title: FFmpeg audio channel rematrix (5.1 to stereo)
body: <p>I've got a (very large!) Apple ProRes 422 file with 16ch audio. To be able to use it on a less powerful computer, I'd like to convert it to a standard MP4 file (h264 video codec, stereo audio). The original file has 16 channels of audio, however, and FFmpeg gives an error when I try to convert it ('Rematrix is needed'). How do I do rematrixing of audio channels? Are there additional steps needed?</p>
  
  <p>This is the FFmpeg command I used:</p>
  
  <pre><code>ffmpeg -y -i Capture0000.mov -vcodec libx264 -vb 2000k -pix_fmt yuv420p -coder 0 -profile:v baseline -acodec libfaac -ab 128k /tmp/out.mp4
  </code></pre>
  
  <p>This is the FFmpeg output:</p>
  
  <pre><code>ffmpeg version 2.2.git Copyright (c) 2000-2014 the FFmpeg developers
        built on May  8 2014 14:17:45 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
        configuration: --prefix=/usr/local/ffmpeg_build --extra-cflags=-I/usr/local/ffmpeg_build/include --extra-ldflags=-L/usr/local/ffmpeg_build/lib --bindir=/usr/local/bin --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-nonfree --enable-x11grab --enable-libxvid --enable-libfaac
        libavutil      52. 81.100 / 52. 81.100
        libavcodec     55. 60.103 / 55. 60.103
        libavformat    55. 37.102 / 55. 37.102
        libavdevice    55. 13.101 / 55. 13.101
        libavfilter     4.  5.100 /  4.  5.100
        libswscale      2.  6.100 /  2.  6.100
        libswresample   0. 18.100 /  0. 18.100
        libpostproc    52.  3.100 / 52.  3.100
      Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Capture0000.mov':
        Metadata:
          creation_time   : 2014-08-17 18:30:31
        Duration: 01:01:22.52, start: 0.000000, bitrate: 202001 kb/s
          Stream #0:0(eng): Video: prores (apch / 0x68637061), yuv422p10le, 1920x1080, 183513 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 2500 tbn, 2500 tbc (default)
          Metadata:
            creation_time   : 2014-08-17 18:30:31
            handler_name    : Apple Alias Data Handler
            encoder         : Apple ProRes 422 (HQ)
          Stream #0:1(eng): Audio: pcm_s24le (lpcm / 0x6D63706C), 48000 Hz, 16 channels, s32, 18432 kb/s (default)
          Metadata:
            creation_time   : 2014-08-17 18:30:31
            handler_name    : Apple Alias Data Handler
      [auto-inserted resampler 0 @ 0x3943da0] [SWR @ 0x39441a0] Rematrix is needed between 16 channels and 5.1 but there is not enough information to do it
      [auto-inserted resampler 0 @ 0x3943da0] Failed to configure output pad on auto-inserted resampler 0
      Error opening filters!
  </code></pre>
  
  <p>This is the FFprobe output:</p>
  
  <pre><code>ffprobe version 2.2.git Copyright (c) 2007-2014 the FFmpeg developers
    built on May  8 2014 14:17:45 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
    configuration: --prefix=/usr/local/ffmpeg_build --extra-cflags=-I/usr/local/ffmpeg_build/include --extra-ldflags=-L/usr/local/ffmpeg_build/lib --bindir=/usr/local/bin --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-nonfree --enable-x11grab --enable-libxvid --enable-libfaac
    libavutil      52. 81.100 / 52. 81.100
    libavcodec     55. 60.103 / 55. 60.103
    libavformat    55. 37.102 / 55. 37.102
    libavdevice    55. 13.101 / 55. 13.101
    libavfilter     4.  5.100 /  4.  5.100
    libswscale      2.  6.100 /  2.  6.100
    libswresample   0. 18.100 /  0. 18.100
    libpostproc    52.  3.100 / 52.  3.100
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Capture0000.mov':
    Metadata:
      creation_time   : 2014-08-17 18:30:31
    Duration: 01:01:22.52, start: 0.000000, bitrate: 202001 kb/s
      Stream #0:0(eng): Video: prores (apch / 0x68637061), yuv422p10le, 1920x1080, 183513 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 2500 tbn, 2500 tbc (default)
      Metadata:
        creation_time   : 2014-08-17 18:30:31
        handler_name    : Apple Alias Data Handler
        encoder         : Apple ProRes 422 (HQ)
      Stream #0:1(eng): Audio: pcm_s24le (lpcm / 0x6D63706C), 48000 Hz, 16 channels, s32, 18432 kb/s (default)
      Metadata:
        creation_time   : 2014-08-17 18:30:31
        handler_name    : Apple Alias Data Handler
  </code></pre>