Notes · Music TechnologyUK · A-Levels

Synthesis

Synthesis generates sound electronically rather than recording it, and it is central to the technology-based composition. This topic builds a synthesiser from its parts: the oscillators and their waveforms and harmonic content, the filter with its cut-off and resonance, the ADSR envelope that shapes amplitude over time, and the LFOs and envelopes that modulate them. It then compares the main synthesis methods - subtractive, FM, additive and wavetable - and the performance controls (mono/poly, portamento, arpeggiator) that make a synth playable.

6 sections~22 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 2

T·0888 / 16

Exam profile

C2 · Build and program a synthesised sound from oscillators, filter, amplifier and modulationC3 · Relate waveform, harmonic content, filter and envelope settings to a heard timbreC2 · Compare synthesis methods and use performance controls to shape a sound

Operators:describeexplaindesigncompareidentify

basic level

The AS foundation is the waveforms, the filter, and the ADSR envelope.

higher level

The full A-level adds resonance, modulation routing, a comparison of synthesis methods and the performance controls.

Depth

Text

Contents · 6 sections

Synthesis

Oscillators and waveforms#

FoundationPearson 9MT0 - synthesis

The four basic waveforms

Fig. 1The four basic oscillator waveforms (stacked for clarity): the sine has only the fundamental, the triangle and square only odd harmonics, and the sawtooth all harmonics - richest and brightest.

The oscillator is the sound source of a synthesiser, generating a repeating waveform at a chosen pitch, and its waveform determines the raw harmonic content - the tone colour - before any further shaping. Four basic waveforms are the building blocks, and each has a distinctive harmonic make-up that you should be able to recognise by sound and by name. The waveform is the first timbral decision, exactly as microphone choice is in recording.

The sine wave is the simplest possible sound: a single frequency with no harmonics at all, giving a pure, smooth, hollow tone (think of a tuning fork or a gentle whistle). Because it has only the fundamental, it is the building block from which all other sounds can be added up, and it is used for sub-basses and pure tones. The triangle wave contains only the odd harmonics, and weakly, so it is a little brighter than a sine but still soft and mellow.

The square wave contains only the odd harmonics (fundamental, 3rd, 5th, 7th...) at greater strength, giving a hollow, woody, clarinet-like tone; narrowing it into a pulse wave adds the even harmonics too and thins the sound. The sawtooth wave contains all the harmonics - both odd and even - falling off gently in strength, making it the brightest and buzziest of the four, rich enough to be the starting point for strings, brass and classic synth leads because there is so much harmonic material for a filter to carve.

This link between waveform and harmonic content is the key idea: a bright sound needs a harmonically rich waveform (sawtooth), a hollow sound an odd-harmonic one (square, triangle), a pure sound a sine. In subtractive synthesis you deliberately start with a rich waveform and then remove harmonics with a filter, so the choice of oscillator waveform sets the palette of harmonics available to shape.

For the exam, be able to name the four waveforms, describe their harmonic content (sine: fundamental only; triangle: weak odd harmonics; square: odd harmonics; sawtooth: all harmonics) and their characteristic sound, and explain why a rich waveform such as the sawtooth is the usual starting point for subtractive synthesis. Recognising a waveform from its tone, and reasoning from harmonic content to timbre, is regularly tested.

Harmonic content of sawtooth and square waves

Fig. 2Harmonic spectra: the sawtooth contains every harmonic, falling as 1/n; the square contains only the odd harmonics. More harmonics means a brighter sound.

Worked example

From waveform to timbre

A student wants a bright, buzzy synth-string sound with lots of harmonic material to filter, and separately a pure sub-bass. Which waveform suits each and why?

The synth strings
Bright, buzzy strings need a harmonically rich source; the sawtooth contains all harmonics, so it gives the most material for a filter to shape.
The sub-bass
A pure, deep sub-bass needs no harmonics that could clutter the low end; the sine wave has only the fundamental, giving a clean tone.
Reason from content
The choice follows directly from harmonic content: rich (sawtooth) for bright and shapeable, pure (sine) for clean.
Next step
The sawtooth would then be filtered and enveloped to become the string sound, while the sine sub-bass needs little further shaping.

Result: Use a sawtooth for the bright, filterable synth strings and a sine for the pure sub-bass - the harmonic content dictates the choice.

Typical mistakes

Saying a square wave contains all harmonics - it contains only the odd harmonics; the sawtooth contains all of them.
Confusing brightness with loudness - a sawtooth is brighter than a sine because of its harmonics, not because it is louder.

Active revision

Identify which waveform you would choose as the starting point for (a) a pure sub-bass, (b) a hollow, clarinet-like tone, and (c) a bright synth-string sound, and justify each by its harmonic content.

