ADSR envelope
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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, a pipe organ, when a key is pressed, plays a note at constant volume, the sound ending virtually as soon as the key is released. The sound of a guitar, by contrast, is loudest immediately after it is played, and fades with time. Other instruments have their own characteristic volume patterns. The ADSR envelope is a way to specify the appropriate behaviour for a "voice" created by the synthesizer. Although usually applied to volume, it is also common to control other sound elements such as filter frequencies or oscillator pitches via ADSR envelope.
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, some for some popular synthesized "voices" that don't mimic real instruments, this parameter is slowed down.
- Decay: How quickly the sound reduces in volume 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 after the end of the note (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 that 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 volume level of sustain not being 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.