International projectionist (Jan-Dec 1948)

New Century Sound Systems Feature Fundamental Reproducer Advances DEVELOPMENT of two new sound amplification systems has recently been completed by Century Projector Corp., and these, together with the Century Master and Standard reproducers give a selection of four distinctive and different sound systems, anyone of which may be obtained in power ratings from 15 to 500 watts. These reproducers incorporate fundamental improvements in film drives which result in better film motion, reduced flutter problems, simplified film threading and equipment adjustments. The improved film drive filter mechanism is a new and novel development by C. C. Davis of the Electrical Research Products Division of Western Electric Co. It was adopted by Century for the foundation of these new reproducers. The development received recognition from the Academy of Motion Picture Arts and Sciences in an award (Class II, Plaque) for the year 1947 as follows: Academy Award Citation "This mechanism is a fundamental improvement in film drive, resulting in better film motion in any type of studio sound recording and studio or theatre sound reproducing equipment. It has reduced flutter problems, simplified film threading and equipment adjustments, and requires no critical manufacturing tolerances. "As this device is suitable to theatre as well as studio equipment, its application has a definite influence on the industry as it results in improved quality in the theatre." The device for which the award was granted differs considerably in many respects from earlier mechanisms. The flywheel is driven by deliberately tensioned film with oil damping applied to the tensioning device. The advantages of this arrangement include provision of optimum bearing conditions of rotational parts, while applying damping to a non FIG. 1. Single-arm filter. SOFT ROLLER R2 FLYWHEEL AND MACNETIC DRAG / FILM COMPLIANCE SOUND O IsPROCKET INTERNAL FILTER CIRCUIT rotating element. In addition, the extent of damping is little affected by varying quahty or buckled condition of the film, as firm contact is established between the film and the drum. In operation, the film is deflected from a straight path between the drum and sound sprocket by a spring-tensioned idler arm and roller with which the damper is associated. Any tendency to flutter which may be induced in the film by the driving sprocket is passed to the compliant, oil-damped arm, rather than appearing as irregular motion of the flywheel. Four Good Design Fundamentals This combination fulfills four desirable requirements for good design: {a) Optimum bearing conditions of rotational parts. (b) Application of damping to a nonrotating element. (c) A value of compliance largely independent of the varying quality or bent condition of the film. (d) Firm contact between film and scanner drum. The compliance element or pliability introduced between the driving sprocket and drum has in general been provided by the nature of the film itself in the form of S-shaped loops. This is the type of compliance used in many machines because of its simplicity and is usually referred to as a kinetic scanner or rotary stabilizer. However, it offers no facilities for the introduction of complete damping and control of film motion at the point of scanning. The tensional path of the film in the new filter, together with the idler arm and roller, constitutes a compliance as in the case of the S-shaped loop, although the two appear considerably different. Since substantially equal amounts of tension in some form must exist at either side of the translation point, tension must be introduced to offset that of the lower film loop. Two methods have been developed to accomplish this. Fig. 1 shows the single-arm filter used in the Century Master reproducers. Fig. 2 shows the double-arm filter used in the Century Standard reproducers. The single-arm type shown in Fig. 1 uses a magnetic drag producing a completely free film loop above the scanner. The double arm type shown in Fig. 2 differs from the single-arm type in that the magnetic drag and pressure pad roller are replaced by an idler arm and plain roller similar to that used in the lower film loop. This makes possible a single unit containing the arms and damper which perform all the functions necessary to drive the drum and flywheel by straight forward belt action. The loops of a film are tensioned by a differential action, through a single linking or spring. The only reactance opposing flywheel motion is that resulting indirectly from changes in the working angle through which the spring tension is apphed to the loops. The cutoff frequency and the natural oscillatory period of the filter circuit, therefore, are functions of the geometry of the arms and loops and the loading LIGHT BEAM, COMMON SPRING CI . LOWER SPROCKET O J si -INTERNAL. FILTER CIRCUIT FIG. 2. Double-arm filter. tension of the spring and are not directly a function of the spring constant. This allows for a compact and practical design of the reproducer. Flutter Suppression Action The transient response of the singlearm and the double-arm filters is that of a series resonant circuit shown below each schematic in Figs. 1 and 2. They may be computed on the basis of the similar electrical circuits. While increased damping decreases transient response, optimum flutter performance results from an amount of damping determined by recurring disturbances. Disturbances caused by the transient nature of film splices are minimized by the relatively well-damped nature of the circuit and by the fact that there is little change in compliance in the presence of large disturbances. It is interesting to note that the lower sprocket of the Standard reproducer has practically no load on it, as the film tension of the filter is almost exactly equal and opposite to the tension of the film from the takeup of the lower magazine. In operation, wide divergence from optimum adjustments has little effect on sound reproducing results, and interference from sprocket holes, bent film, gear teeth, etc., has been virtually eliminated. INTERNATIONAL PROJECTIONIST July 1948