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.
Each
switch simu!taneously connects
a
source
to
one
modulation buss while grounding the
input
to
the other modulation
buss.
For example,
in the position
shown,
5120
routes
LFO
from Pi103 to R108
and
UIQI-12,
the
Direct
Source Buffer input. This buffer has a gain
of
2.
At
the
same
time,
5120
grounds
R107
to
U101-3, the
W-MOD
Source
Summer
input.
U101-1
also amplifies the mixed
modulation
CV
by
about
6,
before supplying
it
to
R2
MOD
WHEEL.
Ul01-7
buffers
the
MOD
WHEEL
output.
As
mentioned above,
it
is
redundant
to
use
the
filter
envelope through
both
R1190
FILTER
ENVELOPE
AMOUNT knob
-
and
RlOl
MODULATION
FILT
ENV
AMT
to 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
the
pulse
width
at
50%,
giving
a
square
wave,
whose
edge
is
sharpened
by
feedback through
RlJ06.
The
LFO
output circuit is similar
to
OSC
8.
2-1
0
MICROCOMPUTER
We
now
turn to the Pro-Oners
digital
circuitry,
of
which
the
Intel
8021
single-chip
Microcomputer is
the
chief component. The
8021
contains
1K
x
8
bits
of
ROM
which
is
mask-programmed
with
instructions, and
64
x
8
bits
of
dynamic
RAM.
LlOl
controls
the
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
dears the internal registers and
sets
the
program
counter
to
0.
Dl14
discharges
C
153 on power
off.
All
input
to
the
microcomputer
results
from
switch closures
either from the
keyboard,
or
the
mode
controls.
The
keyboard
is
merely
a
bank
of
switches
which the computer
scans
in
fundamentally
the
same
way
it
would
scan
a
calcuEator
keyboard.
Each
key
has
a
corresponding memory bit
which
is
set
(1)
or
reset
(0)
if
the
key
is
on
or
off.
This
is
how
sequences can
be
remembered
and
the arpeggiatar latched, The
key
closures are processed into
a
single binary number
sent
to the
DAC.
The
changing
keyboard
"status"
also
produces the
TRIG
and
GATE
signals
which
articulate the voice.
The
remaining switches activate different areas
of
the microcomputer program,
resulting
in
a
change
of
modes.
The
computer peripheral interface lines
are
configured
as
a
data
driver
(POO-PO51
which sends
5
bits of data
to
the matrix "rows"
and
6
bits
;Yo
the
DAC,
an
&-bit
data
receiver
(P
10-P
17)
for
the
switch
matrix McolumnsM, plus odtput einds
which
refresh
the
DAC
(P20),
activate
the control switches
(P211,
and
output the
GATE
(P22)
and
TRIGGER
(P23)
signals
to
the
voice.
The
T1
input
at
pin
13
accepts clock signals which
may
arise
from
the
LFO,
an
external
gate
source,
or
an
external audio source
I-AGATE).
To Next Page
Loading...
