VLC plays 5.1 PCM S24 LE audio in a MOV container out of the wrong speakers

[jimbobuk]

I recently had to master a video file into 5.1 audio with Apple Pro Res 422 video. Everything went well until I tried playing back that video in VLC, only to discover that the speakers were playing the wrong channels out of the wrong speakers. At the time it was all hectic and we didn't have the time to identify VLC playback as the issue, we rather thought that the video file had been output incorrectly.

Further testing though demonstrated that it was purely a VLC playback issue. I've tested this on Windows and Mac OS X with the latest versions of VLC. I even took the nightly development build around a week ago and found it had the same playback behaviour so it's not something that's been fixed yet.

To help anyone test the problem I've provided a test asset that we used to identify the issue. Unfortunately i'm not on a 5.1 system at the moment so I can only describe what I'm hearing with stereo playback. You can hear that its incorrect, but obviously you cannot tell front and rear channels apart when played back properly.

The files are as follows

• http://jimtreats.com/4VLC/SU&RROUND%20T ... %20fix.zip
  • This zip file contains the source wave file of audio
  • reported by VLC as being codec PCM S24 LE (s24l)
  • Channels: 3F2R/LFE
  • The content of the file is someone reading out the channel names in each particular channel. VLC reads this file out correctly with each description coming correctly out of the corresponding speaker. The channel order read out is "Front Left, Front Right, Centre, LFE (inaudible without an LFE speaker), Surround Left, Surround Right"
• http://jimtreats.com/4VLC/5.1%20audio%2 ... ls.mov.zip
  • This zip file contains the .mov file created with the above wave audio embedded within it
  • reported by VLC as being codec PCM S24 LE (lpcm)
  • Channels: 3F2R/LFE
  • The content of the file plays out incorrectly. You hear descriptions coming out of speakers which don't match their descriptions. As I said I'm not on a 5.1 system at the moment so can only describe them here in how they sound wrong on stereo speakers
    • Front Left: played out of the left speaker
    • Front Right: played out of the right speaker
    • Centre: (incorrectly) played out of the left speaker
    • LFE: (incorrectly) played out of the right speaker
    • Surround left: (incorrectly) played out of both speakers
    • Surround right: (incorrectly) inaudible coming out of the LFE speaker
• http://jimtreats.com/4VLC/5.1%20audio%2 ... annels.mp4
  • This is an .mp4 transcode of the earlier .mov file which played back incorrectly
  • It's audio is Codec: MPEG AAC Audio (mp4a)
  • Channels: 3F2R/LFE
  • This file plays back correctly out of each speaker, sounding identical to the original source wave file

Hopefully this will be enough for anyone to confirm the issue. I thought I'd ask on here first but I presume I should just go ahead and make a bug for this issue?

It would be great if this could be fixed so that these types of files could be played back correctly without having to be transcoded out of the PCM master format.

Thanks for your time.

Cheers

Jim

[lrnk85]

Yes! Thank you for posting this. Just having the exact same trouble now. Was very worried, but after some testing all signs pointed to this, and you just confirmed it. So strange how it only happens with ProRes and uncompressed 5.1 WAV. If I play the uncompressed WAV without the video it plays correctly, and a recompression of the ProRes file plays correctly. The ProRes file plays back correctly in QuickTime too. Such an odd bug, how has this not been fixed yet?

[TACarp]

I am also having this problem but with MOV with PCM 7.1. Here are my mapping issues
Front right plays front right
Front left plays front left
Center plays left surround
LFE plays right surround
Left surround plays center
Right surround plays LFE
Right back plays right back
Left back plays left back

I am not sure if this is a bug or VLC was never set with correct channel mapping for PCM. This should either be fixed to work automatically or manual channel mapping settings added.

[Lotesdelere]

You should create a new ticket on the VLC Trac with your above report, a link to a sample file and the link to this thread:
https://code.videolan.org/videolan/vlc/-/issues