If you have a DVD player and a Comcast cable box connected separately to your television, you can't use the DVD player unless you change the channel on your TV to the "line input" or similar channel. After you're finished watching a DVD, you have to change the channel back to 3 or 4 so you can watch cable. But you don't need to go through all that hassle--you can connect your DVD player to your Comcast cable box and avoid having to change television channels.
Turn off the DVD player and disconnect the DVD player connection cables from your television. Keep the cables connected to the DVD player. Your connection cable should consist of three separate cables--a yellow video cable, a white audio cable and a red audio cable.
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Turn off the Comcast cable box and turn it around so you can see the audio/video connection jacks. Look for "Video In" and "Audio In."
Plug the yellow cable (video) into the "Video In" jack on the back of the Comcast cable box. Next, plug the white cable (audio) into the "Audio In - L" jack on the back of the cable box. Then plug the red cable (audio) into the "Audio In - R" jack on the back of the cable box.
Check your connections and make sure the TV cables are pushed in tightly.
Turn on the Comcast cable box, the television and the DVD player. On your Comcast remote, press the "Input/TV/VCR" button to change to the channel for viewing the DVD player connected to the cable box. You should see your DVD player's logo on the television. If you don't see your DVD's logo, check your connection cables on both the DVD player and the cable box.