A voltage-controlled filter is an electronic filter whose operating characteristics can be set by an input control voltage. Voltage controlled filters are widely used in analogue music synthesizers. A music synthesizer VCF allows its cutoff frequency, and sometimes its Q factor, to be continuously varied. The filter outputs often include a lowpass response, and sometimes highpass, bandpass or notch responses. Some musical VCFs offer a variableslope which determines the rate of attenuation outside the bandpass, often at 6dB/octave, 12dB/octave, 18dB/octave or 24dB/octave. In modular analog synthesizers, VCFs receive signal input from signal sources, including oscillators and noise, or the output of other processors. By varying the cutoff frequency, the filter passes or attenuates partials of the input signal. In some popular electronicmusic styles, "filter sweeps" have become a common effect. These sweeps are created by varying the cutoff frequency of the VCF. Controlling the cutoff by means of a transient voltage control, such as an envelope generator, especially with relatively fast attack settings, may simulate the attack transients of natural or acoustic instruments. Historically, musical VCFs have included variable feedback which creates a response peak at the cutoff frequency. This peak can be quite prominent, and when the filter's frequency is swept by a control, partials present in the input signal resonate. Some filters are designed to provide enough feedback to go into self-oscillation, and it can serve as a sine-wave source. ARP Instruments made a multifunction voltage-controlled filter module capable of stable operation at a Q over 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 a classic analog computerstate variable "loop", which provided outputs in quadrature. A VCF is an example of an activenon-linear filter. The characteristic musical sound of a particular VCF depends on both its linear frequency response and its non-linear response to larger amplitude inputs.
