Voltage-controlled filter. A variable DC voltage is used to control a parameter of a filter. Often the parameter controlled is the cut-off frequency. (An envelope generator or LFO might be used). Other controllable parameters: degree of resonance aka Q, bandwidth, etc. Found in such products as synthesizers and multimode transceivers.

In the world of synthesisers - in particular that of subtractive synthesis, whereby the raw sound of oscillators is sculpted into something pleased to the ear - a voltage controlled filter is a filter which can be controlled by one or more voltage inputs, typically directly via one or more sliders, and additional with an external input. This control occurs in real time, such that a varying voltage produces a proportional effect on one or more aspects of the filter.

Typically, a VCF allows for a control voltage to alter the cut-off frequency and the resonance frequency (for more information on filters and their characteristics, see filter). Much less commonly, a VCF might also allow for a control voltage to alter the basic frequency type, whether low-pass, band-pass, notch, high-pass or others. Often the filter will allow for the cut-off and resonance to be controlled individually, although this is not a universal rule; indeed, the famous ARP 2600 synthesiser did not even allow for voltage control of resonance at all, whilst certain digital filter models of the 1980s simply did not include resonance (notably the 'phase distortion' synthesis of the Casio CZ-101).

The term 'voltage control' derives from the early days of synthesis, when discrete electronic components talked to each other with voltages; rather than, as today, with digital information. VCFs are typically modulated with both the synthesisers envelope generators, and also one or more LFOs. The former modulation allows for the classic filter 'twang', as the cut-off is lowered over time (the effect can be emphasised by increasing the resonance over the same period), whilst the latter modulation can produce an equally classic undulating filter effect beloved of ambient musicians.

Nowadays, although the term VCF is still used, filters are not typically controlled with voltages; they are modelled in software, and controlled with digital information. Certain synthesis models do not generally include filters at all, most notably the FM synthesis which became widespread in the 1980s, with the Yamaha DX7.

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