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THE PHOTOGRAPHIC TREATMENT OF VARIABLE AREA SOUND FILMS
J. A. MAURER*
One of the most important of the problems which arise in the photographic recording of sound is that of defining for the film laboratory workman the procedure by which he will obtain the most satisfactory results. This problem as it presents itself in the case of the variable density record has been the subject of several papers which have been read before this Society. The purpose of the present paper is to present an analysis for the case of the variable area record.
This type of record is much less critical in its requirements than the variable density type because there is no need to consider linearity in the photographic tone scale. Since the purpose of the film is merely to reproduce the profile, or outline, of the sound waves, the record is inherently free from non-linear distortion, regardless of what contrast is reached in the development of negative and positive or of what their transmissions may turn out to be.
But this does not mean that the variable area record has no photographic problems worth studying, for there are two other important properties of the record which may be affected by improper photographic procedure. These are the response at high frequencies and the range of volume which can be reproduced.
Fortunately the requirements of the two turn out to be not very different, though they have to do with entirely different properties of the photographic materials.
PHOTOGRAPHIC CONSIDERATIONS AND VOLUME RANGE
When one first looks at a number of variable area sound tracks it is difficult to resist the impression that the best record is the one that looks like A in Fig. 1, that is, a track the exposed half of which is jet black. But, as will soon appear, the sort of track shown at B will give practically as high a level in reproduction as the one at A , while even as weak-appearing a one as C may be acceptable if handled
* RCA Photophone, Inc., New York. 636