[MPlayer-users] CBR Problem with x264opts

Eli Crumrine zybhjk at verizon.net
Thu Jun 5 21:49:54 CEST 2008

> Hi,
> I am trying to use mencoder to get a constant bitrate h264
> stream of 1.6mbps.
> I'm using following command.
> mencoder.exe -noaspect -oac lavc -srate 44100 -af
> lavcresample=44100 -lavcopts acodec=mp2:abitrate=192 -af
> volume=15:0 -ovc x264 -x264encopts
> log=3:partitions=all:bitrate=1600:vbv_maxrate=1800:vbv_bufsize=1800:scenecut=60:keyint=30:bframes=2:subq=7:nofast_pskip:frameref=5
> -o output.avi input.vob
> When i run this output.avi using vlc player, i can see the
> stream bitrate shoots up to 4 mbps as well as goes down to 800
> kbps.
> So it seems mencoder is giving me 1.6 mbps average bitrate.
> How can i get a constant bitrate of 1.6 mbps(CBR) calculated
> over few seconds.

This is how video encoding should (and does) work. Some scenes (little
motion, black screen) require very few bits to encode with a smart codec
like x264, while others will require much more. Forcing a constant bitrate
for every second of a video will decrease the quality.

If you have space constraints, you should decide upon a bitrate by
dividing the total size (in kb) by the length (in s).

Unless the encoder is missing your target size or you are playing it back
on a max-bitrate limited device (I think ipods have issues with this), I'm
not sure why you would want to change this behavior.

More information about the MPlayer-users mailing list