Inter-Channel Coherence Reduction for Stereophonic and Multichannel Acoustic Echo Cancellation

• US 20150030172A1
• Filed: 07/18/2014
• Published: 01/29/2015
• Est. Priority Date: 07/24/2013
• Status: Active Grant

• Current Assignee: mh acoustics LLC
• Original Assignee: mh acoustics LLC
• Inventors: Gaensler, Tomas F., Diethorn, Eric J.
• Granted Patent: US 9,445,196 B2
• Time in Patent Office: Days
• US Class Current: 381/71.2
• CPC Class Codes: H04M 9/082 using echo cancellers echo cancellers per se H04B3/23
• CPC Class Codes: H04R 3/005 for combining the signals of two or more microphones specially adapted for hearing aids H04R25/407
• CPC Class Codes: H04R 3/02 for preventing acoustic reaction , i.e. acoustic oscillatory feedback specially adapted for hearing aids H04R25/453
• CPC Class Codes: H04R 3/12 for distributing signals to two or more loudspeakers specially adapted for hearing aids H04R25/407

Abstract

In an audio system having acoustic echo, incoming audio signals are decorrelated by applying both phase modulation and amplitude modulation. The resulting decorrelated signals are rendered by loudspeakers, and co-located microphones generate outgoing audio signals that include echo signals caused by the existence of acoustic echo paths from the loudspeakers to the microphones. Acoustic echo cancellation (AEC) is applied to the outgoing audio signals to reduce the amount of echo in those signals. Decorrelating the incoming audios signals using amplitude modulation in addition to phase modulation can increase the effectiveness of the AEC processing. In a frequency-domain implementation, the amount of amplitude modulation can be different for different frequency bands (e.g., greater amplitude modulation for lower frequencies corresponding to human speech and/or no amplitude modulation for higher frequencies).

Claims

1. A method for reducing echo in audio signals, the method comprising:

(a) receiving a first plurality of audio signals from a first side of an audio communication;

(b) applying phase modulation and amplitude modulation to at least one of the audio signals to generate a plurality of decorrelated audio signals, wherein the plurality of decorrelated audio signals are rendered at a second side of the audio communication;

(c) receiving a second plurality of audio signals from the second side, wherein the second plurality of audio signals were generated based on the plurality of decorrelated audio signals; and

(d) applying acoustic echo cancellation to the second plurality of audio signals to reduce the echo in the second plurality of audio signals.

2. The invention of claim 1, wherein the second plurality of audio signals were generated by rendering the plurality of decorrelated audio signals using a plurality of loudspeakers at the second side as acoustic signals that traverse acoustic echo paths to a plurality of microphones at the second side that generate the second plurality of audio signals.

3. The invention of claim 1, wherein:

the first plurality of audio signals are a first pair of stereo audio signals;

the plurality of decorrelated audio signals are a pair of decorrelated stereo audio signals; and

the second plurality of audio signals are a second pair of stereo audio signals.

4. The invention of claim 3, wherein step (b) comprises applying phase modulation and amplitude modulation to each of the first pair of stereo audio signals to generate the pair of decorrelated stereo audio signals.

5. The invention of claim 1, wherein step (b) comprises:

(b1) converting the at least one audio signal from a time domain into a frequency domain;

(b2) applying different phase modulation and amplitude modulation to different frequency-domain audio signals to generate modulated frequency-domain audio signals; and

(b3) converting the modulated frequency-domain audio signals from the frequency domain into the time domain to generate the plurality of decorrelated audio signals.

6. The invention of claim 5, wherein greater amplitude modulation is applied to selected low-frequency audio signals than to selected high-frequency audio signals.

7. The invention of claim 6, wherein no amplitude modulation is applied to frequency-domain audio signals above a specified high-frequency threshold.

8. The invention of claim 7, wherein no amplitude modulation is applied to frequency-domain audio signals below a specified low-frequency threshold.

9. The invention of claim 5, wherein greater amplitude modulation is applied to frequency-domain audio signals corresponding to human speech than to other frequency-domain audio signals.

