[mythtv-users] spdif output 5.1 and 2.0 at same time [solved]

[Alen Edwards]

This email documents my working solution to getting 5.1 surround for my 
pre-pro and 2.0 for my TV.  It isn't elegant, but it is working.  More 
work can probably be done on this and maybe I will do that if I can find 
more information on the Ubuntu sound drivers for HDA.

Background.  I tried ALSA which didn't work per the wiki but as I found 
that it is said to do a poor job of rate conversion for spdif out so I 
stopped working on it.

Next I looked at pulseaudio.  I found they decode then encode to get 
spdif.  I wanted clean passthrough so that was not going to work for me.

It looks like the HDA chips can do passthrough and decode 2.0 at the 
same time, but I can find no driver documentation.  The chip maker has 
an undocumented linux driver and the MB mfg has no linux driver.  That 
led me to look for the Ubuntu driver but I could not find it.  My hope 
was that there would be options that could do this.  I can't find it and 
nobody would/could tell me where to look.

So, I ended up with an external AC3 decoder similar to the ones on ebay 
for $20.  Mine is a Technics Digital Surround Processor SH-AC500D.  I 
had it lying around so there was no cost to this for me.  This one does 
AC3 and DTS.  There is a model 300 that does not do DTS.  If I had to 
buy one, I would skip the DTS for my application.  Who wants 2.0 DTS 
anyway?  The only issue would be if you really wanted to listen to both 
5.1 and 2.0 at the same time, which was not my application.

For the inputs, I used the rear panel S/PDIF coax connector for one 
output and used the internal spdif 4 pin connector on the MB for the 
other.  I measured this connection and found 1.5 volts DC on it so 
assumed it was TTL.  I made a network to convert TTL to spdif levels. 

input from 
MB----||------/\/\/\/\/-----------0-----/\/\/\/\/\/------------ground
                        150nF    330 ohm       output     100 ohms
                      
All worked well.

Some day I hope to eliminate the SH-500D but until I can find more on 
the driver, this is working.


Allen