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NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
make the pulling sprocket directly below the gate revolve at a uniform velocity by the use of elaborate mechanical filters between the driving motor and the sprocket. In the best of these systems some ripple was present in the sound reproduction from the film, due to uneven motion of the film resulting from imperfect mechanical parts. A serious flutter or rapid variation in the motion resulted from the use of a sprocket to pull the film through the gate by means of the perforations in
the film.
The rotary stabilizer type of sound head was designed to eliminate all the objections of the gate type. This was accomplished by the use of a rotating drum to
FIG. 1. Rotary stabilizer type of sound head.
which is attached the rotary stabilizer elements. Special care was taken to see that the film as it passes through the sound head is bent into as wide a curve as possible, insuring that it would lie flat and all points of the sound-track would be in focus. The free-running sound drum shaft is mounted in ball bearings so that the friction is reduced to such a small amount that it is possible for the film to drive this drum without appreciable tension. This tension is so light that the film is never pulled taut except at the start. The film being in contact with the drum, rotates with it. This prevents any possibility of film scratching. The film assumes a curved path after leaving the sound drum. The stiffness of the film serves as a compliance which, in conjunction with the mass of the rotating