I have been trying to use FFmpeg utility to convert a avi file using DNxHD to mxf format. I am using "FFmpeg" with params as following:

 ffmpeg -i ccvt_box.avi -vcodec dnxhd -video_size 1920x1080 -r 24 -b:v 115m  ex.mxf        

The error it is giving :

ffmpeg version N-43737-g76c3fff Copyright (c) 2000-2012 the FFmpeg developers
  built on Aug 20 2012 18:50:42 with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
  configuration: 
  libavutil      51. 70.100 / 51. 70.100
  libavcodec     54. 53.100 / 54. 53.100
  libavformat    54. 25.104 / 54. 25.104
  libavdevice    54.  2.100 / 54.  2.100
  libavfilter     3. 11.101 /  3. 11.101
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 15.100 /  0. 15.100
Input #0, avi, from 'ccvt_box.avi':
  Duration: 00:00:10.00, start: 0.000000, bitrate: 691 kb/s
    Stream #0:0: Video: indeo5 (IV50 / 0x30355649), yuv410p, 340x344, 10 tbr, 10 tbn, 10 tbc
    Metadata:
      title           : bob.avi
[dnxhd @ 0x7fcd60818e00] video parameters incompatible with DNxHD
Output #0, mxf, to 'ex.mxf':
    Stream #0:0: Video: dnxhd, yuv422p, 340x344, q=2-1024, 90k tbn, 24 tbc
    Metadata:
      title           : bob.avi
Stream mapping:
  Stream #0:0 -> #0:0 (indeo5 -> dnxhd)
Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height