[vlc] Center channel of AC3 5.1 soundtracks lost in stereo downmix?

[Reid Fosa]

Thanks a lot for the superb job all you developers and video mavens are doing :-)

I've been playing with VLC 0.6.2 for a week or so on a Mac, running OS X 10.2.8,
and noticed a few oddities in its behaviour:

1) Center channel lost when downmixing an AC3 5.1 channel to stereo?

I've tried transcoding a VOB from a DVD containing a 5.1 AC3 soundtrack to
MPEG stereo sound at an audio bit rate of 256Kb/s:

./VLC foo.VOB --sout \
   '#transcode{acodec=mpga,ab=256}:std{mux="ps",access="file",url="/tmp/foo.mpg"}'

Listeining to foo.mpg's sound track, the center channel containing the speech
seems to have disappeared from the downmix.  There's background music in the
resulting 256Kb/s MPEG soundtrack, but no dialogue is audible at all.

Listening to the original VOB file with VLC, the dialogue is very much present
and comes through quite clearly from the computer's speakers.
Is there a known issue with demuxing 5.1 to MPEG -- e.g. an attempt to encode
Pro-Logic phase correlation information across the MPEG stream's left and right
sound channels that has backfired and totally cancels the center channel?
Why is there such a significant difference between the downmix stereo track
played by VLC through the computer's speakers, and the MPEG stereo downmix
produced by "transcode" ?


2) Transport Stream produced by VLC doesn't contain Service IDs?

I've also tried converting a VOB file from a DVD to transport stream format:

./VLC foo.VOB --sout-acodec ac3 --sout file/ts:/tmp/foo.ts

The resulting transport stream file "foo.ts" can be played back by VLC, and
the AC3 soundtrack in the TS file sounds quite fine in VLC.
Looking more closely at the TS file, however, it seems that it doesn't contain
any of the "wrapping" information one expects in a TS -- e.g. a Service ID,
a Program Map Table...  Is there a way to force VLC to generate a (dummy) SID
in the TS files it produces, so that such files more closely resemble the
structure of TS files being broadcast by satellites ?  Let me hasten to add
that I know *nothing* about TS and MPEG, and that my question might thus
just not make sense.  It's just that the tuxbox DVB receiver I use requires
supplying a SID/Service ID in the Transport Stream it plays back.


3) Accelerated transcoding

I the transcoding quality dependent on the processing power of the machine
doing the VLC transcoding?  It seems VLC's transcoding will take precisely
as much time as the "real time" a stream is supposed to run at.  Is there
an option to accelerate -- for machines with powerful CPUs -- or slow down
-- for machines with little processing power -- VLC's transcoding process
when the output is directed to a file?  Powerful machines would then finish
the transcoding faster, while slow machines might run at less than the "real
time" wall clock rate, but produce pristine data files without any dropped
frames.


Hoping that my questions or suggestions elicit the interest of {net|vlc}.gods,

Reid Fose <vlcbox at postmaster.cco.uk>



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