Coaxial SPDIF output for computer motherboard

So, I was walking around the shops with my girlfriend and I see a Battlefield 4 poster. I ask my g/f " This game is coming out sometime soon, right?"  "I don't really know" she probably says. I really don't remember these kind of answers. When I get home I search the net and find out it is released on 31 Oct (2013). It is 30'th. Something needs upgrading, My computer was state of the art years ago. So here come all sort of charts , cpu and gpu performances, and I decide to get a new cpu and motherboard, and hopefully get away without a new graphic card.

All that worked out quite well, except for the crappy socket 1150 cpu heatsink. Intel has changed the hole pattern by about 1mm, so all your old coolers that were designed for 125W cpus and would work fine on a 50W cpu are now rubbish. The stock cpu does get very hot. It got today to within 2 deg Celsius of it's limit, and it's only spring here in Australia. But that will get fixed as soon as I get my Noctua cooler mounting kit. It's free, too. Now that's good customer support.

This is about the sound issues. It seems in the time between my old motherboard and this one the industry has, for some strange reason, changed from the coaxial SPDIF output system to the apparently more fashionable optical system. I mean why use a coaxial cable with RCA plugs when you can use an optical cable that has no other use whatsoever, and it's even cumbersome to use. Times any rca cables have accidental become unplugged: zero .Times TOSLINK cables have become unplugged for no reason: at least one. And this is not only computers. When I was looking for a car stereo, the only one with an digital output was optical, too. Not so only a few years ago. Too bad they were way out of my price range.

Luckily there is an internal connector marked as digital output on the motherboard I have purchased. All I have to do now is drill some holes , make an adapter and hopefully we have sound. Yes, I could use the onboard sound, but it's really really terrible, especially if you plug headphones in it, like I had to do for the couple of days in which playing the game was a *much* higher priority.

The good news is it was easy to make such an adapter. It's only a couple of resistors, and the rca plug. All I had to do is measure the output to find out the voltage, and modify it to suit the coax SPDIF consumer standard which states 0.6V peak to peak signal.

So, after balancing the scope precariously on my computer chair, I had to measure the signal quickly. That is because my girlfriend was complaining about the noise the scope makes. In those days they had to put fans on things, or else they might not last 40 years. But, as usual, something was not working right. There was no signal. I , of course, start checking if there is a ground, if the scope is actually working (it was), etc. My girlfriend was not impressed. Eventually I figured out that, unlike my old realtek soundcard which played the output to both digital and analog simultaneously, this one only plays to one, or the other. Of course, this was one of the things I checked, but it turns out I had to reopen the player for the changes to work. Simple, really.

So, the signal is close to 3.3V, which means I needed a simple voltage divider and an rca plug. The ringing in the picture is from the scope. The divider works out to 75 and 330Ohms.

Note: Vin is on the motherboard side.  Vout is the rca plug middle pin. Refer to your motherboard manual to find out the pins.

The only other advice is to use in insulated rca socket in order to avoid a ground loop. Computers are really noisy and any ground loop will add jitter to the signal. Perhaps an optical sort of plug is required...

All in all this whole ordeal lasted only about , say, an hour. An adapter from ebay is surely, under $20.
I would like to believe an hour of my time is worth more than $20. The only thing I saved was the waiting time from ebay.

P.S. imho optical cables (really, the receiving socket) are very slow, and noisy, which is not good if low jitter is desired. My audio system is not earthed, and a coaxial cable does not introduce any ground loops. Jitter may or may not be an issue in audio, but I can measure it, and I can reduce it, too.
It is even possible that some jitter can be desirable, by masking and other mechanisms. The jury is out for now.