Hard sync resets a slave oscillator's phase every master cycle, locking its pitch and generating bright harmonics

Oscillator hard sync connects a slave oscillator to a master: whenever the master completes a cycle (its rising edge), it forces the slave to reset to the start of its waveform, regardless of where the slave was in its own cycle. This truncation locks the slave’s perceived pitch to the master, while the slave’s own (usually higher) tuning reshapes the waveform — cutting a different, generally non-integer number of slave cycles per master cycle. The result is a bright, buzzy, aggressive, harmonically rich timbre: the classic ‘sync lead’. Sweeping the slave’s pitch (or a Sync parameter setting the master-slave offset) under a fixed master sweeps through the harmonic series, producing the characteristic tearing sync sweep, especially when driven by an envelope or LFO. Soft sync differs: it only resets the slave when the slave is already near the end of its cycle, so it ignores the sync signal unless the two are tuned close to an octave — a gentler, less predictable effect.

Examples

Master VCO at 110 Hz to slave VCO’s sync in; slave tuned to ~300 Hz and swept upward by an envelope: the pitch stays at 110 Hz but the timbre tears and brightens. In Surge XT (Classic/Modern), the Sync parameter sets the master-slave offset in semitones; map the filter envelope to Sync at 12–24 semitones for a sweeping lead — pitch constant, harmonics changing. Sync = 0 gives the plain waveform. In soft-sync mode the same patch only locks when the slave is tuned near an octave of the master.

Assessment

Explain why hard sync makes the slave track the master’s pitch even when the slave knob is detuned, and why a synced oscillator produces more harmonics than an unsynced one at the same pitch. Describe the audible difference between hard sync and soft sync as the slave is swept across a wide pitch range.