International projectionist (Jan 1943-Dec 1944)

signed especially for the microphones to be used, since the impedance of the several types differ so widely. To secure minimum mixing loss, the input impedance of the mixing circuit facing each source should equal the source impedance, but considerable variation is often permissible. A dynamic microphone ordinarily may be worked into any reasonable load resistance provided it is not less than the microphone impedance. In the case of a pre-mixing amplifier, a load impedance greatly different from the optimum, affects the frequency of the amplifier and may limit seriously the power output of the amplifier with tolerable harmonic distortion. Control Range of 45 db Normal In any mixing circuit the range of control available is determined in the design of the mixing controls. The controls in general use have a range of about 45 db, often supplemented by one or two large steps next to the "off" position. The "off" position should theoretically give infinite attenuation or complete cutoff. Actually this is impossible, partly because of the mutual impedance in the control and because its wiring cannot be reduced to zero and more often because of the presence of electrical leakage. Completely independent operation of each mixing control is the ideal. While it is closely approached in some circuit designs some interaction of control functions is inevitable. The cause of this interaction, when present, is the use of a type of control whose output impedance varies with the setting. Such controls may, however, be used in some circuits without causing this effect. The transmission of speech from a microphone to the power amplifier through stray capacitances or spurious couplings, not subject to control by the mixer, is called "leakage." Leakage limits the degree of cutoff obtainable when the controls are in "off" position. Since leakage is greater at the higher frequencies the result is "tinny" quality. The same forms of stray coupling, which give rise to leakage, also make the mixing circuit more vulnerable to noise pickup from stray currents flowing in the ground system. This sometimes causes troublesome power hum or, if the system is near a radio transmitter, radio-frequency pickup, resulting in cross-talk or singing. Circuits which are entirely above ground potential are more subject to leakage and noise pickup caused by spurious coupling. Since a mixing circuit usually contains only resistances it should not affect directly the frequency response of the over-all system. Indi rectly it may, however, through improper or widely varying terminal impedances, upset the normal frequency response of associated amplifiers. Appreciable leakage may give the system a rising response characteristic when the controls are set at near maximum attenuation. Proper design will minimize both of these defects. There are a number of types of controls available. The type whose operation is easiest to analyze and which least affects the properties of the overall circuit is the most complicated in construction, while the types which are simpler and cheaper to build and which have, therefore, been more widely used, in general complicate the theoretical design of the circuit through the introduction of additional variable factors. In order of their simpbcity of appli cation the principal types of controls are: (1) the variable "T"; (2) the ladder; (3) the compensated potentiometer, and (4) the simple potentiometer. Types (2) and (3) are fundamentally interchangeable. The variable "T" control is so-called because the schematic form of the control appears like that letter. It has a shunt resistor and two series resistors, one either side of the junction with one end of the shunt resistor. In one form all three elements are variable and three sliders are required. In another form the two series arms are variable and the shunt resistor is tapped. As the two contacts are moved they connect to resistors in series (Continued on page 29) Projectionist a Potent Factor in Success of War Loan Drive By HUBERT L. VOIGHT WAR ACTIVITIES COMMITTEE, MOTION PICTURE INDUSTRY EVERY projectionist in the nation was this week called upon by the War Activities Committee for assistance in driving the motion picture industry's phase of the Fourth War Loan to a smashing success. In asking the projectionists to cooperate in the campaign, the war activities heads stressed the importance of the four trailers which will be released to all theatres, starting January 18, with the Ginger Rogers footage. This will be followed with a trailer in which Ann Sothern appears as Boxoffice Maisie. and Bob Hope in a serious war-bond appeal will appear in a third release. The fourth release, the United States Treasury Department Fourth War Loan technicolor shield, will be made available to each theatre participating in the drive. Getting the trailers on the screen and WE BOUGHT EXTRA WAR BONDS y WAR LOAN 4th War Loan Shield awarded to theatres participating in drive. keeping them there is a problem laid before America's projectionists. During the Third War Loan, it is reported, many trailers — shipped to theatres — were returned with seals unbroken. In some houses, trailers were exhibited one or two days, and taken off. In others, the sequence of trailer showings was not maintained for the greatest effectiveness. Practically every theatre in this country is expected to cooperate in the Fourth War Loan activities of the industry. "Sell a Bond for Every Seat" is the showman's slogan and goal. In this campaign, the projectionist is recognized as a potent factor to the success of the huge undertaking. Get those trailers on your screen — and keep 'em there! (ED'S NOTE: We wish to direct the attention of the gentlemen connected with the War Activities Committee, Motion Picture Industry, to a few pertinent facts regarding the return of unused trailers in the Third War Loan drive. The showing of patriotic trailers in the majority of motion picture theatres depends solely upon the manager or owner of the theatre. Many theatre managers, not excepting those of the de luxe houses, are concerned chiefly with the reduction of operating costs and instruct their projectionists to "cut" the patriotic trailers so as to avoid any possible overtime charges. Motion picture projectionists have at all times shown their willingness to cooperate with the authorities in the various war loan drives, and investigation will prove that in almost all cases where trailers were returned with unbroken seals the projectionists were not permitted to run them.) 14 INTERNATIONAL PROJECTIONIST