A voltage-controlled filter (VCF) is a processor, a filter whose operating characteristics (primarily cutoff frequency) can be controlled by means of a control voltage applied to control inputs. It can be considered to be a frequency-dependent amplifier. Although popularly known for their use in analog music synthesizers, in general, they have other applications in military and industrial electronics. A VCF allows its cutoff frequency and Q factor (resonance at the cutoff frequency), to be continuously varied; the signal outputs may include a lowpass response, a highpass response, a bandpass response, and a notch response. The filter may offer a variable slope which determines the rate of attenuation outside the bandpass, often at 6dB/octave, 12dB/octave (a '2 pole' filter) or 24dB/octave (a '4 pole' filter). This is also varied by the Q. In modular analog synthesizers, filters receive signal input from signal sources, including oscillators and noise, or the output of other processors. By varying the cutoff frequency, the instrument passes or attenuates partials. In some popular electronic music styles, "filter sweeps" have become a common effect. These sweeps are created by varying the cutoff frequency of the VCF (sometimes very slowly). 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, VCFs have included variable feedback which creates a response peak (Q) 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 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 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 a classic analog computer state variable "loop", which provided outputs in quadrature. 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.