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242 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
location will be encountered if the edge-guide roller bearings are not kept scrupulously clean and properly lubricated.
It is apparent that with 16-mm machines such an edge-guiding arrangement is unsuitable, since the film must be guided regardless of the direction of travel through the machine. Because of this the edge guides on the Maurer machine are located symmetrically on either side of the translation point on the recording drum. To reduce the motion-disturbing effects of edge friction and for the sake of simplicity, Maurer has chosen to use a " channel-type " guide in preference to an "edgepressure" guide. In general, a channel-type guide is satisfactory when the width of the guide is but slightly larger than the widest film to be encountered, and when the length of the guide channel is large compared with the film width. Although, theoretically, the film may "wobble about" between the two guiding limits, the amount of friction introduced in practice is sufficient to damp out most oscillatory side motion. The "drift" in track location during the "settling-down" interval when the machine is coming up to speed is quite small in the Maurer design compared with 35-mm RCA design. Deterioration in film motion resulting from guiding at the translation point is so small that it is difficult to measure ; the penalty paid for this form of edge guiding is a slight difference (about 0.001 in.) in track location between propulsion in one direction as compared with propulsion in the other direction. It is possible that the track-location difference could be reduced if all film manufactured were slit more accurately for width ; inaccurate slitting is more likely to occur on certain non-American-made film than upon film made by the major American film manufacturers. The dimensional accuracy and dimensional stability of such films as EK 5372 are to be admired by sound-recording machine designers.
The Light Modulator
The light modulator of a sound recording optical system is a device that transfers power from an electrical system into movement or into intensity change of a light beam of a character than can produce a sound film record when these light variations are transmitted through the optical system to the film. There are several common types of light modulators in use.
(1) A string oscillograph type of galvanometer — such as the Einthoven or the Duddell — in which a current passing through a conductor loop upon which a small mirror is cemented causes the mirror to twist due to the reaction of current upon the