Voltage-controlled filter
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A voltage-controlled filter (VCF) is a signal processing device in an analog synthesizer positioned after the oscillator. The oscillator generates the signal, the pitch and the overtones. The purpose of the filter is to cut off the unwanted overtones, and leave only the wanted frequencies of the sound.
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.
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 quickly signals outside the pass band become attenuated - usually 12dB/octave (a '2 pole' filter) or 24dB/octave (a '4 pole' filter).