A multi-mode biquad cascade filter with options for lowpass, bandpass, and highpass at either 12dB/oct, 24dB/oct, or 36dB/oct. Modeled after the equations outlined here by Robert Bristow-Johnson.

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.
type/mode	Set the filter type and mode.
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.