Subtractive synthesis

Subtractive synthesis is the classic analog recipe, and most synths with a filter use some version of it. An oscillator makes a bright wave, a filter removes part of it, and an amplifier with an envelope sets how loud the note is from moment to moment.

The filter does the subtracting. A sawtooth on A2 (110 Hz) carries the note plus harmonics at 220, 330, 440 Hz and on up. Put a low-pass filter on it with the cutoff at 660 Hz, the 6th harmonic, and everything up to there comes through while the harmonics above get quieter the higher they go. Turn the cutoff down and the note goes dark and muffled; turn it up and the buzz comes back. Resonance boosts the harmonics right around the cutoff, which adds the squelchy, vocal edge of acid bass lines.

Movement comes from sending an envelope to the cutoff. With a fast decay, each note starts bright and closes down within a fraction of a second, the shape of a pluck or a snappy bass. With a slow attack the filter opens gradually, like a brass swell. An LFO on the cutoff gives a repeating wah or wobble.

Because a filter can only take harmonics away, the starting wave matters. A sine has nothing to remove, so subtractive patches usually start on a saw, square or pulse. The Minimoog Model D, released in 1970, is the textbook example: three oscillators into a 24 dB per octave low-pass ladder filter.

In Gelato Synth, the Filter page has a Low, Band or High pass switch, Cutoff, Resonance, Drive and Envelope to filter, so you can set Osc A to a saw and shape it the subtractive way. How cutoff and resonance work has settings to try, and FM vs subtractive synthesis compares the methods.