# Patent

## Title
Systems and methods for adaptive noise cancellation by adaptively shaping internal white noise to train a secondary path

## Abstract

A processing circuit may include: (i) an adaptive filter having a response that generates an anti-noise signal from a reference microphone signal, wherein the response is shaped in conformity with the reference microphone signal and a playback corrected error, and wherein the playback corrected error is based on a difference between an error microphone signal and a secondary path estimate; (ii) a secondary path estimate filter configured to model an electro-acoustic path of a source audio signal and having a response that generates a secondary path estimate from the source audio signal; (iii) a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error; and (iv) a noise injection portion for injecting a noise signal into the source audio signal, wherein the noise signal is shaped based on the playback corrected error.

## Background
FIELD OF DISCLOSURE
The present disclosure relates in general to adaptive noise cancellation in connection with an acoustic transducer, and more particularly, to detection and cancellation of ambient noise present in the vicinity of the acoustic transducer, including biasing an anti-noise level for anti-noise generated by adaptive noise cancellation.
BACKGROUND
Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events. Because the acoustic environment around personal audio devices such as wireless telephones can change dramatically, depending on the sources of noise that are present and the position of the device itself, it is desirable to adapt the noise canceling to take into account such environmental changes.
A typical adaptive noise cancellation (ANC) system may include a reference microphone for providing a reference microphone signal indicative of ambient audio sounds proximate to a personal audio device and an error microphone in proximity to a transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer. The typical ANC system may further include an adaptive feedforward filter that generates an anti-noise signal from the reference microphone signal to counter 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 to minimize the ambient audio sounds in the error microphone signal based on a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and a secondary path estimate. In addition, the typical ANC system may include an adaptive secondary path estimate filter for modeling an electro-acoustic path of the source audio signal that generates a secondary path estimate from a source audio signal by adapting the response of the secondary path estimate adaptive filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate. The typical ANC system may combine the anti-noise signal with the source audio signal to generate an audio signal provided to the transducer.
Such an ANC system requires the source audio signal in order to properly adapt or “train” the response of the secondary path estimate filter. However, a disadvantage of training with a source audio signal may be that such signals may not have the persistence or spectral density required to effectively train the response of the secondary path estimate filter, in that a source audio signal may have silent intervals or may lack content in particular ranges of frequencies. Such disadvantage may particularly be present in stereo playback modes, as each channel of the stereo signal may convey only a portion of the source audio signal.
SUMMARY
In accordance with the teachings of the present disclosure, the disadvantages and problems associated with existing approaches to adaptive noise cancellation may be reduced or eliminated.
In accordance with embodiments of the present disclosure, a personal audio device may include a personal audio device housing, a transducer, a reference microphone, an error microphone, and a processing circuit. The transducer may be coupled to the housing for reproducing an audio signal including both a source audio signal 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. The reference microphone may be coupled to the housing for providing a reference microphone signal indicative of the ambient audio sounds. The error microphone may be coupled to 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. The processing circuit may include an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, wherein the response is shaped in conformity with the reference microphone signal and a playback corrected error, and wherein the playback corrected error is based on a difference between the error microphone signal and a secondary path estimate. The processing circuit may also include a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates a secondary path estimate from the source audio signal, a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, and a noise injection portion for injecting a noise signal into the source audio signal, wherein the noise signal is shaped based on the playback corrected error.
In accordance with these and other embodiments of the present disclosure, a method for canceling ambient audio sounds in the proximity of a transducer of a personal audio device may include receiving a reference microphone signal indicative of the ambient audio sounds. The method may also include receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The method may further include generating a source audio signal for playback to a listener. The method may additionally include generating an anti-noise signal, from a result of the measuring with the reference microphone, 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 to minimize the ambient audio sounds in the error microphone signal. The method may also include adaptively generating a secondary path estimate, from a source audio signal, by filtering the source audio signal with a secondary path estimate adaptive filter configured to model an electro-acoustic path of the source audio signal and adapting the response of the secondary path estimate adaptive filter to minimize a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate. The method may further include injecting a noise signal into the source audio signal, wherein the noise signal is shaped based on the playback corrected error. The method may additionally include combining the anti-noise signal with the source audio signal to generate an audio signal provided to the transducer.
In accordance with these and other embodiments of the present disclosure, an integrated circuit for implementing at least a portion of a personal audio device may include an output, a reference microphone input, an error microphone input, and a processing circuit. The output may be for providing a signal to a transducer including both a source audio signal for playback to a listener and an anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the transducer. The reference microphone input may be for receiving a reference microphone signal indicative of the ambient audio sounds. The error microphone input may be for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The processing circuit may include an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, wherein the response is shaped in conformity with the reference microphone signal and a playback corrected error, and wherein the playback corrected error is based on a difference between the error microphone signal and a secondary path estimate. The processing circuit may also include a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates a secondary path estimate from the source audio signal, a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, and a noise injection portion for injecting a noise signal into the source audio signal, wherein the noise signal is shaped based on the playback corrected error.
Technical advantages of the present disclosure may be readily apparent to one skilled in the art from the figures, description and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are not restrictive of the claims set forth in this disclosure.



BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
FIG. 1 is an illustration of an example wireless mobile telephone, in accordance with embodiments of the present disclosure;
FIG. 2 is a block diagram of selected circuits within the wireless telephone depicted in FIG. 1, in accordance with embodiments of the present disclosure; and
FIG. 3 is a block diagram depicting selected signal processing circuits and functional blocks within an example adaptive noise canceling (ANC) circuit of a coder-decoder (CODEC) integrated circuit of FIG. 2, in accordance with embodiments of the present disclosure.



DETAILED DESCRIPTION
The present disclosure encompasses noise canceling techniques and circuits that can be implemented in a personal audio device, such as a wireless telephone. The personal audio device includes an ANC circuit that may measure the ambient acoustic environment and generate a signal that is injected in the speaker (or other transducer) output to cancel ambient acoustic events. A reference microphone may be provided to measure the ambient acoustic environment and an error microphone may be included for controlling the adaptation of the anti-noise signal to cancel the ambient audio sounds and for correcting for the electro-acoustic path from the output of the processing circuit through the transducer.
Referring now to FIG. 1, a wireless telephone 10 as illustrated in accordance with embodiments of the present disclosure is shown in proximity to a human ear 5. Wireless telephone 10 is an example of a device in which techniques in accordance with embodiments of the invention may be employed, but it is understood that not all of the elements or configurations embodied in illustrated wireless telephone 10, or in the circuits depicted in subsequent illustrations, are required in order to practice the invention recited in the claims. Wireless telephone 10 may include a transducer such as speaker SPKR that reproduces distant speech received by wireless telephone 10, along with other local audio events such as ringtones, stored audio program material, injection of near-end speech (i.e., the speech of the user of wireless telephone 10) to provide a balanced conversational perception, and other audio that requires reproduction by wireless telephone 10, such as sources from webpages or other network communications received by wireless telephone 10 and audio indications such as a low battery indication and other system event notifications. A near-speech microphone NS may be provided to capture near-end speech, which is transmitted from wireless telephone 10 to the other conversation participant(s).
Wireless telephone 10 may include ANC circuits and features that inject an anti-noise signal into speaker SPKR to improve intelligibility of the distant speech and other audio reproduced by speaker SPKR. A reference microphone R may be provided for measuring the ambient acoustic environment, and may be positioned away from the typical position of a user's mouth, so that the near-end speech may be minimized in the signal produced by reference microphone R. Another microphone, error microphone E, may be provided in order to further improve the ANC operation by providing a measure of the ambient audio combined with the audio reproduced by speaker SPKR close to ear 5, when wireless telephone 10 is in close proximity to ear 5. Circuit 14 within wireless telephone 10 may include an audio CODEC integrated circuit (IC) 20 that receives the signals from reference microphone R, near-speech microphone NS, and error microphone E, and interfaces with other integrated circuits such as a radio-frequency (RF) integrated circuit 12 having a wireless telephone transceiver. In some embodiments of the disclosure, the circuits and techniques disclosed herein may be incorporated in a single integrated circuit that includes control circuits and other functionality for implementing the entirety of the personal audio device, such as an MP3 player-on-a-chip integrated circuit. In these and other embodiments, the circuits and techniques disclosed herein may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller or other processing device.
