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

[Joshua Tidsbury]

You should probably explicitly define the channel layout. Here are some notes from an internal app of mine that should assist in translating the various Dolby terminology into ffmpeg terminology:

ac3_center_dmix (-center_mixlev -ltrt_cmixlev -loro_cmixlev; float)
ac3_surround_dmix (-surround_mixlev -ltrt_surmixlev -loro_surmixlev; float)
ac3_copyright_bit (-copyright; 0 or 1)
ac3_dialnorm (-dialnorm; negative int)
ac3_original_stream (-original; 0 or 1)
ac3_dmix_mode (-dmix_mode; 0,1,2)

ac3_channel_l = 1
ac3_channel_r = 2
ac3_channel_c = 4
ac3_channel_lfe = 8
ac3_channel_ls = 16
ac3_channel_rs = 32
ac3_channel_lsb = 512
ac3_channel_rsb = 1024

-channel_layout: 1_0 = 4, 2_0 = 3, 2_1 = 11, 3_0 = 7, 3_1 = 15, 5_0 = 55, 5_1 = 63, 7_0 = 1591, 7_1 = 1599

And an example for converting a poly bwav into a fully formatted ac3 with metadata:

ffmpeg -i T-Vox\ Channel\ Ident.WAV -vn -acodec ac3 -ac 6 -ar 48000 -ab 448k -dialnorm -24 -dsur_mode 0 -original 1 -dmix_mode 2 -channel_layout 63 result.ac3

Hope that helps,
Josh


--------------
Josh Tidsbury | CTV
m: 416.433.3968

On 2012-05-26, at 10:13 AM, "Chase Patterson" <chapatt at gmail.com> wrote:

> 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
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