A VCF (voltage-controlled filter) is a module that allows users to shape the frequency content of an audio signal using a control voltage. VCFs are commonly used to shape the timbre of a sound, such as creating a brighter or darker tone, or removing unwanted frequencies.
VCFs typically have an input jack for the incoming signal, a control voltage input jack, and an output jack for the processed signal. The incoming signal is filtered based on the frequency response of the VCF, which is controlled by the control voltage input. VCFs can have a variety of frequency responses, such as low-pass, high-pass, band-pass, or notch.