I have an ASUS P5B-VM motherboard for my newish home theatre system. It has an onboard header for S/PDIF output, but no connector. The header has three pins: +5, GND, and S/PDIF-OUT.

ASUS makes a separate accessory back panel connector board for use with this, but I don't have one of those (yet).

Here's a photograph of it (attached, blurry) from the ASUS estore site. In this, one can see that the S/PDIF output (top right) is merely an RCA jack connecting *almost* directly to the feed from the motherboard.

But before it gets to the jack, it passes through a capacitor, and then a voltage divider (two resistors in series between the the S/PDIF-OUT signal and GND, with the RCA jack tapping from the middle point between the two resistors.

My questions are: what values should I use for the capactor and the resistors, in making my own connector board? Let's assume the motherboard S/PDIF-OUT signal uses TTL logic levels (I can verify this.. just haven't done it yet).

???

Further to this, S/PDIF over RCA/coax is supposed to use 0.5V - 1.0V signal levels, so the voltage divider should be easy enough to size up -- just not sure of the current draw requirements for S/PDIF, and thus the magnitude of restance to use. Say, 3kOhm and 1kOhm for a divide-by-4 ? Or a similar ratio with smaller resistors?

And I imagine that pretty much any capacitor in the pico-farad range would probably be fine for that component (?).

Try a 100nF cap, 330 ohm resistor and a 100 ohm resistor. I'm assuming the board already has a buffer in it somewhere. I made a few of these for various things a while ago and they were the values I used.

Code:

---

TTL ----||-----[ 330ohm ]------+---- S/PDIF
                               |
                           100 Ohm
                               |
Gnd ---------------------------+----

---

Indeed, our digital out board uses similar values. 374 and 90.9 ohm resistors and also a 100nf cap.

You might also wish to add an isolation transformer to prevent hum from leaking in if you have ground loops.

Stu

If you can wait a few days (I'm in the process of moving), I have one of these that came with an ASUS motherboard. I can read/measure the values for you...