The ADSR extends the AR envelope by adding separate Decay and Sustain stages, enabling fast attack with slow decay

The AR (Attack-Release) envelope uses a single time constant for both attack and release, so it cannot set a fast attack independently of a slow decay. That suits symmetric flute-type fade-in/out sounds but not a guitar/pluck, which needs a fast attack, a slower decay, and a quick release. The ADSR extends the AR with switching logic that selects different resistances and target levels for the capacitor charging, giving four independently controllable parameters: Attack time, Decay time, Sustain level, and Release time. Crucially, Sustain is a level (held until the gate goes off), not a time like the other three.

Examples

Pluck/guitar patch: Attack ~1 ms, Decay ~200 ms, Sustain low or 0, Release ~50 ms. Organ patch: Attack 0, Decay 0, Sustain max, Release 0 — an instant on/off. The AR alone can make the organ or flute but not the pluck.

Assessment

How does Sustain differ from the other three ADSR parameters? What sound shape suits an AR envelope, and what shape requires a full ADSR and why?