An envelope generator outputs a time-varying control signal that shapes another module's parameter over each note

An envelope generator (EG) is a module whose sole purpose is to output a time-varying control signal — the envelope — not audio. On a trigger/gate it produces a voltage that evolves over time according to a defined shape: typically rising to a peak, then falling, holding a level while the gate is held, and falling again when released. The classic four-stage shape is ADSR: Attack (time to rise to peak), Decay (time to fall to the sustain level), Sustain (level held while the gate is open), Release (time to fall to zero after the gate closes). The EG output is patched to another module’s parameter input — commonly a VCA to shape amplitude, a VCF to shape cutoff, or (more rarely) a VCO to shape frequency — and that connection means the EG controls that parameter. Without an envelope, a synthesized sound has no life cycle: it sounds identical from first to last, which is unnatural. Reading EG connections is foundational to interpreting patch diagrams.

Examples

Patch one EG output simultaneously to a VCA CV input and a VCF frequency input: a short attack punches the sound in, while a longer decay lets the brightness fade after the initial hit. In an Ableton-style ADSR, A=0ms, D=200ms, S=50%, R=300ms gives a typical pluck.

Assessment

Describe what each stage of ADSR controls. Given a patch where an EG output is connected to a VCF frequency input, predict the timbral behaviour across a held note. Which parameter does an EG-to-VCA connection control, and why does routing the same EG to a filter cutoff (not just a VCA) produce a more expressive voice?