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[[File:ADSR parameter.svg|thumb|300px|The amplitude over time of an ADSR envelope]] Sound synthesis techniques often employ an '''envelope generator''' that controls some parameters of a signal or control voltage at any point in its duration.
An '''envelope generator''' is a device that produces an envelope voltage. When it controls a [[VCA]] these together form an '''envelope shaper'''.<ref>''Synthesizers for musicians'' by R A Penfold, PC Publishing, 1989, ISBN 1870775015, p.21</ref>▼
==ADSR==
The contour of an ADSR envelope is specified using four parameters:
;Attack time
:The time taken for initial run-up of level from nil to peak, beginning when the key is first pressed.
;Decay time
:The time taken for the subsequent run down from the attack level to the designated sustain level.
;Sustain level
:The level during the main sequence of the sound's duration, until the key is released.
;Release time
:The time taken for the level to decay from the sustain level to zero after the key is released.
A common variation of the ADSR on some synthesizers, such as the [[Korg MS-20]], was ADSHR (attack, decay, sustain, hold, release). By adding a "hold" parameter, the system allowed notes to be held at the sustain level for a fixed length of time before decaying. The [[General Instrument AY-3-8910]] IC included a hold time parameter only; the sustain level was not programmable. Another common variation in the same vein is the AHDSR (attack, hold, decay, sustain, release) envelope, in which the "hold" parameter controls how long the envelope stays at full volume before entering the decay phase.
Certain synthesizers also allow for a delay parameter before the attack. Modern synthesizers like the [[DSI Prophet 8]] have DADSR (delay, attack, decay, sustain, release) envelopes. The delay setting determines the length of silence between hitting a note and the attack.
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==Usage==
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