Internal dynamic range control in an adaptive noise cancellation (ANC) system

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 dynamic range of signal pathways is improved by filtering images.

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 adaptive noise canceling (ANC) using a reference microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal having an adaptive characteristic into the output of the device to cancel the ambient acoustic events.

The dynamic range of digital audio signal processors, such as the ANC system described above, is set by the width of the signal pathways, which provides a trade-off in circuit complexity, power consumption, and area. Under certain ambient conditions, the dynamic range requirement of an ANC system may be much greater than under nominal conditions, but in order to avoid clipping distortion, the dynamic range of the signal pathways must be sufficient to support the range of signals encountered during operation.

Therefore, it would be desirable to provide a personal audio device, including a wireless telephone that provides noise cancellation that has dynamic range sufficient to avoid clipping distortion, while maintaining low power operation and without requiring significantly larger circuit area.

The above-stated objectives of providing a personal audio device having adaptive noise cancellation (ANC) without clipping distortion while maintaining low power operation and without requiring significantly larger circuit area, 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. One or more microphones are mounted on the device housing to provide a signal indicative of the ambient audio sounds and optionally the output of the transducer. The personal audio system further includes an ANC processing circuit for adaptively generating an anti-noise signal from the one or more microphone signals, such that the anti-noise signal causes substantial cancellation of the ambient audio sounds. One or more adaptive filters are used to generate the anti-noise signal from the one or more microphone signals, which are quantized by a delta-sigma analog-to-digital converter (ADC), a separate delta-sigma noise shaper, or both. The ANC processing circuit further implements a low-pass filter that removes quantization noise images at the output of the adaptive filter to reduce the dynamic required at the output of the adaptive filter.

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.

FIG. 1A is an illustration of a wireless telephone 10 coupled to an earbud EB, which is an example of a personal audio device in which the techniques disclosed herein can be implemented.

FIG. 1B is an illustration of electrical and acoustical signal paths in FIG. 1A .

FIG. 2 is a block diagram of circuits within wireless telephone 10 and/or earbud EB of FIG. 1A .

FIG. 3A is a block diagram depicting one example of an ANC circuit 30 A that can be used to implement ANC circuit 30 of CODEC integrated circuit 20 of FIG. 2 .

FIG. 3B is a block diagram depicting another example of an ANC circuit 30 B that can be used to implement ANC circuit 30 of CODEC integrated circuit 20 of FIG. 2 .

FIG. 4 is a block diagram depicting signal processing circuits and functional blocks that can be used to implement the circuits depicted in FIG. 2 and FIGS. 3A-3B .

FIG. 5 is a waveform diagram depicting signals within the circuits depicted in FIG. 2 and FIGS. 3A-3B .

FIG. 6 is another waveform diagram depicting signals within the circuits depicted in FIG. 2 and FIGS. 3A-3B .

The present invention encompasses noise-canceling techniques and circuits that can be implemented in a personal audio system, such as a wireless telephone and connected earbuds. The personal audio system includes an adaptive noise canceling (ANC) circuit that measures the ambient acoustic environment at the earbuds or other output transducer and generates a signal that is injected in the speaker (or other transducer) output to cancel ambient acoustic events. One or more microphones are provided to measure the ambient acoustic environment, which is used to generate an anti-noise signal provided to the speaker to cancel the ambient audio sounds. One or more adaptive filters are used to generate the anti-noise signal from the one or more microphone signals, which are quantized by a delta-sigma analog-to-digital converter (ADC), a separate delta-sigma noise shaper, or both. The ANC processing circuit further implements a low-pass filter that removes quantization noise images at the output of the adaptive filter to reduce the dynamic required at the output of the adaptive filter. Since ANC performance is strongly affected by the latency of the anti-noise signal path, inserting filters in series with the adaptive filter will reduce performance due to increased latency. Therefore, there is a tradeoff between the dynamic range required to represent the output of the adaptive filter without clipping, and the latency of an ANC system that includes filtering of the adaptive filter output. The corner frequency of the low-pass filter is chosen to provide the best compromise between the dynamic range margin available for the anti-noise signal, and/or other internal signal paths that have quantization noise images, and the latency of the ANC system.

