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DRUM
ROLLER
SPROCKET
ROLLER
OIL DASH POT
DRUM
Fig. 4. Twin-drum recorder with drums in vertical plane, one film sprocket.
drive mechanism described in the last illustration given. This addition, made to provide film motion at the monitor head as nearly equal to that at the record head as possible, was a second impedance drum. The geometry of the film path, considering for the moment just the film sprocket and the two impedance drums, is basically an equilateral triangle in shape with the sprocket at the apex.
A sprung roller added to each of the two equal sides of the basic triangle serves two purposes. Each serves to alter the film path in such a manner as to increase the film wrap about its adjacent drum to the degree desired for film-pulled drum operation. The two sprung roller arm assemblies with associated tie spring are also essential elements in the twin-drum film-drive filter system. The relatively light sprung roller arms tend to absorb any disturbances introduced into the film motion through the film-drive mechanism. This results from the fact that
the film is held in tight contact with the rotating drum surface and the much greater inertia effects of the rotating flywheel mass of the impedance drums make these elements relatively insensitive to such film motion disturbances. Damping of any tendencies of the sprung roller arms and the drums to oscillate is provided by the oil-type dashpot which is linked mechanically to one of the sprung arms. The oil used in the dashpot is a selected grade of temperature-stable silicone.
Film tensioning is furnished through the force exerted by the tie or center spring connected to the two sprung or tensioning arms when the film is threaded properly in the film path shown. Ground springs shown in the illustration are used principally for mechanical purposes and have little effect as functional members of the filter system. Without the ground springs, the sprung roller arms tend to rotate to one or the other extremity of their arc of travel, depending on direction of film motion.
326
April 1952 Journal of the SMPTE Vol. 58