1. Field of the Invention
The present invention relates generally to personal audio devices such as headphones that include adaptive noise cancellation (ANC), and, more specifically, to architectural features of an ANC system in which a secondary path estimating response is trained using ambient noise.
2. Background of the Invention
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 reference 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.
Noise canceling operation can be improved by measuring the transducer output of a device at the transducer to determine the effectiveness of the noise canceling using an error microphone. The measured output of the transducer is ideally the source audio, e.g., downlink audio in a telephone and/or playback audio in either a dedicated audio player or a telephone, since the noise canceling signal(s) are ideally canceled by the ambient noise at the location of the transducer. To remove the source audio from the error microphone signal, the secondary path from the transducer through the error microphone can be estimated and used to filter the source audio to the correct phase and amplitude for subtraction from the error microphone signal. However, when source audio is absent, the secondary path estimate cannot typically be updated. In particular, at the beginning of a telephone conversation, the secondary path estimate may be incorrect and there is no source audio available for training the secondary path estimate until downlink speech commences.
Therefore, it would be desirable to provide a personal audio device, including wireless telephones, that provides noise cancellation using a secondary path estimate to measure the output of the transducer and that can adapt the secondary path estimate independent of whether source audio of sufficient amplitude is present.
The above-stated objective of providing a personal audio device providing noise cancelling including a secondary path estimate that can be adapted whether or not source audio has been present, is accomplished in a personal audio device, a method of operation, and an integrated circuit.
The personal audio device includes a housing, with a transducer mounted on the housing for reproducing an audio signal that includes both source audio for plackback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer. An error microphone is mounted on the housing to provide an error microphone signal indicative of the transducer output and the ambient audio sounds. The personal audio device further includes an adaptive noise-canceling (ANC) processing circuit within the housing for adaptively generating an anti-noise signal from the error microphone signal such that the anti-noise signal causes substantial cancellation of the ambient audio sounds. The processing circuit controls adaptation of a secondary path adaptive filter for compensating for the electro-acoustical path from the output of the processing circuit through the transducer, wherein the processing circuit removes source audio as shaped by the secondary path response from the error microphone signal to provide an error signal. The processing circuit provides ambient noise to the secondary path adaptive filter's coefficient control circuit as a training signal for adapting the secondary path response. The ambient noise provided to the coefficient control circuit may be the anti-noise signal generated from the reference microphone signal, and the ambient noise present at the error microphone removed from the error microphone signal using a primary path modeling adaptive filter to generate an error signal that contains only the components of the error microphone signal due to the anti-noise reproduced by the transducer. The response of the primary path modeling adaptive filter is earlier adapted using the error microphone signal and the reference microphone signal, so that components of the error microphone signal appearing in an output of the primary path adaptive filter are minimized.
The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.
The present disclosure reveals 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 adaptive noise canceling (ANC) circuit that measures the ambient acoustic environment and generates a signal that is injected into the speaker (or other transducer) output to cancel ambient acoustic events. A reference microphone is provided to measure the ambient acoustic environment, and an error microphone is included to measure the ambient audio and transducer output at the transducer, thus giving an indication of the effectiveness of the noise cancellation. A secondary path estimating adaptive filter is used to remove the playback audio from the error microphone signal, in order to generate an error signal. However, depending on the presence (and level) of the audio signal reproduced by the personal audio device, e.g., downlink audio during a telephone conversation or playback audio from a media file/connection, the secondary path adaptive filter may not be able to continue to adapt to estimate the secondary path response. Further, at the beginning of a telephone conversation, not only may downlink audio be absent, but any previous secondary path model may be inaccurate due to a different position of the wireless telephone with respect to the user's ear. The techniques disclosed herein use ambient noise to provide enough energy for the secondary path estimating adaptive filter to continue to adapt, in a manner that is unobtrusive to the user. The anti-noise signal may be provided to the secondary path adaptive filter, in order to provide a training signal for adapting the secondary path response estimate. The error microphone signal is corrected to remove components due to ambient noise present at the error microphone, leaving only components due to the anti-noise signal. The components due to ambient noise are removed using a primary path response modeling adaptive filter that has been previously adapted to model the primary path response.
