On Wed, Mar 02, 2005 at 02:47:45PM -0800, Corey Hickey wrote:
Oooh, I see what's going on now. I was under the impression that the NTSC clock rate was exactly 60 Hz. I understand what you mean now. Basing my research on that, I've finally come up with a useful page out of google:
http://www.edlmax.com/NTSC3.html
Some of the numbers on this page are incorrect; somebody messed up the precision. For instance, 4.5 * 10^6 / 286 = 15734.265734 265734 ... (repeating) and not 15,734.265734265700000.
If the rest of the page is correct, though, it seems that 60000/1001 is actually based upon the RF frequency range available. NTSC was 60 Hz before color, and the addition of a color subcarrier necessitated a slight adjustment to the frequency.
Also, if I follow the math correctly, the NTSC vertical frequency is exactly 60000/1001, so digital NTSC video is exactly the correct framerate.
OK, I think you understand it better than me now. I'll have to read that article you found.
Anyway, the 1001 factor is not a "stupid hack" either. It's part of the real fieldrate of NTSC, which is 59.94... fields per second, not 60 fields per second. I'm told that NTSC was originally 60 fields per second in black & white, then they had to lower the framerate slightly to make bandwidth available for the color carrier. Not sure if this is true though, but it makes sense..
Yes, it seems that the 1001 is derived from a very intelligent hack. :)
:) Rich