Journal of the Society of Motion Picture Engineers (1930-1949)

Record Details:

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524 S. S. A. WATKINS AND C. H. FETTER [J. S. M. p. K. having the ribbons broken. A valve can be strung, spaced, and tuned ready for use in less than ten minutes and of course there are always sufficient spares available so that a defective one may be replaced in fifteen or twenty seconds. Film Technic. — For the recording of sound on film we use positive stock. Positive stock is slower than negative stock but inasmuch as our intensity is well within economical limits the use of positive stock has two distinct advantages; it is cheaper and it has a finer grain. In the recording machine the film receives exposures which are in linear relation with the sound by means of the operation of the light valve. In order to obtain satisfactory reproduction in a projection machine this film must be developed and printed in such a way that the transmission of light through the positive will have the same variations as the original exposures of the negative. The photo-electric cell in the reproducing machine will then reproduce electrically the same variations which were used in the film recording machine to operate the light valve. Ordinarily in photographic work, particularly for screen presentation, light values on the screen are quite different from those in the original picture. Due to limitations in the camera, the emulsion, and in the projection apparatus it is usually necessary for pleasing effects to obtain in screen pictures far greater contrasts than actually exist in the original object photographed. In the variable density system of sound film recording it is necessary to develop the negative and positive in such a way that the light transmitted through the positive contains the exact relations of the original light to which the negative has been exposed. Here the contrast must be such that a true picture of the recorded light is viewed by the photo-electric cell in the projection machine. Some means is necessary therefore by which the contrast in development of both negative and positive sound track may be measured. The gauge used is fundamentally a curve first derived by Hurter and Driffield, commonly called an H and D curve. This is a graph which shows for a particular emulsion and development the relation between various exposures and the respective densities. It is characterized by a toe, a linear portion, and a knee corresponding, respectively, to under, correct, and overexposures of the film. When this curve is plotted between density and the logarithm of exposure its slope is our familiar and sometimes much criticized friend gamma. For a given emulsion the slope of this curve will