NTSC line numbers explained in detail

I've finished examining the NTSC video signal from a vertical line perspective and believe that I understand it as well as I would ever need to for Dexter/Daphne-related problems.

I will now document what I've learned so that when I forget it, I can refer back to this blog post :)

Here is the even->odd field transition:

This is straightforward.  The VBI is 20 lines long (lines 1-20), making line 21 the first "visible" line.  Technically, the visible lines are 21-263.5; however, in practice, lines 22-262 may be considered the 240 lines that NTSC is commonly associated with.

And here is the odd->even field transition:

This is the tricky one and I am going to describe it according to how I visualize (not necessarily how other people describe it).

At the very end of the odd/top field is half of line 263.  Then the VBI for the even field starts (if you assume it starts when the pre-equalization pulses start) and could be considered 20 lines long.

The result is that the vsync pulses will be exactly 262.5 lines apart from each other and thus have a stable frequency.  This is pretty useful to know.

The 525 total lines may be split up like this:

1-20: 20 non-visible lines
21-263.5: 242.5 visible lines
263.5-283.5: 20 non-visible lines
283.5-525: 242.5 visible lines

So the interval from even->odd may be slightly shorter than the interval from odd->even.  Weird.

Therefore, it may be useful to imagine that the VBI period is 20 lines long for both odd and even.