Oberheim ob-8 Instruction Manual

INTRODUCTION
The 0B-8
is the
latest development
in
the evolution of
Oberheim
synthesizers.
The
design philosophy behind
it
was to
toke all of the
features of the OB-Xe, add as
many new features as
were economically
feasable, and redesign oil of the
circuitry
for increased reliability
and lower cost. The result is a
synthesizer with
90 fewer calibrations
and
11
less circuit boards, and with many of the remaining
calibrations
microprocessor assisted. The
circuitry
and the trimmers that were
eliminated have been replaced with software functions
in the
microprocessor which simulate
the replaced hardware.
Some of these
functions
are explained below.
VCO TRACKING
The volts per octave (scaling) parameters of the 16
oscillators
in
the OB-8
are
adjusted by
the
microprocessor whenever the AUTO button is
pressed.
The
processor samples
5
diff eren
t
frequencies for
each
oscillator and calculates the
proper correction voltage
to
bring each
oscillator in tune.
This voltoge changes depending on the final pitch
desired from
the
oscillator. There is
a
rough trimmer adjustment for
each oscillator's volts per octave in
order to get the
oscillator within
the range in which the
processor can calibrate
it. If
this calibration
is
out of the acceptible range,
it
may be noticed by
either AUTO TUNE
failing that oscillator,
or
excessive beating between
it
and other
oscillators while
it is
BETWEEN half
steps.
The reason
for
this
is
that
the processor compensates for scaling
of the
oscillators
for
each half
step, requiring that the
oscillator be
callibrated well enough
so
that
the tracking between half steps
is
a
volt per octave.
If
this
is
out of
calibration
it can also be
percived as
steps
in
the LFO modulation,
since
the oscillator will jump slightly
in
pitch
as the
modulation
amount reaches each half step.
LFOs
All
of the LFOs
in
the OB-8 are
generated
in
software.
What this
means
is,
the voltage necessary
at
any given time to simulate LFO
modulation
is
output through the DAC
to
whatever destination
is
selected.
If, for
example, the
triangle wave of the LFO should be at
its peak,
then a high voltage will be
output to the
destination. When
the
LFO should ramp down, the voltage
is
decreased. All of the
electronic switches
and
VCAs normally required with
a
hardware LFO in
order
to determine
its
destination and
amplitude are
eliminated since
these
functions
are now accomplished
in
software.
INTRODUCTION/I
PITCH BEND
The pitch bend and vibrato levers are scanned
by the microprocessor
and their relative positions ore calculated
to
determine
the voltage
necesary
to
bend
a
pitch up or down, or how much vibrato
to add. This
eliminates
all
problems associated with matching
100k resistors
in
order
to send equal voltages to all
voices, op emp offsets, and scaling
trimmers.
There are
2
rough trimmers used
to bring the levers into
range
so that the
processor can read them. Once they are
in range, the
processor re-calculates the center dead-zone
of
each lever each time
AUTO
is
pressed.
4-POLE FILTER
The OB-Xa had seperate 2-pole and 4-pole filters for each voice,
each filter utilizing
a
CEM3320 with different external components
to
implement the filter functions. In the OB-8, There is one 3320 per
voice, and
the
external components
are
electronically switched
to
generate either
a
2-pole
or a
4-pole slope.
This eliminates the need to
calibrate the two filters seperately, since they are now
the
same
filter.
OP AMP OFFSETS
Many sample -and hold op
amps can affect
a
parameter if their offset
is large. To correct for this, the
processor uses
a
software
calibration procedure described later
to
assist
in setting the offset to
0
volts.
This
adjustment
is
most critical for the envelope
time
parameters, since
the
CEM3310 envelope chips require
a
control voltage
range from 0 to 300 millivolts. The software
is
able
to correct for the
offsets by outputting
a
voltage
to the sample end holds that will
compensate for the offset.
•'
If an op
amp has +30 millivolts
of offset,
the processor. will output 30 millivolts
less than the final voltage
required for that sample and hold, effectively canceling
the offset.