Filters: carving the recipe
A filter removes harmonics you already have. Cutoff chooses where the knife goes; resonance sharpens its edge.
Lesson 2 left you with a stack of harmonics — the recipe. A filter is a tone-shaping tool that turns some of that stack down. It cannot add anything: run a sine through a filter and almost nothing happens, because there is only the fundamental to touch. This subtraction is so central that the whole classic method is named after it — subtractive synthesis: start with a bright wave, carve away until the character appears.
Three directions of the knife
- Low-pass (LP) — lets lows through, removes highs above the cutoff. The most used filter in electronic music: turning cutoff down is the universal gesture for "darker, rounder, further away".
- High-pass (HP) — the mirror: removes lows below the cutoff. Thins a sound out; the standard tool for clearing mud away from a bassline.
- Band-pass (BP) — keeps only a band around the cutoff and removes both sides. Nasal, radio-like, telephone-like.
Cutoff and resonance
Cutoff is where the carving starts, in Hz — on the same axis as the harmonics from lesson 2, which is why sweeping it feels like sliding a curtain across the spectrum. Resonance feeds a bit of the output back, boosting a narrow peak right at the cutoff: a little adds presence and squelch, a lot makes the filter sing at its own frequency. Almost every famous "acid" sound is a low-pass filter with high resonance being swept.
The yellow line is the filter's own shape; the green bars are what survives of the source. Sweep the cutoff and watch the curtain slide through the harmonics — then switch to the noise source (every frequency at once) and the curve becomes perfectly audible. Push resonance up and sweep slowly.
Every device carries this lesson on its FLTR page — FREQ is the cutoff, RESO the resonance, with a TYPE knob for the knife's direction. Digitakt II and Syntakt add a second, Elektron-flavored take on the same idea: the base-width filter on FLTR page 2 is a high-pass and a low-pass in series — BASE sets where the floor starts, WIDTH how much survives above it.