Asus ESC1000 Personal SuperComputer - Manual (Page 61)

ASUS ESC1000 3-15
10. 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.
11. Digital audio connector (4-1 pin SPDIF_OUT)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF)
port(s). If you are using ASUS HDMI-equipped graphics card, connect the
HDMI card to this connector with a S/PDIF out cable.
The S/PDIF out cable is purchased separately.