based on code by Victor Lazzarini and Antti Huovilainen
resonance and distortion are related. can become nasty when cutoff, res, and dist are all high. this is based on code by Josep M Comajuncosas meant to emulate the TB303
Implements the filter Y{n} =a Y{n-1} + b Y{n-2} + d Y^2{n-L} + X{n} - C
described in Dobson and Fitch with no, linear, or cubic interpolation and sigmoid clipping.
Victor Lazzarini's string resonator.