D Richard Felker III wrote:
On Tue, Mar 01, 2005 at 05:14:30PM -0800, Corey Hickey wrote:
D Richard Felker III wrote:
My understanding is that 60000/1001 is just the only rational number within the range of tolerance of the NTSC refresh clock, without having a ridiculously huge denominator. Thus it was standardized as the "official" refresh for NTSC with the advent of digital video. 30000/1001 and 24000/1001 are just the refresh converted from fields to field-pairs or telecined frames.
I'm sure you can find better references on this somewhere if you like...
Rich
Hmm. That makes sense. So, I understand the number now, but I still don't know why a different number had to be chosen. What was wrong with 60/1, 30/1, and 24/1?
They're significantly wrong. In fact the error is so great you'd have desync after a few seconds if you just treated the video as if it were one of these rates. 5994/100, 2997/100, and 23976/1000 also have a significant margin of error. On the other hand, 60000/1001 and friends fall within the margin of tolerance for the actual clock used in NTSC. Or that's my understanding, anyway. Measure it on an oscilloscope if you really want to see! :)
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.
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. :) -Corey