In the early decades of tv the Vacuum tube B&W sets used the voltage pulse sent out by the AC power. US grids have a cycle of 60Hz and PAL countries are 50Hz where the generators in the power stations turn 50 times a second. The voltage going up to 220v (UK), then down to zero volts on its way to 220V in the other direction provided the jolt of electric power these tvs needed to get the CRT to display one FIELD. Two fields each 312 lines (PAL) make one frame. Eyes cant see past 25ish frames in a second so motion picture cameras only go to 24ish a sec to save film. Just trying to find a thread I didnt finnish reading.