Circuit merit figure OVER-ALL CIRCUIT MERIT Circuit condition Corresponding percentage intelligibility based on word tests C 1 C 2 Hopelessly uncommercial; little or nothing intelligible. 0-25 25-60 C 3 C 4 Fair commercial; connected speech generally intelligible. 60-85 85-100 S 3 Field so weak as to render circuit uncommercial. S 4 Field so weak as to render circuit hopelessly uncommercial. EFFECT OF RADIO INTERFERENCE I 0 Absent. I 1=Present, but effect negligible. I 2 Effect appreciable, but limited. Alone would render circuit with merit 3. I 3=Effect enough to render circuit uncommercial. I 4 Effect enough to render circuit hopelessly uncommercial. EFFECT OF NOISE N 0 Absent. N 1-Present, but effect negligible. N 2=Effect appreciable, but limited. Alone would render circuit with merit 3. N 3=Effect enough to render circuit uncommercial. N 4 Effect enough to render circuit hopelessly uncommercial. EFFECT OF FADING V 0 Absent. V 1 Present, but effect negligible. V 2 V 3 Effect appreciable, but limited. Alone would render circuit with merit 3. Effect enough to render circuit uncommercial. V 4 Effect enough to render circuit hopelessly uncommercial. EFFECT OF QUALITY DEPRECIATION Q 0 Absent. Q 1=Present, but effect negligible. Q 2=Effect appreciable, but limited. Alone would render circuit with merit 3. Q 3=Effect enough to render circuit uncommercial. Q 4 Effect enough to render circuit hopelessly uncommercial. Annex 3 SUGGESTIONS FOR SYNCHRONIZATION EXPERIMENTS This discussion is limited to stations which transmit the same program. Since we are concerned primarily with the requirements or limits which should govern experimental operations in a field where there is a scarcity of accurate or systematic data, the principal object is to formulate restrictions allowing useful experiments with the least possible risk. It is important that such experimentation be done in such a way as to furnish conclusive results. The experiments must therefore be carefully controlled and fully checked. Methods of synchronization.-The following possible methods of synchronization have been proposed: 1. The sending of a control frequency by wire from a single point to each station. 2. Independent standards in each station: (a) Temperature-controlled piezo-oscillator; (b) Tuning fork and harmonic amplifier.. 3. Broadcast-frequency pick-up; (a) Manual control; (b) Automatic control. 4. Low-frequency pick-up. 5. High-frequency pick-up: (a) Beats between two; (b) Step up from modulation frequency; (c) Step down. Synchronized broadcast transmitters in general fall into the following two classes: (I) Two or more transmitters emitting carrier frequencies which are supplied from, or automatically controlled by the same source. (II) Two or more transmitters, in each of which the carrier frequency is individually controlled by its own source. Method 2 applies to systems of class (II), whereas the others except 3 (a) are used in systems of class (I), at least in principle. Of the nine methods, only the first has been applied in practice in the United States, and it in only one instance (Stations WBZ and WBZA). Prohibitive expense will probably prevent its general use over distances much greater than in this instance, viz, 90 miles. No engineering data are available as to the interference effects resulting from this synchronized operation at distances of reception in which fading ordinarily occurs. In any type of synchronization, the waves reaching the listener will be of varying phase, so that some types of interference may be expected. Experimentation should be encouraged and the broadcast audience at the same time protected from the objectionable effects of unsuccessful experiments. The goal of the experiments should be to show, first, the attainment of the necessary control of frequencies; second, the nature and amount of interferences produced; and third, the net increase or decrease of program service to the radio audience. Inasmuch as the physical and other means for synchronization are not yet fully developed, and as few stations have extensive experience in extremely accurate measurement work, it seems necessary to specify in detail the steps necessary before stations could be authorized to operate synchronously with program service to the public. The experimental work which must precede authorized broadcasting on a synchronized basis must therefore include (a) demonstration that the emitted waves can be synchrorized under practical operating conditions, (b) demonstration that there is a substantial net increase of service to listeners through synchronization. Each of these involves extensive engineering investigations. The degree of synchronization requires control of frequency to an extraordinary constancy and accuracy, as yet attained by very few laboratories. Preliminary requirements on systems of class (I).-Systems of class (I), wherein both carriers are automatically controlled from a common source, approach absolute synchronization over long periods. Experiments with systems of class (I) are probably less likely to cause trouble or inconvenience to the listening public than experiments under class (II). Restrictions on method may properly be directed mainly toward the satisfaction of the commission that this method is actually being carried out. Thus in connection with method the following limitations, or their equivalents, should be imposed; (1) A common source of frequency shall be provided for the stations. This may be an audio-frequency or a radio-frequency depending upon convenience. (2) Currents from the common source must be supplied to the transmitters of all stations engaged, by either wire or radio channel. (3) Evidence must be supplied to the administration that the carrier waves emitted by all stations engaged in the experiment are continuously and automatically controlled by frequency of the common source during the periods of transmission. Under these conditions it is believed that the stations may safely be permitted to broadcast the same program during regular hours, in experimental periods sufficient in the opinion of the administration to justify the issue of a license. These experimental periods may well be of the same type and duration as those specified in the memorandum. Preliminary requirements on systems of class (II).—Continuous synchronization is not feasible in systems of class (II) with existing commercial apparatus. It is always possible for a finite difference in frequency to exist between transmitters which are individually controlled. Continuous synchronization may prove to be necessary for satisfactory operation in the future, although conclusive evidence on that point is not yet available. Until the requirements for successful operation are more definitely determined, experiments under the conditions, which are associated with class (II), are justifiable. It is necessary, in the first place, that the applicant send the administration a detailed description of the proposed method and a particularly detailed description of the apparatus proposed to be used. This should include not only description of the frequency-control apparatus, but also sufficient data on the entire transmitting equipment to show that the transmitter is capable of being maintained at all times in invariable relation to the frequency-standard. The administration may approve or disapprove the beginning of the experiment on the basis of this showing. In the following, reference is made specifically to the use of piezo-oscillators. Analogous requirements should be specified for other