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.

Oberheim OB-8 Specifications

General

Type	Analog
Polyphony	8 voices
Waveforms	Sawtooth, Square, Pulse, Triangle
LFO	1 LFO with multiple waveforms
Envelopes	2 ADSR envelopes
Memory	120 patches
Control	MIDI (optional)
Year Released	1983
Oscillators	2 per voice (VCOs)
Keyboard	61 keys
Filter	2-pole/4-pole switchable resonant low-pass filter