Overscan
（重定向自Underscan）
Overscan is the situation in which not all of a televised image is present on a viewing screen. It exists because television sets from the 1930s through to the early-2000s were highly variable in how the video image was positioned within the borders of the cathode ray tube (CRT) screen. The solution was to have the monitor show less than the full image i.e. with the edges "outside" the viewing area of the tube. In this way the image seen never showed black borders caused by either improper centering or non-linearity in the scanning circuits or variations in power supply voltage, all of which could cause the image to "shrink" in size and reveal the edge of the picture.