The filter envelope: brightness over time

Lesson 3's contour, aimed at lesson 4's knob — the single move behind most "movement" in synth sounds.

You hold two ideas from earlier lessons. An envelope draws a contour across a note's life. A cutoff decides how much of the harmonic stack survives. Wire the first to the second and the note changes brightness while it plays. Acoustic instruments do this without being asked: a plucked string starts with energy spread far up the series and loses its top partials first, so it dulls as it rings.

A synthesizer has to be told. The filter envelope is that instruction, and it is the most recognizable move in subtractive synthesis. Two settings cover most of the ground. A bass with the cutoff parked at 300 Hz and an envelope that opens it to 3 kHz for 200 ms gives the standard plucked bass. A pad with the cutoff at 600 Hz and a 1.5 s attack up to 5 kHz arrives dull and brightens as it settles.

Same ADSR, a different target

Nothing about the envelope itself changes. Attack, decay, sustain, and release mean what they meant in lesson 3, and the useful times are the same: under 5 ms for a hard front, 100–300 ms of decay for a pluck, several hundred milliseconds of attack before a swell becomes obvious. What changes is where the contour is sent. This time it moves the cutoff, lifting it from wherever FREQ is parked and letting it fall back.

One consequence trips up almost everyone at first. The filter envelope has no say in whether a note exists — the amp envelope still decides that. Give the amp a 100 ms decay to silence and a filter envelope with a 2 s decay never gets to show its second half, because nothing is left to hear. When a filter envelope seems to do nothing, compare its times against the amp page before touching anything else.

ENV: how far the contour pushes

A filter envelope always arrives with a knob the amp envelope has no use for. It is called ENV on these four devices and depth or amount elsewhere, and it scales how far the contour moves the cutoff. At zero the envelope still runs and the cutoff never moves, which is the off switch. Turned up, it decides whether the note opens by half an octave or by five.

ENV is bipolar on all four devices, and the negative half is worth as much as the positive one. Positive depth adds the contour to the FREQ setting, so the note opens and then closes. Negative depth subtracts it: the note starts at FREQ, ducks darker, and climbs back. A short negative decay makes a sound that starts bright and clamps shut, which is behind most reverse-sounding stabs and a lot of plucked electric-piano tones.

Amounts that hold up

Start around a third of the knob's positive travel with a 200 ms decay and sustain at zero. That is a clean pluck on most material, and it leaves room to move in either direction. Past about two-thirds the peak of the contour usually pushes the cutoff to the ceiling, where it stops moving; the attack flattens out and the envelope starts to sound blunt rather than deeper. When high depth sounds wrong, the fix is almost always to back it off.

Resonance and depth work on each other. With resonance near zero the sweep reads as a change in tone. Push resonance to a peak of 12 dB or so and the same sweep reads as a pitch moving through the sound, because the peak itself is audible as it travels. That is the whole recipe behind an acid line: a resonant low-pass, a short envelope, and a cutoff parked low enough that the contour has somewhere to go.