[FFmpeg-trac] #7862(undetermined:reopened): Default channel layout for 6 channels

[FFmpeg]

#7862: Default channel layout for 6 channels
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             Reporter:  lukasf  |                    Owner:
                 Type:  defect  |                   Status:  reopened
             Priority:  normal  |                Component:  undetermined
              Version:  4.1     |               Resolution:
             Keywords:          |               Blocked By:
             Blocking:          |  Reproduced by developer:  0
Analyzed by developer:  0       |
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Changes (by lukasf):

 * status:  closed => reopened
 * version:  unspecified => 4.1
 * resolution:  needs_more_info =>


Comment:

 Replying to [comment:1 cehoyos]:
 > Replying to [ticket:7862 lukasf]:
 > > Normally, 5.1 audio has side left/right channels, not back left/right.
 > Why do you think so?

 I have tested a lot of 5.1 files and all of them report side channels, not
 back channels. Surround back channels were only introduced with appearance
 of 7.1 standards. 7.1 AV receivers have surround back channels, 5.1
 receivers don't. The normal surround left/right channels from 5.1 times
 are interpreted as side left/right today (to map the 5.1 speakers into a
 generic 7.1-capable interface).

 > This ticket shows the reason why we always require a command line
 including a complete, uncut console output: There is an ugly regression
 iirc, but your explanation doesn't explain it.

 I use ffmpeg as library, I do not know how to reproduce the problem via
 command line. If I find a way, I will post it here. I used "n4.1.2" in my
 testing, but "n4.1.3" does not seem to have a change in that area.

 When looking at the code, it is easy to see why AV_CH_LAYOUT_5POINT1_BACK
 is used, because it is listed before AV_CH_LAYOUT_5POINT1 and the list is
 searched from beginning to end.

 Actually I think that it was even done by intention. But the question is,
 why has it been done like that, and shouldn't it be changed. When setting
 channel layout from the command line, channel layout "5.1" will use
 AV_CH_LAYOUT_5POINT1_BACK, only channel layout "5.1(side)" will use
 AV_CH_LAYOUT_5POINT1. So even from those command line values,
 AV_CH_LAYOUT_5POINT1_BACK is used as the "default" 5.1 layout, although it
 is a rather exotic configuration. And AV_CH_LAYOUT_5POINT1 is treated as
 special case, although it is the (IMHO) "normal" layout for a 5.1 file. In
 my opinion, it should be opposite: AV_CH_LAYOUT_5POINT1 should be the
 default and AV_CH_LAYOUT_5POINT1_BACK should be a special case, like
 "5.1(back)".

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Ticket URL: <https://trac.ffmpeg.org/ticket/7862#comment:2>
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