[FFmpeg-user] libmp3lame -ab 128k produces a VBR stream

[Bill Gates]

Hello,

I encode in MPEG4/MP3 to AVI files with the following command:

ffmpeg -i input.avi -vcodec mpeg4 -vtag XVID -s 640x480 -qscale 4 -g 50 -bf
2 -acodec libmp3lame -ab 128k -y output.avi

The files will play fine on the PC, but not on the standalone player.

When I tried opening one of these files with VirtualDub I got the message:

"VirtualDub has detected an improper VBR audio encoding in the source AVI
file and will rewrite the header with standard CBR values during processing
for better compatibility.........."

If I do so, rewriting the file with VirtualDub by direct stream copy of both
audio and video, the result will play fine on the standalone player! So
VirtualDub did the right thing with the audio header.

Why AVI files with an mp3 audio stream produced by ffmpeg are marked in the
AVI header as VBR audio instead of CBR?

Does "-ab 128k" produce a CBR stream, or do I need to code another option in
the command-line to force CBR?

Thanks,