If you want to enjoy the full viewing capability of your HDTV, you must run an HDTV signal through it. Do so by upgrading your Comcast cable box to an HD DVR. Once the new DVR is hooked up, adjust your DVR's HDMI display settings to maximize your HDTV viewing experience.
Turn off the power to the DVR by aiming the remote at the DVR, pressing the cable button on the top of the remote and then pressing the power button. You should see the display go black on the DVR.
Press and hold the "Menu" button immediately after the DVR powers off. In about two seconds, you should see a User Settings screen on your TV. If it doesn't come up, try a couple of times until it does.
Look for the aspect ratio setting (it should be the first one) once you get the User Settings screen uploaded. It will be listed at either 16:9 or 4:3. If you have an HDTV, choose "16:9."
Look for the HDMI setting. Here you will be able to choose from a variety of resolution settings, such as 480i, 720p and 1080i. Depending on your television, choose the one that your television is rated at.
Set the display for non-HD programs and channels under the HDMI setting. It should be set at 480i for the best picture. If you prefer watching those programs without the black bars, then change the setting to "Stretch."
Point the remote at the DVR and press the power button once all your settings are configured. In a few seconds, your DVR will power up with the correct settings and your picture will look the way it was meant to.