Radio Broadcast (May-Oct 1925)

As the Broadcaster Sees It 77 mental fault with which nine announcers out of ten are afflicted: they talk too much. How and Why Stations Heterodyne One Another THE problem of heterodyne interference between broadcasting stations has the same origin as the rush hour jam in a large city: too many people are in the same place at the same time. Class B stations are supposed to be spaced ten kilocycles apart, with an effort being made to reduce the separation to 7J cycles, in order to create additional channels for new stations. These are theoretical separations, conditioned on all the stations keeping their exact assigned frequency. Unfortunately, they vary. When they get a few thousand cycles apart, all the listeners within range of both hear a beat-note, a continuous melancholy whistle, varying slightly in pitch from minute to minute, and well calculated to drive sensitive persons crazy — although, through the psychological phenomenon of auditory fatigue, some people get used to the beat and hardly hear it after a time, unless it is very loud. As a matter of fact, any two — or any number of stations, for that matter— have a beat note in any receiver capable of picking up their waves. If they are 10 kilocycles apart, they have a beat note of 10 kilocycles, or 10,000 cycles, which is over twice as high as the highest note on a piano. This pitch is too high to pass effectively through the audioamplifying circuits of a receiver, and what little does get through is suppressed by the loud speaker and the human ear, neither of which is designed to respond ardently to such an acute note. In short, nothing is heard. But as the two stations, through the deviation of one or the other, or both, from its or their assigned frequency, approach each other, the beat note between them passes into the band of audible and essential frequencies to which the acoustic apparatus of the receiver and listener responds. The resulting interference may be anything from a very shrill whistle up around 4000 cycles, scarcely audible to one not listening for it, down to an angry recurrent growl on either side of zero-beats, if the two stations happen to be right on the same wavelength. Or, it may be a loud, continuous whistle of medium musical pitch. Loudness depends on the strength of the electric fields of the two interfering stations at the heterodyning location; pitch depends on running* Aowiv statioiv interference their respective frequencies and the variations therein. The ultimate result is telephone and telegraph calls from irate listeners. Unfortunately, a station which is too weak to produce a workable signal in a given neighborhood, is perfectly capable of giving rise to heterodyne interference with stations supplying a powerful signal to the locality on which it depends for program service. This is one of the inherent traits of heterodyne amplification, the same which enables an oscillating receiver to hold a very audible beatnote with a distant station, while, in the nonoscillating condition, the modulation of the station in question is inaudible — a characteristic which is therefore responsible for the reprehensible use of beat reception by users of radiating receivers. Great is heterodyne amplification, and, like many other great things, it is also capable of causing a deal of mischief. The result is that in, say, New York City, you may be listening to one of the local stations five or ten miles distant, employing enough amplification to get a comfortable signal; and although in that condition your set could not possibly hear a Chicago broadcaster of the same power, when that Chicago station climbs on to your New York station's wavelength, you get a beat-note of perhaps a quarter the intensity of the local station's signal. (Living in New York, and be u> responsible for the technical operation of c\y 1 of the local broadcasters, I naturallv assure and stand ready to prove it with wavemetiiS and firearms, that the Chicago st?t on is ?X