[MPlayer-users] 2 questions about 2-Pass DivX encoding with mencoder

[Tim U.]

On Tue, Jan 08, 2002 at 11:28:06AM -0400, Troy D. Strum wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> Hi folks...
> 
> (I'm using the 0.60 release)
> 
> When I encode a video using the two pass method I know I can safely delete
> the AVI file from the first pass. Knowing this, can you just send that first
> file into /dev/null or something? ...or would this have little positive
> impact?

I do exactly that, simply to save disk space.  It would be nice to have a 
-ac null option so that no time is spent decoding or encoding the audio on the 
first pass.  The only reason I can think of to keep the first pass AVI is to 
check if the video looks all right before starting the second pass, since, 
unlike with Windows encoders, the first pass AVI produced by mencoder is 
playable.

> Secondly, I caught some mentions of NTSC DVDs having issues. I ripped The
> Prisioner box set (4 titles in total) with no problems (NTSC, 29.97fps)...
> am I just lucky or is there something else I should be on the lookout for?
> (I noted some brief discussion on -ofps but would like some more info)

I have noticed problems with most NTSC DVDs (the only kind available to me), 
all are problems with the inverse telecine, or 3:2 pulldown (pullup?), or 
whatever you want to call it.  Usually it is noticable in just one or two 
scenes, sometimes a lot more.  You're supposed to use 23.976 FPS when encoding 
29.97 FPS NTSC DVDs, because thats exactly the frame rate you get when you take 
away the duplicated fields.  For a good explination of telecine, read:

http://www.divx.com/support/guides/ntsc_pal.php?cid=1&gid=3

When the movie is not completely inverse telecined, you have too many video 
frames, and that can cause the video to fall behind the audio, with the delay 
increasing as more frames that weren't inverse telecined are played.  A'rpi 
recommended that I use the option "-mc 0" once, to prevent A-V sync 
correction.  I guess without that option the audio would not get out of sync, 
but the non-inverse-telecined frames would still be there.

mencoder has inverse telecine problems with "War Games" and "The Saint", and 
probably others that I can't remember right now.  I also tried encoding those 
movies in Windows using mpeg2avi, and DVD2AVI->VFAPI->VirtualDub.  Using 
DVD2AVI->VFAPI->VirtualDub produced the exact same inverse telecine problems 
as mencoder.  mpeg2avi produced less inverse telecine problems, but still 
enough that I didn't want to save the AVI.  The quality of the video produced 
by mencoder is also noticably nicer looking than that produced by VirtualDub 
and mpeg2avi, but maybe that was due to some option I didn't choose.

I hope you can find something useful in my reply.
   - Tim U.