Abstract:
Systems and methods of the present disclosure include analyzing and comparing transfer functions associated with a plurality of electro-acoustic paths for transducers of a personal audio device to determine proximity of the transducers to respective ears of a listener of the personal audio device, quality of acoustic seals associated with the transducers, and for one or more other purposes.

Description:
FIELD OF DISCLOSURE 
       [0001]    The present disclosure relates in general to adaptive noise cancellation in connection with an acoustic transducer, and more particularly, to sharing information between audio channels in an adaptive noise cancellation system. 
       BACKGROUND 
       [0002]    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. 
         [0003]    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. For example, many adaptive noise canceling systems utilize an error microphone for sensing acoustic pressure proximate to an output of an electro-acoustic transducer (e.g., a loudspeaker) and generating an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer. When the transducer is close to a listener&#39;s ear, the error microphone signal may approximate the actual acoustic pressure at a listener&#39;s eardrum (a location known as a drum reference point). However, because of the distance between the drum reference point and the location of the error microphone (known as the error reference point), the error microphone signal is only an approximation and not a perfect indication of acoustic pressure at the drum reference point. Thus, because noise cancellation attempts to reduce ambient audio sounds present in the error microphone signal, performance of a noise cancellation system may be the greatest when the distance between the drum reference point and the error reference point is small. As the distance increases (e.g., transducer held against the ear at a lower pressure), the performance of the noise cancellation system may degrade, partly because the gain of the transfer function from the error reference point to the drum reference point decreases with such increased distance. This degradation is not accounted for in traditional adaptive noise cancellation systems. 
       SUMMARY 
       [0004]    In accordance with the teachings of the present disclosure, the disadvantages and problems associated with improving audio performance of a personal audio device may be reduced or eliminated. 
         [0005]    In accordance with embodiments of the present disclosure, an integrated circuit for implementing at least a portion of a personal audio device may include a first output, a first error microphone input, a second output, a second error microphone input, and a processing circuit. The first output may provide a first output signal to a first transducer including both a first source audio signal for playback to a listener and a first anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the first transducer. The first error microphone input may receive a first error microphone signal indicative of the output of the first transducer and the ambient audio sounds at the first transducer. The second output may provide a second output signal to a second transducer including both a second source audio signal for playback to the listener and a second anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the second transducer. The second error microphone input may receive a second error microphone signal indicative of the output of the second transducer and the ambient audio sounds at the second transducer. The processing circuit may implement a first secondary path estimate adaptive filter for modeling an electro-acoustic path of the first source audio signal through the first transducer and having a response that generates a first secondary path estimate signal from the first source audio signal, a first coefficient control block that shapes the response of the first secondary path estimate adaptive filter in conformity with the first source audio signal and a first playback corrected error by adapting the response of the first secondary path estimate filter to minimize the first playback corrected error, wherein the first playback corrected error is based on a difference between the first error microphone signal and the first secondary path estimate signal, a second secondary path estimate adaptive filter for modeling an electro-acoustic path of the second source audio signal through the second transducer and having a response that generates a second secondary path estimate signal from the second source audio signal, a second coefficient control block that shapes the response of the second secondary path estimate adaptive filter in conformity with the second source audio signal and a second playback corrected error by adapting the response of the second secondary path estimate filter to minimize the second playback corrected error, wherein the second playback corrected error is based on a difference between the second error microphone signal and the second secondary path estimate signal, a first filter that generates the first anti-noise signal to reduce the presence of the ambient audio sounds at the acoustic output of the first transducer based at least on the first playback corrected error, a second filter that generates the second anti-noise signal to reduce the presence of the ambient audio sounds at the acoustic output of the second transducer based at least on the second playback corrected error, and a comparison block that compares the response of the first secondary path estimate adaptive filter and the response of the second secondary path estimate adaptive filter. 
