Korg MS-50 - voltage controlled filter

3. The modules and their functions ^

(2) VOLTAGE CONTROLLED FILTER (VCF)

[vcf module]
- voltage controlled filter

- VCF frequency response

	CUT-OFF FREQUENCY (Fc) This adjusts the frequency at which the filter starts to shave off the upper harmonics of the input signal. The filter removes frequencies above this cut-off point (Fc) while allowing lower frequencies to pass through.
	PEAK Use this knob to adjust the amount of emphasis at the cut-off point (Fc). When the peak is set to 8 or above, the VCF will begin oscillating by itself, thereby acting as another sound source. This self oscillation or "singing" produces an extremely regular sine wave, that has a frequency that depends on the Fc setting.
	CUT-OFF FREQUENCY MODULATION INTENSITY (FcM) This adjusts the intensity of cut-off frequency modulation (FcM) effects (such as growl and expand) that use the MG, EG, MS-01, or other control signals to vary the cut-off frequency (Fc).
	SIGNAL INPUT Use this input for the VCO output signal or other audio frequency (AF) signals (from an audio amp, for example). The VCF will operate best if the external signal you connect to this jack has a +3Vp-p intensity. Test it on the volt meter through the AC IN jack.
	FcM INPUT Connect MG, EG, MS-01, or other outputs to this jack for cut-off frequency modulation effects (growl, expand). When the FcM intensity knob (3) is adjusted to its highest sensitivity setting, a 1V change in the control signal will give a 2-octave variation in the cut-off frequency point (Fc).
	SIGNAL OUTPUT (for sound signal) This jack provides the VCF sound signal output. It is usually connected to the VCA module's SIG IN jack.

As shown in the figure above, the Fc knob determines the point above which the upper harmonics of the signal passing through the VCF are sharply attenuated. The PEAK knob controls the intensity of the frequencies around the "shoulder" of the Fc. Taking a sawtooth wave as an example, let's see how the Fc and peak will affect the waveform after it leaves the VCO and is transferred through the VCF.

[watergate model of vcf]
- the watergate model of VCF operation

In the figure above you see a model of VCF operation based on the way a watergate changes the shape of a series of waves on the surface of a stream of water. (To mix metaphors, you could also compare these waves to your pulse as you heart beats blood through your arteries.) The original shape of these waves corresponds to the VCO output waveform (which could just as well be a triangle or square wave instead of the sawtooth wave shown here.) The VCF acts like a comb-shaped watergate that can be adjusted to cut off parts of the tops of the waves.

How high you raise the watergate will determine how much of the sharp top points you shave off; the more you shave off, the rounder the waveform becomes. The height of the watergate is therefore like the cut-off frequency (Fc) of the VCF.

When the tips of the waves contact the tips of the teeth of our comb-shaped watergate, the teeth begin to vibrate. These vibrations cause new ripples on top of the rounded-off waves; this is because of the sympathetic between the filter and the signal.

In our model, the length of the teeth determines the intensity of the resonance peak that is created. The peak knob on the VCF module lets you adjust how strong a resonance appears at the Fc.

In the "expand" effect, an envelope signal is used to temporarily raise the Fc and then lower it simultaneously with volume fluctuations. (The same envelope signal can modulate both the Fc and the VCA gain.) Similarly, "growl" and "wow" effects are created when you use the MG (modulation generator) output signal to modulate the Fc so that it rises and falls repeatedly. This is like repeatedly raising and lowering the watergate in our example.

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