Journal of the Society of Motion Picture and Television Engineers (1950-1954)

agreed upon so as to accommodate the various types of projectors in service. Some of the advantages of the topmounted soundhead may be listed briefly here: (1) The photographic soundhead may be left unmodified. This carries the advantages that: (a) The photographic sound performance need not be affected. (b) A magnetic-photographic soundhead must have compromise performance, because the magnetic sound stabilizer must operate in the presence of head friction. (c) A photographic soundhead cannot have many parts added to it without becoming excessively overcrowded. (d) Any system which converts existing photographic soundheads for magnetic sound must provide for the conversion of an impractically large number of soundheads of all types. (2) The magnetic-hum-field-sensitive sound mechanism can be moved far from the strong hum fields of motors and selsyns. (3) The threading of -the soundhead is done at convenient eye level. (4) The film at the new magnetic scanning point has not been subject to the momentary frame deformation or buckling caused by the absorption of heat energy while in the picture gate. The Simplex Magnetic soundhead adds only 4 in. to the overall projector height and does not affect the normal operation of the projector. Films with photographic sound tracks are threaded to bypass the magnetic pickup system and films with magnetic sound tracks may be threaded to bypass the photographic sound scanning system. The magnetic soundhead is entirely filmdriven and uses a tight-loop stabilizing system to provide extremely uniform film motion. One function of the single sprocket is to maintain the length (and thereby the tension) in the tight loop. The filmpath is shown in Fig. 2, passing over two tension rollers and two stabilizing drums on whose shafts are mounted flywheels. To insure proper side guiding of the film, the upper stabilizer drum (directly ahead of the magnetic pickup head) is flanged. A tension spring beyond the pivots of the tension roller arms maintains the film tension, and a centering spring attached to one of the arms maintains the rollers centered. A rotary viscous damper employing a sector of a cylinder rotating in a silicone fluid inside a cylindrical drum provides damping for the filter system. The damping is applied directly to one of the filter roller arms in order to avoid the backlash inherent in any mechanical linkage. The natural period of the filter is about ^ of a cycle, and excellent flutter performance results. Because the single sprocket acts passively, the filter system is well isolated from external disturbances. Such disturbances are "passed on" to the pulling sprocket by the soundhead sprocket, and the filter system exists in a sort of backwater or eddy off to one side of the main filmpath. All parts of the soundhead which contact the magnetic coating on the film (except the magnetic pickup head) are of nonmagnetic material, including the stabilizer drums and shafts, which are of stainless steel. The magnetic pickup head is mounted on a bracket which is held on a slide by a spring and a lock screw. This slide permits lateral adjustment of the head relative to the film, with a stop screw and lock nut for holding the correct location. Three set screws in the base of the magnetic head permit azimuth and tip adjustment of the head relative to the bracket, with the three head-mounting screws locking this adjustment. An early form of an annealed mumetal front shield, formed over a cold rolled steel form is shown in these 224 March 1954 Journal of the SMPTE Vol. 62