aac 6 channels -> ac3 6 channels fails (Back Centre, Cb not supported)

[Andres Rohr]

If I use the full build of current ​https://github.com/BtbN/FFmpeg-Builds/releases/tag/latest of 2023-02-18 (Windows version) to convert a video that has a 6 channels aac stream to an ac3 6 channels stream it says "Channel layout change is not supported" and "Not yet implemented". But if I use a nearly 2 year old build from ​https://github.com/mannyamorim/ffmpeg-windows-builds it can do it. I suggest to compare the 6 channel processing of the two releases and if possible incorporate the working code from the old release into the most recent build.

Output of ​https://github.com/BtbN/FFmpeg-Builds/releases/tag/latest of 2023-02-18

---

"C:\Program Files\ffmpeg-master-latest-win64-gpl 2023-02-18\bin\ffmpeg.exe" -hide_banner -y -i "D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example.mp4" -map 0:0 -map 0:1 -c:v:0 h264 -c:a:0 ac3 -b:v:0 2500k -pix_fmt:0 yuv420p -f mp4 "D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example (2).mp4" (20.02.2023 17:15:43)

Guessed Channel Layout for Input Stream #0.1 : 5.1
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example.mp4':

Metadata:

major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf60.2.100

Stream #0:0[0x1](und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], 2733 kb/s, 24 fps, 24 tbr, 12288 tbn (default)

Metadata:

handler_name : VideoHandler
vendor_id : [0][0][0][0]

Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 6 channels, fltp, 394 kb/s (default)

Metadata:

handler_name : SoundHandler
vendor_id : [0][0][0][0]

Stream mapping:

Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (aac (native) -> ac3 (native))

[Parsed_anull_0 @ 000000000305ee40] Channel layout change is not supported
Error while filtering: Not yet implemented in FFmpeg, patches welcome
Failed to inject frame into filter network: Not yet implemented in FFmpeg, patches welcome
Error while processing the decoded data for stream #0:1
Conversion failed!

---

Output of
​https://github.com/mannyamorim/ffmpeg-windows-builds (which redirects to ​https://sourceforge.net/projects/ffmpeg-windows-builds/files/release/) of 2021-04-27

"C:\Program Files\ffmpeg 4.4 alt\ffmpeg.exe" -hide_banner -y -i "D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example.mp4" -map 0:0 -map 0:1 -c:v:0 h264 -c:a:0 ac3 -b:v:0 2500k -f mp4 "D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example (7).mp4" (20.02.2023 19:03:21)

Guessed Channel Layout for Input Stream #0.1 : 5.1
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example.mp4':

Metadata:

major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf60.2.100

Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 2733 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc (default)

Metadata:

handler_name : VideoHandler
vendor_id : [0][0][0][0]

Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 394 kb/s (default)

Metadata:

handler_name : SoundHandler
vendor_id : [0][0][0][0]

Stream mapping:

Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (aac (native) -> ac3 (native))

[libx264 @ 000000000319e840] using SAR=1/1
[libx264 @ 000000000319e840] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000000000319e840] profile High, level 4.0, 4:2:0, 8-bit
[libx264 @ 000000000319e840] 264 - core 161 r3049M 55d517b - H.264/MPEG-4 AVC codec - Copyleft 2003-2021 - ​http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 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=2 keyint=250 keyint_min=24 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=abr mbtree=1 bitrate=2500 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'D:\Video Tests für MMX\Konversionsprobleme\6 channels\6 channel conversion problem example (7).mp4':

Metadata:

major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.76.100

Stream #0:0(und): Video: h264 (avc1 / 0x31637661), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 2500 kb/s, 24 fps, 12288 tbn (default)

Metadata:

handler_name : VideoHandler
vendor_id : [0][0][0][0]
encoder : Lavc58.134.100 libx264

Side data:

cpb: bitrate max/min/avg: 0/0/2500000 buffer size: 0 vbv_delay: N/A

Stream #0:1(eng): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, 5.1, fltp, 448 kb/s (default)

