I a s t month, we promised to talk j about FM and give you an ex planation of what it actually is. First, the fact that there are no more AM (conventional) channels left in many areas, and in others none available without costly litigation, has encour aged many Christian broadcasters to seek out Frequency Modulation (FM) radio. There are a number of advan tages to FM. 1. FM eliminates static and inter ference no matter the weather circum stances. (A thunderstorm directly overhead will not even affect the programs). 2. The FM band is 200 kilocycles wide compared to a band 10 kilocycles wide for AM. This makes possible the brilliant, full fidelity tone. 3. FM can be used for stereophonic broadcasting and is the system which uses multiplexing ( to be discussed in a future column). 4 FM broadcasting hours are un limited. Some AM stations must go off the air when the sun goes down. Now, without getting too technical, let’s discuss the system itself. All radio transmission is based on the gen eration and interception of electro magnetic carrier waves.' (See figure
mission. The result is what we call static or background noise, never ap parent with FM. Now, let’s go one step further, and ask the question, “Why is FM sound more realistic?”
FIGURE THREE
Overtones normally cannot be heard on AM broadcasts, since AM is limit ed to sounds no higher than 5,000 cycles. On FM, however, the full range of audible sound from 20 to 20,000 cycles may be transmitted. Thus the delicate overtones are re ceived on FM, enabling the listener to obtain full, rich sound . . . exactly as “picked up,” or played from the studio. (See figure four). Each FM
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station is assigned a channel 200 cycles wide (as against 10 cycles in AM broadcasts). Also, channels are not assigned to other facilities in near by areas so that the problem of two stations close to each other is elimi nated. People in steel and concrete buildings are also happy to discover that where once radio reception with AM was disturbing and poor, with FM, it is clear and static-free. We have tried to present some basic truths regarding FM since the value of and interest in this medium is growing every day. If you have any questions, be sure to let us know. If you hear of news concerning Christian stations and broadcasts, we would appreciate knowing about it. In a future issue, we will discuss the matter of “ stereo” and other new advances in the field of broadcasting. Before you purchase an FM re ceiver, contact the Biola Book Room, 560 South Hope Street, for special prices on sets sold through the store to Christian friends.
FIGURE ONE
one). In order to transmit different sounds, we must add to or subtract from the otherwise uniform waves of the carrier. This is called modulation. In AM transmission the height of the waves is modulated (amplitude modulation), but the frequency re mains uniform. (See figure two).
FIGURE TWO
In FM transmission, the spacing of the waves is modulated (frequency modulation), while the amplitude re mains uniform. (See figure three). Lightning, motors, street cars (where there are some left), X-ray, ultra violet devices, oil burners and other electrical equipment generate waves of the same nature as AM radio trans-
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