Perception · two channels · one trick

Two ways up
that never end — and never arrive.

Hue is a circle: push red up in frequency and you eventually loop back to red through magenta. A Shepard tone is the same circle for pitch: push a note up and it loops back through itself. Both keep climbing forever because the climb is a rotation. Channel A drives the hue hand; Channel B drives the pitch‑class hand. The two spectrum read‑outs are stacked so the shared move — a peak rises, fades at the top, and a fresh one reappears at the bottom — lines up between light and sound.

Channel A · Light
current colour
red · 0°
λ ≈ 750 nm
The spectrum of a hue. Most hues are one peak that slides up in frequency. Magenta has no wavelength — it only exists as red + violet together, so two peaks appear, the violet one fading as the red one returns. That hand‑off is what lets the colour wheel close.
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Channel B · Sound
C
current pitch class
C · 0°
centre ≈ 370 Hz
Seven sine partials, one octave apart, under a fixed loudness envelope (dashed). As the comb slides up, the top partial fades out exactly as a new one fades in at the bottom — so pitch rises endlessly while the spectrum never moves. Releasing the slider holds the tone; Stop silences it.
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