Radio Broadcast (May 1928-Apr 1929)

6 RADIO BROADCAST MAY, 1928 means that their temporary or permanent failure will bring heterodyne whistles of much greater intensity than are experienced under present conditions. (2) The system must not demand an order of operating skill beyond that obtainable by average broadcasting station staffs. (3) The first cost of equipment required and its maintenance expense must not be so high as to place it beyond the financial capacity of average broadcasting stations. (4) The adoption of the system must not require any substantial alteration in transmitting and receiving equipment and must entail no sacrifice in quality of reproduction. We will examine each proposal in the light of these four qualifications. The first general class of methods concerns those intended to eliminate the carrier heterodyne or whistling interference with which all listeners are now painfully familiar. The pitch of the carrier whistle depends upon the difference in frequency between the twoor more carriers simultaneously actuating the receiving set. Suppose we have two stations assigned to a million cycles, or 300 meters, one precisely on the assigned frequency; the other one half of one per cent, above it, or on 1,005,000 cycles. The resultant effect will be to impose a 5000-cycle note upon the programs of both stations. If one station deviates a hundredth of 1 per cent, from the assigned frequency, the resultant heterodyne will be a hundred cycles. Accuracy of one part in a hundred thousand is therefore essential if the carrier heterodyne is to be reduced to a point below audibility. In that case, the maximum heterodyne note would be 20 cycles, assuming that both stations have deviated in opposite directions from the assigned frequency. Note the extraordinary stability necessary to permit two carriers to overlap without heterodyne. The intensity of the whistle heard at a receiving point is dependent upon the carrier energy received from the more distant station. The degree to which it mars reception depends somewhat upon the ratio of the carrier whistle to the amount of modulation received from the nearer station. An understanding of these two statements will reveal why carrier synchronization will effec real relief under present conditions. You have frequently heard a heterodyne of considerable intensity and waited for the local station to sign off, in the hope that you could identify the distant station causing the whistle. But you find it impossible to hear the slightest sound from the distant station. This is due to thefact that the carrier spreads from thirty to forty times the distance that a high-grade program signal is heard and .also, because of the square law operation of the detector tube, the carrier is subject to much greater amplification than the audio-frequency modulation impressed on it WHY STATIONS ARE SPACED IN PRACTICE, this condition accounts for the 1 great spacing required between stations of moderate power, if the service area of each of them is to enjoy undisturbed reception. The maximum high-grade service range of a 500-watt station is 30 miles, but its average carrier range is at least 1000 miles and often over 2000 miles. Under average conditions, it delivers a distinguishable program signal to sensitive receivers for perhaps 350 or 400 miles. If two stations on the same channel are perfectly synchronizer.', they would have to be spaced only 400 to 500 miles apart, without suffering audio-frequency distortion within their respective local service ranges. But, under present conditions, 1 500-mile separation is necessary to reduce carrier heterodyne to the point that local reception is not noticeably affected and complaints are often registered with respect to heterodynes caused by 500-watt stations 2000 miles distant from the receiving point. Four methods have been suggested for stabilizing the carriers of broadcasting stations so as to eliminate the possibility of carrier whistle: (1) Stable precision crystal oscillators; (2) Remote manual control of carrier frequency; (3) Radio transmission of a reference frequency; and (4) Wire synchronization of carriers. The zero-beat method, employing crystal control oscillators, is now widely used. The station operator wears a headphone through which courses the output of the crystal oscillator and also the station's radiated carrier frequency. The frequency of the station is adjusted until the two are in exact synchronism so that no heterodyne whistle is heard. In preparing to write this article, the author maintained a broadcasting station on its frequency by the zero-beat method for several programs. When utilizing a crystal oscillator, installed at the station, the comparison signal is constant and powerful. The amount of skill required and the cost of maintenance of the equipment needed are within the reach of any broadcasting station. Independent crystal control, however, has been described as too inaccurate and too unstable to permit the perfect synchronization of two carriers. As a matter of fact, there is no inherent fault in the crystal oscillator which cannot be corrected. What are needed are perfected means of supplying crystal oscillators with absolutely constant voltages and means of maintaining the crystal at an absolutely constant temperature. A change of one degree centigrade varies the frequency of a crystal oscillator by sixty to a hundred cycles. The crystal oscillator is usually installed in a penthouse on the roof of a building where the transmitter is installed. Heat supply is often uncertain in such exposed locations and temperature variations of twenty degrees, during operating hours, are not uncommon. Such a change is sufficient to cause a 2000-cycle variation in the frequency of a crystal oscillator. Crystals have been submitted to laboratories by broadcasting station owners with a view to finding out why they do not hold the station to its assigned frequency. Among these are ordinary quartz lenses, crudely scratched and insecurely mounted in contacting clamps. These worsethan-useless crystals have been sold to broadcasting stations with the expectation that they will stabilize carrier frequencies. The fact that a station uses crystal control is no guarantee whatever that it will remain accurately on its frequency any more than providing an aviator with a compass assures that he will arrive safely at a distant destination. Proponents of the crystal oscillator method have sometimes proved their case by setting up two such oscillators in the laboratory, both using a slab from the same quartz crystal. Such demonstrations, however, prove nothing because both oscillators are then working under exactly the same conditions. When one of the oscillators is shipped to a distant station to control its carrier, varying temperature conditions cause sufficient deviation to produce annoying heterodynes. With equipment now commercially available, the crystal oscillator does not possess sufficient stability to eliminate the heterodyne whistle between two stations operating on the same channel. Nevertheless, development of precision oscillators, with accurate temperature control, is a most promising line of research. MANUAL CONTROL OF FREQUENCY A NUMBER of enthusiasts have loudly heralded their success in synchronizing their station's carrier with a single interfering station by checking the heterodyne with a receiving set remote from the broadcasting station. This is the second method listed. By means of a wire connection with the station, the carrier frequency is varied until the observed heterodyne / Circle No 1 High-Grade Service Range \ / Circle No 2 Satisfactory Service Range \ / Circle No 3 Interference Range \ / \ / \ / / / i r \ i i \ \ \ \ \ \ \ / / / / / \ \ SERVICE AREA AND INTERFERENCE RANGE The circles show the relative areas of high-grade service range, satisfactory service range, and interference range, of a 50,000-watt broadcasting station. Note how large the interference range is in comparison to the service range. The actual service area of this station does not extend beyond the area enclosed by circle No. 2, for at greater distances, static and fading will interfere with good reception. The much larger area of circle No. 3 extends far beyond the area of fair reception. Within this large area the station can create interference by generating a heterodyne with the carrier wave of another station, supposedly operating on the same frequency, but actually transmitting on a frequency slightly higher or lower. Accurate stabilization of the carrier frequencies of stations operating on similar frequencies — perhaps by the quartz crystal method — will prevent heterodyne interference but will not prevent interference arising from the clashing of sidebands