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 the update of one or more acoustical path estimates is tailored to avoid instability due to external changes.
2. Background of the Invention
Telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as personal audio 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. Other audio devices may also benefit from noise canceling, or may be provided for the purpose of noise canceling.
Since the acoustic environment around personal audio devices 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. In some cases, adaptive noise canceling circuits can generate undesirable results under certain circumstances.
Therefore, it would be desirable to provide a personal audio device, including a telephone that provides robust noise cancellation that is effective and/or does not generate undesirable responses when external conditions change.
The above-stated objectives of providing a personal audio device having robust performance in response to changing external conditions is accomplished in a personal audio system, a method of operation, and an integrated circuit.
The personal audio device includes an output transducer for reproducing an audio signal that includes 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 personal audio device also includes the integrated circuit to provide adaptive noise-canceling (ANC) functionality. The method is a method of operation of the personal audio system and integrated circuit. A microphone is mounted on the device housing to provide a microphone signal indicative of the ambient audio sounds at the output of the transducer. An ANC processing circuit adaptively generates an anti-noise signal in conformity with the microphone signal, so that ambient audio sounds are canceled. The processing circuit adapts the response of the adaptive filter by adjusting the coefficients of the at least one adaptive filter according to an error signal generated from the microphone signal. If the magnitude of the error is greater than a threshold value, the processing circuit ceases updating of the coefficients of the at least one adaptive filter, reducing disruption of operation by samples that might otherwise de-stabilize the control of the adaptive filter or otherwise generate an undesirable response. The threshold value may be determined from an average value of the error signal.
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.
Noise canceling techniques and circuits that can be implemented in a personal audio device, such as a wireless telephone, are disclosed. 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 using at least one adaptive filter. A microphone is provided to measure the ambient acoustic environment at the transducer output giving an indication of the effectiveness of the noise cancelation. An error signal generated from the microphone output is used to control adaptation of the response of the adaptive filter to minimize the error signal. An additional secondary path estimating adaptive filter may be used to remove the playback audio from the error microphone signal in order to generate the error signal. In order to prevent improper adaptation or instabilities in one or both of the adaptive filters, the cost function of the adaptive filters is modified, such that if the magnitude of the error signal is greater than a threshold value for an update, the update is skipped. The threshold may be determined as a measurement of ambient noise, so that in high noise conditions, the error is allowed to be larger while still updating the filter coefficients.
Referring now to
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 at an error microphone reference position ERP, when wireless telephone 10 is in close proximity to ear 5. Exemplary circuits 14 within wireless telephone 10 include 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 a RF integrated circuit 12 containing the wireless telephone transceiver. In alternative 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 of the present invention 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. 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 at error microphone E, i.e. at error microphone reference position ERP. 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) 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. The coupling between speaker SPKR and error microphone E 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. Since the user of wireless telephone 10 actually hears the output of speaker SPKR at a drum reference position DRP, differences between the signal produced by error microphone E and what is actually heard by the user are shaped by the response of the ear canal, as well as the spatial distance between error microphone reference position ERP and drum reference position DRP. At higher frequencies, the spatial differences lead to multi-path nulls that reduce the effectiveness of the ANC system, and in some cases may increase ambient noise. While the illustrated wireless telephone 10 includes a two microphone ANC system with a third near speech microphone NS, some aspects of the techniques disclosed herein may be practiced in a system that does not include separate error and reference microphones, or a wireless telephone using 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 can be omitted.
Referring now to
Referring now to
To implement the above, an adaptive filter 34A has coefficients controlled by a SE coefficient control block 33, which updates based on correlated components of downlink audio signal ds and an error value. SE coefficient control block 33 correlates the actual downlink speech signal ds with the components of downlink audio signal ds that are present in error microphone signal err. Adaptive filter 34A is thereby adapted to generate a signal from downlink audio signal ds, 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 in error signal e.
Under certain conditions, such as near speech or wind noise entering reference microphone R and/or error microphone E, or when mechanical events occur such as the listener's fingernails scratching on the housing of wireless telephone 10, response W(z) can become unstable, and the coefficient values produced by W coefficient control block 31 can quickly deviate from values that will provide proper noise cancellation.
Where ƒ(e(n)) is the cost function that is minimized by the adaptive filter control loop. The resulting operation prevents sudden events such as near speech and the mechanical noises and wind noise mentioned above, from reacting to error e(n) having a magnitude that exceeds threshold Δ, which adds to robustness of the ANC operation. Because thresholds Δ and −Δ are applied to the computed error, the reaction of W coefficient control block 31 and SE coefficient control block 33 can be on a per-update basis, which could be as frequent as once-per-input-sample.
Effectively, samples that would cause the error e to exceed the threshold values Δ or −Δ will be discarded, preventing them from contributing to error and instability. A larger group of samples could be used, so that a control of the size of a tolerated disturbance could be adjusted. The technique described herein provides a measure of a peak-to-average ratio of the error, and other such measurements could be used. In one implementation, observing the error with two different time constants gives a measure of change due to instability. For example, the comparison of individual samples of the error to the local average error can be used to trigger rejection of samples containing a disturbance. Non-linear filtering, e.g., rules such as: “ignore the next n samples when the threshold has crossed” could be used to provide additional filtering. Threshold Δ can be variable, and set according to the amplitude of ambient noise. Similarly, the same sort of threshold application, with potentially different thresholds, can be applied within SE coefficient control block 33, in which case the threshold can be based upon the magnitude of the training signal, e.g., source audio (ds+ia).
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 and 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. 61/787,802 filed on Mar. 15, 2013.
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Number | Date | Country | |
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61787802 | Mar 2013 | US |