Radio Broadcast (May 1928-Apr 1929)

8 RADIO BROADCAST MAY, 1928 only a weak and distorted signal could be received. A thorough investigation with loop receivers and field strength measuring equipment revealed that two signals from weaf, apparently coming in from two different directions and, at some points, exactly 180 degrees out of phase, tended to cancel each other. Similar effects would be experienced when two stations radiate identical programs on the same channel. If a chain*of twenty stations were synchronized, the resultant reception, at all points beyond the high-grade service range of the synchronizing stations, would at least lack that clearness and purity of tone which characterizes modern broadcasting and might be sufficiently confused to be almost unrecognizable. Because of this consideration, desirable synchronization of chain programs is limited to stations widely separated geographically so that no listener is within the practical range of more than one synchronized station. CHAIN STATION SYNCHRONISM THE practical application of synchronization of chams is thereby limited to the establishment of two or three groupings per chain rather than placing all the members of a chain on a single channel. This would not effect radical saving of cleared channels, a maximum of four or five for each chain being possible. A further barrier to extended chain synchronization is the fact that it can conserve channels only if the stations involved broadcast chain programs exclusively. Since most of the subscribing stations use chain programs only occasionally, an hour or two each evening at the most, permanent assignments to synchronized channels is now impossible. The cost of wire lines and chain features is altogether too high to require subscribers to chain programs to utilize only chain programs. Yet, only under those conditions, would chain synchronization effect any economy of channels. Most chain stations broadcast local programs and perform local services during a majority of their broadcasting time. These services would have to be discontinued or transferred to minor stations under any plan of widespread chain program synchronization. Considering that the problem is to find comfortable room for nearly 700 stations in a band of 89 channels, reliance upon the single measure of chain station synchronization does not offer any promise of real relief, even if the difficulties cited could be overcome immediately. A trend of development which holds some promise is improvement in the permissible percentage of modulation without resultant distortion. This is the third general measure of relief given at the beginning of this article. Its effect is to increase the percentage of the high-grade service area to the total carrier interference range area. The experimental fiftykilowatt transmitter at Whippany, maintained by the Bell Laboratories, utilizes a new method of combining carrier with program signal, said to effect one hundred per cent, modulation. While the results, so far as increased service range for a given carrier power is concerned, are not startling, they are, nevertheless, appreciable. The engineering ideal to be attained in this direction is that the carrier shall cease to radiate at the edge of the station's high-grade service area. The problem of setting up an intense wave motion of any kind and making it cease abruptly at the limit of its usefulness is a tremendous challenge to engineering ingenuity. In radio transmission, there is room for so much improvement in reducing the ratio of carrier spread to useful service range that some progress in this direction may be hoped for. But that this development will have material bearing in the present situation is not within the expectation of the most sanguine workers in this field of research. Considering the immediate possibilities of all the proposed measures of relief, none holds greater promise than the development of highprecision crystal oscillators with accurate temperature control. Realizing the value of a source of constant-frequency oscillation, many laboratories have been concentrating on this problem during the last few months. The writer has seen, in the development stage, a new type of quartz crystal precision oscillator for broadcasting stations which will probably be marketed by the R. C. A. This device will consist of two accurately ground and matched quartz crystals mounted in a constant temperature chamber. The temperature is kept constant by means of a thermostat and maintained at a given setting WIDTH OF CHANNEL WIDTH OF CHANNEL CARRIER PLUS SIDE BANDS CARRIER PLUS SIDE BANDS Station Station A_ B 10,000 Cycles 1 1 1 1 1 1 CO| "! .21 Si SI 31 ">.i 5>l SI i §1 ST £ 1 rier •rier Cai 0 1 1 1 1 1 ~w /, r. .51 y 1.2 Si CARRIERS AND SIDEBANDS Stations assigned to adjacent broadcasting channels transmit on carrier frequencies differing by 10,000 cycles. When programs are transmitted, two sidebands are produced which introduce into the transmitted wave, frequencies up to 5000 cycles above and 5000 cycles below the carrier frequency; the station therefore uses a band of