Convert rawvideo / Y800 / gray to something AviSynth can read

avconv -i in.avi -vcodec mpeg4 -b 100000k out.avi

or in parallel:

find ./ -maxdepth 1 -name "*.avi" -type f |  xargs -I@@ -P 8 -n 1 bash -c "filename=@@; avconv -i \$filename -vcodec mpeg4 -b 100000k \${filename/.avi/}-mpeg.avi"