FFMpeg and channel layout

[alexander121]

Hello,
can somebody tell me please, for which audio format the -channel_layout parameter of FFMPEG is supported or how to find out ?

At this webpage :

http://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/AVCodecContext_AVOptions#-channel_layout

it says :

-channel_layout

FFMPEG: Encode, Decode, Audio
Full Name:
Variables: Integer 64bit
Codecs: all
Web sites:
Description:

why do you think many audio converters don't have channel layout as parameter parameter ?

[JVRaines]

Channel_layout is used by some filters but not by any codecs or formats (muxer/demuxers). Go to the filters documentation and search for all the filters that implement it.

[alexander121]

It seems to be used when i encode an AAC audio file.

Check out the following :

c:\1>ffmpeg -i natgeo.mp4 -vn -strict -2 -c:a aac -channel_layout SR a.aac
ffmpeg version N-68625-g1728659 Copyright (c) 2000-2014 the FFmpeg developers
  built on Dec 19 2014 22:02:36 with gcc 4.9.2 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-lib
modplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrw
b --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinge
r --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --en
able-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis
 --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
libx265 --enable-libxavs --enable-libxvid --enable-decklink --enable-zlib
  libavutil      54. 15.100 / 54. 15.100
  libavcodec     56. 16.100 / 56. 16.100
  libavformat    56. 16.101 / 56. 16.101
  libavdevice    56.  3.100 / 56.  3.100
  libavfilter     5.  4.100 /  5.  4.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'natgeo.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 1
    compatible_brands: isom
    creation_time   : 2012-07-09 11:58:52
  Duration: 00:01:00.29, start: 0.000000, bitrate: 1932 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/
unknown/unknown), 720x406, 1800 kb/s, 25 fps, 25 tbr, 25k tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2012-07-08 08:37:43
      handler_name    : Video
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, flt
p, 128 kb/s (default)
    Metadata:
      creation_time   : 2012-07-08 08:37:49
      handler_name    : GPAC ISO Audio Handler
File 'a.aac' already exists. Overwrite ? [y/N] y
Output #0, adts, to 'a.aac':
  Metadata:
    major_brand     : isom
    minor_version   : 1
    compatible_brands: isom
    encoder         : Lavf56.16.101
    Stream #0:0(eng): Audio: aac, 48000 Hz, 1 channels (SR), fltp, 128 kb/s (def
ault)
    Metadata:
      creation_time   : 2012-07-08 08:37:49
      handler_name    : GPAC ISO Audio Handler
      encoder         : Lavc56.16.100 aac
Stream mapping:
  Stream #0:1 -> #0:0 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
size=     964kB time=00:01:00.28 bitrate= 131.0kbits/s
video:0kB audio:944kB subtitle:0kB other streams:0kB global headers:0kB muxing o
verhead: 2.046242%

[JVRaines]

It's not documented at http://www.ffmpeg.org/ffmpeg-codecs.html#aac. Channel_layout is listed as a global codec option but without any possible values. If the docs are incomplete, I guess you'll have to search the source code.

[Selur]

If the docs are incomplete, I guess you'll have to search the source code.

Channel names:

{
     [0] = { "FL",        "front left"            },
     [1] = { "FR",        "front right"           },
     [2] = { "FC",        "front center"          },
     [3] = { "LFE",       "low frequency"         },
     [4] = { "BL",        "back left"             },
     [5] = { "BR",        "back right"            },
     [6] = { "FLC",       "front left-of-center"  },
     [7] = { "FRC",       "front right-of-center" },
     [8] = { "BC",        "back center"           },
     [9] = { "SL",        "side left"             },
    [10] = { "SR",        "side right"            },
    [11] = { "TC",        "top center"            },
    [12] = { "TFL",       "top front left"        },
    [13] = { "TFC",       "top front center"      },
    [14] = { "TFR",       "top front right"       },
    [15] = { "TBL",       "top back left"         },
    [16] = { "TBC",       "top back center"       },
    [17] = { "TBR",       "top back right"        },
    [29] = { "DL",        "downmix left"          },
    [30] = { "DR",        "downmix right"         },
    [31] = { "WL",        "wide left"             },
    [32] = { "WR",        "wide right"            },
    [33] = { "SDL",       "surround direct left"  },
    [34] = { "SDR",       "surround direct right" },
    [35] = { "LFE2",      "low frequency 2"       },
};

source: http://www.ffmpeg.org/doxygen/2.0/channel__layout_8c.html#a455b3328b02048db7255da22bea6de6d
Channel layout names:

static const struct {
    const char *name;
    int         nb_channels;
    uint64_t     layout;
} channel_layout_map[] = {
    { "mono",        1,  AV_CH_LAYOUT_MONO },
    { "stereo",      2,  AV_CH_LAYOUT_STEREO },
    { "2.1",         3,  AV_CH_LAYOUT_2POINT1 },
    { "3.0",         3,  AV_CH_LAYOUT_SURROUND },
    { "3.0(back)",   3,  AV_CH_LAYOUT_2_1 },
    { "4.0",         4,  AV_CH_LAYOUT_4POINT0 },
    { "quad",        4,  AV_CH_LAYOUT_QUAD },
    { "quad(side)",  4,  AV_CH_LAYOUT_2_2 },
    { "3.1",         4,  AV_CH_LAYOUT_3POINT1 },
    { "5.0",         5,  AV_CH_LAYOUT_5POINT0_BACK },
    { "5.0(side)",   5,  AV_CH_LAYOUT_5POINT0 },
    { "4.1",         5,  AV_CH_LAYOUT_4POINT1 },
    { "5.1",         6,  AV_CH_LAYOUT_5POINT1_BACK },
    { "5.1(side)",   6,  AV_CH_LAYOUT_5POINT1 },
    { "6.0",         6,  AV_CH_LAYOUT_6POINT0 },
    { "6.0(front)",  6,  AV_CH_LAYOUT_6POINT0_FRONT },
    { "hexagonal",   6,  AV_CH_LAYOUT_HEXAGONAL },
    { "6.1",         7,  AV_CH_LAYOUT_6POINT1 },
    { "6.1",         7,  AV_CH_LAYOUT_6POINT1_BACK },
    { "6.1(front)",  7,  AV_CH_LAYOUT_6POINT1_FRONT },
    { "7.0",         7,  AV_CH_LAYOUT_7POINT0 },
    { "7.0(front)",  7,  AV_CH_LAYOUT_7POINT0_FRONT },
    { "7.1",         8,  AV_CH_LAYOUT_7POINT1 },
    { "7.1(wide)",   8,  AV_CH_LAYOUT_7POINT1_WIDE_BACK },
    { "7.1(wide-side)",   8,  AV_CH_LAYOUT_7POINT1_WIDE },
    { "octagonal",   8,  AV_CH_LAYOUT_OCTAGONAL },
    { "downmix",     2,  AV_CH_LAYOUT_STEREO_DOWNMIX, },
};

source: http://www.ffmpeg.org/doxygen/2.0/channel__layout_8c.html#a70c71aa3e5d98eefb119900642b2cddb

why do you think many audio converters don't have channel layout as parameter parameter ?

Would be just too confusing, since the user would need to know the channel layout of the input and output formats.
Some tools allow remixing (changing the channel composition), which basically allows you to specific the channel layout you want for the output, but still requires you to know the channel layout of the input and output formats, which can get messy for when you use 6 or more channels,...