Method and apparatus for providing background acoustic noise during a discontinued/reduced rate transmission mode of a voice transmission system

• US 6,782,361 B1
• Filed: 03/03/2000
• Issued: 08/24/2004
• Est. Priority Date: 06/18/1999
• Status: Expired due to Fees

• Current Assignee: McGill University
• Original Assignee: McGill University
• Inventors: El-Maleh, Khaled Helmi, Kabal, Peter
• Primary Examiner(s): Knepper, David D.
• Application Number:
  US09/518,242
• Time in Patent Office: 1,635 Days
• Field of Search: 704/226-228, 704/200, 704/202, 704/219-223, 704/210, 704/215, 704/258, 704/262, 704/264
• US Class Current: 704/226
• CPC Class Codes: G10L 19/012 Comfort noise or silence coding

Abstract

Natural-quality synthetic noise will replace background acoustic noise during speech gaps and will achieve a better representation of the excitation signal in a noise-synthesis model by classifying the type of acoustic environment noise into one or more of a plurality of noise classes. The noise class information is used to synthesize background noise that sounds similar to the actual background noise during speech transmission. In some embodiments, the noise class information is derived by the transmitter and transmitted to the receiver which selects corresponding excitation vectors and filters them using a synthesis filter to construct the synthetic noise. In other embodiments, the receiver itself classifies the background noise present in hangover frames and uses the class information as before to generate the synthetic noise. The improvement in the quality of synthesized noise during speech gaps helps to preserve noise continuity between talk spurts and speech pauses, and enhances the perceived quality of a conversation.

Claims

1. In a digital communications system comprising a transmitter and a receiver, the transmitter interrupting or reducing transmission of a voice signal during intervals absent speech and the receiver inserting synthetic noise into the received voice signals during said intervals, a method comprising the steps of assigning acoustic background noise in the voice signal to one or more of a plurality of noise classes, selecting a corresponding one of a plurality of excitation vectors each corresponding to at least one of the classes, using at least part of the selected excitation vector to synthesize the synthetic noise, and outputting the synthetic noise during a said interval.

2. A method according to claim 1, wherein the noise classification step is performed at the transmitter, a noise class index corresponding to the one or more noise classes is transmitted to the receiver, and, at the receiver, the noise class index is detected and used to select the corresponding one or more excitation vectors.

3. A method according to claim 1, wherein the plurality of excitation vectors are stored at the receiver and one or more of the vectors selected in dependence upon the noise class index.

4. A method according to claim 1, wherein at least some of said vectors comprise mixture vectors corresponding to different mixtures of a plurality of said noise classes, and the classification step determines a particular one of said mixtures as corresponding to the background noise and transmits a corresponding noise index identifying the corresponding vector, and, at the receiver, the noise index is used to select the corresponding mixture vector.

5. A method according to claim 1, wherein the noise classification step determines that the background noise corresponds to one of a plurality of mixtures of said excitation vectors, the mixtures comprising different proportions of said vectors, and transmits a noise index representing said one of the mixtures, and, at the receiver, the noise index is used to determine the proportions and the step of synthesizing the synthetic noise mixes the excitation vectors in said proportions.

6. A method according to claim 1, wherein said at least part of the selected excitation vector is generated at the receiver upon receipt of the corresponding noise class index.

7. A method according to claim 1, wherein a series of hangover frames are encoded at the transmitter and transmitted to the receiver and the noise class index is determined in the receiver by analyzing background noise present in received hangover frames.

8. A method according to claim 7, wherein background noise parameters are encoded at the transmitter and transmitted to the receiver in hangover frames at the beginning of said interval and the excitation vectors are updated on the basis of the background noise parameters extracted from the received hangover frames.

9. A method according to claim 1, wherein said at least part of the selected excitation vector is used to excite a synthesis filter to synthesize the synthetic noise.

10. A method according to claim 1, wherein the noise classification step is performed on a frame-by-frame basis using relatively long segments of the input voice signal and using line spectral frequencies (LSF) of the signal.

11. A digital communications system comprising a transmitter and a receiver, the transmitter having means for interrupting or reducing transmission of a voice signal during intervals absent speech and the receiver having means for inserting synthetic noise into the received voice signals during said intervals, there being provided means for assigning acoustic background noise in the voice signal to one or more of a plurality of noise classes, selecting a corresponding one of a plurality of excitation vectors each corresponding to at least one of the classes, using at least part of the selected excitation vector to synthesize the synthetic noise, and inserting the synthetic noise into the received signal during a said interval.

12. A system according to claim 11, wherein the noise classification means is at the transmitter, and transmits to the receiver a noise class index corresponding to the one or more noise classes, and the receiver comprises means for detecting the noise class index and, in dependence thereupon, selecting the corresponding one or more excitation vectors.

13. A system according to claim 11, wherein the receiver comprises storage means storing the plurality of excitation vectors and selector means for selecting one or more of the vectors in dependence upon the noise class index.

