Noise suppression system

• US 4,811,404 A
• Filed: 10/01/1987
• Issued: 03/07/1989
• Est. Priority Date: 10/01/1987
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Barlo, Joseph J., Lindsley, Brett L., Gerson, Ira A., Vilmur, Richard J.
• Primary Examiner(s): Ng, Jin F.
• Assistant Examiner(s): Kim, David H.
• Application Number:
  US07/103,857
• Time in Patent Office: 523 Days
• Field of Search: 381/94, 381/47, 381/71, 381/102, 381/107, 381/68, 381/68.2, 381/58, 381/57, 381/158, 455/303, 455/305, 455/306, 455/312, 328/167
• US Class Current: 381/94.3
• CPC Class Codes: G10L 2021/02085 Periodic noise
• CPC Class Codes: G10L 2021/02168 the estimation exclusively taking place during speech pauses
• CPC Class Codes: G10L 2025/786 Adaptive threshold
• CPC Class Codes: G10L 2025/937 Signal energy in various frequency bands
• CPC Class Codes: G10L 21/0208 Noise filtering

Abstract

An improved noise suppression system (800) is disclosed which performs speech quality enhancement upon the speech-plus-noise signal available at the input (205) to generate a clean speech signal at the output (265) by spectral gain modification. The improvements of the present invention include the addition of a signal-to-noise ratio (SNR) threshold mechanism (830) to reduce background noise flutter by offsetting the gain rise of the gain tables until a certain SNR threshold is reached, the use of a voice metric calculator (810) to produce more accurate background noise estimates via performing the update decision based on the overall voice-like characteristics in the channels and the time interval since the last update, and the use of a channel SNR modifier (820) to provide immunity to narrowband noise bursts through modification of the SNR estimates based on the voice metric calculation and the channel energies.

Claims

1. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels;

means for generating estimates of the signal-plus-noise energy and the noise energy in each individual channel;

means for producing a gain value for each individual channel in response to said channel energy estimates, said gain values having a minimum gain value for each channel, said gain value producing means including threshold means for allowing gain values above said minimum gain value to be prodeced only when said signal-plus-noise energy estimates exceed said noise energy estimates by a predetermined amount; and

means for modifying the gain of each of said plurality of pre-processed signals in esponse to said gain values to provide a plurality of post-processed signals.

2. The noise suppression system according to claim 1, wherein said gain value producing means produces gain values based upon the signal-to-noise ratio (SNR) of said channel energy estimates, and wherein said SNR estimates are compared with a predefined SNR threshold such that channels having SNR estimates below said SNR threshold produce minimum gain values.

3. The noise suppression system according to claim 2, wherein said predefined SNR threshold corresponds to an SNR value within the range of 1.5 dB to 5 dB SNR.

4. The noise suppression system according to claim 3, wherein said predefined SNR threshold corresponds to an SNR value of approximately 2.25 dB SNR.

5. The noise suppression system according to claim 1, wherein said gain modifying means provides a maximum amount of attenuation of the pre-processed signal in a particular channel having a minimum gain value.

6. The noise suppression system according to claim 1, wherein gain values produce a higher amount of attenuation for high frequency channels than low frequency channels.

7. The noise suppression system according to claim 1, wherein said gain value producing means further includes a plurality of gain tables, each gain table having predetermined individual channel gain values corresponding to said individual channel energy estimates, and gain table selection means for automatically selecting one of said plurality of gain tables as a function of the overall average background noise level of said input signal.

8. The noise suppression system according to claim 1, further includes means for combining said plurality of post-processed signals to produce said noise-suppressed output signal.

9. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels;

means for generating and storing an estimate of the background noise power spectral density of said pre-processed signals, said background noise estimate generating means including means for modifying said background noise estimate in response to a timing parameter indicative of the time interval since the previous background noise estimate modification;

means for generating an estimate of the signal-to-noise ratio (SNR) in each individual channel based upon said modified background noise estimates;

means for producing a gain value for each individual channel in response to said channel SNR estimates; and

means for modifying the gain of each of said plurality of pre-processed signals in response to said gain values to provide a plurality of post-procesed signals.

