[FFmpeg-trac] #11503(avcodec:new): AC-3 downmix levels defaulting to 1.414 with recent decoder changes

[FFmpeg]

#11503: AC-3 downmix levels defaulting to 1.414 with recent decoder changes
-------------------------------------+-------------------------------------
             Reporter:  Wallboy      |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avcodec      |                  Version:  git-
             Keywords:  ac3          |  master
  resampling downmix                 |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 **Summary of the bug:**

 With the recent commits to AC-3 decoder, I've noticed the default mixing
 levels (when no metadata present) have changed from 0.707 to 1.414 for
 both clev and slev, respectively.

 The ATSC A/52 spec does have these 1.414 values listed in the Alternate
 Bit Stream Syntax, but only for the center channel.

 **How to reproduce:**

 Run a command similar to the following on the latest master branch with a
 5.1 channel AC-3 file (here using AAC codec so we get fltp sample format,
 so the rematrixing code doesn't normalize the mixing levels):

 {{{
 ffmpeg -hide_banner -loglevel debug -i someac3file -vn -c:a aac -frames:a
 1 -ac 2 -f null /dev/null
 }}}

 Relevant debug output:

 {{{
 [auto_aresample_0 @ 000001c6d9787100] Mix levels: center 1.414214 -
 surround 1.414214 - lfe 0.000000.
 [auto_aresample_0 @ 000001c6d9787100] [SWR @ 000001c6d9787fc0] Using fltp
 internally between filters
 [auto_aresample_0 @ 000001c6d9787100] [SWR @ 000001c6d9787fc0] Matrix
 coefficients:
 [auto_aresample_0 @ 000001c6d9787100] [SWR @ 000001c6d9787fc0] FL:
 FL:1.000000 FR:0.000000 FC:1.414214 LFE:0.000000 SL:1.414214 SR:0.000000
 [auto_aresample_0 @ 000001c6d9787100] [SWR @ 000001c6d9787fc0] FR:
 FL:0.000000 FR:1.000000 FC:1.414214 LFE:0.000000 SL:0.000000 SR:1.414214
 }}}

 Run again on a release version of ffmpeg such as 7.1:

 {{{
 [auto_aresample_0 @ 00000228732b7440] [SWR @ 00000228734caf80] Matrix
 coefficients:
 [auto_aresample_0 @ 00000228732b7440] [SWR @ 00000228734caf80] FL:
 FL:1.000000 FR:0.000000 FC:0.707107 LFE:0.000000 SL:0.707107 SR:0.000000
 [auto_aresample_0 @ 00000228732b7440] [SWR @ 00000228734caf80] FR:
 FL:0.000000 FR:1.000000 FC:0.707107 LFE:0.000000 SL:0.000000 SR:0.707107
 }}}

 which I believe these are the expected defaults if the metadata doesn't
 explicitly specify otherwise.

 If this change was intentional, feel free to close this ticket.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11503>
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