Patent Application
20170053638
The present disclosure relates in general to adaptive noise cancellation in connection with an acoustic transducer, and more particularly, to a hybrid adaptive noise cancellation system with a filtered error microphone signal to correct for misalignment between a reference microphone signal and an error microphone signal caused by a feedback filter of the hybrid adaptive noise cancellation system.
Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
In many noise cancellation systems, it is desirable to include both feedforward noise cancellation by using a feedforward adaptive filter for generating a feedforward anti-noise signal from a reference microphone signal configured to measure ambient sounds and feedback noise cancellation by using a fixed-response feedback filter for generating a feedback noise cancellation signal to be combined with the feedforward anti-noise signal. However, using traditional approaches, when a gain of the feedback path is strong, the response of the feedforward adaptive filter may diverge, thus rendering the adaptive system unstable.
In accordance with the teachings of the present disclosure, the disadvantages and problems associated with instability of existing approaches for implementing hybrid adaptive noise cancellation may be reduced or eliminated.
In accordance with embodiments of the present disclosure, a 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 a source audio signal for playback to a listener and an anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the transducer, a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds, an error microphone input for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer; and a processing circuit. The processing circuit may implement a feedforward filter having a response that generates at least a portion of the anti-noise signal from the reference microphone signal, a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates a secondary path estimate from the source audio signal, a feedback filter having a response that generates at least a portion of the anti-noise signal based on the error microphone signal, an alignment filter configured to correct misalignment of the reference microphone signal and error microphone signal by generating a misalignment correction signal; a feedforward coefficient control block that shapes the response of the feedforward filter by adapting the response of the feedforward filter to minimize the ambient audio sounds in the error microphone signal; and a secondary path coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the misalignment correction signal in order to minimize the misalignment correction signal.
In accordance with these and other embodiments of the present disclosure, a method for canceling ambient audio sounds in the proximity of a transducer of a personal audio device may include receiving a reference microphone signal indicative of the ambient audio sounds, receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer, generating a source audio signal for playback to a listener, generating a feedforward anti-noise signal component from the reference microphone signal by adapting a response of an adaptive filter that filters the reference microphone signal to minimize the ambient audio sounds in the error microphone signal, generating a feedback anti-noise signal component based on the error microphone signal for countering the effects of ambient audio sounds at an acoustic output of the transducer, generating a misalignment correction signal to correct misalignment of the reference microphone signal and error microphone signal, generating the secondary path estimate from the source audio signal by adapting a response of a secondary path estimate filter that models an electro-acoustic path of the source audio signal and filters the source audio signal to minimize the filtered playback corrected error, and combining the feedforward anti-noise signal component and the feedback anti-noise signal component with a source audio signal to generate an audio signal provided to the transducer.
In accordance with these and other embodiments of the present disclosure, an integrated circuit for implementing at least a portion of a personal audio device may include an output for providing a signal to a transducer including both a source audio signal for playback to a listener and an anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the transducer, a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds, an error microphone input for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer, a noise input for receiving an injected, substantially inaudible noise signal, and a processing circuit. The processing circuit may implement a feedforward filter having a response that generates at least a portion of the anti-noise signal from the reference microphone signal, a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates a secondary path estimate from the source audio signal, a feedback filter having a response that generates at least a portion of the anti-noise signal based on the error microphone signal, an effective secondary estimate filter configured to model an electro-acoustic path of the anti-noise signal and have a response that generates the filtered noise signal from the noise signal, a feedforward coefficient control block that shapes the response of the feedforward filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the feedforward filter to minimize the ambient audio sounds in the error microphone signal, a secondary path coefficient control block that shapes the response of the effective secondary path estimate filter in conformity with the noise signal and the error microphone signal in order to minimize the playback corrected error, and a secondary estimate construction block that generates the response of the secondary estimate filter from the response of the effective secondary estimate filter.