Metadata:

handler_name : SoundHandler
vendor_id : [0][0][0][0]
encoder : Lavc58.134.100 ac3

video:3303kB audio:548kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.195004%
[libx264 @ 000000000319e840] frame I:3 Avg QP:11.21 size: 78722
[libx264 @ 000000000319e840] frame P:67 Avg QP:13.44 size: 24313
[libx264 @ 000000000319e840] frame B:171 Avg QP:15.30 size: 8867
[libx264 @ 000000000319e840] consecutive B-frames: 1.2% 2.5% 29.9% 66.4%
[libx264 @ 000000000319e840] mb I I16..4: 33.1% 39.9% 27.1%
[libx264 @ 000000000319e840] mb P I16..4: 1.4% 3.2% 0.5% P16..4: 35.0% 9.1% 4.1% 0.0% 0.0% skip:46.8%
[libx264 @ 000000000319e840] mb B I16..4: 0.1% 0.3% 0.0% B16..8: 31.0% 2.4% 0.4% direct: 0.9% skip:64.8% L0:44.7% L1:53.2% BI: 2.1%
[libx264 @ 000000000319e840] final ratefactor: 17.93
[libx264 @ 000000000319e840] 8x8 transform intra:53.9% inter:79.6%
[libx264 @ 000000000319e840] coded y,uvDC,uvAC intra: 55.0% 71.4% 48.1% inter: 8.8% 9.3% 1.0%
[libx264 @ 000000000319e840] i16 v,h,dc,p: 45% 19% 10% 26%
[libx264 @ 000000000319e840] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 18% 20% 29% 5% 6% 7% 5% 6% 5%
[libx264 @ 000000000319e840] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 19% 14% 17% 12% 12% 8% 7% 7% 5%
[libx264 @ 000000000319e840] i8c dc,h,v,p: 58% 18% 16% 8%
[libx264 @ 000000000319e840] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 000000000319e840] ref P L0: 66.1% 4.1% 19.0% 10.8%
[libx264 @ 000000000319e840] ref B L0: 80.5% 15.6% 3.9%
[libx264 @ 000000000319e840] ref B L1: 96.6% 3.4%
[libx264 @ 000000000319e840] kb/s:2693.89

Success!

[Balling]

"aac 6 channels" Your little file has Cb channel in place of LFE. That channel is Back Centre. We do support this, but AC3 does not, so it needs convertion and we do not support that.

See this for clarity, last MediaInfo Name says that Cb means back center that is right behind your head perfectly. ​https://mediaarea.net/AudioChannelLayout

Older version just creates L R C Ls Rs file, i.e. 5.0 file.

[Balling]

so it needs convertion and we do not support that.

On the other hand what convertion can it be? Are we even allowed to add so different channel into other channels?

[Andres Rohr]

Thanks for the quick response.

I see. Tested that specifying the output option "-ac 5" drops the Back Centre and solves the problem. But the average user can't understand why the processing fails. He doesn't know which channels are inside. If there would be a clearer info that only one channel is making all the trouble and that reducing the channels with "-ac 5" would solve the problem, would be a nice addition.

[Balling]

Well, this is "fixed" now... Except it writes LFE channel now instead of Cb. Weird stuff.

So not really fixed, but broken...

-c:a:0 aac still says

Unsupported channel layout "6 channels"

This is more correct:

ffmpeg -i input.mp4 -c:v copy -af "pan=5.1|c0=c0|c1=c1|c2=c2|c3=c3|c4=c4+0.5*c6|c5=c5+0.5*c6" -c:a ac3 -b:a 640k output.mp4

c4=c4+0.5*c6: Maps the 5th input channel (Surround Left) and mixes in 50% of the 7th channel (Back Center).

c5=c5+0.5*c6: Maps the 6th input channel (Surround Right) and mixes in 50% of the 7th channel (Back Center).

[Balling]

This whole back center is what Dolby Surround EX is. ​https://en.wikipedia.org/wiki/Surround_channels#/media/File:6_1_channels_surround_sound.svg

Dunno this is still not recognised as back center ffmpeg even after all recent aacdec changes.