Updated December 8, 2015: Charter subscribers report success with similar hardware and technique
If you subscribe to Comcast/Xfinity cable TV service, chances are you have a set top box from the company connected to your television using an HDMI cable. Too many times I've encountered these boxes in a misconfigured state that negatively affected picture quality. Given how expensive a year's worth of HD cable TV service costs, it's worth spending 5-minutes checking some basic settings in that box's 'hidden' menu.
Note: this information applies to regular Comcast/Xfinity set top boxes and not the newer X1 hardware.
For this operation, you will need the Comcast/Xfinity remote control and the remote control for your TV. If you haven't already, easily configure your Comcast/Xfinity remote to control your TV and other home theater gear!
Turn your TV ON and make sure the Comcast/Xfinity box is OFF.
Press the MENU button on the Comcast/Xfinity remote.
At this point, the front of the set top box should display the current video aspect ratio (16:9 or 4:3).
The TV screen should now display the 'hidden' setup menu.
If for some reason the TV Type setting is anything other than 16:9, change it to 16:9 to match the shape of widescreen televisions. Next, I would verify the settings under the Additional HDMI Settings menu.
The configuration pictured above is my recommended setup for all modern HD and UHD televisions. Next, Return to Main Menu and configure HDMI/YPbPr Output to Native or 1080i.
The Native output setting passes the video signal to the TV unaltered. Use of the Native setting enables a sub-menu of Native Mode Settings that needs some attention.
In the menu pictured above, select all video formats that the TV supports. Most modern televisions will accept all of the above listed video formats, but some TVs balk at receiving a 480i video signal over HDMI (leave unchecked if this is the case). An alternative to using the set top box's Native video mode is to configure it for 1080i output. Use of the 1080i output option will have the set top box convert all standard and high definition channels into the 1080i video format. The 1080i format is the maximum resolution currently used by broadcasters, and today's TVs are quite good at processing this video format resulting in terrific picture quality. Not only does 1080i convert almost perfectly into the 1080p format, a 1080i signal containing content originally captured at 24 frames-per-second (most movies and primetime TV shows) is easily processed to recreate the source material's natural film-like look.
You are done! Press the Comcast/Xfinity remote's Menu button again to exit. If you decided to use the box's Native video mode, confirm that your TV is configured to eliminate overscan with 720p and 1080i channels. Often, a TV's overscan setting (aka Picture Size) is custom for all video formats so eliminating overscan with 1080i channels like NBC and PBS doesn't automatically eliminate overscan with 720p channels like Fox and ESPN.
For more information about TV settings related to optimal picture quality, I've created a short video that will help clarify what needs to be done.