Envelopes
An ADSR envelope runs once each time a note starts. Attack, decay, and release are times. Sustain is the level held while the note is held.
An envelope is a contour that runs once each time a note starts. ADSR is the four stages you meet on most synths.
| Stage | What it sets |
|---|---|
| Attack | Time to rise after the note starts. |
| Decay | Time to fall from that peak to the sustain level. |
| Sustain | The level held while the note is held. |
| Release | Time to fall away after the note ends. |
A short attack, a short decay, and a low sustain finish while the key is still down. That is a pluck or a drum. A long attack and a high sustain fade in and then stay. That is a pad. Release is what you hear after you let go. A long release on a loud sound runs one note into the next.
An amplitude envelope shapes loudness. A filter envelope moves the cutoff, so it shapes brightness. They can be set apart: a fast level attack with a slow filter attack is a click, then a tone that brightens. If the sound ignores how long you hold the key, look at sustain. At zero, the note finishes during the decay.
Compressors also have attack and release. Those are how fast a level detector reacts, and there is no sustain stage. An envelope follower, on Autofilter, is a third case. It draws a contour from audio that is already playing, and uses that contour to move a control.