Similar to a VCA, except the control voltage affects a filter circuit. The filter portion can take on a variety of different characteristics (low pass, resonant, etc.), but the general concept is the same. It is a filter circuit whose value is controlled by a voltage applied to it. In keyboards, where VCA’s are common, the voltage is often either tied to the note number played (higher notes might open a low pass filter) or to attack velocity (the harder the note is played the more the filter is opened). VCF’s are also sometimes found in single ended noise reduction units. These work by opening or closing a high pass filter based on the amplitude of high frequency signals.