Decoding the Signal
A jumbo TV that is 60 feet (20 meters) high has to do the same thing that a normal television set does -- it has to take a video signal and convert it into points of light. If you have read How Television Works, then you know how a television that uses a cathode ray tube (CRT) does this. Here is a quick summary of how a black-and-white TV works:
- The electron beam in a CRT paints across the screen one line at a time. As it moves across the screen, the beam energizes small dots of phosphor, which then produce light that we can see.
- The video signal tells the CRT beam what its intensity should be as it moves across the screen. You can see in the following figure the way that the video signal carries the intensity information.
- The initial five-microsecond pulse at zero volts (the horizontal retrace signal) tells the electron beam that it is time to start a new line. The beam starts painting on the left side of the screen, and zips across the screen in 42 microseconds. The varying voltage following the horizontal retrace signal adjusts the electron beam to be bright or dark as it shoots across.
- The electron beam paints lines down the face of the CRT, and then receives a vertical retrace signal telling it to start again at the upper right-hand corner.
[A color screen does the same thing, but uses 3 separate electron beams and 3 dots of phosphor (red, green and blue) for each pixel on the screen. A separate color signal indicates the color of each pixel as the electron beam moves across the display.]
As the electron beam paints across the screen, it is hitting the phosphor on the screen with electrons. The electrons in the electron beam excite a small dot of phosphor and the screen lights up. By rapidly painting 480 lines on the screen at a rate of 30 frames per second, the TV screen allows the eye to integrate everything into a smooth moving image.
CRT technology works great indoors, but as soon as you put a CRT-based TV set outside in bright sunlight, you cannot see the display anymore. The phosphor on the CRT simply is not bright enough to compete with sunlight. Also, CRT displays are limited to about a 36-inch screen. You need a different technology to create a large, outdoor screen that is bright enough to compete with sunlight.