bpro@bpro-MacPro:~$ ffmpeg -f rawvideo -s hd1080 -r 30 -i /dev/blackmagic/card0 -b 768k -f h264 test3.avi -vframes 300
ffmpeg version git-2012-12-12-f18c873 Copyright (c) 2000-2012 the FFmpeg developers
  built on Dec 12 2012 12:49:50 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-gpl --enable-libfaac --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-librtmp --enable-libtheora --enable-libvorbis --enable-libvpx --enable-x11grab --enable-libx264 --enable-nonfree --enable-version3
  libavutil      52. 12.100 / 52. 12.100
  libavcodec     54. 79.102 / 54. 79.102
  libavformat    54. 49.101 / 54. 49.101
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 26.102 /  3. 26.102
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
[rawvideo @ 0xb2316a0] Estimating duration from bitrate, this may be inaccurate
Input #0, rawvideo, from '/dev/blackmagic/card0':
  Duration: N/A, bitrate: 746496 kb/s
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 1920x1080, 746496 kb/s, 30 tbr, 30 tbn, 30 tbc
Please use -b:a or -b:v, -b is ambiguous
File 'test3.avi' already exists. Overwrite ? [y/N] y
using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle SSE4.2
[libx264 @ 0xb23ab00] profile High, level 4.0
Output #0, h264, to 'test3.avi':
  Metadata:
    encoder         : Lavf54.49.101
    Stream #0:0: Video: h264, yuv420p, 1920x1080, q=-1--1, 768 kb/s, 90k tbn, 30 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo -> libx264)
Press [q] to stop, [?] for help
/dev/blackmagic/card0: Invalid argument
frame=    0 fps=0.0 q=0.0 Lsize=       0kB time=00:00:00.00 bitrate=   0.0kbits/s
video:0kB audio:0kB subtitle:0 global headers:0kB muxing overhead -nan%
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
[libx264 @ 0xb23ab00] final ratefactor: 35.77