ARP Omni-2 - Upper Voicing Board; Master Oscillator and Top Octave Divider

The
OMNI
-2's
tone
generator
circuitry
consists
of
a
master
oscillator
at
500
kHz.,
which
drives
a
large
scale
integrated
circuit
top
octave
divider.
The
top
octave
divider
produces
the
highest
twelve
tones
in
the
instrument.
Frequency
dividers
derive
the
remaining
pitches
from
the
top
octave
divider.
The
squarewave
outputs
of
each
divider
are
waveshaped
10
a
sawtooth
form
(the
waveform
enhancement
alters
the
waveshape
to
a
differentiated
squarewave).
The tones from the waveform generator are fed to
transistor
gating
arrays
which
route
the
signal
to
the.
string
section
and
the
synthesizer
section.
The
trans
istor gating arrays are ‘keyed’ on by an RC circuit
connected to each key. The release time of each
key
(how
long
the
note
remains
after
a
key
release)
is determined by the release capacitor on each gating
input.
The outputs of all the gating arrays are summed and
routed to the string section and the synthesizer
section.
The String Section consists of three parallel phase
shifters which modulate the sawtooth waveforms
from the gating circuits. The phaser outputs are
processed through a Voltage Controlled Amplifier
to control the attack characteristics. (The release
is controlled by the release capacitor on each gating
circuit)
The
Synthesizer
Section
processes
the
gate
outputs
of
the
arrays
through
a
Voltage
Controlled
Filter
and
Voltage
Controlled
Amplifier,
both
of
which
are
controlled
by
an
ADSR
Envelope
Generator.
Thi
synthesizer
output
can
be
routed
through
the
string
section
phasers
when
the
Chorus
Phaser
Switch
is
selected.
The
speed
of
the
phasers
is
reduced
when
the
Chorus
Phaser
is
selected.
The
outputs
of
the
3.1 Upper Voicing Board
GENERAL: The Upper Voicing Board contains the
Master Oscillator, Top Octave Divider I.C., Frequency
Dividers, Wave Shaping for tones C7 through C4,
Keying Circuits for keys 21 through 49 and six of the
ten
Gate
Circuit
1.C.'s.
The
remaining
Wave
Shaping,
Keying Circuits and Gate I.C.% are located on the
Lower Voicing Board. (Note: There are two sche
matics for this board.)
string section and the synthesizer section are summed
together in the Mix Circuitry and routed to the
‘output of the instrument, Separate outputs are pro-
vided for stereo effects.
The Bass Voice Section and the String Bass and
Cello
Voices
derive
their
frequencies
from
the
8°
pitches that come from the frequency dividers
(before
the
waveshaping
circuit).
These
square
wave tones will represent the first 20 notes on the
keyboard
and
are
separately
processed
through
a
monophonic low note priority bass circuit. Once an
8"
pitch
has
been
selected,
it
is
divided
again
to
pro-
vide a 16º pitch.
The
String
Bass
and
Cello
single
note
pitches
(8'
and
167)
are
processed
through
the
phaser
and
are
mixed
with
the
String
Voices
at
the
String
УСА.
‘The Bass Voice single note pitches (8° and 16") are
processed through their own preset synthesizer and
envelope section to provide a distinctive sound
different than the String Bass Voices. Furthermore,
their preset dynamics can be altered by the Staccato
Bass Voice Switch. Its output can be taken directly
at the Bass Output Jack; otherwise it is mixed at
the Main Output.
‘The amplitude of the String Bass Voices and the Bass
Voices (8° and 16 are controlled by the Bass Vol:
ume Slider.
‘The Synthesizer Section can be selected (via single
trigger
switch)
to
be
triggered
only
on
the
first
key
depression. This will allow the strings to be played
without retriggering the Synthesizer Envelope
Generator.
3.1.1 MASTER OSCILLATOR AND TOP OCTAVE
DIVIDER
(Refer to Upper Voicing Schematic, sheet 1 of 2.)
L1 and 01 produce a O to -15 volt sine wave at
approximately 500kHz. The frequency of the oscil-
lator can be varied by adjusting the core of the coil
(externally accessible). 21 buffers and squares the
waveform from the Master Oscillator and supplies it
to the Top Octave Divider, 22. 22 із a LSI (Large
Scale Integrated Circuit) divider which produces the