Radio Broadcast (May 1928-Apr 1929)

mitter, however, may cause the "lefts" and "rights" to be reversed, or the picture upside down, or both. We next need to standardize the speed of the disc, a speed of 450 r.p.m. or 7J pictures per second is very slow, causes a bad flicker, and requires a synchronous motor with a large number of poles. One broadcasting station uses this speed, mainly to keep its modulated frequencies within 5 kc. required limits, and one uses a speed of 1200 r.p.m. or 20 pictures per second, but the majority use a speed of 900 r.p.m. or 15 pictures per second, which seems a wise choice from many standpoints and might as well be ours. The easiest way to secure very constant speed is by the use of a synchronous motor. Such a motor may be made by the amateur without much trouble or expense by cutting out eight slots in an 875 r.p.m. squirrel-cage motor of about | h.p. The illustrations show several types of non-synchronous motors that have been slotted and rewound to run synchronously at 900 or 1800 r.p.m. In some cases the rotor is slotted and in some cases the stator gets divided up, depending upon which has the copper-bar squirrel-cage construction. There seems to be no data as to how much metal should be cut away. The writer has varied the width of the cut from 10 to 30 per cent, with little effect. The 1750 r.p.m. motor shown at the right, was slotted at two places and assembled; it runs synchronously at 1800, although it should have four slots for more power at synchronous speed. Similarly one 1725 motor illustrated on the left was rewound to have eight poles of the same total number of turns as before on four poles but only four slotted places instead of eight and it rims nicely at 900 r.p.m. synchronously on the transmitter. The other motor illustrated in this group shows the 900 r.p.m. "sink" motor used at w2buo for reception. With all of these motors the power is about 40 per cent, of its former rating which is adequate for television purposes if a ; h.p. motor frame is employed and a small directmounted disc is used. With too great a load the motor gets out of step and throbs or "hunts" with grunting noises at the rate of two or three per second. A small type g-10 A view of Phelps short-ivave transmitter neon lamp held behind the scanning disc will show four black bars moving towards or away from the center if the motor is not running synchronously, otherwise the bars appear to stand stationary. If more power is desired, or if the motor does not keep in step, a winding may be put in the slots cut out and fed with direct current. Each pole so wound should be wound in the opposite direction so as to produce alternate north and south poles and greatly increase the power delivered before falling back out of step. The Remaining Question. THE remaining question to be decided is the number of holes we are to use in the disc and this may not be so easily settled. Scanning discs of 24, 30, 48, and 60 holes are now in use among the broadcasters and experimental stations of the big laboratories, but the tendency is to standardize on 48 holes. The greater the number of holes in the disc the finer grained the picture will be and the better detail it will show, but the difficulties in realization mount up rapidly as the number of holes is increased. For the same size picture, the 48-hole disc will have holes of half the diameter of the 24-hoIe disc, and these holes will let through only one-quarter the amount of already weak light that the larger holes pass. Therefore, expensive and very sensitive photo-electric cells are necessary for a 48-hole transmitter; intense illumination, unusual amplifier shielding, and supply filtering also add to the difficulties immensely as compared to 24-hole systems. Mr. Kruse points out that the optics with 24 holes is far simpler and I am inclined to agree that with so many new problems for the experimenter in this new field that the 24-hole disc should be the standard for a while. Possibly later we may tackle 48 holes, but by that time the broadcasters may develop some new agreement which we may use as a standard. Decently recognizable faces can be obtained with 24-hole discs and many other factors and misadjustments may easily produce worse quality with 48-hole systems, especially when considering the number of things that can go wrong. Then, to do justice to 48-hole discs the efficiency and amplification through a total of eight to eighteen stages must be uniform up to about 50.000 cycles whereas with a 24-hole disc picture quality crosswise equal to that vertically may be had without going above 9000 cycles, and even 5000 cycles as the upper limit does not give an entirely hopeless result. This makes possible the use of iron-core audio transformers with their immense gain over the usual resistancecoupled stages. While not wishing to appear dogmatic in proposing the above rules, I believe the beginner will find them helpful as much is to be learned and will be learned in developing 24-hole television to a high degree. A possible future transition respecting an increase in holes does not mean throwing away equipment and starting in all over again. The experience gained with 24-hole television will result in a saving in time, tools, material and temper. None of the recommendations february, 1929 . . . page 248 • above will hamper future development for a long time. They permit either of the two basic methods of scanning and great latitude for individual ingenuity along many lines, and best of all they promote cooperation and close friendship amongst experimenters. In the old 200-meter spark days amateur conventions were meeting places for acquaintances made over the air and nightly maintained to get together. Nowadays the appeal of great dx so easily gotten with low power, together with the fact short waves skip over much of one's own state so that the closest friends over the air are the farthest away, conventions have not increased in interest as they otherwise might. As a result conventions are used for one amateur to boastingly acclaim how many foreign countries he communicated with the evening before or to display his choice cards reporting his signals as "loudest in America," — an often used expression. Television on 150-175 meters may bring back the good old days, not the crashing spark, but the fellowship, which is easily half of the game. Late Wednesday afternoon November 28, 1928, the writer transmitted his first television schedule to W erner Olpe, w2buo, about two miles away, and Robert S. Kruse, wIoa, West Hartford, Conn., about a hundred miles distant. Mr. Kruse did not have his scanning disc finished at the time so he was only able to report on the signal strength, fading, interference, etc., but Mr. Olpe succeeded in reproducing the images excellently before his whole family. Easily recognizable shadowgraphs were produced and the hammer used to tack up some test charts was recognized and described. Incidentally, it was not realized that the hammer was in the field of vision at the transmitter — pulling down the shades is soon not going to be sufficient in the modern home. On subsequent tests a few days later Mr. Kruse got fleeting glimpses of moving images under combinations of most all the difficulties mentioned at the first of this article, — -fading, low signal level and broadcast harmonics. One broadcast harmonic was so bad that a transmitter retune was necessary. The First Amateur Television, AFTER the manner of proclaiming world champions in various new athletic classifications in which they have not yet contested, and consequently are unbeaten, the writer follows suit in calling the above the first amateur television. A sufficiently detailed description of the transmitter to permit it being copied by the layman would take considerable space. Some parts of the construction would not be ethical or convenient to try to copy as parts on hand were used wherever possible. However, some idea of the equipment necessary may be gained from a description of what was used for these tests although the apparatus is almost daily "subject to change without notice." To start with, two 500-watt Mazda bulbs with tin reflectors produce the light source, flood-lighting the subject or victim at close range, not unlike a doctor's baking lamp in action. A few sittings and one should be immune from rheumatism of the face for at least 200 years! Plug-in victims with plenty of spares and an ashpit beneath might be just the ticket. Because of this and because the writer's experiments have been largely oneman affairs, test charts and drawings have been substituted, one devilish horned figure being particularly able to stand the heat is