The HDMI interface is electrically identical and compatible with the video-only DVI interface, which came first. For example, if a cable box or PC has DVI out, but the TV or monitor only has HDMI in, a DVI-to-HDMI adapter cable is used to connect the video. The audio must be cabled separately as DVI transfers only video. The HDMI circuit detects the DVI signals and switches to the DVI protocol.
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