A voltage-controlled filter (VCF) is a filter whose operating characteristics (primarily cutoff frequency) can be controlled by means of a control voltage applied to one or more inputs. Although popularly known for their use in analog music synthesizers, in general, they do have other applications in military and industrial electronics.
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 music synthesizers, filters typically receive audio from the oscillator(s). The oscillator generates an audio waveform, which (except for noise waveforms) includes a fundamental pitch and a series of harmonic partials. By varying the cutoff frequency (the maximum frequency passed by the filter), the musician can add or remove some of the partials 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 some of the oscillator's partials. Controlling the cutoff by means of an envelope generator, especially with relatively fast attack settings, simulates the attack transients of natural or acoustic instruments.
From the very beginnings of modular analog synthesizers, VCFs often include variable feedback which creates a response peak at a frequency just below the cutoff slope. This peak can be quite prominent, and when the filter's frequency is swept by an envelope generator, it creates "chirps" that are very popular and a trademark of traditional analog synth. sounds. Some synthesizers permit enough feedback to let the filter oscillate, and it can serve as a sine-wave source, although not necessarily with accurate pitch.
ARP Instruments made a multifunction voltage-controlled filter module capable of stable operation at a Q greater than 100; it could be shock-excited to ring like a vibraphone bar. Q was voltage-controllable, in part by a panel-mounted control. Its internal circuit was the classic analog computer state variable "loop", which provided outputs in quadrature (although they did not emphasize that).