Introduction to Sound Synthesis
An interactive guide to sound synthesis, from the oscillator and filter to FM, sampling, effects and mastering
This guide explains how synthesisers make sound, starting from the basics and assuming no prior knowledge. The ideas apply to synthesisers of every kind, hardware and software, keyboard and modular.
The first parts cover what a sound is made of: signals, waveforms and filters. The next cover pitch, timing, and how a note is shaped over its length. Those come together in the classic subtractive voice, a layout that many synthesisers use. The later parts change how the parts of that voice are connected, introduce other methods such as FM, wavetables, sampling and physical modelling, and then cover sequencers, ways of routing modulation, effects, and the mixing and mastering that turn a sound into a finished recording. Each chapter has controls to play with, so you can hear an idea while you read about it, and the glossary defines the terms the guide uses.
The guide grew out of an introductory workshop at Synth Library Portland, and the workshop's slide deck is still available if you want the material in one sitting.
Signals
A signal is a level that changes over time: a voltage in analogue hardware, a stream of numbers in software. The same kind of signal can carry a note or move a control.
- Chapter 1
What a signal is
A signal is a level that changes over time: a voltage in analogue hardware and a stream of numbers in software. It is described by its amplitude, its frequency and its waveshape.
- Chapter 2
Control signals and audio signals
A control signal and an audio signal are the same kind of signal running at different speeds. Whether a signal is heard or moves a setting depends on where it is connected.
Timbre
The tone of a steady sound depends mostly on its harmonics: which ones the oscillator produces, and how much of each the filter passes.
- Chapter 3
Waveforms
The four standard waveforms (sine, sawtooth, square and triangle) contain different sets of harmonics, which is why they sound different at the same pitch.
- Chapter 4
Harmonics and overtones
A pitched sound can be described as a set of sine waves called partials. How quickly their levels fall from the lowest partial to the highest sets the brightness. Their phases change the shape of the waveform much more than the sound, and partials that are not whole-number multiples of the lowest often sound metallic.
- Chapter 5
Filters
A filter reduces the harmonics above or below a chosen frequency. Cutoff sets that frequency, resonance boosts the harmonics around it, and slope sets how steeply the level falls beyond it.
Pitch
The pitch of a note is set by the frequency its oscillator runs at. A keyboard, a sequencer and a random source are three common ways of setting that frequency.
- Chapter 6
Pitch and frequency
Pitch follows frequency, and the ear hears frequency by ratio, so doubling a frequency raises the pitch by an octave wherever it starts. Semitones and cents measure those ratios, and note names label particular frequencies.
- Chapter 7
Noise
Noise is a random signal with no single pitch. White, pink and brown noise spread their energy differently across the spectrum, and filtering them produces sounds like wind and surf.
- Chapter 8
One volt per octave
One volt per octave is the pitch standard most analogue synthesisers and modular systems use, so that equipment from different makers can play in tune together. Each volt added to a pitch input raises the pitch by one octave.
- Chapter 9
Where pitch comes from
A keyboard, a sequencer and a sample and hold each produce a control signal that sets an oscillator's pitch. Connecting all three to the same oscillator lets you compare them directly.
Time and dynamics
A clock sets when things happen and a gate sets how long each note lasts. An envelope gives each note a shape, and a VCA makes that shape audible by controlling the oscillator's level.
- Chapter 10
Clocks
A clock is a steady stream of pulses, and other parts of a synthesiser act each time a pulse arrives. Its tempo sets how often the pulses come, swing delays every second one, and a divider turns one clock into several related ones.
- Chapter 11
Gates and triggers
A gate is a signal that goes high when a note starts and stays high until the note ends. Used on its own to control the level, it switches the sound fully on and off, with a click at each end.
- Chapter 12
Envelopes
An envelope turns each gate into a control signal that rises and falls in four stages: attack, decay, sustain and release.
- Chapter 13
The VCA
A VCA sets how much of a signal passes through according to a control signal. Driven by an envelope, it turns an oscillator that never stops into separate notes.
