# Patent

## Title
Systems and methods for adaptive noise cancellation including secondary path estimate monitoring

## Abstract

In accordance with methods and systems of the present disclosure, a processing circuit may implement at least one of: a feedback filter having a response that generates at least a portion of an anti-noise component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate; and a feedforward filter having a response that generates at least a portion of the anti-noise signal from a reference microphone signal. The processing circuit may also implement a secondary path estimate filter configured to model an electro-acoustic path of a source audio signal and have a response that generates a secondary path estimate from the source audio signal and a secondary path estimate performance monitor for monitoring performance of the secondary path estimate filter in modeling the electro-acoustic path.

## Background
RELATED APPLICATION
The present disclosure claims priority to U.S. Provisional Patent Application Ser. No. 61/812,384, filed Apr. 16, 2013, which is incorporated by reference herein in its entirety.
The present disclosure claims priority to U.S. Provisional Patent Application Ser. No. 61/813,426, filed Apr. 18, 2013, which is incorporated by reference herein in its entirety.
The present disclosure also claims priority to U.S. Provisional Patent Application Ser. No. 61/818,150, filed May 1, 2013, which is incorporated by reference herein in its entirety.


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 using both feedforward and feedback adaptive noise cancellation techniques and including monitoring of a secondary path estimate adaptive filter for modeling an electro-acoustic path for the acoustic transducer.
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.
In a traditional hybrid adaptive noise cancellation system that includes both feedforward anti-noise and feedback anti-noise, an error microphone is used to generate an error microphone signal that measures a combined acoustic pressure at an acoustic transducer (e.g., loudspeaker) including playback of a source audio signal and ambient sounds. The error microphone signal is used to generate feedback anti-noise as well as adapt a feedforward adaptive filter for generating feedforward anti-noise from a reference microphone signal configured to measure ambient sounds.
In generating the feedback anti-noise, it is critical that the feedback noise cancelling system cancel only ambient noise at the error microphone, but not the playback signal. Accordingly, a feedback adaptive noise cancellation system will often generate a playback corrected error signal equal to the error microphone signal that is typically reduced by a filtered version of the source audio signal, wherein the filter estimates the secondary path, which is the electro-acoustic path of the source audio signal through an acoustic transducer. If modeled correctly, the playback corrected error signal will be approximately equal to the ambient noise level present at the acoustic transducer.
In traditional approaches, the secondary path is estimated using offline testing and characterization, on the assumption that the secondary path does not significantly change from user to user. However, in actual application, the acoustic environment around an audio device can change dramatically, depending on the sources of noise that are present, the position of the device itself, and the physical characteristics of the user, and it may be desirable to adapt noise cancellation to take into account such environmental changes.
SUMMARY
In accordance with the teachings of the present disclosure, the disadvantages and problems associated with detection and reduction of ambient noise associated with an acoustic transducer 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 implement a feedback filter having a response that generates a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, and wherein the anti-noise signal comprises at least the feedback anti-noise signal component, 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, and a secondary coefficient control block that shapes the response of the secondary path estimate adaptive filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate adaptive filter to minimize 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 a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, countering the effects of ambient audio sounds at an acoustic output of the transducer, wherein an anti-noise signal comprises at least the feedback anti-noise signal component. The method may also include adaptively generating the secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate adaptive filter modeling an electro-acoustic path of the source audio signal and adapting the response of the secondary path estimate adaptive filter to minimize the playback corrected error. The method may further 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 implement a feedback filter having a response that generates a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, and wherein the anti-noise signal comprises at least the feedback anti-noise signal component, 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, and a secondary coefficient control block that shapes the response of the secondary path estimate adaptive filter in conformity with the source audio signal and the playback corrected error by adapting the response of the secondary path estimate adaptive filter to minimize the playback corrected error.
In accordance with these and other embodiments of the present disclosure, a personal audio device may include a personal audio device housing, a transducer, 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 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 implement a feedback filter having a response that generates a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, and wherein the anti-noise signal comprises at least the feedback anti-noise signal component; 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; and a programmable feedback gain, wherein an increasing programmable feedback gain increases the feedback anti-noise signal component and a decreasing programmable feedback gain decreases the feedback anti-noise signal component.
