Voltage-controlled filter

A voltage-controlled filter (VCF) is a filter whose operating characteristics can be controlled by means of a control voltage applied to one or more inputs.^[1]

Depiction of cutoff frequency of a low-pass filter

A VCF allows its cutoff frequency and Q factor to be continuously varied; it usually gives a lowpass response, but may also be switchable to allow highpass, bandpass or even notch responses. The filter may offer a switchable slope which determines how intensely signals outside the pass band become attenuated, usually 12dB/octave (a '2 pole' filter) or 24dB/octave (a '4 pole' filter).

In analog synthesizers, which are commonly used to make electronic music, VCFs are commonly positioned after the oscillator(s). The oscillator generates an audio waveform, which (except for noise waveforms) includes a fundamental pitch and a series of harmonic overtones. By varying the cutoff frequency (the maximum frequency passed by the filter), the synth operator can add or remove some of the overtones to create more interesting and textured sounds.

In much electronic music, "filter sweeps" have become a common effect. These sweeps are created by varying the cutoff frequency of the VCF (sometimes very slowly) to reveal or conceal the oscillator's overtones. Controlling the cutoff by means of an envelope generator, especially with relatively fast attack settings, simulates the attack transients of natural or acoustic instruments.

A VCF is an example of an active non-linear filter: however, if its control voltage is kept constant, it will behave as a linear filter.

See also

• Electronic filter
• Non-linear filter
• Self oscillation
• Subtractive synthesis
• Voltage-controlled amplifier
• Voltage-controlled oscillator

References

External links

• Schematics and PCBs for building your own VCF