Systems and methods for bandlimiting anti-noise in personal audio devices having adaptive noise cancellation

An integrated circuit for implementing at least a portion of a personal audio device, comprising:

• an output for providing a signal to a 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;
• a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds;
• an error microphone input for receiving an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer;
• and
• a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, wherein:
• the processing circuit shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds present in the error microphone signal;
• the response of the adaptive filter is further adjusted independent of the adapting by combining injected noise with the reference microphone signal and the processing circuit further implements a copy of the adaptive filter to receive the injected noise so that the response of the copy of the adaptive filter is controlled by the adaptive filter adapting to cancel a combination of the ambient audio sounds and the injected noise;
• the processing circuit further controls the response of the adaptive filter with the coefficients adapted in the copy of the adaptive filter, whereby the injected noise is not present in the anti-noise signal;
• and
• each of a sample rate of the copy of the adaptive filter and a rate of adapting of the adaptive filter is significantly less than a sample rate of the adaptive filter and the sample rate of the copy of the adaptive filter is significantly less than the rate of adapting of the adaptive filter.