Oscillator sync is a feature in some synthesizers with two or more VCOs, DCOs, or "virtual" oscillators. As one oscillator finishes a cycle, it resets the period of another oscillator, forcing the latter to have the same base frequency. This can produce a harmonically rich sound, the timbre of which can be altered by varying the synced oscillator's frequency. A synced oscillator that resets other oscillator is called the master; the oscillators which it resets are called slaves. There are two common forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync. According to Sound on Sound journalist Gordon Reid, oscillator sync is the least understood feature for many users of a synthesizer.
Hard Sync
The master oscillator's pitch is generated by user input, and is arbitrary. The slave oscillator's pitch may be tuned to 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. If the tuning of the slave oscillator is swept, one may discern a harmonic sequence. 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.
Soft Sync
There are several other kinds of sync which may also be called Soft Sync. In a Hard Sync setup, the slave oscillator is forced to reset to some level and phase with every cycle of the master regardless of position or direction of the slave waveform, which often generates asymmetrical shapes. In some cases, Soft Sync refers to a process intended to nudge and lock the slave oscillator into the same or an integer or fractional multiple of the master oscillator frequency when they both have similar phases, similar to a phase-locked loop.
Reversing Sync
This form of oscillator sync is less common. This form is very similar to Hard Sync, with one small difference. In Reversing Soft Sync, rather than resetting to zero, the wave is inverted; that is, its direction is reversed. Reversing Soft Sync is more associated with analog triangle core oscillators than analog sawtooth core oscillators.
Threshold or Weak Sync
Several kinds of Soft Sync use comparison thresholds:
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.
Phase Advance 'Sync'
The phase of the slave is advanced by some amount when the master oscillator level crosses some threshold. Used for audio synthesis, this may give an audible effect similar to Soft Sync.
When the master oscillator crosses some threshold, the normal reset of the slave is disabled: it will stick at its final level, positive or negative. When the master crosses back over some threshold, the slave is reset.
Overlap Sync
In this method, the current wave completes but a new waveform is generated at the sync pulse. The tail of the old wave and the new wave are output summed if they overlap.
Digital Implementation Aspects
Naive approaches to sync in digital oscillators will result in aliasing. To prevent this, band-limited methods such as additive synthesis, BLIT or BLEP must be adopted to avoid aliasing. In a digital oscillator, best practice is that the slave will not be reset to the identical phase each cycle, but to a phase advanced by an equivalent time to the phase of the master at the reset. This prevents jitter in the slave frequency and provides truer synchronization. For digital oscillators, Reversing Sync may less frequently generate aliasing. This effect may be naively implemented by measuring the zero axis crossings of the master oscillator and reversing the slope of the slave oscillator after every other crossing. For digital implementation, note that none of the Threshold or Weak Sync methods actually synthesize the waveform in a way different from Hard Sync. Overlap sync is primarily a digital technique with simple implementation, such as used in FOF; an analog implementation could be a highly damped sine oscillator excited by the reset pulse.
