[MPlayer-users] converting 25 fps -> 23.976 fps?

[*]

--- D Richard Felker III <dalias at aerifal.cx> wrote:
> > [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> > Hello, I noticed that when I encode a vob stream at 25 fps
> > to an avi at 23.976 fps (using -ofps 23.976) there are
> > subtle jerks in action about every second.  I assume this is
> > due to the way mencoder does the conversion (drops a frame
> > occasionally as opposed to interpolating smoothly between
> > frames)?
> >
> > If this is true, is there any way to make it interpolate
> > smoothly?
>
> What would you have it do to "interpolate smoothly"?? This is a very
> hard problem, and there's no way to do it correctly, since video is
> grossly undersampled in the temporal direction.
>
Okay!  I assumed mencoder would do what you mentioned in your second
reply- that it was tracking the motion (fg and bg) internally and
would use that data.  But I don't know the theory.

> For making this conversion, you should instead use -speed 24/25 (or
> 24000/25025 to be more precise) to slightly speed up both audio and
> video. Keep in mind that you might have to resample audio too in order
> to end up with an output rate that your codec will accept.
>
> Actually, I just checked and -speed only works in mplayer, not
> mencoder. If you _really_ need it, someone might be able to add it to
> mencoder.
>
Sorry, it's not important.  I just assumed it was a common conversion
(one of the most common conversions!) going from PAL <-> NTSC.


> > Or is that not desirable for some other reason?
>
> Unless you have a _VERY_ good reason, you should not be changing fps.
>
> > Likewise, should I not try downconverting vobs at 29.976 fps
> > to 23.976 fps?  Thanks!
>
> VOBS at 29.976 fps don't exist (you mean 29.96 fps), and 29.97 fps is
> rare since most vobs come from movies. If the movie is 29.97 fps,
> chances are it's hard-telecined, and you need to apply inverse
> telecine or it will look horrible. As a bonus, this will inherently
> reduce the framerate to 23.976 fps.
>
You're right- I meant 29.97 and now I see it's hard-telecined.  Could
I ask if the accepted practice is to use -fps 29.97 -ofps 23.976 for
24fps progressive NTSC?  I tried all 4 inverse-telecine filters (detc,
ivtc, pullup, filmdint) and got lots of duplicate frames for all but
pullup (which says it will output more than 24 fps with -ofps 23.976)
and filmdint (which says it will result in an uneven framerate during
playback).  So maybe no filter is correct for this?  It looks okay
with -fps 29.97 -ofps 23.976.

> BTW, why are you so interested in changing framerate?
>
Almost all the movies I come across on the internet have fps 23.976,
maybe because most of them were encoded in the NTSC part of the world.
I thought it would be better to have fps 23.976 to save the bits for
more frame quality.  But perhaps any quality you would gain by that
would be lost in the interpolation.

> Rich
>
Thanks for the replies!


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