FIG. 1A shows a wireless telephone 10 proximate to a human ear 5 . Illustrated wireless telephone 10 is an example of a device in which the techniques herein may be employed, but it is understood that not all of the elements or configurations illustrated in wireless telephone 10 , or in the circuits depicted in subsequent illustrations, are required. Wireless telephone 10 is connected to an earbud EB by a wired or wireless connection, e.g., a BLUETOOTH™ connection (BLUETOOTH is a trademark or Bluetooth SIG, Inc.). Earbud EB has a transducer, such as a speaker SPKR, which reproduces source audio including distant speech received from wireless telephone 10 , ringtones, stored audio program material, and injection of near-end speech (i.e., the speech of the user of wireless telephone 10 ). The source audio also includes any other audio that wireless telephone 10 is required to reproduce, such as source audio from web-pages or other network communications received by wireless telephone 10 and audio indications such as battery low and other system event notifications. A reference microphone R is provided on a surface of a housing of earbud EB for measuring the ambient acoustic environment. Another 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 earbud EB is inserted in the outer portion of ear 5 . While the illustrated example shows an earspeaker implementation of a noise-canceling system, the techniques disclosed herein can also be implemented in a wireless telephone or other personal audio device, in which the output transducer and reference/error microphones are all provided on a housing of the wireless telephone or other personal audio device.

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. 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 embodiments of the invention, 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. Alternatively, the ANC circuits may be included within a housing of earbud EB or in a module located along a wired connection between wireless telephone 10 and earbud EB. For the purposes of illustration, the ANC circuits will be described as provided within wireless telephone 10 , but the above variations are understandable by a person of ordinary skill in the art and the consequent signals that are required between earbud EB, wireless telephone 10 and a third module, if required, can be easily determined for those variations. A near speech microphone NS is provided at a housing of wireless telephone 10 to capture near-end speech, which is transmitted from wireless telephone 10 to the other conversation participant(s). Alternatively, near speech microphone NS may be provided on the outer surface of a housing of earbud EB, or on a boom (microphone extension) affixed to earbud EB.

FIG. 1B shows a simplified schematic diagram of audio CODEC integrated circuit 20 that includes ANC processing, as coupled to reference microphone R, which provides a measurement of ambient audio sounds Ambient that is filtered by the ANC processing circuits within audio CODEC integrated circuit 20 . Audio CODEC integrated circuit 20 generates an output that is amplified by an amplifier A 1 and is provided to speaker SPKR. Audio CODEC integrated circuit 20 receives the signals (wired or wireless depending on the particular configuration) from reference microphone R, near speech microphone NS and error microphone E and interfaces with other integrated circuits such as RF integrated circuit 12 containing the wireless telephone transceiver. In other configurations, 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. Alternatively, multiple integrated circuits may be used, for example, when a wireless connection is provided from earbud EB to wireless telephone 10 and/or when some or all of the ANC processing is performed within earbud EB or a module disposed along a cable connecting wireless telephone 10 to earbud EB.

In general, the ANC techniques illustrated 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 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) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR. The estimated response includes the coupling between speaker SPKR and error microphone E in the particular acoustic environment which is affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to earbud EB. Leakage, i.e., acoustic coupling, between speaker SPKR and reference microphone R can cause error in the anti-noise signal generated by the ANC circuits within CODEC IC 20 . In particular, desired downlink speech and other internal audio intended for reproduction by speaker SPKR can be partially canceled due to the leakage path L(z) between speaker SPKR and reference microphone R. Since audio measured by reference microphone R is considered to be ambient audio that generally should be canceled, leakage path L(z) represents the portion of the downlink speech and other internal audio that is present in the reference microphone signal and causes the above-described erroneous operation. Therefore, the ANC circuits within CODEC IC 20 include leakage-path modeling circuits that compensate for the presence of leakage path L(z). While the illustrated wireless telephone 10 includes a two microphone ANC system with a third near speech microphone NS, a system may be constructed that does not include separate error and reference microphones. Alternatively, when near speech microphone NS is located proximate to speaker SPKR and error microphone E, near speech microphone NS may be used 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 FIG. 2 , circuits within wireless telephone 10 are shown in a block diagram. The circuit shown in FIG. 2 further applies to the other configurations mentioned above, except that signaling between CODEC integrated circuit 20 and other units within wireless telephone 10 are provided by cables or wireless connections when CODEC integrated circuit 20 is located outside of wireless telephone 10 . In such a configuration, signaling between CODEC integrated circuit 20 and error microphone E, reference microphone R and speaker SPKR are provided by wired or wireless connections when CODEC integrated circuit 20 is located within wireless telephone 10 . CODEC integrated circuit 20 includes 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. CODEC integrated circuit 20 also includes 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 error microphone signal. CODEC IC 20 generates an output for driving speaker SPKR from amplifier A 1 , which amplifies the output of a delta-sigma modulated digital-to-analog converter (DAC) 23 that receives the output of a combiner 26 . Combiner 26 combines audio signals is from internal audio sources 24 , and the anti-noise signal anti-noise generated by an 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 . Combiner 26 also combines an attenuated portion of near speech signal ns, i.e., sidetone information st, so that the user of wireless telephone 10 hears their own voice in proper relation to downlink speech ds, which is received from a radio frequency (RF) integrated circuit 22 . Near speech signal ns is also provided to RF integrated circuit 22 and is transmitted as uplink speech to the service provider via antenna ANT.

