Abstract:
In accordance with method and systems of the present disclosure, a processing circuit may implement an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, a coefficient control block that shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, and a coefficient bias control block which biases coefficients of the coefficient control block towards zero in a range of frequencies outside of a frequency response of the source audio signal.

Description:
RELATED APPLICATION 
       [0001]    The present disclosure claims priority to U.S. Provisional Patent Application Ser. No. 61/811,915, filed Apr. 15, 2013, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    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 by dynamically biasing coefficients of an adaptive noise cancellation system. 
       BACKGROUND 
       [0003]    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. 
         [0004]    Adaptive noise cancellation may be used in many elements of personal audio devices, including headphones. Headphones that provide adaptive noise cancellation to a listener may also be used to play audio content to the headphones in a variety of cases. For example, in a phone call, audio content may occupy a telephone speech band of between 300 Hz and 3.4 kHz, inclusive, or in a high-fidelity audio playback situation, the audio content may occupy a frequency range of 20 Hz to 20 kHz, inclusive, for some audio tracks, or 100 Hz to 8 kHz for some compressed audio content. An adaptive noise cancellation system must be stable under all conditions, regardless of the bandwidth of the ambient noise or the bandwidth of a source audio signal. Any adaptive system that depends on a model of an electro-acoustic path of the source audio signal through a transducer, for example a filtered-X least-mean-square feedforward adaptive system, must comprehend the frequency spectra of the various signals involved in such a way that instability in adaptation is avoided. 
       SUMMARY 
       [0005]    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 transducer, a reference microphone, an error microphone, and a processing circuit. The transducer may reproduce an audio signal including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer. The reference microphone may provide a reference microphone signal indicative of the ambient audio sounds. The error microphone may be located in proximity to the transducer and may provide 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 an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, a coefficient control block that shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, and a coefficient bias control block which biases coefficients of the coefficient control block towards zero in a range of frequencies outside of a frequency response of the source audio signal. 
         [0006]    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 adaptively generating an anti-noise signal from a result of the measuring with the reference microphone countering the effects of ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters an output of the reference microphone to minimize the ambient audio sounds in the error microphone signal. The method may additionally include biasing coefficients for controlling the response of the adaptive filter towards zero in a range of frequencies outside of a frequency response of the source audio signal. In addition, the method may include combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer. 
         [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 an output, a reference microphone input, an error microphone input, and a processing circuit. The output may provide 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 receive a reference microphone signal indicative of the ambient audio sounds. The error microphone input may receive an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The processing circuit may implement an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, a coefficient control block that shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, and a coefficient bias control block which biases coefficients of the coefficient control block towards zero in a range of frequencies outside of a frequency response of the source audio signal. 
         [0008]    In accordance with these and other embodiments of the present disclosure, a personal audio device may include a transducer, a reference microphone, an error microphone, and a processing circuit. The transducer may reproduce an audio signal including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer. The reference microphone may provide a reference microphone signal indicative of the ambient audio sounds. The error microphone may be located in proximity to the transducer and may provide 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 feedforward filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, a secondary path estimate adaptive 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, a coefficient control block that shapes the response of the secondary path estimate adaptive filter in conformity with the source audio signal and a playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate, and a coefficient bias control block which biases coefficients of the coefficient control block towards zero in a range of frequencies outside of a frequency response of the source audio signal. 
         [0009]    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 an 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 an output of the reference microphone. The method may additionally include adaptively generating a 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 a playback corrected error based on a difference between the error signal and the secondary path estimate. In addition, the method may include biasing coefficients for controlling the response of the secondary path estimate adaptive filter towards zero in a range of frequencies outside of a frequency response of the source audio signal. The method may further include combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer. 
         [0010]    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 provide 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 receive a reference microphone signal indicative of the ambient audio sounds. The error microphone input may receive 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 feedforward filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, a secondary path estimate adaptive filter for modeling an electro-acoustic path of the source audio signal having a response that generates a secondary path estimate from the source audio, a coefficient control block that shapes the response of the secondary path estimate adaptive filter in conformity with the source audio signal and a playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate, and a coefficient bias control block which biases coefficients of the coefficient control block towards zero in a range of frequencies outside of a frequency response of the source audio signal. 
         [0011]    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. 
