[MEncoder-users] Dolby downmix

[Rolf Ernst]

<snip>
> I suspect the reason for not using "-channels 2" instead of "-channels 6 -af
> channels=2" would be to hope that the filter does anything smart but the mplayer
> manual clearly mentions that the superfluous channels are just truncated.
>
> A simpler method is to use:
> ffmpeg -i inputfile -ac 2 audio.wav
>
> Why aren't you posting on mplayer-users?
>
> Carl Eugen
Carl,

I did not explain myself well enough. Here is my predicament: I am 
taking recordings made with the Hauppage HD PVR (h.264 AC3 5.1) and try 
to reencode them. They come in as 59.97 fps and I am using mencoder's 
fine filter chain: -vf 
tinterlace=4,softskip,harddup,filmdint==io=30:24/fast=0 -fps 600001 
-ofps 24000/1001 which produces *flawless* 23.976 video (for analog film 
source, of course). The HD PVR actually digitizes off a high quality 
component analog signal and one of the quirks is that every once in a 
while it misses a beat (very rarely, but maybe 3 times in an hour 
unnoticeably so) and only the time stamps keep it in sync. Since 
mencoder knows how to thread the combined video and audio together none 
of these glitches matter, which is also why ffmpeg is useless for 
processing these a/v streams.

Since both streams and timestamps are so dependent upon each other it is 
*not* possible to demux, convert using some other tool and then 
multiplex again which *will* consistently lead to a sync error once the 
first glitch is encountered. As you point out (and is clear to me) the 
5.1->2 channel 'conversion' is merely discarding the lfe and back 
speaker sound signals. Thus I can either opt for that or keep the 5.1 
sound which I guess is ok but it would be nice to downmix if possible 
(input, btw, is either m2ts or ts stream format).

As you see, this is really an mencoder challenge but pondering it I ran 
across this mplayer method which may or may not have worked. Simply 
multiplexing it into matroska seemed to cause some sync problems, might 
be fixable or not, didn't pursue because I did not know in the first 
place whether I was getting Dolby Surround (I'd like to have PLII if 
possible).

Do you see my dilemma? Any suggestions on how to possibly approach it?

/re