OTA cascade filter

ATMOSYNTH built-in module

A low-pass that sings, built as the Prophet-5's and the Juno-106's are: four poles in a row, each an OTA charging a capacitor with a buffer after it, and a feedback round all four. It is the smoother cousin of the ladder: the tone it sings is rounder, and the bass under its peak stays as Resonance is turned up — from nothing to 0.9 Resonance the low end falls by about four decibels where the ladder's falls by thirteen, so a resonant sweep keeps its body. Below one Resonance emphasises what is at the cutoff, at one the filter rings on for as long as it is left, and above one it holds a tone of its own with nothing played into it at all — a round one, nearly a sine, which keeps its pitch as it swells. 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, so Resonance means the same at every slope. Every stage saturates, as an OTA does, on what it has still to catch up with, so a loud sound driven in thickens and slows. Drive is how hard the sound is pushed into the filter: at one gently, and above it into a thick, compressed sound whose resonance is held down. 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. Cutoff and Cutoff depth work as they do on the ladder, in cents and negative to invert, 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. Resonance is an input as well, and its depth how far a signal there moves it. However far it is pushed, it stays inside the knob's own range. Its knobs are the ladder's, Slope and all, and at the default Resonance it plays at the ladder's level, so a patch moves between the two by changing the module; with no resonance it is quieter than the ladder, and turned high a little louder, by the bass it keeps. 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.