d field-interlacing. Interlaced fields of video are sent at a rate of 59.94 fps. Why divide an image in half? Why not send complete frames with the odd and even lines combined? The answer is in the eye of the beholder. Although the brain retains the perception of light for one-tenth of a second, the image starts to fade instantly. If complete video frames are sent at a rate of 30 per second, the eye senses the change in light intensity and images appear flicker. For most people, images fuse at frame rates faster than 40 per second. To solve flicker problems, each image is displayed twice using a shutter speed of 60 images per second. Shadow mask Three-hole control grid Focus grid Three Three-hole Convergence Cathodes screen grid plates Face plate Figure 5-11. Color Picture Tube (Michel Bayard). Starting at the top-left, the spot sweeps horizontally to the right, and completes the task in about 50 microseconds (us). At that point, for 10 us, the beam shuts off and the electron gun executes a carriage return or a horizontal blanking interval. After the blanking interval, the beam starts up again on a new line. The process continues until the spot reaches the lower right-hand corner of the screen where it shuts off again during the vertical blanking interval. Next, retrace occurs as the reproducing spot returns to the top of the screen. This process continues left to right, top to bottom, synchronized by oscillators that keep the electron operating strictly on schedule. As the horizontal oscillator drives the electron stream laterally, the vertical oscillator moves slowly downward. In that manner, still-image frames create the illusion of motion. Video frame rates vary enormously and, as they increase, the bandwidth necessary to transmit them also increases. The more the video frames per second, the smoother and more natural the appearance of the motion sequences. Video communications systems designers aim to make motion sequences appear as smooth and natural as The History and Technology of Analog Video the ones vi