Referring now to FIG. 3A , details of ANC circuit 30 A are shown that can be used to implement ANC circuit 30 of FIG. 2 . A combiner 36 A removes an estimated leakage signal from reference microphone signal ref, which in the example is provided by a leakage-path adaptive filter 38 having a response LE(z) that models leakage path L(z). Combiner 36 A generates a leakage-corrected reference microphone signal ref′. A delta-sigma shaper 35 A is used to quantize leakage-corrected reference microphone signal ref′, which reduces the width of subsequent processing stages. Details of a system architecture in which delta-sigma shapers are employed to decrease the width of filters are disclosed in U.S. Patent Application Publication U.S. 20120308025A1 entitled “AN ADAPTIVE NOISE CANCELING ARCHITECTURE FOR A PERSONAL AUDIO DEVICE”, the disclosure of which is incorporated herein by reference. An adaptive filter 32 receives delta-sigma modulated leakage-corrected reference microphone signal ref′ and under ideal circumstances, adapts its transfer function W(z) to be P(z)/S(z) to generate anti-noise signal anti-noise, which is provided to an output combiner that combines the anti-noise signal with the audio to be reproduced by speaker SPKR, as exemplified by combiner 26 of FIG. 2 . The coefficients of adaptive filter 32 are controlled by a W coefficient control block 31 A that uses a correlation of two signals to determine the response of adaptive filter 32 , which generally minimizes the error, in a least-mean squares sense, between those components of leakage-corrected reference microphone signal ref′ present in error microphone signal err. The signals processed by W coefficient control block 31 A are the leakage-corrected reference microphone signal ref′ shaped by a copy of an estimate of the response of path S(z) (i.e., response SE COPY (z)) provided by filter 34 B and another signal that includes error microphone signal err. By transforming leakage-corrected reference microphone signal ref′ with a copy of the estimate of the response of path S(z), response SE COPY (z), and minimizing error microphone signal err after removing components of error microphone signal err due to playback of source audio, adaptive filter 32 adapts to the desired response of P(z)/S(z).

The output of adaptive filter 32 is processed by a digital low-pass filter 33 A that removes signal energy that exists above the operational band of adaptive filter 32 , i.e., above the audio frequency range to which W coefficient control block 31 A adapts the response of adaptive filter 32 . Since response W(z) may have a high gain at some frequencies, at higher audio frequencies when response S(z) has low amplitude as when wireless telephone 10 is off-ear, the amplitude of anti-noise signal anti-noise is increased. Anti-noise signal anti-noise contains not only audio components, but the quantization noise introduced by delta-sigma shaper 35 A as multiplied by images of response W(z) repeated at frequency intervals corresponding to the sample rate of adaptive filter 32 divided by the oversampling ratio of the signal at the input to the adaptive filter 32 . Thus, an increase in the gain of adaptive filter 32 not only increases the amplitude of in-band components of anti-noise signal anti-noise, but out-of-band quantization noise, as well. Referring to FIG. 5 , an illustration of the frequency distribution of quantization noise 50 is shown with respect to a wideband response 52 of adaptive filter 32 . A detail 54 of wideband response 52 of adaptive filter 32 is shown to illustrate a condition in which a high amplitude peak 56 is present in response W(z) due to wireless telephone 10 being off-ear. Such a condition is an example of a condition in which wideband response 52 of adaptive filter 32 might cause clipping due to the product of wideband response 52 and quantization noise 50 which will have significant energy above an audio band of interest bw. Typically, quantization noise in anti-noise signal anti-noise would not be filtered, since transducer SPKR would not be able to physically reproduce those out-of-band components. However, due to the wide dynamic range that response W(z) may have to assume under different ambient conditions, low-pass filter 33 A provides a mechanism to reduce the impact of increases in the magnitude of W(z) on the dynamic range of anti-noise signal anti-noise, which could cause clipping if insufficient digital signal width were unavailable to reproduce the full spectrum of anti-noise signal anti-noise. Referring to FIG. 6 , a graph showing a first peak amplitude 60 of anti-noise signal anti-noise without low-pass filter 33 A and a second peak amplitude 62 of anti-noise signal anti-noise with low-pass filter 33 A illustrates an improvement in dynamic range headroom for anti-noise signal anti-noise of 20 dB.

