Overscan
An area at the outer edges of a video image , the visibility of which cannot be guaranteed, is referred to as an overscan . In the case of projections, especially with CRT screens , the edges of the image are covered by a mask or screen in order to conceal inaccuracies and the geometrical deviations (which are practically unavoidable with analog tubes) at the edge and to guarantee a clean, straight image finish.
In the production of films, videos, DVDs and television programs, this overscan area of around 6% must be taken into account, which must not contain any relevant image elements (e.g. overlays or menus).
In the case of screens with active pixels (e.g. LCD or plasma ), the image can be displayed with pixel accuracy, so that no overscan area is required. However, some flat-screen televisions enlarge the image delivered by approx. 6% and then crop the edge by 6%. This means a loss of resolution, so that more or less strong scaling artifacts occur in the case of filigree elements such as fonts or fine lines. This and the fact that something has been cut off from the edge of the picture is particularly annoying when the television is operated as a computer monitor.
In contrast to the blanking interval, the overscan area is normally not used for external signals. However, there are analog video production systems that fade in a digital time code at the top, which can slide into the visible image area as a “string of pearls” made of white dots if the overscan of the screen is too tight.
When computers were still generating TV-compatible video signals, a margin was also left free on all four sides as a precaution, which was not written with information. With some computers, however, it was quite possible to program the graphics hardware - sometimes using tricks - so that it also displayed the desired content there. On the Commodore C64 it became an art to use overscan sprites in games. On the Commodore Amiga , the use of the overscan area in later operating system versions was even officially supported by it. Likewise with Amstrad CPC . By using the overscan areas, higher resolutions can be achieved.
Now that the graphic output of computers is more and more displayed by LCD monitors that do not need a safety edge, the techniques related to overscan are practically meaningless. In the field of digital flat screens for the TV sector, the overscan is technically only of historical significance. Paradoxically, however, many manufacturers still use it for purely digital sources.