A method of altering the timbre of a VCO by having its phase synchronized to the frequency of a master oscillator. Oscillator sync is usually divided into "hard sync" and soft sync. In hard sync, each time the master oscillator reaches a certain level, it forces the slave oscillator to reset to the initial point in its cycle. The audible effect of this on the slave oscillator varies depending on the difference in frequencies between the two oscillators, and the slave oscillator's waveform. If the master is much slower than the slave, the audible effect is periodic clicking or glitching. As the slave oscillator becomes faster, but still less than half of the master oscillator frequency, the effect becomes similar to amplitude modulation. When the two oscillators are near the same frequency, an effect similar to phase shifting results. As the slave becomes much faster than the master, the output takes on the frequency of the slave.
Soft sync is a generic term used for a variety of techniques that tend to push the frequency of the slave oscillator "closer" to the master. Some soft sync algorithms cause a reset of the slave oscillator on only some cycles; some others change the phase of the slave by a certain amount without forcing a full reset.