         [0006]    In accordance with these and other embodiments of the present disclosure, a method for canceling ambient audio sounds in the respective proximities of transducers associated with a personal audio device may include receiving a first error microphone signal indicative of an output of a first transducer and the ambient audio sounds at the first transducer. The method may also include receiving a second error microphone signal indicative of an output of a second transducer and the ambient audio sounds at the second transducer. The method may also include generating a first secondary path estimate signal from a first source audio signal by filtering the first source audio signal with a first secondary path estimate filter for modeling an electro-acoustic path of the source audio signal through the first transducer, wherein a response of the first secondary path estimate adaptive filter is shaped in conformity with the first source audio signal and a first playback corrected error by adapting the response of the first secondary path estimate filter to minimize the first playback corrected error, wherein the first playback corrected error is based on a difference between the first error microphone signal and the first secondary path estimate signal. The method may additionally include generating a second secondary path estimate signal from a second source audio signal by filtering the second source audio signal with a second secondary path estimate filter for modeling an electro-acoustic path of the second source audio signal through the second transducer wherein a response of the second secondary path estimate adaptive filter is shaped in conformity with the second source audio signal and a second playback corrected error by adapting the response of the second secondary path estimate filter to minimize the second playback corrected error, wherein the second playback corrected error is based on a difference between the second error microphone signal and the second secondary path estimate signal. The method may additionally include generating a first anti-noise signal to reduce the presence of the ambient audio sounds at the acoustic output of the first transducer based at least on the first playback corrected error. The method may further include generating a second anti-noise signal to reduce the presence of the ambient audio sounds at the acoustic output of the second transducer based at least on the second playback corrected error. The method may further include comparing the response of the first secondary path estimate adaptive filter and the response of the second secondary path estimate adaptive filter. 
         [0007]    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 a first output, a first error microphone input, a first reference microphone input, a second output, a second error microphone input, a second reference microphone input, and a processing circuit. The first output may provide a first output signal to a first transducer including both a first source audio signal for playback to a listener and a first anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the first transducer. The first error microphone input may receive a first error microphone signal indicative of the output of the first transducer and the ambient audio sounds at the first transducer. The first reference microphone input may receive a first reference microphone signal indicative of the ambient audio sounds at the acoustic output of the first transducer. The second output may provide a second output signal to a second transducer including both a second source audio signal for playback to the listener and a second anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the second transducer. The second error microphone input may receive a second error microphone signal indicative of the output of the second transducer and the ambient audio sounds at the second transducer. The second reference microphone input may receive a second reference microphone signal indicative of the ambient audio sounds at the acoustic output of the second transducer. The processing circuit may implement a first adaptive filter that generates the first anti-noise signal from the first reference microphone signal to reduce the presence of the ambient audio sounds at the acoustic output of the first transducer, a second adaptive filter that generates the second anti-noise signal from the second reference microphone signal to reduce the presence of the ambient audio sounds at the acoustic output of the second transducer, a first coefficient control block that shapes the response of the first adaptive filter in conformity with the first error microphone signal and the first reference microphone signal by adapting the response of the first adaptive filter to minimize the ambient audio sounds in the first error microphone signal, a second coefficient control block that shapes the response of the second adaptive filter in conformity with the second error microphone signal and the second reference microphone signal by adapting the response of the second adaptive filter to minimize the ambient audio sounds in the second error microphone signal, and a comparison block that compares the response of the first adaptive filter and the response of the second adaptive filter. 
         [0008]    In accordance with these and other embodiments of the present disclosure, a method for canceling ambient audio sounds in the respective proximities of transducers associated with a personal audio device may include receiving a first error microphone signal indicative of an output of a first transducer and the ambient audio sounds at the first transducer, receiving a second error microphone signal indicative of an output of a second transducer and the ambient audio sounds at the second transducer, receiving a first reference microphone signal indicative of the ambient audio sounds at the acoustic output of the first transducer, and receiving a second reference microphone signal indicative of the ambient audio sounds at the acoustic output of the second transducer. The method may also include generating, by a first adaptive filter, a first anti-noise signal from the first reference microphone signal to reduce the presence of the ambient audio sounds at the acoustic output of the first transducer and generating, by a second adaptive filter, a second anti-noise signal from the second reference microphone signal to reduce the presence of the ambient audio sounds at the acoustic output of the second transducer. The method may additionally include shaping, by a first anti-noise path coefficient control block, a response of the first filter in conformity with the first error microphone signal and the first reference microphone signal by adapting the response of the first filter to minimize the ambient audio sounds in the first error microphone signal and shaping, by a second anti-noise path coefficient control block, a response of the second filter in conformity with the second error microphone signal and the second reference microphone signal by adapting the response of the second filter to minimize the ambient audio sounds in the second error microphone signal. The method may further include comparing the response of the first adaptive filter and the response of the second adaptive filter. 
         [0009]    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. 
