ADSR is just so last century. Things that create sound in nature don’t tend to follow the ADSR envelope. What sounds go on forever? Yes, we don’t always want to create sounds that sound natural, but what if we do?
A lot of synth manufacturers add in a second decay step, which requires its own level, but this does seem to complicate the envelope unnecessarily:
Access solved the problem in a perfectly elegant way on the Virus TI. They have a single value that describes the slope that the sustain level follows. If it is set to zero, then the envelope behaves in the normal ADSR way, if it is set to a negative value, the sustain level drops to zero over a period of time determined by the magnitude of the negative value. Genius!
The two-decay model does give us control over the shape of the decay slope, but there are better ways to do this. The Hydrasynth allows us to change the slope type of the ADSR component between linear and exponential in either direction.
My old Moog Prodigy doesn’t even have full ASDR! It has AD/RS. Decay and release have a single knob, so are the same value, but there is a switch to kill the release time completely.
A lot of modern synths allow values for initial delay (before attack starts) and hold (between attack and decay) and these are welcome in my view, although I think tempo sync is essential for these.
The ignore key-off option is for when a drum pad is used to trigger the sound, but you don’t want the envelope to go straight to the release part of the sound, but follow the attack, decay, release route. This is available on the Montage when setting up drum kits (but not anything else!)
So, the ideal envelope (in my humble opinion)?
Delay (optionally tempo-synced)
Attack (curve selectable, optionally tempo-synced)
Hold (optionally tempo-synced)
Decay (curve selectable, optionally tempo-synced)
Sustain slope (curve selectable)
Release (curve selectable, optionally tempo-synced)
Repeat (on, off)
Ignore key-off (on, off)