Active recall

Recall the key points — then reveal.

Sources: Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification (Pearson Edexcel)

Filter, cut-off and resonance#

StandardPearson 9MT0 - synthesis

Low-pass filter response with and without resonance

Fig. 3A low-pass filter passes lows and rolls off highs above the cut-off. Adding resonance builds a peak at the cut-off, giving a vocal, emphasised character.

The filter is the heart of subtractive synthesis: it removes harmonics from the oscillator's rich raw sound to sculpt the timbre. The most common is the low-pass filter, which passes frequencies below a chosen cut-off frequency and progressively attenuates those above it. Lowering the cut-off removes the upper harmonics and darkens the sound; raising it lets them through and brightens it. Sweeping the cut-off is the classic synth gesture, opening and closing the tone.

The cut-off frequency is where the filter begins to act, and the slope (roll-off) - measured in decibels per octave, typically 12 or 24 dB/octave - sets how steeply it removes frequencies above it. A steeper slope makes a more dramatic distinction between the passed and removed regions. High-pass filters do the opposite (removing lows, keeping highs) and band-pass filters keep only a band around the cut-off; the low-pass is the workhorse for taming a bright oscillator.

Resonance (also called Q or emphasis) is a boost applied to the frequencies right at the cut-off point. As resonance is increased, a peak grows at the cut-off, emphasising those frequencies and giving the filter a vocal, whistling, or squelchy character; at extreme settings the filter can self-oscillate, producing a pure tone at the cut-off frequency. Resonance turns a simple tone control into an expressive, characterful shaper, and a resonant filter sweep is one of the most recognisable synth sounds.

Cut-off and resonance are usually the most-modulated controls on a synth. An envelope applied to the cut-off makes the tone evolve over each note (bright at the attack, darkening as it decays - like a plucked or struck sound); an LFO on the cut-off makes it sweep rhythmically (a wobble or a filter-sweep pad); a keyboard-tracking amount opens the filter more for higher notes so they stay bright. The filter is therefore not a static setting but a dynamic, played parameter.

For the exam, be able to describe the low-pass filter and its cut-off and slope, explain resonance as a peak at the cut-off (up to self-oscillation), and show how modulating the cut-off with an envelope or LFO shapes the sound over time. Reading a filter response graph - identifying the passband, the cut-off, the roll-off slope and a resonant peak - is a standard skill.

Worked example

A plucked, vocal filter sound

Design the filter behaviour for a synth sound that starts bright and darkens quickly over each note, with a slightly vocal, emphasised character. Describe the settings.

Start bright
Use a sawtooth into a low-pass filter, and apply an envelope to the cut-off so it opens high at the note's start, letting the harmonics through for a bright attack.
Darken over time
Set the envelope's decay so the cut-off falls quickly after the attack, removing the upper harmonics and darkening the sound - the plucked/struck behaviour.
Add resonance
Raise the resonance moderately so a peak sits at the moving cut-off, giving the sweep a vocal, emphasised character as it closes.
Result
Each note opens bright and closes dark with a resonant, vocal quality, all created by an envelope modulating a resonant low-pass cut-off.

Result: A sawtooth through a resonant low-pass, with a fast-decaying envelope on the cut-off, gives a bright-to-dark plucked sound with a vocal filter character.

Typical mistakes

Confusing cut-off (which frequencies are removed) with resonance (a boost at the cut-off point).
Thinking a low-pass filter removes the low frequencies - it passes the lows and removes the highs above the cut-off.

Active revision

Describe the filter cut-off and resonance settings, and any modulation, you would use to make a bright sound that darkens over each note with a vocal-sounding filter character.

Active recall

Recall the key points — then reveal.

Sources: Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification (Pearson Edexcel)

The ADSR envelope#

StandardPearson 9MT0 - synthesis

The ADSR amplitude envelope

Fig. 4The ADSR envelope: attack rises to the peak, decay falls to the sustain level, sustain is held while the key is down, and release falls to silence after key-off.

An envelope shapes how a parameter changes over the lifetime of a single note, and the most common is the ADSR envelope, which shapes the amplitude (loudness) from the moment a key is pressed to after it is released. Its four stages - attack, decay, sustain and release - describe the contour that distinguishes, say, a percussive pluck from a slow-swelling pad, and reading or designing an ADSR is one of the defining skills of synthesis.

The four stages work as follows. Attack is the time taken to rise from silence to the peak level when the key is pressed - short for a percussive, instant onset, long for a slow swell. Decay is the time to fall from the peak to the sustain level. Sustain is a level (not a time): the level held for as long as the key stays down. Release is the time to fall from the sustain level back to silence after the key is let go. Attack, decay and release are times; sustain is a level.

