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 ---------------------------+----