The Secret Sauce of Synthesizer Sounds At its core, a Voltage - Control Filter (VCF) is an electric circuit that shapes the timbre of a sound by selectively removing frequencies. Think of it as an audio equalizer on steroids. While EQs can boost or cut specific frequencies, a VCF is "voltage - controlled," meaning its settings - like the cut off frequency and resonance - can be changed automatically by control voltage (CV) from other parts of the synthesizer, such as an en