Any way to determine what audio channels are mapped to in a video file?

[Elomo64]

I am currently working with an MP4 file that is set to 5.1 audio.

Using FFMPEG, I can get this information from the audio stream:

Stream #0:1[0x2](und): Audio: eac3 (ec-3 / 0x332D6365), 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
Stream #0:2[0x3](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 384 kb/s (default)

But it doesn't tell me what each channel is mapped to. So far, I've looked for multiple solutions, but I just can't find anything that will give me the info I need. Is there a way to check this?

Thanks!

[Elomo64]

From documentation, I found this:

5.1 FL+FR+FC+LFE+BL+BR
5.1(side) FL+FR+FC+LFE+SL+SR

Does this mean that both streams have this mapping, in numerical order?

So 5.1 is FL=Ch1 FR=Ch2 FC=Ch3 etc.?

[El Heggunte]

MediaInfo

[Elomo64]

Thanks, the info I can get from Media Info for the 5.1(side) stream is:

Channel(s) :
6 channels
Channel positions :
Front: L C R, Side: L R, LFE
Channel positions :
3/2/0.1
Channel layout :
L R C LFE Ls Rs

But I'm unsure if there's a numeric value I can assign, as I see the positions and layout are different.

Though I assume it's "Channel Layout" so I could say that it would be:

FL:1, FR:2, C:3, LFE:4, SL:5, SR:6

?

[ProWo]

5.1 channels layout is:

Ch 1 - Front Left
Ch 2 - Front Right
Ch 3 - Front Center
Ch 4 - Low-frequency Effects
Ch 5 - Left Surround
Ch 6 - Right Surround

Read: https://mediaarea.net/AudioChannelLayout

[Elomo64]

Thank you, though this leads to my next question:

Is it possible for someone to manually select the channels? Basically, I'm just trying to determine if this video I have has been encoded with the audio channels mapped correctly.

[jack_666]

Have a look here. May be of help


https://forum.videohelp.com/threads/408460-How-to-change-audio-channels-without-re-enc...ng#post2680076

[hello_hello]

The standard wave file mapping of

Ch 1 - Front Left
Ch 2 - Front Right
Ch 3 - Front Center
Ch 4 - Low-frequency Effects
Ch 5 - Left Surround
Ch 6 - Right Surround

isn't used by every codec.

What should happen though (and these days it's pretty standard) is multichannel audio is sent to an encoder using the wave file channel order, and the encoder remaps it to the order it uses internally. When it's later decoded, it's remapped back to the wave file channel order by the decoder.

Here's a channel order list for AAC, which is mostly the same as DTS, I think.
https://github.com/nu774/qaac/wiki/Multichannel--handling

5.1 FL+FR+FC+LFE+BL+BR
5.1(side) FL+FR+FC+LFE+SL+SR

The two layouts above are exactly the same in respect to the channel order they're encoded in. Formats like AC3 and DTS have "surround" channels, which don't exist in the wave file layout, so for 5.1 most encoders will accept either the side or back channels as "surround", and the channels they're decoded to later is partly convention, but it doesn't matter.

If you want to play around with channel mapping, try this version of foobar2000.
https://forum.videohelp.com/threads/396860-foobar2000-portable-(for-audio-encoding)
The output meters will show you which channels are being used (if you right click on the output meter you can run it full screen) and there's a Matrix Mixer DSP included you can add to the playback chain (or the conversion chain) and change the channel layout with it.

Swapping the Center and LFE channels, just to demonstrate.


[Attachment 69275 - Click to enlarge]

PS When it's using them, the BL & BR channels on foobar2000's output meter are labelled RL & RR (rear) for some reason, but they're the same channels.