Updated on October 24, 2014
If you purchased a gallon of gas and it measured 5% less than advertised, would you still call it a gallon? When it comes to viewing high definition (HD) video on a high definition television (HDTV), many HDTVs have a nasty habit of hiding (literally destroying) visual information along the border of the video image reducing the detail and clarity of the remaining visible picture that fills the screen. The excessive scaling of video beyond the edge of a TV's screen is referred to as overscan and it is often expressed as a percentage of the sacrificed video picture - a TV that exhibits 5% overscan fails to display the outermost 5% of the video border.
Examples of Overscan
The following pictures show the lower right corner of a 1080p flat panel HDTV's screen including a bit of its black bezel. This HDTV has a default overscan of about 6% as well as an underscan display mode that reveals every pixel of the incoming video signal. The left half of each picture shows the TV screen with its default overscan setting, and the right half shows the exact same portion of the TV's screen and the same frame of video but with overscan disabled - an underscanned picture. In each picture I've highlighted the details concealed by TV's default overscan mode in yellow.
Why Overscan is Bad
The relatively small percentages of video overscan I've mentioned may not sound like much, but the losses incurred scale with screen size. A 65 inch 1080p RPTV with 6% overscan (a typical amount) is missing 1.7 inches of video data from the left and right sides of an HD image - a total of 3.4 inches of lost visual information along the horizontal axis. With full-frame 1080p video, a 6% overscan equals a loss of over 124,000 video pixels - regardless of screen size. Any amount of video overscan introduced by a display device can soften the picture and it always results in significant loss of detail.
Common picture size settings that eliminate overscan are labeled as Just Scan, Dot-by-Dot, Screen Fit, and sometimes simply Overscan with a related on/off setting. Many new TVs default to a picture size setting labeled 16:9 that unintuitively produces an overscanned image. Also, overscan controls are often configured per video format. So turning off overscan for 1080i/p video sources doesn't automatically turn off overscan for 720p or standard definition sources.
Fix it or upgrade
It doesn't matter if the video source is a favorite HD channel, a next-generation game console, upconverted DVD, or Blu-ray movies, a good HDTV doesn't waste a single pixel - you should see everything as the content creator intended! Of course a HDTV can degrade image quality in any number of other ways, but video overscan shouldn't be one of them.