BANDLIMITING ANTI-NOISE IN PERSONAL AUDIO DEVICES HAVING ADAPTIVE NOISE CANCELLATION (ANC)

• US 20120308024A1
• Filed: 12/21/2011
• Published: 12/06/2012
• Est. Priority Date: 06/03/2011
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Alderson, Jeffrey, Kwatra, Nitin, Kamath, Gautham Devendra, Melanson, John L., Abdollahzaden Milani, Ali
• Granted Patent: US 8,948,407 B2
• Time in Patent Office: Days
• US Class Current: 381/71.11
• CPC Class Codes: G10K 11/17815 between the reference signals and the error signals, i.e. primary path
• CPC Class Codes: G10K 11/17854 the filter being an adaptive filter
• CPC Class Codes: G10K 11/17855 for improving speed or power requirements
• CPC Class Codes: G10K 11/17881 the reference signal being an acoustic signal, e.g. recorded with a microphone
• CPC Class Codes: G10K 11/17885 additionally using a desired external signal, e.g. pass-through audio such as music or speech
• CPC Class Codes: G10K 2210/108 Communication systems, e.g. where useful sound is kept and noise is cancelled
• CPC Class Codes: G10K 2210/1081 Earphones, e.g. for telephones, ear protectors or headsets
• CPC Class Codes: G10K 2210/1082 Microphones, e.g. systems using "virtual" microphones
• CPC Class Codes: G10K 2210/3017 Copy, i.e. whereby an estimated transfer function in one functional block is copied to another block
• CPC Class Codes: G10K 2210/3028 Filtering, e.g. Kalman filters or special analogue or digital filters
• CPC Class Codes: G10K 2210/3035 Models, e.g. of the acoustic system
• CPC Class Codes: G10K 2210/3049 Random noise used, e.g. in model identification
• CPC Class Codes: G10K 2210/3056 Variable gain
• CPC Class Codes: G10K 2210/511 Narrow band, e.g. implementations for single frequency cancellation

Abstract

A personal audio device, such as a wireless telephone, includes noise canceling circuit that adaptively generates an anti-noise signal from a reference microphone signal and injects the anti-noise signal into the speaker or other transducer output to cause cancellation of ambient audio sounds. An error microphone may also be provided proximate the speaker to measure the output of the transducer in order to control the adaptation of the anti-noise signal and to estimate an electro-acoustical path from the noise canceling circuit through the transducer. A processing circuit that performs the adaptive noise canceling (ANC) function also either adjusts the frequency response of the anti-noise signal with respect to the reference microphone signal, and/or by adjusting the response of the adaptive filter independent of the adaptation provided by the reference microphone signal.

Claims

1. A personal audio device, comprising:

a personal audio device housing;

a transducer mounted on the housing for reproducing an audio signal including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer;

a reference microphone mounted on the housing for providing a reference microphone signal indicative of the ambient audio sounds;

an error microphone mounted on the housing in proximity to the transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer; and

a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds at the error microphone, wherein a response of the anti-noise signal to the reference microphone signal has an additional shaped frequency response independent of the adapting to alter the anti-noise signal component of the acoustic output of the transducer as heard by the listener.

2. The personal audio device of claim 1, wherein the processing circuit implements a first fixed filter having a predetermined response acting in functional series with the adaptive filter, wherein the predetermined response provides the shaped frequency response.

3. The personal audio device of claim 1, wherein the processing circuit implements a secondary path adaptive filter having a secondary path response that shapes the source audio and a combiner that removes the source audio from the error microphone signal to provide an error signal indicative of the combined anti-noise and ambient audio sounds delivered to the listener, and wherein the processing circuit further implements a copy of the secondary path adaptive filter that filters the reference microphone signal to provide a correlation input to the adaptive filter that is correlated with the error signal to control the adapting of the adaptive filter, wherein the processing circuit adapts the adaptive filter to minimize components of the error signal that are correlated with an output of the copy of the secondary path adaptive filter, and wherein the processing circuit further implements a second filter having a response identical to the predetermined response of the first fixed filter that shapes the reference microphone signal to stabilize control of the response of the adaptive filter.

4. The personal audio device of claim 3, wherein the second filter further includes a low-pass response that prevents the control of the adaptive filter from adapting to remove the predetermined response of the first fixed signal from the anti-noise signal, and wherein the processing circuit implements a third filter having the low-pass response that filters the error signal.

5. The personal audio device of claim 2, wherein the predetermined response is a response shaped to remove a particular problem frequency from the anti-noise signal.