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, receiving an error microphone signal indicative of an output of the transducer and the ambient audio sounds at the transducer, generating a source audio signal for playback to a listener, generating a feedforward anti-noise signal component from the reference microphone signal by adapting a response of an adaptive filter that filters the reference microphone signal to minimize the ambient audio sounds in the error microphone signal, generating a feedback anti-noise signal component based on the error microphone signal, generating the filtered noise signal from a noise signal by adapting a response of an effective secondary path estimate filter that models an electro-acoustic path of the anti-noise signal and filters the noise signal to minimize the error microphone signal, generating the secondary path estimate from the source audio signal by applying a response of a secondary path estimate filter wherein the response of the secondary estimate filter is generated from the response of the effective secondary estimate filter, and combining the feedforward anti-noise signal component and the feedback anti-noise signal component with a source audio signal to generate an audio signal provided to the transducer.
Technical advantages of the present disclosure may be readily apparent to one of ordinary skill in the art from the figures, description and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are not restrictive of the claims set forth in this disclosure.
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:
The present disclosure encompasses noise canceling techniques and circuits that can be implemented in a personal audio device, such as a wireless telephone. The personal audio device includes an ANC circuit that may measure the ambient acoustic environment and generate a signal that is injected in the speaker (or other transducer) output to cancel ambient acoustic events. A reference microphone may be provided to measure the ambient acoustic environment, and an error microphone may be included for controlling the adaptation of the anti-noise signal to cancel the ambient audio sounds and for correcting for the electro-acoustic path from the output of the processing circuit through the transducer.
Referring now to
Wireless telephone 10 may include ANC circuits and features that inject an anti-noise signal into speaker SPKR to improve intelligibility of the distant speech and other audio reproduced by speaker SPKR. A reference microphone R may be provided for measuring the ambient acoustic environment, and may be positioned away from the typical position of a user's mouth, so that the near-end speech may be minimized in the signal produced by reference microphone R. Another microphone, error microphone E, may be provided in order to further improve the ANC operation by providing a measure of the ambient audio combined with the audio reproduced by speaker SPKR close to ear 5, when wireless telephone 10 is in close proximity to ear 5. In different embodiments, additional reference and/or error microphones may be employed. Circuit 14 within wireless telephone 10 may include an audio CODEC integrated circuit (IC) 20 that receives the signals from reference microphone R, near-speech microphone NS, and error microphone E and interfaces with other integrated circuits such as a radio-frequency (RF) integrated circuit 12 having a wireless telephone transceiver. In some embodiments of the disclosure, the circuits and techniques disclosed herein may be incorporated in a single integrated circuit that includes control circuits and other functionality for implementing the entirety of the personal audio device, such as an MP3 player-on-a-chip integrated circuit. In these and other embodiments, the circuits and techniques disclosed herein may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller or other processing device.
In general, ANC techniques of the present disclosure measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and by also measuring the same ambient acoustic events impinging on error microphone E, ANC processing circuits of wireless telephone 10 adapt an anti-noise signal generated from the output of reference microphone R to have a characteristic that minimizes the amplitude of the ambient acoustic events at error microphone E. Because acoustic path P(z) extends from reference microphone R to error microphone E, ANC circuits are effectively estimating acoustic path P(z) while removing effects of an electro-acoustic path S(z) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR including the coupling between speaker SPKR and error microphone E in the particular acoustic environment, which may be affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, when wireless telephone 10 is not firmly pressed to ear 5. While the illustrated wireless telephone 10 includes a two-microphone ANC system with a third near-speech microphone NS, some aspects of the present invention may be practiced in a system that does not include separate error and reference microphones, or a wireless telephone that uses near-speech microphone NS to perform the function of the reference microphone R. Also, in personal audio devices designed only for audio playback, near-speech microphone NS will generally not be included, and the near-speech signal paths in the circuits described in further detail below may be omitted, without changing the scope of the disclosure, other than to limit the options provided for input to the microphone covering detection schemes.
