[MEncoder-users] problem re-encoding 5.1 audio ... channels getmixed up

Nico Sabbi nsabbi at email.it
Fri Jan 13 14:51:33 CET 2006

Phil P wrote:

> Hi,
> I have a file with 5.1 audio that I want to re-encode as 5.1, 
> basically so I can apply the "volnorm" filter and raise the sound level.
> I use the following command:
> mencoder <input-file> -ovc lavc -oac lavc -lavcopts 
> acodec=ac3:abitrate=448:vbitrate=1350:vpass=1 -ffourcc DIVX -channels 
> 6 -af volnorm -o <output-file>
> The command completes correctly but when I play the file on my 5.1 
> speakers I find the channel outputs have been transposed ie. the 
> original file plays fine but when playing my re-encoded file the 
> centre channel appears to be front-right, front-right is now 
> rear-right... etc...
> As I'm not applying any channel filters I wouldn't have expected this 
> to happen.
> Does anyone know why this occurs?
> Is it a common problem?

it sounds like an unlikely bug to me (volnorm shouldn't change the 
channel mapping,
but I can barely believe it does it). Anyway, re-compressing sound after 
having normalized
volume was always considered a bad practice by people more aware of 
audio processing
than me

> Is there any workaround?

-af volnorm,pan=.... (but it's not so easy to use)

> Is there an alternative method to normalize the volume on the original 
> stream?
> I've searched the archive and couldn't locate a similar question.
> Much thanks,
> Phil.

good luck
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