ADSR envelope

An ADSR envelope is a component of many synthesizers, samplers, and other electronic musical instruments. Its function is to modulate some aspect of the instrument's sound — often its volume — over time. The envelope may be a discrete circuit or module (in the case of analog devices), or implemented as part of the unit's software (in the case of digital devices).

When a mechanical musical instrument produces sound, the relative volume of the sound produced changes over time. The way that this varies is different from instrument to instrument. For instance, when a pipe organ's key is pressed, it plays a note at constant volume; the sound dies quickly when the key is released. By contrast, the sound of a guitar is loudest immediately after it is played, and fades with time. Other instruments have their own characteristic volume patterns.

A synthesizer's ADSR envelope is a way to tailor a "voice" (the technical term is timbre) for the synth, sometimes to help it sound more like a mechanical instrument. A quick attack with little decay helps it sound more like an organ; a slower decay helps it sound more like a guitar. While envelopes are most often applied to volume, they are also commonly used to control other sound elements, such as filter frequencies or oscillator pitches.

The contour of the ADSR envelope is specified using four parameters:

Attack : How quickly the sound reaches full volume after the sound is activated (the key is pressed). For most mechanical instruments, this period is virtually instantaneous. However, for some popular synthesized "voices" that don't mimic real instruments, this parameter is slowed down. 'Slow attack' is commonly part of sounds called 'pads'.
Decay : How quickly the sound drops to the sustain level after the initial peak.
Sustain : The "constant" volume that the sound takes after decay until the note is released. Note that this parameter specifies a volume level rather than a time period.
Release : How quickly the sound fades when a note ends (the key is released). Often, this time is very short. An example where the release is longer might be a percussion instrument like a glockenspiel, or a piano with the sustain pedal pressed.

While ADSR envelopes are a useful first approximation to the volumes of real instruments, they are not a complete substitute. Woodwind and brass instruments give the player the ability to vary the sound arbitrarily throughout a note, for instance. Many synthesizers, therefore, offer more flexible facilities for volume control which can be used if desired.

On older synthesizers, such as the Korg MS-20, a common variation on the ADSR 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 length of time before decaying. The General Instruments AY-3-8912 sound chip included the hold time 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.

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