Different sounds have characteristic envelopes. A plucked or struck sound (a guitar, a mallet) has an instant attack, a fairly fast decay, a low or zero sustain and a short release - it starts immediately, dies away, and does not hold. A bowed or blown pad (strings, a soft synth pad) has a slow attack (the swell), a high sustain (it holds while the key is down) and a slow release (it fades gently after release). Recognising a sound's envelope from its behaviour, and vice versa, is regularly tested.

Envelopes are not limited to amplitude. The same ADSR can modulate the filter cut-off (as in the previous section), the pitch, or almost any parameter, so a synth typically has several envelopes. A filter envelope with a fast decay and low sustain gives the bright-to-dark pluck; an amplitude envelope with a slow attack gives the pad swell; combining them shapes both the loudness and the tone contour of each note independently.

For the exam, be able to define the four ADSR stages (attack, decay and release as times; sustain as a level), sketch or read an envelope, and design an appropriate envelope for a described sound - a slow pad versus a plucked sound being the classic contrast. Being explicit that sustain is a level while the others are times is a frequent discriminator between strong and weak answers.

Worked example

Envelopes for a pad and a pluck

Design amplitude ADSR settings for (a) a slow, evolving pad and (b) a sharp plucked sound, and justify each stage.

Pad - attack and decay
Long attack (a slow swell up to peak, say around a second) and a moderate decay; the sound must not begin abruptly.
Pad - sustain and release
High sustain level (it holds at near-full level while the key is down) and a long release (it fades gently after key-off), so the pad lingers.
Pluck - attack and decay
Instant (near-zero) attack so it starts immediately, and a fast decay so it dies away quickly like a plucked string.
Pluck - sustain and release
Low or zero sustain (it does not hold) and a short release, so once the note has decayed there is nothing left to sustain.

Result: Pad: long attack, high sustain, long release; pluck: instant attack, fast decay, low sustain, short release - the envelope defines the two characters.

Typical mistakes

Calling sustain a time - it is the level held while the key is down; attack, decay and release are the times.
Giving a pad an instant attack or a pluck a long sustain, contradicting the sound being described.

Active revision

Design ADSR settings (short/long, high/low) for (a) a slow, sustained pad and (b) a sharp, plucked sound, and explain each stage's value.

Active recall

Recall the key points — then reveal.

Sources: Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification (Pearson Edexcel)

Modulation: LFOs and envelopes#

StandardPearson 9MT0 - synthesis

The synth voice and its modulation

Fig. 5The classic synth voice: oscillator to filter to amplifier, with an amplitude envelope on the amplifier, a filter envelope on the cut-off, and an LFO adding cyclic movement.

Modulation is the automatic movement of one parameter by another, and it is what brings a synth sound to life - static settings sound lifeless, while modulated ones evolve and breathe. The two main modulation sources are the envelope (a one-shot contour per note, as in the previous section) and the low-frequency oscillator (LFO), a slow oscillator - usually below 20 Hz, so below the audible range - whose output cyclically moves a target parameter up and down.

The LFO's effect depends on what it modulates. An LFO on pitch produces vibrato (a gentle wobble in pitch); on amplitude it produces tremolo (a pulsing in loudness); on the filter cut-off it produces a rhythmic filter sweep or wobble; on pulse width it produces a shimmering, thickening movement. The LFO has its own waveform (a sine for smooth movement, a square for an on/off jump, a sawtooth for a ramp), a rate (how fast it cycles, often syncable to the tempo) and a depth (how far it moves the target).

The classic synth voice architecture ties the sound source, shaper and modulators together: the oscillator generates the waveform, the filter (a voltage-controlled filter, VCF) shapes its harmonics, and the amplifier (a voltage-controlled amplifier, VCA) sets its level - and modulation sources feed into these. Typically an amplitude envelope controls the VCA (shaping loudness over the note), a filter envelope controls the VCF cut-off (shaping tone over the note), and one or more LFOs add cyclic movement to pitch, filter or amplitude.

Modulation routing - deciding which source controls which destination, and by how much - is where sound design happens. A modest LFO-to-pitch adds warmth; a deep envelope-to-filter gives a dramatic sweep; an LFO-to-filter synced to the tempo gives a rhythmic pulse. Many synths have a flexible modulation matrix allowing almost any source to reach any destination, so the same few oscillators and filters can produce a vast range of evolving sounds.

