Feedback adaptive noise cancellation (ANC) controller and method having a feedback response partially provided by a fixed-response filter

1. An adaptive noise cancellation (ANC) controller, comprising:

a fixed filter having a predetermined fixed transfer function (B(z)) that relates to and maintains stability of a compensated feedback loop, wherein the fixed filter contributes to an ANC gain of an ANC system; and

a variable-response filter coupled to the fixed filter, wherein a response of the variable-response filter compensates for variations of a transfer function of a secondary path that includes at least a path from a transducer of the ANC system to a sensor of the ANC system, so that the ANC gain is independent of the variations in the transfer function of the secondary path, wherein the response of the variable-response filter is an inverse of the transfer function of the secondary path.

2. The ANC controller of claim 1 , wherein the fixed filter causes the ANC gain to be a uniform feedback gain that depends on the predetermined fixed transfer function.

3. The ANC controller of claim 1 , wherein the response of the variable response filter is controlled in conformity with a control output of an adaptive filter of the ANC system.

4. The ANC controller according to claim 3 , wherein the variable-response filter is the adaptive filter, whereby the response of the variable-response filter is dependent on frequency content of a signal provided as an input to the variable response filter to which the response of the variable-response filter is applied.

5. The ANC controller according to claim 3 , wherein the adaptive filter is an adaptive filter of a feed-forward portion of the ANC system that adapts to cancel the effects of the secondary path on a component of a signal reproduced by the transducer of the ANC system.

6. The ANC controller according to claim 1 , wherein the sensor is a microphone and the transducer is a speaker.

7. An integrated circuit (IC) for implementing at least a portion of an audio device including acoustic noise canceling, the integrated circuit comprising:

an output for providing an output signal to an output transducer including 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 and that contains a component due to the acoustic output of the transducer; and

a processing circuit that adaptively generates the anti-noise signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit implements a feedback filter having a response that generates at least a portion of the anti-noise signal from the at least one microphone signal, the feedback filter comprising a fixed filter having a predetermined fixed transfer function (B(z)) and a variable-response filter coupled to the fixed filter, wherein a response of the variable-response filter compensates for variations of a transfer function of a secondary path that includes at least a path from the transducer to the at least one microphone, wherein the response of the variable-response filter is an inverse of the transfer function of the secondary path.

8. The integrated circuit of claim 7 , wherein the fixed filter causes an ANC gain of the system formed by the feedback filter, the transducer, the at least one microphone and the secondary path to be a uniform feedback gain that depends on the predetermined fixed transfer function.

9. The integrated circuit of claim 7 , wherein the response of the variable response filter is controlled in conformity with a control output of an adaptive filter implemented by the processing circuit that models the secondary path.

10. The integrated circuit of claim 9 , wherein the variable-response filter is the adaptive filter, whereby the response of the variable-response filter is dependent on frequency content of a signal provided as an input to the variable response filter to which the response of the variable-response filter is applied.

11. The integrated circuit of claim 9 , wherein the processing circuit further implements a feed-forward adaptive filter that generates another portion of the anti-noise signal, and further implements a secondary path adaptive filter that adapts to cancel the effects of the secondary path on a component of a source audio signal reproduced by the transducer of the ANC system.

12. A method of canceling effects of ambient noise, the method comprising:

adaptively generating an anti-noise signal to reduce the presence of the ambient noise;

providing the anti-noise signal to a transducer;

measuring the ambient noise with a sensor of an ANC system; and

filtering an output of the sensor with a fixed filter having a predetermined fixed transfer function (B(z)) that relates to and maintains stability of a compensated feedback loop, wherein the fixed filter contributes to an ANC gain of the ANC system and a variable-response filter coupled to the fixed filter, wherein a response of the variable-response filter compensates for variations of a transfer function of a secondary path that includes at least a path from a transducer of the ANC system to the sensor, so that the ANC gain is independent of the variations in the transfer function of the secondary path, wherein the response of the variable-response filter is an inverse of the transfer function of the secondary path.

13. The method of claim 12 , wherein the filtering causes the ANC gain to be a uniform feedback gain that depends on the predetermined fixed transfer function.

14. The method of claim 12 , further comprising controlling the response of the variable response filter in conformity with a control output of an adaptive filter of the ANC system.

15. The method of claim 14 , wherein the variable-response filter is the adaptive filter, wherein the response of the variable-response filter controlled in dependence on frequency content of a signal provided as an input to the variable response filter to which the response of the variable-response filter is applied.

16. The method of claim 14 , wherein the adaptive filter is an adaptive filter of a feed-forward portion of the ANC system that adapts to cancel the effects of the secondary path on a component of a signal reproduced by the transducer of the ANC system.

17. The method of claim 12 , wherein the sensor is a microphone and the transducer is a speaker.