Sallen-Key filter pair

ATMOSYNTH built-in module

A resonant high-pass and then a resonant low-pass, built as the Korg MS-20's are: two filters of two poles each, one after the other, each with its own cutoff and its own resonance. With the high-pass at the bottom, where it starts, it is a low-pass of twelve decibels an octave; brought up under the low-pass's cutoff, the two are a band-pass as wide as the gap between them; and either can be made to sing. 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. Turning it up takes nothing away from the bass, as it does on the ladder, so the peak stands out over a full low end. What holds a singing filter at a level is a pair of diodes clipping its feedback, so the tone it sings has an edge on it, and a loud sound played into a resonance turned high is clipped at the peak while the rest of it comes through — the MS-20's scream. Drive is how hard the sound is pushed into the filter: at one a sound at full scale passes nearly clean, and above it it thickens and compresses. 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, and the high-pass's cutoff has a depth and an input of its own; Key tracking moves both cutoffs with the note played, which needs the note routed here. Resonance is an input as well, for each filter, and its depth how far a signal there moves it. However far it is pushed, it stays inside the knob's own range. The low-pass's knobs are the ladder's, without Slope, and it plays at about the ladder's level, so a patch moves between the two by changing the module. 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.