Oberheim Xpander Owner's Manual

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FM
Synthesis
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Frequency
Modulation
or FM is a basic
analog concept.
The evolution
of
Linear FM as a
technique
for sound
synthesis
has become
possible
in
recent
years because
of
major
break-
throughs in
digital signal
processing and
oscillator stability.
The
fundamental idea
of
FM is that if
you
take two
oscillators and
modulate
the
first
(called
the
earner)
with the
second (called the
modulator),
a
series of
overtones
related
to
the
fre-
quency and
amplitude
of the
modulation will
be
generated. The
electronic
configuration
needed to
accomplish
this is quite simple.
One oscillator
modulates the
frequency
of
a
second
oscillator.
Period. The
trick is using
linear
rather than
exponential
control, and
being
able to
precisely
control the
modulation.
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Terms of
Modulation
In the
Xpander, VC02 is used to
modulate
VCOL
VCOl is called
the
Carrier VC02
is
called
the
Modulator
Use
a
Triangle
waveform
on VCOl
Turn off
all
the
waveforms
of VC02.
Its
triangle
wave is
patched
separately
for FM
Access the
FM/LAG
page
and turn up the
FM
AMPIitude.
Then
go
to
the VC02
page
and
play
with the
FREOuency
control What
you should
hear are
some
of the rudest
sounds
ever
to come out
of
a
synthesizer.
Subtlety
With
a
small
modulation
amplitude,
the
overtones
generated
will be
the
frequencies
of the
two
oscillators as
well as the
sum and
the
difference
of
those
two
frequencies.
With increas-
ing
amplitudes,
other
overtones
will
be
generated as
well
More
of these
mostly
non-harmonic
overtones
will
be
generated if the
modulating
frequency
is
lower than
the
carrier The lower
the
modulator,
the more
spread out
these
overtones
become.
Changing
the
modulation
depth
(amplitude)
changes
the volume
and
number of
the over-
tones, not
unlike
filtering. The
depth can
itself
be
modulated
by
any
modulating
source
on
the
Xpander; Envelopes,
LFOs.
Levers, Velocity, etc.;
routed
directly or
through
VCAs.
Track-
ing
Generators, or
the Lag
Processor.
The
VCF
instead of VC01
can
be used
as the
carrier,
for other
tirnbral
variations. Set the
filter
resonance to 63 so
that
the filter
will oscillate.
Mathematics
If the
frequencies
of the two
oscillators
are not
in an exact
ratio, some
wonderfully
noisy
sounds
can
be
generated
But because
natural
harmonics are
in
exact
mathematical
ratios,
in-tune
harmonics
will result
only
when the
carrier
and
modulator
oscillators
are
in
basic
exact
frequency
ratios
(unison, octaves,
fifths, etc.).
UWWMVft-
63
Xpander
Owners Manual
Synthesthosia

Oberheim Xpander Specifications

General

Type	Analog Synthesizer
Polyphony	6 voices
Oscillators	2 VCOs per voice
Control	MIDI
Synthesis Type	Analog Subtractive
VCA	1 per voice
Memory	100 patches
Modulation	Extensive modulation matrix
Year Released	1984
Filters	Multi-mode filter per voice
Envelopes	3 per voice
Keyboard	No keyboard
Inputs	CV/Gate
Outputs	Individual voice outputs