Referring again to FIG. 3A , in addition to error microphone signal err, the other signal processed along with the output of filter 34 B by W coefficient control block 31 A includes an inverted amount of the source audio (ds+ia) including downlink audio signal ds and internal audio ia. Source audio (ds+ia) has also been processed by a delta-sigma shaper 35 B that is similar to delta-sigma shaper 35 A, reduces the required width of the filters that follow in the signal path, including leakage path adaptive filter 38 and a secondary path adaptive filter 34 A. Source audio (ds+ia) is processed by secondary path adaptive filter 34 A having response SE(z), of which response SE COPY (z) is a copy. Filter 34 B is not an adaptive filter, per se, but has 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 secondary path adaptive filter 34 A. By injecting an inverted amount of source audio (ds+ia) that has been filtered by response SE(z), adaptive filter 32 is prevented from adapting to the relatively large amount of source audio (ds+ia) present in error microphone signal err. By transforming the inverted copy of downlink audio signal ds and internal audio ia with the estimate of the response of path S(z), the source audio (ds+ia) that is removed from error microphone signal err before processing should match the expected version of downlink audio signal ds and internal audio ia reproduced at error microphone signal err. The source audio (ds+ia) matches the amount of source audio (ds+ia) present in error microphone signal err because the electrical and acoustical path of S(z) is the path taken by source audio (ds+ia) to arrive at error microphone E.

To implement the above, secondary path adaptive filter 34 A has coefficients controlled by a SE coefficient control block 31 B, which processes the source audio (ds+ia) and error microphone signal err after removal, by a combiner 36 C, of the above-described filtered downlink audio signal ds and internal audio ia, that has been filtered by adaptive filter 34 A to represent the expected source audio delivered to error microphone E. Adaptive filter 34 A 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). Similarly, a LE coefficient control block 31 C also is adapted to minimize the components of source audio (ds+ia) present in leakage-corrected reference microphone signal ref′, by adapting to generate an output that represents the source audio (ds+ia) present in reference microphone signal ref.

As with adaptive filter 32 , both secondary path adaptive filter 34 A and leakage path adaptive filter 38 have images that can increase the amplitude of quantization noise introduced by a delta-sigma shaper 35 B. Therefore, another low-pass filter 33 B is introduced between leakage path adaptive filter 38 and combiner 36 A and a low-pass filter 33 C is introduced between secondary path adaptive filter 34 A and a combiner 36 C. Each of low-pass filters 33 B and 33 C will generally have the same type of amplitude response as low-pass filter 33 A, e.g., a first-order low-pass response with a corner frequency above the audio band of interest of the ANC system. Alternatively, higher-order filters could be used. Low pass filters 33 A, 33 B and 33 C are in series with, and thus can be merged with, adaptive filter 32 , secondary path adaptive filter 34 A, and leakage path adaptive filter 32 , respectively. W coefficient control block 31 A, SE coefficient control block 31 B and LE coefficient control block 31 C are prevented from causing the responses of adaptive filter 32 , secondary path adaptive filter 34 A, and leakage path adaptive filter 32 , respectively, to adapt to cancel the responses of low pass filters 33 A, 33 B and 33 C, respectively, since W coefficient control block 31 A, SE coefficient control block 31 B and LE coefficient control block 31 C are operating at the baseband sample rate and not the oversampled rate at which adaptive filter 32 , secondary path adaptive filter 34 A, and leakage path adaptive filter 32 operate. Further the respective feedback signals that control W coefficient control block 31 A, SE coefficient control block 31 B and LE coefficient control block 31 C are filtered and decimated down to the baseband rate. If significant phase shift is present in the audio band of interest due to any of low-pass filters 33 A- 33 C, corresponding phase-shifts may be introduced as needed to compensate. An exemplary response for low-pass filters 33 A- 33 C might be a single pole roll-off with a corner frequency of 5 times the maximum frequency of the audio band of interest, e.g., 100 kHz for an ANC system with a potential maximum cancellation frequency of 20 kHz.

