Opened 6 years ago

Last modified 3 years ago

#7384 reopened defect

FFmpeg 4.0 does not set 5.1 surround sound into audio stream when converting mkv video with 5.1 surround sound

Reported by: itrdev Owned by:
Priority: important Component: avcodec
Version: git-master Keywords: aac regression
Cc: Blocked By:
Blocking: Reproduced by developer: no
Analyzed by developer: no

Description

Summary of the bug:

I upgraded ffmpeg to 4.0 version and found and issue that was not present in 3.4.2 version. When I convert MKV video with 5.1 surround sound audio ffmpeg gives an mp4 with audio that has channel_layout=unknown. As a result Windows and IE/Edge/Firefox browsers cannot play audio.
FFpmeg 3.4.2 gives a converted video with channel_layout=5.1. And audio is played without errors.

How to reproduce:

% ffmpeg.exe -noautorotate -i MKV.mkv -c:v libx264 -crf 31 -preset veryfast -y Mkv.mp4
ffmpeg version 4.0.2 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 7.3.1 (GCC) 20180722
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
  libavutil      56. 14.100 / 56. 14.100
  libavcodec     58. 18.100 / 58. 18.100
  libavformat    58. 12.100 / 58. 12.100
  libavdevice    58.  3.100 / 58.  3.100
  libavfilter     7. 16.100 /  7. 16.100
  libswscale      5.  1.100 /  5.  1.100
  libswresample   3.  1.100 /  3.  1.100
  libpostproc    55.  1.100 / 55.  1.100
[aac @ 000001229524e580] This stream seems to incorrectly report its last channel as SCE[1], mapping to LFE[0]
Input #0, matroska,webm, from 'MKV.mkv':
  Metadata:
    encoder         : libebml v0.7.5 + libmatroska v0.7.7
    creation_time   : 2005-08-22T17:01:27.000000Z
  Duration: 00:00:30.85, start: 0.000000, bitrate: 1128 kb/s
    Stream #0:0: Video: mpeg4 (Simple Profile) (XVID / 0x44495658), yuv420p, 360x240 [SAR 3:2 DAR 9:4], SAR 1:1 DAR 3:2, 15 fps, 15 tbr, 1k tbn, 15 tbc (default)
    Metadata:
      title           : XviD Video Stream
    Stream #0:1(eng): Audio: ac3, 48000 Hz, 5.1(side), fltp, 192 kb/s (default)
    Metadata:
      title           : AC3 5.1 Test Audio
    Stream #0:2(eng): Audio: aac (HE-AAC), 48000 Hz, 5.1, fltp
    Metadata:
      title           : AAC 5.1 Test Audio
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg4 (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (ac3 (native) -> aac (native))
Press [q] to stop, [?] for help
[aac @ 0000012295300e40] Using a PCE to encode channel layout
[mpeg4 @ 0000012295303b80] Video uses a non-standard and wasteful way to store B-frames ('packed B-frames'). Consider using the mpeg4_unpack_bframes bitstream filter without encoding but stream copy to fix it.
[libx264 @ 00000122956600c0] using SAR=1/1
[libx264 @ 00000122956600c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 00000122956600c0] profile High, level 1.2
[libx264 @ 00000122956600c0] 264 - core 155 r2901 7d0ff22 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=1 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=2 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=0 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=1 keyint=250 keyint_min=15 scenecut=40 intra_refresh=0 rc_lookahead=10 rc=crf mbtree=1 crf=31.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'Mkv.mp4':
  Metadata:
    encoder         : Lavf58.12.100
    Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 360x240 [SAR 1:1 DAR 3:2], q=-1--1, 15 fps, 15360 tbn, 15 tbc (default)
    Metadata:
      title           : XviD Video Stream
      encoder         : Lavc58.18.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1(side), fltp, 394 kb/s (default)
    Metadata:
      title           : AC3 5.1 Test Audio
      encoder         : Lavc58.18.100 aac
frame=  462 fps=136 q=-1.0 Lsize=    1658kB time=00:00:30.72 bitrate= 442.0kbits/s dup=1 drop=0 speed=9.03x
video:343kB audio:1298kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.030490%
[libx264 @ 00000122956600c0] frame I:14    Avg QP:26.16  size:  1752
[libx264 @ 00000122956600c0] frame P:179   Avg QP:28.83  size:  1026
[libx264 @ 00000122956600c0] frame B:269   Avg QP:30.51  size:   530
[libx264 @ 00000122956600c0] consecutive B-frames: 17.5% 10.8% 11.0% 60.6%
[libx264 @ 00000122956600c0] mb I  I16..4: 32.5% 56.3% 11.2%
[libx264 @ 00000122956600c0] mb P  I16..4: 17.6% 24.2%  2.5%  P16..4: 11.3%  5.7%  2.6%  0.0%  0.0%    skip:36.0%
[libx264 @ 00000122956600c0] mb B  I16..4:  2.0%  4.9%  0.3%  B16..8: 13.6%  6.4%  0.8%  direct: 3.3%  skip:68.7%  L0:40.2% L1:42.7% BI:17.1%
[libx264 @ 00000122956600c0] 8x8 transform intra:57.2% inter:34.3%
[libx264 @ 00000122956600c0] coded y,uvDC,uvAC intra: 29.5% 18.8% 2.1% inter: 4.5% 5.9% 0.1%
[libx264 @ 00000122956600c0] i16 v,h,dc,p: 38% 34%  9% 19%
[libx264 @ 00000122956600c0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 15% 39% 28%  3%  2%  3%  3%  4%  3%
[libx264 @ 00000122956600c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 22% 23% 17%  7%  6%  6%  6%  7%  5%
[libx264 @ 00000122956600c0] i8c dc,h,v,p: 67% 23%  8%  2%
[libx264 @ 00000122956600c0] Weighted P-Frames: Y:7.8% UV:4.5%
[libx264 @ 00000122956600c0] kb/s:91.08
[aac @ 0000012295300e40] Qavg: 31339.592

