[MEncoder-users] AC3 5.1 Probably Bug (downmixing not working/not there)

[Jean-Michel Bruenn]

Hey again,

this is a old mail, i'm know now a bit more, here's the issue:

I'm since two or three days (dont remember) in the mplayer channel and 
watched in the www, i never found an answer nor something else which 
could explain me that behaviour. I know how to "solve" that myself, 
though i don't think this is correct. Lemme explain: I have recently 
played around with 5.1 ac3 in Linux using mplayer. If i start those 
files with no parameter (mplayer file.name) it defaults to 2ch output 
even if the file got 6ch. This is no Problem - I guess mplayer will 
downmix from 6 channels to 2 channels. I understand downmixing as: It 
will combine all 6 channels to 2 Channels. But this seems not to be the 
case.

If i play the file as it is, i can hear voices on the right speaker. 
There's sound/music on both, voice only on the right. The Channel Layout 
for ac3 5.1 is: LF, C, RF, LS, LR, LFE - Thus mplayer is NOT downmixing 
- It's just picking the first two channels (LF and C). Those ac3's i got 
here contains no voice on LF, RF, LS, LR. It only got voice on Center. 
Thus its quite clear, that playing a 5.1 ac3 with no option (which 
defaults to 2 channel output) gives me the Front Left and Center speaker 
as output. Somehow i don't believe this is correct.

as i said, i know how to fix it for myself. i still dont think this is 
correct.

some answered they had no problems with it:

the question is, whether its really correct. i guess you got voice on 
all 3 channels (LF, RF, C)
thus you won't hear the difference.
but in fact it will give you LF and C
instead of LF and RF

Jean-Michel Bruenn wrote:
> Hey,
>
> i hope i'm on the right maillinglist here. I'm wondering whether the 
> mplayer/mencoder ac3 channel mapping at 5.1 is wrong or correct. This 
> is quite difficult, because nearly every page i found related to this 
> is saying something else. Some say it's LF, LR, C, LFE, LS, LR, others 
> say it's LF, C, RF, LS, LR, LFE (which seems to be the correct 
> variant, as my tests on different hw and sw players have shown and the 
> dolby page at wikipedia explains) and Mplayer/Mencoder seems to use a 
> totally different variant, it seems Center is on the 5th Channel.
>
> I'd be really glad if someone could write down the correct 
> channel-mapping for ac3 and stop the overall confusion related to this..
>
> Cheers
> Jean
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