Tonewheels

Tonewheels most usually refer to optical tonewheels.

One of the better known ones will be the optigan, which is basically PCM-like in optical form.

There are various similar ideas. The Hammond organ, for example, uses gears and magnetic pickups as a (near-sine) tonewheel.

And many more experimental things http://www.umatic.nl/tonewheels_historical.html