Asus M3A78-T - Manual (Page 56)

2-32 Chapter 2: Hardware information
10. Optical drive audio connector (4-pin CD)
These connectors allow you to receive stereo audio input from sound sources
such as a CD-ROM, TV tuner, or MPEG card.
11. Digital audio connector (4-1 pin SPDIF_OUT)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF)
port(s). Connect the S/PDIF Out module cable to this connector, then install
the module to a slot opening at the back of the system chassis.
The S/PDIF module is purchased separately.