Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, June, 1930 Page 17 ture is surprising. In the case of a close-up, it is possible to represent say a policeman's face, clearly showing the cap with shield. One instantly knows it is a policeman's face. Or again, a really good looking girl may be reproduced— features, hair, necklace, attractive eyes. If the action requires more of the subject than just a close-up, a silhouette study with as many as three characters may be reproduced, with good story-telling possibilities. Voice or sound accompaniment, however, helps immeasurably and should be included whenever possible. So much for the program end, but what is beiug done for the home end? The answer is, plenty. Recently, engineers have developed simple equipment which serves well enough for the existing technique. The equipment is roughly divided into three different classes, namely: (1) Equipment for the avowed layman, who is interested solely in what he can see, and cares positively nothing for the experimental end. (2) Equipment for the layman who does not mind adding or changing the equipment a bit as time goes on, in keeping with advances in the art, yet desires to start out with something tangible. (3) Equipment for the experimenter who gets the real thrill out of building his own from tried and proven parts. In the first class, there are products now available for the average home in the form of simple radiovisors and special radiovision short-wave receivers. One popular model is in the form of an attractive cabinet, with recessed opening or shadowbox at the front, and control panel below. As many as eight persons can view the pictures at one time, due to the large magnifying lens. A companion device is a special shortwave receiver without regeneration and with sufficiently broad tuning so as to take in the necessary width of side bands for good radiovision detail. The receiver includes the power pack for full a-c. operation. In the second class is a simple, stripped outfit, with parts entirely exposed. One type now becoming popular makes use of a simple form of Faraday eddy current motor, which runs silently. A 48-line disc is supplied, although 60hole and 24-hole discs are interchangeable, with corresponding speed rotors. A third class is in the form of a kit of components, including all parts machined, drilled and threaded, so that the average boy or man can assemble his own radiovisor. The problem of synchronism, once considered serious, is no longer looked upon as insurmountable. For the present, the usual a-c. power line serves as the synchronizing agency particularly when the transmitter is on the same power system. On different a-c. power systems, the receiving scanning disc, operating synchronously from the power system, may require some manual adjustment from time to time, although it is not difficult to maintain synchronism with the transmitted images. Of course the present equipment is far from perfection. It is not in the same class as the usual home movie equipment. It is far from the relative perfection attained by sound broadcasting after ' ten years of constant development. But even so — even allowing all the criticisms against radio television — even after disillusioning those who expect to see the World's Series Games broadcast on a screen the size of a picture theatre screen — we still maintain that radiovision is about ready for commercial exploitation. And why should it not be entitled to the same consideration from the sympathetic public, as sound broadcasting received in its pioneer days? The Chief Problems of Television By George H. Robinson * AS may be surmised from the title, this article does not attempt to offer suggestions on how to build a television apparatus, nor does it attempt to describe the functionings of such an apparatus. It is written, rather, for the purpose of conveying to the engineer, as well as the layman, a plain story about the chief difficulties that today are engaging the attention of experts in the subject. The problem of television in its general sense is converting signals into electrical signals and then back into light signals. After these two essentials have been accomplished it remains to develop the required degree of sensitiveness, speed, efficiency and accuracy. A theoretically possible television set might be expected to duplicate the functioning of the eye. The image is formed upon the retina, a sensitive screen, which consists of a multitude of light-sensitive elements. Each element is connected to a nerve fibre which goes directly to the brain. This University of Louisville. group of nerve fibres constitute the optic nerve. Thus, a large number of photosensitive elements could be connected each with an individual channel leading to a distant point and signals sent simultaneously from the various elements, and used simultaneously to re-create the scene at the distant point. It is immaterial to the eye, however, whether the whole scene is surveyed at once or all of its various elements viewed individually in less than one-sixteenth of a second. This phenomonon is explained by the functioning of the human eye. Function of the Scanning Disc In television, the scanning disc, which runs over the elements of the image in sequence, transmits the various elementary signals individually. Sometimes, when more than one channel for transmission is to be used, the signals may be broken into several series of simultaneous signals. An intense light is reflected from the image onto the scanning disc. This beam of light passes through the holes of the scanning disc and is registered on the photoelectric cell. The light falling on the photoelectric cell is greatly diminished in brilliancy, due to the speed of the rotating disc and to the fact that the light falling on the image must not be so bright that it will be a hazard tc the image, in case it is a human. Of course, the signal coming from the cell may be amplified, but this must be done so that tube noises and other sources of interference will not seriously interfere with the desired results. Assuming that the desired electrical signal is produced, the next step is the transmission of this signal. Formerly, in the system of the A. T. and T. Co., for the transmission of still pictures, a frequency band of 400 cycles was used to transmit a picture of 5 inches by 7 inches, the operation taking seven minutes. On a television set using a scanning disc and obtaining a picture of the same size and fineness of grain it would be necessary to use a frequency band of nearly 7000 times as great as now used, or, in other words, a frequency of approximately 3,000,000 cycles. Wire circuits are not designed to utilize frequencies higher (Concluded on page 19)