         [0010]    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 
         [0011]    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: 
           [0012]      FIG. 1A  is an illustration of an example personal audio device, in accordance with embodiments of the present disclosure; 
           [0013]      FIG. 1B  is an illustration of an example personal audio device with a headphone assembly coupled thereto, in accordance with embodiments of the present disclosure; 
           [0014]      FIG. 2  is a block diagram of selected circuits within the personal audio device depicted in  FIGS. 1A and 1B , in accordance with embodiments of the present disclosure; 
           [0015]      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; 
           [0016]      FIG. 4  is a block diagram depicting selected circuits associated with two audio channels within the personal audio device depicted in  FIGS. 1A and 1B , in accordance with embodiments of the present disclosure; and 
           [0017]      FIG. 5  is a flow chart depicting an example method for controlling generation of anti-noise by an ANC system based on comparison of secondary path information between audio channels of the personal audio device. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring now to  FIG. 1A , a personal audio device  10  as illustrated in accordance with embodiments of the present disclosure is shown in proximity to a human ear  5 . Personal audio device  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 personal audio device  10 , or in the circuits depicted in subsequent illustrations, are required in order to practice the invention recited in the claims. Personal audio device  10  may include a transducer such as speaker SPKR that reproduces distant speech received by personal audio device  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 personal audio device  10 ) to provide a balanced conversational perception, and other audio that requires reproduction by personal audio device  10 , such as sources from webpages or other network communications received by personal audio device  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 personal audio device  10  to the other conversation participant(s). 
         [0019]    Personal audio device  10  may include adaptive noise cancellation (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&#39;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 personal audio device  10  is in close proximity to ear  5 . Circuit  14  within personal audio device  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 personal audio device 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. 
         [0020]    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 personal audio device  10  adapt an anti-noise signal generated out 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 personal audio device  10 , when personal audio device  10  is not firmly pressed to ear  5 . While the illustrated personal audio device  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 personal audio device 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 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. 
         [0021]    Referring now to  FIG. 1B , personal audio device  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  18 A, and a right headphone  18 B. 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&#39;s ear or ear canal, and includes without limitation earphones, earbuds, and other similar devices. As more specific non-limiting examples, “headphone,” may refer to intra-canal earphones, intra-concha earphones, supra-concha earphones, and supra-aural earphones. 
         [0022]    Combox  16  or another portion of headphone assembly  13  may have a near-speech microphone NS to capture near-end speech in addition to or in lieu of near-speech microphone NS of personal audio device  10 . In addition, each headphone  18 A,  18 B may include a transducer such as speaker SPKR that reproduces distant speech received by personal audio device  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 personal audio device  10 ) to provide a balanced conversational perception, and other audio that requires reproduction by personal audio device  10 , such as sources from webpages or other network communications received by personal audio device  10  and audio indications such as a low battery indication and other system event notifications. Each headphone  18 A,  18 B 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 to a listener&#39;s ear when such headphone  18 A,  18 B is engaged with the listener&#39;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. 
         [0023]    The various microphones referenced in this disclosure, including reference microphones, error microphones, and near-speech microphones, may comprise any system, device, or apparatus configured to convert sound incident at such microphone to an electrical signal that may be processed by a controller, and may include without limitation an electrostatic microphone, a condenser microphone, an electret microphone, an analog microelectromechanical systems (MEMS) microphone, a digital MEMS microphone, a piezoelectric microphone, a piezo-ceramic microphone, or dynamic microphone. 
         [0024]    Referring now to  FIG. 2 , selected circuits within personal audio device  10 , which in other embodiments may be placed in whole or part in other locations such as one or more headphone assemblies  13 , are shown in a block diagram. CODEC IC  20  may include an analog-to-digital converter (ADC)  21 A for receiving the reference microphone signal and generating a digital representation ref of the reference microphone signal, an ADC  21 B for receiving the error microphone signal and generating a digital representation err of the error microphone signal, and an ADC  21 C 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 Al, 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 , and a portion of near speech microphone signal ns so that the user of personal audio device  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. 
         [0025]    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  34 B 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 SE COPY (z), 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  34 B 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 SE COPY (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 and 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 before comparison 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. As shown in  FIGS. 2 and 3 , W coefficient control block  31  may also reset signal from a comparison block  42 , as described in greater detail below in connection with  FIGS. 4 and 5 . 
