Envelope generator

An envelope generator is a module that controls how a sound changes over time. The most common type has four stages: attack, decay, sustain, and release (ADSR). Attack is the initial build-up, decay is the drop after the peak, sustain is the level held while a gate stays high, and release is the fade-out after the gate ends. Envelopes are most often used to shape amplitude (volume), filter cutoff frequency, or pitch. Stage combinations vary, including D, AD, AR, ASR, ADSR, and AHDSR.