This is just a quick little hack that I did on my MythTV box some time ago. The motherboard that I have did not come with a cable for the S/PDIF audio output and it would have been $10 – $20 to purchase one. So, I did some searching around and found out it is actually quite easy to make one myself.
This particular motherboard has a three pin header that actually allows a coax or optical out. The coax connection is easy enough, if I needed optical out I would just buy the cable. The pin-out of the board looks like this:
For a coax connector, you just need the SPDIF_OUT and Ground pins. The +5V is only used for an optical connection. So, with the parts I had lying around, I found an RCA jack, metal bracket, connector and some wire. The connector I had was actually four pins, but I was able to cut off one of them. It did not have any keying on it, so it is possible to insert it backward, so I just had to be conscious of that when plugging it in. Then, nothing fancy, I just wired it up. The SPDIF_OUT pin goes to the tip connection of the RCA, the Ground goes to the ring. Here are a couple pics of how it turned out. I’ve been using it for a while and it works great!
yorkspace.com 
Would you happen to know what the wiring would be for the optical output. Must need some kind of coax to optical converter?
Don,
In this case, you would not need a coax to optical converter. It would be possible to directly connect an optical transmitter to the motherboard. I did a little bit of searching, and I think something like the Toshiba TOTX142L(F) part would work. It is available from DigiKey for $5.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=TOTX142LF-ND
Thanks for this. Works perfectly with the Intel Atom 330 motherboard and Apex Mi-008 case. There is even a spot at the back of the case to fit the SPDIF jack!
Just great, I’ve need to do this for ages and now i found this post, Great Work 🙂
would it work if connected directly to rca input of home theatre system.