Adaptive virtual microphone for earbuds
Disclosed are systems and methods for adaptive noise cancellation (ANC) at a virtual microphone located at the eardrum of a wearer of an earphone. The techniques estimate the level of leakiness of ambient noise in the earphone from the magnitude of the gain of the transfer function of the feed forward loop filter of the ANC. Based on the estimated leakiness, the techniques estimate the transfer function of the feed forward loop filter to compensate for the difference in the effect of noise cancellation at the virtual microphone and error microphone. A bias term that represents the difference in the frequency response of the attenuated noise at the virtual and error microphone may be added to the adaptive feed forward algorithm. The ANC operation may apply the bias term to the adaptive feed forward loop filter to reduce the ambient noise at the eardrum rather than at the error microphone.
1. A method for performing adaptive noise cancellation (ANC) for an earphone, the method comprising:
generating, using an adaptive filter of the ANC, an anti-noise signal from ambient noise captured by a reference microphone of the earphone to attenuate leakage noise from the ambient noise, wherein the adaptive filter is being updated by an adaptive filter controller;
determining an estimated level of the leakage noise based on the adaptive filter;
determining a bias term for the adaptive filter based on the estimated level of the leakage noise;
applying the bias term to the adaptive filter to change the anti-noise signal to attenuate the leakage noise received at an ear drum of a user wearing the earphone; and
reducing an error term that is being input to the adaptive filter controller, the error term comprises an error noise signal captured by an error microphone, wherein the error term is reduced by subtracting an estimated change to the error noise signal that is due to the bias term being applied to the adaptive filter.
2. The method of claim 1 , wherein applying the bias term comprises:
adjusting a gain of the adaptive filter for generating the anti-noise signal to attenuate the leakage noise at the eardrum.
3. The method of claim 2 , wherein applying the bias term further comprises:
adapting a transfer function of the adaptive filter by a least-mean-squared (LMS) algorithm to drive the error term toward zero.
4. The method of claim 2 , wherein the error term comprises the error noise signal captured by the error microphone modified to estimate an attenuated noise resulting from the leakage noise attenuated by the anti-noise signal at the ear drum.
5. The method of claim 1 , wherein determining the estimated level of the leakage noise comprises:
determining a relationship between a magnitude of a gain of the adaptive filter, when the adaptive filter is adapted to attenuate the leakage noise at an error microphone, and the leakage noise for a range of levels of the leakage noise; and
estimating the level of the leakage noise in the earphone based on the relationship and the magnitude of the gain of the adaptive filter when the adaptive filter is adapted to reduce the leakage noise at the error microphone.
6. The method of claim 1 , wherein determining the bias term for the adaptive filter comprises:
determining a relationship between a difference in a gain of the adaptive filter when the adaptive filter is adapted to attenuate the leakage noise at an error microphone and at the ear drum for a range of levels of the leakage noise; and
estimating the bias term for the adaptive filter based on the relationship and the estimated level of the noise leakage.
7. The method of claim 1 , wherein applying the bias term to the adaptive filter comprises:
generating an explicit bias to an output of the adaptive filter based on the bias term and the ambient noise captured by the reference microphone; and
adding the explicit bias to the output of the adaptive filter to generate the anti-noise signal.
8. The method of claim 1 , wherein applying the bias term to the adaptive filter comprises:
adapting a transfer function of the adaptive filter to introduce the bias term; and
generating the anti-noise signal based on the transfer function that is adapted and the ambient noise captured by the reference microphone.
9. The method of claim 1 , further comprising:
estimating a first transfer function of an acoustic path for the leakage noise from an error microphone to the ear drum based on the estimated level of the leakage noise;
estimating a second transfer function of an acoustic path for the anti-noise signal from the error microphone to the ear drum based on the estimated level of the leakage noise;
estimating an attenuated noise resulting from the leakage noise attenuated by the anti-noise signal at the ear drum based on the first transfer function, the second transfer function, and the error noise signal captured by the error microphone; and
adapting a transfer function of the adaptive filter to drive the attenuated noise at the ear drum toward zero.
10. The method of claim 1 , wherein applying the bias term to the adaptive filter to change the anti-noise signal to attenuate the leakage noise at the ear drum comprises attenuating the leakage noise at a virtual microphone that is virtually located at the ear drum.
