Amateur talking pictures and recording (1933)

SOUND-ON-FILM EQUIPMENT 143 Normal Take-up Sprocket lamp which emits a bright light which is collected by a lens assembly which throws an extremely fine beam of light on to the sound gate, through the sound track to the photo-electric cell, which is coupled to an amplifier and loud speaker. Perhaps it is not yet quite clear how the sound gate functions. Let us, therefore, examine (C) which shows in a schematic manner the sound gate looking in the direction of the X in diagram (B). For our purpose we can consider the face of the sound gate as consisting of an opaque material in which is cut a small slit which corresponds in position with the sound track on the film. It is on to this slit that the fine beam from the lens assembly is projected, and passes through it to the sound track and thus to the photoelectric cell. The actual width of the beam of light falling through the slit is in the region of one thousandth of an inch or less, and is actually made as fine as possible consistent with brilliancy of illumination. A little consideration will show that the width of this light beam corresponds to the point of a gramophone needle in the disc method of reproduction. The highest frequencies which can be reproduced from a gramophone record depend to a large extent upon the smallness of the point of the needle, simply because if the needle has a blunt point it cannot follow round some of the fine curves of the sound wave. So it is with sound-on-film, and obviously the light beam cannot pick out frequencies having peaks smaller than its own width. There are naturally a good many mechanical variations of the details shown at (B) and (C) in Fig. 69. Thus, for instance, the positions of the photoelectric cell and the exciter lamp might easily be reversed. Slit 'Movietone ' ~ Sprocket Fig. 69c Position of Sound Gate slit