Whenever I buy a Bluray, I convert it to an mp4 using a combination of makemkv and ffmpeg. This allows me to store my films on a single large hard drive without using up shelf space I don't have. I then discard the original disc, to stay legal.

However, because ffmpeg has a very poor aac encoder, I usually convert the audio to ALAC first, and then use Apple's built-in afconvert utility to convert said ALAC to aac audio.

afconvert DayOfTheDocotor-alac.m4a -d aac -b 256000 -f m4af DayOfTheDoctor.m4a

I then mux the video and audio into a single mp4 with ffmpeg.

This method works great for stereo sound, but whenever I use afconvert on a 5.1 audio track, it gives me the following warning message:

*** warning: Transcoding multichannel audio may not handle channel layouts correctly (0)

It then proceeds to convert the audio, and as far as I can tell on my stereo speakers, everything is working fine. I don't actually have a 5.1 sound system at the moment, but I might get one someday, and when I do, I want to be able to play my existing movies.

So my question is, what does this warning message mean, and is afconvert properly transcoding my 5.1 audio?

  • I'm not certain, but you might have to specify your layout -l - see developer.apple.com/library/mac/technotes/tn2237/_index.html – Tetsujin Feb 13 '16 at 12:22
  • Thanks for the link. In my case, the audio is being encoded to a 5.1 aac (as I want), so is it safe to assume everything is working properly? Again, I can't actually test by listening because I only have a stereo system. – Wowfunhappy Feb 14 '16 at 19:25
  • I can't say for certain, sorry - it's not a workflow I use, I just found the man page & thought it may be useful. – Tetsujin Feb 14 '16 at 19:28

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