[old_tv_nut]

I believe the drum has three spirals of scanning holes, so it can be 3 times smaller than a drum that has all the holes in one big spiral. Then, the disc has spiral slots that select one of the spirals of holes on the drum for each revolution of the drum. It then takes three revolutions of the drum to create one picture.
Edit: the scanning pattern would be the ordinary left to right, top to bottom, but the top third of the picture comes from the top spiral of holes, the middle third from the middle spiral, etc. If the picture rate was, for example, 10 frames per second, the drum would turn at 30 rps or 1800 rpm.