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 including receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The method may also include generating a source audio signal for playback to a listener. The method may further include generating a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, countering the effects of ambient audio sounds at an acoustic output of the transducer, wherein an anti-noise signal comprises at least the feedback anti-noise signal component. The method may additionally include generating the secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate filter modeling an electro-acoustic path of the source audio signal. The method may also include applying a programmable feedback gain to a path of the feedback anti-noise signal component, wherein an increasing programmable feedback gain increases the feedback anti-noise signal component and a decreasing programmable feedback gain decreases the feedback anti-noise signal component. The method may further include combining the anti-noise signal with a 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 and output, 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 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 implement a feedback filter having a response that generates a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, and wherein the anti-noise signal comprises at least the feedback anti-noise signal component; 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; and a programmable feedback gain, wherein an increasing programmable feedback gain increases the feedback anti-noise signal component and a decreasing programmable feedback gain decreases the feedback anti-noise signal component.
In accordance with these and other 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 implement a feedback filter having a response that generates a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, a feedforward filter having a response that generates a feedforward anti-noise signal component from the reference microphone signal, wherein the anti-noise signal comprises at least the feedback anti-noise signal component and the feedforward anti-noise signal component, wherein the feedforward filter is configured to be disabled from generating the feedforward anti-noise signal component responsive to a disturbance in the reference microphone signal, and 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.
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 a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, countering the effects of ambient audio sounds at an acoustic output of the transducer, wherein an anti-noise signal comprises at least the feedback anti-noise signal component. The method may also include generating the secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate filter modeling an electro-acoustic path of the source audio signal. The method may further include generating a feedforward anti-noise signal component, 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 filtering with a feedforward filter an output of the reference microphone, wherein the anti-noise signal comprises at least the feedback anti-noise signal component and the feedforward anti-noise signal component. The method may additionally include disabling the feedforward filter from generating the feedforward anti-noise signal component responsive to a disturbance in the reference microphone signal. The method may also include combining the anti-noise signal with a 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 implement a feedback filter having a response that generates a feedback anti-noise signal component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, a feedforward filter having a response that generates a feedforward anti-noise signal component from the reference microphone signal, wherein the anti-noise signal comprises at least the feedback anti-noise signal component and the feedforward anti-noise signal component, wherein the feedforward filter is configured to be disabled from generating the feedforward anti-noise signal component responsive to a disturbance in the reference microphone signal, and 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.
In accordance with these and other 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 implement at least one of: a feedback filter having a response that generates at least a portion of the anti-noise component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate; and a feedforward filter having a response that generates at least a portion of the anti-noise signal from the reference microphone signal. The processing circuit may also implement 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 and a secondary path estimate performance monitor for monitoring performance of the secondary path estimate filter in modeling the electro-acoustic path.
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, comprising at least one of: generating a feedback anti-noise signal component comprising at least a portion of the anti-noise signal from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate, countering the effects of ambient audio sounds at an acoustic output of the transducer; and generating a feedforward anti-noise signal component comprising at least a portion of the 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 filtering an output of the reference microphone. The method may also include generating the secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate filter modeling an electro-acoustic path of the source audio signal. The method may further include monitoring with a secondary path estimate performance monitor performance of the secondary path estimate filter in modeling the electro-acoustic path. The method may additionally include combining the anti-noise signal with a 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 implement at least one of: a feedback filter having a response that generates at least a portion of the anti-noise component from a playback corrected error, the playback corrected error based on a difference between the error microphone signal and a secondary path estimate; and a feedforward filter having a response that generates at least a portion of the anti-noise signal from the reference microphone signal. The processing circuit may also implement 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 and a secondary path estimate performance monitor for monitoring performance of the secondary path estimate filter in modeling the electro-acoustic path.