         [0012]    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 
         [0013]    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: 
           [0014]      FIG. 1A  is an illustration of an example wireless mobile telephone, in accordance with embodiments of the present disclosure; 
           [0015]      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; 
           [0016]      FIG. 2  is a block diagram of selected circuits within the wireless telephone depicted in  FIG. 1 , in accordance with embodiments of the present disclosure; and 
           [0017]      FIG. 3  is a block diagram depicting selected signal processing circuits and functional blocks within an example adaptive noise canceling (ANC) circuit of a coder-decoder (CODEC) integrated circuit of  FIG. 2 , in accordance with embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]    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 present disclosure 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 inventions 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). 
         [0020]    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&#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 wireless telephone  10  is in close proximity to ear  5 . In these and other embodiments, additional reference microphones 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 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. 
         [0021]    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. 
         [0022]    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  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. 
         [0023]    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 wireless telephone  10 . In addition, each headphone  18 A,  18 B 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  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 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. 
         [0024]    Referring now to  FIG. 2 , selected circuits within wireless telephone  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 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. 
         [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 (as modified by a noise-injection signal by combiner  35 A as described in greater detail below) and another signal that includes error microphone signal err (as modified by a noise-injection signal by combiner  37 A as described in greater detail below). By transforming reference microphone signal ref with a copy of the estimate of the response of path S(z), response 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 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  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. 
         [0026]    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 (as modified by a noise-injection signal by combiner  35 B as described in greater detail below) with a playback corrected error equal to 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  (and which may be modified by a noise-injection signal by combiner  37 B as described in greater detail below). 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  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. 
         [0027]    As depicted in  FIG. 3 , ANC circuit  30  may include a coefficient bias control block  40  which biases coefficients of one or more of W coefficient control block  31  and SE coefficient control block  33  towards zero in one or more particular ranges of frequencies, as described in further detail below. In some embodiments, coefficient bias control block  40  may have structure and/or functionality identical or similar to that disclosed in U.S. patent application Ser. No. 13/333,484 entitled “Methods for Bandlimiting Antinoise in Earpiece Active Noise Cancel Headset,” and filed on Dec. 21, 2011, which is incorporated herein by reference thereto. For purposes of clarity and exposition of the present disclosure, the level of detail disclosed in U.S. patent application Ser. No. 13/333,484 regarding certain functionality of coefficient bias control block  40  is not repeated herein, but rather is summarized to describe implementation details pertinent to the present disclosure. 
         [0028]    As shown in  FIG. 3 , coefficient bias control block  40  may include a noise source  42 , a bandpass filter  44 , a frequency bias selector  46 , a filter  32 A configured to apply a response which is a copy of the response of adaptive filter  32 , and a filter  34 C configured to apply a response which is a copy of the response of adaptive filter  34 A. In operation, noise source  42  may generate white noise (e.g., an audio signal with a constant amplitude across all frequencies of interest, such as those frequencies within the range of human hearing) which is filtered by band pass filter  44  to generate an injected noise signal. The bandpass range of frequencies of the white noise passed by bandpass filter  44  to generate the injected noise signal may be controlled by frequency bias selector  46 , which may select an upper bound and lower bound of the bandpass range based on reference signal ref, a source audio signal (e.g., downlink speech signal ds and/or internal audio signal ia), and/or frequency limits of a transducer (e.g., speaker SPKR) for playing back the source audio signal, as described in greater detail below. In some embodiments, the injected noise signal may be combined (e.g., by combiner  35 A) with reference microphone signal ref as filtered by filter  34 B and communicated to W coefficient control block  31 . In these and other embodiments, the injected noise signal may be combined (e.g., by combiner  35 B) with a source audio signal (downlink speech signal ds and/or internal audio signal ia) and communicated to SE coefficient control block  33 . 
         [0029]    In addition, filter  32 A may filter the injected noise signal with the response W COPY (z), which is a copy of the response W(z) of adaptive filter  32 , to generate a W-filtered noise injection signal. Filter  32 A may not be an adaptive filter, per se, but may have an adjustable response that is tuned to match the response of adaptive filter  32 , so that the response of filter  32 A tracks the adapting of adaptive filter  32 . In some embodiments, the W-filtered noise injection signal and the injected noise signal may be combined (e.g., by combiner  37 A) with the playback corrected error signal and communicated to W coefficient control block  31 . 
         [0030]    In these and other embodiments, filter  34 C may filter the injected noise signal with the response S COPY2 (z), which is a copy of the response SE(z) of adaptive filter  34 A, to generate a SE-filtered noise injection signal. Filter  34 C 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 C tracks the adapting of adaptive filter  34 A. In some embodiments, the SE-filtered noise injection signal and the injected noise signal may be combined (e.g., by combiner  37 B) with the playback corrected error signal and communicated to SE coefficient control block  33 . 
