Korg POLYSIX Owner's Manual page 6

The VCF is perhaps the single most expressive module in the Polysix. Its basic function is to modify the tonal
quality (timbre) of the waveforms produced by the VCOs by eliminating certain harmonics (overtones) and
emphasizing other ones. Understanding how to use it is a key part of getting the most of your instrument.
There are 6 VCFs in the Polysix, one for each voice. Each VCF is a 4 pole, 24dB/octave low pass filter with
voltage controlled cutoff frequency, variable resonance, positive and negative EG modulation ("enveloping"), and
variable keyboard tracking. Each of these terms will be explained in detail below.
How does a filter modify timbre? There are two basic functions: cutoff frequency and resonance.
1) CUTOFF
The cutoff frequency is the point in the audio spectrum where the filter starts to affect sounds passing through
it. The Polysix filters are termed Low Pass, which means that they pass frequencies which are lower than the
cutoff point.
Any frequencies above the cutoff frequency of the filter are progressively rolled off, or reduced in level. The
higher a frequency is above the cutoff point, the more it gets rolled off. This rolloff can occur at different rates. The
Polysix filters have a 24dB/octave rolloff. This means that a harmonic an octave above the cutoff point would be
about 16 times softer coming out of the filter than going in. At two octaves above the cutoff point, a harmonic
would be about 256 times softer! This very sharp rolloff helps to produce realistic imitations of most instruments, a
wide variety of punchy synth voicings, and many special effects. Since the cutoff point can be varied throughout
the audio spectrum, you can choose to remove no harmonics, upper harmonics only, everything but the
fundamental, or anything in between. It can also remove everything, silencing the instrument, if the cutoff point
goes much below the fundamental pitch.
There are five ways to vary the cutoff frequency of the six VCFs: manually, via the VCF CUTOFF control; or
automatically via each voice's Envelope Generator, the Keyboard Tracking function, or the Modulation Generator;
or via any external device plugged into the rear panel VCF fcM IN jack. The actual cutoff frequency of the VCF
will be proportional to the sum of the voltages (control signals) from these five sources at any moment.
VCF CUTOFF control
This determines the initial cutoff frequency of the filter. At 0, the cutoff is so low that no sound will be passed.
As you turn the control clockwise (towards higher numbers), the cutoff frequency increases and the note will begin
to sound, first with a dark, muted timbre and then with a brighter one. At 10, the cutoff point is above any audible
harmonic and the sound will be very bright.
2) RESONANCE
The second way that a filter modifies timbre is by adding resonance, (also known as "Q", "Peak", or
"Emphasis") at the cutoff point. Resonance is useful for a wide variety of sounds, such as organ, french horn,
classic synth bass and lead sounds, and many special effects. The RESONANCE control allows you to vary the
amount of resonance from 0 to self-oscillation.
At 0, there is no resonance, and frequencies above the cutoff point are rolled off smoothly. Changing the cutoff
point with no resonance simply varies the brightness or high frequency content of the sound.
Advancing the Resonance control clockwise (towards higher numbers) creates a resonant "peak" which serves
to strongly emphasize the frequencies near the cutoff point (frequencies above the cutoff point are still rolled off).
This results in much more dramatic changes to the timbre of the sound. Medium resonance produces the
characteristic "waa-waa" sound when the filter cutoff is swept by the EG, footpedal or manually.
At high resonance, the filter will self-oscillate, producing a pure sine wave at the cutoff frequency. This can be
used as a second sound source along with the VCO, or by itself if the VCO is shut off. The purest tone is
generated at about 7.
6
VCF
1 2 3
CUTOFF RESONANCE EG INTENSITY
VCF
4
KBD TRACK