Abstract:
In accordance with systems and methods of this disclosure, a method may include generating a feedforward anti-noise signal component from a result of measuring with the reference microphone countering the effects of ambient audio sounds at an acoustic output of a transducer by filtering an output of the reference microphone, adaptively generating a feedback anti-noise signal component from a result of measuring with an error microphone for countering the effects of ambient audio sounds at the acoustic output of the transducer by adapting a response of a feedback adaptive filter that filters a synthesized reference feedback to minimize the ambient audio sounds in the error microphone signal, wherein the synthesized reference feedback is based on a difference between the error microphone signal and the feedback anti-noise signal component.

Description:
RELATED APPLICATION 
       [0001]    The present disclosure claims priority to U.S. Provisional Patent Application Ser. No. 61/812,823, filed Apr. 17, 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 using both feedforward and feedback adaptive noise cancellation techniques. 
       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. 
         [0004]    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. However, adaptive noise canceling circuits can be complex, consume additional power, and can generate undesirable results under certain circumstances. For example, as depicted in  FIG. 1 , some noise canceling circuits employ hybrid adaptive noise cancellation, including both: (i) an adaptive feedforward system  102  for generating a feedforward anti-noise signal component from a reference microphone signal ref provided by a reference microphone R and indicative of ambient audio sounds; and (ii) an adaptive feedback system  104  including an adaptive filter  110  and a coefficient control block  112  for generating coefficients for adaptive filter  110 , wherein adaptive feedback system  104  generates a feedback anti-noise signal component from a synthesized reference feedback signal synref, the synthesized reference feedback signal based on a difference between an error microphone signal err and an anti-noise signal, wherein the anti-noise signal is equal to the sum of the feedforward anti-noise signal component and the feedback anti-noise signal component, and wherein error microphone signal err is provided by an error microphone E and is indicative of an acoustic output of a transducer  106  (e.g., loudspeaker) and the ambient audio sounds at transducer  106 . Before being subtracted from error microphone signal err to generate synthesized reference feedback signal synref, the anti-noise signal is filtered by a secondary path estimate filter  108  for modeling an electro-acoustic path of a source audio signal through transducer  106 . 
         [0005]    In such approach, synthesized reference feedback signal synref synthesizes the ambient noise seen by error microphone E and is thus independent of the effect of adaptive feedforward system  102 . The consequence is that adaptive feedback system  104  is unable to determine the frequency regions that feedforward system  102  has cancelled and adapts to reduce noise in the same regions, causing performance of the adaptive noise cancellation system to suffer. 
       SUMMARY 
       [0006]    In accordance with the teachings of the present disclosure, the disadvantages and problems associated with detection and reduction of ambient narrow band noise associated with an acoustic transducer may be reduced or eliminated. 
         [0007]    In accordance with embodiments of the present disclosure, a personal audio device may include a personal audio device housing, a transducer mounted on the housing for reproducing an audio signal including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer, a reference microphone mounted on the housing for providing a reference microphone signal indicative of the ambient audio sounds, an error microphone mounted on the housing in proximity to the transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer, and a processing circuit. The processing circuit may implement a feedforward filter having a response that generates a feedforward anti-noise signal component from the reference microphone signal. The processing circuit may also implement a feedback adaptive filter having a response that generates a feedback anti-noise signal component from a synthesized reference feedback, the synthesized reference feedback based on a difference between the error microphone signal and the feedback anti-noise signal component, and wherein the anti-noise signal comprises the feedforward anti-noise signal component and the feedback anti-noise signal component. The processing circuit may also implement a feedback coefficient control block that shapes the response of the feedback adaptive filter in conformity with the error microphone signal and the synthesized reference feedback by adapting the response of the feedback adaptive filter to minimize the ambient audio sounds in the error microphone signal. 
         [0008]    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 measuring ambient audio sounds with a reference microphone to produce a reference microphone signal, measuring an output of the transducer and the ambient audio sounds at the transducer with an error microphone, 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 an output of the reference microphone, adaptively generating a feedback anti-noise signal component from a result of the measuring with the error microphone for countering the effects of ambient audio sounds at the acoustic output of the transducer by adapting a response of a feedback adaptive filter that filters a synthesized reference feedback to minimize the ambient audio sounds in the error microphone signal, wherein the synthesized reference feedback is based on a difference between the error microphone signal and the feedback anti-noise signal component; and combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer. 