10. The invention of claim 1, wherein:

the phase modulation has a phase-modulation rate; and

the amplitude modulation has an amplitude-modulation rate, wherein the phase-modulation and amplitude-modulation rates are selected such that image shift due to the phase modulation is out of phase with image shift due to the amplitude modulation.

11. The invention of claim 10, wherein the amplitude-modulation rate is substantially equal to the phase-modulation rate.

12. The invention of claim 1, wherein step (b) further comprises adaptively generating the amplitude modulation.

13. The invention of claim 12, wherein step (b) comprises:

(b1) determining whether the first plurality of audio signals comprise substantially background noise; and

(b2) reducing the amplitude modulation if step (b1) determines that the first plurality of audio signals comprise substantially background noise.

14. The invention of claim 13, wherein step (b1) determines that the first plurality of audio signals comprise substantially background noise when (i) the first plurality of audio signals have a low signal level and (ii) the first plurality of audio signals have a low inter-channel coherence level within a specified frequency range.

15. The invention of claim 12, wherein step (b) comprises:

(b1) generating a background-noise coefficient for the first plurality of audio signals; and

(b2) generating the amplitude modulation based on the background-noise coefficient.

16. The invention of claim 15, wherein step (b1) comprises generating the background-noise coefficient based on signal level and coherence level of the first plurality of audio signals.

17. The invention of claim 1, wherein:

the second plurality of audio signals were generated by rendering the plurality of decorrelated audio signals using a plurality of loudspeakers at the second side as acoustic signals that traverse acoustic echo paths to a plurality of microphones at the second side that generate the second plurality of audio signals;

step (b) comprises;

(b1) converting the at least one audio signal from a time domain into a frequency domain;

(b2) applying different phase modulation and amplitude modulation to different frequency-domain audio signals to generate modulated frequency-domain audio signals; and

(b3) converting the modulated frequency-domain audio signals from the frequency domain into the time domain to generate the plurality of decorrelated audio signals;

greater amplitude modulation is applied to frequency-domain audio signals corresponding to human speech than to other frequency-domain audio signals;

no amplitude modulation is applied to frequency-domain audio signals above a specified high-frequency threshold;

no amplitude modulation is applied to frequency-domain audio signals below a specified low-frequency threshold;

the phase modulation has a phase-modulation rate;

the amplitude modulation has an amplitude-modulation rate, wherein the phase-modulation and amplitude-modulation rates are selected such that image shift due to the phase modulation is out of phase with image shift due to the amplitude modulation;

the amplitude-modulation rate is substantially equal to the phase-modulation rate; and

step (b) further comprises adaptively generating the amplitude modulation by;

(b4) generating a background-noise coefficient for the first plurality of audio signals based on signal level and coherence level of the first plurality of audio signals; and

(b5) generating the amplitude modulation based on the background-noise coefficient.

18. The invention of claim 17, wherein:

the first plurality of audio signals are a first pair of stereo audio signals;

the plurality of decorrelated audio signals are a pair of decorrelated stereo audio signals;

the second plurality of audio signals are a second pair of stereo audio signals; and

step (b) comprises applying phase modulation and amplitude modulation to each of the first pair of stereo audio signals to generate the pair of decorrelated stereo audio signals.

19. An audio signal processing system for reducing echo in audio signals, the system comprising:

a coherence reduction (CR) module configured to (i) receive a first plurality of audio signals from a first side of an audio communication and (ii) apply phase modulation and amplitude modulation to at least one of the audio signals to generate a plurality of decorrelated audio signals, wherein the plurality of decorrelated audio signals are rendered at a second side of the audio communication; and

an acoustic echo cancellation (AEC) module configured to (i) receive a second plurality of audio signals from the second side, wherein the second plurality of audio signals were generated based on the plurality of decorrelated audio signals, and (ii) apply acoustic echo cancellation to the second plurality of audio signals to reduce the echo in the second plurality of audio signals.