[MEncoder-users] losing 5.1 audio on -oac copy transcode

[Dan Shoup]

I have an mpg file that uses AC3 5.1 surround sound.  I am trying to transcode, but having a difficult time getting 5.1 sound in the output file.  My transcoding settings are:
 
Mencoder.exe -i "E:\GBPVR\Dir\filename.mpg” -of avi -oac copy -ovc xvid -xvidencopts fixed_quant=4 -vf scale=1024:576 -o “E:\GBPVR\Dir\filename.avi”
 
This workes (file transcodes and is playable play), but the file plays with 2-channel audio in most players.  If I play it with mplayer, it also plays with 2-channel audio by default, but I can get 5.1 if  I force it with -afm hwac3 -channels 6.  The original file does not have any problems playing in 5.1 on any audio player (except mplayer, but will work if I specify -afm hwac3 -channels 6), so it seems something in the transcoding process is causing this problem.  I’m guessing it simply has the wrong header information (Mencoder identifies the original file as AC3 2 channel when it starts transcoding despite the fact that the 6 channels are actually there…so it is probably passing this misinformation to the transcoded file).  
 
Can anyone suggest a way to solve this problem (e.g., is there a way to force mencoder to identify the audio stream as 6-channel ac3)?
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