with latest SVN kdenlive (from PPA), using this command line:
f=mp4 acodec=libfaac ab=128k ac=2 ar=48000 pix_fmt=yuv420p vcodec=libx264 threads=0 crf=24 aspect=%dar vpre=veryfast
produces a no video file (only audio).
Using constant bitrate it works.
I've used this setting for many encode with previous versions.
Hey guys, I know you're very busy developing in these days, thank you!