14. A system according to claim 11, wherein at least some of said vectors comprise mixture vectors corresponding to different mixtures of a plurality of said noise classes, and the classification means determines a particular one of said mixtures as corresponding to the background noise and transmits a corresponding noise index identifying the corresponding mixture vector, the receiver comprising means responsive to the noise index to select the corresponding mixture vector.

15. A system according to claim 11, wherein the noise classification means comprises means for determining that the background noise corresponds to one of a plurality of mixtures of said excitation vectors, the mixtures comprising different proportions of said excitation vectors, and transmitting a noise index representing said one of the mixtures, and the receiver comprises means responsive to the noise index for determining the proportions, the means for generating the synthetic noise mixing the excitation vectors in said proportions.

16. A system according to claim 11, wherein the receiver comprises means for generating said at least part of the selected excitation vector upon receipt of the corresponding noise class index.

17. A system according to claim 11, wherein the transmitter comprises means for transmitting a series of hangover frames to the receiver at the beginning of a said interval and the receiver comprises means for analyzing background noise present in the received hangover frames to determine the noise class index and supplying the noise class index to the means for selecting said one or more excitation vectors.

18. A system according to claim 17, wherein the transmitter comprises means for encoding background noise parameters and transmitting the encoded parameters to the receiver in hangover frames at the beginning of said interval and the receiver comprises means for extracting the background noise parameters from the received hangover frames and updating the excitation vectors on the basis thereof.

19. A system according to claim 11, wherein the receiver comprises a synthesis filter for excitation by said at least part of the selected excitation vector to generate the synthetic noise.

20. A system according to claim 11, wherein the transmitter performs noise classification on a frame-by-frame basis using relatively long segments of the input voice signal using line spectral frequencies (LSF) of the signal.

21. A transmitter for use in the system of claim 11, comprising classification means for classifying acoustic background noise in the voice signal to one or more of a plurality of noise classes, and transmitting to the receiver a noise class index corresponding to the one or more excitation vectors corresponding to the noise classes.

22. A transmitter according to claim 21, wherein the classification means determines the background noise to correspond to one of a plurality of different mixtures of a plurality of said excitation vectors, and transmits a corresponding noise index identifying the corresponding excitation vector mixture to the receiver.

23. A transmitter according to claim 21, wherein the noise classification means comprises means for determining that the background noise corresponds to one of a plurality of mixtures of said excitation vectors, the mixtures comprising different proportions of said vectors, and transmitting a noise index representing said one of the mixtures to the receiver.

24. A transmitter according to claim 21, further comprising means for transmitting a series of hangover frames to the receiver at the beginning of a said interval.

25. A transmitter according to claim 21, further comprising means for encoding background noise parameters and transmitting the encoded parameters to the receiver in at least some of said hangover frames.

26. A transmitter according to claim 21, wherein the noise classification means operates on a frame-by-frame basis using relatively long segments of the input voice signal and using line spectral frequencies (LSF) of the signal.

27. A receiver for a digital communications system according to claim 11, comprising means for selecting at least one of a plurality of excitation vectors each corresponding to at least one of the classes, using at least part of the selected excitation vector to generate the synthetic noise, and outputting the synthetic noise during a said interval.

28. A receiver according to claim 27, for a system according to claim 12, further comprising means for detecting the noise class index and, in dependence thereupon, selecting the corresponding one or more excitation vectors.

29. A receiver according to claim 27, further comprising storage means for storing the plurality of excitation vectors and selector means for selecting one or more of the vectors in dependence upon a noise class index received from the transmitter.

30. A receiver according to claim 27, for use in a system according to claim 13, wherein at least some of said vectors comprise mixture vectors corresponding to different mixtures of a plurality of said noise classes, and the classification means determines a particular one of said mixtures as corresponding to the background noise and transmits a corresponding noise index identifying the corresponding mixture vector, the receiver comprising means responsive to the noise index to select the corresponding mixture vector.

31. A receiver according to claim 27, for a said system wherein the noise classification means comprises means for determining that the background noise corresponds to one of a plurality of mixtures of said excitation vectors, the mixtures comprising different proportions of said vectors, and transmitting a noise index representing said one of the mixtures, the receiver comprising means responsive to the noise index for determining the proportions and means for mixing the excitation vectors in said proportions before application to the means for generating the synthetic noise.

32. A receiver according to claim 27, further comprising means for generating said at least part of the selected excitation vector upon receipt of the corresponding noise class index.

33. A receiver according to claim 27, for a system wherein the transmitter comprises means for transmitting a series of hangover frames to the receiver at the beginning of a said interval, the receiver comprising means for analyzing background noise present in the received hangover frames to determine the noise class index and supplying the noise class index to the means for selecting said one or more excitation vectors.

34. A receiver according to claim 27, for a system wherein the transmitter comprises means for encoding background noise parameters and transmitting the encoded parameters to the receiver in hangover frames at the beginning of said interval, the receiver comprising means for extracting the background noise parameters from the received hangover frames and updating the excitation vectors on the basis thereof.

35. A receiver according to claim 27, comprising a synthesis filter for excitation by said at least part of the selected excitation vector to generate the synthetic noise.