[mythtv-users] SPDIF and Stereo Analog at same time anyone?

Allen Edwards allen.p.edwards at gmail.com
Tue Feb 10 07:57:07 UTC 2009

On Mon, Feb 9, 2009 at 9:22 AM, Allen Edwards <allen.p.edwards at gmail.com>wrote:

> With the fury of activity lately on sound setups, I thought I would bring
> up an old question I worked on unsuccessfully a few months ago.  Has anyone
> figured out how to get spdif passthrough and analog 2 channel out at the
> same time?
> My setup has Intel HDA on an Asus M3A.  I have a projector and surround
> system that we use together .  But most of the time I use the TV and its
> speakers. The surround amp runs 200 watts at idol so I don't like to use it
> to listen to Jepordy and the news.  I presently have an old Technics decoder
> just for the TV in addition to the surround processor.  That works but I
> want to use the processor in another room to get Pandora to play the same
> thing in that room as what is playing in the living room.  I have a coax
> cable that I can use to run the spdif between the rooms.
> That brings me back to the question I spent so much time trying to figure
> out previously.  How to set up the HDA chip to put out analog at the same
> time it is putting out the unprocessed spdif signal.  Reading the data sheet
> on the sound chip I am almost positive it can be done.  I have tried the
> earphone jack, tried both AC'97 and HDA and nothing works.  If anyone has
> solved this please let me know what you did.
> Allen
I have worked some more on this today and think I am stuck.  Please someone
let me know if the following is correct.

I can change mythtv to put sound into alsa default.  I can set up alsa to
put the sound into the spdif type hw:0:1 and that works fine.  I can also
set default up as a plug and slave to a section that puts the sound into
hw:0,1 and that works.  My thought was to split the signal there and have
one continue to go to the spdif port hw:0,1 and have the other section
decode the spdif and send it to the analog output hw:0,1.  But, I do not
think alsa can decode the spdif.  I think it expects a pcm signal and can
encode to spdif.  So, to get combined analog and digital from alsa I would
not use passthrough but send pcm to alsa which would send one signal to the
analog port and the other encode and send to the spdif port.  But this is
not passthrough, will not do DTS for example, and isn't what I want.  My
thought is that it is the applications responsibility to form the pcm signal
for the sound chip to do the DAC on.  The sound chip is not a spdif decoder,
that is the applications responsibility.

So, I think it cannot be done.

Am I correct?

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