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An adaptive noise canceling (ANC) circuit adaptively generates an anti-noise signal that is injected into the speaker or other transducer output to cause cancellation of ambient audio sounds. At least one microphone provides an error signal indicative of the noise cancellation at the transducer, and

An adaptive noise canceling (ANC) circuit adaptively generates an anti-noise signal that is injected into the speaker or other transducer output to cause cancellation of ambient audio sounds. At least one microphone provides an error signal indicative of the noise cancellation at the transducer, and the adaptive filter is adapted to minimize the error signal. In order to prevent improper adaptation or instabilities in one or both of the adaptive filters, spikes are detected in the error signal by comparing the error signal or its rate of change to a threshold. Therefore, if the magnitude of the coefficient error is greater than a threshold value for an update, the update is skipped. Alternatively the step size of the updates may be reduced. Similar criteria can be applied to a filter modeling the secondary path, based on detection applied to both the source audio and the error signal.

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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 effects of ambient audio sounds in an acoustic output of the tran

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 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; anda processing circuit that adaptively generates the anti-noise signal from the reference microphone signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, 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 the error signal, and wherein the processing circuit adapts first coefficients of the first adaptive filter according to the reference microphone signal and the error signal and adapts second coefficients of the secondary path adaptive filter according to the error signal, and wherein if a magnitude of a value derived from the error microphone signal has a rate of change that exceeds a threshold value indicating a spike in the ambient audio sounds, the processing circuit alters adaptation of the first adaptive filter to reduce disruption in values of the coefficients caused by the spike in the ambient audio sounds. 2. The personal audio device of claim 1, wherein the processing circuit determines an average level of the ambient audio sounds from an average of the value derived from the error microphone signal, and determines the rate of change of the magnitude of the value derived from the error microphone signal from a difference between the average level of the value derived from the error microphone signal and an instantaneous value of the magnitude of the value derived from the error microphone signal. 3. The personal audio device of claim 1, wherein the processing circuit determines an average level of the ambient audio sounds from an average of a value derived from the reference microphone signal, and determines the rate of change of the magnitude of the value derived from the error microphone signal from a difference between the average level of the value derived from the reference microphone signal and an instantaneous value of the magnitude of the value derived from the error microphone signal. 4. The personal audio device of claim 3, wherein the processing circuit further implements a controllable filter controlled by a coefficient control of the secondary path adaptive filter that filters the reference microphone signal to apply a copy of the secondary path response to the reference microphone signal, wherein the processing circuit determines the average level of the ambient audio sounds from an average value of the output of the controllable filter. 5. The personal audio device of claim 1, wherein the processing circuit compares the magnitude of the value derived from the error microphone signal to the threshold value at each sample of the error microphone signal, wherein the processing circuit skips updates due to samples for which the magnitude of the value of derived from the error microphone signal exceeds the threshold value. 6. The personal audio device of claim 1, wherein the processing circuit alters adaptation of the first adaptive filter by freezing adaptation of the first coefficients of the first adaptive filter. 7. The personal audio device of claim 1, wherein the processing circuit alters adaptation of the first adaptive filter by reducing a step size of the first adaptive filter until the spike is absent from the value derived from the error microphone signal. 8. The personal audio device of claim 1, wherein the processing circuit implements a counter that sustains the altering of the adaptation of the first adaptive filter after the rate of change of the value derived from the error microphone signal is less than the threshold value for a number of samples equal to or greater than a filter length of the first adaptive filter. 9. The personal audio device of claim 1, wherein the processing circuit further alters adaptation of the secondary path adaptive filter in response to the magnitude of the value derived from the error microphone signal having a rate of change that exceeds the threshold value indicating the spike in the ambient audio sounds. 10. The personal audio device of claim 1, wherein the processing circuit further determines if the source audio signal has a rate of change that exceeds a second threshold value indicating a spike in the source audio, the processing circuit alters adaptation of the secondary path adaptive filter to reduce disruption in values of the second coefficients that control adaptation of the secondary path adaptive filter caused by the spike in the source audio. 11. The personal audio device of claim 10, wherein the processing circuit determines an average level of the source audio, and determines the rate of change of the source audio from a difference between the average level of the source audio and an instantaneous value of the magnitude of the value derived from the error microphone signal. 