Radio Broadcast (May 1928-Apr 1929)

MAY, 1928 WILL NEW TRANSMITTING METHODS BE THE REMEDY? 7 disappears. Because, in isolated instances, two stations ha\u employed this method successfully, it has been hailed as a panacea. In those cases where two stations, assigned to the same channel, cause a heterodyne sufficiently loud to permit of easy elimination by the zero-beat method, they both suffer audio-frequency distortion, due to the interaction of their programs, even within their immediate service areas. Although the heterodyne whistle may be eliminated, the system does not permit of accurate synchronization unless the receivers used respond to frequencies below sixty cycles. Furthermore, the scheme merely accomplishes approximate synchronization between two stations and affords no assurance that either station is on its assigned frequency. Should any number of stations on the same channel use this plan, that frequency would become a raving bedlam, each station trying to follow the others, each not knowing what changes to expect in the frequency of the others. The difficulties of manual frequency control can best be appreciated by setting up three or four regenerative receivers in neighboring houses, adjusting them all in an oscillating condition upon a predetermined broadcasting station, and maintaining them there without permitting an audible whistle. The third plan listed is theoretically most attractive. A short-wave station is to radiate a national synchronizing signal, to be used as a reference frequency by all broadcasting stations. This is accomplished by radiating a 10,000-cycle note, impressed by modulation on a short-wave carrier. A receiving set, installed at each broadcasting station, would pick up this signal, supply it to a harmonic producer which multiplies the received note to the assigned frequency of the station. The station's carrier would then be adjusted until it zero-beat with the output frequency of the harmonic producer. The transmitted synchronizing signal cannot be higher than 10,000 cycles because it must be a multiple of every frequency used as a broadcast carrier. The manual control of a broadcasting station's frequency is difficult enough when a local crystal oscillator at the station itself furnishes the reference frequency. But to use for this purpose a weak and varying national synchronizing signal, transmitted in most cases more than a third of the way across the country, is like trying to balance an egg on your nose. The carrier frequency of a broadcasting station is constantly subject to slight variations, due to changing temperatures of vacuum tubes, voltage changes in the main power supply, and the effect of modulation peaks. Each of these variations must be compensated by readjustment of the carrier frequency. The source of the reference or comparison frequency must therefore be perfectly stable. A NATIONAL SYNCHRONIZING SIGNAL? THE principal difficulty with the national synchronizing signal plan is that the entire country cannot be successfully blanketed by the output of a single short-wave station at all hours of the day and night. The received signal must be sufficiently strong and stable to actuate a harmonic producer and produce a steady reference frequency for every broadcasting station in the United States. The system does not meet the requirement of reliability. The cost of maintaining a national synchronizing station in continuous operation, even if divided among 700 broadcasters, and the rather elaborate receiving equipment needed at each station is a serious, though by no means insurmounta-ble, barrier to the plan. The fourth method is the employment of wire lines for synchronizing stations. In the case of Energy from Station No 1 Energy from Station No 2 Resultant Energy Intercepted by Receiving Sets Resultant Audible Heterodyne Whistle in the output of the Receiver WHEN TWO STATIONS INTERFERE Station 1 may be operating on its correct frequency, but Station 2, which we will assume should be on the same frequency, may be slightly off its assigned frequency. A receiver tuned to Station 1 also receives energy from Station 2. The third curve shows what happens in the detector circuit: the two waves combine and in the loud speaker there is a heterodyne whistle, constant in pitch, indicated by the fourth curve. If the whistle is loud enough, it will completely ruin reception. Accurate stabilization of the carrier frequencies of all broadcasting stations within an accuracy of at least .05 per cent, is absolutely essential if reception is to be free of heterodyne interference due to this cause chain broadcasters, it might be possible to use the order wire circuit interconnecting chains, but, since chain circuits are set up for only an hour or two each evening, the contribution to stability which this would afford is quite negligible. There are not enough telephone wire facilities to spread a national synchronizing signal to every city where a station exists. The cost of interconnecting hundreds of stations would run into several million dollars annually. The economic burden which wire synchronization on a national scale would impose is entirely beyond the capacity of the broadcasting industry to bear. 18 Per Cent. 69 Per Cent 13 Percent, 28 Per Cent. 45 Per Cent. 27 Per Cent . Fair Service V/////A Good Service Very Good Service STATIONS SHOULD BE OUTSIDE CITIES The drawings show the relative effectiveness of two transmitters of equal power, one located in a city and the other outside. As many listeners as possible should be included within the good service area of a station, which is possible by locating the station outside of a city. Under such conditions, 69 per cent, are located in the area of good service compared with 45 per cent, in the previous city location. Locating a station outside of a city distributes more evenly the field strength of the signals because the absorption effect of steel buildings is removed. (Data from Bell System Technical Journal, Jan. 1927) With respect to the second problem, the synchronization of both program and carriers on the part of chain stations, many of the considerations already discussed apply. The outstanding example of carrier and program synchronization has been the successful simultaneous operation of wbz in Springfield, Massachusetts, and wbza in Boston. A special channel is utilized to transmit a synchronizing signal so that both stations take their carrier frequency from the same frequency source. Both stations invariably broadcast the same program. The results of this experiment have been satisfactory and the question is often asked why all the stations of the Red Network, for instance, do not synchronize their carrier frequencies in the same way so that, instead of occupying ten or twelve channels, they would use but one. The task of carrier and program synchronization of weaf and wlw, for example, as compared with the synchronization of wbz and wbza, presents some curious problems, the importance of which is not generally realized. In the first place, wbz and wbza are separated by only seventy miles, while weaf and wlw are 570 miles apart. This eight-folds the wire leasing costs for synchronizing the latter two stations, making a truly imposing financial burden. Secondly, the two stations do not continuously and invariably radiate the same program. Were they to radiate two different programs, audiofrequency distortion of both programs would be sufficient to cripple the entertainment value of both stations, even well within their local service areas. Third, since large areas receive signals from both weaf and wlw in appreciable amounts, the received signals in such areas would cause phase distortion. The reason that phase distortion is not experienced more generally in the wbz-wbza combination is that, because of an inexplicable ether wall, there are few points where an appreciable signal is received from both stations. Considering that radio waves travel 186,000 miles a second, it is hard to conceive appreciable lag in the reception of the same program radiated simultaneously from two different stations at varying distances from a receiving point. But, even in the hypothetical case of weaf and wlw this lag may cause serious distortion. A listener at Staunton, Virginia, where both weaf and wlw are received with good volume, is approximately 272 miles from Cincinnati and 365 miles from Bellmore. The distance from Bellmore is 93 miles greater than the distance from Cincinnati and therefore, theoretically at least, the program from Bellmore would lag S-J0 of a second behind that from Cincinnati. This would cause serious distortion. Some frequencies in the musical scale would be exaggerated and others reduced in intensity. Experience with the reception of several signals from the same station, through the effect of reflection and the influence of bodies of water resulting in phase differences, offers valuable evidence, tending to confirm the distorting effect of synchronized chain broadcasting where the receiver responds appreciably to signals from more than one broadcasting station. Ordinarily, the reception of two or three signals from the same station does not seriously affect quality because one of the signal sources usually predominates over the others sufficiently to make their influence negligible. But there are many known cases where phase distortion accounts for the poor quality with which highgrade stations are heard in some areas. W hen weaf broadcast from Walker Street, several years ago, listeners in Pelham, New York, but 16 miles airline distance from the transmitter, complained that, even with the best Of receivers.