Pearson Edexcel · 1.3 Synthesis · Component 4
Two oscillators into a resonant low-pass filter, shaped by twin ADSR envelopes — the classic subtractive voice. Start rich, subtract what you don't want, then shape what's left in time. The waterfall shows the whole story at once: frequency across, level up, the last two seconds receding into depth.
In the exam
Subtractive synthesis starts with a harmonically rich source (saw or square), removes harmonics with a filter (cutoff + resonance), and shapes the result in time with envelopes — plus an LFO for cyclic movement.
Each idea is one control section on the right — and one visible behaviour in the waterfall. If you can point at it, you can write about it.
The flat spectrum is one slice of time — just now. The waterfall shows that slice and the previous two seconds receding into depth, so the shape of an ADSR envelope is drawn into the surface itself.
Hold a note. Watch the front edge bloom on the attack, settle into a sustained ridge while you hold, then fall away into a release tail that recedes into depth. Drag vertically while you hold to morph between Osc 1 and Osc 2 — with the defaults, pulling down collapses the stack to a lone fundamental (sine); pulling up grows the full sawtooth harmonic ladder.
Height is level. Depth is time. And colour is frequency, on the thermal palette real audio software uses — deep indigo at the sub-bass end, plum and crimson through the bass, vermillion and burnt orange through the mids, gold at the top.
So the fundamental of a C3 note (131 Hz) sits in plum on the left; the 2–8 kHz presence band glows orange; brilliance sparkles gold. Silence melts into the paper — colour only comes up where there is energy. When the exam asks what a low-pass filter did to the timbre, this is the picture to describe: the golds and oranges went first.
[SPEC] Pearson Edexcel A-Level Music Technology · Unit 4 §1.3 Synthesis — oscillators & waveforms, filters (cutoff, resonance), ADSR envelopes, LFO (rate, depth, targets) · [PP] past papers 2017–2023 as cited above · audio is real Web Audio synthesis, the curve is the filter's true measured response, and the envelope graphs are the same maths the sound uses.