The HDMI connection technology (HDMI stands for High Definition Multimedia Interface) is the only connection that supports HD video and audio transmission in the same cable. Because of this, it is the most commonly used cable type for connecting HD sources (e.g. a Comcast High Definition cable box) to home theater receivers and/or HDTVs. Connecting the cable itself is not a difficult process.
Plug one end of the HDMI cable into your Comcast box's "HDMI-out" port.
Connect the other end of the cable to your HDTV or home theater system.
Turn on your HDTV and home theater system (if applicable) and switch to the "HDMI" input mode. You can now view the video signals from your Comcast box in full HD on your HDTV.