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1945 Theatre Catalog, 4th Edition, Page 594 (566)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 594
Page 594

1945 Theatre Catalog, 4th Edition, Page 594

ning disc. The scanning is accomplished by causing electric currents to pass through the focusing coils in a way that will displace the electron image at the anode in a regular order and will permit the constantly flowing, variable stream of electrons to flow through the aperture and fall upon another electrode which amplifies their effect. This method is known as the udirect pick-up" scanning system, since the projected image of the scenerto be transmitted causes electrons to be emitted and moved in a stream on the anode during the entire scanning period. The resultant amplified current is then similar to that leaving the iconoscope and is similarly used to modulate the transmitter antenna current.

Operation of Receiver

Let us consider the television receiver, but with the clear understanding that it is only a part of a system.

The television-receiver antenna is energized by the traveling waves which cause corresponding currents to fiow from the antenna to the receiver. These very small currents are amplified by vacuum tubes, and, when of a useful intensity, they operate a device designed to convert them into light images, This device is known as the kinescope or oscillight.

The receiver tube has a plate with a beam of electrons playing upon it just as does the pick-up tube. In the receiver, however, the plate, or screen, is actually the distal end of the tube itself, made nearly flat, and coated on the inside with a very thin layer of iiuorescent material, which gives off light wherever the electrons strike it. Some fluorescent materials will glow for some time, but the peculiar compound chosen permits the glow to die out shortly after the electron beam moves away and before it returns again.

The tiny electron beam in the receiver tube, whenever it is not moving, and, therefore, striking the screen in one spot causes a bright glowing spot on the screen at the point of contact. This spot is about the size of a pinhead. Although the glow is really on the inside of the tube, it is visible on the outside because the end of the tube is clear glass, and the screen of iiuorescent material is very thin. The brightness of the spot

FIGURE 4.-How an alternating current from a microphone modulates the constant high-frequency antenna current is shown here. With a dead microphone, the antenna current is of constant intensity, the wave forms being of uniform height. The height of these wave forms is varied by the microphone current. The reception

depends on the strength of the electron beam and varies as the strength of the beam varies. That spot of light is used to reproduce each Spot of the picture, one at a time, by moving around all over the picture area. It must be moved in exactly the same way that the pick-up tube beam is moved, which, in the modern systems, is in horizontal parallel lines from top to bottom. As this beam moves all over the picture in regular fashion, and repeating the circuit thirty times a second, it varies in brightness continually, tipainting" the highlights and shadows of each tiny element of the picture. To our slowly reacting human eyes, the spot will not be visible because it is moving so fast, but we must remember that, actually, the- light and the scenes are caused by one tiny spot of light, fiying over the screen and varying in brightness as it goes.

Much of the receiver apparatus is for controlling the movement of the beam: feeding to it the currents which have been received from the transmitter in order to vary the strength of the beam and, therefore, the brightness of the flying spot. The beam of the receiver tube and that of the pick-up tube many miles away, must be accurately synchronized and kept in perfect step. Thus it is that a picture image, optically focused on the plate of a pick-up tube, is reproduced on the fluorescent screen of the receiver tube.


Many requirements of a technical nature must be satisfied before television can occupy its destined place in the electronic arts. It is desirable to consider the more important so that the reader will be able better to appreciate the present state of the art as well as some of-the obstacles that have had to be overcome and how television is really possible.

As in motion pictures, the degree of technical perfection of the reproduced image may be measured in part by the detail it contains. To produce a system that will transmit and reproduce pictures of acceptable detail has been one of the most severe problems in the development of television. *

The amount of detail that can be transmitted by a television system depends upon the number of picture ele ments resulting from the scanning process. The number of picture elements, then, depends upon the number of horizontal lines by which a complete picture is scanned. A picture element is substantially round and has a diameter equal to the distance between the centersof the adjacent scanning lines. i

Rate of Scanning

As a yardstick in determining comparative definition, it is interesting to note that photographs reproduced in the better newspapers are made up of approximately 4,000 picture elements to the square inch. Rotogravure pictures of excellent quality have as many as 10,000, and the half-tone illustrations in this publication range between 10,000 and 14,400 picture elements to the square inch.

As mentioned previously, there must always be a compromise between picture definition and equipment potentialities. During the experimental stages of television the number of scanning lines (horizontal) to the picture has been gradually increased as the apparatus was perfected. When television was first commercially introduced in 1940, a-picture with 441 horizontal lines was used. Shortly thereafter, when various groups in the industry created serious agitation for reconsideration of standards, a detailed study and analysis of all of the problems involved was made, and a standard of 525 lines was established. This, however, is specincally limited to black-and-white images, as a different standard may be desirable for images in color.

Images with 525 horizontal lines compare very favorany in quality with images of equal size projected from 16mm. film. (Figure 5.)

In television, as in motion pictures, two considerations are involved in determining the rate at which the scanning operation must be repeated. The rate of repetition must be enough to give the appearance of reasonable continuous and natural motion in the reproduced scene, and must be great enough to minimize unsteadiness, or fiicker, in the reproduced picture. Continuity of motion may be maintained with a repetition rate of 16 pictures or frames a second. This is the rate used in the days of the silent motion picture. At least 48 frames 3.

at the radio waves involves the generating of electric currents in the receiving antennas when it is struck by the traveling waves of varying strength. These currents, after being suitably amplified, actuate the loudspeaker, which converts these electric waves into the some sound waves that originally reached the microphone.


1945 Theatre Catalog, 4th Edition, Page 594