[mythtv-users] Audiophile and TV audio?

allen edwards allen.edwards at oldpaloalto.com
Sun Jun 22 16:48:52 UTC 2008

I am on a quest to get Audiophile S/PDIF for my surround processor and 
L+R analog for my TV out of Myth so I can get sound with my dual monitor 
system (TV and Projector). 

I was under the impression that it worked on my  Prototype system.  I 
remember that I set myth to AC bypass and got both S/PDIF and analog out 
of the MB. However, I just tried to verify this on the prototype and it 
does not work with either Mythbuntu 7.10 or 8.04.  So much for believing 
something just because you saw it with your own eyes. 

So, I went on a quest to figure this out.

First, ALSA.  
http://www.mythtv.org/wiki/index.php/Configuring_Digital_Sound says it 
will work.  Although I could not get the wiki to work, I now see this:

"The upsampling in ALSA is unfortunately very low quality ... This is 
very easy to overcome, by sending all your audio directly to your 
soundcards SPDIF or IEC958 Connector".

This basically rules out using ALSA, except as a pass through device, so 
I am not going to work more on that.

Next thing I looked at was PulseAudio.  Pulseaudio is set up as a device 
in ALSA so audio will go to it.  Then you either send it back to ALSA 
(obviously no good given the problems with ALSA) or you set up sinks in 
Pulaseudio.  I did searches on spdif and pulseaudio and mostly found bug 
reports.  Most interesting is this thread 
http://ubuntuforums.org/showthread.php?t=663151 which is an attempt to 
get people to say how they got spdif passthrough to work with no 
responses.  It ultimately terminates by linking to a thread where 
someone explains how they got surround to work but it is not pass 
through, it is using Pulseaudio to decode the signal which is then 
encoded and sent back out.  I followed a few of the guidelines for 
setting up Pulseaudio and was unable to get anything to work.  In the 
process of debugging this, I came to the conclusion that Pulseaudio 
cannot do spdif passthrough, that it is a decode-encode process, so I 
quit wasting time on it.

Next is the Azalia driver itself.  In searching I find that it is 
claimed this chip can do the job:
"It (Azalia) also understands the concept of multiple isochronous 
streams, say a 6-channel 5.1 program out of one jack and a simultaneous 
stereo program out of another".

I looked at the Intel spec 
http://download.intel.com/design/chipsets/manuals/30234903.pdf  (a 160 
page technical spec)
This is a bit and register level spec that would be used by people 
programming the driver.  It does, however, look like the internal codec 
can decode the spdif input into "front" (L+R) and send the unmodified 
bitstream to the spdif output.  This is exactly what I want.

Next I went looking at the drivers.  ASUS has no Audio Linux driver for 
the M3A MB and the Realtek ALC883 linux driver is undocumented.  I am 
not even sure these are used with Mythbuntu anyway.  I did see that 
there are people adding options to the sound drivers but I cannot find 
any documentation on this.

Here is my hope.  Add options to the sound driver to configure the 
Azalia chip to put out "front" from the analog codec and spdif 
unmodified from the spdif jack.

Anyone have a clue where I should go next?


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