In general, ANC techniques of the present disclosure measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and by also measuring the same ambient acoustic events impinging on error microphone E, ANC processing circuits of wireless telephone 10 adapt an anti-noise signal generated at the output of speaker SPKR from the output of reference microphone R to have a characteristic that minimizes the amplitude of the ambient acoustic events at error microphone E. Because acoustic path P(z) extends from reference microphone R to error microphone E, ANC circuits are effectively estimating acoustic path P(z) while removing effects of an electro-acoustic path S(z) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR including the coupling between speaker SPKR and error microphone E in the particular acoustic environment, which may be affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, when wireless telephone 10 is not firmly pressed to ear 5. While the illustrated wireless telephone 10 includes a two-microphone ANC system with a third near-speech microphone NS, some aspects of the present invention may be practiced in a system that does not include separate error and reference microphones, or a wireless telephone that uses near-speech microphone NS to perform the function of the reference microphone R. Also, in personal audio devices designed only for audio playback, near-speech microphone NS will generally not be included, and the near-speech signal paths in the circuits described in further detail below may be omitted, without changing the scope of the disclosure, other than to limit the options provided for input to the microphone covering detection schemes. In addition, although only one reference microphone R and one error microphone E is depicted in FIG. 1, the circuits and techniques herein disclosed may be adapted, without changing the scope of the disclosure, to personal audio devices including a plurality of reference microphones and/or error microphones.
Referring now to FIG. 2, selected circuits within wireless telephone 10 are shown in a block diagram. CODEC IC 20 may include an analog-to-digital converter (ADC) 21A for receiving the reference microphone signal and generating a digital representation ref of the reference microphone signal, an ADC 21B for receiving the error microphone signal and generating a digital representation err of the error microphone signal, and an ADC 21C for receiving the near speech microphone signal and generating a digital representation ns of the near speech microphone signal. CODEC IC 20 may generate an output for driving speaker SPKR from an amplifier A1, which may amplify the output of a digital-to-analog converter (DAC) 23 that receives the output of a combiner 26. Combiner 26 may combine audio signals is from internal audio sources 24, the anti-noise signal generated by ANC circuit 30, which by convention has the same polarity as the noise in reference microphone signal ref and is therefore subtracted by combiner 26, the injected noise from ANC circuit 30, and a portion of near speech microphone signal ns so that the user of wireless telephone 10 may hear his or her own voice in proper relation to downlink speech ds, which may be received from radio frequency (RF) integrated circuit 22 and may also be combined by combiner 26. Near speech microphone signal ns may also be provided to RF integrated circuit 22 and may be transmitted as uplink speech to the service provider via antenna ANT.
Referring now to FIG. 3, details of ANC circuit 30 are shown in accordance with embodiments of the present disclosure. Adaptive filter 32 may receive reference microphone signal ref and under ideal circumstances, may adapt its transfer function W(z) to be P(z)/S(z) to generate the anti-noise signal, which may be provided to an output combiner that combines the anti-noise signal with the audio to be reproduced by the transducer, as exemplified by combiner 26 of FIG. 2. The coefficients of adaptive filter 32 may be controlled by a W coefficient control block 31 that uses a correlation of signals to determine the response of adaptive filter 32, which generally minimizes the error, in a least-mean squares sense, between those components of reference microphone signal ref present in error microphone signal err. The signals compared by W coefficient control block 31 may be the reference microphone signal ref as shaped by a copy of an estimate of the response of path S(z) provided by filter 34B and a playback corrected error, labeled as “PBCE” in FIG. 3, based at least in part on error microphone signal err. The playback corrected error may be generated as described in greater detail below.
By transforming reference microphone signal ref with a copy of the estimate of the response of path S(z), response SECOPY(z) of filter 34B, and minimizing the difference between the resultant signal and error microphone signal err, adaptive filter 32 may adapt to the desired response of P(z)/S(z). In addition to error microphone signal err, the signal compared to the output of filter 34B by W coefficient control block 31 may include an inverted amount of modified source audio signal (e.g., downlink audio signal ds and/or internal audio signal ia, which may be combined with a noise signal generated by noise injection portion 40), that has been processed by filter response SE(z), of which response SECOPY(z) is a copy. By injecting an inverted amount of source audio signal, adaptive filter 32 may be prevented from adapting to the relatively large amount of source audio signal present in error microphone signal err. However, by transforming that inverted copy of source audio signal with the estimate of the response of path S(z), the source audio that is removed from error microphone signal err should match the expected version of the source audio signal reproduced at error microphone signal err, because the electrical and acoustical path of S(z) is the path taken by the source audio signal to arrive at error microphone E. Filter 34B may not be an adaptive filter, per se, but may have an adjustable response that is tuned to match the response of adaptive filter 34A, so that the response of filter 34B tracks the adapting of adaptive filter 34A.
