The filter envelope: brightness over time
Lesson 3's contour, aimed at lesson 4's knob — the single move behind most "movement" in synth sounds.
You now own two separate ideas: an envelope draws a contour over a note's life, and a cutoff sets how bright the sound is. Point the first at the second and you get the defining gesture of subtractive synthesis: a note that changes color as it plays. Acoustic instruments do this naturally — a plucked string starts bright and dulls as it rings. A filter envelope is how a synthesizer earns that liveliness.
Same ADSR, different target
Nothing about the envelope itself changes — attack, decay, sustain, release mean exactly what they meant in lesson 3. What changes is the destination: instead of pushing loudness from silence to full and back, the contour pushes the cutoff from closed to open and back. A fast-decay filter envelope on a bass makes the classic pluck-with-a-"pew"; a slow-attack one makes a pad that blooms open.
The same envelope editor as lesson 3 — with a destination switch. Hold a note on amp, then flip to filter and hold the same note: the contour is identical, but now it draws brightness instead of loudness (the level stays put while the color moves). Try the pluck preset on both targets.
Depth: how far the envelope pushes
On real hardware the filter envelope always comes with one more knob the amp envelope does not need: depth (ENV on these devices). It scales how far the contour moves the cutoff — and on every one of these four machines it is bipolar: negative depth flips the contour upside down, so the note starts bright and closes upward motion into a reverse sweep. Depth at zero is the off switch: the envelope runs, but the knife never moves.
On all four devices the filter envelope lives right on the FLTR page next to the cutoff it drives: its own ATK/DEC/SUS/REL row plus the bipolar ENV depth knob. Open ATK and ENV side by side and this lesson is under your fingers: