Cellulophone

One disc an octave: twelve rings of slots, C innermost, B outermost, each with as many slots as its note needs at the disc's speed, rounded to whole slots. The windows sit in a line above the axle; a lit ring is a key whose mirror sends its light to the cell. Drawn turning a fortieth as fast.

Top: the screen in the window, clear where it lets light through, with a slot crossing it. Below: the light that reaches the cell over one slot pitch (the note's period), averaged over the slot's width; the dashed line is its mean, the steady light that the coupling capacitor takes away. Push the keys back (the mod wheel) to slide the slit in.

Each ring's slots, and how far whole slots leave its note from the tuning: the motor is set so that A is exact, and every octave's disc turns at exactly twice the speed of the one below, so each note is off by the same cents in every octave.

Every note is a steady stack of harmonics as long as its mirror is in the beam. Narrow slots keep the high ones; wide slots lose them. With the lamps on the mains, a 100 Hz line and sidebands come with every note; a worn disc fringes each line with faint sidebands at its turning rate.