Asus Pundit-R Optical S/PDIF output to Dolby Digital Processor

I've had an Asus Pundit-R system setup running MCE2005 for a little
while now.

Only just tried to connect up the optical S/PDIF output to my Yamaha
Home Cinema Amp.

I can get audio out over the connection but only ever 2 channel PCM
sound, the amp does not detect a Dolby Digital AC3 stream. This is
from the MCE2005/Nvidia DVD playback and from WinDVD playback.

I'm using the latest AD1888 SoundMAX driver and have 5.1 sound turned
on everywhere I can see.

Is there any secret to getting DD working over the optical connection.