A state variable filter that mixes between lowpass, bandpass, and highpass. Modeled after the algorithm designed by Andrew Simper outlined here.

Output is tanh limited to give distortion at higher gain levels. This is particularly useful when pinging the filter at high resonance.

Input	Description
unit input	The signal to filter.
V/oct	Volt per octave offset from f0.
f0	The base cutoff frequency.
Q	Resonance, scaled exponentially. Small values are usually enough, but higher values come in handy when pinging the filter.
gain	Input gain to push the output against the tanh limiter and cause distortion.
mix	Filter type control. 0 is lowpass, 0.5 is bandpass, and 1 is highpass.