Wireless telephone 10 includes adaptive noise canceling (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 is provided for measuring the ambient acoustic environment and is positioned away from the typical position of a user's mouth, so that the near-end speech is minimized in the signal produced by reference microphone R. A third microphone, error microphone E, is 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. An exemplary circuit 14 within wireless telephone 10 includes an audio CODEC integrated circuit 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 an RF integrated circuit 12 containing the wireless telephone transceiver. In other implementations, the circuits and techniques disclosed herein may be incorporated in a single integrated circuit that contains 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 general, the ANC techniques disclosed herein measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and also measure the same ambient acoustic events impinging on error microphone E. The ANC processing circuits of illustrated 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 present at error microphone E. Since acoustic path P(z) extends from reference microphone R to error microphone E, the ANC circuits are essentially estimating acoustic path P(z) combined with removing effects of an electro-acoustic path S(z). Electro-acoustic path S(z) 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. Path S(z) is 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, other systems that do not include separate error and reference microphones can implement the above-described techniques. Alternatively, near speech microphone NS can be used to perform the function of the reference microphone R in the above-described system. Finally, 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 can be omitted.
Referring now to
To implement the above, secondary path adaptive filter 34A has coefficients controlled by a SE coefficient control block 33, which processes the source audio (ds+ia) and error microphone signal err after removal, by a combiner 36B, of the above-described filtered downlink audio signal ds and internal audio ia, that has been filtered by secondary path adaptive filter 34A to represent the expected source audio delivered to error microphone E. Secondary path adaptive filter 34A is thereby adapted to generate an error signal e from downlink audio signal ds and internal audio ia, that when subtracted from error microphone signal err, contains the content of error microphone signal err that is not due to source audio (ds+ia). However, if downlink audio signal ds and internal audio ia are both absent, e.g., at the beginning of a telephone call, or have very low amplitude, SE coefficient control block 33 will not have sufficient input to estimate acoustic path S(z). Therefore, in ANC circuit 30A, when source audio has not been present, the secondary path estimate is updated by using the ambient noise-based SE training mode mentioned above, which uses ambient noise measured by reference microphone R as a training signal for updating response SE(z) of secondary path adaptive filter 34A.
When SE coefficient control 33 needs to be updated, e.g., at the start of a telephone conversation, and a source audio detector 37 indicates that source audio (ds+ia) has insufficient amplitude for training the secondary path response SE(z), control circuit 39 asserts control signal sel to select the ambient noise-based training mode. In order to provide a copy of the ambient noise training signal referenced at the location of error microphone E, an adaptive filter 50 is used to model acoustic path P(z). During an initial training phase with ANC turned off, which is accomplished by de-activating (muting) a controllable amplifier stage 35 in response to de-assertion of a control signal haltPE, adaptive filter models path P(z) by filtering reference microphone signal ref with adaptive filter 50 and subtracting the output of adaptive filter 50 from error microphone signal err using a combiner 36A. Control signal haltSE is also asserted to prevent adaptation of secondary path response SE(z) during adaptation of the primary path response PE(z) of adaptive filter 50. The output of combiner 36A is compared with reference microphone signal err in a PE coefficient control block 51 which is generally a least-mean-squared (LMS) control block, which causes adaptive filter 50 to adapt primary path response PE(z) to match acoustic path P(z). After primary path response PE(z) is adapted, control signal haltPE is asserted, causing PE coefficient control block to maintain primary path response PE(z) at its current value. Subsequently, adaptive filter 50 filters reference microphone signal ref to provide an output that is representative of the ambient noise component of error microphone signal err. Control signal setW is also set to cause coefficient control block 31 to set the response of adaptive filter 32 to a predetermined response for generating the ambient noise training signal, generally a response that should provide some noise cancelling effect while response SE(z) of adaptive filter 34 is being trained, since the ambient noise training signal will be audible as the anti-noise signal anti-noise while secondary path adaptive filter 32 is being adapted. A combiner 36C is used in the ambient noise-based SE training mode (sel=1) to subtract the output of adaptive filter 50 from error microphone signal err. Combiner 36C thus effectively removes the ambient noise component from error microphone signal err, so that error signal e will contain only a component due to anti-noise signal anti-noise, since source audio (ds+ia) is absent or very low in amplitude. During this time, anti-noise signal anti-noise is provided to the input of adaptive filter 34A via selector 38A and control signal haltSE is de-asserted so that SE coefficient control block 33 is allowed to update coefficients to train response SE(z). Once response SE(z) is adapted, control signal sel is de-asserted and control signals haltW and setW are also de-asserted to allow response W(z) to adapt by updating coefficient control block 31.
Referring now to
Referring now to
While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing, as well as other changes in form and details may be made therein without departing from the spirit and scope of the invention.
This U.S. patent application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application Ser. No. 61/787,641 filed on Mar. 15, 2013.
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Number | Date | Country | |
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20140270224 A1 | Sep 2014 | US |
Number | Date | Country | |
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61787641 | Mar 2013 | US |