he consumer video, and 70-90 times per second for com- display, moving to the right edge of the display. The puter displays. Special timing information, called verti- scanning then moves down one line, and repeats scan- cal sync, is used to indicate when a new image is ning left-to-right. This process is repeated until the starting. entire screen is refreshed, as seen in Figure 7.1-2. Each still image is also composed of scan lines, lines of In the early days of television, a technique called data that occur sequentially one after another down the "interlacing" was used to reduce the amount of infor- display, as shown in Figure 7.1-1. Additional timing in- mation sent for each image. By transferring the odd- formation, called horizontal sync, is used to indicate numbered lines, followed by the even-numbered lines (as when a new scan line is starting. shown in Figure 7.1-3), the amount of information sent The vertical and horizontal sync information is usually for each image was halved. transferred in one of three ways: Given this advantage of interlacing, why bother to use 1. Separate horizontal and vertical sync signals progressive? With interlace, each scan line is refreshed half as often 2. Separate composite sync signal as it would be if it were a progressive display. Therefore, 3. Composite sync signal embedded within the video to avoid line flicker on sharp edges due to a too-low frame signal rate, the line-to-line changes are limited, essentially by Introduction to video CHAPTER 7.1 VERTICAL HORIZONTAL for a 16:9 display). Of course, this results in a lower SCANNING SCANNING value for widescreen (16:9) displays, which goes against intuition. Standard-definition Standard-definition video is usually defined as having 480 or 576 interlaced active scan lines, and is commonly Figure 7.1-2 Progressive displays "paint" the lines of an image called "480i" and "576i," respectively. consecutively, one after another. For a fixed-pixel (non-CRT) consumer display with a 4:3 aspect ratio, this translates into an active 