Ladder filter

ATMOSYNTH built-in module

A low-pass that sings, built as a transistor ladder is. Four poles with a feedback around them, and Resonance is how much: below one it emphasises what is at the cutoff, at one the feedback exactly makes up for what the poles take off, and above one the filter holds a tone of its own with nothing played into it at all — the whistle the Moogs are asked for, and the one thing the ordinary Filter cannot do however far its resonance is pushed. It sings at the cutoff the knob names, and starts to at one, however high the cutoff is set, so the whistle is in tune wherever it is. Slope is which pole the sound is taken after: 24 decibels an octave after the fourth, and 18, 12 or 6 after the third, second or first. The feedback goes round all four whichever it is, as it does on a Sub 37, so Resonance means the same at every slope and a gentler one keeps more of what is above the cutoff around the same peak. Nothing makes up the bass the feedback takes away: the further Resonance is turned the thinner the low end, which is how the filters this one follows sound. Cutoff and Cutoff depth work as they do on the ordinary Filter, in cents and negative to invert, and the depth reaches eight octaves either way, so an envelope can open a filter set low all the way to the top; Key tracking moves the cutoff with the note played, which needs the note routed here. Drive is how hard the sound is pushed into the filter, and each of its four stages saturates, as a transistor ladder's do: at one gently, and above it into a thick, compressed sound whose resonance swells and is held down by every stage it goes round rather than clipped on the way in. It is kept out of the feedback, so driving the filter does not move the point at which it starts to sing. What its curves leave off centre on a sawtooth or a narrow pulse is taken off below any note, so what comes out sits on centre. This module carries code: nothing in the vocabulary can be fed back into itself a sample at a time, which is the whole of what a filter like this is. Resonance is an input as well, and its depth how far a signal there moves it — an envelope into it makes the filter sing at the start of a note and settle as it decays. However far it is pushed, it stays inside the knob's own range.