trackdrum-friction-reference #

Ground truth for the friction voice, so a port can be diffed instead of argued about.

trackdrum-friction-reference.swift is the continuous-friction block and the setDrumSkinScratch position→material mapping lifted verbatim out of slab/menuband/Sources/MenuBand/MenuBandPercussion.swift. No GUI, no audio engine: it renders the same maths for a simulated gesture straight to raw float32, so you can measure the real thing.

cd pop/cult/reference
swiftc -O trackdrum-friction-reference.swift -o ref && ./ref
ffmpeg -f f32le -ar 48000 -ac 1 -i ref.f32 ref.wav

What it settled #

Three rounds of tuning by ear went the wrong way — the skids were called buzzy, then harsh, then still harsh — because every fix was aimed at an assumption. Measured against this reference, the assumption was backwards:

The real TrackDrum friction is bright and tiny. Bright-and-quiet reads as a surface you feel; bright-and-loud reads as harsh; dull is mud. The DSP port was already correct — what was wrong was where the gestures travelled: paths that hug the pad centre sit on skin at 175 Hz cutoff and sound dull by construction, so they were moved out to snare/rim where the reference drag goes.

And the distinction that took longest to find: bright is the cutoff, metallic is the resonance. Past surface distance 0.62 the resonance ramp climbs 185 → 560 Hz and rings. Paths stop short of it, which buys the brightness without the ring.