[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      |                    Owner:  (none)
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:  avcodec
              Version:  git-master   |               Resolution:
             Keywords:  ac3          |               Blocked By:
  resampling downmix                 |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
Description changed by Wallboy:

Old description:

> **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.

New description:

 **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.

 ----

 **UPDATE:** Furthermore, if you manually specify `-clev` and `-slev`
 values, they are ignored in the latest git master:


 {{{
 [auto_aresample_0 @ 0000026f4df79680] [debug] Setting 'slev' to value
 '0.707'
 [auto_aresample_0 @ 0000026f4df79680] [debug] Setting 'clev' to value
 '0.707'
 ...
 [auto_aresample_0 @ 0000026f4df79680] [verbose] Mix levels: center
 1.414214 - surround 1.414214 - lfe 0.000000.
 [auto_aresample_0 @ 0000026f4df79680] [SWR @ 0000026f4e03c900] [debug]
 Using fltp internally between filters
 [auto_aresample_0 @ 0000026f4df79680] [SWR @ 0000026f4e03c900] [debug]
 Matrix coefficients:
 [auto_aresample_0 @ 0000026f4df79680] [SWR @ 0000026f4e03c900] [debug] FL:
 FL:1.000000 FR:0.000000 FC:1.414214 LFE:0.000000 SL:1.414214 SR:0.000000
 [auto_aresample_0 @ 0000026f4df79680] [SWR @ 0000026f4e03c900] [debug] FR:
 FL:0.000000 FR:1.000000 FC:1.414214 LFE:0.000000 SL:0.000000 SR:1.414214
 }}}

 Probably related.

--
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Ticket URL: <https://trac.ffmpeg.org/ticket/11503#comment:2>
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