Is it possible to write the fps with drawtext off avconv, i can't find information about it.
drawtext=fontfile=arial.ttf:text=
The full command i use without drawtext:
avconv -f rawvideo -pix_fmt gray -r 8 -s 640x480 -i - -an -t 00:00:30 -f rawvideo -r 8 -y \"/dev/xillybus_write_32\"