Technical advantages of the present disclosure may be readily apparent to one of ordinary skill 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. 1A is an illustration of an example wireless mobile telephone, in accordance with embodiments of the present disclosure;
FIG. 1B is an illustration of an example wireless mobile telephone with a headphone assembly coupled thereto, in accordance with embodiments of the present disclosure;
FIG. 2 is a block diagram of selected circuits within the wireless telephone depicted in FIG. 1A, 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 active noise canceling (ANC) circuit of a coder-decoder (CODEC) integrated circuit of FIG. 3, 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. 1A, 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. In different embodiments, additional reference and/or error microphones may be employed. 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 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.
Referring now to FIG. 1B, wireless telephone 10 is depicted having a headphone assembly 13 coupled to it via audio port 15. Audio port 15 may be communicatively coupled to RF integrated circuit 12 and/or CODEC IC 20, thus permitting communication between components of headphone assembly 13 and one or more of RF integrated circuit 12 and/or CODEC IC 20. As shown in FIG. 1B, headphone assembly 13 may include a combox 16, a left headphone 18A, and a right headphone 18B. As used in this disclosure, the term “headphone” broadly includes any loudspeaker and structure associated therewith that is intended to be mechanically held in place proximate to a listener's ear canal, and includes without limitation earphones, earbuds, and other similar devices. As more specific examples, “headphone,” may refer to intra-concha earphones, supra-concha earphones, and supra-aural earphones.
Combox 16 or another portion of headphone assembly 13 may have a near-speech microphone NS that may capture near-end speech in addition to or in lieu of near-speech microphone NS of wireless telephone 10. In addition, each headphone 18A, 18B 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. Each headphone 18A, 18B may include a reference microphone R for measuring the ambient acoustic environment and an error microphone E for measuring of the ambient audio combined with the audio reproduced by speaker SPKR close a listener's ear when such headphone 18A, 18B is engaged with the listener's ear. In some embodiments, CODEC IC 20 may receive the signals from reference microphone R, near-speech microphone NS, and error microphone E of each headphone and perform adaptive noise cancellation for each headphone as described herein. In other embodiments, a CODEC IC or another circuit may be present within headphone assembly 13, communicatively coupled to reference microphone R, near-speech microphone NS, and error microphone E, and configured to perform adaptive noise cancellation as described herein.
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 ia 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, 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.
As shown in FIG. 2, signals ds and/or ia may first be filtered by compensating filter 28 with a response CPB(z). As explained in greater detail below, compensating filter 28 may boost a source audio signal comprising signals ds and/or ia within a frequency range responsive to a determination by a secondary path estimate performance monitor 48 of ANC circuit 30 that a secondary path estimate adaptive filter 34A of ANC circuit 30 (depicted in FIG. 3) is not sufficiently modeling an electro-acoustic path of the source audio signal for the frequency range of sound, as described in greater detail below.
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 a feedforward anti-noise component of the anti-noise signal, which may be combined by combiner 38 with a feedback anti-noise component of the anti-noise signal (described in greater detail below) to generate an anti-noise signal which in turn may be provided to an output combiner that combines the anti-noise signal with the source audio signal 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 another signal that includes error microphone signal err. By transforming reference microphone signal ref with a copy of the estimate of the response of path S(z), response SECOPY(z), and minimizing the ambient audio sounds in the error microphone signal, 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 downlink audio signal ds and/or internal audio signal ia that has been processed by filter response SE(z), of which response SECOPY(z) is a copy. By injecting an inverted amount of downlink audio signal ds and/or internal audio signal ia, adaptive filter 32 may be prevented from adapting to the relatively large amount of downlink audio and/or internal audio signal present in error microphone signal err. However, by transforming that inverted copy of downlink audio signal ds and/or internal audio signal ia with the estimate of the response of path S(z), the downlink audio and/or internal audio that is removed from error microphone signal err should match the expected version of downlink audio signal ds and/or internal audio signal ia reproduced at error microphone signal err, because the electrical and acoustical path of S(z) is the path taken by downlink audio signal ds and/or internal audio signal ia 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 downlink audio signal ds and/or internal audio signal ia and error microphone signal err after removal of the above-described filtered downlink audio signal ds and/or internal audio signal ia, that has been filtered by adaptive filter 34A to represent the expected downlink audio delivered to error microphone E, and which is removed from the output of adaptive filter 34A by a combiner 36 to generate a playback-corrected error, shown as PBCE in FIG. 3. SE coefficient control block 33 may correlate the actual downlink speech signal ds and/or internal audio signal ia with the components of downlink audio signal ds and/or internal audio signal ia that are present in error microphone signal err. Adaptive filter 34A may thereby be adapted to generate a signal from downlink audio signal ds and/or internal audio signal ia, that when subtracted from error microphone signal err, contains the content of error microphone signal err that is not due to downlink audio signal ds and/or internal audio signal ia.