10. The noise suppression system according to claim 9, wherein said background noise estimate modifying means includes means for producing said timing parameter, and means for comparing said timing parameter to a predetermined timing threshold such that a background noise estimate modification is performed when said timing parameter exceeds said timing threshold.

11. The noise suppression system according to claim 10, wherein said predetermined timing threshold is in the range of 0.5 second to 4 seconds.

12. The noise suppression system according to claim 11, wherein said predetermined timing threshold is approximately equal to 1 second.

13. The noise suppression system according to claim 10, wherein said background noise estimate modifying means further includes means for generating an estimate of the energy in each individual channel, and means for producing a multi-channel energy parameter in response to the total value of all individual channel energy estimates.

14. The noise suppression system according to claim 13, wherein said background noise estimate modifying means further includes means for comparing said multi-channel energy parameter to a predetermined energy threshold such that a background noise estimate modification is performed when said multi-channel energy parameter is less than said energy threshold.

15. The noise suppression system according to claim 13, wherein said multi-channel energy parameter is generated by translating said individual channel SNR estimates into individual channel voice metrics and summing the individual channel voice metrics, the voice metric sum being a measurement of the overall voice-like characteristics of the energy in all channels.

16. The noise suppression system according to claim 14, wherein said background noise estimate modifying means modifies said background noise estimates in response to said timing parameter regardless of said multi-channel energy parameter.

17. The noise suppression system according to claim 13, wherein said multi-channel energy parameter producing means accommodates for minor variations in individual channel energy estimates such that said minor variations do not significantly affect said multi-channel energy parameter.

18. The noise suppression system according to claim 14, wherein said predetermined energy threshold is set such that a background noise estimate modification is performed if all channels exhibit individual SNR values less than 6 dB SNR.

19. The noise suppression system according to claim 14, wherein said predetermined energy threshold is set such that a background noise estimate modification is not performed if any single channel exhibits an SNR value of at least 6 dB SNR.

20. The noise suppression system according to claim 9, wherein said gain value producing means further includes a plurality of gain tables, each gain table having predetermined individual channel gain values corresponding to various individual channel SNR estimates, and gain table selection means for automatically selecting one of said plurality of gain tables as a function of the overall average background noise level of said input signal.

21. The noise suppression system according to claim 9, further includes means for combining said plurality of post-processed signals to produce said noise-suppressed output signal.

22. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of a number N of selected frequency channels

means for generating an estimate of the energy in each individual channel;

means for monitoring said channel energy estimates and for distinguishing narrowband noise bursts from speech energy and background noise energy, thereby producing a modification signal;

means for selectively modifying said channel energy estimates in response to said modification signal such that channel energy estimates representative of narrowband noise bursts are modified;

means for producing a gain value for each individual channel in response to each modified channel energy estimate; and

means for modifying the gain of each of said plurality of pre-processed signals in response to said gain values to provide a plurality of post-processed signals.

23. The noise suppression system according to claim 22, wherein said modification signal is indicative of the total number of individual channels having energy estimates exceeding a predetermined energy threshold.

24. The noise suppression system according to claim 23, wherein said predetermined energy threshold corresponds to a signal-to-noise ratio (SNR) value within the range of 4 dB to 10 dB SNR.

25. The noise suppression system according to claim 24, wherein said predetermined energy threshold corresponds to an SNR value of approximately 6 dB SNR.

26. The noise suppression system according to claim 23, wherein said channel energy estimate modifying means includes means for comparing said modification signal to a predetermined count threshold such that a channel energy estimate modification is performed when said total number of individual channels is less than said count threshold.

27. The noise suppression system according to claim 26, wherein said predetermined count threshold corresponds to less than 40% ×

N.

28. The noise suppression system according to claim 22, wherein said gain modifying means provides a maximum amount of attenuation of the pre-processed signal in a particular channel having a modified channel energy estimate.

29. The noise suppression system according to claim 22, wherein said gain value producing means further includes a plurality of gain tables, each gain table having predetermined individual channel gain values corresponding to various individual channel energy estimates, and gain table selection means for automatically selecting one of said plurality of gain tables as a function of the overall average background noise level of said input signal.

30. The noise suppression system according to claim 22, further includes means for combining said plurality of post-processed signals to produce said noise-suppressed output signal.

