Oscillator sync

Oscillator sync is a feature in some synthesizers with two or more VCOs, DCOs, or "virtual" oscillators. One oscillator will restart the period of another oscillator, so that they will have the same base frequency. This can produce a musical sound, rich with harmonics. The timbre can be altered on the synched oscillator by varying its frequency input. A synched oscillator that resets the other oscillator(s) is called the master, and any synched oscillator that is reset by another oscillator is called a slave. There are two common forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync. Soft Sync is a term used for a variety of mechanisms. Other sync mechanisms exist which are also discussed. According to Sound on Sound journalist Gordon Reid, oscillator sync is the least understood feature for many users of a synthesizer.

The master oscillator's pitch is generated by user input (typically the synthesizer's keyboard), and is arbitrary. The slave oscillator's pitch may be tuned to (or detuned from) this frequency, or may remain constant. Every time the master oscillator's cycle repeats, the slave is retriggered, regardless of its position. If the slave is tuned to a lower frequency than the master it will be forced to repeat before it completes an entire cycle, and if it is tuned to a higher frequency it will be forced to repeat partway through a second or third cycle. This technique ensures that the oscillators are technically playing at the same frequency, but the irregular cycle of the slave oscillator often causes complex timbres and the impression of harmony.

This effect may be achieved by measuring the zero axis crossings of the master oscillator and retriggering the slave oscillator after every other crossing.

This form of oscillator sync is more common than soft sync, but is prone to generating aliasing in naive digital implementations.

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