Prevention of ANC instability in the presence of low frequency noise
AssigneeApple Inc.
InventorsVladan Bajic, Thomas M. Jensen
Life of this patent
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Abstract
An active noise control (ANC) processor has an adaptive filter that uses a reference signal to produce an anti-noise signal, and an error signal to evaluate cancellation performance. An adaptive filter algorithm engine configures the filter coefficients of the adaptive filter, in accordance with pre-shaped versions of the error and reference signals. The pre-shaping filter has a high-pass transfer function and enables the adaptive algorithm engine to increase noise cancellation performance in a high frequency band during the presence of focused or narrow-band noise energy in a low frequency band. Other embodiments are also described and claimed.
Claims10
An audio apparatus comprising:
- a reference microphone to produce a reference signal;
- an error microphone to produce an error signal;
- and
- an active noise control (ANC) processor having
- an adaptive filter to use the reference signal and produce an anti-noise signal,
- an adaptive filter algorithm engine to configure digital filter coefficients of the adaptive filter in accordance with signals at a first input and a second input,
- a first pre-shaping filter to filter the reference signal at the first input,
- a second pre-shaping filter to filter the error signal at the second input,
- wherein each of the first and second pre-shaping filters has a high pass transfer function that is designed to suppress energy in a low frequency band, relative to a high frequency band, by having a knee between 100 Hz and 400 Hz and a roll-off slope between 10 dB/octave and 25 dB/octave.
A method for active noise control (ANC) that enables an adaptive filter algorithm engine that adapts a digital adaptive filter to produce anti-noise in a high frequency band during the presence of focused or narrowband noise energy in a low frequency band, comprising:
- pre-shaping a reference microphone signal in accordance with a transfer function with a high-pass section having a knee between 100 Hz and 500 Hz and a roll-off slope between 10 dB/octave and 25 dB/octave;
- pre-shaping an error microphone signal in accordance with a transfer function with a high-pass section having a knee between 100 Hz and 500 Hz and a roll-off slope between 10 dB/octave and 25 dB/octave;
- and performing an adaptive filter process to configure digital filter coefficients of an adaptive filter in accordance with the pre-shaped reference microphone signal and the pre-shaped error microphone signal.
A mobile phone comprising:
- a mobile phone handset housing having therein
- an earpiece speaker;
- an audio source to produce an audio user content signal;
- a reference microphone to produce a reference signal;
- an error microphone positioned closer to the earpiece speaker than the reference microphone to produce an error signal;
- and
- an active noise control (ANC) processor having an adaptive filter to use the reference signal to produce an anti-noise signal that is combined with the audio user content signal to drive the earpiece speaker, a first pre-shaping filter to filter the reference signal, a second pre-shaping filter to filter the error signal, an adaptive filter algorithm engine to configure filter coefficients of the adaptive filter in accordance with the filtered reference and error signals, wherein each of the first and second pre-shaping filters has a high pass transfer function that is designed to suppress energy in a low frequency band, relative to a high frequency band, to prevent instability of the ANC processor in the high frequency band during the presence of narrowband noise energy in the low frequency band, wherein the low frequency band is 5 Hz to 400 Hz, and the high frequency band is 400 Hz to 1 kHz, and wherein the high pass transfer function has a knee between 100 Hz and 400 Hz and a roll-off slope between 10 dB/octave and 25 dB/octave.
Dependencies are inferred from the claim text.