12. A method of countering effects of ambient audio sounds by a personal audio device, the method comprising: adaptively generating an anti-noise signal from a reference microphone signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error microphone signal and the reference microphone signal;combining the anti-noise signal with source audio;providing a result of the combining to a transducer;generating the reference microphone signal indicative of the ambient audio sounds with a reference microphone;generating the error microphone signal indicative of audio reproduced by the transducer the transducer and the ambient audio sounds with an error microphone;filtering the source audio with a secondary path adaptive filter having a secondary path response to produce filtered source audio;removing the filtered source audio from the error microphone signal to generate an error signal;adapting first coefficients of the first adaptive filter according to the reference microphone signal and the error signal;adapting second coefficients of the secondary path adaptive filter according to the error signal;detecting a spike in the ambient audio sounds by determining whether the magnitude of a value derived from the error microphone signal has a rate of change that exceeds a threshold value; andresponsive to the detecting having detected a spike, altering the adapting of the first coefficients and the second coefficients to reduce disruption in values of the coefficients caused by the spike. 13. The method of claim 12, further comprising: determining an average level of the ambient audio sounds from an average of the value derived from the error microphone signal; anddetermining the rate of change of the magnitude of the value derived from the error microphone signal from a difference between the average level of the value derived from the error microphone signal and an instantaneous value of the magnitude of the value derived from the error microphone signal. 14. The method of claim 12, further comprising: determining an average level of the ambient audio sounds from an average of a value derived from the reference microphone signal; anddetermining the rate of change of the magnitude of the value derived from the error microphone signal from a difference between the average level of the value derived from the reference microphone signal and an instantaneous value of the magnitude of the value derived from the error microphone signal. 15. The method of claim 14, further comprising: filtering the reference microphone signal with a controllable filter controlled by a coefficient control of the secondary path adaptive filter to apply a copy of the secondary path response to the reference microphone signal; anddetermining the average level of the ambient audio sounds from an average value of the output of the controllable filter. 16. The method of claim 12, further comprising: comparing the magnitude of the value derived from the error microphone signal to the threshold value at each sample of the error microphone signal; andthe adapting of the first coefficients of the first adaptive filter skipping updates due to samples for which the magnitude of the value of derived from the error microphone signal exceeds the threshold value. 17. The method of claim 12, further comprising altering adaptation of the first adaptive filter by freezing adaptation of the first coefficients of the first adaptive filter. 18. The method of claim 12, further comprising altering adaptation of the first adaptive filter by reducing a step size of the adapting of the first coefficients of the first adaptive filter until the spike is absent from the value derived from the error microphone signal. 19. The method of claim 12, further comprising implementing a counter that sustains the altering of the adapting of the first coefficients of the first adaptive filter after the rate of change of the value derived from the error microphone signal is less than the threshold value for a number of samples equal to or greater than a filter length of the first adaptive filter. 20. The method of claim 12, further comprising altering the adapting of the second coefficients of the secondary path adaptive filter in response to the magnitude of the value derived from the error microphone signal having a rate of change that exceeds the threshold value indicating the spike in the ambient audio sounds. 21. The method of claim 12, further comprising: determining if the source audio signal has a rate of change that exceeds a second threshold value indicating a spike in the source audio; andaltering the adapting of the second coefficients of the secondary path adaptive filter to reduce disruption in values of the second coefficients caused by the spike in the source audio. 22. The method of claim 21, further comprising: determining an average level of the source audio; anddetermining the rate of change of the source audio from a difference between the average level of the source audio and an instantaneous value of the magnitude of the value derived from the error microphone signal. 23. An integrated circuit for integration within a personal audio device, comprising: an output for providing an output signal to an output transducer including both source audio for playback to a listener and an anti-noise signal for countering 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 acoustic output of the transducer and the ambient audio sounds at the transducer; anda processing circuit that adaptively generates the anti-noise signal from the reference signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, 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 the error signal, and wherein the processing circuit adapts first coefficients of the first adaptive filter according to the reference microphone signal and the error signal and adapts second coefficients of the secondary path adaptive filter according to the error signal, and wherein if a magnitude of a value derived from the error microphone signal has a rate of change that exceeds a threshold value indicating a spike in the ambient audio sounds, the processing circuit alters adaptation of the first adaptive filter to reduce disruption in values of the coefficients caused by the spike in the ambient audio sounds. 