Overscan is a behavior on certain television sets in which part of the input image is displayed outside the visible boundaries of the screen. The outer edges of a video image that are typically cut off by consumer television sets in order to ensure that the image fills the entire display.
Existing cathode ray tube (CRT) television from the 1930s through the 2000s was highly variable in how the video image was positioned within the confines of the screen. Later it became common practice to have video signals with black edges around the picture, which the television is intended to beat in this way.
In fact, the content creators have defined three areas of all screens so they can make sure all content is displayed correctly:
Tile Safe: The Space Nearly All Television Will Show Confirms. No Text Will Be Interrupted.
Action Case: Most of the Viewing Area Defined by the Highest TV Set Calibration.
Bottom View: Full Picture.
This type of standardization provided a guide to producers and directors that nothing of value was lost, but that nothing was left to show on the screen for televisions that showed more pictures than others later on.
In other words: it is complex and none of the same rules apply today. But overscan still exists.
So Why Do Modern Televisions Use Overscan?
Overscan is not required for modern "fixed pixel" high-definition televisions such as LCDs. In fact, overscan's crop and zoom method often degrades picture quality, making it not only unnecessary but also undesirable.
Overscan also serves another lesser known purpose. Since the outer space is still (mostly) invisible, it is used to house important data for analog-to-digital converters.Because analog cannot add additional information to the picture, such as digital (metadata), this data is neatly crammed into things like flashing pixels or scanlines - think of it as Morse code for TVs.
Although the majority of everything is now fully end-to-end digital, some analog-to-digital conversions are still going on. The problem that old technology has been widely adopted and used for this long: It's nearly impossible to get rid of it completely.