Radio Broadcast (May 1928-Apr 1929)

248 RADIO BROADCAST SEPTEMBER, 1928 -70 K.C.Channel 30 20 K.C. K.C. I.F. I.F. J{ 20 30 K.C. K.C. I.F. I.F. 620 640 660 680 700 FREQUENCY IN K.C. FIG. 6 should be possible to form arj opinion of the likelihood of their future adoption. The advantages of the double modulation system are: 1. Greater selectivity and freedom from static because of the double tuning feature at the receiver. 2. The advantage of semi-secrecy, if desired. Here is a system which could answer the question, "How can programs be sold to the listener?" It is very doubtful, however, if the listening public of the United States would react favorably to the proposition of paying for their radio programs. The disadvantages of double modulation are: 1. The number of simultaneous broadcasts (or the number of conversations) would have to be greatly reduced because the width of radio channel per conversation is more than double that required for the present system of broadcasting. 2. The amount of local interference set up by a double modulation transmitter would be very serious if harmonics from the oscillators generating the carrier and intermediate waves were allowed to be radiated. 3. More costly and complicated transmitting and receiving apparatus due to extra tuning controls, tubes, and amplifiers. Minor technical disadvantages have been omitted since these could probably be overcome by engineering development work. So far as the writer knows, there are no radio stations in our country making use of the straight double modulation system for other than experimental purposes. If this is the case, in spite of the fact that the system has been known for many years, we would infer that the disadvantages outweigh the advantages. In concluding that our present system will not be supplanted by that of straight double modulation we see that the underlying reasons for this conclusion are based principally on the technical characteristics of this little-known radio system. It has been the object of this article to explain, as simply as possible, these characteristics. THE SINGLE SIDE-BAND SYSTEM MOST of the advantages incorrectly attributed to the double modulation system can be realized in the "single side-band" system. Great interest surrounds any workable system which allows the same waveband in the ether to carry, without mutual interference, twice as many conversations as it can at present. This is what single side-band transmission will do. The following paragraphs answer the questions of: What is this system? How does it work? and What will it do?" Single side-band transmission is the transmission of modulation frequencies by the radiation of only one side-band, the other side-band and carrier wave being suppressed at the transmitter. Refer for a moment to Fig. 3 showing the spectrum lines of the ordinary transmitter. At the instant Fig. 3 was recorded we assumed that a constant musical tone of 1000 cycles (1 kc.) was impressed upon the microphone. During a musical selection the audio frequencies, as we know, may vary erratically and rapidly from 50 to 5000 cycles (or more). To represent the resulting movement of the side-bands from moment to moment, they are shown in Fig. 8 as dotted lines occupying a frequency space of 4950 cycles. Also, in this figure it is indicated that the carrier-wave and lower side-band are suppressed, leaving only the upper side-band to be radiated into the ether. The width of radio channel required for this system of broadcasting is less than 5 kc, or half of that demanded by the present system. This is an important step in the right direction. Let us see how such a telephone signal can be received. Due to the absence of a transmitted carrier wave, messages from such a transmitter would not be understandable on our ordinary receivers. However, the carrier wave, which is steady in frequency, can be supplied locally at the receiver by an oscillating tube. Its transmission through the ether is thus made unnecessary. In receiving signals from a single side-band transmitter, using the receiver arrangement shown in Fig. 7, the local oscillator supplying the carrier-wave (which is no longer a "carrier") would be set by the operator at exactly 660 kc. to correspond in frequency with the suppressed carrier. This must be done by ear, since if the frequency of this local beat oscillator is even very slightly off, the received signal will not have its natural quality. For instance, the voice of the best announcer could, by misadjustment, be made to resemble that of an old woman! Oscillator ^-supplying ^ carrier-wave R.F.Tuner & Detector Microphone TRANSMITTER ^ (irk* RECEIVER FIG. 7 660 FREQUENCY IN K.C. FIG. 8 To complete the answer to "How does it work?" brief mention will be made of the apparatus at the transmitter which accomplishes the suppression of the side-band and the carrier. Fig. 7 illustrates the schematic outline of one type of transmitter. The voice energy and that from the master r.f. oscillator is fed into a pushpull modulator, so connected as to suppress the carrier-wave (the frequency of which is determined by the master r.f. oscillator). Then the output of this modulator is passed through a filter and the undesired side-band is removed, leaving only one side-band (which varies in frequency from 50 to 5000 cycles as the voice frequency varies) to be amplified by the power amplifier and finally to be radiated from the antenna. CAN IT BE PERFECTED? THIS system affords radio telephony requiring only one-half the channel width required at present. It allows an increase in sharpness of tuning at the receiver without reducing fidelity, thus providing more selectivity. Much less power is necessary at the transmitter, since the carrier wave is not radiated. Generally there is less distortion and variation in received signal strength due to fading, because the locally generated carrier is steady. These are some of the advantages that occur with single side-band transmission. When it is coupled with double modulation still other advantages appear, one of which is the possibility of a high degree of secrecy when certain combinations are used. The disadvantages are: Increased complication of apparatus; more skill required in the operation of the receiver; and, in our present broadcast band, the disadvantage that all our receivers would require modification. The most serious of these disadvantages is the difficulty of setting and maintaining the local oscillator at the desired frequency. Assuming transmission to take place at 1000 kc. the exactness with which the oscillator must be set is I part in 100,000. In the present state of technical development, this would require a highly skilled operator and precision instruments. Development work is needed to overcome the demand for such accuracy in order to make ordinary single side-band reception practical for everyone's use in the broadcast waveband. It is the single side-band system (combined with double modulation) that has been selected for use in the American Tel. & Tel. Co.'s transatlantic radio link between the United States and Great Britain. The two photographs show parts of the apparatus used. It seems logical to suppose that single sideband transmission will grow in use for point-topcint communication and the time will come when it will be used for the broadcasting of speech, music, and vision.