Adaptive LPC noise reduction system

• US 20080285773A1
• Filed: 05/17/2007
• Published: 11/20/2008
• Est. Priority Date: 05/17/2007
• Status: Active Grant

• Current Assignee: Blackberry Limited
• Original Assignee: QNX Software Systems Limited (Canada) (Blackberry Limited)
• Inventors: Nongpiur, Rajeev, Hetherington, Phillip A.
• Granted Patent: US 8,447,044 B2
• Time in Patent Office: Days
• US Class Current: 381/94.2
• CPC Class Codes: G10L 21/0208 Noise filtering
• CPC Class Codes: G10L 21/0232 Processing in the frequency domain
• CPC Class Codes: G10L 25/12 the extracted parameters being prediction coefficients

Abstract

A noise suppression system reduces low-frequency noise in a speech signal using linear predictive coefficients in an adaptive filter. A digital filter may update or adapt a limited set of linear predictive coefficients on a sample-by-sample basis. The linear predictive coefficients may be used to provide an error signal based on a difference between the speech signal and a delayed speech signal. The error signal represents an enhanced speech signal having attenuated and normalized low-frequency noise components.

Claims

1. A noise suppression system comprising:

a sampling circuit adapted to sample an input signal at a predetermined sampling rate;

a plurality of delay circuits configured to sequentially delay the sampled input signal;

an adaptive processor configured to update a plurality of linear predictive coefficient (LPC) values on a sample-by-sample basis, based on an error signal;

the error signal based on a difference between the sampled input signal and the sequentially delayed signal; and

the LPC values configured to flatten the error signal across a frequency region of interest to provide an enhanced signal having reduced low-frequency components.

2. The system of claim 1, further comprising a conversion circuit configured to convert the error signal to an analog signal as an enhanced output signal having reduced low-frequency components.

4. The system of claim 1, where between 2 and 20 LPC values are updated on a sample-by-sample basis.

6. The system of claim 1, where the error signal represents enhanced sampled speech.

7. The system of claim 6, where noise components of the enhanced sampled speech are normalized in amplitude, and an average amplitude of the noise components is reduced.

11. The system of claim 1, further comprising a voice activity detector configured to detect presence of a speech signal and inhibit updating of the LPC values in the presence of the speech signal.

12. The system of claim 11, where the detection of the speech signal is based on an average energy level of the sampled input signal.

13. The system of claim 1, further comprising a high-pass filter and low-pass filter.

14. The system of claim 13, where the low-pass filter passes low-frequency components of the sampled input signal to the adaptive processor, and blocks higher-frequency components of the sampled input signal.

15. The system of claim 14, where the low-frequency components are flattened in amplitude.

18. The system of claim 15, further comprising a wind buffet detector configured to detect presence of a wind buffet, and inhibit adaptation of the LPC values when wind buffets are not detected.

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19. A noise suppression system comprising:

a sampling circuit adapted to sample an input signal at a predetermined sampling rate;

a plurality of delay circuits configured to sequentially delay in time the sampled input signal;

an adaptive processor configured to update a plurality of linear predictive coefficient (LPC) values on a sample-by-sample basis, based on an error signal;

the error signal based on a difference between the sampled input signal and the sequentially delayed signal;

a voice activity detector configured to detect presence of a speech signal and inhibit updating of the LPC values in the presence of the speech signal; and

the LPC values configured to flatten the error signal across a frequency region of interest to provide the error signal as an enhanced speech signal having reduced low-frequency components.

21. The system of claim 19, where the detection of the speech signal is based on an average energy level of the sampled input signal.

23. The system of claim 19, where the adaptive processor loosely models a human vocal tract.

24. The system of claim 19, where the error signal represents enhanced sampled speech.

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25. A method for enhancing a signal provided to a user device, the method comprising:

sampling an input signal at a predetermined sample rate;

delaying the sampled input signal by multiple levels of delays to provide sequentially delayed signals;

processing the sequentially delayed signals in an adaptive filter;

adaptively updating linear predictive coefficient (LPC) values on a sample-by-sample basis based on an error signal, the error signal based on a difference between the sampled input signal and the sequentially delayed signals, where the LPC values cause the error signal to have a normalized amplitude across a frequency region of interest, and

providing the error signal as an enhanced signal having a flattened low-frequency spectrum.

26. The method according to claim 25 further comprising converting the error signal to an analog signal and outputting the error signal as an enhanced signal to the user device.

29. The method according to claim 25, where the adaptive processor loosely models a human vocal tract.

31. The method according to claim 25, further comprising detecting presence of a speech signal and inhibiting updating of the LPC values in the presence of the speech signal.

33. The method according to claim 25, further comprising providing a high-pass filter and a low-pass filter.

34. The method according to claim 33, where the low-pass filter passes low-frequency components of the sampled input signal to the adaptive processor, and blocks higher-frequency components of the sampled input signal.

35. The method according to claim 34, where the low-frequency components are flattened in amplitude.

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36. A computer-readable storage medium having processor executable instructions to provide a noise-reduced signal by performing the acts of:

sampling an input signal at a predetermined sample rate;

delaying the sampled input signal by multiple levels of delays to provide sequentially delayed signals;

processing the sequentially delayed signals in an adaptive filter;

adaptively updating linear predictive coefficient (LPC) values on a sample-by-sample basis based on an error signal, the error signal based on a difference between the sampled input signal and the sequentially delayed signals, where the LPC values cause the error signal to have a normalized amplitude across a frequency region of interest; and

providing the error signal as an enhanced signal having a flattened low-frequency spectrum.

39. The computer-readable storage medium of claim 36, further comprising processor executable instructions to cause a processor to perform the act of detecting presence of a speech signal and inhibiting updating of the LPC values in the presence of the speech signal.

40. The computer-readable storage medium of claim 39, further comprising processor executable instructions to cause a processor to perform the act detecting the speech signal based on an average energy level of the sampled input signal.

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