The simplest representation of digital video is a sequence of frames, each consisting of a rectangular grid of picture elements, or pixels. Each pixel can be a single bit, to represent either black or white. The quality of such a system is similar to what you get by sending a color photograph by fax—awful. (Try it if you can; otherwise photocopy a color photograph on a copying machine that does not rasterize.)
The next step up is to use 8 bits per pixel to represent 256 gray levels. This scheme gives high-quality black-and-white video. For color video, good systems use 8 bits for each of the RGB colors, although nearly all systems mix these into composite video for transmission. While using 24 bits per pixel limits the number of colors to about 16 million, the human eye cannot even distinguish this many colors, let alone more. Digital color images are produced using three scanning beams, one per color. The geometry is the same as for the analog system of
Fig. 7-14 except that the continuous scan lines are now replaced by neat rows of
discrete pixels. To produce smooth motion, digital video, like analog video, must display at least 25 frames/sec. However, since good-quality computer monitors often rescan the screen from images stored in memory at 75 times per second or more, interlacing is not needed and consequently is not normally used. Just repainting (i.e., redrawing) the same frame three times in a row is enough to eliminate flicker.
In other words, smoothness of motion is determined by the number of different images per second, whereas flicker is determined by the number of times the screen is painted per second. These two parameters are different. A still image painted at 20 frames/sec will not show jerky motion, but it will flicker because one frame will decay from the retina before the next one appears. A movie with 20 different frames per second, each of which is painted four times in a row, will not flicker, but the motion will appear jerky.
The significance of these two parameters becomes clear when we consider the bandwidth required for transmitting digital video over a network. Current computer monitors most use the 4:3 aspect ratio so they can use inexpensive, massproduced picture tubes designed for the consumer television market. Common configurations are 1024 × 768, 1280 × 960, and 1600 × 1200. Even the smallest of these with 24 bits per pixel and 25 frames/sec needs to be fed at 472 Mbps. It would take a SONET OC-12 carrier to manage this, and running an OC-12 SONET carrier into everyone’s house is not exactly on the agenda. Doubling this rate to avoid flicker is even less attractive. A better solution is to transmit 25 frames/sec and have the computer store each one and paint it twice. Broadcast television does not use this strategy because television sets do not have memory.
And even if they did have memory, analog signals cannot be stored in RAM without conversion to digital form first, which requires extra hardware. As a consequence, interlacing is needed for broadcast television but not for digital video.
