Figure 48 shows a schematic diagram of the galvanometer and optical system. The light from the lamp is focused through a condensing lens onto the mirror of the galvanometer. The light stop between the lens and the mirror is for the purpose of cutting off the fringe of distorted light so as to give a clear cut beam of light. The galvanometer window is tilted at a slight angle to keep the light reflected from its surface from entering the optical system. The reflected light from the mirror passes through the cylindrical lens, which condenses the light in one direction only. It then passes through a spherical condensing lens which reduces the beam size still more. The beam from this lens is focused on the slit in the aperture plate (this slit is 0.003 of an inch wide). The light which passes through the aperture plate slit is focussed on the film through a microscope objective lens system. This objective reduces the size of the beam by a 4 to 1 ratio in both directions so that the resultant beam on the film is 0.070 of an inch long by 0.00075 of an inch wide.

When the mirror is at rest, only one-half of the sound track is exposed. The other half of the light beam is cut off by a screen. When the mirror is moved to its extreme position in one direction, the light beam is shifted off of the screen so that the beam covers the entire sound track. When moved in the other direction to its extreme position, the beam moves over so that it is cut off entirely by the screen and strikes no part of the sound track. When referring to the extreme position which the mirror moves it is not meant that this extreme is the maximum movement which can be obtained by the mirror, but only the maximum position to which it should ever move when recording. When the film moves through the recorder it is exposed to this fine line of light which varies in length as the mirror vibrates in response to the sound currents flowing through the loop supporting it. When the film is developed the part of the film which was exposed to the light beam will be opaque while the remainder of the sound track will be transparent. See Figure 49. The juncture of the opaque and transparent parts of the sound track form a wavy line which is practically an exact representation of the sound pressures of the original sound waves. When this sound track moves through the sound head of the projector the amount of light passing through the film will vary with the width of the transparent part of the track. This varying amount of light produces a varying current through a photo-electric cell. The effect of this current is amplified in a vacuum tube amplifier and the amplified variations are sent to the loud speakers which reproduce the sound.

68. Synchronizing Sound on Film. -The sound film is usually run on an entirely separate machine from the camera for practical reasons, and the two films must be so synchronized that when they are printed together they will be in synchronism throughout the length of the film. This is accomplished by running the camera and the recorder at exactly the same speed. They are both driven by synchronous motors connected to the same power supply, and this keeps them always in synchronism. Some kind of marking is required so that the picture and sound track can be lined up for printing. This is sometimes taken care of by marking the film by means of a small marker lamp which shines on the film outside of the sprocket holes. Since the sound gate of the projector is 19 frames from the picture in the light aperture it is necessary to displace the sound track by 19 frames when they are printed together. The advantage of sound-on-film is that the sound will always be in synchronism with the picture if the projector is properly threaded. The breaking of the film does not interfere with the synchronous action, so that a blank patch is not necessary.

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