[FFmpeg-user] Converting a 23.98p source to 29.97i ProRes w/interlaced 3:2 pulldown?
krueger at lesspain.de
Tue Oct 27 10:02:31 CET 2015
On Mon, Oct 26, 2015 at 8:42 PM, Mel Matsuoka <melmatsuoka at gmail.com> wrote:
> > On Oct 26, 2015, at 9:15 AM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:
> > I believe (strongly) that what you like as output
> > framerate is not 29.97 but 30000/1001
> Hi Carl,
> I guess I'm looking for the "why", rather than the "what", as far as this
> syntax is concerned.
> In NTSC broadcast land, 29.97 frame rate is *always* defined and referred
> to as just that: 29.97. No professional video tool that I'm aware of
> requires the user to define that frame rate with the level of precision
> that dividing 30000 by 1001 gives you.
> And I suppose this begs the question, where do the numbers 30000 and 1001
> come from to begin with?
Just some heuristics. As explained by Moritz, the correct rate is
30000/1001 due to its analogue heritage and most tools (software, cameras)
I know produce timestamps (and thus a frame rate) based on that. However,
some don't. E.g. Final Cut Pro 7 and earlier and most Apple Tools from that
time even used the rounded version 29.97 in the files they produced. In
theory the difference is so marginal, that it should not cause problems (it
takes more than nine hours for the rounding difference to produce 1 frame
A/V desync if a tool interpretes a 29.97 file as 30000/1001 which
definitely happens in some cases). My experience has been, that the safer
side is to use 30000/1001, simply because more tools use it and even
Apple's new tools (like FCPX use 30000/1001 internally). You will probably
not run into these problems if you use 29.97 but if you do, errors might be
very subtle and hard to diagnose.
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