Code excitation linear predictive (CELP) encoder and decoder and code excitation linear predictive coding method

• US 5,727,122 A
• Filed: 02/09/1995
• Issued: 03/10/1998
• Est. Priority Date: 06/10/1993
• Status: Expired due to Term

• Current Assignee: OKI Electric Industry Company Limited
• Original Assignee: OKI Electric Industry Company Limited
• Inventors: Aoyagi, Hiromi, Katsuragawa, Hiroshi, Ariyama, Yoshihiro, Hosoda, Kenichiro
• Primary Examiner(s): MacDonald, Allen R.
• Assistant Examiner(s): SAX, ROBERT L
• Application Number:
  US08/379,653
• Time in Patent Office: 1,125 Days
• Field of Search: 395/2.32, 395/2.31, 395/2.25, 395/2.26, 395/2.27, 395/2.75
• US Class Current: 704/223
• CPC Class Codes: G10L 19/07 Line spectrum pair [LSP] vocoders
• CPC Class Codes: G10L 19/10 the excitation function being a multipulse excitation
• CPC Class Codes: G10L 19/12 the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders

Abstract

There is provided a code excitation linear predictive (CELP) coding or decoding apparatus in which a code vector, which is transmitted by a codebook such as a stochastic codebook, is converted adaptively in accordance with vocal tract analysis information (LPC) so that a high quality reproduction speech is obtained at a low coding rate. Further, in order to obtain a similar effect, a pulse-like excitation codebook formed of an isolated impulse is provided in addition to the adaptive excitation codebook and stochastic excitation codebook so that either the stochastic excitation codebook or the pulse-like excitation codebook is selectively used to provide a vocal tract parameter as a linear spectrum pair parameter.

Claims

1. A code excitation linear predictive coding apparatus comprising:

excitation codebook means for selectively outputting an excitation code vector as an excitation source information of a speech signal; and

code vector conversion circuit means for converting the excitation code vector selectively output from the excitation codebook means into a frequency characteristic determined at the time of output of said excitation code vector.

2. A coding apparatus according to claim 1, wherein the code vector conversion circuit means generates an impulse response of a transfer function which is determined in accordance with a vocal tract parameter of an input speech signal, and convolves the excitation code vector with the impulse response.

3. A coding apparatus according to claim 2, wherein the impulse response of the transfer function which is determined in accordance with the vocal tract parameter is represented by:

space="preserve" listing-type="equation">H(Z)=(1-Σ

A.sup.j ajZ.sup.-j)/(1Σ

B.sup.j ajZ.sup.-j)

where aj (j is 1 to p) is a linear predictive coefficient;

p is a vocal tract analysis order; and

A and B are in the ranges;

0<

A<

1 and 0<

B1.

4. A coding apparatus according to claim 1, wherein the code vector conversion circuit means generates an impulse response of a transfer function which is determined in accordance with an excited pitch lag, and convolves the excitation code vector with the impulse response.

5. A coding apparatus according to claim 4, wherein the impulse response of the transfer function which is determined in accordance with the excited pitch lag is represented by:

space="preserve" listing-type="equation">H(Z)=1/(1-ε

Z.sup.-L)

where ε

is a constant satisfying a range of 0<

ε

≦

1; and

L is a pitch lag signal.

6. A coding apparatus according to claim 1, wherein the code vector conversion circuit means convolves the excitation code vector with the impulse response of the transfer function which is determined in accordance with transfer functions represented by:

space="preserve" listing-type="equation">H(Z)=(1-Σ

A.sup.j ajZ.sup.-j)/(1-Σ

B.sup.j ajZ.sup.-j)

and

space="preserve" listing-type="equation">H(Z)=1/(1-ε

Z.sup.-L)

where aj (j is 1 to p) is a linear predictive coefficient;

p is a vocal tract analysis order;

A, B and ε

are in the ranges;

0<

A<

1, 0<

B<

1 and 0<

ε

≦

1; and

L is a pitch lag signal.

7. A code excitation linear predictive decoding apparatus comprising:

excitation codebook means for selectively outputting an excitation code vector as an excitation source information of a speech signal; and

code vector conversion circuit means for converting the excitation code vector selectively output from the excitation codebook into a frequency characteristic determined at the time of output of said excitation code vector.

8. A decoding apparatus according to claim 7, wherein the code vector conversion circuit means generates an impulse response of a transfer function which is determined in accordance with a vocal tract parameter of an input speech signal, and convolves the excitation code vector with the impulse response.