6. The personal audio device of claim 5, wherein the particular problem frequency is a multipath null in the frequency range between 2 kHz and 5 kHz that is present in an acoustic path between the reference microphone and the error microphone.

7. The personal audio device of claim 2, wherein the processing circuit implements a secondary path adaptive filter having a secondary path response that shapes the source audio and a combiner that removes the source audio from the error microphone signal to provide an error signal indicative of the combined anti-noise and ambient audio sounds delivered to the listener, wherein the processing circuit further subtracts the output of the first fixed filter and adds the output of the adaptive filter to the source audio provided to the secondary path adaptive filter to remove the effect of the first fixed filter from the error signal.

8. The personal audio device of claim 7, wherein the processing circuit further implements a second fixed filter having a phase response matching a predetermined phase response of the first filter, but having an amplitude response that passes frequencies across a frequency band in which the predetermined response of the first fixed filter has substantial attenuation, wherein the processing circuit filters the output of the adaptive filter that is added to the source audio with the second fixed filter, so that the phase response of the first fixed filter does not cause error in the adapting of the adaptive filter, and wherein the processing circuit further implements a third fixed filter having a response matching the response of the second fixed filter, wherein the processing circuit further filters the reference microphone signal supplied to the copy of the secondary path adaptive filter with the third fixed filter.

9. The personal audio device of claim 1, wherein the personal audio device is a wireless telephone further comprising a transceiver for receiving the source audio as a downlink audio signal.

10. The personal audio device of claim 1, wherein the personal audio device is an audio playback device, wherein the source audio is a program audio signal.

11. A method of canceling ambient audio sounds in the proximity of a transducer of a personal audio device, the method comprising:

first measuring ambient audio sounds with a reference microphone to produce a reference microphone signal;

second measuring an output of the transducer and the ambient audio sounds at the transducer with an error microphone;

adaptively generating an anti-noise signal from a result of the first measuring and the second measuring for countering the effects of ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters an output of the reference microphone;

combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer; and

shaping a frequency response applied to the generated anti-noise signal independent of the adapting of the response of the adaptive filter to reduce error between an anti-noise signal component of the acoustic output of the transducer as heard by the listener and an anti-noise signal component of the acoustic output of the transducer.

12. The method of claim 11, wherein the shaping is performed by filtering a result of the adaptively generating with a first fixed filter having a predetermined response.

13. The method of claim 12, further comprising:

shaping a copy of the source audio with a secondary path response;

removing the result of the shaping the copy of the source audio from the error microphone signal to produce an error signal indicative of the combined anti-noise and ambient audio sounds delivered to the listener; and

second filtering the reference microphone signal with a response identical to the predetermined response of the first fixed filter and another response according to a copy of the secondary path adaptive filter to provide an input to the adaptive filter.

14. The method of claim 13, further comprising applying a low-pass response to the result of the second filtering to prevent the adaptively generating from adapting to cancel the shaping, and further comprising filtering the error signal with another filter having the low-pass response.

15. The method of claim 13, wherein the predetermined response is a response shaped to remove a particular problem frequency from the anti-noise signal.

16. The method of claim 15, wherein the particular problem frequency is a multipath null in the frequency range between 2 kHz and 5 kHz that is present in an acoustic path between the reference microphone and the error microphone.

17. The method of claim 12, further comprising:

shaping a copy of the source audio with a secondary path response;

removing the result of the shaping the copy of the source audio from the error microphone signal to produce an error signal indicative of the combined anti-noise and ambient audio sounds delivered to the listener;

second filtering the reference microphone signal with a response according to a copy of the secondary path adaptive filter to provide an input to the adaptive filter; and

subtracting the output of the first fixed filter and adding the output of the adaptive filter to the source audio provided to the secondary path adaptive filter to remove the effect of the first fixed filter from the error signal.

18. The method of claim 17, further comprising:

filtering a the portion of the output of the adaptive filter that is added to the source audio with a second fixed filter having a phase response matching a predetermined phase response of the first fixed filter, but having an amplitude response that passes frequencies across a frequency band in which the predetermined response of the first fixed filter has substantial attenuation, so that the phase response of the first fixed filter does not cause error in the adaptively generating; and

filtering the reference microphone signal supplied to the second filtering with a third fixed filter having a response equal to the response of the second fixed filter.

19. The method of claim 13, wherein the personal audio device is a wireless telephone, and wherein the method further comprises receiving the source audio as a downlink audio signal.

20. The method of claim 13, wherein the personal audio device is an audio playback device, wherein the source audio is a program audio signal.

