[MPlayer-users] spurious blockiness when transcoding high-bitrate mpeg2 to med-bitrate mpeg4

[Simen Thoresen]

Hi team,

I've made a 16Mpbs mpeg2 grab of a 640x480 29.9fps Laserdisk, and now want 
to transcode the 6G 'perfect' mpeg2 file to a more managable mpeg4 file for 
sotrage. I'm transcoding using mencoder and lavc mpeg4, and the result is 
imho very good (altho suggestions for improvement are appreciated). What 
bugs me, is that on a few points in the mpeg4 stream, I get sudden bursts of 
artifacts for a half a second or so. These do not coincide with obvious 
damages in the original mpeg2-stream, nor do they occur in scenes that are 
markedly different than what is being encoded without issue.

So I'm wondering - what is causing this, and how do I avoid it?

A few details more - re-encoding an excerpt of the original file with 
generous lead-in and lead-out show the blockiness appearing differently than 
from the original encode. Different excerpts cause differences in the 
apparent blockiness, altho they always happen in the same scenes.
Re-encoding the same excerpts with only single-pass (default) encoding also 
causes the blockiness to appear visually different, as does encoding it with 
different (default) lavc:keyint settings.

I'm using MEncoder 1.0pre3, built with gcc 2.95.3 on an Athlon XP box.

The mencoder line I intended using, was

mencoder partial.mpg -o partial.avi -mc 0 -ovc lavc -lavcopts
vcodec=mpeg4:vbitrate=1200:mbd=2:v4mv:keyint=132:vb_strategy=1:vpass=1 -oac 
mp3lame -lameopts preset=192 -vf pp=hb/vb/dr/al/lb

mencoder partial.mpg -o partial.avi -mc 0 -ovc lavc -lavcopts 
vcodec=mpeg4:vbitrate=1200:mbd=2:v4mv:keyint=132:vb_strategy=1:vpass=2 -oac 
mp3lame -lameopts preset=192 -vf pp=hb/vb/dr/al/lb

I can upload a 33M mpeg2 source-excerpt and a 4M mpeg4 transcoding of the 
same excerpt if anyone wants to take a look.

Anyone?

Yours,
-S

-- 
Simen Thoresen, Wulfkit Support, Dolphin ICS
http://www.tysland.com/~simentt/cluster