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.
The basic analog VCO is built around a core, which generates one waveform; waveshaping circuitry produces the other waveforms. There are two basic types of core circuits. The most common type generates a sawtooth wave; an alternate type generates a triangle wave. There are often subtle differences in the exact harmonic content of the resulting waveforms, which give the two types a somewhat different character.
Because the musical octave is an exponential phenomenon, most VCOs include an exponential converter on the control voltage input circuit, which provides a Volts/octave response to an input control voltage; that is, an increase of a certain amount of voltage produces an output frequency change of one octave, and this is true throughout the VCO's scaling range. (Most analog VCOs will scale properly through a range of about five octaves.) 1 volt/octave is an industry standard used by all modern and most vintage equipment; see List of VCOs with non-standard scaling. Some VCOs, notably those on early Yamaha and Korg gear, do not have exponential converters and so the control voltage input produces a Volts/Hz response, where a given change in the input voltage produces a given Hz change in the output. This does not correspond to the musical octave.
A digitally controlled oscillator (DCO) is one in which the audio signal path is still analog, but the frequency is controlled by a digital timing mechanism.