Modulation
Modulation is one signal changing a setting somewhere else in the synthesiser. An LFO sent to pitch gives vibrato, sent to a filter's cutoff it gives a repeating sweep, and sent to an amplifier it gives tremolo.
- Chapter 14
The LFO
An LFO is an oscillator that runs below the audio range and moves a setting up and down. Sent to pitch, cutoff or level, it produces vibrato, a repeating filter sweep or tremolo.
- Chapter 15
Envelopes as modulation
An envelope can be sent to the same destinations as an LFO. Instead of repeating, it moves the setting once for each note, which is how a note can start bright and then darken.
The classic voice
The parts covered so far combine into the layout the Minimoog used in 1970: an oscillator, a filter and an amplifier, with envelopes and an LFO controlling them. Most subtractive synthesisers since have used the same layout.
- Chapter 16
The classic voice
The classic subtractive voice is an oscillator, a filter and an amplifier, with two envelopes and an LFO controlling them. The Minimoog used this layout in 1970, and most subtractive synthesisers since have kept it.
- Chapter 17
Build it in seven steps
The classic voice is built one part at a time, and you can hear the sound after each step. At step four an envelope is added and the sound does not change, because the envelope is not yet connected to anything.
Sequencers
A sequencer stores notes and plays them back in time with a clock. Step sequencers, trigger sequencers and piano rolls store the same kind of information in different layouts.
- Chapter 18
Step sequencers
A step sequencer stores a pitch and a gate for each step and moves to the next step on every clock pulse. Length, gate length, ties, slides and ratchets change how each step plays.
- Chapter 19
Trigger sequencers
A trigger sequencer stores only whether each step fires, in a grid with one row per sound. It is the usual way of programming drums.
- Chapter 20
Piano rolls
A piano roll stores notes with any start time and length on a grid of pitch against time, rather than one note per fixed step.
Shaping control signals
Control signals can be processed on their way to a destination. A slew limiter smooths them, a switch chooses between them, and logic combines gates and triggers into new rhythms.
- Chapter 21
Slew
A slew limiter limits how fast a signal can change, so steps become ramps. It is how glide is made, and it turns a stepped random signal into a smooth one.
- Chapter 22
Switches
A sequential switch passes one of several signals and moves to the next on each trigger. It can route one source to several destinations or choose between several sources.
- Chapter 23
Hocketing
Hocketing shares a single line of notes between several voices, so each voice plays only some of the notes. A switch or a sequencer's gate outputs can do the sharing.
- Chapter 24
Logic
Logic modules combine gates and triggers. AND, OR and XOR produce new rhythms from existing ones, and a flip-flop divides a clock by two.
Routing modulation
Larger patches send several modulation sources to several destinations, change the amount of one modulation with another, and gather many settings under a single control.
- Chapter 25
The modulation matrix
A modulation matrix connects any modulation source to any destination by an amount. It does the job of patch cables on a synthesiser with fixed connections.
- Chapter 26
Modulating the modulation
The amount, rate or shape of one modulation can itself be modulated by another source, so that vibrato fades in or an LFO speeds up over a note.
- Chapter 27
Macro controls
A macro control moves several settings at once, each by its own amount, so that turning one knob changes several aspects of the sound together.
Breaking it apart
The same building blocks can be connected in ways the classic voice does not use: an LFO fast enough to change the timbre, an envelope driving pitch, a sequencer that clocks itself. Each produces sounds the standard layout does not.
- Chapter 28
Modulation at audio rate
When an LFO runs faster than about 20 Hz, its modulation is no longer heard as movement. It adds new frequencies to the sound and changes its timbre.
- Chapter 29
An envelope as a pitch source
An envelope sent to an oscillator's pitch makes each note start high and fall. A large, fast fall of this kind is how most synthesised kick drums and toms are made.
- Chapter 30
Patches that drive themselves
A patch can send a module's output back into its own input, as in a sequencer that clocks itself. A patch built this way can keep changing without any outside input.
Shaping timbre further
Besides lowpass and highpass filters, a comb filter, a ring modulator and a wavefolder also change a sound's harmonics, each in a different way.
- Chapter 31
Comb filters
A comb filter mixes a signal with a very short delayed copy of itself. Its response has evenly spaced peaks and notches, and with feedback it rings at a pitch set by the delay.
