[FFmpeg-user] How can I convert mpeg2 5.1 audio to ac3?

Carl Eugen Hoyos cehoyos at ag.or.at
Tue Nov 4 19:47:11 CET 2014

 <bluesky <at> caramail.com> writes:

> >> But as noted, if I play it in MythTV then all the
> >> audio comes from the correct spearlers
> > You mean you don't hear the "S" in "Surround" coming
> > (to some degree) from the front speaker while the
> > rest of the word is clearly coming from the rear
> > speaker? I may have a cheap receiver...
> In MythTV, the surround sound comes from both the 
> front and rear speakers for the specified side, but 
> the volume is higher in the rear channels (might 
> just be the way I have my speakers balanced).  

Using MPlayer (and setting my receiver to PLII) I can 
clearly hear the sound as surround sound, but the "S" 
of "Surround" comes from the front instead of the rear 
This doesn't change if I reencode the audio and send 
it directly to the receiver.
My assumption was that this simply shows the deficits 
of the PLII system.

> Whereas in XBMC, a much lower volume is heard from 
> the rear speakers and the channels aren't properly 
> separated.

Unfortunately, you can get neither support for XBMC 
nor MythTV here (the relevant question to them is 
probably: Are you reencoding my MPEG-2 two-channel 
audio to AC3 instead of sending it as PCM to give 
the receiver a better chance to know what to do?), 
I will hopefully open a ticket for PLII decoding, 
I have no idea how difficult / trivial this is 
given that we support encoding...

> > Don't worry about MPEG-2 Multichannel, your sample
> > is certainly Dolby PLII.
> Thanks.  Is there a particular tool you used to 
> determine that?

Iirc (and I think Wikipedia also explains this), there 
are a few very old DVDs that use MPEG-2 Multichannel, 
I severely doubt that it was formalized for DVB.
You can check the bitrate to be sure: I would expect 
that 256k are not enough for Multichannel.

> Mediainfo doesn't seem to know anything about it.

And as explained, Mediainfo output is rarely relevant 
on this mailing list.

Carl Eugen

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