5.1 AC3 audio.. no sound from left channel in VLC or mpv, fine in Audacity

[arcooke]

I have a video file (a movie) that is having audio issues. If I open the file in VLC or mpv .. there is no speech coming out of the left channel. I can only hear talking out of the right channel. But I can hear background sounds and music just fine.

I exported the audio track with mkvtoolnix.. and when I load it up into Audacity and hit play.. everything works perfectly fine, all channels are working as they should. If I play the same extracted audio track (.mka) in VLC or mpv .. it's still missing left channel speech.

What's the best way to fix this? I don't have a clue what could be wrong.


Audio is 5.1 AC3. Codec on vlc reads:

Codec: A52 Audio (aka AC3) (a52 )
Channels: 3F2R/LFE
Sample Rate: 48000Hz
Bits per Sample: 32

mediainfo output:

Format                                   : Matroska
Format version                           : Version 4 / Version 2
File size                                : 238 MiB
Duration                                 : 1h 26mn
Overall bit rate mode                    : Constant
Overall bit rate                         : 385 Kbps
Encoded date                             : UTC 2016-12-22 00:26:11
Writing application                      : mkvmerge v9.6.0 ('Slave To Your Mind') 64bit
Writing library                          : libebml v1.3.4 + libmatroska v1.4.5

Audio
ID                                       : 1
Format                                   : AC-3
Format/Info                              : Audio Coding 3
Mode extension                           : CM (complete main)
Format settings, Endianness              : Big
Codec ID                                 : A_AC3
Duration                                 : 1h 26mn
Bit rate mode                            : Constant
Bit rate                                 : 384 Kbps
Channel(s)                               : 6 channels
Channel positions                        : Front: L C R, Side: L R, LFE
Sampling rate                            : 48.0 KHz
Frame rate                               : 31.250 fps (1536 spf)
Compression mode                         : Lossy
Stream size                              : 238 MiB (100%)
Language                                 : English
Default                                  : Yes
Forced                                   : No

EDIT: Important update. If I export to uncompressed WAV, I get the same issue I get when playing the AC3 audio in VLC/mpv. So that leads me to believe the channel ordering is all screwy or something. The selected channel in this screenshot is the speech.. and in the uncompressed WAV, I can only hear this channel through the right ear.

[hello_hello]

Yeah the channel order is wrong.

The order (when imported into Audacity) should be front left, front right, centre, LFE, surround left, surround right.

You appear to have front left, centre, front right, surround left, surround right, LFE.

Click on the drop down arrow for the second channel and move it down one, move the last channel up two places, and then export as 5.1ch something. Whatever format you prefer. Or you can switch the channel assignments using the mapping dialogue that opens when you use the export audio menu. Just make sure Audacity isn't set to always downmix to stereo under Input/Output in Preferences.

PS. The correct order is different to the order displayed by MediaInfo. It can't determine if the channels are correct as such anyway, and it displays the channel order according to speaker placement, not the channel order in the audio file itself.

Different formats (AC3, AAC, DTS etc) use their own channel ordering but when audio is imported into a program (or decoded) it's always imported using the standard wave file channel order. It's exported the same way and the encoder should remap the channels according to it's specific layout, if need be. The process is something you normally don't have to think about, unless somehow it ends up incorrect as is the case with your file.

The wave file channel order can be found here. https://msdn.microsoft.com/en-us/library/windows/hardware/dn653308%28v=vs.85%29.aspx