(n)*w(n),

e_w (n)=e(n)*w(n),

e(n)=said difference,

e(n)=the output signal of said spectral envelope filter,

w(n)=the impulse response characteristic of each of said first and second weighting filters,

h(n)=the impulse response of said spectral envelope filter, and the symbol * representing convolutional integration, wherein said index and gain signals deriving means includes means for computing the relation given by "g" and selecting a result of the computations that minimizes the following relation;

##EQU6##

9. A speech encoder as claimed in claim 1, wherein said transmitting means comprises a multiplexer for multiplexing said first, second and third coded signals and said index and gain signals.

10. A speech decoder comprising:

means for receiving a signal through a communication channel, said signal containing a first coded signal representative of a fine structure of the pitch of discrete speech samples, a second coded signal representative of a spectral characteristic of said speech samples, a third coded signal representative of amplitudes and locations of main excitation pulses, an index signal and a gain signal;

a code book for storing auxiliary excitation pulses and retrieving the stored auxiliary excitation pulses with said index signal;

gain determination means responsive to said gain signal for modifying the amplitudes of said auxiliary excitation pulses retrieved from said code book;

a pulse generator for reproducing said main excitation pulses in accordance with said third coded signal;

a pitch synthesis filter having a pitch characteristic variable with said first coded signal for modifying said reproduced main excitation pulses with said pitch characteristic;

means for combining the outputs of said pitch synthesis filter and said gain determination means; and

a spectral envelope filter having a spectral envelope characteristic variable with said second coded signal for modifying the combined outputs with said spectral envelope characteristic.

11. A speech decoder as claimed in claim 10, wherein said received signal further contains a disabling signal representative of the presence of a voiced sound component in said speech samples, and wherein said gain determination means and said code book are disabled in response to said disabling signal.

12. A speech decoder as claimed in claim 10, further comprising a second pitch synthesis filter having a pitch characteristic variable with said first coded signal for modifying the output of said gain determination means and applying the modified output to said combining means.

13. A speech decoder as claimed in claim 10, wherein said received signal further contains a select signal representative of different constituents of consonants of said speech samples, further comprising a second code book for storing auxiliary excitation pulses of different characteristic from those stored in the first-mentioned code book and means for selecting one of said first and second code books in response to said select signal.

14. A speech decoder as claimed in claim 10, wherein said received signal further contains a disabling signal which indicates that said gain and index signals are ineffective, and wherein said gain determination means and said code book are disabled in response to said disabling signal.

15. A coded speech communication system comprising:

means for analyzing a series of discrete speech samples and generating a first signal representative of a fine structure of the pitch of said speech samples and a second signal representative of a spectral characteristic of said speech samples;

means for deriving amplitudes and locations of main excitation pulses from said first and second signals and generating a third signal representative of said determined pulse amplitudes and locations;

means for generating a fourth signal representative of auxiliary excitation pulses;

means for transmitting said first, second, third and fourth signals from a transmit end of a communication channel to a receive end of the channel;

means for receiving said first, second, third and fourth signals at said receive end;

means for deriving a replica of said main excitation pulses from said received first and third signals;

means including a code book for deriving a replica of said auxiliary excitation pulses from said code book in response to said received fourth signal; and

means for modifying said replicas with said second signal to recover a replica of said speech samples.

16. A coded speech communication system comprising:

a speech encoder comprising;

means for analyzing a series of discrete speech samples and generating a first coded signal representative of a fine structure of the pitch of said speech samples and a second coded signal representative of a spectral characteristic of said speech samples;

means for determining amplitudes and locations of main excitation pulses from said first and second coded signals as well as from a feedback signal, generating a third coded signal representative of said determined pulse amplitudes and locations, detecting a difference between said speech samples and said main excitation pulses as said feedback signal and controlling the process of the determination of said amplitudes and locations so that said difference is minimized;

a first code book for storing auxiliary excitation pulses in locations addressable as a function of an index signal;

means for deriving said index signal from said difference and retrieving auxiliary excitation pulses from said first code book with said index signal and deriving a gain signal and controlling the amplitude of the retrieved auxiliary excitation pulses with the gain signal so that the amplitude-controlled auxiliary excitation pulses approximate said difference; and

means for transmitting said first, second and third coded signals, said index signal and said gain signal through a communication channel, and

a speech decoder comprising;

means for receiving said first, second and third coded signals, said index signal and said gain signal through said communication channel;