For the exam, be able to describe the LFO (a sub-audio oscillator producing vibrato, tremolo or filter sweeps, with waveform, rate and depth), distinguish it from an envelope (cyclic movement versus a one-shot per-note contour), and lay out the oscillator-filter-amplifier voice with its envelope and LFO modulation. Being able to say which modulation source and destination produce a described movement is a common question.

Worked example

Routing modulation for movement

You want a synth lead with a gentle vibrato and a slow, tempo-synced filter wobble. Describe the modulation routing and the controls you would set.

Vibrato
Route an LFO (sine waveform) to the oscillator pitch with a small depth for a gentle wobble; set a moderate rate (around 5-6 Hz) for a natural vibrato.
Filter wobble
Route a second LFO to the filter cut-off; sync its rate to the tempo (say a quarter note) so the sweep pulses in time, and set the depth for the amount of sweep.
Shape the note
Keep an amplitude envelope on the amplifier so notes start and stop cleanly, and a filter envelope for the per-note tone contour underneath the LFO movement.
Balance
Set the two LFO depths so the vibrato is subtle and the filter wobble is the prominent, rhythmic movement.

Result: An LFO on pitch (small depth, ~5 Hz) gives vibrato; a tempo-synced LFO on the filter cut-off gives the rhythmic wobble - each set by its rate and depth.

Typical mistakes

Confusing an LFO (cyclic, continuous movement) with an envelope (a one-shot contour triggered per note).
Mixing up vibrato (LFO on pitch) and tremolo (LFO on amplitude).

Active revision

State which modulation source and destination produce (a) vibrato, (b) tremolo and (c) a tempo-synced filter wobble, and name the control that sets how strong each effect is.

Active recall

Recall the key points — then reveal.

Sources: Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification (Pearson Edexcel)

Synthesis methods#

AdvancedPearson 9MT0 - synthesis

Synthesis methods

Fig. 6The main synthesis methods and how each generates sound - the palette from which a sound designer chooses.

There are several fundamentally different ways to generate a synthesised sound, and knowing the main methods lets you recognise and choose the right approach. Subtractive synthesis - the method built up in the previous sections - starts with a harmonically rich waveform and removes harmonics with a filter. It is intuitive and the basis of most analogue and virtual-analogue synths, ideal for warm basses, leads and pads where a filter sweep is the defining gesture.

Additive synthesis works the opposite way: it builds a sound by adding together many sine waves of different frequencies and amplitudes (recalling that any sound is a sum of harmonics). By controlling the level and evolution of each partial independently, additive synthesis can create precise, evolving timbres, though it needs many oscillators and careful control, which historically made it demanding. It is the theoretical inverse of subtractive synthesis - building up from pure tones rather than carving down from a rich one.

FM (frequency modulation) synthesis generates complex, harmonically rich timbres by using one oscillator (the modulator) to modulate the frequency of another (the carrier) at audio rate. Small changes in the modulator's frequency and depth produce dramatically different, often metallic, bell-like or electric-piano timbres that are hard to make subtractively. FM is powerful for bright, clangorous and digital-sounding tones, but its controls are less intuitive because the relationship between settings and result is complex.

Wavetable synthesis stores a series of single-cycle waveforms in a table and scans through them over time, so the oscillator's waveform itself evolves and morphs. It excels at moving, digital, evolving sounds and is central to much modern electronic music. Other methods exist (granular, sample-based, physical modelling), but subtractive, additive, FM and wavetable are the core families to compare, each suited to different timbres.

For the exam, be able to compare the methods: subtractive (rich waveform, filtered down - warm, intuitive); additive (sum of sines, built up - precise, complex); FM (one oscillator modulates another - bright, metallic, bell-like); wavetable (scanning evolving waveforms - moving, modern). Choosing a method for a target timbre, and contrasting subtractive with FM in particular, are standard questions.

Subtractive versus FM synthesis

Fig. 7Subtractive filters a rich waveform (warm, intuitive); FM modulates one oscillator with another (bright, metallic). Both generate sound electronically for composition.

Worked example

Choosing a method for a timbre

Choose the best synthesis method for (a) a warm, filter-swept bass and (b) a bright, bell-like electric-piano tone, and justify each.

The bass
A warm bass whose defining gesture is a filter sweep is a natural fit for subtractive synthesis: a sawtooth through a resonant low-pass, swept by an envelope or LFO.
The electric piano
Bright, metallic, bell-like electric-piano tones are the signature of FM synthesis, which readily produces clangorous, inharmonic-tinged timbres that are hard to make by filtering.
Why not swap them
Subtractive struggles to make convincing metallic bell tones (there is no filter setting that adds those inharmonic partials), while FM is unintuitive for a simple warm, swept bass.
Conclusion
Match method to timbre: subtractive for the warm swept bass, FM for the bright bell/electric-piano.

