Pravetz 82

For resons of getting the best piture from the available bandwidth, all analouge broadcast TV systems use interlaced scanning. And that pretty much requires a 'half line' in every field, so a total of an odd number of lines. The sync pulses are prodcued, of course, but dividing down a master clock. And if you have an odd nymber of lines, you can't have a /2 (simple flip-flop) involved anywhre. It's interesting to see how the broadcast standfard were set up to use simple-ish divider chains 243 lines (very old UK standard) = 3^5 (so 5 cascaded /3 circuits) 405 lines (old UK System A) = 3^4*5 (replace one of those /3s with a /5) 525 lines (US system) = 3 * 5^2 * 7 625 lines (UK system) = 5^4 Of coruse the French had ot be a bit eccentric :-) They had a monochrome-only 819 line standard (System E). It involves a /13 circuit :-) -tony