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STANDARD SOUND DEVICES 49
The exact timing of picture and sound is photographed and fixed regardless of any handling of the film or the projector and irrespective of any cutting or splicing of the film. A few frames of film can be lost, and the film can be spliced, without any noticeable effect on either the picture or the sound reproduction.
The four principal types of R. C. A.-Photophone equipment for theatres have the following type designations and will serve audiences of the following number of persons in any average auditorium or theatre:
Type A up to 6,000 persons B up to 3,000 persons C up to 1,500 persons D up to 750 persons
The R. C. A. manufactures a projector which is equipped with a sound-reproducing head, which reproduces sound of all existing types of synchronized sound films. The film path is indicated by guide lines within the machine, which facilitate accurate threading.
The path of the sound light beam, which in the projector passes through the slit optical system and film sound track, falls on the photo-electric cell. In this cell the varying light gives rise to the corresponding electric currents which, after being greatly magnified by the amplifiers used in this system, operate the loudspeakers that reproduce the original sound on the stage. The action of reproducing the sound is instantaneous, and the sound-reproducing system is entirely automatic in operation.
The lights which are used to illuminate the sound track are provided in duplicate. Therefore, if one burns out during a performance, the touch of a lever brings a second, prefocussed lamp into position and lights it automatically without causing interruption in the performance. (It is in