As shown in FIG. 3, ANC circuit 30 may also comprise a disturbance detect block 42. Disturbance detect block 42 may include any system, device, or apparatus configured to detect a signal disturbance based on sound incident at reference microphone R, error microphone E, and/or near-speech microphone NS. As used herein, the term “signal disturbance” may include any sound impinging on reference microphone R, error microphone E, and/or near-speech microphone NS that might be expected to falsely influence generation of the feedforward anti-noise component, and may include speech or other sounds occurring close to the reference microphone, error microphone E, and/or near-speech microphone NS, the presence of ambient wind, physical contact of an object with the reference microphone error microphone E, and/or near-speech microphone NS, a momentary tone, and/or any other similar sound. As shown in FIG. 3, disturbance detect block 42 may detect such a signal disturbance based on reference microphone signal ref, error microphone signal err, and/or near-speech microphone signal NS. However, in these and other embodiments, disturbance detect block 42 may detect such a signal disturbance based on any other sensor associated with wireless telephone 10. If disturbance detect block 42 detects a disturbance, it may communicate a signal to feedforward adaptive filter 32 that may disable feedforward adaptive filter 32 from generating the feedforward anti-noise component, such that ANC circuit 30 generates only the feedback anti-noise component during the time in which a signal disturbance is present.
As depicted in FIG. 3, ANC circuit 30 may also comprise feedback filter 44. Feedback filter 44 may receive the playback corrected error signal PBCE and may apply a response FB(z) to generate a feedback anti-noise component of the anti-noise signal based on the playback corrected error which may be combined by combiner 38 with the feedforward anti-noise component of the anti-noise signal to generate the anti-noise signal which in turn may be provided to an output combiner that combines the anti-noise signal with the source audio signal to be reproduced by the transducer, as exemplified by combiner 26 of FIG. 2. Also as depicted in FIG. 3, a path of the feedback anti-noise component may have a programmable gain element 46, such that an increased gain will cause increased noise cancellation of the feedback anti-noise component, and decreasing the gain will cause reduced noise cancellation of the feedback anti-noise component. In instances when feedback filter 44 transitions from a state in which it is disabled from generating the feedback anti-noise component to a state in which it is enabled to generating the feedback anti-noise component (or vice versa), such gain may be smoothly ramped between two gain values to prevent an impulsive or fast change in the feedback anti-noise component which may negatively affect listener experience. Additionally or alternatively, in some embodiments, the gain of gain element 46 may be listener-configurable, for example via one or more user interface elements present on wireless telephone 10 and/or combox 16. In these and other embodiments, responsive to a determination that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path in a frequency range (as described in greater detail below), secondary path estimate performance monitor 48 may disable feedback filter 44 from generating the feedback anti-noise component and/or reduce the effective gain of feedback filter 44 (e.g., relative to the effective gain employed when secondary path estimate adaptive filter 34A is sufficiently modeling the electro-acoustic path) by modifying the gain of gain element 46.
Although feedback filter 44 and gain element 46 are shown as separate components of ANC circuit 30, in some embodiments some structure and/or function of feedback filter 44 and gain element 46 may be combined. For example, in some of such embodiments, an effective gain of feedback filter 44 may be varied via control of one or more filter coefficients of feedback filter 44.
As shown in FIG. 3, ANC circuit 30 may also comprise secondary path estimate performance monitor 48. Secondary path estimate performance monitor 48 may comprise any system, device, or apparatus configured to compare error microphone signal err to the playback-corrected error microphone signal, thus giving an indication of how efficiently secondary path estimate adaptive filter 34A is modeling the electro-acoustic path of the source audio signal over various frequencies, as determined by the efficiency by which secondary path estimate adaptive filter 34A causes combiner 36 to remove the source audio signal from the error microphone signal in generating the playback-corrected error over various frequencies.
Responsive to a determination by a secondary path estimate performance monitor 48 that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path of the source audio signal for a frequency range of sound, one or more components of CODEC IC 20 may perform an action. For example, responsive to a determination that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path in a frequency range, compensating filter 28 may boost a source audio signal comprising signals ds and/or is within the frequency range. As another example, responsive to a determination that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path in a frequency range, secondary path estimate performance monitor 48 may disable feedback filter 44 from generating the feedback anti-noise component and/or reduce the effective gain of feedback filter 44 (e.g., relative to the effective gain employed when secondary path estimate adaptive filter 34A is sufficiently modeling the electro-acoustic path) by modifying the gain of gain element 46. As another example, responsive to a determination that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path in a frequency range, secondary path estimate performance monitor 48 may disable adaptive filter 32 from adapting, may mute adaptive filter 32 (e.g., disable it from generating the feedforward anti-noise component), and/or may reset adaptive filter 32.
To determine whether or not secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path of the source audio signal, secondary path estimate performance monitor 48 may calculate a secondary index performance index (SEPI) defined as:

