[mythtv] Digital audio passthrough

[James Courtier-Dutton]

On 27 September 2010 02:49, Jean-Yves Avenard <jyavenard at gmail.com> wrote:
> On 15 September 2010 19:44, James Courtier-Dutton
> <james.dutton at gmail.com> wrote:
>> As you can see, none of the sensible options are "default".
>> It should also be noted, that if pulseaudio is being used, you cannot
>> do AC3 pass through, and the "default" device name might be
>> redirecting sound to pulseaudio.
>
> As you're an ALSA developer, you probably now this as I couldn't find
> definitive documentation on the matter.
>
> ALSA uses a different channel ordering than SMPTE which is what ffmpeg output.
>
> For 5.1 that order is well defined. For 7.1 however, I haven't found
> any documentations. As such I have assumed that it's 5.1 + surround.
>
> From some reports, it seems to be wrong. Could you tell me what 7.1
> channel order is for ALSA?
>

Channel ordering is contained in:
/usr/share/alsa/pcm/surround71.conf
#  Samples must be positioned:
#    chn0 - front left
#    chn1 - front right
#    chn2 - rear left
#    chn3 - rear right
#    chn4 - center
#    chn5 - lfe
#    chn7 - side left
#    chn8 - side right

The choice of channel outputs is based on what a majority of the sound
cards out there use in hardware.