Radio Broadcast (May 1928-Apr 1929)

246 RADIO BROADCAST Oscillator Modulator Oscillator FIG. I. A DOUBLE MODULATION SYSTEM This diagram, which is reproduced from Morccroffs "Principles of Radio Communication," shows a circuit for transmitting by the double modulation system cies picked up by the microphone. The device which allows the voice to he superimposed upon the radio wave is called the "modulator." The idea of double tuning or double modulation applied to a radio transmitter was shown in a patent by John Stone Stone more than 17 years ago. Just who was first to disclose a complete workable system, the writer does not know; but a sytsem similar to that described in the press reports was built by the Western Electric Company for the U.S. Navy prior to 1919. A description of this multiplex radio telephone system was given by Craft and Colpitts in the Proceedings of the A.I.E.E. (about 1 9 1 9). For several years preceding and following this date, John Hays Hammond, Jr. was interested in double modulation systems, and his engineers developed complete multiplex radio telegraph installations which were tested by the U.S. Signal Corps and the U.S. Navy. It was while doing development work and research on these sets that the writer first became familiar with double modulation systems. In Morecroft's Principles of Radio Communication (pages 680-83, 1st ed.) written in 1921, complete diagrams for a double modulation system are shown. One of these diagrams is reproduced in Fig. 1. If the reader is interested in the actual connections to use in building such a transmitter or receiver, he is referred to these pages. Just a few random references to the use of multiplex radio are given here. The engineer recently advocating its use, being a well-known pioneer in the radio field, famous for his inventions, must not have intended to claim novelty for this system. However, reporters of his address did infer that "double tuning" is new. Before answering the next question, "Will double modulation multiply all existing radio channels?" it is necessary to describe briefly the arrangement of our present broadcasting system so as to form a background against which different systems may be viewed and compared. WNAC WJZ FREQUENCY IN K.C FIG. 2 WHAT IS A RADIO CHANNEL? A RADIO channel is a band of radio frequencies, the width of which is determined by the type of transmission. Two such channels located in the broadcast "spectrum" are shown diagramatically in Fig. 2. The word "spectrum," which is often employed in the study of light waves, has found increasing use in connection with radio waves. This is because a continuous electric wave appears in the radio spectrum as a straight line, located at a certain point in the frequency scale, exactly as a light line having a single color stands out in the light spectrum. Fig. 2 shows the location of the two radio channels assigned by the Federal Radio Commission to stations wjz and wnac. Each channel is designated by a number, which corresponds to the frequency (in kilocycles) or to the wavelength (in meters), of the point at which the center of the channel is located. Thus, looking at Fig. 2 we can say station wjz (New York) operates on the 660-kc. channel and wnac (Boston) on the 650-kc. channel. Since the width of radio channel depends upon the type of transmission, a list of channel widths, in frequency, required by three well-known types of transmission is given as follows: (a) Radio telephony (broadcasting), 10 kc; (b) Interrupted or tone-modulated c.w. telegraphy, about 2 kc; (c) Unmodulated c.w. telegraph, less than 0.3 kc, depending upon the keying speed (words per minute). These widths remain the same whether transmission takes place in the longer or shorter wavebands, assuming precautions are taken to maintain the frequency of the radio generator constant. The present broadcast spectrum extends from 550 kc. to 1500 kc. In order to provide for the maximum number of transmitting stations each channel is limited to 10 kc, the minimum width which will give satisfactory reproduction of music. The total number of channels available on this basis is 95. With our present serious station interference and hundreds of would-be broadcasters waiting for space in the ether, the question of the most economical use of this band is important, and it is this problem which we are considering. THE RESULT OF MODULATION WITH the help of Fig. 3, which shows one of the radio channels of Fig. 2 magnified, let us examine the nature of the wave disturbance caused in the ether by a radio telephone transmitting station. We will suppose that the spectrum resulting from a transmitter (such as wjz) is shown here. When the microphone is idle, that is, no modulation taking place, the radio SEPTEMBER, 1928 frequency carrier wave only is radiated. This is accurately located at the assigned frequency of 660 kc, and is represented by a single line in the spectrum. Now suppose the microphone at the station is energized by a steady musical tone of, say, 1000 cycles; the result is the formation of so-called "side-bands" shown in Fig. 3 by the shorter lines on either side of the central carrierwave. Digressing for a moment, if this is a picture of the frequency spectrum from a transmitter, how is it that we can hear at the receiver the musical tone impressed on the microphone? The reason we hear the 1000-cycle tone is because after the carrier-wave and either side -band has entered the receiver, they "beat" together in the detector (or de-modulator) circuit, and produce a tone of the original modulating frequency. This explanation of why the radio telephone receiver "works" is somewhat different from that usually given in popular radio articles which omit the important point of "beats" in the detector circuit and do not explain the need of the presence of the carrier-wave at the receiver. The process of modulation at a radio transmitter is fairly complex, but it is not necessary for the reader to go into the mechanism of modulation in order to understand some of the simpler results of the process. It is necessary, however, to accept the fact that when a carrier-wave is modulated, upper and lower side-bands appear as shown in Fig. 3. Each of these is spaced from the carrier-wave by an amount equal to the frequency of the modulating tone. From this fact it is evident that, the higher the pitch of the musical note striking the microphone, the broader will be the band in the ether occupied by the transmitter at that moment. Those persons complaining that one local broadcasting station is much "broader" than another should note this point. Usually the alleged "broadness of wave" of a particular transmitter is due to the broader tuning of the listener's receiver at this frequency or to the greater power radiated by this particular transmitter. If neither of the stations shown in Fig. 2 ever used modulation frequencies above the highest on the ordinary piano keyboard or the highest reached by the piccolo (that is, fundamental frequencies of about 4600 cycles) there would be no overlapping of one station into the channel of the other — provided, of course, both carrier waves were on their assigned frequencies. In many transmitters sudden modulation peaks may cause momentary shifting of the carrier frequency or other effects, resulting in intermittent interference being noted in neighboring radio channels. It is understood, of course, that from the tone quality standpoint it is desirable, if permitted, to transmit modulation frequencies of higher than 5000 cycles because harmonics of various musical instruments range above this figure. And these add a great deal to the naturalness of broadcast music We have now defined a radio channel, shown 1* 10 K.C.Chan nel "I Carrier-wave 1 1 1 ' Lower side. Upper side |: 1 j Band Band 1 1 1 ■ 1 1 1 -1 1 1 ! 1 1 1 1 1 1 1— 654 656 658 660 662 664 666 FREQUENCY IN K.C. FIG. 3