A positive going GATE signal arrives at U402 and provides both a positive and a negative logic drive signal (pins 1 and 7 respectively). The positive output (U402-pin 1) sets U403(Y) to the DECAY/SUSTAIN position and, via a pulse generated through C402/R402, sets the flip-flop U404 causing U403B to switch to the ATTACK position (pin 15 and pin 1 closed).
In this position the envelope capacitor C401 is charged via the ATTACK pot P405. Opamp U401B is a voltage follower and the exponential charge on C401 is presented to the ENVELOPE OUT (U401B pin 7). A portion of this signal is fed through the trimmer P407 to a `reset' pin on the flip-flop so that when the ATTACK curve reaches a predefined peak, it causes the flip-flop to reset. U403(X) then switches to the DECAY/SUSTAIN/RELEASE position (pin 15 and pin 2 closed) and, with U403(Y) now in the DECAY/SUSTAIN position (pin 14 and pin 13 closed), the envelope capacitor starts to discharge through P403 and the opamp U401A. The envelope capacitor will, however,only discharge until the voltage on the capacitor equals the SUSTAIN level at the output of U401A (as set by the SUSTAIN pot P402).
With the GATE still on at this stage, the envelope capacitor C401 will remain charged at the SUSTAIN level until the GATE is released at which point U403(Y) returns to the RELEASE position (pin 14 and pin 12 closed) and connects the RELEASE pot P404 to the envelope capacitor. The envelope capacitor now discharges fully providing the RELEASE curve of the envelope.
The above description assumes that the GATE-on time is greater than the total of the ATTACK and DECAY times added together, in which case a full ADSR envelope is achieved with the SUSTAIN-on time being equal to the sum of the GATE time minus the ATTACK + DECAY times
i.e SUSTAIN(on) = GATE(on) - ATTACK(on) - DECAY(on)
Releasing the GATE signal earlier will result in only a portion of the Attack-Decay-Sustain curves being completed. The RELEASE curve is ALWAYS initiated as soon as the GATE is released. Depending upon the GATE-on time the resultant envelope will be either AR, ADR or ADSR.