1 · Supply reel — unwinds, ω = v / r
2 · Take-up reel — slip-clutch tension
3 · Erase head
4 · Play head — moving flux → voltage
5 · Capstan + pinch roller — set 4.76 cm/s
6 · Flywheel — inertia smooths speed
7 · Motor + drive belt
8 · Magnetic tape — oxide stripes = audio
WALKMAN · TAPE TRANSPORT
CASSETTE MECHANISM · LIVE PHYSICS
■ STOP 0:10 / 1:30
play-head output
drag to orbit · pinch / ⌘+scroll to zoom · orbit behind to see the belt drive
Press PLAY: the head bridge slides up and the pinch roller grips the tape against the capstan.
Pitch follows tape speed — slide left for the dying-battery warble. Switch the flywheel off to hear wow & flutter.
How a Walkman moves the tape
- Motor → belt → flywheel. A small DC motor drives a heavy brass flywheel through a rubber belt (orbit behind the deck to see it). The belt also isolates motor vibration from the tape.
- The flywheel is the metronome. Its rotational inertia irons out every little speed wobble. Switch it off in the controls and you'll hear the result — wow & flutter, the warble of a worn-out deck.
- Capstan + pinch roller set the speed. The capstan is the thin precision shaft on the flywheel's axle. On PLAY, the rubber pinch roller squeezes the tape against it, so the tape moves at exactly 4.76 cm/s — no matter how full either reel is. This is the single key idea of every tape machine.
- The reels never set the speed. They only manage slack. Since the rim must match the tape speed, each reel turns at ω = v / r: the supply reel spins faster as it empties, the take-up slower as it fills (r₁² + r₂² stays constant — watch them trade). The take-up spindle is driven through a felt slip clutch: its drive wheel turns faster than needed and the felt slips, applying just gentle tension.
- The tape is the recording. A plastic ribbon coated with iron-oxide particles, magnetised in stripes (orange/cyan = north/south here, drawn from the actual melody). Four tracks fit on the 3.81 mm width: stereo for side A, stereo for side B.
- The play head reads it by induction. As magnetised tape slides over the head's tiny gap, the changing flux through the coil induces a voltage (Faraday's law) — microvolts, amplified to your headphones. No power touches the tape itself.
- Speed = pitch. Because the signal lives as a spatial pattern, playback frequency is tape speed ÷ wavelength. Weak batteries → slow capstan → everything flat and droopy. Try the speed slider.
- FF / REW. The head bridge and pinch roller retract and a reel is driven directly at high speed. The faint chipmunk chirp while winding is "cue/review" — the tape still lightly brushing the head. The erase head (left of the play head) only powers up when recording.
Note: this demo cassette is only 90 s long so you can watch the reel radii change; a real C-60 runs 30 min per side. Drive-train ratios are simplified for visibility.
Try: slow the tape and hear the pitch sag · kill the flywheel · rewind and listen for the chirp · X-ray off for the full case · watch the supply reel speed up as it empties.
Try: slow the tape and hear the pitch sag · kill the flywheel · rewind and listen for the chirp · X-ray off for the full case · watch the supply reel speed up as it empties.