Radio broadcast .. (1922-30)

No. 343 RADIO BROADCAST Laboratory Information Sheet Speech Power and Its Measurement April, 1930 IN CONNECTION with a paper entitled ''Speech Power and Its Measurement" by L. J. Sivian, published in October, 1929, Bell System Technical Journal, curves and data are given on two devices useful in many ways. Curves and circuits of the two units—a volume indicator and an impulse meter— appear on " Laboratory Information Sheet" No. 344 and the following are some notes regarding their use. The volume indicator meter has been widely used for controlling amplification in radio broadcasting, in phonograph and film recording of speech and music, and for rapid measurement and control of speech levels. Essentially the volume indicator is a three-element vacuum-tube voltmeter with a rapid- action d.c. galvanometer in the plate circuit}. The tube is operated on a part of its characteristic such that the rectified plate current is proportional to the square of the voltage input. The meter combined with the electrical circuit has a dynamic characteris- tic as shown by the curve on "Sheet" No. 344, which gives the maximum deOection as a function of the duration of the a.c. input voltage. For inputs lasting more than about 0.18 seconds the maximum deflection remains the same, and, since the av( i r;tu r <- syllable duration in speech is of the order of 0.2 seconds, it follows that the maximum deflection is approximately proportional to the mean power. The impulse meter is essentially a peak-reading voltmeter and the circuit is designed so as to cause the plate current to reach its ultimate value with an input of as short a duration as possible. The time required for the galvanometer to reach its maximum deflection is determined by the dynamic characteris- tic of the meter and its associated plate circuit as well as by the time constant of the condenser- charging circuit connected to the grid of the tube. The curve, therefore, shows the rate at which the potential on the blocking condenser builds up and by reference to the curve it will be noted that the plate current reaches 80 per cent, of its ultimate value with an a.c, input of only approximately 0.015 seconds. (Continued from page 359) No. 345 RADIO BROADCAST Laboratory Information Sheet Regarding Grounds for A. C. Sets April, 1930 IN THE INSTALLATION and use of a.c. radio receivers it is frequently found that more volume is ob- tained without any wire connected to the ground terminal than is obtained with a ground connection to the binding post. This effect has evidently given quite a few servicemen the impression that there was something wrong with the receiver. The fact is, however, that this quite common effect does not necessarily indicate that the receiver is defec- tive. The volume obtained from a receiver depends upon the ability of the antenna system to pick up signals and upon the gain of the radio receiver. Modern high-gain receivers have to be very care- fully designed from the standpoint of shielding, filtering, and grounding to make them absolutely stable and if any one of these points is neglected the set will have some regeneration. On the other hand, if the set depends for some of its amplifica- tion on regeneration its performance will depend somewhat upon the conditions under which it is operated. Proper grounding is an important point in the prevention of regeneration and the lack of a ground or a comparatively poor ground may cause an otherwise perfectly stable receiver to regenerate slightly. This is the effect which is responsible for the peculiar operation of a.c. receivers with and without proper ground connections. With a proper ground .iTi- set has a gain approximating that which its makers intended it should have. If, however, no ground is used some regeneration will exist which will generally tend to increase the gain and, as a result, more volume is obtained. The disadvantage of not using a ground, however, is that this in- creased gain may only be obtained over a small part of the dial and at other points the set may tend to oscillate or the first tuned circuit may be thrown out of alignment so that the selectivity is impaired. For these reasons it is always advisable to operate a receiver with a ground if it is intended that it should have one. If for some reason the receiver must be operated without a ground it is worth while to try reversing the plug in the light socket in order to determine the position which gives the most satisfactory operation. type are " background tendency to "drift." capacity" and Conclusion Radio is doing much toward aiding aerial navigation and making aviation a safe commercial possibility. Radio is doing much toward making aviation fool- proof and making it possible to fly in in- clement weather with safety. Meteorologi- cal reports, radio beacon service, and direct communication are doing much toward making aviation a dependable institution. Radio control of airport land- ing lights will make night Hying safer. As planes become larger and space and pay- load become less important, more depend- able and larger apparatus can be used to advantage. All sciences are interdependent, and thus commercial aviation is benefited directly by radio development. Aviation, with the cooperation of radio, will give us the safest, most rapid, and cheapest transporation ever known to mankind. MEASURING PERCENT- AGE MODULATION (Continued from page 335) cycles. In Fig. 1 A the percentage of mod- ulation is 18 per cent.; in Fig. 4n, 41 per cent., and in Fig. Ic the transmitter is seriously overloaded, since part of the cycle is completely suppressed. It is easy to see that the modulation no longer has a sinusoidal shape. The remark may be made that, even without the rotating mir- ror, the beginning of overloading can be seen directly upon the stripped picture on the tube. The unmodulated part of the carrier wave appears with great brilliance, since it affects the cathode ray twice as long as the ends of the stripe. The width of this bright part of the middle of the stripe becomes less and less with in- creasing degrees of modulation and shrinks to a lighted point in the stripe with more than 100 per cent. With a little practice the percentage modulation can be evaluated direct from the stripe. An oscillograph of a broadcast trans- mitter is shown in Fig. 4D. This picture was taken with the arrangement shown in the heading illustration, i.e., with a h.f. am- plifier in front of the tube. These pictures, which are of considerable value for the control of the transmission, show that nearly 100 per cent, modulation is present. With a vacuum-tube voltmeter and an indicating instrument for the determina- tion of the degree of excitation, momentary overloads can hardly be seen due to the inertia of the instruments used. The in- stantaneous and easily readable cathode- ray oscillograph is, therefore, to be preferred for a continuous watch in the transmitting station. The WGY-KGO Problem In a report to the Commission, Martin Rice states that, since WGY has been operating simultaneously with KGO, 62 per cent, of reports from listeners in New York, Ohio, Indiana, Illinois, and Iowa complain of poor reception, while only 8 p_er cent, from that territory raised objec- tions before the dual assignment went into effect. In Iowa, Nebraska, Colorado, Wyoming, Idaho, Utah, Nevada, Arizona, and Texas, 72 per cent, complain of re- ception quality, while prior to the present allocation, 88 per cent, in those states re- ported good quality. In California, where KGO used to receive no complaints, 32 per cent, of those writing in now report un- satisfactory reception. 360 • RADIO BROADCAST FOR APRIL •