I have an audio design using the ADAU1772. It seems to be working perfectly well from the hardware point of view as the DSP is biasing an electret microphone and outputting the signal to a speaker. The application is Active Noise Reduction. The microphone and speaker naturally cause feedback as they are close to each other in the cup of the headphone. Now, if I create a very simple schematic that feeds the microphone input (0) to the output (0), through a single filter element, all is fine (as in, there is still positive audio feedback squealing, but the hardware is working fine). However, if I insert 2 or more filters in series, I get a chopped up "white noise" signal. Sometimes it is continuous. Now, I cannot see there is a problem with the hardware, as it works fine as described above. And I SHOULD only get a single tone of feedback (and harmonics thereof), not white noise. I've take a recording of the sound to describe it. The signal I suspect is clipped though, but I think it gives the idea correctly. Firstly, why would adding other filters or element (even adding a Mute button function causes this!) create this problem, and secondly, why on earth is it intermittent?
Any help would be appreciated please!