Oscillators: where every sound starts

Every patch begins with something that makes a sound, and almost always that's an oscillator. It's the simplest module in the rack, and it holds most of the ideas the rest of synthesis builds on. By the end of this guide you'll know why a note has a pitch, why some waves sound bright and others soft, and why synth players stack two oscillators slightly out of tune.

New to patching? Take the 5-minute guided build first. It walks you through adding modules, patching cables, and unplugging and rerouting them, which every Try it box here assumes you can do.

Each Try it box opens the synth with its steps pinned on the rack, so you can read and listen at the same time. When you are done, Back to the guide (or your browser’s Back button) brings you back to the same spot.

A sound is something wobbling

When your speaker plays a sound, its cone pushes the air forwards and backwards. If that push repeats at a steady rate, your ear hears a pitch. Repeat it 110 times a second and you hear a low A. Repeat it 440 times a second and you hear the A a tuning fork plays. That rate is the frequency, measured in hertz (Hz): repeats per second.

An oscillator is just a thing that repeats a shape, over and over, forever. That's the whole job. In Kables the classic one is the VCO, short for voltage-controlled oscillator.

That last part surprises everyone. An oscillator doesn't play notes; it just runs. Deciding when a sound starts and stops is a separate job, done by a VCA and an envelope. That split is the heart of modular: each module does one thing, and you decide how they work together. They get their own guide, next.

The shape is the colour

Play two oscillators at exactly the same pitch, one making a smooth curve and one making a jagged ramp, and they sound completely different. Why, if the pitch is the same?

Because any repeating shape is secretly a stack of simpler waves. There's the main one, at the pitch you hear (the fundamental), plus quieter ones at 2×, 3×, 4× that frequency and so on, called harmonics. You don't hear them as separate notes; you hear them as the colour of the sound. The shape of the wave decides which harmonics are there and how loud they are.

The VCO has the four classic shapes:

This is why the sawtooth is everyone's favourite starting point. It has the most harmonics, so it gives a filter the most to carve away. Start bright, then shape it darker. That's called subtractive synthesis, and it's how most classic synth sounds are made (more in the filters guide).

See the shapes

Hearing the difference is one thing; seeing it makes it click. The Scope module draws whatever you patch into it, so you can watch a wave's shape while you listen to it.

The scope shows a wave over time. To see the harmonics themselves, use the Spectrum module, which shows a sound over frequency: bass on the left, treble on the right.

Pitch works in octaves

Here's something odd about ears: we don't hear pitch in hertz, we hear it in ratios. Going from 110 Hz to 220 Hz sounds like the same step up as going from 440 Hz to 880 Hz, even though the second jump is four times as many hertz. Each doubling sounds like "the same note, higher": an octave.

So modular synths control pitch per octave. The VCO's V/OCT input works like this: add 1.0 and the pitch goes up one octave (the frequency doubles). Add 1/12 (about 0.083) and it goes up one semitone, the step between neighbouring keys on a piano. That's why a keyboard can play a VCO just by sending numbers down a cable.

The knobs follow the same idea:

• Tune sets the starting pitch. It defaults to middle C (261.6 Hz), so keyboards play in tune.
• Oct jumps by whole octaves.
• Fine nudges by cents, hundredths of a semitone. Tiny, but it matters more than you'd think (next section).

Two oscillators are bigger than one

Remember the scope figure turning when you nudged Fine? When two oscillators are almost the same pitch, their waves drift in and out of step. When they're in step they add up and get louder; when they're out of step they partly cancel. The result is a slow, shimmering swell called beating, and it's the secret behind the classic "fat" synth sound.

A few cents of detune between two or three sawtooth oscillators is how big analog leads, pads and basses are made.

Beyond the four shapes

Once the basics feel familiar, two modules take the idea further.

The Wavetable oscillator can morph smoothly between four shapes, and its Width control squeezes one half of the wave and stretches the other. Patch a slow LFO into either, and the shape changes while the note plays, so the sound evolves instead of sitting still (lesson 17).

Noise is the opposite of an oscillator. It never repeats, so it has no pitch at all, just every frequency at once. That makes it perfect for things without a note: wind, surf, the hiss of a hi-hat (lesson 11).

Some sources copy real objects instead of drawing waves. The Plucked String fires a puff of noise into a loop one pitch-period long, and it rings like a guitar string (lesson 18). The Resonator rings at the oddly spaced frequencies of bells, bars and gongs when something strikes it (lesson 22). Different methods, same idea: something vibrates, and its pattern of harmonics is what you hear.

What to remember

• An oscillator repeats a shape forever. How fast it repeats is the pitch; the shape is the colour.
• Brighter waves have more harmonics. Sine has none, saw has all of them, which is why saws are great to filter.
• Pitch works in octaves: +1 on V/OCT is one octave up.
• Oscillators never stop on their own. Making notes start and end is the next job, for an envelope and a VCA.
• Two oscillators slightly out of tune beat against each other and sound bigger than either one alone.