Generates an ADSR (Attack, Decay, Sustain and Release) envelope.
Driven by a gate signal - if the level of the signal goes higher than the Trigger Threshold, the attack stage begins, proceeds to the decay stage and then holds at the sustain level. The release stage begins when the gate falls below this theshold - even if the previous stages have not completed.
The output is a signal between 0.0 (rest) and 1.0 (peak) and the transitions are linear.
""" .