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398 CINEMATOGRAPHIC ANNUAL.
V3 is a rectifier tube and furnishes direct current to the field of the 720 cycle generator.
By these means the motor speed is kept, under ordinary conditions, to within two-tenths of a per cent.
Switch S j when closed toward the right will make the motor run unregulated except for the hand adjusted potentiometer F1 and the motor speed may be adjusted to whatever the operator might want.
VOLUME CONTROL
It is necessary to have some means of varying sound levels in theatres, first because of the variation in sound energy requirements in theatres, second because of the variation in levels of recorded sound, third because of the variation in the size of the audiences and fourth the desirability of level control during reproduction for the purpose of emphasis.
Most reproducing systems make use of two means of controlling sound levels. The first of these methods is the use of the gain control, with which the gain amplifier is equipped. The other is the use of an attenuating device known as the fader, which is usually connected electrically just ahead of the gain amplifier. The first method is generally used to adjust the amplifier system to the requirements of the particular theatre, while the second method is used by the operator during the show.
CAUSES OF DISTORTION AND FACTORS AFFECTING THE FIDELITY OF REPRODUCTION
Factors causing distortion in a reproducing system may be classified under three heads. First, those affecting the response at various frequencies (called frequency characteristics) ; second, those which cause the type of distortion known as non-linear; third, those which cause the addition of extraneous noises. In the case of the Western Electric system, if the apparatus is properly maintained and operated, the frequency characteristic is quite uniform from slightly below 60 cycles to somewhere above 5,000.
The type of distortion known as non-linear is the type which occurs when an amplifier or other part of the system is overworked. Such distortion may get into the reproduction in the theatre from one or more of the following causes: improper