Envelope generator

Sound synthesis techniques often employ an envelope generator that controls some parameters of a signal or control voltage at any point in its duration. When it controls a VCA these together form an envelope shaper.

ADSR
Most often the envelope generator is an ADSR (Attack Decay Sustain Release), which may be applied to overall amplitude, frequency, or filter. It is usually triggered by a gate signal from the keyboard.

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.

Usage
For shorter envelopes at higher pitch, as happens in acoustic instruments, a master CV is taken from the same voltage as used for VCO pitch.

Kits

 * MFOS ADSR Envelope Generator
 * MFOS Dual Attack Release Envelope Generator
 * Serge Extended ADSR Envelope Generator
 * Oakley Voltage Controlled ADSR and VCA module
 * Oakley Looping ADSR with integral VCA

Schematics
Readily available analogue IC and discrete component based CEM IC based
 * Formant music synthesizer by C. Chapman, Elektor, 1978, p. 64, using op-amps and common discrete components
 * yusynth ADSR has a 555 core with a few op-amps.
 * Digisound 80 80-10 Voltage Controlled Envelope Generator, uses CEM3310. Electronic Music Circuits by Barry Klein,1982, p. 62-3, includes Charles Blakey's independent trigger input modification.
 * Digisound 80 80-8 Dual ADSR, uses CEM3310
 * Digisound 80 80-18 Dual Multi-Function Envelope Generator, uses CEM3310