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1945 Theatre Catalog, 4th Edition, Page 593 (565)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 593
Page 593

1945 Theatre Catalog, 4th Edition, Page 593

irrespective of how complex each sound may be. Various tones may compose it, but it has only one sound, and it can be represented by only one current. An instant later, another sound can be represented by another current, and we can thus transmit faithfully by radio a progression of single instantaneous sounds. A picture, however, even on an instantaneous flash, is not a single thing of any sort, and certainly cannot be described or represented by one electric current or one anything else. It is composed of many little elements, one for each area of that size which the eye can just distinguish. For example, if we look at a scene the dimensions of which are 10 feet (120 inches) on each side, from a distance at which the eye can just distinguish objects one inch in diameter, there are nearly 15,000 (120 times 120) small, one-inch square areas which must be described individually to convey the whole picture. The description probably will be in terms of light intensity.

Still vs. Moving Transmission

Television, the transmission of moving scenes, requires transmission of many separate pictures each second. Just as in motion pictures, it isnecessary to transmit individual still pictures with sufficient successive rapidity to make use of itpersistence of vision" by which the spectator is given the illusion of a continuously moving scene.

Sound motion pictures are projected at the rate of twenty-four different still pictures a second. To obtain comparable results in television, therefore it is necessary to find a way to transmit at least tw0 dozen pictures a second-to send out information about each small element of each picture, repeating the process many times each second.

There in a nutshell we have the primary cause of practically all the television engineering problems. It may be described as a requirement of transmitting an enormous amount of information with great accuracy in a very short space of time. Let us now see how this may be accomplished.

Of course, we have only light to start with. Any object or scene is visible because of the light waves which are reflected from it to the eye. For television we desire to capture these light waves with some kind of device which will con. vert them into electric waves which can then be used to modulate, or control, the radio currents a generator feeds into the transmitting antenna. The best way to convert light waves into electric waves is by means of some sort of photo-electric device.

There are two different devices in television used to convert light waves into electric waves-the iconoscope and the image dissectorewhich are comparable to the microphone in sound transmission.

The lconoscope

The iconoscope described here is the pick-up device used by the Radio Corporation of America in its television system.

The iconoscope has two main parts. One is a plate upon which is focused, by







FIGURE 3.-A radio telephone transmitting station is shown in the manner of Figure 2. Of course the microphone is not directly in the antenna circuit, in practice, but is coupled to it through amplifiers.

ordinary optical means, the scene to be televised. It corresponds exactly to the plate of film in a photographic camera; however, its surface is covered not with a photographic emulsion but with microscopic light-sensitive elements, or photoelectric cells. Each one separate from the other, generates electric voltage when light strikes it, with the voltage being proportional to the strength of the light.

When a picture is focused on this plate, with various parts of the picture at various degrees of brightness, the tiny photo-electric cells that have no light upon them generate no voltage; those with bright light upon them generate a strong voltage, and those with an intermediate degree of light, of course, generate an intermediate voltage. The problem, then, is simply to collect the various voltages off the plate in order to use them.

One method of doing this might be to employ some sort of mechanical system which causes a tiny wire to brush against the plate and sweep all over it with uniform strokes, thus making contact with the Whole area bit by bit. However, since it is necessary to sweep the entire plate so fast and so many times a second, it is impossible to devise any mechanical system light enough to be so moved. So, a brush which has no measurable weightea beam of electrons-is used for this purpose.

The second main element of the iconoscope is an arrangement for generating this small beam and directing it so that it falls on the plate in one tiny spot. Other electric arrangements cause this spot to move all over the plate, in regular fashion, sweep by sweep. The greater the number of sweeps (called lines) the more accurately will the picture detail be reproduced. However, the more lines used, the more information has to be transmitted in the same length of time, and this makes it more difficult. The number of lines chosen, therefore, must be a compromise between

the opposing factors of desired picture quality and possible apparatus effectiveness.

Electron "Searchlight" Beam

The little electron "searchlight" beam, sweeping across the plate with its regular brush strokes, acts just as a wire brush would, and collects electricity from the cells on the plate as it passes over them. The electron beam sweeping across the plate hits at any one moment only one little spot, which, incidentally, is smaller than a pinhead. After the beam has completed its travel all over the plate, it comes back to the starting point to collect the voltage subsequently developed. In fact, the beam sweeps across the entire plate thirty times a second, collecting electricity wherever there is any present-which means, of course, wherever there is any light.

The electron beam originates in that part of the iconoscope called the electron gun or cathode. The cathode is covered with certain chemical compounds which give off electrons when heated, and the cathode in the iconoscope may be heated readily by a current, just as a lamp filament. Therefore, the cathode is a part of the electron beam; and if we connect the cathode and the plate to external apparatus, we can draw off the electricity which the beam collects from the plate having the light image focused upon it.

The electric currents thus obtained from the image plate by the electron beam are, of course, very small. But they can be amplified to useful intensities, and then we would have currents carrying intelligence representing the light pictures; and these currents can be used to control the transmitter antenna current.

In short, we have a system which is operating to pick up scenes, a spot at a time, and covering these spots so quickly and the whole scene over and over again so many times a second, that if we arrange a reproducing system to act in reverse fashion and to deliver light images corresponding to the spot currents, our very slow human senses will not follow the details of the process and will perceive only the average total result, which is the completed picture.

Image Dissector

The image dissector is the basis of the Farnsworth television system. In this device, the picture to be scanned is focused through an ordinary camera lens upon a translucent, photo-sensitive cathode located at one end of a cylindrical glass tube. As the whole scene falls upon this cathode at once, myriad electrons gyrate backwards through the tube toward the anode.

The focusing coils around the outside of the tube straighten out the electrons and move them in orderly, parallel lines, so they end up raining upon the anode with a distribution of electrons corresponding to the distribution of light intensity upon the cathode. There is a small aperture in the center of the anode which corresponds to the hole in a scan
1945 Theatre Catalog, 4th Edition, Page 593