Korg PS-3300 - VCF (Voltage Controlled Filter)

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@VCF (Voltage Controlled Filter)
When you look at the block diagram of a
synthesizer, you will almost always see this
series: VCO ~ VCF ~ VCA. These three
basic modules control the three elements
of sound: pitch, timbre, and volume.
The VCO generates a waveform (with a
certain tone color) of a certain pitch
(determined by its frequency).
By shaving off upper harmonic elements of
the waveform, the VCF adds roundness to
the sound.
A number of unique synthesizer effects are
created by automatically varying the cut-
off frequency in proportion to volume.
These volume changes are controlled by
the envelope signal. In other words, in this
so-called “expand” effect the envelope
control voltage varies the volume and cut-
off frequency at the same time.
Other VCF effects based on variation of
tone color are “wow” and “growl”. These
are obtained by using a modulation genera-
tor output signal (the MG is also known as
an LFO or low frequency oscillator). This
cyclic control voltage signal is used to mod-
ulate the VCF. If the operation of the VCF
seems too abstract to you, think of it as a
kind of watergate.
LL,
The
VCO
sends
out
a
sawtooth-shaped
pul-
sating
wave.
(Of
course
this
could
also
be
a
square
wave,
triangle
wave,
and
so on.)
The
VCF
is a watergate
with
teeth
on
it like
a
comb.
You
can
see
that
when
the
saw-
tooth
wave
tries
to get
by
the
watergate,
the
top
of the
wave
will
be
more
or less
shaved
off
depending
on
the
watergate’s
height;
when
the
top
is shaved
off,
it will
be
more
rounded.
In
other
words,
the
height
of the
watergate
corresponds
to the
cut-off
frequency
(Fe)
of the
VCF.
As the wave goes through the teeth of the
watergate, a resonance is created at the
cut-off frequency because of the interac-
tion between the teeth and the points cre-
ated on the top of the wave as parts of it
are shaved off. The longer the teeth, the
stronger the resonance. In other words, the
length of the teeth corresponds to the
PEAK function of the VHF.
To understand the expand effect, think of
the watergate being temporarily raised and
lowered
as
the
volume
goes
up
and
down.
With cut-off frequency modulation (FeM),
the watergate is repeatedly raised and low-
ered in proportion to the cyclic low-
frequency signal from the MG (modulation
generator) section. (This gives “‘wow" and
“growl” effects.)
fig.6 Watergate theory of VCF operation.
(APYTVCF & HAS)