In analog synthesis, a Voltage Controlled Oscillator (VCO) is an oscillator whose output frequency is determined by an input control voltage1. The frequency (pitch note) and amplitude (the power of the signal) are often modulated directly by the synth’s keyboard.

The term “beat frequency” in this context typically refers to the phenomenon that occurs when two VCOs are operating at slightly different frequencies. The resulting sound fluctuates in volume at a rate equal to the difference in frequencies of the two VCOs. This fluctuation, or “beating,” can be used creatively in sound synthesis to add richness and depth to the sound.