An electrical circuit that emphasizes or eliminates some frequencies from a signal. Filters are used in electronic music to alter the harmonic content of a signal, which changes its timbre. Many of the filters used in synthesizers are voltage controlled filters (or a digital equivalent), which allows the filter to be controlled by a signal generated elsewhere in the synthesizer, in addition to or instead of a panel knob.

The essential parameters of a filter are its cutoff frequency and its slope. The cutoff frequency is, basically, the demarcation between frequencies that the filter allows to pass, and frequencies that it tries to eliminate. A filter's cutoff frequency is determined by a panel knob, or, if it is a VCF, a control voltage input. The slope is determined by the design of the circuitry. Slope is specified either in decibels per octave or in "poles", where (to simplify somewhat), one "pole" is equivalent to 6 dB/octave. Two-pole and four-pole filters are commonly used in synthesizers. Many filters used in synths also use resonance to sharpen the response of the filter. Resonance basically means feeding back some of the filter's output to its input, so that part of the signal gets run through the filter multiple times. Resonance tends to make the cutoff response sharper, and also produces a response peak at the cutoff frequency. Excessive resonance may cause the filter to break into self-oscillation; that is, it produces an output tone at or near the cutoff frequency, without any input applied. Performers disagree on the desirability of this; some find the ability to self-oscillate a useful trait in a synth filter.

Although there are an infinite number of possible response curves that a filter can have, the most common types used in synths are:

  • Low pass, which tries to eliminate all frequencies above the cutoff frequency.
  • High pass, which tries to eliminate all frequencies below the cutoff frequency.
  • Band pass, which tries to eliminate all frequencies that are not within a specified band on either side of a center frequency. (A bandpass filter can be thought of as a low pass filter and a high pass filter back to back, allowing frequencies between the two cutoff frequencies and eliminating those above the low pass and below the high pass.)
  • Notch or band reject, which tries to eliminate only the frequencies that are within a specified band on either side of a center frequency.
  • Comb, which consists of a series of notches.