SEPI=10 log 10(PE/PCE)

where PE is an estimated power of error microphone signal err and PCE is the power estimate of the playback corrected error PBCE. The above equation for SEPI may be rewritten as:

SEPI=10 log 10[(PAmbient+P(PB·S(z)))/(PAmbient+P(PB·S(z)−SE(z)))]

where PAmbient is an estimated power of the ambient noise and “PB” connotes the power is related to the source audio signal. When ambient noise is low, SEPI is directly related to the secondary path estimation SE(z). Thus, the higher SEPI, the better the secondary path estimate adaptive filter 34A (e.g., SE(z)) is modeling the electro-acoustic path of the source audio signal (e.g., S(z)). When ambient noise is not low:

SEPI=10 log 10[(1+P(PB·S(z))/PAmbient)/(1+P(PB·S(z)−SE(z))/PAmbient)]

which may be rewritten as:

SEPI=10 log 10[(1+SNR)/(1+SNR·Model Error)]

where SNR is a signal-to-noise ratio wherein “signal” refers to the playback corrected error signal and “noise” refers to any other signal sensed by the error microphone E, and the Model Error is a value indicative of the error between SE(z) and S(z). When the Model Error is higher, SEPI is lower, and vice versa. Thus, by monitoring SEPI, secondary path estimate performance monitor 48 is effectively monitoring the signal-to-noise ratio of error microphone signal err together with the difference between SE(z) and S(z).

