Radio Broadcast (Nov 1926-Apr 1927)

462 RADIO BROADCAST MARCH, 1927 according to a pre-arranged succession, so as to make a continuous orderly picture consisting of 300,000 separated or blended light impressions. In the words of Doctor Alexanderson: "When we embark on such an ambitious program as television, it behooves us to reason out, as far as it is possible, whether the results we expect to get are going to be worth while even if our most sanguine hopes are fulfilled. We are dealing with the photoelectric cell, the amplifier, the antenna, and the radio wave. The photo-electric cell and the amplifier employ the medium of the electron which is extremely fast, but the use of the radio wave itself imposes certain speed limitations on account of the limited scale of available wavelengths. The question therefore remains what quality of reproduction may we ultimately expect in a television system if we succeed to take full advantage of the ultimate working speed of the radio wave? An experimental study of the problem, and the conclusions, may be illustrated by the comparison, of some pictures made at different speeds. "The three pictures (shown on this page) were made with the selective shade process under conditions similar in character to that of one of our long -wave transatlantic transmitting stations, with a wavelength of 12,000 meters, or a wave frequency of 25,000 cycles. The picture to the left is the result we get if the time of transmission is two minutes. For the middle picture, the transmission time is four minutes and, for the picture to the right, eight minutes. Everything else in the three cases is identical. Relatively these pictures represent the effect of sluggishness of the tuned antenna upon the sharpness in the reproductions. The two-minute picture is not as sharp as the eight-minute picture. ; "By using a wavelength of twelve meters instead of 12,000 rheters (a wave frequency of 25,000,000 cycles instead of 25,000 cycles), the sluggishness can be reduced. If now, the photo-electric cell and the amplifier and the light control can keep Up with this pace, the radio wave will do its part and transmit a picture in i-ioooth part of two minutes, i. e., in one-eighth of a second. We are thus able to predict that it will be possible to transmit a good picture in a space of time which is of the order of magnitude of the time required for moving picture operation, the exact figure being one-sixteenth of a second." THE MODEL PROJECTOR IN DOCTOR. Alexanderson's laboratory, *■ there is a model of a television projector, consisting of a source of light, a lens, and a drum carrying a number of mirrors. When the drum is stationary, a spot of light, reflected from one of the mirrors, is focused on a spot on the picture to be transmitted. This spot of light is the brush that paints the picture. When the drum revolves, the spot of light passes across the picture, as a new mirror, which is set at a slightly different angle, comes into line to reflect the beam. Thus the light spot passes over the picture until the whole is covered. If we expect to paint a light picture of fair quality, the least we can be satisfied with is ten thousand separate strokes of the brush. This means that thespot of lightshould pass over the picture to be transmitted in one hundred parallel paths and that it should be capable of making one hundred separate impressions of light and darkness in each path. If we now repeat this process of painting the picture over and over again sixteen times in a second to obtain satisfactory television, it means that we require 160,000 independent strokes of the brush of light in one second! To work at such a speed seems at first inconceivable; moreover, a really good picture requires that the speed be raised to something like 300,000 picture units per second. Doctor Alexanderson says: "Besides havingthe theoretical possibility of employing waves capable of high speed signalling, we must have a light of such SUCCESSFUL TRANSMISSION ON LONG WAVES Is not so rapid as on the short ones, as far as radio photograph transmission is concerned, due to a lag element which creeps in. These pictures were transmitted with 12,000-meter (25,000-kc.) apparatus. That on the left was sent in two minutes; that in the center, in four minutes; and That on the right, in eight minutes "The gain in using seven beams of light in multiple is twofold. In the first place, we get the direct increase of illumination of 7 to 1, but we have the further advantage that the speed at which each light beam must travel on the screen has been reduced at a rate of 7 to 1, because each light spot has only 24 tracks to cover instead of 170. While the light itself may travel at any conceivable speed, there are limitations of the speed at which we can operate a mirror drum or any other optical device, and the drum with 24 mirrors has already been designed for the maximum permissible speed. A higher speed of the light spot can therefore be attained only by making the mirrors correspondingly smaller, and a mirror one-seventh as large will reflect only one-seventh as much light. The brilliancy of the light spot would therefore be only one-seventh of what we realize by the multiple beam system, which gives seven light spots seven times as bright, or 49 times as much total light. "There is another advantage in the use of the multiple light beam. Each light beam needs to move only one seventh as fast and therefore needs to give only about 43,000 instead of 300,000 independent impressions per second. A modulation speed of 43,000 per second is high with our present radio practice, but yet within reason, being only ten times as high as we use in broadcasting." There is ample promise in the fruition of Doctor Alexanderson's work. By the use of seven television carrier waves, spaced 100 kilocycles, available, for example, by using the band between 20 and 21 meters (1 5,000 and 14,284 kc), and an improvement of existing equipment, television is entirely within reason. Howlongthis process of refinement will require, Doctor Alexanderson is unwilling to predict, but it seems to be in prospect within a relatively short period. brilliancy that it will illuminate the screen effectively, although it stays in one spot only one three-hundred thousandths of a second. This is one of the serious difficulties because, even if we take the most brilliant arc light we know of, and no matter how we design the optical system, we cannot obtain sufficient brilliancy to illuminate a large screen with a single spot of light. The model television projector was built in order to study this problem and to demonstrate the practicability of a new system which promises to give a solution to this difficult)'. The result of this study is briefly that, if we employ seven spots of light instead of one, we will get 49 times as much useful illumination. Off hand, it is not so easy to see why we gain in light by the square of the number of light spots used, but this can be explained with reference to the model. The drum has twenty-four mirrors and, in one revolution of the drum, one light spot passes over the picture twenty-four times, once for each mirror; and when we use seven sources of light and seven light spots we have a total of 168 light spot passages over the screen during one revolution of the drum. SIMPLE SYSTEM BEING DEVELOPED ANOTHER line of experiment being carried out by Doctor Alexanderson, dependent not upon accurate modulation and demodulation, is a simple system of radio photography (not television) which promises to bring it soon within reach of amateur experimenters. The system is not subject to the vagaries of fading. It does not depend on accurate modulation as do the transmission of music and full tone radio photography, but upon interruption, as does code radio telegraphy. The amateur radio photographer — and there will soon be many of his kind — will obtain a permanent proof of the reception every time he uses his photo-receiver. Unlike the broadcast fan, he cannot say that he heard with perfect clarity and comfortable volume a distant station, if he did not do so. The radio photograph gives unmistakable evidence of reception and of the distortion-free operation of the receiver.