Where did 625 line television come from?

[Kalee20]

Frequency division was hard back then... Synchronised blocking oscillators were the standard way to do frequency division and that meant small divisors to give reliable operation.

So, not charge-pump dividers as Amie had been thinking (which would divide over a wide range of input frequencies), but oscillators synced-up to a subharmonic of incoming frequency (which works only over a narrow range)?

Thus if you want to divide 1kHz by 5, you'd make a blocking oscillator or multivibrator, free-running at 190Hz and then inject the 1kHz pulses. It won't trip after the 4th pulse, but the 5th pulse is so near the avalanche point that it will, thus pulling frequency up to 200Hz...?