Multichannel AAC (again)

[cmw]

Hey there!

This bug has been known now for years, that VLC can't handle multichannel AAC correctly. However, when i tried the recent nightly, I noticed that opposed to the behaviour of earlier version, namely displaying that a 5.1 mutlichannel audio track has 8 channels while switchting some and completely dropping others, this has changed now.

With the recent nightly, it correctly displays 6 channels, and plays all of them, only the channels are mapped wrong. Maybe someone did something that actually solved the underlying problem and now the only thing left to do would be mapping the channels correctly. Can you please look into it? I know, multichannel AAC is not the most common way to encode something, but it's been getting more and more popular and I know quite a lot of people who encode their video/audio that way. It's the only thing that keeps VLC being the most awesomest player ever

Best regards,
cmw

Samples:
6ch AC3 file (correct channel mapping): http://molkemon.mo.funpic.de/samples/6chac3.rar (RAR-compressed because of file size restriction on webhost)
6ch raw AAC file (wrong channel mapping) http://molkemon.mo.funpic.de/samples/6chaac.aac
6ch mp4 contained AAC file (wrong channel mapping) http://molkemon.mo.funpic.de/samples/6chmp4.mp4

I could provide more samples, e.g. AAC in MKV container, or AAC in AVI container, but it's all the same --> wrong channel mapping

[Jean-Baptiste Kempf]

So this is better than before, right ?

What is the mapping problem exactly ?
I think there is already a ticket on this issue

[cmw]

Yes, it's better than before, and there has been a ticket for the past three years or so. Originally, the Ticket was set to be resolved with 0.8.5 or sth, but then was de-prioritized and put to the "bugs pardise" where it will probably not solved in reasonable time.

The initial problem was the following: Whenever there was multichannel AAC in a file (or raw aac itself), VLC would falsley display that the file has 8 channels (while there were only 6). 3 of the channel (front left, center, front right) were "hearable", allthough all in the wrong spots (front left was back right, center swas front right and front right was front left). The other channels would not be hearable.

Now, since I tried with the nightly, the problem has become a bit different: VLC correctly displays 6 channels for the file, and all the channels are played, allthough still switched around (this should be reproduceable on any system, OS doesn't matter, so if you have a 5.1 sound system, just try out yourself).

The problem also effects the down-muxing in case the 5.1 source has to be downmuxed to stereo (the sound is just wrong then... dialogue too quite, effects far too loud etc...).

2 channel AAC itself works flawlessly.

I still don't see why it is such a problem to correct this. As I understand, VLC uses FAAD for aac decoding. However, when I use ffdshow for aac decoding, with faad set as the decoder, it works without any problem...

[Jean-Baptiste Kempf]

Can you tell me exactly waht the mapping is ?

[cmw]

Sure:

Channel which should be used = Channel that actually outputs

Left Front = Right Front
Center = Left Front
Right Front = Left Surround
Right Surround = Center
Left Surround = Right Surround
LFE = LFE

[Jean-Baptiste Kempf]

See #493 on trac.

[cmw]

Thanks for looking into it... as you can see from the opening date, this bug has been around for ages... I have to admit I really don't know much about coding and stuff, but given you use faad just like some other programs, and it works with them, the bug should be pin-downable without unreasonable effort I hope :/

Best regards

Edit: Btw, as I can see from the ticket, someone says that for some unknown reason, faad doesn't seem to work with every type of AAC (never heard of that before), but if that is true, couldn't you try realaac instead (like in ffdshow, there is an option to switch between the two).

[Jean-Baptiste Kempf]

Fixed in development versions.
Thanks sooo much.