Result: Use subtractive synthesis for the warm, filter-swept bass and FM for the bright, bell-like electric piano - each method suits the timbre it makes best.

Typical mistakes

Describing FM as a filter method - FM uses one oscillator to modulate another's frequency; it has no filter at its core.
Confusing additive (adding sines) with subtractive (filtering a rich waveform) - they are opposites.

Active revision

Explain which synthesis method you would choose for (a) a warm, filter-swept bass and (b) a bright, metallic electric-piano/bell tone, and why.

Active recall

Recall the key points — then reveal.

Sources: Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification (Pearson Edexcel)

Performance controls: mono/poly, portamento, arpeggiator#

AdvancedPearson 9MT0 - synthesis

Synth performance controls

Fig. 8The performance controls that determine how a synth plays: voice mode, glide, unison and the arpeggiator - each shaping the musical result as much as the sound design.

Beyond generating and shaping a sound, a synthesiser has performance controls that determine how it plays, and these strongly affect the musical result. The most basic is the voice mode: monophonic synths play only one note at a time (ideal for basses and lead lines, and for the sliding, expressive playing that single-voice instruments allow), while polyphonic synths play several notes at once (needed for chords and pads). Choosing mono or poly is a decision about the part's role.

Portamento (or glide) makes the pitch slide smoothly from one note to the next rather than jumping instantly, and its time control sets how long the slide takes. It is most associated with monophonic lead and bass playing, where the sliding pitch is expressive and characterful (the classic synth-lead glide). On a polyphonic synth glide behaves less predictably, which is another reason expressive glide lines are usually played monophonically.

Unison stacks several oscillators (or voices) on each note, slightly detuned from one another, to thicken and fatten the sound - the big, wide 'supersaw' lead being the extreme case. Unison trades polyphony for thickness (all the voices go into one note), so it is often combined with monophonic mode for a huge single-line lead. The detune amount sets how wide and chorused the stacked sound becomes.

The arpeggiator automatically plays the notes of a held chord one after another in a pattern (up, down, up-down, random) and in time with the tempo, turning a held chord into a rhythmic sequence. Its controls - the pattern, the rate (note value), the octave range and whether it latches - make it a compositional tool in its own right, generating moving, hypnotic lines from simple held chords, and it is a staple of electronic music.

For the exam, be able to describe these performance controls and their musical uses: mono versus poly (single line/bass versus chords/pads), portamento/glide (expressive pitch slides, especially mono), unison (detuned stacking for thickness) and the arpeggiator (held chords turned into tempo-synced patterns). Knowing which control produces a described playing behaviour, and how it suits a part, is regularly assessed.

Worked example

Setting up a fat lead line

Set up the performance controls for a big, fat, expressive monophonic synth-lead line with pitch slides.

Voice mode
Set the synth to monophonic, since it is a single-line lead and mono enables predictable glide.
Glide
Turn on portamento with a short-to-medium glide time so notes slide expressively into one another.
Unison
Engage unison with several detuned voices to fatten and widen the single line into a big lead sound.
Leave the arpeggiator off
The arpeggiator is for turning chords into patterns; for a played lead line it is left off.

Result: Monophonic mode, portamento glide and detuned unison give a big, fat, expressively sliding lead line; the arpeggiator stays off.

Typical mistakes

Trying to play a chord on a monophonic synth - it sounds only one note at a time.
Confusing unison (detuned oscillators stacked on one note for thickness) with polyphony (separate notes for chords).

Active revision

Describe the performance-control settings (voice mode, glide, unison, arpeggiator) you would use for a big, fat monophonic synth-lead line with expressive slides.

Active recall

Recall the key points — then reveal.

Sources: Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification (Pearson Edexcel) · Ofqual - GCE AS and A level qualifications (subject-level conditions and requirements) (Ofqual)

References & sources

Sources

Pearson Edexcel

Pearson Edexcel Level 3 Advanced GCE in Music Technology (9MT0) Specification

Ofqual

Ofqual - GCE AS and A level qualifications (subject-level conditions and requirements)

Synthesis

6 sections~22 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 2

Synthesis

Key points

From waveform to timbre

Key points

A plucked, vocal filter sound

Key points

Envelopes for a pad and a pluck

Key points

Routing modulation for movement

Key points

Choosing a method for a timbre

Key points

Setting up a fat lead line

Sources

Synthesis

Key points

From waveform to timbre

Key points

A plucked, vocal filter sound

Key points

Envelopes for a pad and a pluck

Key points

Routing modulation for movement

Key points

Choosing a method for a timbre

Key points

Setting up a fat lead line

Sources