Radio and Television Today (Jan-Nov 1941)

1 1 I "43 154T 42 MC. 23 121 , 25 I I I I 27l F ■" o> 5 in -* Q m or CD in -* > > Z t Q r» -r < w S 0 s ur-5' 29131 33 37 I 43 l 47 , 35 , 39J4I , 45 , 4S 1^ 53 57 51 , 5,5 59|6I 63 67 i I 73 I 77 W7» ■ 7g A 79181 J _* ■ 8? i 93 50 97 91 , 9j5 , 991 The FM portion of the radio spectrum, with the dial positions of stations operating and building. will be found on page 48. Complete list of all stations THE ABC OF FM WHAT TO TELL THE CUSTOMER • WHERE TO FIND FM STATIONS Everybody is talking about FM — and asking questions of the men who sell receivers for this new form of radio. Why does the FM receiver not reproduce the noise that an AM set does? Why do I hear more "tones" from an FM receiver than from a conventional set? What is "dynamic range? Why should FM have more "dynamic range"? Won't an AM set costing as much as an FM set, be just as noise-free and have the same tone quality? Is it necessary to put-up the special FM antenna? These are typical queries which confront the FM salesman— and, in many cases, his selling success will depend upon how well he answers them. Naturally, the salesman is not expected to give an engineering treatise on the subject of FM, but he should be ready with a simple, and accurate, statement which will be understood by the lay customer. NO NOISE Why isn't FM affected by "static," electrical noises, telephone clicks, and the million and one other sources of disturbance which mar or destroy ordinary broadcasting? This is one of the most often-asked questions — and rightly too, for the radio public has experienced many disappointments, in the past, with so-called "static eliminators." The important point to drive home is that noise-free or static-free reception is characteristic of a correctly designed system of frequency modulation. "FM" is not simply some special gadget on the receiver which might also be applied to an AM set. SEPARATING THE SOUND Lightning, neon signs, electric motors, switches, and numerous other causes create radio waves which may travel very short or very long distances just as ordinary radio programs do. These interfering waves are for the most part, amplitude modulated, that is, they vary in strength or intensity. Furthermore, these unwanted radio waves act as carriers for the high pitched whine of an electric motor, the raucous buzz of a neon sign, the click of switches. Thus they are called "noise" waves or signals. Another characteristic which increases the bad features of noise signals, is the fact that they appear not at a single spot on the radio dial, but instead cover wide "bands" of the spectrum. Thus they do not blot out a single station but ruin reception of scores of stations at a time. Because the noise signals cover wide "swaths" in the radio band, a receiver, FM or AM, which is tuned to some particular station will also receive any noise signals blanket ing the part of the dial to which the set is tuned. Notice that the noise signal is present in both the FM as well as the AM receiver. But noise is not heard from the FM receiver because the noise can be separated from the FM carrier wave without also losing the speech or music program. With the conventional AM system, however, the noise cannot be separated from program carrier wave and therefore is reproduced along with the speech or music. The reason why the noise can be separated and sidetracked in the FM set and not the AM, is best shown with accompanying chart. At the FM transmitter, the sound or program wave is used to change not the amplitude but the frequency of the radio carrier wave ; the lounder the sound, the more the frequency is shifted, and the higher the pitch of the sound, the faster the frequency is shifted. From the AM transmitter, the radio carrier wave sent out is a constant frequency and the sound is used to change the amount of power sent out from the station's antenna. The louder the sound, the greater the power. And the higher the pitch of the sound, the faster the power is changed from maximum to minimum. LIMITER ERASES The noise signals which are changing in strength (amplitude modulated) combine with both FM and AM carrier waves as shown by the jagged peaks on the top of the carriers. Thus the FM carrier is no longer only frequency modulated by the sound but also is amplitude modulated by the noise. The AM carrier is further amplitude modulated by the noise. Note carefully that the noise and the sound have the same effect on the AM carrier, namely, to change its amplitude, or power. Now in the FM receiver, the amplitude modulated component caused by the noise can be eliminated with limiter circuits which prevent changes in amplitude of the signal from going on to the loudspeaker. HEARING MORE Of course, these amplitude changes due to the noise in an AM carrier cannot be "erased" with such a limiter circuit, for it would also "erase" the desired sound or program, which is also carried by the amplitude modulations, The FM receiver reproduces more high tones than an AM set because the FM transmitter is not limited by "room" in the radio spectrum as is the AM transmitter. More spe 26 RADIO TODAY