Motion Picture News (Oct-Dec 1929)

1234 Motion Picture News EQUIPMENT |)EFEREINCE PACES OPERATION K by JOHN F. RIDER O O MOTION PICTURE NEWS A. C. 800.41 voltages and the measurement of such EFFECTIVE AND PEAK A. C. VOLTAGES ■\17TIAT is to follow is of interest in connection with * * voltages. The value of D. C. potential indicated upon a D. C. voltmeter is the maximum value found in the circuit and this value remains constant during cadi instant that the potential is present across any two points in the circuit. The same, however, cannot be said about A. C. potentials. A .lu.lv of the usual illustration of a sine wave of A. C. potentials shows that it varies between zero and maximum ami has three points of zero during a complete cycle and two points of maximum. At all other instants 11 ls oi some value between zero and maximum. The same is true with A. C. current. Under the circumstances it is necessary to decide upon a certain value of potential or current which may be represented as the value of potential or current which actually does work in the circuit, since the voltage and current are ever changing in magnitude. In other words it isnecessary to decide upon an effective numerical value of current or voltage, the value usually indicated upon an A. C. current or potential meter. If the above is true it is evident that both A.C. current and voltage have two values. First the maximum or peak value and second, the effective value. Assuming the application of A. C. potential to resistance, A. C. curren! will flow through the resistance, the magnitude of A. ('. current varying in accordance with the instantaneous values of applied potential. Yet, an A. C. current meter gives a steady deflection. . . . The heating effect of current beingindependent of direction of flow gives a very convenient method of deciding upon the value of A. ('. voltage and current which actually does work in a circuit. This method of procedure involves a ,010 pan. on between alternating and direct or continuous currents. A a matter of fact the unit alternating current is chosen as the current which will produce the same heating effect as unit direct current. With respect to current flow in a circuit, such as the filament eircuit of an amplifier, it is unnecessary to consider the "peak" value of current, since the "effective" value of current causes the heat and the diameter of the wire is chosen on the basis of the "effective" value of current in the circuit. Hence for all calculation, the current indicated upon a current meter is used as tin basis. The same is true of the voltage in such circuits, since the effective value of voltage governs the effective value of current. The only place that it 1. necessary to consider the "peak" value of voltage present in the circuit is where rupture of insulation or a dielectric is the hazard. Such cases are rectifiers, rectifier tube., condensers, the insulation of cables which carry A.C. voltages, transformer insulation, etc. A special item is the grid bias of a tube. The bias is equal to the "peak" signal voltage which mav be applied to the tll'ie. o o MOTION PICTURE NEWS FILTER SYSTEMS. 800.7 THE occasion frequently arises when it i. necessary to employ a filter system to eliminate an undesired frequency, such as the scratch frequency in sound on disc -ystems or perhaps a peak in the loud speaker response curve. The application of such filters is simple and the calculation is likewise not difficult. In view of the circuit structure and the need to eliminate just one frequency or to attenuate just one frequency, series resonance is employed-, the resonant circuit being connected aero-, the pick-up unit or across lie loud speaker coil. Since the function of this circuit is to offer a path for the current at the resonant frequency, the action is to bypass some or all of the current, so that less or very little will How through the regular element. Absolutely short circuit is impossible because of resistance present in the coil of ih efilter, but very satisfactory current attenuation is available. In the case of the speaker filler it is necessary to know the amount of attenuation desired, in other i-ords, the required reduction of current flow through llu speaker. Where the response curve of the peaker 1 available and Ho amplitude of the peak is known, in T. 0. or HI!, tin impedances are so ■ ranged that the current divides between the speaker coil and the filter. Because of this impedance re lation the filter structure is slightly altered. A resistance is connected in series with the filter. The value I" resistanci i government by the tiller impedai required for tin work and the 1). C. resistance of the coil. Suppose thai we are working with a speaker coil rated at 21 hms at 600 cycles and that ii is neces 0 reduce the aeighl of the respon e curve at tin. frequency. Suppose further that the current in ill- speaker coil must be reduced to one half of its normal value. This mean, that the impedance of the at iv onance moo cycles) -1 he equal to the impedance of the -peaker. II' the coil is a 100 mil henrj 1 lycomb type of unit, the capacity required lor resonance at 600 cycles is approximately .7'Jo mfd. However the resistance of this circuit would 1.. very low, an dsince a 200 ohm impedance 1 required, the -..line of the series resistance would In 2 'Inn less the If C. resistance "I the .oil