21. An integrated circuit for implementing at least a portion of a personal audio device, comprising:

an output for providing a signal to a transducer including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer;

a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds;

an error microphone input for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer; and

a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, wherein a response of the anti-noise signal to the reference microphone signal has an additional shaped frequency response independent of the adapting to alter the anti-noise signal component of the acoustic output of the transducer as heard by the listener.

22. The integrated circuit of claim 21, wherein the processing circuit implements a first fixed filter having a predetermined response acting in functional series with the adaptive filter, wherein the predetermined response provides the shaped frequency response.

23. The integrated circuit of claim 22, wherein the processing circuit implements a secondary path adaptive filter having a secondary path response that shapes the source audio and a combiner that removes the source audio from the error microphone signal to provide an error signal indicative of the combined anti-noise and ambient audio sounds delivered to the listener, and wherein the processing circuit further implements a copy of the secondary path adaptive filter that filters the reference microphone signal to provide a correlation input to the adaptive filter that is correlated with the error signal to control the adapting of the adaptive filter, wherein the processing circuit adapts the adaptive filter to minimize components of the error signal that are correlated with an output of the copy of the secondary path adaptive filter, and wherein the processing circuit further implements a second filter having a response identical to the predetermined response of the first fixed filter that shapes the reference microphone signal to stabilize control of the response of the adaptive filter.

24. The integrated circuit of claim 23, wherein the second filter further includes a low-pass response that prevents the control of the adaptive filter from adapting to remove the predetermined response of the first fixed signal from the anti-noise signal, and wherein the processing circuit implements a third filter having the low-pass response that filters the error signal.

25. The integrated circuit of claim 22, wherein the predetermined response is a response shaped to remove a particular problem frequency from the anti-noise signal.

26. The integrated circuit of claim 25, wherein the particular problem frequency is a multipath null in the frequency range between 2 kHz and 5 kHz that is present in an acoustic path between the reference microphone and the error microphone.

27. The integrated circuit of claim 22, wherein the processing circuit implements a secondary path adaptive filter having a secondary path response that shapes the source audio and a combiner that removes the source audio from the error microphone signal to provide an error signal indicative of the combined anti-noise and ambient audio sounds delivered to the listener, wherein the processing circuit further subtracts the output of the first fixed filter and adds the output of the adaptive filter to the source audio provided to the secondary path adaptive filter to remove the effect of the first fixed filter from the error signal.

28. The integrated circuit of claim 27, wherein the processing circuit further implements a second fixed filter having a phase response matching a predetermined phase response of the first filter, but having an amplitude response that passes frequencies across a frequency band in which the predetermined response of the first fixed filter has substantial attenuation, wherein the processing circuit filters the output of the adaptive filter that is added to the source audio with the second fixed filter, so that the phase response of the first fixed filter does not cause error in the adapting of the adaptive filter, and wherein the processing circuit further implements a third fixed filter having a response matching the response of the second fixed filter, wherein the processing circuit further filters the reference microphone signal supplied to the copy of the secondary path adaptive filter with the third fixed filter.

29. A personal audio device, comprising:

a personal audio device housing;

a transducer mounted on the housing for reproducing an audio signal including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer;

a reference microphone mounted on the housing for providing a reference microphone signal indicative of the ambient audio sounds;

an error microphone mounted on the housing in proximity to the transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer; and

a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds at the error microphone, wherein the response of the adaptive filter is further adjusted independent of the adapting, in order to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds.

30. The personal audio device of claim 29, wherein the adaptive filter comprises:

a first fixed portion of the adaptive filter; and

a second adaptive portion of the adaptive filter, wherein the first fixed portion and the second adaptive portion operate together to generate the response that shapes the anti-noise signal, and wherein the second adaptive portion has a leakage characteristic that restores the response of the second adaptive portion to an initial response of the second adaptive portion over time.

31. The personal audio device of claim 29, wherein the response of the adaptive filter is adjusted by combining injected noise with the reference microphone signal so that the response of the adaptive filter is controlled by the adaptive filter adapting to cancel the injected noise, whereby the response of the adaptive filter is reduced in frequency regions in a frequency range of the injected noise.

32. The personal audio device of claim 29, wherein the response of the adaptive filter is adjusted independent of the adaptation of the adaptive filter by the processing circuit implementing a copy of the adaptive filter to receive the injected noise so that the response of the copy of the adaptive filter is controlled by the adaptive filter adapting to cancel a combination of the ambient audio sounds and the injected noise, and wherein the processing circuit further controls the response of the adaptive filter with the coefficients adapted in the copy of the adaptive filter, whereby the injected noise is not present in the anti-noise signal.

