Many voltage-controlled oscillators for electronic music are based on a capacitor charging linearly in an op-amp integrator configuration.
This provides stable digital pitch generation by using the leading edge of a square wave to derive a reset pulse to discharge the capacitor in the oscillator's ramp core.
[2] Whilst this was an issue for monophonic synthesizers, the limited number of oscillators (typically 3 or fewer) meant that keeping instruments tuned was a manageable task, often performed using dedicated front panel controls.
With the advent of polyphony, tuning problems became worse and costs went up, due to the much larger number of oscillators involved (often 16 in an 8-voice instrument like the Yamaha CS-80[3] from 1977 or Roland Jupiter-8[4] from 1981).
Engineers working on the problem looked to the frequency division technology used in electronic organs of the time and the microprocessors and associated chips that were starting to appear, and developed the DCO.
The leading edge of this square wave is used to derive a reset pulse to discharge the capacitor in the oscillator's ramp core.