CLAVIA DMI AB CLAVIA NORD Rack 2X [84/114] Cutoff frequency

Page 82 About Subtractive Synthesis

Cutoff Frequency

The most important parameter for a filter is its cutoff frequency, which is the setting that determines

where in the frequency material it should start cutting. If the cutoff frequency in a low pass filter is set to

a very low value, only the lowest harmonics (the bass) will pass through. If you raise the cutoff all the

way up, all frequencies will be let through, as the figure below illustrates.

Changing the cutoff frequency is often referred to as “sweeping the filter”. This is probably one of the

most important ways of shaping the timbre of a synthesizer sound. By using an envelope you can for

example have a high cutoff at the beginning of a sound which is then gradually lowered (the filter “clos-

es” as the sound decays). This would emulate the way most plucked string sound (piano, guitar etc) be-

have; the amplitude of the harmonics decreases as the sound decays.

Key Tracking

When you play different pitches, the oscillators produce different frequencies. This means that the over-

tones in the waveform appear at different frequencies. The cutoff frequency of the filter however, is

fixed. This means that different overtones will be cut off at different pitches. To be more precise, the fur-

ther up the keyboard you play, the muddier the sound will be.

To remedy this problem many synthesizers have a parameter called Filter Keyboard Tracking. When this

is activated, the filter Cutoff Frequency varies with which key you play, just as the oscillator frequency

does. This ensures a constant harmonic spectrum for all keys.

Resonance

Resonance in a filter is created by connecting the output of the filter to its input, in other words setting

up a “feedback loop”. The amount of feedback is then controlled with a Resonance parameter on the

front panel of the instrument.

Amplitude

Frequency

Filter Frequency

Amplitude

Frequency