33. A method of canceling ambient audio sounds in the proximity of a transducer of a personal audio device, the method comprising:

first measuring ambient audio sounds with a reference microphone to produce a reference microphone signal;

second measuring an output of the transducer and the ambient audio sounds at the transducer with an error microphone;

adaptively generating an anti-noise signal from a result of the first measuring and the second measuring for countering the effects of ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters an output of the reference microphone;

combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer;

adjusting a response of the adaptive filtering independent of the adaptively generating, in order to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds; and

providing a result of the combining to the transducer to generate the acoustic output.

34. The method of claim 33, wherein the adaptive filter comprises a first fixed portion of the adaptive filter, and a second adaptive portion of the adaptive filter, and wherein the method further comprises operating the first fixed portion and the second adaptive portion together to perform the adaptive generating, and wherein the method further comprises restoring the response of the second adaptive portion to an initial response of the second adaptive portion over time to cause leakage.

35. The method of claim 33, further comprising adjusting the response of the adaptive filter by combining injected noise with the reference microphone signal so that the adaptively generating adapts to cancel the injected noise, whereby the response of the adaptive filter is reduced in frequency regions in a frequency range of the injected noise.

36. The method of claim 33, wherein a response of the adaptively generating adaptive filter is adjusted independent of the adaptively generating by:

filtering the injected noise with a duplicate response substantially identical to the response of the adaptive filter, whereby the duplicate response is controlled by the adaptively generating adapting to cancel a combination of the ambient audio sounds and the injected noise; and

controlling the response of the adaptive filter with coefficients adapted in the duplicate response, whereby the injected noise is not present in the anti-noise signal.

37. An integrated circuit for implementing at least a portion of a personal audio device, comprising:

an output for providing a signal to a transducer including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer;

a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds;

an error microphone input for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer; and

a processing circuit that implements an adaptive filter having a response that shapes the anti-noise signal to reduce the presence of the ambient audio sounds heard by the listener, wherein a response of the adaptive filter is adjusted independent of the adaptation of the adaptive filter to the ambient audio sounds, in order to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds.

38. The integrated circuit of claim 37, wherein the adaptive filter comprises:

a first fixed portion of the adaptive filter; and

a second adaptive portion of the adaptive filter, wherein the first fixed portion and the second adaptive portion operate together to generate the response that shapes the anti-noise signal, and wherein the second adaptive portion has a leakage characteristic that restores the response of the second adaptive portion to an initial response of the second adaptive portion over time.

39. The integrated circuit of claim 37, wherein the response of the adaptive filter is adjusted by combining injected noise with the reference microphone signal so that the response of the adaptive filter is controlled by the adaptive filter adapting to cancel the injected noise, whereby the response of the adaptive filter is reduced in frequency regions in a frequency range of the injected noise.

40. The integrated circuit of claim 37, wherein the response of the adaptive filter is adjusted independent of the adaptation of the adaptive filter by the processing circuit implementing a copy of the adaptive filter to receive the injected noise so that the response of the copy of the adaptive filter is controlled by the adaptive filter adapting to cancel a combination of the ambient audio sounds and the injected noise, and wherein the processing circuit further controls the response of the adaptive filter with the coefficients adapted in the copy of the adaptive filter, whereby the injected noise is not present in the anti-noise signal.

41. A personal audio device, comprising:

a personal audio device housing;

a transducer mounted on the housing for reproducing an audio signal including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer;

a reference microphone mounted on the housing for providing a reference microphone signal indicative of the ambient audio sounds;

an error microphone mounted on the housing in proximity to the transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer; and

a processing circuit that implements a filter having a fixed frequency response and an adjustable gain that adjusts a magnitude of the anti-noise signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit adjusts the gain of the filter to minimize the ambient audio sounds at the error microphone.

42. A method of canceling ambient audio sounds in the proximity of a transducer of a personal audio device, the method comprising:

first measuring ambient audio sounds with a reference microphone to produce a reference microphone signal;

second measuring an output of the transducer with an error microphone and the ambient audio sounds at the transducer;

adaptively generating an anti-noise signal from a result of the first measuring and the second measuring for countering the effects of ambient audio sounds at an acoustic output of the transducer by adjusting a gain of a filter that filters an output of the reference microphone;

combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer; and

providing a result of the combining to the transducer to generate the acoustic output.

43. An integrated circuit for implementing at least a portion of a personal audio device, comprising:

an output for providing a signal to a transducer including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer;

a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds;

an error microphone input for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer; and

a processing circuit that implements a filter having a fixed frequency response and an adjustable gain that adjusts a magnitude of the anti-noise signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit adjusts the gain of the filter to minimize the ambient audio sounds at the error microphone.