Voltage controlled filter

The voltage-controlled filter (VCF) is an electronic filter whose operating characteristic can be accurately voltage controlled over a wide audio range.

Common filter types used in audio synthesis include: There are many ways to implement most filters, and each has its own specific audio characteristics or flavour. Some common features across most filter types include:
 * Low-pass (high-cut) filter - removes higher frequencies
 * High-pass (low-cut) filter - removes lower frequencies
 * Band-pass filter - removes frequencies outside a given band
 * Notch filter - removes frequencies within a given band
 * Shelf filters - raise or lower frequencies above or below a cut off point.
 * Peak filters - raise frequencies within a given band
 * Formant filters - raise multiple peaks, often in such a way that they mimic the human voice.
 * State variable filters - e.g. include simultaneous low-pass, high-pass and band pass outputs as well as voltage controllable cut off frequency and resonance (Q)
 * Comb filter - adding a delayed version of a signal to itself, causing constructive and destructive interference.
 * A cut-off frequency - the frequency at which the filter begins to remove frequencies. This is often voltage-controllable and can changes over time (e.g. controlled by an envelope, or an LFO).
 * Resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable.
 * Phase shift - frequencies beyond the cut-off often have their phase affected.