Radio Broadcast (May-Oct 1925)

78 Radio Broadcast fault.) But if the New York broadcaster then takes his carrier off the air, and you let your receiver alone, there is silence. You have to bring up your amplification in order to hear Chicago. This has an important bearing on the problem of running down heterodyne interference when it does occur. The only receiver which can be safely used in such work is one which has a volume control independent of the frequency adjustments. If the tuning and intensity controls are electrically interlinked, I should say that the receiver is worthless for detective equipment. If you are near one of the heterodyning transmitters, you are not likely to be able to identify the more distant one unless Number i takes his carrier off the air. If the interference is serious, and the transmission of the station is properly monitored from a point outside the studio, this is likely to be done. The engineers are waking up to the fact that they can best solve their station-interference problems by direct action, by exchange of telegrams between the broadcasters involved, as soon as the trouble starts. The telegraph companies are generally willing to give priority to such messages. It is preferable to take the carrier off the air for a few minutes, for the purpose of identifying the interfering station, and to send him a telegram explaining the situation, rather than to suffer the condition to continue and to allow the program to be hashed up, in greater or less degree, for the entire evening. It is customary, when shutting down for this purpose, to take the listeners into one's confidence and to solicit their aid, for, with the great natural variations in receiving conditions, quite possibly some outsider will be able to do the job better than the one or two members of the station personnel engaged in chasing down the trouble. The purpose of this article is to give listeners some data which will make their testimony reliable in this regard. The rules of the game may be summarized as follows: 1. When the announcement goes out, tune your set precisely to the wavelength of the local broadcaster who complains of the interference. This can usually be done in the few remaining seconds of transmission. 2. If you are receiving on a loud speaker, change to head telephones; your chances with the phones are obviously better. 3. When the carrier goes off the air, bring up your volume control till the interfering station is readable. Don't touch the frequency controls. 4. If you are able, under these conditions, to make a positive identification, and you feel inclined to do that much for the cause, dispatch a telegram to the party of the first part who has gone off the air. The next best thing is to write a letter. 5. If the intensity and wavelength (frequency) controls of your set are not perfectly free from interaction, or if you are not confident that your set tunes very sharply, you can be of greatest service by staying out of the controversy. Your testimony will only confuse the issue. Of course, if you have a set accurately calibrated in kilocycles, it may be permissible to try to determine the actual frequencies of the stations involved, but with ordinary equipment one is not justified in testifying that Station Number 2 was actually on the wavelength of Station Number 1 unless the frequency-determining elements of the receiver remained unchanged. Even this, of course, is only a beginning, for Station Number 1 may have been off his wave. Once it has been established, however, which stations have been involved, it is usually possible to clear up the situation for the time being, and the accurate calibration of their frequency indicators must be left to the Federal radio supervisors. The necessity of instructing the listeners in the above procedure, if they are to be of service in these situations, was brought home to me by a recent incident. I was listening at my home when one of the stations in which I am interested developed heterodyne interference early in the evening. Program complications made it inadvisable to interrupt the service later, so I telephoned immediately and had the carrier taken off the air for four minutes. Before these instructions could be carried out the interfering station shifted his wavelength, and the beat-note ceased. It was too late to cancel the order, and the carrier went off. I listened on the chance that the distant station might come in again during the four-minute period of observation, but heard nothing except two extremely distant transmitters heterodyning each other, and a spark station in the English Channel. As soon as the carrier went back on the air, Dr. Alfred N. Goldsmith telephoned me to say that he also had heard nothing to indicate that any one was on our wavelength, and that on his specially calibrated super-heterodyne both of the out-of-town stations which occasionally clash with us were on their assigned frequencies. Dr. Goldsmith is the chief broadcast engineer of the largest radio company in the world, and has been making precision measurements in radio for about fifteen years, so that what he says must be accepted as ex