WP-20 MODULE DESCRIPTIONS
ENVELOPE GENERATOR (See figure 2)
(Also referred to as Attack/Decay Generator or
A.D. Generator)
The envelope generator is comprised of IC1,
IC2A and associated components. IC1 (a 555 timer)
is used in a modified monostable mode.
When S3 is depressed, the trigger input of
IC1 (pin 2) is momentarily brought low. This
causes pin 3 to go high and simultaneously turns
off the internal clamping transistor which allows
C2 to charge. Current flows through forward bi-
ased Dl, through R36 (Attack Control) and charges
C16. This voltage is fed to high impedance buf-
fer amp IC2A whose output is fed through R2 to
IC1 (pins 6 and 7). When this voltage charges
to 2/3 of the power supply voltage, pin 3 goes
low (it's current sinking state). Now, D2 which
is forward biased, discharges C16 through R37
(Decay Control). This completes one Attack/Decay
cycle. When S2 (Sustain Switch) is closed, pin
3 will stay high as long as S3 (Trigger Switch)
is depressed.
For the repeat function to operate, S2 must
be open and S5 (Repeat Switch) closed. This
causes the Attack/Decay Generator to retrigqer
itself via R32 at the end of each decay cycle.
The A.D. output is taken from the output of IC2A.
Low Frequency Oscillator (LFO)-(See figure 3)
The Low Frequency Oscillator is comprised
of IC4A, IC4B, IC4C (all CMOS NOR gates) and
associated components. It produces a squarewave
of about 50% duty cycle at the output of IC4C.
R21 and C9 integrate the squarewave into a tri-
angle wave. When S9 is closed, D3 quickly dis-
charges C9 (if IC4C is low) resulting in a ramp-
type waveform. Switches S10, S11 and S12 dis-
tribute either the squarewave or the triangle/
rampwave to the VCA, VCF and VCO modulation se-
lect switches.
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