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

[Chase Patterson]

So, the command I figured out (ffmpeg -i Session.flac -s:a 48k -ab
640k -acodec ac3 -ac 6 Session.ac3) works, but even though it does in
fact have 6 channels, it plays in stereo. Am I missing an argument
that maps the channels correctly? I also tried converting from a wav
version of the file (that's also encoded in 5.1 surround, and plays as
such), but with the same result.

Does anyone have any idea what I'm missing?


Thanks,
Chase

On Thu, May 24, 2012 at 7:28 PM, Chase Patterson <chapatt at gmail.com> wrote:
> 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