A black-and-white television picture is generated by exciting the phosphor on the television screen using an electron beam whose intensity is modulated to generate the image we see. The path that the modulated electron beam traces is shown in Figure 18.4. The line created by the horizontal traversal of the electron beam is called a line of the image in order to trace a second line, the electron beam has to be deflected back to the left of the screen. During this period, the gun is turned off in order to prevent the retrace from becoming visible. The image generated by the traversal of the electron gun has to be updated rapidly enough for persistence of vision to make the image appear stable. However, higher rates of information transfer require higher bandwidths, which translate to higher costs
In order to keep the cost of bandwidth low it was decided to send 525 lines 30 times a second. These 525 lines are said to constitute a frame. However, a thirtieth of a second between frames is long enough for the image to appear to flicker. To avoid the flicker, it was decided to divide the image into two interlaced fields. A field is sent once every sixtieth of a second. First, one field consisting of 262.5 lines is traced by the electron beam. Then, the second field consisting of the remaining 262.5 lines is traced between the lines of the first field.
The situation is shown schematically in Figure 18.5. The first field is shown with solid lines while the second field is shown with dashed lines. The first field begins on a full line and ends on a half line while the second field begins on a half line and ends on a full line. Not all 525 lines are displayed on the screen. Some are lost due to the time required for the electron gun to position the beam from the bottom to the top of the screen. We actually see about 486 lines per frame.
In a color television, instead of a single electron gun, we have three electron guns that act in unison. These guns excite red, green, and blue phosphor dots embedded in the screen. The beam from each gun strikes only one kind of phosphor, and the gun is named according to the color of the phosphor it excites. Thus, the red gun strikes only the red phosphor, the blue gun strikes only the blue phosphor, and the green gun strikes only the green phosphor. (Each gun is prevented from hitting a different type of phosphor by an aperture mask.)