One major feature that you’ll find on a variety of different synthesizers is oscillator sync. This refers to the process where one oscillator restarts the period of another oscillator, which results in both having the same base frequency. The timbre on the synched oscillator can be adjusted by varying its frequency input.
The synched oscillator that resets the other oscillator is referred to as the master. The slave oscillators are the ones that are being reset.
Hard Sync and Soft Sync are the two basic types that are found on synthesizers.
Hard Sync is found far more often than soft sync, but it often generates aliasing. In hard sync, the pitch of the master oscillator is arbitrary and is created by user input, most often through the synthesizer’s keyboard. You can keep the pitch of the slave oscillator constant or tune it to or detune it from a certain frequency.
Each time the cycle of the master oscillator repeats, the slave is triggered again no matter what position it is in. When a slave that is tuned to a lower frequency than the master, it will need to repeat before it completes its full cycle. When a slave is tuned to a higher frequency than the master, it will be forced to repeat partway through the second or third cycle. This ensures that the oscillators will technically be playing at the same frequency. At the same time, the irregular cycle of the slave will often cause aliasing, the impression of harmony, and unnatural timbres.
This effect results from measuring the master oscillator’s zero axis crossings and retriggering the slave after every other crossing. Do note that aliasing is common in digital implementations, like in software synthesizers for example, but it is not an inherent part of the actual method. Analog synthesizers will not exhibit aliasing.
Soft Sync is a less common form of oscillator sync, but it generates aliasing less often. There is one small difference between Hard Sync and Soft Sync. The slave oscillator in a hard sync setup resets to zero with every cycle of the master no matter the position or direction of the slave waveform. This often results in asymmetrical shapes. In Soft Sync the wave is inverted, meaning the direction is reversed rather than being reset to zero.
This effect comes from measuring the master oscillator’s zero axis crossings and reversing the slave oscillator’s slope after every other crossing.
The hard sync is disabled when the amplitude or frequency of the slave crosses over the threshold that is defined by the user. It’s also disabled when the frequency of the slave is lower than that of the master or when the frequency of the slave extends too high above or far below that of the master. Any of these may be referred to as soft sync depending on the manufacturer or synthesizer model. However, these are listed as secondary because none of them synthesize the waveform in a different way than hard synch.