A digitally controlled oscillator, or DCO, is an oscillator circuit that generates an analog signal, but whose frequency is controlled by a digital control input (as opposed to a voltage controlled oscillator, whose frequency is set by a control voltage). The DCO first appeared in the 1980s as a sort of intermediate step between the all-analog VCO, and the all-digital implementations that came later. Several DCO circuit designs exist, but most work by synchronizing an analog sawtooth core to a digital countdown timer driven by a high-frequency, highly precise clock. The value loaded into the countdown timer determines the output frequency.
A DCO has the virtue that it stays in tune, unlike an all-analog VCO circuit. However, depending on the design, its waveform characteristcs are not precisely like an analog VCO. The DCO can behave differently when the frequency is being slurred, such as by a pitch wheel or portamento. And if it is not calibrated properly, its output level can vary with frequency. Contrarywise, some performers feel that the DCO, like the all-digital implemention, is too precise and does not possess the "thick" or "fat" sound of a VCO. After the end of the 1980s, for many years there were no synths in production that use DCOs, but Dave Smith Instruments brought back the DCO with the Evolver in 2002. Two of the best-selling synths of the 1980s, the Roland Juno-106 and the Korg Poly-800, used DCOs.