6 – Raster scanning

Publisher Summary This chapter outlines the basics of raster scanning. In analog video, information in the image plane is scanned left to right at a uniform rate during a fixed, short interval of time that is represented as active line time. Scanning establishes a fixed relationship between a position in the image and a time instant in the signal. Successive lines are scanned at a uniform rate from the top to the bottom of the image, so there is also a fixed relationship between vertical position and time. In an analog video interface, synchronization information (sync) is conveyed during the blanking intervals. In principle, a digital video interface can omit blanking intervals and use an interface clock corresponding just to the active pixels. However, this would be impractical because it would lead to two clock domains in equipment that require blanking intervals, and this would cause unnecessary complexity in logic design. Instead, digital video interfaces use clock frequencies chosen to match the large blanking intervals of typical display equipment.