Automatic equalization for consistent headphone playback
Automatic equalization for consistent headphone may take place in a playback mode of operation in which ANC is turned off and there is no direct feedback from an internal microphone (to the input of a speaker). An automatic user content equalization process is active during that mode of operation which adapts a filter AEQ to restore a flat or other desired frequency response at the output of the speaker despite variation in headphone fit. An estimate of a transfer function of a path S is determined, wherein the path S is from i) the input of the speaker of the headphone to the internal microphone signal. The filter AEQ is adapted based on the estimate of the transfer function of the path S while it filters user content audio that drives the input of the speaker of the headphone. Other embodiments are also described and claimed.
1. An audio signal processing method for a headphone, the method comprising:
determining a transfer function of a path from i) an input of a speaker of the headphone to ii) an internal microphone signal, and updating the transfer function by an adaptive filter algorithm on a per audio frame basis while a user content audio signal is driving the input of the speaker of the headphone;
filtering the user content audio signal that is driving the input of the speaker of the headphone, by a filter cascade that comprises a first filter and a second filter; and
adapting the second filter and not the first filter while the filter cascade is filtering the user content audio signal that is driving the input of the speaker of the headphone, based on the updated transfer function.
2. The method of claim 1 wherein while the user content audio signal is driving the input of the speaker, no anti-noise is being produced by the headphone and there is no feedback path from an internal microphone of the headphone directly to the input of the speaker of the headphone.
3. The method of claim 1 further comprising performing acoustic noise cancellation using the speaker while the user content audio signal is driving the speaker.
4. The method of claim 3 further comprising:
filtering an output of the first filter to produce a contribution by the user content audio signal; and
removing the contribution from the internal microphone signal.
5. The method of claim 1 wherein adapting the second filter comprises altering a transfer function of the second filter from 10 Hz to 10 kHz.
6. The method of claim 1 wherein the second filter 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 instance of the transfer function of the path.
7. The method of claim 6 wherein the second filter has a transfer function that is an inverse of a minimum phase version of the transfer function of the path.
8. The method of claim 1 wherein adapting the second filter and not the first filter comprises maintaining the first filter fixed while adapting the second based on the updated transfer function.
9. 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
determine a transfer function of a path from i) an input of a speaker of the headphone to an internal microphone signal, and update the transfer function by an adaptive filter algorithm on a per audio frame basis while a user content audio signal is driving the input of the speaker of the headphone,
filter the user content audio signal that is driving the input of the speaker of the headphone, by a filter cascade that comprises a first filter and a second filter, and
adapt the second filter and not the first filter based on the updated transfer function while the filter cascade is filtering the user content audio signal.
10. The headphone audio system of claim 9 wherein the processor and memory are integrated in the headphone housing.
11. The headphone audio system of claim 9 wherein the second filter comprises a plurality of biquads, and the processor adapts the second filter by i) determining a least squares fit of the transfer function of the path using a plurality of template functions, and computing filter coefficients of the biquads based on the least squares fit.
12. The headphone audio system of claim 9 wherein processor adapts the second filter by altering a transfer function of the second filter from 10 Hz to 10 kHz.
13. The headphone audio system of claim 9 wherein while the user content audio signal is driving the input of the speaker, no anti-noise is being produced by the headphone and there is no feedback path from the internal microphone directly to the input of the speaker.
14. The headphone audio system of claim 9 wherein the second filter 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 instance of the transfer function of the path.
15. The headphone audio system of claim 9 wherein the second filter has a transfer function that is an inverse of a minimum phase version of the transfer function of the path.
16. An article of manufacture comprising
a non-transitory machine readable medium having stored therein instructions that configure a processor to:
determine a transfer function of a path from i) an input of a speaker of a headphone to an internal microphone signal of the headphone, and update the transfer function by an adaptive filter algorithm on a per audio frame basis while a user content audio signal is driving the input of the speaker of the headphone;
filter the user content audio signal that is driving the input of the speaker of the headphone, by a filter cascade that comprises a first filter and a second filter; and
adapt the second filter and not the first filter while the filter cascade is filtering the user content audio signal that is driving the input of the speaker of the headphone, based on the updated transfer function.
17. The article of manufacture of claim 16 wherein the second filter comprises a plurality of biquads, and adapting the second filter comprises:
computing a plurality of gain parameters using the transfer function;
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.
18. The article of manufacture of claim 16 wherein adapting the second filter comprises performing a least squares estimation to directly produce digital filter coefficients of the second filter.
19. The article of manufacture of claim 16 wherein adapting the second filter comprises
altering a transfer function of the second filter from 10 Hz to 10 kHz.
20. The article of manufacture of claim 16 wherein adapting the second filter and not the first filter comprises
maintaining the first filter fixed while adapting the second based on the updated transfer function.