FFprobe output for ffpmeg 4.0

% ffprobe -show_streams MKV.mp4
ffprobe version 4.0.2 Copyright (c) 2007-2018 the FFmpeg developers
  built with gcc 7.3.1 (GCC) 20180722
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
  libavutil      56. 14.100 / 56. 14.100
  libavcodec     58. 18.100 / 58. 18.100
  libavformat    58. 12.100 / 58. 12.100
  libavdevice    58.  3.100 / 58.  3.100
  libavfilter     7. 16.100 /  7. 16.100
  libswscale      5.  1.100 /  5.  1.100
  libswresample   3.  1.100 /  3.  1.100
  libpostproc    55.  1.100 / 55.  1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'MKV.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.12.100
  Duration: 00:00:30.80, start: 0.000000, bitrate: 440 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 360x240 [SAR 1:1 DAR 3:2], 91 kb/s, 15 fps, 15 tbr, 15360 tbn, 30 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 6 channels, fltp, 345 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[STREAM]
index=0
codec_name=h264
codec_long_name=H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10
profile=High
codec_type=video
codec_time_base=1/30
codec_tag_string=avc1
codec_tag=0x31637661
width=360
height=240
coded_width=368
coded_height=240
has_b_frames=2
sample_aspect_ratio=1:1
display_aspect_ratio=3:2
pix_fmt=yuv420p
level=12
color_range=unknown
color_space=unknown
color_transfer=unknown
color_primaries=unknown
chroma_location=left
field_order=unknown
timecode=N/A
refs=1
is_avc=true
nal_length_size=4
id=N/A
r_frame_rate=15/1
avg_frame_rate=15/1
time_base=1/15360
start_pts=0
start_time=0.000000
duration_ts=473088
duration=30.800000
bit_rate=91261
max_bit_rate=N/A
bits_per_raw_sample=8
nb_frames=462
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=1
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
DISPOSITION:timed_thumbnails=0
TAG:language=und
TAG:handler_name=VideoHandler
[/STREAM]
[STREAM]
index=1
codec_name=aac
codec_long_name=AAC (Advanced Audio Coding)
profile=LC
codec_type=audio
codec_time_base=1/48000
codec_tag_string=mp4a
codec_tag=0x6134706d
sample_fmt=fltp
sample_rate=48000
channels=6
channel_layout=unknown
bits_per_sample=0
id=N/A
r_frame_rate=0/0
avg_frame_rate=0/0
time_base=1/48000
start_pts=0
start_time=0.000000
duration_ts=1474560
duration=30.720000
bit_rate=345780
max_bit_rate=394000
bits_per_raw_sample=N/A
nb_frames=1441
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=1
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
DISPOSITION:timed_thumbnails=0
TAG:language=eng
TAG:handler_name=SoundHandler
[/STREAM]