Referring now to
Combox 16 or another portion of headphone assembly 13 may have a near-speech microphone NS that may capture near-end speech in addition to or in lieu of near-speech microphone NS of wireless telephone 10. In addition, each headphone 18A, 18B may include a transducer, such as speaker SPKR, that reproduces distant speech received by wireless telephone 10, along with other local audio events such as ringtones, stored audio program material, injection of near-end speech (i.e., the speech of the user of wireless telephone 10) to provide a balanced conversational perception, and other audio that requires reproduction by wireless telephone 10, such as sources from webpages or other network communications received by wireless telephone 10 and audio indications, such as a low battery indication and other system event notifications. Each headphone 18A, 18B may include a reference microphone R for measuring the ambient acoustic environment and an error microphone E for measuring of the ambient audio combined with the audio reproduced by speaker SPKR close a listener's ear when such headphone 18A, 18B is engaged with the listener's ear. In some embodiments, CODEC IC 20 may receive the signals from reference microphone R, near-speech microphone NS, and error microphone E of each headphone and perform adaptive noise cancellation for each headphone as described herein. In other embodiments, a CODEC IC or another circuit may be present within headphone assembly 13, communicatively coupled to reference microphone R, near-speech microphone NS, and error microphone E, and configured to perform adaptive noise cancellation as described herein.
Referring now to
Referring now to
To implement the above, adaptive filter 34A may have coefficients controlled by SE coefficient control block 33, which may compare downlink audio signal ds and/or internal audio signal ia and error microphone signal err after removal of the above-described filtered downlink audio signal ds and/or internal audio signal ia, that has been filtered by adaptive filter 34A to represent the expected downlink audio delivered to error microphone E, and which is removed from the output of adaptive filter 34A by a combiner 36 to generate a playback-corrected error (shown as PBCE in
As depicted in
As mentioned above, ANC circuit 30A may also include an alignment filter 42. In the presence of feedback filter 44, an effective secondary path Seff(z) for adaptive filter 32 may be given by Seff(z)=S(z)/[1+H(z)S(z)], and a playback-corrected error PBCEFB(z) with feedback filter 44 present (e.g., H(z)≠0) may be different than a playback-corrected error signal PBCE(z) without feedback filter 44 present (e.g., H(z)=0), as may be given by ErrFB=Err(z)/[1+H(z)S(z)]. Accordingly, in the absence of alignment filter 42 (e.g., if playback corrected error PBCE was not filtered by alignment filter 42 and was fed directly into W coefficient control 31 and SE coefficient control 33), the reference microphone signal ref and the playback corrected error PBCE may not be aligned, but may differ by a phase angle of 1/[1+H(z)S(z)]. Thus, alignment filter 42 may be configured to correct such misalignment of reference microphone signal ref, error microphone signal err, the source audio signal, and the playback-corrected error by generating a filtered playback-corrected error (shown as “filtered PBCE” in
Referring now to
As depicted in
In addition, in ANC circuit 30B, an alignment filter 42B may be implemented in place of alignment filter 42 of ANC circuit 30A, such that alignment filter 42B may have a response 1+SE(z)H(z)G that accounts for any misalignment between reference microphone signal ref and error microphone signal err caused by feedback filter 44 and programmable gain element 46 that would be introduced into ANC circuit 30B if alignment filter 42B were not present (e.g., if playback corrected error PBCE was not filtered by alignment error 42 and was fed directly into W coefficient control 31 and SE coefficient control 33).
As shown in
Responsive to a determination by a secondary path estimate performance monitor 48 that secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path of the source audio signal, secondary path estimate performance monitor 48 may control gain element 46 and alignment filter 42B to reduce gain G, and then increase gain G when secondary path estimate adaptive filter 34A is sufficiently modeling the electro-acoustic path. Thus, when secondary path estimate adaptive filter 34A is not well-trained, secondary path estimate performance monitor 48 may reduce gain G and train secondary path estimate adaptive filter 34A. Once secondary path estimate adaptive filter 34A is well-trained, secondary path estimate performance monitor 48 may increase gain G and then update secondary path estimate adaptive filter 34A and/or adaptive filter 32.
