Method and device for coding speech in analysis-by-synthesis speech coders
View Patent ↗The present invention discloses a method of improving the coded speech quality in low bit rate analysis-by-synthesis (AbS) speech coders. This is accomplished by relaxing the waveform matching constraints for non-stationary 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.
1. A method of encoding a speech signal, the method comprising:
obtaining in an encoder a pulse train using a 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, the first excitation codebook having a first position grid;
shifting the pulse locations of a first set of locations in the encoder to obtain a second set of locations in accordance with a second excitation codebook, the second excitation codebook having a second position grid and the first position grid containing a higher population density of pulse positions than the second position grid; and
producing a coded excitation signal.
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 according to claim 1 wherein the method is applied to nonstationary speech segments of the speech signal.
4. 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.
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 “peakiness” value is used to calculate a dispersion value for subsequent phase randomization.
7. A method of transmitting a speech signal from a sender to a receiver comprising the steps of:
obtaining a pulse train using a first excitation codebook in an encoder, wherein the pulse train includes a plurality of pulses located at a first set of locations in accordance with the first excitation codebook, the first excitation codebook having a first position grid;
shifting the pulse locations of the fist set of locations in the encoder to obtain a second set of locations in accordance with a second excitation codebook, the second excitation codebook having a second position grid and the first position grid containing a higher population density of pulse positions than the second position grid;
producing a speech excitation signal in 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.
8. A method according to claim 7 wherein the method is performed by a low bit rate Analysis-by-Synthesis (AbS) speech coder.
9. A method according to claim 7 wherein the method is applied to nonstationary speech segments of the speech signal.
10. A method according to claim 7 wherein the “peakiness” or dispersion information is transmitted from the encoder to the decoder for use in phase randomization of the decoded signal.
11. A method according to claims 7 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.
12. A method according to claim 7 wherein the method is preferably applied to nonstationary speech segments of the speech signal which are determined by detecting a level of “peakiness” that is typically indicative of nonstationary speech.
13. A method according to claim 12 wherein the “peakiness” value is used to calculate a dispersion value for subsequent phase randomization of the decoded signal.
14. An encoder for encoding speech signals wherein the encoder comprises:
means for obtaining a pulse train using a first excitation codebook, wherein the pulse train comprises a plurality of pulses located at a first set of locations in accordance with the first excitation codebook, the first excitation codebook having a first position grid;
means for shifting pulse locations of the first set of locations to obtain a second set of locations in accordance with a second excitation codebook, the second excitation codebook having a second position grid and the first position grid containing a higher population density of pulse positions than the second position grid; and
means for producing a speech excitation signal in an encoder at the sender.
15. An encoder according to claim 14 wherein the encoder is included within a low bit rate Analysis-by-Synthesis (AbS) speech coder.
16. An encoder according to claim 14 wherein the encoder includes means for detecting nonstationary segments in the speech signals.
17. An encoder according claim 14 wherein the encoder includes means for calculating a “peakiness” value of a segment of the speech signal.
18. An encoder according claim 17 wherein the encoder includes means for calculating a dispersion value for subsequent phase randomization from the “peakiness” value.
19. A device comprising a speech coder for encoding and decoding speech signals, wherein the device further comprises:
means for obtaining a pulse train using a 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, the first excitation codebook having a first position grid;
means for shifting pulse locations of the first set of locations to obtain a second set of locations in accordance with a second excitation codebook, the second excitation codebook having a second position grid and the first position grid containing a higher population density of pulse positions than the second position grid; and
means for producing a speech excitation signal in an encoder at the sender.
20. A device according to claim 19 wherein the device includes means for detecting nonstationary segments in the speech signals.
21. A device according to claim 19 wherein the device is a mobile terminal.
22. A device according to claim 19 wherein the device is a radio base station.
23. A device according to claim 19 wherein the device is a voice storage or voice messaging device.