FFprobe output for ffpmeg 3.4.2

% ffprobe -show_streams MKV.mp4
ffprobe version 4.0.2 Copyright (c) 2007-2018 the FFmpeg developers
  built with gcc 7.3.1 (GCC) 20180722
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
  libavutil      56. 14.100 / 56. 14.100
  libavcodec     58. 18.100 / 58. 18.100
  libavformat    58. 12.100 / 58. 12.100
  libavdevice    58.  3.100 / 58.  3.100
  libavfilter     7. 16.100 /  7. 16.100
  libswscale      5.  1.100 /  5.  1.100
  libswresample   3.  1.100 /  3.  1.100
  libpostproc    55.  1.100 / 55.  1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'MKV.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.83.100
  Duration: 00:00:30.80, start: 0.000000, bitrate: 436 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 360x240 [SAR 1:1 DAR 3:2], 91 kb/s, 15 fps, 15 tbr, 15360 tbn, 30 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 341 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[STREAM]
index=0
codec_name=h264
codec_long_name=H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10
profile=High
codec_type=video
codec_time_base=1/30
codec_tag_string=avc1
codec_tag=0x31637661
width=360
height=240
coded_width=368
coded_height=240
has_b_frames=2
sample_aspect_ratio=1:1
display_aspect_ratio=3:2
pix_fmt=yuv420p
level=12
color_range=unknown
color_space=unknown
color_transfer=unknown
color_primaries=unknown
chroma_location=left
field_order=unknown
timecode=N/A
refs=1
is_avc=true
nal_length_size=4
id=N/A
r_frame_rate=15/1
avg_frame_rate=15/1
time_base=1/15360
start_pts=0
start_time=0.000000
duration_ts=473088
duration=30.800000
bit_rate=91261
max_bit_rate=N/A
bits_per_raw_sample=8
nb_frames=462
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=1
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
DISPOSITION:timed_thumbnails=0
TAG:language=und
TAG:handler_name=VideoHandler
[/STREAM]
[STREAM]
index=1
codec_name=aac
codec_long_name=AAC (Advanced Audio Coding)
profile=LC
codec_type=audio
codec_time_base=1/48000
codec_tag_string=mp4a
codec_tag=0x6134706d
sample_fmt=fltp
sample_rate=48000
channels=6
channel_layout=5.1
bits_per_sample=0
id=N/A
r_frame_rate=0/0
avg_frame_rate=0/0
time_base=1/48000
start_pts=0
start_time=0.000000
duration_ts=1474560
duration=30.720000
bit_rate=341279
max_bit_rate=341279
bits_per_raw_sample=N/A
nb_frames=1441
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=1
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
DISPOSITION:timed_thumbnails=0
TAG:language=eng
TAG:handler_name=SoundHandler
[/STREAM]

It looks like a defect because it worked in 3.4.2 version. I'm waiting for response. Thanks

Change History (14)

comment:2 by James, 6 years ago

Component: ffmpeg → avcodec
Keywords: aac added; ffmpeg 5.1 surround audio removed
Priority: normal → important
Status: new → open
Version: unspecified → git-master

Seems to be a regression since fc9dcfe7d50d7f1b38fb287b4d92b5f3fc4bfb05

comment:3 by Rostislav Pehlivanov, 6 years ago

Resolution: → invalid
Status: open → closed

The issue is that the file signals a layout of 5.1(side), but the AAC spec only supports 5.1(rear channels instead of side). Hence, to express that the encoder uses a not-so-universally supported feature to signal the non-standard layout. Firefox 62.0 here supports decoding it just fine, so maybe whatever you're using is old.
You can convert between 5.1(side) and 5.1(rear) by specifying -channel_layout "5.1", which will make the encoder use the standard well understood path.

Last edited 6 years ago by Rostislav Pehlivanov (previous) (diff)

comment:4 by itrdev, 6 years ago

But why it's working fine in ffmpeg 3.4.2? Were there any breaking changes in ffmpeg 4.0 connected to this?

comment:5 by itrdev, 6 years ago

Last edited 6 years ago by itrdev (previous) (diff)

comment:6 by itrdev, 6 years ago

Last edited 6 years ago by itrdev (previous) (diff)

comment:7 by itrdev, 6 years ago

Additional question: what should I do with ffmpeg 4 if input can contain videos with different channels layouts? In 3.4.2 ffmpeg desided what to do but in 4.0 this issue exists.

comment:8 by itrdev, 6 years ago

Resolution: invalid
Status: closed → reopened

I reopened this issue because maybe closed issues are not tracked for additional response. Thanks!

comment:9 by itrdev, 6 years ago

My general question is how I should convert videos with different channels layouts to save theirs layouts with ffmpeg 4.

