[old_tv_nut]

A question came up on today's meeting about the horizontal/vertical sweep rates on the early 405 line/50 Hz and 343 line/60 Hz being related. The answer was that the ratios used were composed of small prime factors to accomodate the countdown circuits of the time.

I just wanted to add that the circuits used were possibly of three types:
1) pulse current accumulation - pulses from the higher frequency would increase the charge on a capacitor by a small step for each pulse, until the voltage reached the trigger level for a following monostable multivibrator
2) monostable timing counter - the output monostable had a dead time once triggered that made it wait somewhat over n-1 pulse times until it was triggerable again, by the nth pulse
3) digital binary counters

Large factors would not be a problem for digital counters, but could be an issue for methods #1 and #2, as increased precision and reduced drift would be needed if n was a large number. While #3 would not suffer from drift, the parts count and cost was much greater than for #1 and #2.

Prime factors of 525: 7,5,5,3
Prime factors of 625: 5,5,5,5

For NTSC color, the color subcarrier is 455/2 of horizontal
Factors of 455: 13,7,5