In order to provide a more accurate measure of the performance of secondary path estimate adaptive filter 34A, secondary path estimate performance monitor 48 may “smooth” its calculation of SEPI in order to filter out variations in the instantaneous calculation of SEPI. Thus, a smoothed SEPI, represented as SEPIsmooth, may equal a low-pass filtered, averaged, or rolling averaged version of instantaneous SEPI calculations. To increase system response speed, the instantaneous SEPI calculation may be used rather than SEPIsmooth when the instantaneous SEPI calculation falls below a predetermined minimum threshold or rises above a predetermined maximum threshold.
When SEPIsmooth is low, such an index value may mean that either the current signal-to-noise ratio is low for the secondary path estimation, or the secondary path estimation is not adequately modeling the electro-acoustic path of the source audio signal. In either event, it may not be desirable to adapt adaptive filter 32 and response W(z) during such time. Thus, when SEPIsmooth is above a minimum performance threshold, secondary path estimate performance monitor 48 may take no actions on other components of CODEC IC 20. However, when SEPIsmooth falls below such minimum performance threshold (e.g., indicating that response SE(z) is not well-adapted), secondary path estimate performance monitor 48 may disable adaptive filter 32 and response W(z) from adapting, as well as taking any or all of the other actions described herein as taking place responsive to a determination that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path, until such time as SEPIsmooth again rises above the minimum performance threshold. If SEPIsmooth further falls below a reset threshold lower than the minimum performance threshold (e.g., indicating that SE(z) is much different than S(z), as may occur when a headphone 18A or 18B is removed from a listener's ear), the response W(z) may be reset and adaptive filter 32 may be disabled from generating the feedforward anti-noise component, as the then-current response W(z) may be based on a largely incorrect SE(z).
To effectively calculate SEPI, secondary path estimate performance monitor 48 requires a source audio signal (e.g., downlink speech signal ds and/or internal audio signal ia). Thus, without a source audio signal, secondary path estimate performance monitor 48 cannot effectively monitor the performance of secondary path estimate filter 34A. However, such inability to monitor may not be problematic in embodiments of ANC circuit 30 in which adaptive filter 32 adapts only when a source audio signal is present. Nonetheless, even in the absence of a source audio signal, it may be desirable to determine whether or not a headphone 18A, 18B has become disengaged from a listener's ear. Thus, to make such determination, secondary path estimate performance monitor 48 may examine a power ratio R(z) between reference signal ref and error microphone signal err at various frequencies. When adaptive filter 32 and secondary path estimate filter 34A effectively model the path between the reference microphone and the error microphone, the value of the power ratio R(z) should be small (e.g., near 1) in the absence of a source audio signal. However, if response SE(z) should change and cease effectively modeling response S(z), the value of power ratio R(z) may increase. By tracking the power ratio R(z) over various frequency bands, secondary path estimate performance monitor 48 may be able to make a determination of whether a headphone 18A, 18B is loose fitting, engaged with a listener's ear, disengaged with a listener's ear, a speaker thereof is covered by a portion of the listener's anatomy, and/or other conditions. As an example, secondary path estimate performance monitor 48 may determine that one or more of such conditions has occurred if the power ratio R(z) exceeds a threshold power ratio T(z) in a particular frequency band, where T(z) is determined by tracking the power ratio R(z) in well-trained settings (e.g., when a source audio signal is available). In response to the occurrence of any of such conditions or a determination that the power ratio R(z) exceeds a threshold power ratio T(z) in a particular frequency band, secondary path estimate performance monitor 48 may take any or all of the other actions described herein as taking place responsive to a determination that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path.
This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example 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 example 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 that implements:
an anti-noise generating filter having a response that generates at least a portion of the anti-noise signal;
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; and
a secondary path estimate performance monitor for monitoring performance of the secondary path estimate filter in modeling the electro-acoustic path based on the error microphone signal and a playback correct error signal, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate.

2. The personal audio device of claim 1, wherein the secondary path estimate filter is an adaptive filter, and the processing circuit further implements a 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 in order to minimize the playback corrected error.

3. The personal audio device of claim 1, wherein the anti-noise generating filter comprises an adaptive feedforward filter that generates at least a portion of the anti-noise signal from the reference microphone signal, and the processing circuit further implements a feedforward coefficient control block that shapes the response of the anti-noise generating filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the anti-noise generating filter to minimize the ambient audio sounds in the error microphone signal.

4. The personal audio device of claim 3, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit disables adaptation of the anti-noise generating filter.

5. The personal audio device of claim 3, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit resets adaptation of the anti-noise generating filter.

6. The personal audio device of claim 1, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit disables the anti-noise generating filter from generating the anti-noise signal.