         [0009]    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 for providing a signal to a transducer including both source audio 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. The processing circuit may implement a feedforward filter having a response that generates a feedforward anti-noise signal component from the reference microphone signal. The processing circuit may also implement a feedback adaptive filter having a response that generates a feedback anti-noise signal component from a synthesized reference feedback, the synthesized reference feedback based on a difference between the error microphone signal and the feedback anti-noise signal component, and wherein the anti-noise signal comprises the feedforward anti-noise signal component and the feedback anti-noise signal component. The processing circuit may also implement a feedback coefficient control block that shapes the response of the feedback adaptive filter in conformity with the error microphone signal and the synthesized reference feedback by adapting the response of the feedback adaptive filter to minimize the ambient audio sounds in the error microphone signal. 
         [0010]    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. 
         [0011]    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 
         [0012]    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: 
           [0013]      FIG. 1  is a block diagram depicting selected signal processing circuits and functional blocks within a hybrid active noise canceling (ANC) circuit including both feedforward and feedback, as is known in the art; 
           [0014]      FIG. 2  is an illustration of a wireless mobile telephone, in accordance with embodiments of the present disclosure; 
           [0015]      FIG. 3  is a block diagram of selected circuits within the wireless telephone depicted in  FIG. 2 , in accordance with embodiments of the present disclosure; and 
           [0016]      FIG. 4  is a block diagram depicting selected signal processing circuits and functional blocks within an ANC circuit of a coder-decoder (CODEC) integrated circuit of  FIG. 4 , in accordance with embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    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. 
         [0018]    Referring now to  FIG. 2 , 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). 
         [0019]    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 . 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. 
         [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 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. 
         [0021]    Referring now to  FIG. 3 , selected circuits within wireless telephone  10  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 A 1 , 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. 
         [0022]    Referring now to  FIG. 4 , details of ANC circuit  30  are shown in accordance with embodiments of the present disclosure. Feedforward 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 signal component, which may be provided to an output combiner that combines the feedforward anti-noise signal component and the feedback anti-noise signal component described below with the audio to be reproduced by the transducer, as exemplified by combiner  26  of  FIG. 3 . The coefficients of feedforward adaptive filter  32  may be controlled by a W coefficient control block  31  that uses a correlation of signals to determine the response of feedforward 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 (e.g., a playback corrected error equal error microphone signal err minus the downlink speech signal ds and/or internal audio signal ia as transformed by the estimate of the response of path S(z), response SE(z)). 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, feedforward adaptive filter  32  may adapt to the desired response of P(z)/S(z). In addition, a filter  37 A that has a response C x (z) as explained in further detail below, may process the output of filter  34 B and provide the first input to W coefficient control block  31 . The second input to W coefficient control block  31  may be processed by another filter  37 B having a response of C e (z). Response C e (z) may have a phase response matched to response C x (z) of filter  37 A. Both filters  37 A and  37 B may include a highpass response, so that DC offset and very low frequency variation are prevented from affecting the coefficients of W(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, feedforward 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 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. 
         [0023]    Feedback adaptive filter  32 A may receive a synthesized reference feedback signal synref and under ideal circumstances, may adapt its transfer function W SR (z) to be P(z)/S(z) to generate a feedback anti-noise signal component, which may be provided to an output combiner that combines the feedforward anti-noise signal component and the feedback anti-noise signal component with the audio to be reproduced by the transducer, as exemplified by combiner  26  of  FIG. 3 . Thus, the feedforward anti-noise signal component and feedback anti-noise signal component may combine to generate the anti-noise for the overall ANC system. Synthesized reference feedback signal synref may be generated by combiner  39  based on a difference between a signal that includes the error microphone signal (e.g., the playback corrected error) and the feedback anti-noise signal component as shaped by a copy SE COPY (z) of an estimate of the response of path S(z) provided by filter  34 C. The coefficients of feedback adaptive filter  32 A may be controlled by a W SR  coefficient control block  31 A that uses a correlation of signals to determine the response of feedback adaptive filter  32 A, which generally minimizes the error, in a least-mean squares sense, between those components of synthesized reference feedback signal synref present in error microphone signal err. The signals compared by W SR  coefficient control block  31 A may be the synthesized reference feedback signal synref and another signal that includes error microphone signal err. By minimizing the difference between the synthesized reference feedback signal synref and error microphone signal err, feedback adaptive filter  32 A may adapt to the desired response of P(z)/S(z). 
         [0024]    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  to generate the playback corrected error. 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. 
         [0025]    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. 
         [0026]    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.