methods, such as temperature-controlled tuning-forks, etc. The piezo-oscillators or other basic standards should be provided at least in duplicate for each station. All piezo-oscillators should be adjusted to the licensed frequency, all at the same time, by the Bureau of Standards. Each piezo-oscillator must be complete with its own independent measuring instruments, tubes, heated cabinet, etc. If automatic piezo-control is used (i. e., piezo-oscillator used as master oscillator of transmitter), it is to be considered as part of the transmitter and is not to be sent to the Bureau of Standards; one or more independent piezo-oscillators must be provided in each station as check standards. The piezo-oscillators must be capable of remaining constant in frequency continuously (i. e., for many months) within 20 cycles. This requires that the filament and plate voltages be always adjusted to the same values, that the coupling be kept constant within certain limits and that spare tubes be available, the effect of the substitution of which upon the frequency has been determined. Four experimental periods should be specified as follows. For each experimental period, a copy of the observational data shall be furnished to the administration. At the termination of each of the periods, the administration may send a government observer to make check observations or may require an additional period of further observations. In special cases, where the applicant has had experience of many years in similar work, the administration may allow two or more of the experimental periods to be concurrent or may shorten them. Experimental period I.-Observations shall be taken at least every two days over a period of at least two months, to determine whether the standards used are capable of remaining constant in frequency continuously within 15 cycles of the licensed frequency. (See second sentence under “Experimental period II.") At the end of this period, besides the checks already mentioned the administration may require new calibrations of the standards. Experimental period II.-Next, it is necessary to demonstrate that the emitted waves from the stations can be held continuously within 15 cycles of the licensed frequency. (At some future time it will probably be necessary to restrict this to a smaller variation, but for the present 15 cycles can be allowed, permitting a maximum variation between synchronized stations of 30 cycles.) The trial and demonstration of this shall be a period of not less than one month of transmissions by the stations during the hours after midnight. In special cases the tests may be authorized also in the daytime. Observations of the relative frequencies of each of the stations shall be made at a distance greater than one mile from any station, at least every two nights for a period of at least two hours. Experimental period III.-For a period of not less than one month of transmissions by the stations during the hours after midnight (or, in special cases authorized, in the daytime), observations shall be made, at least every two nights, of the character of received signals from the stations. These observations shall include observations taken at approximately 20, 50 and 100 miles from each station (and in special cases also at other distances specified by the administration). They shall be made by observing relative signal intensity, fading, quality and other characteristics of the signals, alternately with all stations operating and with one station operating. A government observer may be detailed to participate in this part of the work. This phase of the experimental work is of particular importance for the following reason. The results of theoretical studies and of such partial trials as have been made indicate that, while heterodyne interference will be removed by synchronization, there will be in its place an annoying impairment of the quality and fluctuation of intensity due to a form of interference of the waves from the two stations. While this may not destroy as much of the service area as would heterodyne interference, it will nevertheless reduce the service area of each station. The gain to be expected from synchronization, if and when it is demonstrated, therefore, is that many stations may be placed on one frequency, the sum of their small service areas being greater than the service area of one station if operating alone on the frequency. It is not now known whether this gain will actually be realized. In any event, the service area of a synchronized station may be less than it would have if it operated alone on the frequency. There may be no service in intermediate areas remote from any of the synchronized stations. The use of only one station on a frequency is likely to continue as the only means of giving service in large rural areas. Experimental period IV.-For a period of not less than one month, to be specified by the administration, the stations shall operate with synchronization with no limitation of hours. Observations similar to those during experimental period III shall continue. In addition, the stations, not less than twice each evening, shall announce to the radio audience that they are operating experimentally in synchronization with other stations, naming them. Results. The administration will authorize each experimental period only upon evidence of successful completion of the preceding period. Upon the completion of the last experimental period, if satisfied with the results, the administration will license the stations to operate on full time. With regard to constancy of the standard sources, which is to be checked during the experimental periods I and II, the important point is to maintain these sources at frequencies which depart from each other by less than thirty cycles. If this requirement is fulfilled, the secondary requirement that they be adjusted at the same time to operate, to within the same limits, at the assigned frequency, would appear to be unnecessary. If the primary requirements of synchronization be fulfilled, no further restrictions are required on the absolute value of the carrier frequencies than those set forth in Chapter · III, Section 3.1 Experimental operations with stations of this class may demonstrate that these limits are too wide, or even that systems of class (II) are in general unsatisfactory. The proposals under " Experimental period IV" may be modified to allow the stations to operate with synchronization for not less than one month, and to require that the observations be carried on for at least one month. Provision should be made for extending this last experimental period, if desired, for a considerable time before the administration issues a license, because it is impossible to specify any particular period as sufficient to allow the collection of conclusive data. 66 The proposals under Experimental period III" and "Experimental period IV" outline suitable test procedures and call attention to certain unfavorable results for which the technical observers should watch. It should be emphasized that audio-frequency distortion may occur in the received signal in common service areas of the synchronized stations, even with systems of class (I). With systems of class (II) this distortion may possibly be increased by the occurrence of a beat note between the partially synchronized carriers, even though this beat note be below the normal audible limit. Among the conditions which should be met for ideal operation of synchronized stations of either type, the following may be listed: '[See ante, p. 240.-H. F. W.] |