An analogue NTSC theoretically wants 525 lines of 63.5us across two fields of a (not quite) 30Hz frame.
In practice, most analogue TVs don't really care all that much; component tolerances in the TV are such that they'll sync to anything close. In particular, you'll use frames of either 262 or 263 line to give a 524 or 526 non-interlaced field, and completely ignore the half-sync and equalising pulses on the field sync.
If you assume the picture starts at line 0 - not the way you'd think of it as a video engineer, but it makes the logic easier to understand - then:
- line 0: picture starts
- line 240: front porch (the gap between picture and sync)
- line 243: field sync (three lines)
- line 246: back porch (time for the beam to scan back)
- line 262 = line 1: picture starts again
If you want to use only 176 of the available lines, I would recommend that you position them somewhere in the middle of the actual official active picture. That allows an analogue TV - famously overscanned - to show the entire image without cutoff. Conveniently, 240 -176 is a handy multiple of eight...
So consider now, your output still starts at line 0, but it's 'really' 32 line into the active line. Now your line count looks like this:
- line 0: picture starts
- line 176: front porch
- line 211: field sync
- line 214: back porch
- line 262 = line 0
That's still a bit of a pain to use counters and such; I'd simplify by moving the field sync to line 212 (divisible by four) or 216 (divisible by eight) and making it four lines long. Most TVs won't care about the extended sync pulse, and moving its position merely shifts the output vertically.
I also wouldn't care particularly about the exact frequency for the line scan; the sync detector is looking for the sync pulse and as long as it's the right length or close to it, it'll work. Make it a handy multiple of your system clock. The maximum video frequency you can display on NTSC is around 5MHz, which is a dot clock of 10MHz, though I would go no higher than that to avoid the colour subcarrier frequency filters in the TV. A 1MHz character clock will give you an easy 48 characters across a line (and again, I'd be looking at multiples of 4 or 8 to reduce the counting logic!)
Neil (spent too long in UK broadcasting: I'm a PAL/625 snob
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