Some televisions draw the borders of an image beyond the normally visible area. Additionaly, some TVs have bezels that cover a small percentage of their physical screen. Either of these conditions result in a slight cropping of the visible image on the TV screen. This is known as "overscan".
When an image is sent from Apple TV, OS X or iOS to a TV or HDTV, you may see an option to turn adjust overscan or underscan. For example, if you don't see the OS X menu bar or Dock when mirroring your computer to Apple TV, try adjusting the overscan or underscan setting in the Displays pane of System Preferences.
When you change this setting, the video signal sent from the Apple device compensates by slightly changing the size of the displayed image. On some TVs, changing this setting may create a black border around the image (underscan). This is expected behavior.
Note: Some HDTVs have built-in settings you can also use to adjust overscan or underscan. Consult the documentation that came with your HDTV for more information.
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