posted 25 April 2002 13:59            

Now I understand why Kyma uses direct feedback over its filters to achieve "resonance". The same technique can be applied to any filtering. Whether or not a resonance peak develops depends on choosing appropriate gain and sign of feedback, and the resonance will develop whenever the overall loop gain at some frequency approaches 1.

This is not generally at the same frequency as the "filter cutoff". You can spot where this will occur by looking at the phase response of a filter chain. For a low-pass filter system, the first crossing of 180 degrees phase shows the frequency of potential oscillation with sufficient levels of negative feedback.

The departure from filter cutoff frequency is even more pronounced for high-pass filters. For a long time that puzzled me -- why would SSC do such a thing, given the gross disparity between specified filter cutoff frequency and resonant frequency. Now with a historical context in mind it makes a whole lot more sense.