[mythtv-users] Fedora 10 and SPDIF output

[Allen Edwards]

On Sun, Dec 7, 2008 at 2:04 AM, Alex Butcher <mythlist at assursys.co.uk> wrote:
> Hi -
>
> On Sun, 7 Dec 2008, Paul wrote:
>
>>    I'm installing a friends system and want to use Fedora 10 but before
>> I install MythTV, I wanted to test audio output via SPDIF , but can't
>> seem to find how to do this in F10?
>>
>> Does Pulse audio need to be configured or do I have to remove this as
>> per older Fedora?
>> ie how do I set SPDIF as default output
>
> There's a lot of paranoia about PulseAudio about, but it just worked for me
> with FC8. Getting simultaneous stereo analogue and raw DD/dts bitstream was
> harder - details at
> <http://www.mythtv.org/wiki/index.php/User:Cowbutt#Sound>,
>
> One thing I have noticed is that sometimes, the dts bitstream to my amp will
> drop out when, if I remember correctly, when there's lots going on in the
> video stream (e.g. cutscenes, fast pans). I think I read that's a MythTV
> issue, though.
>
>> Thanks
>
> Best Regards,
> Alex.
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I have some comments regarding using asound.conf to configure alsa to
give spdif out so that you can get analog and spdif output at the same
time.  I have read that the way alsa deals with spdif is that it
decodes the ac3 digital stream, then does the defined assignments,
then encodes it into a new ac3 stream.  In other words, it processes
the signal and does not pass it through.   Evidence of this can be
seen in the conf file if the bit rate is defined.  If alsa was just
passing the spdif bitstream through, it wouldn't know or want the bit
rate defined in the file as it will be different depending on the
source, DVD or ATSC.   I have also read that alsa does not do a very
good job of encoding the audio so you have a loss of quality.  As my
goal is to get the best spdif audio quality possible, I bypass alsa
and use alsa:spdif as the default output device and remove
asound.conf.

That said, I would love to have both spdif and analog output at the
same time.  I believe my Azilia sound chip has the hardware to do this
but I don't know how to access this capability.  I believe that would
be a sound driver setting in other words, at lower level lower than
alsa or myth.

Comments?

Allen