I have no idea. I did a lot of googling and lots of talk about what it is in ffmpeg and not much examples. Lots of people wanted to use cbr and examples for that were given.
I want VBR on my encodes.
Does anyone use the kdenlive gui? I get the feeling people use a lot of commandline.
Here is what I have for encoding from that.
This does 3000 bit something, I have no idea if it is variable or constant.
The line vb=3000k sets a bit rate.
f=mp4 hq=1 acodec=aac ab=%audiobitrate+'k' ar=48000 pix_fmt=yuv420p vcodec=libx264 minrate=0 vb=3000k g=250 bf=3 b_strategy=1 subcmp=2 cmp=2 coder=1 flags=+loop flags2=dct8x8 qmax=51 subq=7 qmin=10 qcomp=0.6 qdiff=4 trellis=1 aspect=%dar pass=%passes movflags=faststart