To implement the above, adaptive filter 34A may have coefficients controlled by SE coefficient control block 33, which may compare a modified source audio signal and a playback corrected error. The modified source audio signal may include the source audio signal (e.g., downlink audio signal ds and/or internal audio signal ia) with injected noise generated by noise injection portion 40 and combined with the sum source audio signal by combiner 42. The playback corrected error may be equal to error microphone signal err after removal of the source audio signal (as filtered by adaptive filter 34A to represent the expected playback audio delivered to error microphone E) by a combiner 36. SE coefficient control block 33 may correlate the actual modified source audio signal with the components of the modified source audio signal that are present in error microphone signal err. Adaptive filter 34A may thereby be adapted to generate a secondary estimate signal from the modified source audio signal, that when subtracted from error microphone signal err to generate the playback corrected error, includes the content of error microphone signal err that is not due to the modified source audio signal.
As shown in FIG. 2, ANC circuit 30 may also generate an injected noise signal that may be combined with near-speech signal ns, the source audio signal, and the anti-noise (e.g., by combiner 26). Generation of such injected noise signal is discussed in greater detail with respect to FIG. 3, below.
As shown in FIG. 3, the noise signal combined with the source audio signal may be generated by noise injection portion 40. Noise injection portion 40 may include a white noise source 44 for generating white noise (e.g., an audio signal with a constant amplitude across all frequencies of interest, such as those frequencies within the range of human hearing). A frequency shaping filter 46 may generate the noise signal by filtering the white noise signal, wherein a response of the frequency shaping filter is shaped by frequency shaping filter coefficient control block 48 in conformity with the playback corrected error. In some embodiments, coefficient control block 48 implements an adaptive linear prediction coefficient system which estimates a frequency spectrum of the playback corrected error. Accordingly, the noise signal generated by frequency shaping filter 46 may comprise the white noise signal filtered such that the white noise signal is attenuated or eliminated in those frequencies within the frequency spectrum of the playback corrected error.
In some embodiments, noise injection portion 40 may include a gain element 50 configured to attenuate the noise signal to an amplitude substantially below an amplitude of the error microphone signal err such that the noise signal is substantially imperceptible to the listener. In these and other embodiments, noise injection portion 40 may include an inverse secondary path estimate filter 52 having a response inverse to the response of the secondary path estimate filter, wherein inverse secondary path estimate filter 52 applies its response to the noise signal before injection of the noise signal into the source audio signal, in order to undo the effect on the noise signal of the secondary path S(z) and the secondary path estimate SE(z) of filter 34A.
As a result, the modified source audio signal including the noise signal may have spectral content in all frequency ranges of interest due to the injected noise signal, but the injected noise signal may be of an intensity low enough such that it is not perceptible to a listener, thus providing a broadband signal which SE coefficient control block 33 may use to adapt the response SE(z), while minimally affecting listener experience.
Although the embodiments shown in FIG. 2 and FIG. 3 contemplate that source audio signal, injected noise signal, and anti-noise signal are combined at combiner 26, in some embodiments, the modified source audio signal (including the combination of the source audio signal and injected noise) may instead be combined with the anti-noise signal at combiner 26.
This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present inventions have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.