         [0031]    As mentioned above, frequency bias selector  46  may select an upper bound and lower bound of the bandpass range of bandpass filter  44  based on reference signal ref, a source audio signal (e.g., downlink speech signal ds and/or internal audio signal ia), and/or frequency limits of a transducer (e.g., speaker SPKR) for playing back the source audio signal. In some embodiments, frequency bias selector  46  may select a lower bound of the bandpass range equal to an approximate upper bound of the frequency content of the source audio signal. In such embodiments, frequency bias selector  46  may dynamically track frequency content of the source audio signal in order to determine the lower bound of the bandpass range based on a recent trend of the upper bound of frequency content of the source audio signal (e.g., a trailing average of the upper bound of the frequency content). In these and other embodiments, frequency bias selector  46  may select an upper bound and a lower bound for the bandpass range such that the bandpass range is within a frequency response of the transducer for playing back the source audio signal (e.g., speaker SPKR) and within a frequency response of ambient audio sounds as indicated by reference microphone signal ref. In such embodiments, frequency bias selector  46  may select an upper bound for the bandpass range equal to an approximate upper bound of frequency response of the transducer or equal to an approximate upper bound of frequency response of the ambient audio sounds. 
         [0032]    Accordingly, for frequency ranges in which the frequency content of the source audio signal, the frequency content of the ambient audio sounds, and the frequency response of the transducer do not “intersect”—in other words, frequency ranges in which at least one of the source audio signal, the ambient audio sounds, and the transducer have content/response but at least one of the source audio signal, the ambient audio sounds, and the transducer do not have content/response—frequency bias selector  46  may cause bandpass filter  44  to bandpass filter white noise generated by noise source  42  within such a frequency range, thus generating an injected noise signal having content only within such frequency range. Thus, when W coefficient control block  31  compares reference microphone signal ref to the playback corrected error, to the extent there exists a frequency range in which the frequency content of reference microphone signal ref and the playback corrected error do not intersect, coefficient bias control block  40  injects white noise into the reference microphone signal ref or the playback corrected error (e.g., by combiners  35 A and  37 A, respectively) within such frequency range, so that the compared signals have content throughout the same intersecting frequency spectrum, and thus biasing adaptation coefficients in the frequency range towards zero. Similarly, when SE coefficient control block  33  compares a source audio signal to the playback corrected error, to the extent there exists a frequency range in which the frequency content of the source audio signal and the playback corrected error do not intersect, coefficient bias control block  40  injects white noise into the source audio signal or the playback corrected error (e.g., by combiners  35 B and  37 B, respectively) within such frequency range, so that the compared signals have content throughout the same intersecting frequency spectrum, and thus biasing adaptation coefficients in the frequency range towards zero. Without the injection of noise as described herein, W coefficient control block  31  and/or S coefficient control block  33  may, in a frequency range in which the frequency content of the comparison signals do not intersect, attempt to nonetheless adapt filter responses in such frequency range, which may lead to adaptation instability. 
         [0033]      FIG. 3  and the foregoing description thereof contemplate injection of noise signal into both of W coefficient control block  31  and SE coefficient control block  33 . However, in some embodiments, ANC circuit  30  may be configured such that coefficient bias control block  40  may inject noise into one of W coefficient control block  31  and SE coefficient control block  33 , but not both. If noise injection is applied to W coefficient control block  31 , as the W(z) response adapts, it may not matter that the SE(z) response is a good model of the secondary path in the frequency range in which noise is injected as the W(z) response adaptation coefficients will be biased towards zero in such frequency range. Similarly, if noise injection is applied to SE coefficient control block  33 , the SE(z) response will not attempt to model the secondary path in the frequency range in which noise is injected, and because the SE(z) response in such frequency range will be small, it does no harm to the stability of the adaptation of the W(z) response in a least-mean-square adaptation system. 
         [0034]    In some embodiments, coefficients of SE coefficient control block  33  may initialize with a bandlimited frequency response for the SE(z) response, thus allowing for a starting point for adaptation of the SE(z) response before any source audio signal for training the SE(z) response appears so that the SE(z) response does not attempt to model the true secondary path beyond any likely initial playback bandwidth. Thus, in case the source audio signal is narrowband (e.g., downlink speech in the telephone voice band), there will be no significant ambient content at higher frequencies being passed through filter  34 B as input to W coefficient control block  31  that might lead to instability. 
         [0035]    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. 
         [0036]    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.