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 U403C 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
curve on C401 is presented to the ENVELOPE OUT (U401B pin 9). A portion
of this signal is fed through the trimmer P406 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. U403B then switches to the DECAY/SUSTAIN/RELEASE
position (pin 15 and pin 2 closed) and, with U403B 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).
If the GATE is still on at this stage, the envelope capacitor will remain
charged to the SUSTAIN level until the GATE is released at which point
U403B 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.