To determine whether or not secondary path estimate adaptive filter 34A is not sufficiently modeling the electro-acoustic path of the source audio signal, secondary path estimate performance monitor 48 may calculate a secondary index performance index (SEPI) defined as:
SEPI=Σi=kn|SE(i)|
where k represents a first coefficient tap of secondary path estimate adaptive filter 34A and n represents a second coefficient tap of secondary path estimate adaptive filter 34A. In some embodiments, the coefficient taps will comprise the coefficient taps representing the longest delay elements of a finite impulse response filter that implements secondary path estimate adaptive filter 34A. For example, in a 256-coefficient filter, k may equal 128 and n may equal 256. Once calculated, the value of SEPI may be compared to one or more threshold values to determine if secondary path estimate adaptive filter 34A is sufficiently modeling the electro-acoustic path of the source audio signal. If the SEPI value is below such a threshold, secondary path estimate adaptive filter 34A may be determined to be sufficiently modeling the electro-acoustic path of the source audio signal
Referring now to
As shown in
In operation, when secondary path estimate performance monitor 48 determines that secondary path estimate filter 34A is sufficiently modeling the electro-acoustic path of the source audio signal, secondary path estimate performance monitor 48 may cause the response SEG(z) to be updated with the response SE(z) on a periodic basis. On the other hand, when secondary path estimate performance monitor 48 determines that secondary path estimate filter 34A is not sufficiently modeling the electro-acoustic path of the source audio signal, secondary path estimate performance monitor 48 may freeze the update of SEG(z). In some embodiments, whenever the response SEG(z) is to be updated, smoothing or cross-fading may be applied to transition the response SEG(z) from its current response to its updated response.
In addition, in some embodiments, secondary path estimate performance monitor 48 may update response SEG(z) at an update frequency dependent upon a value of SEPI. For example, if SEPI is below a first threshold value, secondary path estimate performance monitor 48 may cause response SEG(z) to update at a first update frequency. If SEPI is above the first threshold value but below a second threshold value, secondary path estimate performance monitor 48 may cause response SEG(z) to update at a second update frequency which is lesser than the first update frequency. If SEPI is above the second threshold value, secondary path estimate performance monitor 48 may cause response SEG(z) to cease updating.
Referring now to
As depicted in
Thus, if secondary response SE(z) closely tracks the actual secondary response S(z), then the modified source audio signal will approximately equal the unmodified source audio signal.
The approach set forth in
Referring now to
By transforming reference microphone signal ref with a copy of the estimate of the effective response of path S(z), response SEeff_COPY(z), and minimizing the ambient audio sounds in the error microphone signal, adaptive filter 32 may adapt to the desired response of P(z)/Seff(z). In addition to error microphone signal err, the signal compared to the output of filter 34B by W coefficient control block 31 may include an inverted amount of downlink audio signal ds and/or internal audio signal ia that has been processed by a filter response SE(z). Filter 54B may not be an adaptive filter, per se, but may have an adjustable response that is tuned to match the response of adaptive filter 54A, so that the response of filter 54B tracks the adapting of adaptive filter 54A.
To implement the above, adaptive filter 54A may have coefficients controlled by SE coefficient control block 33B, which may compare an injected, substantially inaudible noise signal nsp and error microphone signal err after removal by combiner 37 of noise signal nsp that has been filtered by adaptive filter 54A having response SE(z) to represent the expected noise signal nsp delivered to error microphone E. Thus, SE coefficient control block 33B may correlate the noise signal nsp with the components of noise signal nsp that are present in error microphone signal err in order to generate response SEeff(z) of adaptive filter 54A to minimize the error microphone signal.
Downlink audio signal ds and/or internal audio signal may be filtered by secondary estimate filter 34A having response SE(z). The filtered downlink audio signal ds and/or internal audio signal may be subtracted from error signal err by a combiner 36 to generate a playback-corrected error (shown as PBCE in
Furthermore, in order to generate response SE(z) of adaptive filter 34A, an SE construction block 58 may determine response SE(z) from response SEeff(z). For example, SE construction block 58 may calculate response SE(z) in accordance with the following equation:
For example, in order to implement a filter that has a response as in the foregoing equation, one may construct a finite impulse response filter directly using the frequency response of terms on the right side of the equation. As another example, one may construct a filter with such a response using several finite impulse response and/or infinite impulse response blocks.
This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present inventions have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.