## Claims

1. A personal audio device comprising:
a personal audio device housing;
a transducer coupled to the housing for reproducing an audio signal including both a source audio signal 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 coupled to the housing for providing a reference microphone signal indicative of the ambient audio sounds;
an error microphone coupled to 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 comprising:
an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, wherein the response is shaped in conformity with the reference microphone signal and a playback corrected error, and wherein the playback corrected error is based on a difference between the error microphone signal and a secondary path estimate;
a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates the secondary path estimate from the source audio signal;
a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error; and
a noise injection portion for injecting a noise signal into the source audio signal, wherein the noise signal is shaped based on the playback corrected error, wherein the noise injection portion comprises:
a noise source;
a frequency shaping filter having a response that generates the noise signal from the noise source and shapes the noise signal in conformity with the playback corrected error; and
a filter that shapes the noise signal in conformity with at least one parameter of the secondary path estimate filter in order to reduce audibility of the noise signal in the audio signal reproduced by the transducer; and
wherein a response of the noise injection portion includes a response that is an inverse of at least a portion of the response of the secondary path estimate filter.

2. The personal audio device of claim 1, wherein the noise injection portion comprises a gain element configured to attenuate the noise signal to an amplitude substantially below an amplitude of the error microphone signal such that the noise signal is substantially imperceptible to the listener.

3. The personal audio device of claim 1, wherein the noise injection portion comprises an inverse secondary path estimate filter having a response inverse to the response of the secondary path estimate filter, wherein the inverse secondary path estimate filter applies its response to the noise signal before injection of the noise signal into the source audio signal.

4. The personal audio device of claim 1, wherein the noise injection portion comprises:
a gain element configured to attenuate the noise signal to an amplitude substantially below an amplitude of the error microphone signal such that the noise signal is substantially imperceptible to the listener; and
an inverse secondary path estimate filter having a response inverse to the response of the secondary path estimate filter, wherein the inverse secondary path estimate filter applies its response to the noise signal before injection of the noise signal into the source audio signal.

5. The personal audio device of claim 1, wherein the coefficient control block is further configured to analyze the error signal to determine frequency content of the error microphone signal and adaptively control a frequency response of the noise shaping filter in conformity with frequency content of the error signal.

6. The personal audio device of claim 5, wherein the at least one parameter comprises parameters determinative of the response of the secondary path estimate filter.

7. The personal audio device of claim 5, wherein a gain of a response of the noise injection portion is set in conformity with an inverse of a magnitude of the response of the secondary path estimate filter over at least a portion of the response of the secondary path estimate filter.

8. A method for canceling ambient audio sounds in the proximity of a transducer of a personal audio device, the method comprising:
receiving a reference microphone signal indicative of the ambient audio sounds;
receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer;
generating a source audio signal for playback to a listener;
generating an anti-noise signal, from the reference microphone signal, 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 to minimize the ambient audio sounds in the error microphone signal;
adaptively generating a secondary path estimate, from the source audio signal, by filtering the source audio signal with a secondary path estimate adaptive filter configured to model an electro-acoustic path of the source audio signal and adapting the response of the secondary path estimate adaptive filter to minimize a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate;
injecting a noise signal into the source audio signal with a noise injection portion, wherein the noise signal is generated by filtering an output of a noise source with a frequency shaping filter in conformity with the playback corrected error and filtering the noise signal in conformity with at least one parameter of the secondary path estimate filter by filtering the noise signal with a response that is an inverse of at least a portion of the response of the secondary path estimate filter in order to reduce audibility of the noise signal in the audio signal reproduced by the transducer; and
combining the anti-noise signal with the source audio signal to generate an audio signal provided to the transducer.