         [0026]    Filter  34 B may not be an adaptive filter, per se, but may have an adjustable response that is tuned to match the response of adaptive filter  34 A, so that the response of filter  34 B tracks the adapting of adaptive filter  34 A. 
         [0027]    To implement the above, adaptive filter  34 A 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  34 A to represent the expected downlink audio delivered to error microphone E, and which is removed from the output of adaptive filter  34 A by a combiner  36 . SE coefficient control block  33  correlates 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  34 A 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. 
         [0028]    Also as depicted in  FIG. 3 , a path of the anti-noise signal may have a programmable gain element  38 , such that an increased gain will cause increase of the anti-noise signal combined at output combiner  26  and a decreased gain will cause decrease of the anti-noise signal combined at output combiner  26 . As described in greater detail below with respect to  FIGS. 4 and 5 , the gain of programmable gain element  38  may vary based on a gain signal received from comparison block  42 . 
         [0029]    For clarity of exposition, the components of audio IC circuit  20  shown in  FIGS. 2 and 3  depict components associated with only one audio channel. However, in personal audio devices employing stereo audio (e.g., those with headphones) many components of audio CODEC IC  20  shown in  FIGS. 2 and 3  may be duplicated, such that each of two audio channels (e.g., one for a left-side transducer and one for a right-side transducer) are independently capable of performing ANC. 
         [0030]    Turning to  FIG. 4 , a system is shown including left channel CODEC IC components  20 A, right channel CODEC IC components  20 B, and a comparison block  42 . Each of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B may comprise some or all of the various components of CODEC IC  20  depicted in  FIG. 2 . Thus, based on a respective reference microphone signal (e.g., from reference microphone R L  or R R ), a respective error microphone signal (e.g., from error microphone E L  or E R ), a respective near-speech microphone signal (e.g., from near-sppech microphone NS L  or NS R ), and/or other signals, an ANC circuit  30  associated with a respective audio channel may generate an anti-noise signal, which may be combined with a source audio signal and communicated to a respective transducer (e.g., SPKR L  or SPKR R ). 
         [0031]    Comparison block  42  may be configured to receive from each of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B a signal indicative of the response SE(z) of the secondary estimate adaptive filter  34 A of the channel, shown in  FIG. 4  as responses SE L (z) and SE R (z), and compare such responses. Comparison of the responses of the secondary estimate adaptive filters  34 A may be indicative of a proximity of each of the transducers SPKR L  and SPKR R  to a respective ear of a listener, indicative of a quality of an acoustic seal between each of the transducers SPKR L  and SPKR R  to a respective ear of the listener, and/or indicative of other physical properties of transducers SPKR L  and/or SPKR R . Based on such comparison, comparison block  42  may generate to one or both of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B a reset signal (e.g., reset L , reset R ) and/or a gain signal (e.g., gain L , gain R ) in order to alter one or both of the anti-noise signals generated by left channel CODEC IC components  20 A and right channel CODEC IC components  20 B. In some embodiments, such alteration may be independent of a response of a filter (e.g., adaptive filter  32 ) generating such anti-noise signal. For example, in such embodiments, a filter (e.g., adaptive filter  32 ) may generate an anti-noise signal for attempting to reduce presence of ambient audio sounds in an audio output signal at a transducer, wherein such anti-noise signal may be altered (e.g., attenuated) by a gain signal generated by comparison block  42  and communicated to gain element  38 . In such embodiments, the adaptive filter  32  generating the anti-signal altered by gain element  38  may be frozen (e.g., prevented from adapting) when the gain of gain element  38  is other than a unity gain, otherwise adaptive filter  32  may attempt to adapt to the attenuated anti-noise signal. To freeze adaptation of the response of adaptive filter  32 , adaptive filter  32  or coefficient control block  31  may be configured to cease adaptation when gain of gain element  38  is non-unity (e.g., as shown in  FIG. 3 , coefficient control block  31  may receive the gain signal from comparison block  42 , and may be configured to cease update of coefficients when the gain signal indicates a non-zero gain). 
         [0032]    In these and other embodiments, such alteration may include altering a response of the filter (e.g., adaptive filter  32 ) generating such anti-noise signal. For example, in such embodiments, coefficients of W coefficient control  31  may be reset to an initial value based on a reset signal generated by comparison block  42 . 
