Asus P5Q-E/WiFi-AP - Manual (Page 59)

ASUS P5Q-E 2-33
12. Optical drive audio connector (4-pin CD)
This connector allows you to receive stereo audio input from sound sources
such as a CD-ROM, TV tuner, or MPEG card.
13. Digital audio connector (4-1 pin SPDIF)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF)
port(s).
The S/PDIF out cable is purchased separately.