A Picture Becomes a Parade
Now, we’re going to find out how television works. Let’s start in the television studio. Suppose we want to televise Linda Darnell. Television has three main jobs to do to bring Linda Darnell in motion into your living room. It must take a continuous series of pictures of her, it must send them through space, and it must reproduce them for you.
The studio from which Miss Darnell is to be televised looks very much like a movie studio. There are a large and experienced crew of technicians, cameras, big lights, sets, props, microphones, and so on.
The television camera, which looks like a studio motion-picture camera, is wheeled up in front of Miss Darnell. The picture is lined up, just as a movie photographer would do it. The sound technician swings a microphone above Miss Darnell’s brunette head, close enough to pick up the sound of her voice, but out of the range of the camera’s lens.
The light reflected from Linda Darnell’s face and figure passes through a system of lenses into the inside of the television camera, which engineers call the “iconoscope.”
Inside the iconoscope, Miss Darnell’s image is projected onto a rubidium metal plate. This plate is called a “mosaic” and is mounted inside the cathode-ray tube.
Although it is smaller than a penny post card, the mosaic contains several hundred thousand electric eyes. Every time a ray of light falls on one of these, it stimulates a minute electric charge, the amount of the charge varying in proportion to the intensity of the light falling on the electric eye.
By this means we have converted the light waves of Miss Darnell’s image into electric charges of varying intensity.
The electron gun
The problem now is to pick these electrical impulses from the mosaic, arrange them in order so that they can be broadcast, and put them back together in your television set so that you will be able in your home to see Linda Darnell as she appears in the studio.
This job is done by the smallest thing yet discovered in the world-the electron. In the neck of the cathode-ray tube, there is an “electron gun” that shoots a steady stream of rapidly moving electrons onto the mosaic. This is called the cathode beam.
Controlled by magnets which bend it from side to side and gradually downward, the cathode beam “scans” the mosaic in successive horizontal lines. One by one and row by row it spotlights the cells of the mosaic just as you are scanning the words on this page right now. The main difference is that the electron beam moves a great deal faster than your eye. If there were 525 lines of type on this page, in-stead of about thirty-five, and if you could read thirty of these pages a second, then the comparison would be exact.
As the cathode beam passes over each cell in turn it releases the tiny charge built up in each one. Remember that where the image is bright the charge is stronger than where the image is dark. The resulting series of small electrical impulses is picked up by a second plate sandwiched to the back of the mosaic and led out of the cathode-ray tube as a current of varying strength. It is this fluctuating current that goes to the television transmitter to be amplified and broadcast.
If television were as simple as this description sounds, we would probably have had it years ago. Actually it took scientists almost a century to develop a satisfactory iconoscope, once the idea for it had been born. The mathematics of what goes on inside the cathode-ray tube is by itself complicated enough to send an Einstein off to the seashore for a vacation.
After Linda Darnell’s picture has been transmitted from the mosaic to the television broadcasting station, it is amplified and sent out into space in a parade of about 10,000,000 impulses a second. These impulses finally reach the television antenna on the roof of your house. They march down the lead-in wire of your set, where they are again amplified by electronic tubes and sent into another funnel-shaped cathode-ray tube called the “kinescope.”
The kinescope has a cathode beam shot from an electron gun just like the one in the iconoscope back in the television studio. But at this end the intensity of the beam, instead of being steady, varies in accord with the incoming impulses.
Instead of a mosaic, the kinescope has a screen at the end of the funnel. This screen is coated with a fluorescent material that glows when electrons strike it. A set of magnets, working in harmony with those of the iconoscope, swings the kinescope’s cathode beam back and forth across the screen at the same time that the electrical impulses are transmitted from the mosaic in the studio miles away.
Depending on the strength of the electron beam, each of the little pieces of Linda Darnell’s picture glows light or dark on the screen. The result is a picture of her that is renewed 30 times every second—a continuous series of pictures of her for you to enjoy just as a motion picture.
How television works
The television camera or television consists of the iconoscope, an electric tube, which converts the light image into an electrical signal. The camera is trained on the person or scene and the image is focused on a photo-sensitive plate at the back of the tube. The tube converts the image into a series of electrical impulses which are amplified and shaped into wave forms by vacuum tubes. The impulses are carried by cable to the antenna and there transmitted.
The television receiver picks up the television waves, retranslates them into a stream of electrons which plays across a fluorescent screen in the large end of a kinescope tube. Electrical impulses are converted by the tube into light. The varying degrees of light form the image of the person or scene on the screen of the television receiver. Actually a series of still pictures are being broadcast at the rate of 30 per second which, when observed, form a smooth continuity of action similar to motion pictures which operate at 24 frames a second.
At the same time, the sound waves are broadcast and received through a radio which is part of the television set, so that you both see and hear the program.
The high speed with which these pictures change gives you very little flicker. The glowing television screen is fairly bright, but the pictures show up best when viewed in a darkened room.
The voice of Linda Darnell, to accompany the picture, is broadcast over another radio system and is received through a separate radio in the same cabinet with the television receiver.
Scientists are working to improve and perfect a method of sending both sound and pictures together. By turning the voice into electrical impulses and projecting it onto the mosaic at the same time that the picture is picked up, sound could be carried along with the picture like a sound track on motion-picture film.
We have followed the image of Linda Darnell from the studio to your living room. But television is not confined to indoor or studio programs alone. Mobile field units, consisting of a power supply, camera, and microphone, and a low-power transmitter have been designed to pick up pictures of outdoor scenes. These units will make news and special events a primary source of television programs.
After the war, you won’t need to have a sports commentator tell you that the ball has been put in play, the pass completed, the tackle made, and so on. If you have a television set, you’ll see it all, as it happens, with your own eyes. Several television stations have mobile transmitters in use today. The University of Pennsylvania football games have been telecast for several years, and television audiences have enjoyed Madison Square Garden events-rodeos, ice shows, tennis tournaments, and boxing bouts. Television audiences have also witnessed political conventions, speeches by the president, the feeding of seals at the Bronx Zoo, Fifth Avenue parades, and other events.
Portable camera and transmitter units may be set up for special events anywhere within a ten-mile radius of the main transmitter. Ten miles is the maximum effective range of the portable relay transmitter. This range is long enough, however, to permit the pickup of most locations within a big city.
What types of programs might be best suited for studio presentation? For remote presentation? Would there be enough interest in such features as feeding the seals at the Bronx Zoo to warrant the expense of setting up mobile equipment?