8. Envelope
An envelope is used to give a sound a shape using 4 major parameters, Attack, Decay, Sustain, Release (ADSR). These controls can be found on other parts of synthesizers and in equalizers. The ADSR is normally effective at every single sample point which leads to more DSP (Digital Signal Processing) power being taken up, but provides a smoother sound.
Attack - The time is takes the tone to raise its maximum level.
Decay - The time it takes the tone to fall to the sustain level. This decay is usually exponential.
Sustain - When a note is played, the message goes through the attack and decay and then waits on the sustain level until the note stops being played.
Release - After the note has passed through the ADS part of the envelope, it passes into the release. This is the time it takes the tone to fall from the sustain level to zero. The release is usually exponential.
(folk.uio.no)
The ADSR points are marked on the graph above. V represents Velocity and T represents Time. The signal always follow through A>D>S>R. However, some envelopes have an 'inverse' button which means that it starts and ends on maximum level. The changing of these parameters can give radically different results to a very simple wave form.