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SPROCKET
ROLLER
Fig. 1. Recorder having single magnetic head in drum.
for the sound drum as described by Collins.1
As may be observed from Fig. 1 since the coated surface of the film was toward the inside of the film loop between sprocket and drum, space limitations permitted the mounting of only the one magnetic head shown at the drum. In this particular adaptation the one head was used for recording the sound track and later for reproducing with no facilities available for monitoring the recorded track at the time of recording.
When used for photographic recording prior to the conversion, this equipment at least equalled any other commercially available equipment in providing flutterfree film motion. The quality of film motion, when used for magnetic recording, was for practical purposes the equivalent of that when used as a photographic recorder. However, experience with the equipment as a magnetic recorder indicated the need of a more desirable location for the magnetic head since head wear tended
ROLLER
Fig. 2. Recorder having two retractable magnetic heads external from drum.
to be uneven due to difference in pressure between film and head across the width of the head. The partial view in Fig. 1 shows in exaggerated form the position the film tends to assume with respect to the head and drum. Necessarily, the head at the magnetic-gap section must protrude slightly beyond the film supporting surface of the drum to provide the desired contact pressure between film and head. Unit area pressure tends to be relatively high at the edge of the head adjacent to the drum and to diminish as the opposite edge is approached. This tendency may be reduced to some extent initially by a slight rotation of the head.
A second illustration of a basically photographic type of recorder modified for use of magnetic film is shown schematically in Fig. 2. The filmpulled drum type, tight-loop filter system with damping applied by means of a dashpot connected to one sprung roller arm was retained since it too provided excellent film motion. In this instance (as well as in the remainder of the systems to be described) the film threading was such that the coated surface faced to the outside of the film loop between sprocket and drum. With only a slight change in the film path,
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April 1952 Journal of the SMPTE VoL 58