Some synth sounds you recognise in a second: that biting, metallic lead that seems to “sweep” across the spectrum as you move a wheel, the signature you associate with certain synth-pop records or screaming solos. Behind many of them lies one precise technique: oscillator sync, better known as hard sync. It is neither an effect nor a separate kind of synthesis: it is a way of forcing two oscillators to behave in a way neither would on its own. Let us look at how, and above all why it sounds the way it does.
Quick recap: what an oscillator is
In a synth, the oscillator is the sound source. It produces a repeating waveform, a cycle that starts over and over at a given frequency, and that frequency sets the pitch of the note. In classic subtractive synthesis, you start from harmonically rich waveforms (sawtooth, square) and sculpt them with a filter and envelopes. Sync, by contrast, acts directly on the oscillators, upstream of everything else.
The principle: one oscillator resetting another
Sync involves two oscillators, traditionally called the master and the slave.
- The master oscillator sets the pitch you actually hear. It gives the note.
- The slave oscillator is the one you listen to, but it is under control: every time the master restarts its cycle, it forces the slave to restart its own from the beginning, instantly, whether or not it has finished its cycle.
That forced restart is what does everything. Because the slave is reset at the master’s rate, the overall period of the heard sound follows the master’s frequency (so the note stays in tune). But the shape of the slave’s wave is chopped, broken mid-cycle. That discontinuity creates a complex wave, rich in new harmonics, with a bright, slightly brash timbre very different from what the slave would produce alone.
Where the famous “sweep” comes from
The real show begins when you vary the slave oscillator’s pitch while the master stays fixed. Normally, raising an oscillator’s frequency raises the note. Not here: the note stays locked to the master’s, since the master sets the period. But by changing the slave’s “free” frequency, you move the point at which its cycle is cut, and so you continuously reshape the sound’s harmonic content. The result is that characteristic timbral sweep: the pitch does not move, but the tone glides, brightens, hardens, like a filter made of pure harmonics.
That is exactly what makes sync so expressive: you hold a stable note while making its colour evolve dramatically. And because that parameter (the slave’s pitch) is an ideal modulation target, you rarely leave it to the hand alone.
How to program it so it sings
A static sync patch is interesting for two seconds; modulation is what brings it to life. Three sources pair especially well with the slave’s pitch.
- An envelope: routing an ADSR envelope to the slave’s frequency gives that sweep that “rises” then falls on every note played. It is the setting behind a good share of 1980s sync leads: sharp attack, the timbre leaps, then settles.
- An LFO: a slow LFO on the slave’s pitch creates a steady ripple in the timbre, ideal for a moving pad or a hypnotic effect.
- The mod wheel or aftertouch: handing the sweep to live playing turns sync into a musical gesture rather than a fixed setting. That is where the sound takes on its full expressive dimension.
As a starting point: take two sawtooths, engage sync, set the master to the note you want, and detune the slave slightly upward. Add an envelope on the slave’s pitch with a medium decay, and play. You will recognise the colour immediately. From there, it is all about dosage.
A few things to keep in mind
Sync is not only a lead recipe. A few useful markers:
- It also works on softer sounds: with subtle modulation, it adds movement without turning into a scream.
- The timbre depends heavily on the waveforms chosen for master and slave. The sawtooth is the most common, but a pulse or a triangle wave give very different colours.
- On a polyphonic synth, each voice has its own synced oscillator pair, so the setting behaves as expected, note by note. To understand how those voices are allocated, see our explainer on polyphony.
- Sync belongs to the family of techniques that enrich the spectrum by waveshaping; in that spirit, it is a cousin of FM synthesis, even if the mechanism is entirely different.
My take
Oscillator sync is one of those tools that instantly adds character, as long as you do not leave it static. Set raw, it impresses for three seconds then tires; modulated by an envelope or handed to the wheel, it becomes a real gesture, that rise in timbre you feel under your fingers. It is also a great way in to grasp that, on a synth, pitch and timbre are not always the same story. Patch an envelope to the slave, play two notes, and you will understand in ten seconds what a long paragraph struggles to describe.
