Asus ROG Xonar Phoebus Quick Start Manual page 72

Q6: Will PCM sound output through S/PDIF be just 2 channels even
with different analog output channels?
Answer:
The S/PDIF protocol specification (IEC-60958) can only carry 2-channel PCM
data or non-PCM AC3/DTS data. So, when a user selects PCM output for S/
PDIF, the Xonar Phoebus audio card will always deliver 2 channel PCM data
through the S/PDIF output port. To attain the 5. channel surround sound,
you can select DDO, which allows the Xonar Phoebus to deliver 5. surround
sound for DVD movies, and even stereo music.
72
ASUS Xonar Phoebus