Modulation Sources; Lfo; Microcomputer - Sequential Pro One Technical Manual

2-8
MODULATION SOURCES
The five modulation destinations were discussed above, The modulation source
circuitry
is
straightforward. See SD121. Each source (LFO, FILT
ENV, and OSC
B)
has
an attenuator
and
DPDT switch. E a c h switch simu!taneously connects a source to one
modulation buss while grounding t h e input to t h e other modulation
buss.
For example,
in t h e position shown, 5120
routes LFO from Pi103 t o R108
and UIQI-12,
the
Direct
Source Buffer input. This buffer has a gain of 2. At
t h e same
t i m e , 5120 grounds R107
to U101-3, t h e W-MOD Source
Summer
input. U101-1 also amplifies t h e mixed
modulation CV by about 6, before supplying it t o
R2
MOD
WHEEL. Ul01-7
buffers the
MOD
WHEEL output.
As mentioned above, it is redundant
t o
use t h e
filter envelope through both R1190
FILTER ENVELOPE AMOUNT knob
-
and RlOl MODULATION FILT ENV AMT t o S121.
The path is intended for Filter Envelope Modulation of OSC A or B FREQ or PW.
2-9
LFO
1
U106 LFO is
a CEM 3340, like OSC
A and 0. Its only variable CV
is from R160 LFO
FREQUENCY through R191. Divider
R 1108/Rll07
at pin 5 fixes t h e pulse width at
50%, giving a square wave, whose edge is sharpened by feedback through RlJ06.
The
L F O output circuit is similar to OSC 8.
2-1 0 MICROCOMPUTER
We now
turn t o t h e Pro-Oners digital circuitry, of which t h e Intel 8021 single-chip
Microcomputer is the chief component. The 8021 contains 1 K x
8
bits of ROM which is
mask-programmed
with
instructions, and 64
x 8
bits of dynamic RAM. LlOl controls
t h e 8021's internal clock circuit, establishing a frequency of about 3 MHz.
When power is first turned
on, U113-17 RESET is pulled
high by C153. The positive-
going pulse d e a r s t h e internal registers and sets t h e program counter to 0. D l 1 4
discharges
C
153 on power off.
All
input t o t h e microcomputer
results from switch closures either from t h e keyboard,
or t h e mode controls. The keyboard
is merely a bank of switches which t h e computer
scans
in fundamentally t h e same way
i t would scan a calcuEator keyboard.
Each key
has
a corresponding memory bit
which is
set
(1) or reset
(0) if t h e
key is on or off.
This is how sequences can be remembered and t h e arpeggiatar latched, The key
closures a r e processed into a single binary number sent t o t h e DAC. T h e changing
keyboard "status" also produces t h e TRIG and GATE signals which articulate t h e voice.
The remaining switches activate different areas of t h e microcomputer program,
resulting in a change of modes.
The computer peripheral interface lines are configured as a d a t a driver (POO-PO51
which sends 5 bits of d a t a to t h e matrix "rows"
and
6 bits
;Yo
t h e DAC,
an
&-bit data
receiver (P 10-P 17) f o r the switch
matrix McolumnsM, plus odtput einds which refresh t h e
DAC (P20), activate t h e control switches (P211,
and
output t h e GATE (P22) and
TRIGGER (P23) signals t o t h e voice.
The T1 input
a t pin
13 accepts clock signals which may arise from t h e LFO, an
external gate source, or an external audio source
I-AGATE).