ARP Omni-2 - Bass Signal Processing and Control

== and Waveshaping for tones C2 through G3.
3.2.1 WAVE SHAPING, KEYING AND GATE
CIRCUITS
(Refer
to
Lower
Voicing
Schematic,
sheet
1
of
3.)
‘The Wave Shaping, Gating and Keying circuits on this
board are a continuation of the circuits on the Upper
Voicing Board. See sections 3.1.2, 3.1.3 and 3.1.4
for detailed descriptions.
32.2 BASS LOW NOTE PRIORITY CIRCUIT
(Refer to Lower Voicing Schematic, sheet 2 of 3.)
The 8" and 16" Bass section of the OMNI-2 is single
note, low note priority. The Bass Section covers the
lowest octave and a half of the keyboard (Keys 1
through 20). The release time of the Bass Section is
fixed, the release control on the front panel has no
control over the Bass Section. Unlike the 4" and 8°
polyphonic tones, the Bass Priority Circuit receives
and generates only square waves.
‘The function of the Bass Low Note Priority circuit is
to route the square wave of the lowest note depressed
(only) in the Bass section to the 8" and 16" Bass Wave
Shaping circuits (through CR41-60) for processing,
‘The
Bass
Wave
Shaping
circuits
are
monophonic;
they
may only accept one waveform at a time.
Square wave tones C2-G3 are routed from the Upper
Voicing Board frequency dividers to one input of a
three input nor gate for each key (233-239). The
outputs of these nor gates are inverted square waves
only
when
the
other
two
inputs
are
a
logic
0
(e.g.
2338 pin 6 is a square wave only if pin 4 and pin 3
are at logic 0.).
The “enable” input of the three input nor gates (e.g.
2338, pin 3) will always be a logic 0 (-15 volts)
provided no keys are depressed to the left of the
circuit under examination.
When a key is depressed, -15 volts from the bus rod
discharges a luf sustain capacitor through keying
transistors (01-020). (eq. Key 2, P1-12 discharges
042 through R7, 02 and CR63 to -15 volts on key
depression.) When a key is released, the voltage on
the capacitor is allowed to charge back to 0 volts
through a 3,3Mohm resistor (e.g. ВБ). This keying
Voltage is routed to an input of the three input nor
gates leg. 2338, pin 4). Thus the keying voltage
permits the square wave to be transmitted from the
input of the three input nor gate to the output
provided that (A) the keying voltage is less than -7.5
volts
and
(B)
the
enable
input
is
a
logic
0
(indicating
no lower keys are being played).
The keying voltage is also processed through a COS-
MOS inverter (245-247) and a nor gate (233-247)
to transmit serially a logic 1 state to all keys higher
than the one depressed.
КҮР
1
on
1/0 indicates normal
logic 1, goes to O on.
key depression.
BASS LOW NOTE PRIORITY CIRCUIT
The output of the three-input NOR (В) will be a
square wave only when the Enable (A) input is a logic
0 (15 volts). Also note that when this occurs, all
audio signals to the right will be prevented from being
processed - hence, low note priority.
NOTE: All logic gates in the Bass Low Note Priority
Section are COSMOS [Complimentary Symmetry
Metal Oxide Semiconductor) devices, For this
particular application, they are operated between
ground and -15 volts. Therefore, a logic 1 is any
voltage between ground and -7.5 volts, logic O is any
voltage between -7.5 volts and -15 volts,
The release time (decay time after the keys are
released) is fixed at about one second, however, an
additional charge path is provided through the 3.3K
resistor and the diode to S2 for those keys previously
depressed to shorten any residual release time left
on that note. 01-020 are off when no keys are
depressed,
3.2.3 BASS KEYING VOLTAGE
(Refer to Lower Voici
g Schematic, sheet 3 of 3.)
‘The T point of the Bass Keying Reference Generator
supplies a voltage to the bass of Q1-20 of the Bass
Low Note Priority Circuit which is at least three
diode drops higher than the keying voltage supplied
to the Lower keyboard bus rod (P1-14). The diode
drops are created using CR114, O30 and 08113.