[FFmpeg-user] AC-3 Dolby Digital 5.1 Encoding

[Chase Patterson]

Hi,

I'm mixing audio for a movie on Ardour (an audio editing software), which
can't export to AC3, but it can export multiple channels to other formats,
e.g. FLAC. So I have it encoded in FLAC with 5.1 surround sound. I'd like
to use ffmpeg to convert it to AC3 at 48kHz and 640kb/s with 5.1 surround
(Dolby Digital).

Here's what I have so far, but I'd like to change (or make sure it's
correct) the channel layout.

ffmpeg -i Session.flac -s:a 48k -ab 640k -acodec ac3 -ac 6 Session.ac3

Which returns

ffmpeg version 0.10.3 Copyright (c) 2000-2012 the FFmpeg developers
  built on May  9 2012 17:51:07 with gcc 4.7.0 20120505 (prerelease)
  configuration: --prefix=/usr --enable-libmp3lame --enable-libvorbis
--enable-libxvid --enable-libx264 --enable-libvpx --enable-libtheora
--enable-libgsm --enable-libspeex --enable-postproc --enable-shared
--enable-x11grab --enable-libopencore_amrnb --enable-libopencore_amrwb
--enable-libschroedinger --enable-libopenjpeg --enable-librtmp
--enable-libpulse --enable-gpl --enable-version3 --enable-runtime-cpudetect
--disable-debug --disable-static
  libavutil      51. 35.100 / 51. 35.100
  libavcodec     53. 61.100 / 53. 61.100
  libavformat    53. 32.100 / 53. 32.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 61.100 /  2. 61.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    52.  0.100 / 52.  0.100
[flac @ 0x1c003a0] max_analyze_duration 5000000 reached at 5016000
Input #0, flac, from 'Session.flac':
  Duration: 00:00:14.00, bitrate: 1010 kb/s
    Stream #0:0: Audio: flac, 48000 Hz, 5.1, s32
Incompatible sample format 's32' for codec 'ac3', auto-selecting format
'flt'
[ac3 @ 0x1c07920] channel_layout not specified
[ac3 @ 0x1c07920] No channel layout specified. The encoder will guess the
layout, but it might be incorrect.
Output #0, ac3, to 'test.ac3':
  Metadata:
    encoder         : Lavf53.32.100
    Stream #0:0: Audio: ac3, 48000 Hz, 5.1(side), flt, 640 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (flac -> ac3)
Press [q] to stop, [?] for help
size=    1095kB time=00:00:14.01 bitrate= 640.0kbits/s
video:0kB audio:1095kB global headers:0kB muxing overhead 0.000000%

That leads me to believe that it's using a channel layout of "5.1(side)"
(maybe assuming the surround speakers are on the sides, rather than behind,
as I want). Also, it automatically sets the sample format to "flt". Is this
what I want, or how can I change it?

Thank you!


Chase