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1102 HANDBOOK OF PROJECTION FOR
It must, of course, be in precise register throughout its length with the line of light which originally illuminated the sound track in the camera.
The foregoing is not only true of Movietone, but also it is true of all others using the variable density method of recording.
WESTERN ELECTRIC PICK-UP. — The photo graphic imprint of sound upon the variable density sound band has already been explained to you, see page 975. It is in the form of horizontal (horizontal as the film is in the projector) lines of varying width, made up or composed of photographic shadings, all of which, taken as a whole, is the photographic representation of sound, in all its combinations and modulations. It is illustrated in magnified form, in Figs. 383 and 384.
The problem in projection is to pick up those lines and shadings in the form of light, at their exact true valuation as expressed lineally upon the sound band, and to translate these light values into electrical impulses without distortion, to amplify them and finally to send them forth into theatre auditoriums in the form of sound which will be undistorted, have correct volume and be in every way a faithful reproduction of the sounds originally "heard" by the microphone.
We will now proceed to examine the Western Electric pick-up. The optical system of the sound pick-up attachment attached to the motion picture projector consists of a straight coil filament light source, contained in a glass globe, known as the "exciting lamp," a small condenser, a "slit," a small objective lens, the sound track upon the motion picture film and the photo-electric cell, the arrangement of which elements with relation to each other is diagrammatically illustrated in Fig. 405.