Channel reordering in pcm files

[Carl Eugen Hoyos]

I will attach a three channel pcm caf file. The file contains left, center and right stream, FFmpeg currently warns that the channel layout is unsupported. It is simple to set the correct channel layout (trivial patch attached), but the problem is that the file has channel order "center, left, right" (MPEG-3-0-B), so that FFmpeg maps the left channel to the right, the right channel to center and the center channel to the left channel on decoding.

How can a demuxer tell the pcm decoder that the decoder has to reorder channels?

ffmpeg -i MPEG30B.caf
FFmpeg version git-N-29301-gab15726, Copyright (c) 2000-2011 the FFmpeg developers
  built on Apr 23 2011 10:48:07 with gcc 4.5.2
  configuration: --cc=/usr/local/gcc-4.5.2/bin/gcc --enable-gpl --enable-libcelt
  libavutil    51.  0. 0 / 51.  0. 0
  libavcodec   53.  1. 0 / 53.  1. 0
  libavformat  53.  0. 1 / 53.  0. 1
  libavdevice  53.  0. 0 / 53.  0. 0
  libavfilter   2.  0. 0 /  2.  0. 0
  libswscale    0. 13. 0 /  0. 13. 0
[caf @ 0x12a0ae0] Unknown channel layout.
[caf @ 0x12a0ae0] Estimating duration from bitrate, this may be inaccurate
Input #0, caf, from 'MPEG30B.caf':
  Duration: 00:00:10.66, start: 0.000000, bitrate: 1151 kb/s
    Stream #0.0: Audio: pcm_s8, 48000 Hz, 3 channels, u8, 1152 kb/s
At least one output file must be specified

[Carl Eugen Hoyos]

This is now a bug because FFmpeg incorrectly reports "3.0" instead of "3 channels, unknown layout".

$ ffmpeg -i MPEG30B.caf
ffmpeg version N-44712-g7200fec Copyright (c) 2000-2012 the FFmpeg developers
  built on Sep 23 2012 21:08:48 with gcc 4.5.3 (GCC)
  configuration: --cc=/usr/local/gcc-4.5.3/bin/gcc --enable-gpl
  libavutil      51. 73.101 / 51. 73.101
  libavcodec     54. 58.100 / 54. 58.100
  libavformat    54. 28.100 / 54. 28.100
  libavdevice    54.  2.101 / 54.  2.101
  libavfilter     3. 17.100 /  3. 17.100
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
[caf @ 0x14f4240] Estimating duration from bitrate, this may be inaccurate
Input #0, caf, from 'MPEG30B.caf':
  Duration: 00:00:10.66, start: 0.000000, bitrate: 1151 kb/s
    Stream #0:0: Audio: pcm_s8 (lpcm / 0x6D63706C), 48000 Hz, 3.0, u8, 1152 kb/s
At least one output file must be specified

I sent a patch last year to fix this ticket:
​http://thread.gmane.org/gmane.comp.video.ffmpeg.devel/132789

[Michael Niedermayer]

more possible solutions: (not saying they are better or not than what was suggested)
the demxuer could just reorder chanels
the demuxer could pass the channel order through extradata or sidedata to the decoder

[Elon Musk]

Proper way is having more generic channels layout representation.

[Balling]

Is not that like this done here ​https://github.com/FFmpeg/FFmpeg/blob/a0ac49e38ee1d1011c394d7be67d0f08b2281526/libavformat/mov_chan.c#L177 and
maybe it should be here ​https://github.com/FFmpeg/FFmpeg/blob/904ab5365cf4881eae1e5cac980f3c4f252b69cd/libavformat/isom.c#L711 after 91b782720fd0df5571775b6591bc41797d6ecf78

[Marton Balint]

Fixed in e6c2c8703732bc46395c65c530038c8146df0deb.