International projectionist (Jan-Dec 1956)

improper adjustment of the cluster unit, the impedance rollers, or the pressure rollers. Magnetic-sound test films having 12,000-cycle tracks are used for checking the high-frequency response of the sound magnets. While magnetic reproducers in perfect condition are capable of reproducing somewhat higher frequencies than 12,000 cycles (as are optical soundheads, also), satisfactory sound reproduction does not require a frequency response exceeding 8,000—10,000 cycles. Nevertheless, a lack of response from 12,000-cycle test tracks indicates a need for immediate replacement of the magnetic pickup cluster. It is absolutely necessary for the fourth, or auditorium sound-effects, track to reproduce 12,000 cycles when auditorium speakers are employed. A signal of this frequency is used as a "control" for the automatic electronic switch that turns the fourth-channel amplifier output on and off. This channel is admittedly an unsatisfactory adjunct to motion-picture sound, and is preferably disconnected from the penthouse reproducers and used, with discontinuance of the electronic switch, for playing phonograph records before the show and during intermissions. Upkeep of Magnetic Heads Even though magnetic soundheads are simple in principle, their maintenance is more costly than the upkeep of optical soundheads. Optical sound reproducers require frequent focus tests (which cost nothing) and periodic replacement of exciter lamps (at the cost of pennies), whereas magnetic sound reproducers require replacement of the magnetic pickup cluster unit after every 3,000,000 feet of film, on the average — once a year in the case of 10 hours of CinemaScope projection per week, per projector. Cluster units cost more than one hundred dollars apiece. The output and frequency-response characteristics of a new pickup cluster improve slowly during the useful life of the unit, afterward deteriorating very suddenly to a point where output is only a small fraction of normal, and all high-frequency response is lost. Sound distortion then becomes intolerable. Several manufacturers of CinemaScope magnetic clusters claim a useful life of about 6,000,000 feet of film; but even so, replacement of these units is inevitable. The photoelectric cells of optical soundheads are considerably cheaper and last much longer. Development of a truly permanent cluster unit is impossible. Even hardplated pole pieces are eventually worn down by the abrasive action of magnetic tracks and the pressure of film running under great tension. It is to be noted that the sound-reproducing performance of sound magnets is impaired by hard platings and the use of hard magnetic alloys in place of soft iron. Damage from Sprockets The use of narrow-tooth sprockets for "Foxhole" prints can be a source of film damage. Still another source of film damage is the increased complexity of the film path, with a number of extra idlers and pressure rollers as well as heavy impedance film drums in the penthouse soundhead. Many soundheads of this type also have a toothed sprocket which is turned by the film, itself, and is thus indirectly driven by the upper feed sprocket of the projector mechanism. When normal upperreel tension is employed to prevent "coasting" of the feed reel, this arrangement is rough on the film perforations. The impedance and tension rollers, in particular, have a tendency to inflict a large number of fine scratches in the emulsion of the film. "Cupping" of the film at the pole pieces of the magnetic pickup will scratch the celluloid side of the film in the picture area. Film tension at this point is greater than at any other point in the entire film path, not excluding the projector gate. No wonder, then, that prints played through magnetic soundheads are sometimes marred by "showers of ,!" Bypass Optical Soundhead It is suggested that magnetic-sound film be threaded to bypass the scanning drum and sound sprocket of the optical soundhead whenever the layout of the soundhead permits direct passage of the film from the lower sprocket of the projector mechanism to the lower takeup sprocket of the optical soundhead. (A shortened film path is usually impossible when old gate-type soundheads are used.) Similarly, optical-track prints should bypass the impedance rollers and magnetic pickup unit of the penthouse soundhead to avoid possible damage. The bypassed film course should be carefully checked, however, since great damage will ensue if the film rubs against any part of the fire-valve box, the penthouse, or the picture-mechanism frame. Double-check the film path while the machine is in operation — the next projectionist to receive the print may dislike scratched film! Contrary to popular opinion, the frequency range offered by CinemaScope magnetic tracks is no greater than that of well-made optical tracks. The practicable top frequency in either process is 1.5,000 cycles, with 12,000 as an easily obtainable norm and 10,000 as the highest frequency to be reproduced by the theatre speaker system. In fact, a top cut between 8,000 and 9,000 cycles by the amplifier filter network is often considered desirable as a noise-reducing expedient. Also contrary to popular opinion, (Continued on page 33) "The film was warped. Chief." Ideal Kincma, London 12 INTERNATIONAL PROJECTIONIST • SEPTEMBER 1956