[mythtv-users] Audio wierdness: AC-3 upconvert. Do I have a problem?

[Jean-Yves Avenard]

On 12 June 2013 23:36, Phil Wild <phil at holobyte.com.au> wrote:

> I did some more testing tonight. bluray track via bluray player, very loud
> and no issues. The track I was playing through mythtv that sounds terrible,
> I played through my Roku3 and Plex and it sounds perfect.
> I am going to switch the hdmi inputs on the back of the receiver to confirm
> that it is not the receiver input. I'm also going to load VLC onto my
> frontend and play the file through it to see if it is OS related rather than
> mythtv.

channel order has little to do with the quality of the playback.

there's a lot of things at play, and i'm not sure how Roku or Plex are doing it.
It could be a difference between playing the audio in plain stereo or
multi-channels.

Decoding is done by FFmpeg; excepted AC3 and DTS; very few codec
decoders will perform any downmixing (conversion from 5.1 to stereo).
So say you are trying to play a 5.1 AAC file ; most video/audio player
that let FFmpeg do the whole lot (and that's most of them), request
FFmpeg to provide stereo from the decoding of the original multi
channels audio file: all you get is the left and right channels. All
the other information contained in the surround channels is lost.
For AC3 and DTS, you will get a proper downmixing because the codec
contains information on how to do it.

As I don't like loosing information, myth does it differently. When
trying to play on a stereo system, when playing a multi-channels file
all channels are decoded, and then myth apply a downmixing matrix to
it (actually, it's the same matrix as Dolby ProLogic-II). so no
information is lost.

Reason I mention the above: say you have an audio file with a
corrupted surround channels info but clean front channels. If you
ignore the surround, it will sound clear, if you matrix it into the
signal: it will sound bad: this could be an explanation why you get ok
sound in one player and bad one in another

When it comes to audio playback quality, there's very little doubt in
my mind that myth has the most advanced and comprehensive system
available from all player available: you *always* get the best
possible data that can be handled by your hardware/software... XBMC
new Audio Engine only caught up in using 24 bits audio internally a
few months ago (while making a big fuss about it) when myth had that
for years (Added in 0.24)

If you have a file sample where you can consistently reproduce the
issue, I'm happy to have a look and see what's going on...