a second code book for storing auxiliary excitation pulses identical to those stored in said first code book and retrieving the stored auxiliary excitation pulses with said received index signal;

gain determination means for modifying the amplitudes of said auxiliary excitation pulses retrieved from said second code book with said received gain signal;

a pulse generator for reproducing said main excitation pulses in accordance with said received third coded signal;

a pitch synthesis filter having a pitch characteristic variable with said received first coded signal for modifying said reproduced main excitation pulses with said pitch characteristic;

means for combining the outputs of said pitch synthesis filter and said gain determination means; and

a spectral envelope filter having a spectral envelope characteristic variable with said received second coded signal for modifying the combined outputs with said spectral envelope characteristic.

17. A coded speech communication system as claimed in claim 16, said speech encoder further comprises means for detecting a voiced sound component from said speech samples, disabling the transmission of said index signal and said gain signal upon detection of said voiced sound component and transmitting a disabling signal representative of the detection of said voiced sound component, and wherein said receiving means receives said disabling signal, and said second code book and said gain determination means are responsive to the received disabling signal to nullify their outputs.

18. A coded speech communication system as claimed in claim 17, wherein said index and gain signals deriving means comprises a first pitch synthesis filter having a pitch characteristic variable in accordance with said first coded signal for modifying the auxiliary excitation pulses retrieved from said first code book with said pitch characteristic, and wherein said speech decoder comprises a second pitch synthesis filter having a pitch characteristic variable with said received first coded signal for modifying the output of said gain determination means and applying the modified output to said combining means.

19. A coded speech communication system as claimed in claim 18, wherein said index and gain signals deriving means further comprises a spectral envelope filter having a spectral envelope characteristic variable in accordance with said second coded signal for modifying the auxiliary excitation pulses retrieved from said first code book with said spectral envelope characteristic.

20. A coded speech communication system as claimed in claim 16, wherein said speech encoder further comprises:

means for detecting whether said speech samples contain a vowel component or a consonant component and disabling the transmission of said index signal and said gain signal upon the detection of said vowel component;

means responsive to the detection of said consonant component for analyzing consonant components of said speech samples and generating a select signal representative of different constituents of said consonant components;

a third code book for storing auxiliary excitation pulses of different characteristic from those stored in said first code book; and

means for selecting one of said first and third code books in accordance with said select signal,

wherein said transmitting means tranmits said select signal through said communication channel,

wherein said receiving means receives said select signal, said speech decoder further comprising a fourth code book for storing auxiliary excitation pulses of different characteristic from those stored in said second code book and means for selecting one of said second and fourth code books in response to said received select signal.

21. A coded speech communication system as claimed in claim 16, wherein said speech encoder further comprises:

means for recovering said auxiliary excitation pulses from said index signal and said gain signal; and

means for determining when the recovered auxiliary excitation pulses are ineffective and disabling the transmission of said index signal and said gain signal,

wherein said receive means receives said disabling signal, said gain determination means and said second code book being responsive to the received disabling signal to nullify their outputs.

22. A coded speech communication system as claimed in claim 16, wherein said index and gain signals deriving means comprises:

a spectral envelope filter having a spectral envelope characteristic variable in accordance with said second coded signal for modifying the auxiliary excitation pulses retrieved from said first code book with said spectral envelope characteristic;

a first weighting filter having a perceptual weighting function variable with said second coded signal for modifying said difference with said perceptual weighting function;

a second weighting filter having a perceptual weighting function variable with said second coded signal for modifying said auxiliary excitation pulses retrieved from said first code book with said perceptual weighting function;

wherein said gain signal is given by "g" which satisfies the following relation;

##EQU7## where, e_w (n)=e(n)*w(n)=n(n)*h(n)*w(n),

e_w (n)=e(n)*w(n),

e(n)=said difference,

e(n)=the output signal of said spectral envelope filter,

w(n)=the impulse response characteristic of each of said first and second weighting filters,

h(n)=the impulse response of said spectral envelope filter, and the symbol*representing convolutional integration, wherein said index and gain signals deriving means includes means for computing the relation given by "g" and selecting a result of the computations that minimizes the following relation;

##EQU8##

23. A coded speech communication system as claimed in claim 16, wherein said transmitting means comprises a multiplexer for multiplexing said first, second and third coded signals and said index and gain signals and said receiving means comprises a demultiplexer for demultiplexing said received signals.