Look at the schematics in:
http://dvb-upload.com/index.php?action=download&pid=2223
it's a circuit to add S/PDIF out to a Sat/TV Decoder.
In few words the digital signal is obtain from the
Crystal D/A converter's pins through a DIT4096 (96Khz Digital Audio Transmitter).
I don't know if it's easy to find the corresponding D/A converter in RIO models.
Hope this helps you.
Valerio