Hard Sync
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 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.
Read more about this topic: Oscillator Sync
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