International projectionist (Jan 1959-Dec 1960)

vide a good output with a minimum of film-grain noise. The most urgent problem confronting the designer of a soundhead is the attainment of absolutely smooth and flutter-free transport of the film past the scanning point. The first soundheads depended upon flywheel-damped "sound sprockets" which pulled the film through sound gates having pressure pads much like those in the gate of a projector mechanism. But direct film drive by means of a sprocket (even though a heavy flywheel is attached to the sprocket shaft) has five serious disadvantages : (1) "Gear flutter" is imparted to the sprocket by the driving gears and the projector motor unless the flywheel be inordinately large and heavy. (2) Slower oscillations, resulting in "wow-wows", are created by the flywheel, itself, the exact frequency of the flutter depending upon the mass (weight) of the flywheel. (3) Very slight departures of the sound sprocket from perfect concentricity with the axis of the shaft to which it is fastened produce "gargle," a more rapid flutter than the wows. (4) The minute "yanks" imparted to the film by the engagement and disengagement of the sprocket teeth (especially if these be worn) create the high-frequency type of flutter known as "whiskers." Whiskers do not affect the lower tones, but make all high notes (violins, soprano voices, etc.) unpleasantly raspy. (5) The length of film between the sound sprocket and the point of scanning allows varying amounts of film shrinkage to produce irregular flutter. None of these disadvantages mar the performance of the more modern type of soundhead employing a specially-damped "sound-takeoff drum" which is driven, not by the gear system of the projector but by the film itself. The "sound sprocket" of this type of soundhead serves only to keep the film moving without regard to its motion past the scanning beam. Because of this, the sound drum and sprocket are rather far apart. The length of film between the drum and the sprocket is never taut, once the sound-drum has been brought up to speed, but forms a springy loop which effectively isolates the film on the drum from possible flutter of the film on the sprocket. Sound-Drum Tension The film is held in place on the sound-drum by a pressure-roller assembly which serves secondarily as a lateral-guiding device to keep the soundtrack centered in the scanning beam. The guide-roller assembly must be kept clean and properly adjusted at all times, because too little tension will delay the acceleration of the sounddrum, causing fluttery sound for several seconds after the changeover; too much tension will tighten the springy film loop and nullify the damping arrangement which controls the rotation of the drum. The sound-drum and shaft are made from one-piece, chrome-nickel steel forging, and are heat-treated to assume the needed rigidity. The sound-drum can never become lopsided in relation to its shaft. To permit the scanning beam to pass through the soundtrack in that portion of the film which contacts the drum, the width of the drum is less than that of the film. The soundtrack and outer perforation margin therefore overhang the edge of the drum. The Rotary Stabilizer Now, the remarkable feature of this type of soundhead is the ingenious flywheel, called a rotary stabilizer, which is attached to the opposite end of the "Spherical aberration" is one of the defects of simple lenses having spherical surfaces. This produces unsharp images because the focal length of the outer zones of such a lens is shorter than the focal length of the centra! zones. Focus for marginal rays Simple spherical lens F°cu= for r I central ray rays 'Aspheric" lens Compound lens having spherical elements sound-drum shaft. An ordinary solid flywheel would "hunt" (or oscillate) with the springy film loop in much the same way that a weight suspended from a coil-spring oscillates under the slightest disturbance. The rotary-stabilizer type of flywheel behaves differently. It absorbs all disturbing forces and dissipates their energy without affecting the smooth rotation of the sound-drum. Even the passage of a splice over the drum or the sprocket fails to cause the slightest flutter in the motion of the film. Rotary Stabilizer Function A rotary-stabilizer consists of a lightweight metal case containing a freefloating, heavy flywheel which does not contact the circular can-like case except through a film of the viscous oil with which the assembly is filled. The sound-drum shaft is attached to the hollow case; the heavy flywheel is attached to nothing but is pivoted so that it is free to revolve as impelled by the revolving outer case through the oil-film. Practically all of the high-quality soundheads manufactured at present utilize the rotary stabilizer. The directsprocket drive of the film through a stationary film gate has been completely abandoned, although many such soundheads, relics of early "talkie" days and "universal bases," still are giving service — better service, even, than might be predicted from a consideration of basic principles alone. Most Dirt From Film Satisfactory soundhead operation demands cleanliness of all parts, and particularly of the exciter lamp, photocell, photocell lens, optical-tube lenses, and sound drum (or sound gate in old-style soundheads). Much of the trouble-causing dirt that collects in soundheads comes from the film, which is notoriously prone to pick up dust and dirt by electrostatic attraction. A wad of film dirt adhering to the sound-drum will cause sound flutter and may project far enough over the edge of the drum to interrupt the scanning beam at each revolution. The result is a periodic thumping noise from the speakers. Although this is more likely to happen in 16-mm than in 35-mm machines, the projectionist should nevertheless lightly wipe the sound-drum with a clean, soft cotton cloth after each reel. Stubborn deposits of emulsion may be removed from the polished surface INTERNATIONAL PROJECTIONIST • OCTOBER 1959