[MEncoder-users] Changing frames per second

[Nico Sabbi]

Il Sunday 30 September 2007 18:13:08 Ross Clement ha scritto:
> I'm considering buying a digital camera with movie mode. This will
> allow me to always have a "video" camera to hand instead of the
> borrowed camera I use sometimes at the moment. However, pretty well
> any still cameras with "movie" mode record at ~30fps instead of the
> 25fps of PAL video.
>
> Clearly I could author all my DVDs as NTSC videos, but then I lose
> some vertical resolution. Also there would be difficulties if I mix
> ~30fps and 25fps content. I think.
>
> I had read that changing frames-per-second of video was problematic
> as dropping a number of frames per second
>
> I had a go at downloading some ~30fps video clips from the internet
> and converted them to 25fps. A sample command is:
>
> mencoder 1_hi.mov -of mpeg -mpegopts format=dvd:tsaf -ovc lavc -oac
> lavc -lavcopts
> vcodec=mpeg2video:acodec=ac3:vbitrate=1000:aspect=4/3 -ofps 25  -o
> junk.mpeg
>
> I was surprised by how good the resulting quality is. I couldn't
> see any unnatural jerkiness of movement.
>
> I've searched through the MPlayer html docs, and can't find
> anything about how frame rate conversions are done. Though I notice
> the frequent "skipping frame" output while encoding. So I presume
> that the conversion hard-drops frames.
>
> Can anyone tell me how frame rates are converted? And are there any
> higher quality techniques built into mencoder that I could use that
> might achieve a higher quality result?

you have just been very lucky, something that doesn't happen often.
Mplayer keeps sync in a scary way that you had better avoid like the
plague (dropping and duplicating frames even if they are keyframes).

If you want better looking video, provided that you need to raise
(not lower) the nominal framerate, avoid any framerate conversion
and use -mepgopts tele_src=x:tele_dest=y; moreover the process
is theoretically reversible (but don't ask me to write the code :) )