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THE ADDITION OF SOUND TO FILMS
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of the diagram and is seen to be of the simplest form. It will be realised that the light spot is rather longer than half the slit width so that only one side of the spot travels across the aperture. This early form of Variable Area recording suffered from difficulties encountered in manufacturing the necessary thin slit and keeping it free from dust particles. To give some idea of this problem it should be remembered that 9,000 cycles, each accurately defined, were required to be photographed within a distance of 12-inches (30* 5-cms); assuming the film speed to be at the rate of 16 pictures per second, that is, at the old silent film speed. This, meant that one complete cycle occupied a distance of 0* 0013-inch (0*033-mm). If such a frequency was to be accurately recorded the slit width would need to be even smaller than this.
The first step made to improve this condition was to adopt a film speed of 24 pictures per second. However, it should not be concluded that the film speed was necessarily raised only to assist the Variable Area system, since those people working on the development of Variable Density tracks were also experiencing similar difficulties. At the new speed one complete cycle, at a frequency of 9,000 per second, now occupied a distance of 0 002inch (0'051-mm). Although this was considerably larger than that previously obtained, it still suffered from mechanical disadvantages, and an improved system was introduced in which the moving light spot was projected onto a large slit, thus producing an enlarged image of the trace over a correspondingly large aperture. This projected image was then optically reduced by a microscope objective lens system to form an image of the required dimensions upon the film.
By 1932 the MirrorGalvanometer system was again changed, this time to increase the sensitivity of the system. The new arrange
•axis of dotation iof mirsor
Fig. 93. — Principle of Variable Area film recording.