7. The personal audio device of claim 1, wherein:
the anti-noise generating filter comprises a feedback filter having a response that generates the anti-noise signal from the playback corrected error; and
responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit disables the anti-noise generating filter from generating the anti-noise signal.

8. The personal audio device of claim 1, wherein:
the anti-noise generating filter comprises a feedback filter having a response that generates the anti-noise signal from the playback corrected error; and
the processing circuit further implements a programmable feedback gain, wherein an increasing programmable feedback gain increases the portion of the anti-noise signal generated by the anti-noise generating filter and a decreasing programmable feedback gain decreases the portion of the anti-noise signal generated by the anti-noise generating filter; and
the processing circuit disables the anti-noise generating filter from generating the anti-noise signal by setting the programmable feedback gain to zero.

9. The personal audio device of claim 1, wherein:
the anti-noise generating filter comprises a feedback filter having a response that generates the anti-noise signal from the playback corrected error; and
the processing circuit further implements a programmable feedback gain, wherein an increasing programmable feedback gain increases the portion of the anti-noise signal generated by the anti-noise generating filter and a decreasing programmable feedback gain decreases the portion of the anti-noise signal generated by the anti-noise generating filter.

10. The personal audio device of claim 9, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit decreases the programmable feedback gain.

11. The personal audio device of claim 1, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path for a particular frequency range of sound, the processing circuit implements a compensating filter to boost the source audio signal within such frequency range to the source audio signal being communicated to the transducer and the secondary path estimate filter.

12. The personal audio device of claim 1, wherein the secondary path estimate performance monitor calculates, responsive to a determination that a source audio signal is present, a performance index based on the ratio between a power of the error microphone and a power of the playback corrected error, and the processing circuit controls at least one of the response of the anti-noise generating filter and the response of the secondary path estimate filter based on the performance index.

13. The personal audio device of claim 1, wherein the secondary path estimate performance monitor calculates, responsive to a determination that no source audio signal is present, a power ratio as a function of frequency between the error microphone signal and the reference microphone signal and the processing circuit controls at least one of the response of the anti-noise generating filter and the response of the secondary path estimate filter based on the performance index.

14. 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 for countering the effects of ambient audio sounds at an acoustic output of the transducer;
generating a secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate filter modeling an electro-acoustic path of the source audio signal;
monitoring with a secondary path estimate performance monitor performance of the secondary path estimate filter in modeling the electro-acoustic path based on the error microphone signal and a playback correct error signal, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate; and
combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer.

15. The method of claim 14, further comprising adapting a response of the secondary path estimate filter to minimize the playback corrected error.

16. The method of claim 14, further comprising generating the anti-noise signal by adapting a response of an adaptive feedforward filter that filters an output of the reference microphone to minimize the ambient audio sounds in the error microphone signal.

17. The method of claim 16, further comprising disabling adaptation of the adaptive feedforward filter responsive to a determination that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path.

18. The method of claim 16, further comprising resetting adaptation of the adaptive feedforward filter responsive to a determination that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path.

19. The method of claim 14, further comprising disabling generation of the anti-noise signal responsive to a determination that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path.

20. The method of claim 14, further comprising:
generating the anti-noise signal from a playback corrected error with a feedback filter;
applying a programmable feedback gain to a path of the anti-noise signal, wherein an increasing programmable feedback gain increases the anti-noise signal and a decreasing programmable feedback gain decreases the anti-noise signal; and
disabling generation of the anti-noise signal by setting the programmable feedback gain to zero responsive to a determination that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path.

21. The method of claim 14, further comprising:
generating the anti-noise signal from the playback corrected error;
applying a programmable feedback gain to a path of the anti-noise signal, wherein an increasing programmable feedback gain increases the anti-noise signal and a decreasing programmable feedback gain decreases the anti-noise signal; and
decreasing the programmable feedback gain responsive to a determination that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path.

22. The method of claim 14, further comprising boosting, within a frequency range, the source audio signal responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path.