FIG. 3A also illustrates another feature that may be optionally included to decrease the change of clipping by reducing out-of-band energy in anti-noise signal anti-noise. A gain block g 1 is optionally included to multiply the amplitude of source audio (ds+ia) by a gain factor, e.g, 20 dB, prior to delta-sigma shaper 35 B. By increasing the gain of the signal path in front of secondary path adaptive filter 34 A and leakage path adaptive filter 38 , after adaptation, the gains of secondary path adaptive filter 34 A and leakage path adaptive filter 38 will be decreased by a corresponding amount. By forcing a lower gain for secondary path adaptive filter 34 A and leakage path adaptive filter 38 , the images of responses SE(z) and LE(z) that would otherwise multiply the high-frequency quantization noise are reduced. An advantage of lowering the gains of secondary path adaptive filter 34 A and leakage path adaptive filter 38 , rather than only providing a low-pass filter at their outputs, is that no additional latency is introduced. Both the gain reduction and low-pass filtering can be applied in combination, which can provide for a higher corner frequency of the low-pass filters to achieve similar dynamic range performance to a system without gain reduction and having a lower corner frequency, thus providing lower latency. Increasing the gain before delta-sigma shaper 35 B could cause clipping if the amplitude of source audio (ds+ia) becomes too great. However, under such conditions, error microphone E and reference microphone R will generally also be in a clipping condition, due to high amplitude output from transducer SPKR. In addition to reducing the potential for clipping at the outputs of secondary path adaptive filter 34 A and leakage path adaptive filter 38 , including gain block g 1 also provides for increased stability and simplifies the design of decimators included in other portions of the signal path as disclosed in the above-incorporated U.S. Patent Application Publication “AN ADAPTIVE NOISE CANCELING ARCHITECTURE FOR A PERSONAL AUDIO DEVICE.” Since there are closed loops present in the ANC system, the decimators must be designed to have unity gain or less for out-of-band energy, so that portions of the ANC system do not become unstable, causing in-band non-linear operation.

FIG. 3B shows another example of details of an alternative ANC circuit 30 B that can be used to implement ANC circuit 30 of FIG. 2 . ANC circuit 30 B is similar to ANC circuit 30 A of FIG. 3A , so only differences between ANC circuit 30 B and ANC circuit 30 A will be discussed below. ANC circuit 30 B implements a feedback noise canceling system in which the anti-noise signal is provided by filtering error signal e with a predetermined response FB(z) using a fixed filter 32 A. As in ANC circuit 30 A of FIG. 3A , low-pass filter 33 A filters anti-noise signal anti-noise to remove energy above the audio band of interest that might otherwise cause clipping under certain conditions.

Referring now to FIG. 4 , a block diagram of an ANC system is shown for implementing ANC techniques as depicted in FIG. 3 , and having a processing circuit 40 as may be implemented within CODEC integrated circuit 20 of FIG. 2 . Processing circuit 40 includes a processor core 42 coupled to a memory 44 in which are stored program instructions comprising a computer-program product that may implement some or all of the above-described ANC techniques, as well as other signal processing. Optionally, a dedicated digital signal processing (DSP) logic 46 may be provided to implement a portion of, or alternatively all of, the ANC signal processing provided by processing circuit 40 . Processing circuit 40 also includes ADCs 21 A- 21 C, for receiving inputs from reference microphone R, error microphone E and near speech microphone NS, respectively. In alternative embodiments in which one or more of reference microphone R, error microphone E and near speech microphone NS have digital outputs, the corresponding ones of ADCs 21 A- 21 C are omitted and the digital microphone signal(s) are interfaced directly to processing circuit 40 . DAC 23 and amplifier A 1 are also provided by processing circuit 40 for providing the speaker output signal, including anti-noise as described above. The speaker output signal may be a digital output signal for provision to a module that reproduces the digital output signal acoustically.

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.