Radio Broadcast (May 1929-Apr 1930)

Will Synchronization Bring a Radio Panacea? WHENEVER an experiment in the synchronization of broadcast station carriers is announced, articles appear in the press to the effect that, should the experiment be successful, the number of stations operating simultaneously in the broadcast band may be increased greatly. In most instances, this impression is entirely erroneous because only approximate carrier-frequency regulation is attempted. Only when absolute synchronization of both carrier and program signal is the objective is there any hope of increasing the number of transmitters which may operate simultaneously. Absolute synchronization has been accomplished successfully only when the stations are linked by wire lines and both carrier and program signals are supplied from a common originating point, as in the case of wbz and wbza. However, there are definite limitations even to absolute synchronization. It is useful only when the signal received at any one point comes from but one of the two synchronized stations. A receiver, so located that it receives equal signals from both synchronized stations, is subject to serious distortion because the two signals are out of phase due to the differing time required to transmit them by wire and back through the air. Usually, because of fading and fieldstrength variations in the received signal, this phase difference is emphasized by a swinging effect. Consequently, undistorted reception with absolute synchronization is possible only when the signals of one of the stations predominate over the other at all reception points within their respective service areas. We are informed of practical experiments conducted in Germany in the field of absolute radio program synchronization, employing wire distribution of both program and carrier. A definite interference pattern, which changed not only from day to day but hour to hour, was found to exist, due to the interaction of the signals received from two points simultaneously but out of phase. Reception, good at one point, would be found to be practically nil at another but a few score yards distant. This experience corresponds to theoretical conceptions of phase difference effects encountered in radio transmission from two different points. Authorities differ on these questions, it is true, but until we have actual demonstration of successful wire-line synchronization under the practical conditions of the present broadcast spectrum, glowing descriptions of this would-be panacea to broadcast allocation ills are rather the expression of a hope than a justified assertion. In the case of approximate synchronization, attempted by means of independent crystals accurately matched, very serious limitations to its application exist which do not appear to be generally understood. The principal object sought in approximate carrier synchronization is to eliminate carrier whistle on regional channels. This is the most annoying type of interference, now widely experienced on all the crowded regional channels. For example, wcco and wpch, now engaging in a synchronization experiment, operate simultaneously only during the day. If their carriers are approximately synchronized, they may also operate simultaneously during such evening hours as the program service areas of the stations do not overlap. If the separation between these stations were reduced by 500 miles, approximate synchronization would still eliminate the carrier whistle, but the distortion due to the simultaneous reception of two programs and the effects of the sub-audible beat note, created by their carrier interaction, would cause disruption of the service of both stations. While a 50 or 60-cycle carrier heterodyne of approximately synchronized stations is not reproduced by the loud speaker, the subaudible beat occurring interacts with the audio-frequency or program component, affecting musical quality. Wpch serves only a small area, surrounding New York, and, during the early evening hours, wcco's signal is of such a low field strength that it would not produce an audible effect in wpch's service area. At the same time, wpch, being a low-powered station, would have no noticeable effect in wcco's territory. But, as the later evening hours approach, and good transmission conditions prevail, wcco may, under certain conditions, deliver sufficient signal in the New York area to affect the quality of wpch's transmissions. The success of the wpch-wcco experiment, therefore, means only that, in certain instances, where a low and high-powered station, widely separated, are paired on the same channel, their respective service may be somewhat improved at those times that their audio-frequency and carrier signals are of a widely diverse field strength within their service areas. Two or three hours' service in the early part of the evening is a valuable addition to wpch's opportunity to serve its audience and the experiment of carrier stabilization is thereby justified. But station managements are warned that that is the maximum effectiveness of the experiment. Amateur allocation experts should realize that approximate synchronization will not increase the number of stations which may be assigned in regional channels. The Federal Radio Commission has issued permission to the Continental Broadcasting Corporation of New York to attempt an experiment in synchronization of two broadcasting stations in Virginia. The frequencies assigned for the purpose are 3257, 3256, and 4795 kc. These high frequencies, when heterodyned, produce a 1539kc. carrier, the frequency of the two stations in the broadcast band associated in the experiment. The employment of two or three highfrequency transmitters for generating a desired broadcast carrier frequency at several separated broadcasting stations by heterodyning may or may not have advantages over the distribution of a single frequency, which is stepped to the desired broadcast frequency by means of a frequency multiplier or harmonic producer. The latter method requires the use of but one high-frequency channel instead of two or three. The conclusion that a demonstration over short distances will make possible long-range synchronization of chain stations is unwarranted because it still remains to be proved that fading and noise effects do not cause instability in reception of the synchronizing frequencies and that skip-distance effects limit synchronization to very long spans only, so that amplifying such a signal to serve as the carrier for broadcasting is impractical. Canada's High-Frequency Allocation AN AGREEMENT has been reached between the State DeZjk partment and the Canadian Government on continental high-L frequency assignments, in accordance with the report of the majority of the American delegation at the recent conference at Ottawa. That report, with which ex-Commissioner 0. H. Caldwell of New York dissented, gives the United States a total of 146 of the 228 general communications channels, of which 112 are exclusive and the remaining 34 shared with Canada and Newfoundland. Canada is allocated 38 exclusive channels, to be shared with Newfoundland, and 48 shared with other nations. Newfoundland received 17 channels, shared with the United States, Cuba five exclusive and 15 shared with Canada; Mexico and other nations, eight exclusive and 16 shared with Canada. Of the 65 channels below 3412 kilocycles, the United States holds 34, shared with Canada and Newfoundland; Canada has 48, shared with other nations; Newfoundland 17, shared with the United States; Cuba 15 shared with Canada, and other nations have 16 shared with Canada. Apparently, the meetings leading to this agreement were not in the nature of a negotiation but rather a presentation of frequencies to Canada. With utter disregard of the future needs of the United States for essential high-frequency communication channels, an extraordinarily liberal award has been made to Canada. Considering that our population and habited area is roughly ten times that of Canada, there is no possible excuse for the present ratio,. which gives Canada more than 70 per cent, of the number of frequencies assigned to the United States. Furthermore, the precedent established by this agreement will be pressed by Canada as applicable in the broadcast band. If our broadcast channels were divided in the same ratio, the American allocations would amount to approximately 48, the Canadian 34, and other neighboring countries 14. The same arguments which swayed the State Department into accepting the Canadian proposal for this disproportionately large share of high-frequency channels are certainly applicable to broadcast channels. • may, 1929 pugc 8 •