Sound motion pictures : from the laboratory to their presentation (1929)

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4 o SOUND MOTION PICTURES placed in or about the studio, out of range of the camera, near enough to the performer to pick up the desired sounds of orchestra, musician, speaker, or vocalist. Sound-proof booths house the cameras so that no outside noises may in- terfere with the procedure. When a scene is to be recorded the camera photographs it while the microphones pick up the desired sounds. These sounds, striking the diaphragm of the micro- phone, cause it to vibrate. The vibrations are then trans- lated into a fluctuating electric current that flows through wires to a control room. There the current passes through an amplifier system onto a recording device, and the fluctuations of the current are changed into mechanical vibrations which are recorded on a disk of soft wax. The procedure is therefore substantially that of making phono- graph records. To make sure that the picture and the sound are perfectly synchronized, the motors which operate the cameras and the turntable carrying the disk are made to run at uniform speed and in perfect unison. These motors are started together; they reach full speed together; they continue to run together. When picture and sound recorded by this method are re- produced in a theatre, the equipment employed is that made by Western Electric and known as the Western Electric Sound Projector System. A standard projector is used for the film. A turntable is added for the sound rec- ord. Both, however, are operated by one and the same motor; and means are provided to control the speed auto- matically. An adaptation of the Western Electric public address system makes it possible to pick up electrical vibrations from the reproducer, then to amplify them, and by means of loudspeaking telephones located in the theatre to transform them into sound. For the average theatre four horns, placed behind the motion picture screen, are sufficient for the purpose.