International projectionist (Jan 1943-Dec 1944)

rate of discharge, until with no resistor we obtain a spark on short circuiting the terminals of the resistor. If the rate of discharge were too rapid in these particular circuits, changeover disturbances ranging from dull thuds to definite clicks would be audible, depending upon whether the rate was just above the lower limit of audibility or considerably above. So the size of capacitor must be selected, consistent with the requirements of the circuit and the value of the resistors established so that the rate of discharge will be such that there will be no lag in sound on changeover on the one hand and no changeover clicks on the other hand. In this connection we might point out that C4 is charged through R6 and therefore the rate at which the operating condition is established also is dependent upon the value of this resistor. All of these factors properly established with respect to each other and with due consideration for the functions of the associated circuits do produce a quiet, rapid changeover of the electronic type. Tuned Filter Circuit Employed In Figure 2, P1 is the main system volume control, having the same range as in Figure 1. We note also that VT2 is connected as a triode in both amplifiers. The cathode resistor R_ is by-passed by C8, which in accordance with our earlier discussion gives some fixed high frequency equalization. Continuing to the right from the plate of VT, in Figure 2 we come to a mesh. C9 and Lx in parallel, constitute a tuned or resonan. circuit, which offers a maximum attenuation at 120 cycles. The system, in which this amplifier is used, employs exciter lamps heated by 60 cycle current. Exciter lamp changeover, with three way switches, is used. This tuned circuit attenuates the slight hum produced by these lampB. It will be remembered th a capacitor presents a high impedance to the passage of low frequency signals and that as the frequency increases the impedance decreases. An inductance (Lj^) functions in reverse. Now when these units are connected in parallel, we have an inductance, which tends to suppress any changes in the current through the circuit or, in other words, it has a lagging current. The capacitor, on the other hand, will store energy as the voltage increases and release it as the voltage decreases. This unit is said to have a leading current. The combination of the units in parallel results in their presenting a maximum impedance at a definite frequency dependent upon the values of the units selected. If two such units were to be connected across the line to accomplish the same result they would be in series. In this instance their char S.M.P.E. Plans Strong Program For 53rd Semi-Annual Meeting AN outstanding program is being planned for the 53rd semi-annual meeting of the Society of Motion Picture Engineers, to be held at the Hotel Pennsylvania, New York, N. Y., May 4 to 6 inclusive. The Papers Committee is assembling an attractive and interesting schedule. Members and others contemplating the preparation of papers to be read at this meeting are urged to communicate immediately with the Chairman of the Papers Committee, at the office of the Society in the Hotel Pennsylvania. Reception and local arrangements are in the hands of a 22-man committee headed by chairman Dr. A. N. Goldsmith. Oscar Neu and his committee are in charge of hotel and transportation arrangements; the publicity committee is headed by Julius Haber, and the luncheon and banquet arrangements are in charge of D. E. Hyndman and a committee of nine. The projection committee is in charge of H. F. Heidegger and includes the officers and members of New York Projectionists Local No. 306. The usual get-together luncheon for members, their families, and guests will be held in the Roof Garden of the hotel on May 4 at 12:30 p.m.; addresses by eminent speakers will be one of the high lights of this luncheon. The banquet and dance will take place in the Georgian Room of the hotel on May 5 at 8 p.m. The tentative program is as follows: Tuesday, May 4 9:00 a.m. Hotel Roof; Registration 10:00 a.m. Salle Moderne; Business and technical session 12:30 p.m. Roof Garden; Get-together luncheon 2:00 p.m. Salle Moderne; Technical session Wednesday, May 5 9:30 a.m. Hotel Roof; Registration 10:00 a.m. Salle Moderne; Technical session 12:30 p.m. Luncheon period 2:00 p.m. Salle Moderne; Technical session 8:00 p.m. Georgian Room; 53rd Semi-Annual banquet and dance Thursday, May 6 9:30 a.m. Hotel Roof; Registration 10:00 a.m. Salle Moderne; Technical session 12:30 p.m. Luncheon period 2:00 p.m. Salle Moderne; Technical session acteristics combine so that at a definite frequency their impedance is a minimum, and is equivalent to a resistance. Thus at this frequency the greatest amount of signal will pass through the shunt path and be diverted from the load. Tuned circuits find many applications throughout the field of electronics and speech transmission and this simple mesh may be said to be the basis for the more complex networks used. The high and low pass networks used for selection of frequencies delivered to the high and low frequency speakers in sound systems is one example. The band pass filter is another type widely used, either to pass a definite range of frequencies or to eliminate a definite band of frequencies. Combinations may be used to obtain a variety of results that are astounding. The resistors R10 and Rn at either end of this tuned circuit, it will be noted, are connected to the plate supply circuit. R10 is actually the plate resistor, while R1X has been added to equalize the voltage across the mesh and thereby improve the operation of the circuit. A further difference between Figures 1 and 2 is the inclusion of C10 in the output circuit of Figure 2. This is a blocking capacitor for the purpose of keeping plate voltage from the input circuit of the power amplifier. The power amplifier used with Figure 1 has its own blocking capacitor. The two amplifiers presented in this discussion are examples of modern types of voltage or volume control amplifiers employing high impedance input and output circuits, resistance coupling and in the case of Figure 1 an electronic changeover. But with the development of the electronic art in the past year, who can say what we may expect in the way of sound system amplifiers in the future? NEW PICTURES REACH ARMY HOSPITALS VIA RED CROSS Patients in U. S. Army hospitals are now viewing first-run pictures right from their bedsides as a result of a new program in the war efforts of the American Red Cross. This hospital motion picture service is carrying 16 mm. movies to bed patient audiences in military hospitals all over the country, with the Red Cross now operating the third largest motion picture chain in the United States. The new program is operating under supervision of the four area directors of the American Red Cross, with the group's recreation staff of each hospital arranging for the showings in cooperation with medical officers and hospital staffs. Before the end of the year the ward circuits will cover more than 350 hospitals. Generally, projectionists will be enlisted men, trained in all phases of motion picture operation and detailed to this Red Cross service. APRIL 1943 11