9. The method of claim 8, further comprising attenuating the noise signal to an amplitude substantially below an amplitude of the error microphone signal such that the noise signal is substantially imperceptible to the listener.

10. The method of claim 8, further comprising applying a response of an inverse secondary path estimate filter response to the noise signal before injection of the noise signal into the source audio signal, wherein the inverse secondary path estimate filter response is inverse to the response of the secondary path estimate filter.

11. The method of claim 8, further comprising:
attenuating the noise signal to an amplitude substantially below an amplitude of the error microphone signal such that the noise signal is substantially imperceptible to the listener; and
applying a response of an inverse secondary path estimate filter response to the noise signal before injection of the noise signal into the source audio signal, wherein the inverse secondary path estimate filter response is inverse to the response of the secondary path estimate filter.

12. The method of claim 8, further comprising analyzing the error signal to determine frequency content of the error microphone signal and adaptively controlling a frequency response of the noise shaping filter in conformity with frequency content of the error signal.

13. The method of claim 12, wherein the at least one parameter comprises parameters determinative of the response of the secondary path estimate filter.

14. The method of claim 12, wherein a gain of a response of the noise injection portion is set in conformity with an inverse of a magnitude of the response of the secondary path estimate filter over at least a portion of the response of the secondary path estimate filter.

15. 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 a source audio signal for playback to a listener and an anti-noise signal for countering the effect 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 comprising:
an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, wherein the response is shaped in conformity with the reference microphone signal and a playback corrected error, and wherein the playback corrected error is based on a difference between the error microphone signal and a secondary path estimate;
a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates the secondary path estimate from the source audio signal;
a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error; and
a noise injection portion for injecting a noise signal into the source audio signal, wherein the noise signal is shaped based on the playback corrected error, wherein the noise injection portion comprises:
a noise source;
a frequency shaping filter having a response that generates the noise signal from the noise source and shapes the noise signal in conformity with the playback corrected error; and
a filter that shapes the noise signal in conformity with at least one parameter of the secondary path estimate filter in order to reduce audibility of the noise signal in the audio signal reproduced by the transducer; and


wherein a response of the noise injection portion includes a response that is an inverse of at least a portion of the response of the secondary path estimate filter.

16. The integrated circuit of claim 15, wherein the noise injection portion comprises a gain element configured to attenuate the noise signal to an amplitude substantially below an amplitude of the error microphone signal such that the noise signal is substantially imperceptible to the listener.

17. The integrated circuit of claim 15, wherein the noise injection portion comprises an inverse secondary path estimate filter having a response inverse to the response of the secondary path estimate filter, wherein the inverse secondary path estimate filter applies its response to the noise signal before injection of the noise signal into the source audio signal.

18. The integrated circuit of claim 15, wherein the noise injection portion comprises:
a gain element configured to attenuate the noise signal to an amplitude substantially below an amplitude of the error microphone signal such that the noise signal is substantially imperceptible to the listener; and
an inverse secondary path estimate filter having a response inverse to the response of the secondary path estimate filter, wherein the inverse secondary path estimate filter applies its response to the noise signal before injection of the noise signal into the source audio signal.

19. The integrated circuit of claim 15, wherein the coefficient control block is further configured to analyze the error signal to determine frequency content of the error microphone signal and adaptively control a frequency response of the noise shaping filter in conformity with frequency content of the error signal.

20. The integrated circuit of claim 19, wherein the at least one parameter comprises parameters determinative of the response of the secondary path estimate filter.

21. The integrated circuit of claim 19, wherein a gain of a response of the noise injection portion is set in conformity with an inverse of a magnitude of the response of the secondary path estimate filter over at least a portion of the response of the secondary path estimate filter.