[MEncoder-users] Xvid bff/tff field order issue

[Oliver Muth]

Hi,

when transcoding an interlaced mpeg-2 file (DVB-T capture) to Xvid ASP5, the output is always bottom field first, no matter what I do, even though the input is top field first. At least mediainfo reports tff for the mpegs and bff for the Xvids.

The resulting videos I take with me to watch them on an analog TV (that's why I keep them interlaced).
Most of the time they look fine, but sometimes I get awfull jitter in high motion scenes. Quickly moving objects seem to kind of bounce back and forth.

Attempts to cure the problem:
A. pre-pending il=s to the filter list partly works: No bouncing, but the video seems to flicker slightly.
B. yadif=0 works, too - but I wanted to keep it interlaced.
C. pre-pending phase=t - yes, thatś it! Looks nice.
(Of course on a progressive display they would all look the same.)

Now my questions:

1. Why am I not seeing this effect with all movies? All input files are reported to be tff by mediainfo. Is that wrong? Or am I just occasionally blind, or the effect is almost invisible in movies with slower motion?
2. What's happening? I assume that after encoding with Xvid the movie is still tff, only the flag is set to bff. When I transcode back to mpeg2 everything looks fine again.
3. Could someone please explain (in simple words) the difference between il=s and phase=t? Does il=s just switch the fields within the same frame, while phase=t pushes the bottom field into the next frame? Why does it flicker with il=s?
4. Would it be safe to add "phase=t" to my standard profile?
Or would I destroy those movies that otherwise were transcoded fine?

Thanks in advance for your answers

Oliver


My mencoder command line:
mencoder -profile "Xvid-asp5-avi-1" -o out.avi in.mpg

The mencoder profile:
[Xvid-asp5-avi]
profile-desc="Xvid/MP3 MPEG4 ASP at L5 encoding constant quantiser"
ovc=xvid=1
xvidencopts=profile=asp5:fixed_quant=4:vhq=4:quant_type=mpeg:chroma_opt=1:threads=2:autoaspect=1:interlacing=1:psnr=1
vf=il=d,hqdn3d=2:1:2,il=i,harddup=1
oac=mp3lame=1
lameopts=vbr=3

System: Debian Squeeze with debian-multimedia repositories.

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