11. A processor of an earphone, the processor configured to perform adaptive noise cancellation (ANC) comprising operations to:
generate, by an adaptive filter of the ANC, an anti-noise signal from ambient noise captured by a reference microphone of the earphone to attenuate leakage noise from the ambient noise, wherein the adaptive filter is being updated by an adaptive filter controller;
determine an estimated level of the leakage noise based on the adaptive filter;
determine a bias term for the adaptive filter based on the estimated level of the leakage noise;
apply the bias term to the adaptive filter to change the anti-noise signal to attenuate the leakage noise received at an eardrum of a wearer of the earphone; and
reduce an error term that is being input to the adaptive filter controller, the error term comprises an error noise signal captured by an error microphone that results from the leakage noise attenuated by the anti-noise signal at the error microphone, wherein the error term is reduced by subtracting an estimated change to the error noise signal that is due to the bias term being applied to the adaptive filter.
12. The processor of claim 11 , wherein to apply the bias term, the operations comprise:
adjust a gain of the adaptive filter to generate the anti-noise signal to attenuate the leakage noise at the ear drum.
13. The processor of claim 12 , wherein the operations to apply the bias term comprise:
adapt a transfer function of the adaptive filter by a least-mean-squared (LMS) algorithm to drive the error term toward zero.
14. The processor of claim 12 , wherein the error term comprises the error noise signal captured by the error microphone modified to estimate an attenuated noise resulting from the leakage noise attenuated by the anti-noise signal at the ear drum.
15. The processor of claim 11 , wherein to determine the estimated level of the leakage noise, the operations comprise:
determine a relationship between a magnitude of a gain of the adaptive filter, when the adaptive filter is adapted to attenuate the leakage noise at an error microphone, and the leakage noise for a range of levels of the leakage noise; and
estimate the level of the leakage noise in the earphone based on the relationship and the magnitude of the gain of the adaptive filter when the adaptive filter is adapted to reduce the leakage noise at the error microphone.
16. The processor of claim 11 , wherein to determine the bias term for the adaptive filter, the operations comprise:
determine a relationship between a difference in a gain of the adaptive filter when the adaptive filter is adapted to attenuate the leakage noise at an error microphone and at the ear drum for a range of levels of the leakage noise; and
estimate the bias term for the adaptive filter based on the relationship and the estimated level of the noise leakage.
17. The processor of claim 11 , wherein the operations further comprise:
estimate a first transfer function of an acoustic path for the leakage noise from an error microphone to the ear drum based on the estimated level of the leakage noise;
estimate a second transfer function of an acoustic path for the anti-noise signal from the error microphone to the ear drum based on the estimated level of the leakage noise;
estimate an attenuated noise resulting from the leakage noise attenuated by the anti-noise signal at the ear drum based on the first transfer function, the second transfer function, and the error noise signal captured by the error microphone; and
adapt a transfer function of the adaptive filter to drive the attenuated noise at the ear drum toward zero.
18. An earphone comprising:
a reference microphone configured to capture ambient noise;
a speaker configured to transmit an anti-noise signal to attenuate leakage noise from the ambient noise;
an error microphone configured to capture an error noise signal resulting from the leakage noise attenuated by the anti-noise signal;
a processor; and
a memory coupled to the processor to store instructions, which when executed by the processor, cause the processor to perform adaptive noise cancellation (ANC) operations comprising:
generate, by an adaptive filter of the ANC operations, the anti-noise signal from the ambient noise captured by the reference microphone, wherein the adaptive filter is being updated by an adaptive filter controller;
determine an estimated level of the leakage noise based on the adaptive filter when the adaptive filter is adapted to reduce an error term derived from the error noise signal;
determine a bias term for the adaptive filter based on the estimated level of the leakage noise;
apply the bias term to the adaptive filter to change the anti-noise signal to attenuate the leakage noise received at an eardrum of a wearer of the earphone;
filter the ambient noise captured by the reference microphone, using the bias term and using a model or estimate of a secondary channel transfer function, to produce a bias term filtered-version of the ambient noise; and
reduce an error term that is being input to the adaptive filter controller, by subtracting the bias term filtered-version of the ambient noise from an error noise signal captured by an error microphone.