How does a subtractive synthesiser create and shape a sound from an oscillator, filter, amplifier and envelope?
Subtractive synthesis: oscillators and waveforms, the voltage-controlled signal path (VCO, VCF, VCA), the filter and resonance, the ADSR envelope, the LFO and modulation, and how these combine to design a synth sound.
A focused answer to the Edexcel 9MT0 subtractive synthesis content, covering oscillators and waveforms, the VCO, VCF and VCA signal path, the filter and resonance, the ADSR envelope, the LFO and sound design.
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What this dot point is asking
Edexcel wants you to explain how a subtractive synthesiser builds a sound: an oscillator generates a harmonically rich waveform, a filter removes harmonics to shape the tone, an amplifier sets the level, and an envelope and LFO control how the sound changes over time. You must define the ADSR stages precisely and describe the voltage-controlled signal path. Synthesis is central to Component 2 and is examined in the listening papers.
The answer
The oscillator and waveforms
The choice of waveform sets the raw harmonic content: a sawtooth (all harmonics) is bright and full, a square (odd harmonics) is hollow, a pulse can be made thin or nasal by adjusting its width.
The voltage-controlled signal path
The names come from analogue synths where each stage was controlled by a voltage, but the same structure underlies software (virtual analogue) synths.
The filter and resonance
The ADSR envelope and the LFO
Examples in context
When a bass synth has a punchy filter sweep on each note, an envelope is moving the filter cutoff. When a lead has a gentle wobble, an LFO is modulating its pitch or filter. When a pad swells in slowly, a slow attack on the amplitude envelope is shaping it. Subtractive synthesis is the most common synthesis method and the foundation of most electronic sounds you will design for Component 2.
Try this
Q1. Why does subtractive synthesis start from a harmonically rich waveform? [2 marks]
- Cue. Because it works by filtering (subtracting) harmonics, so it needs harmonics to remove.
Q2. State what each letter of ADSR stands for, noting which is a level. [2 marks]
- Cue. Attack, decay, sustain, release; sustain is a level, the others are times.
Q3. What does the filter cutoff control in a subtractive synth? [1 mark]
- Cue. Which harmonics are removed, shaping the timbre (brightness).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 9MT0/03 20195 marksDescribe how a subtractive synthesiser produces a sound, naming the oscillator, filter, amplifier and envelope stages and explaining what each contributes.Show worked answer →
Subtractive synthesis starts with a harmonically rich waveform and removes (subtracts) frequencies to shape the tone. The oscillator (VCO, voltage-controlled oscillator) generates the raw waveform and sets the pitch; a sawtooth or square wave is a common starting point because it contains many harmonics. The filter (VCF, voltage-controlled filter), usually a low-pass filter, removes harmonics above its cutoff to shape the timbre, with resonance emphasising frequencies at the cutoff. The amplifier (VCA, voltage-controlled amplifier) controls the loudness over time. The envelope (ADSR: attack, decay, sustain, release) controls how the sound changes over time, most often shaping the VCA's volume so the note fades in and out naturally, and can also be routed to the filter cutoff.
Markers reward oscillator generating a harmonically rich waveform and setting pitch, filter (usually low-pass) removing harmonics to shape tone, amplifier controlling level, and the ADSR envelope shaping the sound over time.
Edexcel 9MT0/03 20224 marksExplain what the ADSR envelope controls, defining each of its four stages, and give an example of an envelope setting that would produce a plucked sound.Show worked answer →
The ADSR envelope shapes how a parameter (usually the volume, via the VCA) changes over the duration of a note. Attack is the time taken to rise from silence to the peak level after the note is triggered. Decay is the time taken to fall from the peak down to the sustain level. Sustain is the level held while the note continues to be held down (a level, not a time). Release is the time taken to fall from the sustain level back to silence after the note is released.
A plucked sound has a fast attack (the note starts instantly), a fairly quick decay, a low or zero sustain level (so it dies away while held, like a plucked string), and a short release. This gives the sharp onset and rapid decay characteristic of a pluck.
Markers reward attack/decay/release as times and sustain as a level, the role of shaping the note over time, and a plucked setting (fast attack, quick decay, low sustain, short release).
Related dot points
- Other synthesis methods: additive synthesis (building from sine waves), FM synthesis (carrier and modulator), wavetable synthesis, the characteristic sounds of each, and how they contrast with subtractive synthesis.
A focused answer to the Edexcel 9MT0 synthesis content, covering additive synthesis from sine waves, FM synthesis with carrier and modulator, wavetable synthesis, their characteristic sounds, and the contrast with subtractive synthesis.
- Sampling and sample-based synthesis: capturing and triggering samples, the sampler and key mapping, looping, time-stretching and pitch-shifting, slicing and reordering, warping to tempo, and creative sample manipulation.
A focused answer to the Edexcel 9MT0 sampling content, covering capturing and triggering samples, the sampler and key mapping, looping, time-stretching, pitch-shifting, slicing and reordering, and creative manipulation.
- MIDI and sequencing: MIDI as performance data not audio, note, velocity and controller messages, real-time and step input, quantisation and groove, programming drums and instruments with velocity and timing for a realistic result.
A focused answer to the Edexcel 9MT0 MIDI content, covering MIDI as performance data versus audio, note, velocity and controller messages, real-time and step input, quantisation and groove, and programming realistic parts.
- The harmonic series and timbre: fundamental and harmonics, how the relative levels of harmonics shape tone, the waveform shapes of basic tones, the frequency spectrum and the phase relationships that create a sound's character.
A focused answer to the Edexcel 9MT0 harmonics content, covering the harmonic series, fundamental and harmonics, how relative harmonic levels shape timbre, the basic waveform shapes, the frequency spectrum and phase.
- Equalisation: the frequency bands, high-pass and low-pass filters, shelving and parametric EQ, cut and boost, the Q (bandwidth) control, and using subtractive EQ to create space and corrective and creative EQ in a mix.
A focused answer to the Edexcel 9MT0 EQ content, covering the frequency bands, high-pass and low-pass filters, shelving and parametric EQ, cut and boost, the Q control, and subtractive, corrective and creative equalisation in a mix.
Sources & how we know this
- Pearson Edexcel A-Level Music Technology (9MT0) specification — Pearson Edexcel (2017)