Subtractive synthesis is the most widely used method of sound design in electronic music. The idea is simple: start with a waveform full of harmonics, then subtract - filter out - the parts you do not want. Nearly every classic analogue synth, and a vast number of software synths, works this way.
Before software instruments, a musician might save for months to buy one synth, then spend years exploring every control until they had replaced its factory sounds with their own. Today it is easy to collect dozens of plug-ins and only ever use their presets. Learning how one subtractive synth works, in depth, will make you more creative on all of them.
The signal path
A classic subtractive synth has three main stages:
- Oscillators - generate the raw sound.
- Filter - shapes its tone by removing frequencies.
- Amplifier - controls its loudness over time.
Envelopes and LFOs modulate each stage. This chapter covers the first two; the next chapter covers the amplifier and modulation.
Oscillators
An oscillator generates a repeating waveform at a set pitch. Most synths offer sine, triangle, square/pulse and sawtooth, and many offer more.
- Octave and coarse tuning - shift pitch in octaves or semitones.
- Fine tuning - small shifts in cents (hundredths of a semitone).
- Pulse width - on a pulse wave, changes the ratio of high to low in each cycle. A 50% pulse is a square wave; narrower pulses sound thinner and more nasal. Modulating pulse width with an LFO gives a rich, moving, chorus-like sound.
Multiple oscillators
Most synths have two or three oscillators that can be mixed together.
- Detune - set two oscillators a few cents apart and they beat against each other, creating a thick, moving sound. The classic fat analogue sound.
- Intervals - tune one oscillator an octave or a fifth above another for a fuller, organ-like tone.
- Different waveforms - a sawtooth for brightness with a square an octave below for weight.
- Sub oscillator - an extra oscillator one or two octaves below the main one, for bass weight.
Unison
Unison stacks multiple copies of each oscillator, detuned and often spread across the stereo field. It creates the huge supersaw sound of trance and EDM leads and the width of modern pads. More voices and more detune mean a bigger, wider, less focused sound.
Noise
Most synths include a noise generator. Mixed in, it adds breath to pads, air to leads, and the body of snares, hi-hats and sound effects. White noise has equal energy across all frequencies; pink noise has more low-frequency energy and sounds softer.
The mixer
The mixer section sets the level of each oscillator and the noise. Pushing oscillator levels hard into the filter can add saturation on many analogue and modelled synths - part of the character of the sound.
Filters
The filter removes frequencies, and it is the most important tone control on a subtractive synth.
Filter types
- Low-pass - lets lows through and cuts highs. The most common type. Closing it makes a sound darker and softer.
- High-pass - lets highs through and cuts lows. Thins a sound out.
- Band-pass - lets a band of frequencies through and cuts both sides. Nasal, telephone-like.
- Notch (band-reject) - cuts a narrow band and passes the rest. Subtle, phasey.
Cutoff
The cutoff frequency is where the filter starts working. On a low-pass filter, frequencies above the cutoff are reduced. Sweeping the cutoff is one of the most expressive moves in electronic music - the classic filter sweep.
Resonance
Resonance (or Q) boosts frequencies right around the cutoff. A little adds character and emphasis; a lot creates a whistling peak. At high settings some filters self-oscillate, producing a sine wave of their own. The squelch of acid house is a resonant low-pass filter being swept on a Roland TB-303.
Slope
The slope describes how steeply the filter cuts beyond the cutoff, in decibels per octave:
- 12 dB/oct (2-pole) - gentler, more open.
- 24 dB/oct (4-pole) - steeper, more dramatic - the classic Moog ladder sound.
Key tracking
Key tracking makes the cutoff follow the notes you play, so higher notes open the filter further. Without it, high notes can sound dull and low notes too bright.
Do this now
- Initialise a synth patch - most have an "init" preset - so you start from a single raw oscillator.
- Switch through every waveform and listen to each with the filter fully open.
- Add a second oscillator, detune it by 7-10 cents, and listen to the thickening.
- Sweep the low-pass cutoff slowly from closed to open; then turn up the resonance and sweep again.
- Compare 12 dB and 24 dB slopes on the same sound.