Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST OCTOBER, 1928 TABLE I: WHO IS ON THE AIR WITH TELEVISION SIGNALS Call Letters Location Wavelength Meters No. of Holes in Disc Speed of Disk. (R. P. M.) No. of Pictures Per Second Schedule of Transmissions (E. S. T.) WGY 2XAF Schenectady, N. Y. Schenectady, N. Y. 379 31.4 24 24 1260 1260 21 \ 21 ) Sunday, 10:15-10:30 P. M. Tuesday, Thursday, Friday, 1:30-2:00 P. M. Monday, Wednesday, Friday, 8-9 P. M. 5-10 minute periods every hour station 2XAD 3XK WRNY Schenectady, N. Y. Washington, D. C. New York City 22 46. 7 326 24 48 48 1260 900 450 21 ' 15 7.5) 2XAL 9XAA WMAC New York City Chicago, 111. Chicago, 111. 32 61 447.5 48 48 45 450 900 900 15 15 is on air 10 to 11 A. M. Daily except Sunday Probably 11:30-12 p. M. Daily 4XA (WREC) lXAY (WLEX) Memphis, Tenn. Lexington, Mass. 62 24 48 900 900 15 15 Irregular 9:30 P. M. Daily 8XAV Pittsburgh, Pa. 62.5 60 960 16 Irregular button short-circuits the resistor so that the motor tends to revolve slightly above the correct speed. Successful synchronizing then becomes a matter of getting the knack of pressing and releasing the button at such intervals as to hold the disc at exactly the correct speed. And this is no small job! More anon about synchronizing. As will be pointed out, Jenkins, in our opinion, has a better method. At the Daven Company's laboratory a pretty young lady was asked to sit in front of the television transmitter located in one corner of the room while the rest of us moved over to the opposite corner where the television receiver was located, the transmitter and receiver being connected electrically by wire. The young lady smiled, winked, smoked a cigarette (these moderns!) and we saw it all in the receiver. The color of the received image is pink — the characteristic glow of a neon lamp. As we recall it, the size of the picture appeared to be about 3" by 3". On August 21, this company held a public demonstration in connection with station wor. A demonstration for the public, interested in practical television, and with little or no interest in experimental television, had to be arranged in the form of a show. In this instance puppets were used, as in Punch and Judy, since they are small and their entire movements can readily be reproduced in a small space. The experimenter doesn't care very much about what is sent, being interested in how good the results are, but this demonstration possibly gives us some idea of the technique that will be developed for the first television programs. A journey to Washington, D. C, to the private laboratory of C. Francis Jenkins, provided one of the most enjoyable days of the investigation. On July 2 Jenkins began the broadcasting of radio movies — i. e., transmitting from a strip of motion picture film — from his experimental short-wave station, 3XK, going on the air every Monday, Wednesday, and Friday night. He has gotten the cooperation of many experimenters, and up to the present his pictures have been received as far west as Chicago and north as far as Boston. Jenkins demonstrated two different types of television receivers. One of these uses a drum type scanning disc, which has the advantage that it can be made much smaller in size than a flat scanning disc designed to give the same size picture. The drum disc which we saw consisted of a cylinder about 7" in diameter and about 4" long, with 48 small scanning holes punched in its peripheral wall and arranged in the form of a four turn helical. A neon light source containing four small plates each about \" square is located in the center, and quartz rods extend from each hole in the wall to the neon tube. This drum with its 48 quartz rods cannot be cheaply made and for the first television experiments a flat scanning disc will prove satisfactory and more economical. We therefore leave a more complete description of it for a later date. Jenkins also had in operation a receiver using an ordinary flat scanning disc from which satisfactory reception could be obtained. This receiver, a picture of which appears in this page, used a small 48-hole scanning disc, the source of light being a small G. E. neon lamp. This lamp gives a picture probably not more than about f " high and about \" wide — quite small. The method of synchronizing is interesting and we found it quite easy to hold the picture stationary. The arrangement used is indicated in Fig. 1. The scanning disc, D, is mounted on a shaft which revolves in the bearing, B. The motor, M, is mounted on a block of wood at a small angle to the disc as indicated, and the mounting block fitted with a slider, fitting into a groove on the baseboard. The screw, S, enables the operator to move the motor to the left or right, parallel to the disc. The end of the motor shaft is fitted with two flanges, F, about 2|" in diameter with a rubber disc, R, clamped between them. This rubber disc may be made by cutting a i\" or 3" diameter disc from an old automobile inner tube. The motor is so located that the rubber disc bears against the scanning disc at a point about 3" from the center of the scanning disc. The motor which may be any type, a.c. or d.c, is connected to the line without the use of a resistor JENKINS SCANNING APPARATUS This rear view of Jenkins' receiving apparatus shows the scanning disc, neon lamp, and driving motor. The screw at the left varies the speed 0/ the disc by moving the friction drive motor along its surface. and the speed of the disc is adjusted by turning the screw, S, thereby moving the motor assembly further away from or nearer to the center of the scanning disc. With this arrangement the motor runs constantly at normal speed; at least, it runs much more uniformly than when synchronizing by means of a resistor in the motor circuit — the method mentioned previously. We recommend that those who decide to do some television experimenting, start off with this method — although since we are experimenting everyone has a perfect right to try any and every method he can think of to obtain easy synchronizing. Jenkins at present transmits silhouettes, although he expects soon to transmit ordinary pictures. Silhouettes were used at first so as to keep the side band frequencies within a limit of about plus or minus 5000 cycles. The short-wave channels now being licensed for experimental television are 100 kc. wide, and in a band of this width it is possible to transmit the wide band of frequencies essential for transmitting high quality half-tone pictures. To date, Jenkins has always sent out the same program — a little girl bouncing a ball. In reception the girl and the ball will show up black, silhouetted against the pink background of the neon glow. A trip was made to Boston a few days later and at this point we succeeded in receiving 3XK, Jenkins' station, and getting recognizable images on a television receiver constructed by James Millen. Static marred reception considerably, but apparently had less effect on television reception than on ordinary broadcast reception which was very poor at the time. At this time we used a large 2' disc made by the National Company and a Raytheon Kino-Lamp, from which combination can be obtained pictures about \\" square. This test at Mr. Millen's home was made on a Friday between the hours of 8 and 9 p.m. E. S. T. during one of Jenkins' regular transmission periods. Static was very bad, and during the latter half of the demonstration there was thunder and lightning. In spite of this, plus considerable fading, what we considered fairly good results were obtained. The transmission started off with an announcement in both code and phone telling what the program consisted of, after which the actual transmission began. The incoming signals contained components of all the frequencies in the audio band, but the characteristic note in the loud speaker seemed to be about 2000 cycles, probably because the ear is most sensitive to this frequency. At various times during the hour, especially during those moments when the signal was strong, the silhouettes of our little girl with the bouncing ball could be easily recognized. The static produced a lot of black spots and lines on the picture but did not prevent one from recognizing the image. In this test a short-wave tuner followed by a three-stage high-quality transformer-coupled amplifier was used. Theoretically, results would have been better with a resistance-coupled amplifier but the improvement would not be noticeable unless good strong signals without any fading were being received. This test is of interest for it gives one some idea of what can be expected, and it also indicates the perhaps obvious fact that television reception will be satisfactory at any point at which a good loudspeaker signal can be received. While at Boston we had hoped to see a demonstration from the local station, wlex, which has obtained a short-wave television license as indicated in Table i, but the new 500-watt short-wave transmitter was not yet ready. Before this article is published wlex will probably be on the air with regular programs.