Method and device for coding speech in analysis-by-synthesis speech coders

• US 20030055633A1
• Filed: 06/10/2002
• Published: 03/20/2003
• Est. Priority Date: 06/21/2001
• Status: Active Grant

• Current Assignee: HMD Global Oy
• Original Assignee: Nokia Corporation
• Inventors: Heikkinen, Ari P.
• Granted Patent: US 7,089,180 B2
• Time in Patent Office: Days
• US Class Current: 704/220
• CPC Class Codes: G10L 19/10 the excitation function being a multipulse excitation

Abstract

The present invention discloses a method of improving the coded speech quality in low bit rate analysis-by-synthesis (AbS) speech coders. In an embodiment of the invention, this is accomplished by relaxing the waveform matching constraints for nonstationary plosive speech segments of speech signals by suitably shifting pulse locations of the coded excitation signal. The shifting results in the coded signal having phase information that does not exactly match original signal in places where it is perceptually insignificant to the listener. Furthermore, a technique for adaptive phase dispersion is introduced to the coded excitation signal to efficiently preserve important signal characteristics such as the energy spread of the original signal.

Claims

1. A method of encoding a speech signal wherein the speech signal is encoded in an encoder using a first excitation codebook having a first position grid and a second excitation codebook having a second position grid to produce a coded excitation signal, wherein the first position grid contains a higher population density of pulse positions than the second position grid.

2. A method according to claim 1 wherein the method is performed by a low bit rate Analysis-by-Synthesis (AbS) speech coder.

3. A method according to claim 1 wherein the encoding comprises the steps of:

obtaining a pulse train using the first excitation codebook, wherein the pulse train includes a plurality of pulses located at a first set of locations in accordance with the first excitation codebook; and

shifting the pulse locations of the first set of locations to obtain a second set of locations in accordance with the second excitation codebook.

4. A method according to according to claim 1 wherein the method is applied to nonstationary speech segments of the speech signal.

5. A method according to according to claim 1 wherein the method is preferably applied to nonstationary speech segments of the speech signal which are determined by detecting the level of “

peakiness”

that is typically indicative of nonstationary speech.

6. A method according to any of the preceding claims wherein the population density of the first excitation codebook is approximately in a range of five to ten times the density as compared to that in the second excitation codebook.

7. A method according to any of the preceding claims wherein the “

peakiness”

value is used to calculate a dispersion value for subsequent phase randomization.

8. A method of transmitting a speech signal from a sender to a receiver comprising the steps of:

encoding a speech excitation signal with an encoder at the sender;

transmitting said encoded excitation signal to the receiver; and

decoding said encoded excitation signal with a decoder to produce synthesized speech at the receiver,

wherein the speech excitation signal is encoded in the encoder using a first excitation codebook having a first position grid and a second excitation codebook having a second position grid to produce a coded excitation signal which is decoded in the decoder using the second excitation codebook, wherein the first position grid contains a higher population density of pulse positions than the second position grid.

9. A method according to claim 8 wherein the method is performed by a low bit rate Analysis-by-Synthesis (AbS) speech coder.

10. A method according to claim 8 wherein the method is applied to nonstationary speech segments of the speech signal.

11. A method according to claim 8 wherein the method is preferably applied to nonstationary speech segments of the speech signal which are determined by detecting the level of “

peakiness”

that is typically indicative of nonstationary speech.

12. A method according to claim 8 wherein the “

peakiness”

or dispersion information is transmitted from the encoder to the decoder for use in phase randomization of the decoded signal.

13. A method according to claims 8 wherein the population density of the first excitation codebook is approximately in a range of five to ten times the density as compared to that in the second excitation codebook.

14. A method according to claims 11 or 12 wherein the “

peakiness”

value is used to calculate a dispersion value for subsequent phase randomization of the decoded signal.

15. An encoder for encoding speech signals wherein the encoder comprises a first excitation codebook and a second excitation codebook for use in encoding said speech signals, wherein the first excitation codebook contains a higher population density of pulse positions than the second excitation codebook.

16. An encoder according to claim 15 wherein the encoder is included within a low bit rate Analysis-by-Synthesis (AbS) speech coder.

17. An encoder according to claim 15 wherein the encoder further comprises:

means for obtaining a pulse train using the first excitation codebook, wherein the pulse train includes a plurality of pulses located at a first set of locations in accordance with the first excitation codebook; and

means for shifting the pulse locations of the first set of locations to obtain a second set of locations in accordance with the second excitation codebook.

18. An encoder according to claim 15 wherein the encoder includes means for detecting nonstationary segments in the speech signals.

19. An encoder according claim 15 wherein the encoder includes means for calculating the “

peakiness”

value of a segment of the speech signal.

20. An encoder according claim 19 wherein the encoder includes means for calculating a dispersion value for subsequent phase randomization from the “

peakiness”

value.

21. A device comprising a speech coder for encoding and decoding speech signals, wherein the device further comprises a first pulse codebook for use with the encoder and a second pulse codebook for use with the decoder, wherein the first codebook contains a higher population density of pulse positions than the second codebook.

22. A device according to claim 21 wherein the device includes means for detecting nonstationary segments in the speech signals.

23. A device according to claim 21 wherein the device further comprises:

means for obtaining a pulse train using the first excitation codebook, wherein the pulse train includes a plurality of pulses located at a first set of locations in accordance with the first excitation codebook; and

means for shifting the pulse locations of the first set of locations to obtain a second set of locations in accordance with the second excitation codebook.

24. A device according to claim 21 wherein the device is a mobile terminal.

25. A device according to claim 21 wherein the device is a radio base station.

26. A device according to claim 21 wherein the device is a voice storage or voice messaging device.