dmoon:~/test/vconv-aac$ ffmpeg -i 3gptest.avi -s 176x144 -b 64000 -r 10 -ab 12 -ac 1 -ar 8000 z.3gp
FFmpeg version SVN-r8568, Copyright (c) 2000-2007 Fabrice Bellard, et al.
  configuration: --enable-amr_nb --enable-amr_wb --enable-libmp3lame --enable-gpl --enable-libfaac --enable-libfaad
  libavutil version: 49.4.0
  libavcodec version: 51.40.2
  libavformat version: 51.11.0
  built on Mar  3 2007 19:27:32, gcc: 3.4.6
Input #0, avi, from '3gptest.avi':
  Duration: 00:00:13.3, start: 0.000000, bitrate: 6042 kb/s
  Stream #0.0: Video: mjpeg, yuvj420p, 352x288, 25.00 fps(r)
  Stream #0.1: Audio: pcm_s16le, 48000 Hz, stereo, 1536 kb/s
File 'z.3gp' already exists. Overwrite ? [y/N] y
PIX_FMT_YUV420P will be used as an intermediate format for rescaling
Output #0, 3gp, to 'z.3gp':
  Stream #0.0: Video: h263, yuv420p, 176x144, q=2-31, 64 kb/s, 10.00 fps(c)
  Stream #0.1: Audio: amr_nb, 8000 Hz, mono, 0 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
[amr_nb @ 0x8435be0]bitrate not supported
Error while opening codec for output stream #0.1 - maybe incorrect parameters such as bit_rate, rate, width or height
dmoon:~/test/vconv-aac$ ffmpeg -i 3gptest.avi -s 176x144 -b 64000 -r 10 -ab 12500 -ac 1 -ar 8000 z.3gp
FFmpeg version SVN-r8568, Copyright (c) 2000-2007 Fabrice Bellard, et al.
  configuration: --enable-amr_nb --enable-amr_wb --enable-libmp3lame --enable-gpl --enable-libfaac --enable-libfaad
  libavutil version: 49.4.0
  libavcodec version: 51.40.2
  libavformat version: 51.11.0
  built on Mar  3 2007 19:27:32, gcc: 3.4.6
Input #0, avi, from '3gptest.avi':
  Duration: 00:00:13.3, start: 0.000000, bitrate: 6042 kb/s
  Stream #0.0: Video: mjpeg, yuvj420p, 352x288, 25.00 fps(r)
  Stream #0.1: Audio: pcm_s16le, 48000 Hz, stereo, 1536 kb/s
File 'z.3gp' already exists. Overwrite ? [y/N] y
PIX_FMT_YUV420P will be used as an intermediate format for rescaling
Output #0, 3gp, to 'z.3gp':
  Stream #0.0: Video: h263, yuv420p, 176x144, q=2-31, 64 kb/s, 10.00 fps(c)
  Stream #0.1: Audio: amr_nb, 8000 Hz, mono, 12 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
[amr_nb @ 0x8435be0]bitrate not supported
Error while opening codec for output stream #0.1 - maybe incorrect parameters such as bit_rate, rate, width or height