Asus M4A78-HTPC/RC - Manual (Page 40)

8. Optical drive audio in 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.
9. Digital audio connector (4-1 pin SPDIF_OUT)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF) ports.
The S/PDIF module is purchased separately.
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