International projectionist (Jan-Dec 1939)

now allocated to all broadcasting in the United States! Another important consideration in television development is the problem of picture brightness. Cathode-ray tubes used in television receivers at present, are as bright as could be desired in a darkened room. Viewed in the daylight, however, or even in a well-lighted living room, their brightness is deficient. While it is always possible to darken motion picture theatres, television receivers will probably be expected to be more versatile, and to operate in bright bght as well. The problem of increasing picture brightness is being attacked in many ways. Operating voltages for instance can be and are being increased. This, however, is undesirable from the standpoint of safety and cost. More efficient luminescent materials are, of course, the most obvious solution, and such materials are constantly under development. Another interesting development in this connection is the direct-viewing tube. This differs from the ordinary tube in that the bombarded side of the screen is viewed, instead of the opposite side, as is customary. Such tubes naturally require a construction of unorthodox shape. However, they may be the tubes of the future, both for reasons of brightness and also for reasons of contrast and detail, as will be pointed out later. Maloff reports a direct-viewing tube having a maximum useful brightness of 100 candles per square-foot. This is more than ten times as bright as the highlights in a high-quality motion picture. • Television Picture Contrast Finally, there must be considered the matters of contrast and detail. The present contrast available in television tubes is quite good, but much still remains to be done. For one thing a cathode-ray tube exhibits the phenomenon of halation. This is the optical effect of the diffusion of light in the screen material, and with it we may also group the internal reflection of light from the walls of the tube. Halation is well known in photography. It decreases the brightness of highlights and diffusely lights up points which are supposed to be dark, particularly in locations near the highlights. The general effect is thus to f'ecrease the available contrast and to limit the possible fine detail. The direct-viewing tube is a very effective means of decreasing halation. When such a tube is used, the increased contrast is very striking. In addition to halation, a cathode-ray tube, also exhibits the phenomenon of "blooming," which is an electrical effect and results in defocusing the spot in the highlights. Improved focusing arrangements can be used to decrease "blooming," but even in the best of modern tubes it still is a problem. Since the contrast desired in a television picture requires an electronic beam of varying density, the focusing of the tube must be so arranged that the focal point does not change with current density, i.e., brilliance. This is not an easy problem. However, it is evident that before 2000-line pictures are ever obtained, great advances must be made in the cure of "blooming." • Propagation of Signal The problem of signal propagation in television assumes an importance which, in many respects, is far more serious than that of the corresponding problem in sound transmission. In the first place, the exceedingly wide frequency channels required in television make it necessary that the signals be transmitted in the ultra-short-wave bands. At these frequencies, as is well known, there exists reliably only line-of-sight transmission, since there is no longer reflection from the Heaviside layer. While this fact limits the area of coverage of any transmitter, it is actually very desirable from the standpoint of interference. Thus there is far less likelihood of multiple images caused by multiple path reception, due to reflections from the Heaviside layer, or of interference from a distant station operating at the same frequency, or from atmospheric "static." The only serious sources of noise at these frequencies are those generators within approximately line-of-sight, of which noteworthy examples are automobile ignition systems and medical diathermy machines. While reflections from the Heaviside layer are negligible, nevertheless, because of the very short waves employed, objects such as steel buildings, water towers, overhead wires, etc., provide efficient reflectors and give rise to "ghost" images. The severity of this problem will be realized much more fully than at present when the general public begins the erection of receiving antennae and the operation of receivers on a large scale. The line-of-sight limitation greatly increases the difficulty of serving a large geographical area with a given program. 46.6 miles is the radius of the area over which reliable coverage can be obtained from the transmitter, provided that the power of the transmitter is sufficiently great. Consider, now what this transmitter power must be, in order to give reliable reception at the required distance from the transmitter. • Power Requirements It is an empirical fact that reliable reception of a television program requires an input signal of about one millivolt. The required transmitter antenna power is 27.4 kw. Actually, at the present time it is not possible to radiate this much power, since no tubes are available to generate it at these ultra-high frequencies. Using two of the latest high-power developmental tubes in push-pull, it is possible . to generate 10 kw. (40 kw. peak) at fifty megacycles. The limiting factor in this case is the fact that the size of high power tubes makes it Receiver Sale Premature — Zenith Prexy By e. f. Mcdonald, jr. PRESIDENT, ZENITH RADIO CORP. "The offering for sale of television receivers at this time ... is in my opinion, unfair to the public and premature, both for economic and technical reasons. Such premature introduction . . . will load the public with undue experimental replacement cost, which in turn, will result in retarding, instead of furthering, development and in unprofitable operations for the companies engaging in such a program. "I do not believe the radio industry should ask the public to pay for its experimentation in television, at least without putting the public on notice that receivers put out at this time are on an experimental basis and may be subject to many costly changes and replacements. A 'Stock Proposition' Now "I still feel . . . that 'general use of television in the homes is just around the corner for the stock salesmen only.' When we have overcome all of our difficulties — and when I say we, I mean the radio in dustry— television will no doubt become a wonderful new industry. I feel that I am as close to the television picture as the next man, and Zenith is prepared at this time to produce and sell television receivers; but I am not ready to take the public's money until television is . . . ready to provide money's worth . . . Obsolescence Important Factor "Television is essentially different from anything with which we have had to deal in the past. The first automobiles . . . sold to the public, unless they have been worn out from use, will still operate on our highways today, and with the gasoline now used. Likewise, the first radio receivers . . . sold to the public will still operate and reproduce programs from the most modern broadcasting stations of today. On the other hand, the television receiving set of one year ago is already obsolete and cannot be operated in the home with the latest television transmitter of today. "What the public should know, but has not been told, about television is that the receivers must be matched to, synchronized with and built on the same standards as the transmitters. Any major change made in the television transmitter will necessitate a change in the receiver." 16 UVTGRNATIONAL PROJECTIONIST