Internal dynamic range control in an adaptive noise cancellation (ANC) system
First Claim
1. A personal audio device, comprising:
- 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;
at least one microphone mounted on the housing for providing at least one microphone signal indicative of the ambient audio sounds;
a delta-sigma modulator for quantizing the at least one microphone signal at an oversampled rate substantially higher than a baseband audio rate of the audio signal; and
a processing circuit that generates the anti-noise signal using an adaptive filter operating at the oversampled rate to reduce the presence of the ambient audio sounds heard by the listener in conformity with the at least one microphone signal, wherein a wideband response of an output of the adaptive filter includes a first lowest-frequency image and multiple higher-frequency images at multiples of the oversampled rate, wherein the processing circuit further implements a digital low-pass filter having an input coupled to an the output of the adaptive filter to remove at least some of the higher-frequency images of the quantized microphone signal that appear in the output of the adaptive filter to reduce the dynamic range required by the output of the adaptive filter, and wherein the digital low-pass filter has a corner frequency greater than a maximum frequency of an the first lowest-frequency image in the output of the adaptive filter.
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Abstract
A personal audio device, such as a headphone, includes an adaptive noise canceling (ANC) circuit that adaptively generates an anti-noise signal using one or more microphone signals that measure the ambient audio. The anti-noise signal is combined with source audio to provide an output for a speaker. The anti-noise signal causes cancellation of ambient audio sounds that appear in the microphone signals. A processing circuit uses the reference microphone to generate the anti-noise signal using one or more adaptive filters. The processing circuit also includes low-pass filters that remove quantization noise images at the output of the adaptive filter to reduce the dynamic range required at the output of the adaptive filter.
278 Citations
24 Claims
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1. A personal audio device, comprising:
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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; at least one microphone mounted on the housing for providing at least one microphone signal indicative of the ambient audio sounds; a delta-sigma modulator for quantizing the at least one microphone signal at an oversampled rate substantially higher than a baseband audio rate of the audio signal; and a processing circuit that generates the anti-noise signal using an adaptive filter operating at the oversampled rate to reduce the presence of the ambient audio sounds heard by the listener in conformity with the at least one microphone signal, wherein a wideband response of an output of the adaptive filter includes a first lowest-frequency image and multiple higher-frequency images at multiples of the oversampled rate, wherein the processing circuit further implements a digital low-pass filter having an input coupled to an the output of the adaptive filter to remove at least some of the higher-frequency images of the quantized microphone signal that appear in the output of the adaptive filter to reduce the dynamic range required by the output of the adaptive filter, and wherein the digital low-pass filter has a corner frequency greater than a maximum frequency of an the first lowest-frequency image in the output of the adaptive filter. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A method of countering effects of ambient audio sounds by a personal audio device, the method comprising:
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adaptively generating an anti-noise signal using an adaptive filter operating at an oversampled rate to reduce the presence of the ambient audio sounds heard by a listener in conformity with at least one microphone signal, wherein a wideband response of an output of the adaptive filter includes a first lowest-frequency image and multiple higher-frequency images at multiples of the oversampled rate; combining the anti-noise signal with source audio; providing a result of the combining to a transducer at a baseband audio rate substantially lower than the oversampled rate of the adaptive filter; measuring the ambient audio sounds with at least one microphone to produce at least one microphone signal indicative of the ambient audio sounds; quantizing the at least one microphone signal at the oversampled rate with a delta-sigma modulator; and filtering the anti-noise signal with a digital low-pass filter to remove at least some of the higher-frequency images of the quantized microphone signal that appear in the output of the adaptive filter to reduce the dynamic range required by the output of the adaptive filter, wherein the digital low-pass filter has a corner frequency greater than a maximum frequency of the first lowest-frequency image in the output of the adaptive filter. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. An integrated circuit for implementing at least a portion of a personal audio device, comprising:
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an output for providing an output signal to an output transducer 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; at least one microphone input for receiving at least one microphone signal indicative of the ambient audio sounds; a delta-sigma modulator for quantizing the at least one microphone signal at an oversampled rate substantially higher than a baseband audio rate of the audio signal; and a processing circuit that adaptively generates the anti-noise signal using an adaptive filter operating at the oversampled rate to reduce the presence of the ambient audio sounds heard by the listener in conformity with the at least one microphone signal, wherein a wideband response of an output of the adaptive filter includes a first lowest-frequency image and multiple higher-frequency images at multiples of the oversampled rate, wherein the processing circuit further implements a digital low-pass filter having an input coupled to the output of the adaptive filter to remove at least some of the higher-frequency images of the quantized microphone signal that appear in the output of the adaptive filter to reduce the dynamic range required by the output of the adaptive filter, and wherein the digital low-pass filter has a corner frequency greater than a maximum frequency of an the first lowest-frequency image in the output of the adaptive filter. - View Dependent Claims (18, 19, 20, 21, 22, 23, 24)
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Specification