Adaptive feedback processing for consistent headphone acoustic noise cancellation

1. An audio signal processing method for a headphone, comprising:

filtering an internal microphone signal of a headphone by i) a first filter G that, as part of an acoustic noise cancellation, ANC, subsystem, produces an anti-noise audio signal, and ii) a second filter C to produce a feedback audio signal, wherein the first filter and the second filter operate based on the same audio frame of the internal microphone signal;

determining an estimate of a transfer function of a path S, wherein the path S is from i) an input of a speaker of the headphone to ii) the internal microphone signal;

adapting the second filter C based on the estimate of the transfer function of the path S; and

driving by the feedback audio signal an input of a speaker of the headphone.

2. The method of claim 1 wherein the second filter C comprises a plurality of biquads, and adapting the second filter C comprises

computing a plurality of gain parameters using the estimate of the transfer function of the path S;

scaling a plurality of template functions by the plurality of gain parameters, respectively; and

computing filter coefficients of the plurality of biquads based on scaling the plurality of template functions.

3. The method of claim 1 wherein adapting the second filter C comprises performing a least squares estimation to directly produce digital filter coefficients of the second filter C.

4. The method of claim 1 wherein adapting the second filter C comprises

altering a transfer function of the second filter C from 10 Hz to 10 kHz.

5. The method of claim 1 further comprising removing from the internal microphone signal a contribution by a user content audio signal, before filtering by the first filter G.

6. The method of claim 1 wherein the second filter C comprises a plurality of infinite impulse response biquad filters coupled in cascade, and wherein every one of the biquad filters is updated based on a same estimate of the transfer function of the path S.

7. The method of claim 6 wherein the second filter C has a transfer function that is an inverse of a minimum phase version of the estimate of the transfer function of the path S.

8. The method of claim 1 further comprising:

filtering an external microphone signal of the headphone by third filter W, to produce a feedforward audio signal; and

combining the feedforward audio signal with the feedback audio signal for driving the input of the speaker.

9. The method of claim 8 wherein the third filter is part an ANC subsystem and produces an anti-noise signal.

10. The method of claim 8 further comprising

combining the feedforward audio signal and the feedback audio signal with a user content audio signal for driving the input of the speaker; and

removing from the internal microphone signal a contribution by the user content audio signal, before filtering by the first filter G.

11. The method of claim 10 further comprising

filtering the user content audio signal by a fourth filter, before combining the filtered user content audio signal with the feedforward and feedback audio signals wherein the fourth filter is fixed or is updated less frequently than the second filter C.

12. The method of claim 1 wherein determining the estimate of the transfer function of the path S comprises performing an adaptive filter algorithm that repeatedly updates the estimate of the transfer function on a per audio frame basis.

13. A headphone audio system comprising:

a headphone housing having integrated therein an internal microphone to produce an internal microphone signal, and a speaker;

a processor; and

memory having stored therein instructions that configure the processor to

filter the internal microphone signal by i) a first filter G that, as part of an acoustic noise cancellation, ANC, subsystem, produces an anti-noise audio signal, and ii) a second filter C to produce a feedback audio signal, wherein the first filter is coupled in cascade with the second filter,

determine an estimate of a transfer function of a path S, wherein the path S is from i) an input of the speaker to ii) the internal microphone signal,

adapt the second filter C based on the estimate of the transfer function of the path S, and

drive the input of the speaker with the feedback audio signal.

14. The headphone audio system of claim 13 wherein the processor and memory are integrated in the headphone housing.

15. The headphone audio system of claim 13 wherein the second filter C comprises a plurality of biquads, and the processor adapts the second filter C by determining a least squares fit of the estimate of the transfer function of the path S using a plurality of template functions, and computing filter coefficients of the biquads based on the least squares fit.

16. The headphone audio system of claim 13 wherein processor adapts the second filter C by altering a transfer function of the second filter C from 10 Hz to 10 kHz.

17. The headphone audio system of claim 13 wherein the processor removes from the internal microphone signal a contribution by a user content audio signal, before filtering by the first filter G.

18. The headphone audio system of claim 13 wherein the second filter C comprises a plurality of infinite impulse response biquad filters coupled in cascade, and wherein every one of the biquad filters is updated based on a same estimate of the transfer function of the path S.

19. The headphone audio system of claim 13 wherein the second filter C has a transfer function that is an inverse of a minimum phase version of the estimate of the transfer function of the path S.

20. The headphone audio system of claim 13 further comprising an external microphone integrated in the headphone housing, and wherein the memory has stored therein further instructions that configure the processor to:

filter an external microphone signal produced by the external microphone by a third filter W, to produce a feedforward audio signal; and

combine the feedforward audio signal with the feedback audio signal for driving the input of the speaker.

21. A processor for use with a headphone, the processor comprising

a processor configured to:

filter an internal microphone signal by i) a first filter that, as part of an acoustic noise cancellation, ANC, subsystem, produces an anti-noise audio signal, and ii) a second filter that produces a feedback audio signal, wherein the first filter and the second filter are coupled in cascade,

determine an estimate of a transfer function of a path S, wherein the path S is from i) an input of a speaker to ii) the internal microphone signal,

adapt the second filter based on the estimate of the transfer function of the path S, and

drive the input of the speaker with the feedback audio signal.

22. The processor of claim 21 wherein the processor is further configured to remove from the internal microphone signal a contribution by a user content audio signal, before filtering by the first filter.