comment:10 by pkv, 5 years ago

The default channel layouts supported by aac encoder are:
mono, stereo, 3.0, 4.0, 5.0 and 5.1
(FC, FL+FR, FL+FR+FC, FL+FR+FC+BC, FL+FR+FC+BL+BR, FL+FR+FC+BL+BR+LFE).
If one of your layouts is one of those, they will be encoded as before.
If your layout is not one of them but is close, for instance 5.1(side), you can do as atomnuker said:
ffmpeg -channel_layout 5.1 -i input etc...


comment:11 by pkv, 5 years ago

Resolution: → invalid
Status: reopened → closed

comment:12 by Carl Eugen Hoyos, 5 years ago

Keywords: regression added
Resolution: invalid
Status: closed → reopened

The additional option should of course not be required, even more so if this is a regression.

in reply to:  3 comment:13 by Balling, 3 years ago

Replying to Rostislav Pehlivanov:

The issue is that the file signals a layout of 5.1(side), but the AAC spec only supports 5.1(rear channels instead of side).

Yes, but looks like 7.1 does support side? See ​https://git.1f0.de/gitweb/?p=ffmpeg.git;a=commit;h=71d762aaa1d96c1a6c4cc83b5a0c418b53028efe;js=1

comment:14 by YetAnotherID, 3 years ago

I've only just bumped into this issue, and found quite a few closed tickets on the subject, but as this ticket is active recently I thought I'd add my 2 cents worth here.
Sorry if I'm getting any of this wrong, but I can't imagine how it's not the problem.

I've always understood that for AAC using a centre channel, the golden rule is to make the first element a single channel element, and to end with a single channel element, if it exists. I think the LFE channel is a specially labelled SCE.
In other words, the channel order begins with the centre channel, then the stereo channels are encoded in pairs from front to back, and the final SCE is the back centre channel (if it exists). Whether the surround channels are "back" or "side" in wave file channel order shouldn't matter. They're encoded as surround channels. If there's only a pair of them it's 5.1ch audio, if there's two pairs it's 7.1ch.

Best as I can tell, ffmpeg is storing the AAC elements in wave file channel order, rather then the AAC order of FC, FL, FR, Ls, Rs, LFE.
​https://wiki.multimedia.cx/index.php/MPEG-4_Audio#Channel_Configurations

For AAC there's "num_front_channel_elements" for specifying the number of front speakers, but the specification seems fairly determined about the order of the elements.

​https://web.archive.org/web/20110713115817/http://jongyeob.com/moniwiki/pds/upload/13818-7.pdf

8.5.1 Data Elements
num_front_channel_elements
The number of audio syntactic elements in the front channels, front center to back center, symmetrically by left and right, or alternating by left and right in the case of single channel elements (Table 25).

This is the other info I dug out of an AAC stream with MediaInfo, so in the order they appear to be stored (ffmpeg encoded).

channel_configuration: 0 (0x0) - (4 bits) -
num_front_channel_elements: 2 (0x2) - (4 bits) - Front: FL FR FC
num_side_channel_elements: 1 (0x1) - (4 bits) - LFE becomes Side: C
num_back_channel_elements: 1 (0x1) - (4 bits) - Back: L R
num_lfe_channel_elements: 0 (0x0) - (2 bits)

The worst part is, I don't think you need to use a PCE for 5.1ch audio if you follow the element order rules. Even if you don't, it'd probably be assumed to be the front centre channel if there's only one SCE.

8.5.2.3 Implicit channel mapping
1) Any number of SCE's may appear (as long as permitted by other constraints, for example profile). If this number of SCE's is odd, then the first SCE represents the front center channel, and the other SCE's represent L/R pairs of channels, proceeding from center front outwards and back to center rear.
If the number of SCE's is even, then the SCE's are assigned as pairs as center-front L/R, in pairs proceeding out and back from center front toward center back.

Or you can use Channel_Configuration to specify a channel layout, and not bother specifying the number of front or side channels etc, although from what I can tell, it has to be used with the elements in the correct order. That's all QAAC seems to do.

channel_configuration: 6 (0x6) - (4 bits) - Front: L C R, Side: L R, LFE

Unfortunately the implicit channel order for 7.1ch audio is the cinema layout (where there's extra stereo channels in the front rather than extra surround channels), and because ffmpeg has encoded 7.1ch as 7.1ch (front), then decoded it as 7.1ch (wide), and QAAC began using PCE's for 7.1ch a long time ago, I've not kept up with any changes to the way ffmpeg does it, so I've no idea if it's using PCE's now, or doing so correctly, but it seems to me this is something that it should be doing according to the spec.

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