Radio Broadcast (May 1928-Apr 1929)

PCV £R HETWORKS DO HOT PROVIDE A SOLUTION TO THE PROBLEM OF SYHCHROHIZIHG TELEVISION RECEIVERS The rapidity of development ma\es it practically impossible to obtain an accurate up-to-date map showing the power networks m the United States Afeuertheless, the shaded districts in the above illustration give some idea of what has been accomplished in this field. In addition many of the networks shown above may be connected if necessary Problems in Synchronizing Television Discs THE development of a system for keeping the scanning disc of a television receiver in exact synchronism with that of the transmitting disc is one of the biggest problems in radio at the present time. The simplest method would be to have the disc at the transmitter on the same shaft with the similar disc at the receiver. For a review of the principles of television, this simplified arrangement will be considered. In diagram A of Fig. i the subject, A, at the left is being viewed by the electrical eye, C, through the holes in the rotating scanning disc, E. The varying electrical impulses in C, produced by different intensities of light reflected from light and dark parts of the face of A, are amplified by a common vacuum-tube amplifier and cause the brilliancy of the flat plate in the neon tube, D, to vary in step. An observer, B, looks through spirally arranged holes in disc F at this plate and, although he actually sees it through only one hole at a time, the rapid rotation of the disc gives him the impression that image, A, is on the flat plate in D. What B sees is shown in diagram B of Fig. i, the dotted line representing the margins of the scene. Each hole in the disc passes across the scene and traces the light-intensity variations on the plate below the path of the previous hole until the last hole in the spiral comes around when the process is repeated. By BOYD PHELPS It will be evident what a mess would be made of the picture if the receiving disc should slip around on the shaft only half a picture width, or if it was half a turn out of phase with the transmitter disc. But such a difficulty would be tame as compared to cutting the shaft between the two discs and running the discs with separate motors at different speeds ! A rain or black and white dots or streaks with no trace of a picture OYNCHRONIZATION is one of the major problems in the accomplishment of television. Although in experimental television, manual control may he used satisfactorily, it is absolutely essential, if any sort of practical television is to be achieved finally, that some method of synchronisation be evolved that will hold the received image exactly stationary on the screen of the television receiver. At one time or another many of us probably have thought over the possibilities of synchronising the television receiver with the transmitter by the use of synchronous motors operated from the power lines of the local power company. A discussion of this method of synchronisation is the subject of this article. The author, Mr. Phelps, after many heated arguments with radio enthusiasts who fell that this method would work satisfactorily, decided that the only way to settle the argument was actually to get the "dope" and the ingenious method which he used to secure the data certainly makes interesting reading. — The Editor. would be the result. With these remarks as an introduction the necessity for precision in the maintenance of exact speed of the receiving disc in television may be more apparent. SYNCHRONOUS MOTORS FOR distance transmission where a shaft connecting transmitter and receiver is impractical, the first method of synchronizing that seems obvious is to use synchronous motors op!D®»? erating on the same alternating-current supply. This works fairly well under some conditions, but we shall speak more of this later. An ordinary 1760 r. p. m. squirrel-cage induction motor will run synchronous at 1800 r.p.m. on 60 cycles if four slots are cut in the rotor, and similarly, motors can be made to run 3600, 1200, 900, etc. Fifteen complete pictures per second means 900 r. p. m. of the scanning disc and is about the minimum speed permissible without flicker. This speed is used at present by C. F. Jenkins at his station 3XK in Washington, D. C, on 46.72 meters (see Table 11). Twenty pictures per second means 1200 r. p. m. of the scanning disc and is the speed used for receiving the wgy pictures on 380 meters from Schenectadv, N. Y. A frequency higher than 60 gives a more accurate lining up of the edges of the picture, so in the Bell Telephone demonstra 123