Journal of the Society of Motion Picture Engineers (1930-1949)

Oct., 1935] THE HIGH-FIDELITY REPRODUCER 297 As the drum rotates, the film is moved past the reproducing point, maintaining its contact with the drum over some considerable part of the circumference beyond this point by virtue of the film tension established by the main pulling sprocket. Since the film tension is a matter of only an ounce or so, the film leaves the drum in a relatively free loop, and engages the pulling sprocket in such a manner that the directions of the film upon leaving the drum and upon entering the pulling sprocket are nearly at right angles to one another. This feature has an important bearing upon the filtering action for the film motion while the film is in contact with the drum. The best method would have been to use a {/-shaped film loop between these points, but as that would have presented design difficulties, the pres FIG. 3. Rotary stabilizer, film drum, and pressure roller system for sound attachment. ent method was chosen. The action may readily be visualized by holding a piece of film in the hand so that the described directions are obtained, and moving one end in a direction perpendicular to that of the other end. The motion imparted to the other end is considerably reduced. It is the existence of the film loops together with the relatively high mechanical impedance acting upon the drum shaft that so definitely eliminates sprocket -hole modulation in this machine. A second sprocket follows the pulling sprocket and is used to aid in isolating the reactions from the take-up magazine, which, because of their possible severity, might otherwise be transmitted through the film loops to the drum. The function of the rotary stabilizer is to provide an inherently damped positive reactance to cooperate with the negative reactance of the film loops in forming a mechanical filter. This device consists