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How Television Works Television: Almost every home has one. They are maligned, lauded, criticized and praised, but mostly they are watched. Years ago it was not uncommon to hear a little kid ask "how do they get all those little people in that box"? Today's kids may be a bit more sophisticated, but the question of how television works remains a mystery to most people. Television is both simple and complex. The basic concepts are fairly simple, but the actual circuits that make up your average television set are quite complicated, having been refined over years of experience. Television started early on in the history of radio, but didn't really take off until the 1950's. When color became common in the early 60's, the stage was set for the explosive growth that followed. Let's take a look at how television works.

(Picture from 1939 RCA Television Handbook Cover)

Electronic Images

No matter how you get the signal from one location to the other, be it radio waves, coaxial cable or fiber optics, the first thing to consider is the method used to convert images to electronic signals and back again. This process is the heart of television.

The first part of the process involves getting the image into an electronic form. How do you send so much information? As the saying goes, "A picture is worth a thousand words." Actually, there is much more information in a picture than there is in a thousand words. So how do we do it? Well, as another sage would say, "How do you eat an elephant? One bite at a time." So, that is what must be done with the picture. It is broken up into small pieces by a scanning process, and send one line at a time, with each line containing hundreds of bits of the picture. This is called the video signal.

In early cameras, various forms of special tubes were used for this process. The tubes had a special layer that was sensitive to light, and as this layer was scanned by an electron beam, the image that was focused on the tube was converted to an electronic voltage which corresponded to the brightness of the image (see diagram at left, also from the 1939 RCA television handbook). Modern cameras use solid state devices that do essentially the same thing. Color images are sent in the same way, except that there must be a signal for each of the primary colors used.

Terrestrial Television Transmission

So how does the signal get from the studio to your home? What carries the video signal from there to here? The obvious answer: Radio waves. Yes, our old friend radio strikes again. The radio waves that carry television pictures to you are the exact same radio waves that carry the news to you in your car. The only difference is the type of information they are carrying. In conventional broadcast radio, the radio waves carry a sound, or audio signal, while in television they carry the video signal.

Regular television stations use a form of AM modulation known as vestigial sideband (VSB) to send the video via radio waves. Similar to AM, VSB filters out duplicate parts of the modulated signal in order to reduce the amount of channel space, or bandwidth necessary to send the signal. Since television signals take up a lot of bandwidth (typically 1000 times that of audio signals) it is important to conserve channel space as much as possible.

Satellite Television There are two major types of satellite television systems today. "Traditional" satellite TV uses microwave frequencies that are relatively low, and require large dishes for best performance. These are the systems that have been available to consumers since the late 70's. These systems use frequency modulation (FM) for sending the video, which reduces interference between the various satellite transponders (satellite circuitry which relays the TV signal). The new DSS (Direct Satellite System) satellite TV units operate at a higher frequency than traditional units, which allows the dishes to be smaller. In addition, the video signals are converted to digital data which is sent over the satellite link. At the receiver the digital signals are converted back to video which is sent on to the TV set. DSS signals are generally of higher quality than conventional satellite TV.

High Definition Television

By the strict definition, high definition television (HDTV) is television with more resolution, allowing pictures to be sent with finer detail. There are more lines in HDTV than in regular television (almost three times as many) and the picture is wider, making viewing more like "going to the movies." The new proposed standards for HDTV in the United States also use digital transmission, which should improve the overall quality of the picture as well. Now if we could only get the programming quality up to the level of the picture quality...