If you have ever been to a sporting event that has a large-screen TV in the stadium, then you have witnessed the gigantic and amazing displays that make the games so much easier to follow. On the TV, they can display instant replays, close-ups and player profiles. You also see these large-screen TVs at race tracks, concerts and in large public areas like Times Square in New York City.
Have you ever wondered how they can create a television that is 30 or 60 feet (10 to 20 meters) high? In this article, we will take a look at the LED technology that makes these huge displays possible!
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.
There are two big differences between a jumbo TV screen that you see at a stadium and the TV in your home:
- Obviously, it is gigantic compared to your TV. It might be 60 feet (20 meters) high instead of 18 inches (0.5 meters) high.
- It is incredibly bright so that people can see it in sunlight.
To accomplish these feats, almost all large-screen outdoor displays use light emitting diodes (LEDs) to create the image.
LEDs are, essentially, little colored light bulbs. Modern LEDs are small, extremely bright and use relatively little power for the light that they produce. Other places you now see LEDs used outdoors are on traffic lights and automobile brake lights.
On a color CRT television set, all of the colors are produced using red, green and blue phosphor dots for each pixel on the screen:
In a jumbo TV, red, green and blue LEDs are used instead of phosphor. A "pixel" on a jumbo TV is a small module that can have as few as three or four LEDs in it (one red, one green and one blue). In the biggest jumbo TVs, each pixel module could have dozens of LEDs. Pixel modules typically range from 4 mm to 4 cm (about 0.2 to 1.5 inches) in size.
To build a jumbo TV, you take thousands of these LED modules and arrange them in a rectangular grid. For example, the grid might contain 640 by 480 LED modules, or 307,200 modules. The size of the ultimate screen depends on the size of the LED modules:
LED Module Size 4 mm
- 2.56 x 1.92 meters screen size
- 8.4 x 6.3 feet screen size
LED Module Size 25 mm
- 16 x 12 meters screen size
- 52.5 x 39.4 feet screen size
LED Module Size 40 mm
- 25.6 x 19.2 meters screen size
- 84 x 63 feet screen size
To control a huge LED screen like this, you use a computer system, a power control system and a lot of wiring.
- The computer system looks at the incoming TV signal and decides which LEDs it will turn on and how brightly. The computer samples the intensity and color signals and translates them into intensity information for the three different LED colors at each pixel module.
- The power system provides power to all of the LED modules, and modulates the power so that every LED has the right brightness. Turning on all of those LEDs can use a lot of power. A typical 20-meter jumbo TV can consume up to 1.2 watts per pixel, or approximately 300,000 watts for the full display.
- Several wires run to each LED module, so there are a lot of wires running behind the screen.
As LED prices have dropped, jumbo TV screens have started to pop up in all sorts of places, and in all sorts of sizes. You now find LED TVs indoors (in places like shopping malls and office buildings) and in all sorts of outdoor environments -- especially areas that attract lots of tourists.
For more information on LED screens and related topics, check out the links on the next page.