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

Carl Eugen Hoyos cehoyos at ag.or.at
Tue Nov 4 13:56:30 CET 2014

 <bluesky <at> caramail.com> writes:

> If I play the same file in XBMC only the front 
> speakers are used.

How is your computer connected to your receiver?
I can choose here between analog, hdmi and 
TOSLINK (optical). If I use hdmi or the optical 
input, my receiver allows me to switch the 
"listen mode": As suspected, the audio uses 
Dolby PLII to encode surround sound via stereo 
channels. If I select "PLII" on my receiver, I 
hear the surround sound (with failures because 
the algorithm cannot be perfect).
If I use the analog input, I have to force the 
receiver to "two-channel" input to make it 
detect PLII: If I set it to 6-cable input 
(which is what I normally use), I cannot choose 

As said, FFmpeg does not support PLII decoding 
(only encoding). Are you sure MythTV contains 
an internal implementation and not does not use 
your receiver's Dolby decoder?

Just to make things less confusing: Above has 
nothing to do with "mpeg2": You can use any 
audio codec that supports two channels and 
use it to store PLII-encoded surround sound.

There is also "MPEG-2 5.1" or "MPEG-2 
Multichannel". This is very rare and FFmpeg 
cannot detect it (nor decode it). I don't 
think your file can use both Dolby PLII and 
MPEG-2 Multichannel at the same time.

Wikipedia articles for Dolby PLII, toslink and 
MPEG-2 Multichannel exist.

Carl Eugen

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