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1945 Theatre Catalog, 4th Edition, Page 595 (567)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 595
Page 595

1945 Theatre Catalog, 4th Edition, Page 595

second are required, however, to, minimize dicker unless some artifice be em ployed.

Persistence of vision is that characteristic of the human eye which causes a retinal image to remain for a short period of time after the actual image focused on the retina of the eye is changed. In other words, the human eye reacts comparatively slowly, so that if the images on the retina be changed with sufiicient rapidity, an impression of continuously moving pictures is created.

Motion pictures are projected at the rate of 24 frames 3. second at the present time, and the artifice to reduce flicker, and also solve certain mechanical problems, takes the form of an additional blade upon the projector shutter that interrupts the light while the film is being moved, or while one picture is being pulled dOWn and the next one moved into place for projection. Thus, as far as flicker is concerned, the projection is in effect 48 frames a second, each still picture on the film being projected twice, since the light is cut off twice by the two-blade shutter, once while the film is moving and once while it is standing still.

Such an artifice is not applicable in television. Some other method must be devised. Interlaced scanning is employed to provide satisfactory freedom from flicker. In this procedure, instead of scanning the picture in adjacent horizontal lines from top to bottom, alternate lines, or every other line, covering the entire area of the picture are first scanned and then the beam returns and scans the omitted lines. The entire picture is scanned 30 times a second, but the picture is covered in alternate lines 60 times a second. This arrangement is most desirable to eliminate flicker because it permits pick-up from motionpicture film which must be run at the standard rate of speed to permit suitable reproduction of a standard sound track.

There is another requirement in television which affects the frame frequency. This is the relation that should exist between the frequency of the power supply to both the transmitter and the receiver and the repetition rate. It is desirable that the repetition rate be an integral divisor or multiple or sub-multiple of the power-line frequency. This is necessary in order to minimize certain non-syn chronous interference effects which otherwise might be detrimental to the picture.

Television transmitters and receivers are designed to operate in this country on a 60-cycle, alternating-current power supply. Thus, a frame frequency or repetition rate of 30 frames a second fulfills the requirements, whereas 24 frames a second would not. This is the smallest sub-multiple of 60, the double of which is above the maximum flicker frequency observable by the eye (48 frames 3 second).

Transmission Data

The transmission electrically of highdefinition images over a single channel, cable, or radio requires very wide fre 1945-THEAYRE


FIGURE 5.-lmages with 525 horizontal lines compare very favorably in quality with images of equal size proiected from 16mm. film, as is evidenced by this photograph from an actual television image. In television, as it is with motion pictures, the scanning rate must be suFfi:iently rapid to give the appearance of continuous and natural motion in the reproduced scene and to minimize unsteadiness or flicker in the reproduced picture.

quency band apparatus and circuits. This is occasioned by the rate at which information must be transmitted relating to brightness of a very large number of picture elements.

For example, a television image with an aspect ratio (the relation of the length to height) of 4 to 3 Will contain about 250,000 picture elements. This is determined by the square of the number of lines times the aspect ratio. For an image with 525 lines with equal horizontal and vertical resolution, or detail, there would be more picture elements than 250,000, but when the standard was changed from 441 to 525 lines, it was recognized that, to maintain the picture elements at about 250,000, there would be some sacrifice of horizontal resolution while fine vertical resolution has been preserved.

When 30 pictures a second are scanned, information must be transmitted concerning the brightness of 30 times 250,000, or 7,500,000 picture elements a second must be transmitted. The total frequency band required for transmitting a picture as described above is about 4,000,000 cycles a second, or about 4 megacycles a second.

Four megacycles a second is the width of the frequency band to accommodate the highest frequency television signal that must be amplified and carried by the apparatus and circuits in the television system. It is that frequency band by which the carrier-wave, or antenna current, of the radio transmitter must be modulated. The total radio transmitting channel will be 8 megacyeles when the carrier is modulated by the picture signal, 4 megacycles above the carrier and the same number below. This is equal to the combined widths of 800 sound broadcasting channels of 10,000 cycles a second each.

Channels of such great width are not available in the frequency spectrum or range now used for radio services. For this and other reasons related to technical requirements, the ultra-high fre quency-or ultra-short waves-are used for television. Frequencies above 30 megacycles, equivalent to below .10 meters (number of cycles a second times

. number of meters always equals 300, 000,000) are employed.

Ultra-short waves have properties in propagation similar to optical waves. The range over which satisfactory high deiinition television pictures may be reliably transmitted by ultra-short waves is limited practically to the distance of the horizon from the height at which the transmitting antenna is placed. Under some abnormal conditions, pictures may be reCeived over greater distances for periods of very short duration, but primarily television stations will serve local areas. The signals from the stations in these local areas will be stable and will have about the same intensity during the day and night hours and during all seasons of the year.

The explanation of this relatively short distance of satisfactory transmission is that these very short radio waves are not reflected by the heaviside layer. The heaviside layer is a layer of air, filled with electrons and ionized gas molecules, which acts like a good reflector for electromagnetic radio waves. It is located about 100 miles from the earthis surface, and its height changes with sunlight conditions. It is higher at night than in the day time.

Radio waves of the order of the regular broadcast band (550 to 1600 kilocycles a second) are reflected and may thus be transmitted over long distances, being reflected from the heaviside layer to the earth and back a number of times. Television waves are not redected by the heaviside layer, so that only direct waves are useful.

In order to economize on the use of the frequency band required, tvestigial side band" transmission is used. In this system, the partial elimination of one side ,band is achieved by the use of band-pass filters which have a range of partial transmission in the range on
1945 Theatre Catalog, 4th Edition, Page 595