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212 SOUND MOTION PICTURES
V. The R. C. A.-Photophone Recording Apparatus
The Photophone studio apparatus used in the recording process consists of camera, a sound recorder, an amplifier, a sound collector, and the necessary power supply devices. Standard cameras are used, except that the synchronous motor is substituted for the hand-crank or variable speed motor; and a device is added to mark on the film the starting and stopping points. The sound recorder is built to operate with little vibration and uniform speed and is also driven by a synchronous motor. The film is carried over a drum, with loops between the drum and sprockets. A small photometer is used to check the exposure. The optical system is somewhat similar to that used in the reproducing device; namely, a concentrated filament lamp and a system of lenses that produce an image of a slit on the film. The variation of exposed area on the film is obtained by means of an electro-magnetic galvanometer, similar to that used in oscillographs.
The moving system comprising a current-carrying strip and a small mirror is so designed that the response is practically uniform from zero to 6,000 cycles. This covers the practical range in speech and music. The sound collectors are of the type known as condenser microphones and may be used singly or in groups. Each microphone is connected to its own two-stage amplifier, and all are connected to a power amplifier adapted to control both the volume of each collector separately and also the volume after combining. The power amplifier may be located near the sound recorder and is equipped with a monitor loudspeaker. Power for the amplifier and the recorder is supplied by a storage battery and a small motor generator set. The recorder and the power amplifiers are preferably located in a separate room or, in outside work, on a truck.