Radio Broadcast (May 1928-Apr 1929)

SEPTEMBER, 1928 CAN WE MULTIPLEX OUR RADIO CHANNELS? 247 that its width for broadcasting is 10 kc., and shown that the side-bands, which vary in response to the voice frequencies from 50 to 5000 cycles, occupy the full width of this channel. We are now ready to consider the more unusual system of double modulation, sometimes called "secret radio," "selective signalling," and "double tuning." THE DOUBLE MODULATION SYSTEM A BRIEF outline of such a system will be mentioned so that the reader will understand how double modulation can be used at an experimental broadcasting station employing this system. Instead of modulating the usual carrier wave by audio frequency currents, an intermediate frequency (i.f.). above the limit of audibility — say 20,000 cycles — is used. This intermediate frequency is, in turn, modulated by speech frequencies. The resultant is a modulated carrier wave superimposed upon another carrier which is radiated in the usual manner. In receiving such a doubly modulated wave, it is necessary to tune to and detect the radiofrequency carrier wave, which could be, say, 660 kc; then tune to and detect the intermediate frequency of 20 kc. and, if desired, amplify the audio-frequency signal resulting from the second detection. A transmitter and receiver that work in the above manner are shown schematically in Fig. 4. The audio frequencies (50 to 5000 cycles) are picked up by the microphone, and are impressed on the i.f. oscillator (or generator) by means of the first modulator. The modulated output of the i.f. oscillator acts on the second modulator arranged to modulate the r. f. oscillator (or generator) so that a doubly modulated wave may be radiated from the antenna. Only the bare fundamentals are shown at the receiver; namely, an r. f. tuner (660 kc.) and detector which feeds its inaudible output into an i.f. tuner (20 kc.) and detector. In the output of the latter are connected telephones in which /"he desired signal can be heard. SEMI-SECRET TRANSMISSION SINCE our radio receivers of to-day are intended for the reception of single-modulated signals, we do not have the second receiver unit of Fig. 4 (i.f. tuner and detector). Hence, by the use of certain combinations at the transmitter, intelligible signals from double modulated transmitters cannot be received with ordinary AT THE RECEIVING END This apparatus is part of that used for the reception of single side-ba>ul signals at the Hoitlton, Maine, station of the American Telephone & Telegraph Co. The operator at the right is adjusting the apparatus used to pick up the signal, and the man at the left is responsible for the wire line to New York receivers. For this reason, engineers term such a system "semi-secret." Some readers will doubtless ask the question, "What about the super-heterodyne — it possesses an intermediate tuner and detector." To convert a super-heterodyne into a double modulation system receiver, as in Fig. 3, it will usually be necessary, first, to alter the fixed-tune i.f. amplifier so that it can be tuned to any desired transmitted intermediate frequency (20 to 100 kc), and, second, to eliminate the r.f. heterodyning oscillator, which is unnecessary. An editorial in Radio News not so long ago stated that the rapid modulations necessary for television could be superimposed on the present carrier wave of a broadcast transmitter by means of double modulation so that no additional channels would be required. As we know, this same double-tuning system has been offered by others as a means for greatly increasing the broadcast channels now available in a given wave-band. The fallacy in both of the above views will be pointed out in the paragraphs to follow. the assigned frequency, which we will take in this example as 660 kc. Spaced equally, on either side, are the upper and lower i.f. waves separated from the carrier by the selected intermediate frequency of 20 kc. Associated with each of these are the usual upper and lower a.f. sidebands separated from the i.f. line by 1000 cycles, since, at the moment this radio spectrum was recorded, it is assumed that a steady 1000cycle note was sounded at the microphone. If the highest fundamental musical note were played (about 5000 cycles), these side-bands would move to a position such that the two outermost bands would reach the dotted lines which indicate the limits demanded by a single channel when the doublemodulation system is used. We note from Fig. 5 that this width is 50 kc. Compare this with the ordinary broadcast channel shown in Fig. 2, the width of which is only 10 kc. Let us take this a step farther, both because it is interesting to see what happens in the ether when a single carrier-wave is multiplexed so as to carry several simultaneous conversations and also because we find that when more than one intermediate frequency is used the frequency band per conversation is less than 50 kc. wide. Fig. 6 shows the location of the spectrum lines resulting from the simultaneous transmission of two independent audio signals, one on an intermediate frequency of 20 kc. (the lines for which f-< 50K. C.Channel <-\ arrier wave Lower t.F. wave Lower side J Band I Upper I.F. wave Upper side Band THE CHANNEL NEEDED FOR DOUBLE MODULATION 'IG. 5 illustrates the radio spectrum lines resulting from the radiation of a doubletransmission. In the carrier-wave located at F1 modulated radiophone center we have the r. f. Microphone Modulator I.F. , Osc. (20 K.C.) Modulator R.F. Osc. (660 l! K.C.) (20 K.C.) TRANSMITTER * R.F.=Tuner « & Detector '(660 K.C.) RECEIVER FIG. 4 630 640 650 660 670 680 690 FREQUENCY IN K.C. FIG. 5 are identical with those of Fig. 5), and the second on, say, 30 kc. The point to note is that the total band width here measures 70 kc, or 20 kc. more than that of Fig. 5, which is for a single conversation. To summarize, we may say the width of band required for the first multiplex channel is 50 kc. with an additional 20 kc. for each additional channel. We conclude, therefore, that the number of broadcasters in any given wavelength band would not be multiplied if double modulation were used but actually decreased by more than one-half. THE PRO AND CON OF DOUBLE MODULATION LACKING the ability to glimpse into the future, we can only guide our speculations by the technical facts that we know about the infant radio developments of to-day. By weighing their advantages and disadvantages it