frequencies 10,000 cycles wide. The left sketch shows the frequency bands used by two adjoining stations. The two carrier frequencies differ by 10,000 cycles; the sidebands meet each other but do not overlap. This holds true when two adjacent stations hold exactly to their assigned frequency. If either station varies, the condition shown at the right obtains, where we have assumed that one station has wandered from its frequency to the extent of 4000 cycles (4 kc.) deviation. This leaves the carrier separation between the two only 6000 cycles (6 kc). Then, in the receiver output, we would hear a 6000-cycle note, which may be loud enough to ruin reception. Interaction between the two sidebands of the stations — shown on the shaded portion of the diagram — also occurs by means of this thermostat, checked by a thermometer. A suitable heating coil is also mounted in the constant temperature chamber. The two crystals, supplied with this oscillator, may be ground to any one frequency in the band of from 550 to 1500 kilocycles. A small selector switch is arranged to select either of the two crystals. Should one of the crystals fail during the operation of a broadcasting station, it is only necessary to throw this selector switch which removes the defective crystal and cuts in the spare crystal, which will be at the right temperature to start operation immediately. The oscillator circuit will consist of a vacuum tube and coil system, the electrical constants of which have been very carefully determined with a view to being suitable to work with the quartz crystals. A monitoring receiver, comprising a suitable detector and two stages of audiofrequency amplification, in a separate box, has also been designed for use in connection with the quartz crystal precision oscillator. When these two boxes (the quartz crystal oscillator and the monitoring receiver) are used in conjunction, a loud speaker may be connected to the output of the last stage of audio-frequency amplification and the quartz crystal frequency beat against the carrier of the broadcasting station. Special precautions will be taken to emphasize the low frequencies so that the zero-beat note will be heard at its greatest efficiency. No definite claims have yet been made as to the stability of this device, but there is no question that two stations, operating on the same channel, both employing the device, will not heterodyne each other seriously. It should eliminate the high pitched squeal and that, alone, will justify its installation. It is not unreasonable to expect that, with continued improvement and experience with precision quartz crystal oscillators, complete carrier synchronization will ultimately be made possible. NEW METHODS OF TRANSMISSION THIS summary would be incomplete if mention were not made of several proposed new methods of transmission, claimed to reduce the width of the channel required by a broadcasting station. These methods are frequently mentioned in public statements, issued as possible measures of relief by persons who must know the objections to their adoption in the broadcast band. One is single side-band transmission, accomplished by suppression of the carrier and one side-band, as utilized in practice in the transatlantic telephone. It has the vital objection that its adoption is predicated upon scrapping every transmitting and receiving equipment in the country. The broadcast receiver necessary to pick up single side-band transmission is expensive and delicate. One of its elements is an oscillator which must be adjusted to within ten or fifteen cycles of the assigned frequency of the station to be received, in order to supply the missing suppressed carrier. To reequip every broadcast listener with a suitable receiver under this system would cost the public not less than half a billion dollars. Another suggestion along these lines is the adoption of a new system of frequency modulation. For this has been claimed the extraordinary virtue of accommodating simultaneously between one and two thousand broadcasting stations in the present band. The basic principle of the system is that the carrier frequency is shifted up and down according to the desired audio signal to be transmitted. The receiving set is tuned with extreme sharpness so that the shifting carrier causes varying energy to actuate the receiving set by reason of the detuning effect. This sytem would also require the scrapping of all receiving sets, unless the carrier is shifted over so wide a scale that no economy of channels is effected. The contention is also raised that a shifting carrier sets up numerous harmonics so that the theoretically narrow band occupied by the frequency modulated carrier would prove to be in practice no narrower than that used by broadcasting .stations operating under the present simple method's of modulation. A most exhaustive study of the entire subject will lead inevitably to the conclusion that progress in increasing the capacity of the broadcasting band will be steady but that no radical developments, sufficient to offer a complete solution to the present problem, are in sight.