Diode ladder filter

ATMOSYNTH built-in module

A low-pass that squelches, built as the TB-303's is: four poles joined by diodes rather than kept apart by transistors, so each pulls on its neighbours and no two of them sit in the same place. With no resonance it starts to fall gently from well below the cutoff and steepens past it, darker than the ladder at the same setting; turned up, a peak rises out of that slope, and an envelope sweeping the cutoff with Resonance high is the rubbery squelch of an acid line. 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. The further Resonance is turned the thinner the low end, as on the ladder and by about as much: the feedback takes the bass, and half of what it takes is made up after the filter. Every stage saturates, as diodes do, so a resonance pushed hard or a loud sound driven in finds the filter slower to move, and the tone it sings sags in pitch as it swells. 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 while the bass under it comes through. 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, without Slope, 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.