9

The Attack, Decay, Sustain, and Release (ADSR) Envelope Controls
An oscillogram of a percussive tone is shown below in which the level rises immediately
to the top of its range and then decays. If you drew a box around the upper half of the
oscillogram, you could consider it the “envelope” of the sound—an image of the level as
a function of time. It is the function of the envelope generator to set the shape of this
envelope.
Attack Decay Release
Sustain
Time that the key is held
The envelope generator usually features four controls—attack, decay, sustain and release,
commonly abbreviated as ADSR.
• Attack: Controls the time it takes for the initial slide from an amplitude of zero to 100%
(full amplitude).
• Decay: Determines the time taken for the subsequent fall from 100% amplitude to the
designated sustain level.
• Sustain: Sets the steady amplitude level produced when a key is held down.
• Release: Sets the time it takes for the sound to decay from the sustain level to an
amplitude of zero when the key is released.
If a key is released during the attack or decay stage, the sustain phase is usually skipped.
A sustain level of zero will produce a piano-like—or percussive—envelope, with no
continuous steady level, even when a key is held.
Using the Envelope to Control Filters
Envelope generators are not limited to controlling signal amplitude. They can also control
the rise and fall of the filter cutoff frequency or modulate other parameters. In other
words, envelope generators can be used as a modulation source—or as a “remote control”
for a given parameter, if you prefer.
This aspect of synthesizers—modulation—is covered in the following section.
362 Appendix Synthesizer Basics