[MPlayer-DOCS] [PATCH] update the mencoder telecine docs

[Corey Hickey]

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