A voltage controlled oscillator, or VCO, is an oscillator whose output frequency is determined by an input control voltage. The VCO, or a digital equivalent of it, is an essential component of all modern synthesizers, and it serves as the primary sound source on most analog synthesizers. Typically, the VCO outputs a selection of waveforms containing harmonics which allow a number of different timbres to be achieved by subsequent filtering. Commonly found waveforms are the triangle wave, sawtooth wave, and pulse wave (often with pulse width modulation also under control of a control voltage). Also sometimes found is the sine wave, which contains no harmonics but is useful in reinforcing other waveforms as well as building up waveforms using the technique of additive synthesis.