- Chapter 32
Ring modulation
A ring modulator multiplies two signals. The output contains their sum and difference frequencies and neither original, which gives metallic and bell-like tones.
- Chapter 33
Wavefolding
A wavefolder reflects the parts of a waveform that pass a threshold back on themselves. Raising the input level folds the waveform more times and adds more high harmonics, which changes the timbre in a different way from a filter sweep.
Beyond subtractive
Subtractive synthesis is one method among several. Additive, FM, phase distortion, wavetable, vector, sampling, granular and physical modelling synthesis each make sound in a different way.
- Chapter 34
Additive
Additive synthesis builds a sound from sine waves, setting the frequency, level and envelope of each one separately.
- Chapter 35
FM
FM synthesis changes one oscillator's frequency with another at audio rate. The ratio between their frequencies and the depth of the modulation, called the index, set the timbre.
- Chapter 36
Phase distortion
Phase distortion reads a cosine wave at an uneven rate within each cycle, which bends it into brighter shapes. An envelope on the amount changes the timbre over each note, as on Casio's CZ synthesisers.
- Chapter 37
Wavetables
A wavetable is a set of single-cycle waveforms stored in order. A position control moves through the set and changes the timbre as it goes.
- Chapter 38
Vector
Vector synthesis mixes four sound sources with one two-dimensional control, usually a joystick, and an envelope can move the mix over each note.
- Chapter 39
Sampling
A sampler plays a recording back as an instrument. Changing the playback speed changes the pitch and the length together, and loops let a short recording sustain.
- Chapter 40
Granular
Granular synthesis plays many short sounds called grains, each with its own envelope. The grains can be cut from a recording or generated by an oscillator.
- Chapter 41
Physical modelling
Physical modelling recreates the sound of a string, a tube or a drum head by calculating how the object vibrates.
Effects
Effects process a finished sound. Most are built from delays: a single repeat is an echo, many dense repeats are reverb, and a short delay that moves gives chorus, flanging and phasing.
- Chapter 42
Delay
A delay plays a signal back after a set time. With feedback each repeat is fed back in, giving a series of echoes that fade away.
- Chapter 43
Reverb
Reverb is the dense pattern of reflections a room adds to a sound. Digital reverbs build it from many delays or from a recording of a real space.
- Chapter 44
Chorus
Chorus mixes a signal with a copy delayed by a few tens of milliseconds and slowly moved by an LFO, which sounds like several instruments playing together.
- Chapter 45
Flanger
A flanger mixes a signal with a copy delayed by a few milliseconds and swept by an LFO. The moving comb filter that results gives a sweeping, jet-like sound.
- Chapter 46
Phaser
A phaser passes a signal through a chain of allpass filters and mixes it with the original. An LFO moves the resulting notches up and down the spectrum.
- Chapter 47
Distortion
Distortion changes the shape of a waveform, usually by clipping it, and this adds harmonics. Soft clipping rounds the peaks and hard clipping cuts them flat.
Mixing and dynamics
An envelope follower turns a sound's level into a control signal, and a compressor uses one to control level automatically. Mixing and mastering combine sounds into a finished piece.
- Chapter 48
Envelope followers
An envelope follower turns the level of an audio signal into a control signal that rises and falls with it, so that one sound can control another.
- Chapter 49
Compression
A compressor turns a signal down when it gets louder than a threshold, by an amount set by the ratio. Attack and release set how quickly it reacts.
- Chapter 50
Mixing
Mixing sets the level, pan and tone of each sound so that they can be heard together. Gain staging keeps the signal at each stage high enough to stay above the noise and low enough not to clip.
- Chapter 51
Mastering
Mastering is the last stage before release. It adjusts the overall tone and loudness of a finished mix for the formats and places it will be played.
Before you start
Sound plays only when you press a button. Headphones or full-range speakers help, since several chapters deal with low frequencies that laptop speakers barely reproduce. If you want the acoustics behind harmonics before you start, the harmonic series guide covers them. The glossary lists the terms the guide uses, and each entry links to the section that explains it.