9. A decoding apparatus according to claim 8, wherein the impulse response of the transfer function which is determined in accordance with the vocal tract parameter is represented by:

space="preserve" listing-type="equation">H(Z)=(1-ε

A.sup.j ajZ.sup.-j)/(1-ε

B.sup.j ajZ.sup.-j)

where aj (j is 1 to p) is a linear predictive coefficient;

p is a vocal tract analysis order; and

A and B are in the ranges;

0<

A<

1 and 0<

B1.

10. A decoding apparatus according to claim 7, wherein the code vector conversion circuit means generates an impulse response of a transfer function which is determined in accordance with an excited pitch lag, and convolves the excitation code vector with the impulse response.

11. A coding apparatus according to claim 10, wherein the impulse response of the transfer function which is determined in accordance with the excited pitch lag is represented by:

space="preserve" listing-type="equation">H(Z)=1/(1ε

Z.sup.31 L)

where ε

is a constant satisfying a range of 0<

ε

≦

1; and

L is a pitch lag signal.

12. A decoding apparatus according to claim 7, wherein the code vector conversion circuit means convolves the excitation code vector with the impulse response of the transfer function which is determined in accordance with transfer functions represented by:

space="preserve" listing-type="equation">H(Z)=(1-ε

A.sup.j ajZ.sup.-j)/(1-ε

B.sup.j ajZ.sup.-j)

and

space="preserve" listing-type="equation">H(Z)=1/(1-ε

Z.sup.-L)

where aj(j is 1 to p) is a linear predictive coefficient;

p is a vocal tract analysis order;

A, B and ε

are in the ranges;

0<

A<

1, 0<

B<

1 and 0<

ε

≦

1; and

L is a pitch lag signal.

13. A code excitation linear predictive coding apparatus comprising:

excitation codebook means for outputting an excitation code vector as an excitation source information of a speech signal; and

pulse-like excitation codebook means for storing a pulse-like excitation code vector composed of an unit impulse.

14. A code excitation linear predictive coding apparatus according to claim 13, further comprising means for generating a pulse-like excitation code vector from the pulse-like excitation codebook means, and means for transmitting information indicative of what pulse-like excitation code vector is selected to a code excitation linear predictive decoding apparatus.

15. A code excitation linear predictive coding or decoding apparatus according to claim 14, further comprising:

code vector conversion circuit means for converting the pulse-like excitation code vector transmitted from the pulse-like excitation codebook into a frequency characteristic determined at the time of output of the pulse-like excitation code vector.

16. A code excitation linear predictive coding apparatus according to claim 13, further comprising means for generating a vocal tract parameter, and transmitting the vocal tract parameter in the form of a linear spectrum pair parameter to a code excitation linear predictive decoding apparatus.

17. A code excitation linear predictive decoding apparatus comprising:

excitation codebook means for outputting an excitation code vector as an excitation source information of a speech signal; and

pulse-like excitation codebook means for storing a pulse-like excitation code vector composed of an unit impulse.

18. A code excitation linear predictive decoding apparatus according to claim 17, further comprising means for selecting the pulse-like excitation code vector in the pulse-like excitation codebook in accordance with selected information transmitted from a corresponding code excitation linear predictive coding apparatus.

19. A code excitation linear predictive decoding apparatus according to claim 17, further comprising means for receiving a vocal tract parameter in the form of a linear spectrum pair parameter used for vocal tract reproduction from a corresponding code excitation linear predictive coding apparatus.

20. A code excitation linear predictive coding method comprising the steps of:

selectively outputting an excitation code vector from an excitation codebook as an excitation source information of a speech signal;

converting the excitation code vector into a converted code vector having a frequency characteristic; and

multiplying the converted code vector by a gain output from a gain codebook.

21. A code excitation linear predictive coding method according to claim 20, wherein the converting step comprises the steps of:

generating an impulse response of a transfer function which is determined in accordance with a vocal tract parameter output from an input speech vector; and

convolving the excitation code signal with the impulse response in order to obtain the converted code vector.

22. A code excitation linear predictive coding method according to claim 20, wherein the converting step comprises the steps of:

generating an impulse response of a transfer function which is determined in accordance with an excited pitch lag obtained from indexes of an adaptive excitation code; and

convolving the excitation code signal with the impulse response in order to obtain the converted code vector.