Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, May, 1930 Page 7 A Practical Television System1 THE purpose of this paper is to present the general outlines of a practical television transmission system that is capable of daily service operation, transmitting the best picture obtainable within the limitations of contributing factors. Such a system recognizes and compensates where possible for deficiencies in the contributing elements. Undoubtedly in its present form the means for production of the television signal has progressed beyond the capabilities of the apparatus employed in its propagation. Considering each component part in a paper would be endless, but a general description of a transmitting system with more or less emphasis on the points found most difficult is believed to be of interest. For this reason I have made but brief mention of the reception of the television signal, leaving that for another time or to someone else. No doubt all are familiar with the structure of the image as it is transmitted through space. In brief review it will be necessary only to recall the fact that the frequencies encountered range from the picture frequency, in our case 15 cycles (we are dealing with a 48-line system having a picture frequency of 15 per second) and capable of a horizontal resolution of 64 elements (the actual resolution we attain is greater) up to the highest frequency involved, which is half the number of picture elements multiplied by the frequency of repetition 48 x 64 x 15 or 23,240 cycles. There is nothing new in Nipkow's disc nor in the basic methods of scanning employed. There are two main systems used in our transmission of scheduled programs — one employing motion-picture film with film recorded, or disc synchronized sound, and the other a direct pickup arrangement, capable of operation with a normal degree of artificial lighting, or, in the field, with the subject illuminated by sunlight alone, or by scanning the subject with a flying point of light and picking up the reflected light with a multiple bank of photocells. By D. E. Replogle * In the film pickup system as substantially shown in Fig. 1, the film is steadily advanced at a speed of fifteen frames per second and is projected, enlarged to about three times its original size, upon an aperture so adjusted as to represent one forty-eighth of the picture height. This aperture is scanned by a disc rotating at a corresponding rate and slotted with fortyeight narrow apertures. Because of the uniform advance of the film no intermittent action is necessary in the advancing mechanism. In addition it will be observed that the disc turntable for sound synchronized material is geared directly to the drive motor that actuates the film advance. I would like to stress the importance of a uniform light source. The arc is rather vague as to light frequency values, is prone to the introduction of shifting images of the crater, and is highly unstable in illumination intensity. On the other hand, the incandescent lamp has little radiation toward the low or ultra-violet end of the spectrum where the cells are most efficient. The ideal source is as yet unknown but research points to the ultimate use of a sturdy ribbon filament worked at an extremely high temperature and used in conjunction with a quartz envelope and lenses to conserve all of the ultra-violet possible. Special Sound Recording Naturally, the speed of 15 frames per second made it impossible for us to employ standard film and our sound matter had to be recorded specially. The purpose of scanning an enlarged picture, as shown in the sketch, is one of increased resolution, there being a lower limit to the width of scanning aperture which we are capable of accurately producing. By scanning an enlarged picture we were able to produce a scanning element so fine when applied to the picture width as to allow us to attain a horizontal definition of the order of a thousand elements, that is, a degree of excellence far surpassing the capabilities of contributing equipment. I mentioned before that this system required the use of special film and likewise special recordings. We, therefore, desired to rearrange our equipment to employ standard film at standard speed so as to utilize sound synchronized material of a normal character. It was neither possible to scan the image cast by the ordinary projection machine, due to the intermittent gear, nor was it desirable to increase the speed of our own equipment so as to advance the film at the usual Fig. 1. Schematic of film pickuo system. t Delivered before the Radio Club of America, March 12, 1930. * Acting Chief Engineer, Jenkins Television Corporation, V/ert Z oovmq At /vice Of Disc