[MPlayer-users] A mencoder question, incorrect bitrate.

D Richard Felker III dalias at aerifal.cx
Wed Apr 21 05:08:56 CEST 2004

On Tue, Apr 20, 2004 at 05:33:47PM -0700, Jack wrote:
> Xabier Rodriguez Calvar wrote:
> >	Hi, I'm a newbie with mencoder and have a question. I'm trying to 
> >	recompress an mpg file with a bitrate to another file with a lower bitrate 
> >to make it smaller. I'm trying to make it in three passes.
> >
> >	First I extract the audio. I don't want to recompress this, so I use 
> >	-oac copy, but when making the second pass, I write the recomended bitrate 
> >to 700MB (874) and when finishing I can see another bitrate (778,505 
> >kbit/s). 
> >
> >Which is the problem? I want to adjust it to 700MB! :-)
> >
> lavc's target bitrate is just that, a target.  It adjusts the bitrate up 
> and down throughout the video based on the current complexity of the 
> video.  In a video with an equal mix of complex and simple portions, the 
> average bitrate over the whole thing will be the rate you specified.  
> For most files, you'll end up with an average bitrate located somewhere 
> kind of near what you gave it for the target, but to get it perfect, you 
> have to adjust your target bitrate.  If you want a true constant 
> bitrate, where it will be exactly what you specified all the time, I 
> believe the option is vrc_eq=1, but that's a bad idea.. you'll have 
> significant quality problems in complex sections of the video.
> So you just have to keep setting the bitrate higher until the actual, 
> average bitrate of the file is what you want.  At least, I think that's 
> the only option.  I'm sure Rich will catch me if I'm wrong.

No. :) The problem here is that you don't need 874 kbit/sec to
compress lousy 352x288 vcd to mpeg4. Quality maxes out at 778kbit in
this particular example. If you really want to get a little more
quality and get the size up to 700 meg, you can use vqmin=1 and lmin=1
to allow some frames to be encoded at the very highest quality, but
q=2 and q=1 are visually almost indistinguishable.


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