23. The method of claim 14, further comprising:
calculating a performance index based on the ratio between a power of the error microphone and a power of the playback corrected error responsive to a determination that a source audio signal is present; and
controlling at least one of a response of an anti-noise generating filter for generating the anti-noise signal and a response of the secondary path estimate filter based on the performance index.

24. The method of claim 14, further comprising:
calculating a power ratio as a function of frequency between the error microphone signal and the reference microphone signal responsive to a determination that no source audio signal is present; and
controlling at least one of a response of an anti-noise generating filter for generating the anti-noise signal and a response of the secondary path estimate filter based on the performance index.

25. 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 that implements:
an anti-noise generating filter having a response that generates at least a portion of the anti-noise signal;
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; and
a secondary path estimate performance monitor for monitoring performance of the secondary path estimate filter in modeling the electro-acoustic path based on the error microphone signal and a playback correct error signal, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate.

26. The integrated circuit of claim 25, wherein the secondary path estimate filter is an adaptive filter, and the processing circuit further implements a 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 in order to minimize the playback corrected error, the playback corrected error based on a difference between the error microphone signal and the secondary path estimate.

27. The integrated circuit of claim 25, wherein the anti-noise generating filter comprises an adaptive feedforward filter that generates at least a portion of the anti-noise signal from the reference microphone signal, and the processing circuit further implements a feedforward coefficient control block that shapes the response of the anti-noise generating filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the anti-noise generating filter to minimize the ambient audio sounds in the error microphone signal.

28. The integrated circuit of claim 27, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit disables adaptation of the anti-noise generating filter.

29. The integrated circuit of claim 27, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit resets adaptation of the anti-noise generating filter.

30. The integrated circuit of claim 25, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit disables the anti-noise generating filter from generating the anti-noise signal.

31. The integrated circuit of claim 25, wherein:
the anti-noise generating filter comprises a feedback filter having a response that generates the anti-noise signal from the playback corrected error; and
responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit disables the anti-noise generating filter from generating the anti-noise signal.

32. The integrated circuit of claim 25, wherein:
the anti-noise generating filter comprises a feedback filter having a response that generates the anti-noise signal from the playback corrected error;
the processing circuit further implements a programmable feedback gain, wherein an increasing programmable feedback gain increases the portion of the anti-noise signal generated by the anti-noise generating filter and a decreasing programmable feedback gain decreases the portion of the anti-noise signal generated by the anti-noise generating filter; and
the processing circuit disables the anti-noise generating filter from generating the anti-noise signal by setting the programmable feedback gain to zero.

33. The integrated circuit of claim 25, wherein:
the anti-noise generating filter comprises a feedback filter having a response that generates the anti-noise signal from the playback corrected error; and
the processing circuit further implements a programmable feedback gain, wherein an increasing programmable feedback gain increases the portion of the anti-noise signal generated by the anti-noise generating filter and a decreasing programmable feedback gain decreases the portion of the anti-noise signal generated by the anti-noise generating filter.

34. The integrated circuit of claim 33, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path, the processing circuit decreases the programmable feedback gain.

35. The integrated circuit of claim 25, wherein responsive to a determination by the secondary path estimate performance monitor that the secondary path estimate filter is not sufficiently modeling the electro-acoustic path for a particular frequency range of sound, the processing circuit implements a compensating filter to boost the source audio signal within such frequency range to the source audio signal being communicated to the transducer and the secondary path estimate filter.

36. The integrated circuit of claim 25, wherein the secondary path estimate performance monitor calculates, responsive to a determination that a source audio signal is present, a performance index based on the ratio between a power of the error microphone and a power of the playback corrected error, and the processing circuit controls at least one of the response of the anti-noise generating filter and the response of the secondary path estimate filter based on the performance index.

37. The integrated circuit of claim 25, wherein the secondary path estimate performance monitor calculates, responsive to a determination that no source audio signal is present, a power ratio as a function of frequency between the error microphone signal and the reference microphone signal and the processing circuit controls at least one of the response of the anti-noise generating filter and the response of the secondary path estimate filter based on the performance index.