[MPlayer-DOCS] [PATCH] update the mencoder telecine docs
D Richard Felker III
dalias at aerifal.cx
Wed Mar 2 06:11:33 CET 2005
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! :)
> I've searched google several more times and still haven't found out why.
> I have a few links that come close.
> This page says it's a stupid hack:
This page is idiotic. Pulldown is used for both NTSC and PAL, whenever
progressive video is displayed on an interlaced device. NTSC just uses
the common 3:2 pulldown known as "telecine" while PAL uses 2:2
pulldown (sped up by a few % so it's 25 fps instead of 24) or
sometimes 3:2:2:2:2:2:2:2:2:2:2:2 pulldown (excact 25/24 ratio).
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..
> This page says "don't ask":
Very naive page.. :)
> This page looks promising but doesn't seem to explain it:
Hmm wonder why...
> That last one does list a few books that at least _ought_ to. I might
> have to go to the library and see if I can find them.
I'm not sure it's worth the trouble. The fact is: this is just how it
is. 60000/1001 is the NTSC fieldrate. :)
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