mencoder: channel layout not supported by encoder

[ffigs]

Summary of the bug:

When transcoding audio to certain lavc codecs, mencoder hits an error and quits, e.g.:

[ac3 @ 0x55837f4ac940]Specified channel layout '2 channels' is not supported by the ac3 encoder
[ac3 @ 0x55837f4ac940]Supported channel layouts:
[ac3 @ 0x55837f4ac940]  mono
[ac3 @ 0x55837f4ac940]  stereo
[ac3 @ 0x55837f4ac940]  3.0(back)
[ac3 @ 0x55837f4ac940]  3.0
[ac3 @ 0x55837f4ac940]  quad(side)
[ac3 @ 0x55837f4ac940]  quad
[ac3 @ 0x55837f4ac940]  4.0
[ac3 @ 0x55837f4ac940]  5.0(side)
[ac3 @ 0x55837f4ac940]  5.0
[ac3 @ 0x55837f4ac940]  2 channels (FC+LFE)
[ac3 @ 0x55837f4ac940]  2.1
[ac3 @ 0x55837f4ac940]  4 channels (FL+FR+LFE+BC)
[ac3 @ 0x55837f4ac940]  3.1
[ac3 @ 0x55837f4ac940]  4.1
[ac3 @ 0x55837f4ac940]  5.1(side)
[ac3 @ 0x55837f4ac940]  5.1
Couldn't open codec ac3, br=224.

How to reproduce:

1. Use ffmpeg to synthesize a test file, which we'll try to re-encode with mencoder:

   $ ffmpeg \
   -f lavfi -i testsrc=duration=10:size=1280x720:rate=30 \
   -f lavfi -i sine=frequency=1000:duration=10 -ac 2 -c:a pcm_s16le \
   vid1.mkv
   

2. Use mencoder to re-encode the audio track with some lavc codec:

   $ mencoder vid1.mkv -ovc copy -oac lavc -lavcopts acodec=$codec -of lavf -o vid2.mkv
   

The problem codecs are ac3, mp2, and li1mp3lame. flac, wmav1, and wmav2 are not affected and work on my system.

Details

The input codec/container doesn't seem to matter, nor does the actual output audio channel layout, since common layouts such as stereo/mono are affected (also 5.1 for AC3). Re-encoding the video or not also makes no difference. So, there's nothing really significant about the test file. It's 100% correlated with the output codec as far as I can tell.

Issue is not present on older mencoder version r38435 (via RPMFusion repo, statically-linked to ffmpeg 4.4.4).

Could it be due to "2 channels" vs "stereo"?

Fedora 39, tested on latest SVN (r38458) with latest ffmpeg as well as ffmpeg 6.1.1 release. Logs are for an ac3 encode with MPLAYER_VERBOSE=9.

[ffigs]

Successful ac3 encode with mencoder r38435

[ffigs]

Unsuccessful ac3 encode using mencoder r38458

[beastd]

Thanks for the report!
I could reproduce.

It's probably related to the adoption of FFmpeg API AVChannelLayout .

I have an idea on how to fix it. Will try it out and if it works, provide a patch for you to test here.

[beastd]

Patch attached as attachment:0001-ae-lavc-Generate-a-guessed-AVChannelLayout-based-on-.patch​

Could you please try and report if it works for you?

[beastd]

I have already committed to MPlayer SVN because I think almost no lavc encoders could work without the fix.

So you could just svn up and r38460 and above should work.

[ffigs]

Thanks for getting this sorted so quickly! Fixed for me as well as of r38460.

[beastd]

You're welcome!

Thanks for reporting back. Closing this as fixed now.