24. The integrated circuit of claim 23, wherein the processing circuit determines an average level of the ambient audio sounds from an average of the value derived from the error microphone signal, and determines the rate of change of the magnitude of the value derived from the error microphone signal from a difference between the average level of the value derived from the error microphone signal and an instantaneous value of the magnitude of the value derived from the error microphone signal. 25. The integrated circuit of claim 23, wherein the processing circuit determines an average level of the ambient audio sounds from an average of a value derived from the reference microphone signal, and determines the rate of change of the magnitude of the value derived from the error microphone signal from a difference between the average level of the value derived from the reference microphone signal and an instantaneous value of the magnitude of the value derived from the error microphone signal. 26. The integrated circuit of claim 25, wherein the processing circuit further implements a controllable filter controlled by a coefficient control of the secondary path adaptive filter that filters the reference microphone signal to apply a copy of the secondary path response to the reference microphone signal, wherein the processing circuit determines the average level of the ambient audio sounds from an average value of the output of the controllable filter. 27. The integrated circuit of claim 23, wherein the processing circuit compares the magnitude of the value derived from the error microphone signal to the threshold value at each sample of the error microphone signal, wherein the processing circuit skips updates due to samples for which the magnitude of the value of derived from the error microphone signal exceeds the threshold value. 28. The integrated circuit of claim 23, wherein the processing circuit alters adaptation of the first adaptive filter by freezing adaptation of the first coefficients of the first adaptive filter. 29. The integrated circuit of claim 23, wherein the processing circuit alters adaptation of the first adaptive filter by reducing a step size of the first adaptive filter until the spike is absent from the value derived from the error microphone signal. 30. The integrated circuit of claim 23, wherein the processing circuit implements a counter that sustains the altering of the adaptation of the first adaptive filter after the rate of change of the value derived from the error microphone signal is less than the threshold value for a number of samples equal to or greater than a filter length of the first adaptive filter. 31. The integrated circuit of claim 23, wherein the processing circuit further alters adaptation of the secondary path adaptive filter in response to the magnitude of the value derived from the error microphone signal having a rate of change that exceeds the threshold value indicating the spike in the ambient audio sounds. 32. The integrated circuit of claim 23, wherein the processing circuit further determines if the source audio signal has a rate of change that exceeds a second threshold value indicating a spike in the source audio, the processing circuit alters adaptation of the secondary path adaptive filter to reduce disruption in values of the second coefficients that control adaptation of the secondary path adaptive filter caused by the spike in the source audio. 33. The integrated circuit of claim 32, wherein the processing circuit determines an average level of the source audio, and determines the rate of change of the source audio from a difference between the average level of the source audio and an instantaneous value of the magnitude of the value derived from the error microphone signal. 34. 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 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; anda processing circuit that adaptively generates the anti-noise signal from the reference signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, 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 the error signal, wherein the processing circuit further implements a copy of the secondary path adaptive filter that filters the reference microphone signal to produce a secondary-path-compensated reference microphone signal, and wherein the processing circuit adapts coefficients of the first adaptive filter according to the secondary-path-compensated reference microphone signal and the error signal, and wherein if a magnitude of the secondary-path-compensated reference microphone signal has a rate of change that exceeds a threshold value indicating a spike in the ambient audio sounds, the processing circuit alters adaptation of the first adaptive filter to reduce disruption in values of the coefficients caused by the spike in the ambient audio sounds. 35. The personal audio device of claim 34, wherein the processing circuit determines an average level of the ambient audio sounds from an average of the secondary-path-compensated reference microphone signal, and determines the rate of change of the magnitude of the secondary-path-compensated reference microphone signal from a difference between the average level of the secondary-path-compensated reference microphone signal and an instantaneous value of the magnitude of the secondary-path-compensated reference microphone signal. 36. A method of countering effects of ambient audio sounds by a personal audio device, the method comprising: generating a reference microphone signal indicative of the ambient audio sounds with a reference microphone;generating an error microphone signal indicative of the ambient audio sounds and audio reproduced by the transducer with an error microphone;adaptively generating an anti-noise signal from the reference microphone signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by a listener in conformity with the error microphone signal and the reference microphone signal;combining the anti-noise signal with source audio;providing a result of the combining to a transducer;filtering source audio with a secondary path adaptive filter having a secondary path response that shapes the source audio to produce filtered source audio;removing the filtered source audio from the error microphone signal to generate the error signal;further implementing a copy of the secondary path adaptive filter that filters the reference microphone signal to produce a secondary-path-compensated reference microphone signal;adapting coefficients of the first adaptive filter according to the secondary-path-compensated reference microphone signal and the error signal; andaltering adaptation of the first adaptive filter to reduce disruption in values of the coefficients caused by the spike in the ambient audio sounds if a magnitude of the secondary-path-compensated reference microphone signal has a rate of change that exceeds a threshold value indicating a spike in the ambient audio sounds. 