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Oscillator sync is a feature in synthesizers. One oscillator will restart the period of another oscillator, so that they will have the same base frequency. 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 basic forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync

This form of oscillator sync is more common than soft sync, but frequently generates aliasing. In this form, 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 unnatural timbres, the impression of harmony, and aliasing.

This effect is 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 less common, but less frequently generates aliasing. This form is very similar to Hard Sync, with one small difference. In a Hard Sync setup, the slave oscillator is forced to reset to zero with every cycle of the master regardless of position or direction of the slave waveform, which often generates asymmetrical shapes. In Soft Sync, rather than resetting to zero, the wave is inverted; that is, its direction is reversed.

This effect is achieved by measuring the zero axis crossings of the master oscillator and reversing the slope of the slave oscillator after every other crossing.

Due to the rarity and ill-defined nature of Soft Sync, several other effects may also be referred to by the same name:

• Hard Sync which is disabled when the frequency or amplitude of the slave crosses a user-defined threshold.
• Hard Sync which is disabled when the frequency of the slave extends too high above or too far below the frequency of the master.
• Hard Sync which is disabled when the frequency of the slave is lower than the frequency of the master.

Soft Sync may accurately refer to any of these, depending on the synthesizer or manufacturer in question. However, because none of them actually synthesize the waveform in a way different from Hard Sync (rather, they selectively deactivate it) they are listed here as secondary.