[sdiy] ADSR Trigger/Retrigger behaviour

[rburnett at richieburnett.co.uk]

While we're on the subject of analogue envelopes, and in particular the 
attack phase, does anyone know if there is any sort of convention 
regarding how far the capacitor is allowed to charge during the attack 
phase, before we switch over to the decay phase.

To put it another way, the decay phase is clearly defined since it 
decays exponentially from the end of the attack level down to wherever 
the sustain level is set to.  Similarly the release phase decays 
exponentially from whatever the sustain level was set to down to zero or 
silence.  Both the decay and release phases have defined start and end 
points and defined time-constants, but the end-point for the attack 
phase seems less well defined.  For example, it might typically consist 
of charging a capacitor from 0V towards 10V with the classic exponential 
charging waveform, but it theoretically takes forever to get to 10V.  So 
at some point we have to say "that's enough waiting" and switch over to 
the decay phase of the envelope.

Various analogue synths seem to set this comparator threshold at 
different levels.  Set the threshold too low and terminate the attack 
phase after less than a time-constant of the exponential growth and the 
attack shape starts to tend towards a linear attack.  Conversely set the 
threshold too high and there is going to be very little perceived 
variation in volume towards the end of the attack phase, particularly as 
we perceive volume logarithmically!

To me as an engineer, it would seem sensible to set the threshold level 
for switching to the decay phase at 63% of the level that the capacitor 
is charging towards.  So for the 0-10V envelope case above, the 
capacitor would charge from 0 to 6.3V during the attack phase before 
moving into the decay phase.  That way the envelope would linger in the 
attack phase for exactly one RC time-constant at whatever the attack 
time-constant is set to.

Is there any convention for this, or has anyone even given the 
implications of this any thought!?!?

Best regards,

-Richie,