Imagine an air or steam jet that impinges on the outer edge of a scanning disc. Due to the resulting boundary layer and viscosity of the moving fluid the disc will start to rotate, much like a small wheel will if you place its edge within a stream of moving water. This is the principle of Nicola Tesla's boundary layer turbine engine.
Alternatively, a scanning disc might have a serrated outer edge so that the fluid air or steam jet impinges on the serrations to rotate the disc. This would create a turbine engine according to the impulse turbine principle of Charles Parsons.
In any case, deflecting the air or steam jet toward or away from the edge of the disc could act as a speed regulator. A servo system could do this automatically. Alternatively, with a fixed air or steam jet a servo-controlled hub braking system could serve this purpose.
This scheme would be ideal for a non-electric television system.