This module is best explained by comparing its use with that of some real sounds or acoustic sounds.
When you strike a bell the sound starts almost immediately. That’s because its attack, the first portion of a sound envelope, is very fast, almost instantaneous in fact, where as a violin has a slower attack. When the bow is drawn across the strings it takes a little while for the sound to reach its maximum volume. It’s still fairly fast but nowhere near as quick as a bell.
If the violins string was plucked however the sound then has a quicker attack. As soon as a bell is hit its volume is at its maximum almost immediately, it has completed its attack phase and it has entered its second phase of a sound envelope and that is its decay phase.
A bell will take quite a while to decay to zero that’s when you can no longer hear anything. Hitting a triangle however would have a similar very fast attack and a much shorter decay than that of a bell and a violin has an even shorter decay .As soon as you lift the bow off the strings the sound stops almost immediately. The third phase of an envelope generator is the sustain phase, most acoustic or natural sounds can’t sustain indefinitely. When you blow a wind instrument such as a trumpet or flute, while you are blowing, that is the sustain phase of the sound envelope, as soon as you stop blowing the sound enters its fourth and last phase and that is the release.
As soon as a sound stops sustaining it goes into the release portion, release is similar to decay, and on a wind instrument such as a trumpet after you stop blowing the release time is very short,
The envelope generator on a synthesiser usually consists of four controls. The attack which controls the attack time before the envelope enters the decay phase, the decay control which alters the decay time the sustain level which controls the level the sound goes to when hold a key down and the release which controls the amount of time the sound takes to die away after the note is released.
If you connect the output of an envelope generator to the control voltage input of a VCA you can alter the parameters of the note when you press a key on the keyboard. Apart from creating a control voltage for the pitch of the VCO the keyboard also creates a signal that indicates whether or not a key is pressed, this is known as the gate signal.
When a key is pressed the gate signal starts the envelope generator going through its attack and decay functions, it then goes to the sustain level while the key is held, once the key is released the envelope generator enters its release phase. This is how an analog synthesiser creates its dynamics and is able to create short blips of sound to huge long decaying bass drones.