I stumbled across this effect while attempting to construct a feedback canceller. It doesn't cancel feedback but it does allow for one input to "duck" another one in individual frequency bands.

Each band of a GraphicEQ is controlled by the inverse of an AnalysisFilter whose center frequency is the same as that of the EQ band. If the spectrum of the input has a lot of energy around 500 hz, for example, the GraphicEQ will attenuate 500 hz.

If the input to the analysis filters and the EQ are the same, you can get some strange filtering effects where the loudest partials are attenuated (usually resulting in a kind of time-varying high pass effect).

If the input to the analysis filters is different from the input to the EQ, you can create a mix where both inputs are audible and do not mask each other. For example, if the input to the analysis filters is a voice and the input to the EQ is a pad, the voice notches out a space for itself in the spectrum of the pad.

I'm hoping people will come up with alternative ways of implementing this idea (as well as any thoughts on the feedback-cancelling puzzle I originally had set out to solve).

-- CarlaScaletti - 21 Nov 2003

Here is a Sound that implements an adaptive feedback squeal cancellation. This Sound is described here.

-- DavidMcClain - 23 Nov 2003

For feedback control, could you use a vocoder channel bank and somehow invert the control signals from the sidechain?

- BillMeadows - 12 Dec 2003

That's a great idea! (would someone have a chance to try this?)

-- CarlaScaletti - 12 Dec 2003