An envelope generator generates a signal that changes through the length of a sound, normally to control the loudness of that sound. An example of its use is to control the volume of a piano sound. It creates a signal that goes from low to high very quickly (the loud bit when the key is pressed), then goes down slowly as the sounds gets quieter. They are also used to drive other parts of the synthesizer, for example the filter.
How the value of a function varies with time, usually defined by 4 factors: Attack, Decay, Sustain and Release (ADSR). The most common usage is with the volume of a synthesised sound in which case the ADSR can be defined as: Attack - the length of time taken from when the note was struck to when the sound reaches it's maximum volume Decay - The length of time taken for the drop of volume to occur after the sound reaches the maximum Sustain - The length of time the sound continues to play while the note is still held Release - The length of time taken for the volume to return to zero once the note has been released.
The outline of a complex waveform showing the maximum and minimum values over time. The attack, decay, sustain, and release regions describe the general musical data acquisition envelope.