Comb filter

ATMOSYNTH built-in module

A filter tuned to a pitch rather than set to a cutoff. It delays the sound by exactly one period of that pitch and adds it back to itself, so everything at the pitch and its harmonics reinforces and everything between cancels — a whole row of peaks and notches at once, which is the sound of a short room, a cardboard tube, or a plucked string depending on how much of it goes round again. Resonance is how much: near one it rings on into a pitch of its own long after the sound that started it, and below nothing the feedback is subtracted instead, which shifts every notch by half a spacing and leaves the hollow, woody sound an octave below where you would expect it. Damping takes a little more of the top off each time round, so the ring dies from the treble down as a string does rather than all at once. Pitch is an input as well as a knob and Pitch depth is how far, in cents — an LFO patched in is a flanger you wired yourself — and Key tracking moves the tuning with the note played, which needs the note routed here: at one the comb rings in tune with whatever is played through it. This module carries code, for the one thing the node vocabulary cannot do: a comb's delay is one divided by its pitch, and nothing in that vocabulary divides. Resonance is an input as well, and its depth how far a signal there moves it; the comb holds what arrives inside the knob's own range, so the ring never runs away.