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A STUDY OF GROUND NOISE IN THE REPRODUCTION OF SOUND BY PHOTOGRAPHIC METHODS*
Otto Sandvik
GROUND noise is a term ordinarily used to designate extraneous sounds which occur on a photographic sound record, or more explicitly, noises which are present on a film that either contains no silver deposit or else has a silver deposit of uniform optical density. The purpose of this paper is to point out some causes of these noises and to show how the noise level increases with repeated runs of the film through the projector.
Some of the earlier workers were inclined to believe that the noise was due to optical inhomogeneity in the film base itself. In order to investigate the validity of this contention a series of experiments was carried out. In these experiments, several kinds of film base besides the standard motion picture film were investigated. One of these experiments, and the only one which will be considered here^was to run the clean film through the sound reproducer in the usual manner. The gain or the volume control of the amplifier was advanced until a barely perceptible noise, caused by the moving film, was heard. This method is rather qualitative in nature; but has the advantages of directness and similarity to conditions which prevail in practice.
The results obtained by several observers showed that there was very little difference in the noise level of different kinds of film base; and moreover, that, with a gain setting as normally used for a sound record, the ground noise was imperceptible.
The amount of noise, however, was increased considerably by running the film through the projector a few times, and was found to be enhanced greatly by handling with the bare fingers, by impressing even minute scratches, or by blowing dust particles into the projector against the film while it was running through.
The next step was to determine how much, if any, the noise level was increased by the several steps involved in manufacturing the film, and the ultimate processes of developing, fixing, washing, and drying.
* Communicatioin No. 374 from the Kodak Research Laboratories.
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