         [0033]    In these and other embodiments, after the anti-noise signal of a particular channel is altered in response to the responses SE(z) of secondary estimate adaptive filters  34 A differing by more than a predetermined threshold, the ANC circuit  30  of such channel may reset coefficients of its respective SE coefficient control block  33  to be substantially equal to those of the other SE coefficient control block  33 , to provide a starting point for adaptation once the condition (e.g., lack of proximity between transducer and listener&#39;s ear) leading to alteration of the anti-noise is remedied. 
         [0034]    Although the foregoing discussion contemplates comparison of responses SE(z) of secondary estimate adaptive filters  34 A and altering a response of an anti-noise signal in response to the comparison, it should be understood that ANC circuits  30  may compare responses of other elements of ANC circuits  30  and alter anti-noise signals based on such comparisons alternatively or in addition to the comparisons of responses SE(z). For example, in some embodiments, comparison block  42  may be configured to receive from each of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B a signal indicative of the response W(z) of the adaptive filter  32 A of the channel, shown in  FIG. 4  as responses W L (z) and W R (z), and compare such responses. Comparison of the responses of the adaptive filters  32 A may be indicative of a proximity of each of the transducers SPKR L  and SPKR R  to a respective ear of a listener, indicative of a quality of an acoustic seal between each of the transducers SPKR L  and SPKR R  to a respective ear of the listener, and/or indicative of other physical properties of transducers SPKR L  and/or SPKR R . Based on such comparison, comparison block  42  may generate to one or both of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B a reset signal (e.g., reset L , reset R ) and/or a gain signal (e.g., gain L , gain R ) in order to alter (e.g., attenuate) one or both of the anti-noise signals generated by left channel CODEC IC components  20 A and right channel CODEC IC components  20 B. 
         [0035]      FIG. 5  illustrates a flow chart depicting an example method  50  for controlling generation of anti-noise by an ANC system based on comparison of secondary path information between audio channels of the personal audio device. According to one embodiment, method  50  may begin at step  52 . As noted above, teachings of the present disclosure may be implemented in a variety of configurations of CODEC IC  20 . As such, the preferred initialization point for method  50  and the order of the steps comprising method  50  may depend on the implementation chosen. 
         [0036]    At step  52 , comparison block  42  or another component of CODEC IC  20  may compare responses SE L (z) and SE R (z) of secondary estimate adaptive filters  34 A and/or compare responses W L (z) and W R (z) of adaptive filters  32 . At step  54 , comparison block  42  or another component of CODEC IC  20  may determine if the responses SE L (z) and SE R (z) differ by more than a predetermined threshold and/or responses W L (z) and W R (z) differ by more than the same or another predetermined threshold. If the responses SE L (z) and SE R (z) differ by more than a predetermined threshold and/or if responses W L (z) and W R (z) differ by more than the same or another predetermined threshold, method  50  may proceed to step  58 , otherwise method  50  may proceed to step  56 . 
         [0037]    At step  56 , responsive to a determination that responses SE L (z) and SE R (z) do not differ by more than a predetermined threshold and/or that responses W L (z) and W R (z) do not differ by more than the same or another predetermined threshold, anti-noise signals generated by each of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B may be unaltered. After completion of step  56 , method  50  may proceed again to step  52 . 
         [0038]    At step  58 , responsive to a determination that responses SE L (z) and SE R (z) differ by more than a predetermined threshold and/or that responses W L (z) and W R (z) differ by more than the same or another predetermined threshold, anti-noise signals generated by one or both of left channel CODEC IC components  20 A and right channel CODEC IC components  20 B may be altered. As mentioned above, such alteration may include varying a gain applied to an anti-noise signal in order to attenuate (including muting by attenuating with a zero gain) the anti-noise signal before it is reproduced by a transducer, and/or may include further altering response W(z) of adaptive filter  32  by resetting coefficients of W coefficient control  31  to a predetermined initial value. After completion of step  58 , method  50  may proceed again to step  52 . 
         [0039]    Although  FIG. 5  discloses a particular number of steps to be taken with respect to method  50 , method  50  may be executed with greater or fewer steps than those depicted in  FIG. 5 . In addition, although  FIG. 5  discloses a certain order of steps to be taken with respect to method  50 , the steps comprising method  50  may be completed in any suitable order. 
         [0040]    Method  50  may be implemented using comparison block  42  or any other system operable to implement method  50 . In certain embodiments, method  50  may be implemented partially or fully in software and/or firmware embodied in computer-readable media. 
         [0041]    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. 
         [0042]    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.