Voltage controlled filter

A voltage-controlled filter (VCF) is an electronic filter whose operating characteristic can be set by a control voltage.

Common filter types used in audio synthesis 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)

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:
 * 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.