[MEncoder-users] Compression too great, bitrate too low on detelecined deinterlaced mpeg2video

[Laine Lee]

On 12/29/08 2:24 AM, "Reimar Döffinger"
<Reimar.Doeffinger at stud.uni-karlsruhe.de> wrote:

> If the quality with vqscale=1 or vqscale=2 is too bad there are only
> two possibilities:
> 1) you did not specify the right so the were ignored
> 2) your source video is already that horrible, and this is not an
> encoding issue.
> 
> I assume you tried playing the video with the filters you use for
> encoding with MPlayer?

Yes, I tried the filters, but there are other options needed to get mencoder
to perform the pullup without duplicating frames that can't be used with
mplayer (apparently mplayer doesn't need them, because the output looked
fine).

I was able to get the bitrate  of my scaled video into the desired range by
lowering the vbitrate value from 8000 to 6600, and to change the values for
dia and predia from -10 to -1. Replacing the vbitrate specification with
vqscale=2 in a command with no other change resulted in a lower bitrate
which was comparable to what I was getting before making the changes to the
vbitrate and dia-predia values. If there's another way that vqscale should
be used in a command line similar to this, please let me know.

One thing that baffles me is that the turbo option for the first pass not
only decreased the bitrate, but it resulted in 1 of every 15 frames (my
keyint specification is 15) being rendered with extreme blockiness in
certain passages whether or not I included the keyint option among the lavc
options in the first pass command string. To eliminate these unwanted
results, I omitted the turbo option. There was a detriment to encoding time
of course, but the bitrate went up, and the blocky 15th frames disappeared.

Possibly the most serious concern for me, though, is that encoding results
for brief excerpts of the video (a minute or two) differed significantly
from the results for the same passages when encoded with the entire file
(110 minutes). Several of the results I am reporting here were only
confirmed after encoding the entire 110 minutes of video several times. This
circumstance has caused my use of mencoder to become rather more academic
than even a tinkerer such as myself would prefer.

Here's the command I'm working with now.

mencoder -oac copy -ovc lavc -lavcopts
vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9200:vbitrate=6600:threads=2
:trell:mbd=2:preme=2:dia=-1:predia=-1:precmp=2:subcmp=3:cbp:vqmin=1:dc=10:lm
in=0.01:keyint=15:vstrict=0:aspect=16/9:vpass=1 -vf
framestep=2,filmdint=fast=0,softskip,hqdn3d=2:1:2,scale=720:480,harddup
-noskip -of mpeg -mpegopts format=dvd:tsaf -fps 60000/1001 -ofps 24000/1001
-o /dev/null infile.mpg && mencoder -oac copy -ovc lavc -lavcopts
vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9200:vbitrate=6600:threads=2
:trell:mbd=2:preme=2:dia=-1:predia=-1:precmp=2:subcmp=3:cbp:vqmin=1:dc=10:lm
in=0.01:keyint=15:vstrict=0:aspect=16/9:vpass=2 -vf
framestep=2,filmdint=fast=0,softskip,hqdn3d=2:1:2,scale=720:480,harddup
-noskip -of mpeg -mpegopts format=dvd:tsaf -fps 60000/1001 -ofps 24000/1001
-o outfile.mpg infile.mpg

Thanks.

-- 
Laine Lee