[FFmpeg-trac] #3160(undetermined:new): ac3 silently chooses 5.1(side) channel layout

[FFmpeg]

#3160: ac3 silently chooses 5.1(side) channel layout
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             Reporter:  llogan       |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  minor        |                Component:
              Version:  git-master   |  undetermined
             Keywords:  ac3          |               Resolution:
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0
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Comment (by cehoyos):

 I (think I) understand the ticket but please note that the ac3 encoder
 does not "choose" {{{5.1(side)}}}, it encodes both {{{5.1}}} and
 {{{5.1(side)}}} as AC3_CHMODE_3F2R (3 front channels, two rear channels)
 which is the only 3-2 configuration supported by ac3 (as you showed) - lfe
 is always encoded independently. This makes sense for performance reasons
 but also because I am not sure if all FFmpeg multichannel audio decoders
 distinguish correctly between {{{5.1}}} and {{{5.1(side)}}}.

 I suggest that if you really believe such a warning makes sense, switch
 "case AV_CH_LAYOUT_5POINT0_BACK:" and "case AV_CH_LAYOUT_5POINT0:" in
 libavcodec/ac3enc.c (line 2087), add a warning in between (and commit) and
 send your patch to ffmpeg-devel.

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Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/3160#comment:1>
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