umber of scanning lines that determines the vertical detail or resolution of a picture. When discussing picture resolution, the aspect ratio is the width-to-height ratio of the video image (see Section 16.2.1). The aspect ratio used almost universally is 4:3.3 In other words, a TV image 12 in. wide would necessarily be 9 in. high. Thus an image divided into 525 (491) vertical elements would then have 700 (652) horizontal elements to maintain an aspect ratio of 4:3. The numbers in parentheses represent the practical maximum active lines and elements. Therefore the total number of elements approaches something on the order of 250,000. We reach this number because, in practice, the vertical detail reproduced is 64-87% of the active scanning lines. A good halftone engraving may have as many as 14,400 elements per square inch, compared to approximately 3000 elements per square inch for a 9 by 12-inch TV image. Motion is another variable factor that must be transmitted. The sensation of continuous motion, standard TV video practice, is transmitted to the viewer by a successive display of still pictures at a regular rate similar to the method used in motion pictures. The regulate rate of display is called the frame rate. A frame rate of 25 frames per second will give the 1 With most European TV systems this value is 625 lines. Resolution deals with the degree to which closely spaced objects in an image can be distinguished one from another. High-definition television (HDTV) is a notable exception. 16.2 AN APPRECIATION OF VIDEO TRANSMISSION Exploring element Imaged scene Communications channel Reproducing Displayed scene Figure 16.1 Scanning process from TV camera to receiver display. viewer a sense of motion, but on the other hand he/she will be disturbed by luminance flicker (bloom and decay), or the sensation that still pictures are "flicking" on screen one after the other. To avoid any sort of luminance flicker sensation, the image is divided into two closely interwoven (interleaving) parts, and each part is presente