High-Definition Multimedia Interface's (HDMIs) are a combined audio and video interface for transmitting digital data. They are an alternative to legacy combined audio and video connections for connecting HDTVs to cable set-top boxes, blue-ray disk players and other home entertainment peripheries. HDMI connections support all video and audio data sources and are supported by Comcast cable set-top boxes. HDMI cables provide you with the added advantage of supporting both HD video, as well as audio which legacy connections were not capable of providing.
Press the "menu key" on your remote control or the front panel of your Comcast set-top box.
Choose the "Output Type" menu option by pressing the "select" key on your remote control when the "Output Type" menu option is highlighted.
Cursor up and down using the "Up" and "Down" arrow buttons on your remote control to choose "1080p" or "720p" for your HDTV screen resolution. If you choose 480I, then your HDMI interface will not be enabled for HDTV transmission to your television.
Select the "OK" button and then choose the "Exit" menu option, using your COMCAST set-top box remote control. Your Comcast HDMI port will now be enabled.