37. The method of claim 36, further comprising: determining an average level of the ambient audio sounds from an average of the secondary-path-compensated reference microphone signal; anddetermining the rate of change of the magnitude of the secondary-path-compensated reference microphone signal from a difference between the average level of the secondary-path-compensated reference microphone signal and an instantaneous value of the magnitude of the secondary-path-compensated reference microphone signal. 38. An integrated circuit for implementing at least a portion of a personal audio device, comprising: an output for providing an output signal to an output transducer including both source audio for playback to a listener and an anti-noise signal for countering 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 acoustic output of the transducer and the ambient audio sounds at the transducer; anda processing circuit that adaptively generates the anti-noise signal from the reference signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, 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 the error signal, wherein the processing circuit further implements a copy of the secondary path adaptive filter that filters the reference microphone signal to produce a secondary-path-compensated reference microphone signal, and wherein the processing circuit adapts coefficients of the first adaptive filter according to the secondary-path-compensated reference microphone signal and the error signal, and wherein if a magnitude of the secondary-path-compensated reference microphone signal has a rate of change that exceeds a threshold value indicating a spike in the ambient audio sounds, the processing circuit alters adaptation of the first adaptive filter to reduce disruption in values of the coefficients caused by the spike in the ambient audio sounds. 39. The integrated circuit of claim 38, wherein the processing circuit determines an average level of the ambient audio sounds from an average of the secondary-path-compensated reference microphone signal, and determines the rate of change of the magnitude of the secondary-path-compensated reference microphone signal from a difference between the average level of the secondary-path-compensated reference microphone signal and an instantaneous value of the magnitude of the secondary-path-compensated reference microphone signal. 40. 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 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; anda processing circuit that adaptively generates the anti-noise signal from the reference signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, 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 the error signal, wherein the processing circuit adapts first coefficients of the first adaptive filter according to the reference microphone signal and the error signal, wherein the processing circuit adapts second coefficients of the secondary path adaptive filter according to the source audio and the error signal, wherein if a magnitude of the source audio has a rate of change that exceeds a threshold value indicating a spike in the source audio, the processing circuit alters adaptation of the secondary path adaptive filter to reduce disruption in values of the second coefficients caused by the spike in the source audio. 41. A method of countering effects of ambient audio sounds by a personal audio device, the method comprising: adaptively generating an anti-noise signal from a reference signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal;combining the anti-noise signal with source audio;providing a result of the combining to a transducer;generating the reference microphone indicative of the ambient audio sounds with a reference microphone;generating the error microphone signal indicative of audio reproduced by the transducer and the ambient audio sounds with an error microphone;filtering the source audio with a secondary path adaptive filter having a secondary path response that shapes the source audio to generate filtered source audio;removing the filtered source audio from the error microphone signal to provide the error signal;adapting first coefficients of the first adaptive filter according to the reference microphone signal and the error signal;adapting second coefficients of the secondary path adaptive filter according to the source audio and the error signal; andaltering adaptation of the secondary path adaptive filter to reduce disruption in values of the second coefficients caused by the spike in the source audio if a magnitude of the source audio has a rate of change that exceeds a threshold value indicating a spike in the source audio. 42. An integrated circuit for integration within a personal audio device, comprising: an output for providing an output signal to an output transducer including both source audio for playback to a listener and an anti-noise signal for countering 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 acoustic output of the transducer and the ambient audio sounds at the transducer; anda processing circuit that adaptively generates the anti-noise signal from the reference signal by adapting a first adaptive filter to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, 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 the error signal, wherein the processing circuit adapts first coefficients of the first adaptive filter according to the reference microphone signal and the error signal, wherein the processing circuit adapts second coefficients of the secondary path adaptive filter according to the source audio and the error signal, wherein if a magnitude of the source audio has a rate of change that exceeds a threshold value indicating a spike in the source audio, the processing circuit alters adaptation of the secondary path adaptive filter to reduce disruption in values of the second coefficients caused by the spike in the source audio.