31. An improved method of attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal in a noise suppression system comprising the steps of:

separating the input signal into a plurality of preprocessed signals representative of a number N of selected frequency channels;

generating an estimate of the energy in each individual channel;

generating and storing an estimate of the background noise power spectral density of said pre-processed signals;

generating an estimate of the signal-to-noise ratio (SNR) in each individual channel based upon said background noise estimates and said channel energy estimates;

producing a gain value for each individual channel in response to said channel SNR estimates, said gain values having a range of minimal values, said gain value producing step including the steps of providing a predefined SNR threshold and comparing said channel SNR estimates to said predefined SNR threshold such that channels having SNR estimates below said SNR threshold produce gain values within said minimal range; and

modifying the gain of each of said plurality of preprocessed signals in response to said gain values to provide a plurality of post-processed signals.

32. The method according to claim 31, wherein said predefined SNR threshold corresponds to an SNR value within the range of 1.5 dB to 5 dB SNR.

33. The method according to claim 31, wherein said gain modifying step provides a maximum amount of attenuation of the pre-processed signal in a particular channel having a gain value within said minimal range.

34. The method according to claim 31, including the step of modifying said background nose estimate in response to a timing parameter indicative of the time interval since the previous background noise estimate modification.

35. The method according to claim 34, wherein said background noise estimate modifying step includes the steps of producing said timing parameter and comparing said timing parameter to a predetermined timing threshold such that a background noise estimate modification is performed when said timing parameter exceeds said timing threshold.

36. The method according to claim 35, wherein said predetermined timing threshold is in the range of 0.5 second to 4 seconds.

37. The method according to claim 34, wherein said background noise estimate modifying step further includes the step of producing a multi-channel energy parameter in response to the total value of all individual channel SNR estimates.

38. The method according to claim 37, wherein said background noise estimate modifying step further includes the step of comparing said multi-channel energy parameter to a predetermined energy threshold such that a background noise estimate modification is performed when said multi-channel energy parameter is less than said energy threshold.

39. The method according to claim 38, wherein said multi-channel energy parameter is generated by translating said individual channel SNR estimates into individual channel voice metrics and summing the individual channel voice metrics, the voice metric sum being a measurement of the overall voice-like characteristics of the energy in all channels.

40. The method according to claim 38, wherein said background noise estimate modifying step modifies said background noise estimates in response to said timing parameter regardless of said multi-channel energy parameter.

41. The method according to claim 38, wherein said predetermined energy threshold is set such that a background noise estimate modification is performed if all channels exhibit individual SNR values less than 6 dB SNR.

42. The method according to claim 38, wherein said predetermined energy threshold is set such that a background noise estimate modification is not performed if any single channel exhibits an SNR value of at least 6 dB SNR.

43. The method according to claim 31, including the steps of monitoring said channel SNR estimates and distinguishing narrowband noise bursts from speech energy and background noise energy thereby producing a modification signal, and selectively modifying said channel SNR estimates in response to said modification signal such that channel SNR estimates representative of narrowband noise bursts are modified.

44. The method according to claim 43, wherein said modification signal is indicative of the total number of individual channel having SNR estimates exceeding a predetermined modification threshold.

45. The method according to claim 44, wherein said predetermined modification threshold corresponds to an SNR value within the range of 4 dB to 10 dB SNR.

46. The method according to claim 44, wherein said channel SNR estimate modifying step includes the step of comparing said modification signal to a predetermined count threshold such that a channel SNR estimate modification is performed when said total number of individual channels is less than said count threshold.

47. The method according to claim 46, wherein said predetermined count threshold corresponds to less than 40% ×

N.

48. The method according to claim 43, wherein said gain modifying step provides a maximum amount of attenuation of the pre-processed signal in a particular channel having a modified channel SNR estimate.

49. The method according to claim 31, wherein said gain value producing step further includes the step of automatically selecting one of a plurality of gain tables as a function of the overall average background noise level of said input signal, each gain table having predetermined individual channel gain values corresponding to various individual channel SNR estimates.

50. The method according to claim 31, further includes the step of combining said plurality of post-processed signals to produce said noise-suppressed output signal.