International projectionist (Jan-Dec 1954)

Some Questions Answered About — Magnetic Sound Reproduction WILLIAM BORBERG Chief Mechanical Design Engineer A. A. LEONARD Applications Engineering Supervisor General Precision Laboratory, Inc., Pleasantville, N. Y. I ESTS SHOW that the present four-track head will improve in response, and therefore in performance, during the first 100 hours of use. The performance will be nearly uniform between 100 and 700 hours. Three and three quarter million feet of film, or more, or over 700 hours of life, can be expected. Head wear will then be observed to affect one of the four tracks, most likely track No. 2, and this sound channel will deteriorate in a few hours completely. Thus, a worn pickup head does not cause a sudden breakdown of the show, and since warning is given, replacement can be made in time. This circumstance has been experienced both in our laboratory and in theatres under actual operating conditions. It is true that improper head adjustment can result in a drastic reduction in head life. However, as maintenance personnel have become more familiar with the criteria for proper adjustment, this factor has lost the importance it had when magnetic sound was first introduced in the theatre. Filings, Emery and Abrasives The sound tracks in a magnetic system consist of carefully compounded, purified, and oriented oxides of iron, supported and cemented to the film base by special adhesives. Probably the compound most nearly like that of the soundtrack is known to industry in general as "rouge." This is a material used to obtain the excellent surfaces and high polish we expect in our projection lenses. It certainly does not scratch. Emery is a fine-grained impure variety of corrundum mixed with other minerals, chiefly magnetite."^ No aluminum oxide or corrundum will be found in the coundtrack. Further, inspection of a head after use will show that the pole shoes are highly polished, usually with a better polish than the original one. Dust, always present, contributes greatly to wear. The film moving at 1% feet per second may also become charged and may atract dust particles. Certainly dust causes a great deal of wear. E. W. Franck, of Reeves Soundcraft Corp., makes the following statement :(Journal of the S.M.P.T.E., April 1953, p. 506) "I think it would clarify the thinking somewhat if we think of head wear as not being caused by the coating but by the dirt, the abrasive dust which is picked up. The film base iself does considerable wearing for this reason — it picks up a lot of dust and dirt. Some types of coatings will pick up more dust or dirt than others and hance wear the head more; but it is the dirt, more than the film base and more than the coatings, which does the damage." Sprocl<et Characteristics, Performance Four-track magnetic sound sprockets do have narrow teeth. This is not their only difference from the previous standard. For a long time it was felt necessary to maintain a tooth spacing to accommodate film shrinkage over a wide range. Modern film base does no shrink as much as the older types, therefore the diameter of the new sprockets has been increasd slightly to permit the pitch or spacing of the teeth to be increased. This results in the teeth entering and leaving the film perforations with less rubbing or "picking." Tests have been run, and proof is available, that film with standard perforations run on new narrow-tooth sprockets will outlast standard film on old sprockets. Of course, the old wide-tooth sprockets could be increased in diameter, too, but the change to the narrow tooth and greater diameter gives essentially the same increase in film life. We refer to the statement by Dr. E. K. Carver, Eastman-Kodak:'^ "In the very beginning, I believe that 20th-Fox realized that any new sprockets they made would have to run at least as well on film with standard perforations as the old sprockets. It turns out that if you do not increase the diameter of the sprockets, you get slightly worse results; but if you use an intermittent sprocket with a diameter of 0.953 inches, you get at least five times as good results as far as wear and tear on the film goes as you would with a 0.935 sprocket. You also get better results than if you use a 0.943 sprocket, which incidentally, is an ASA standard. The 0.953 sprocket more than compensates for any decrease in wear you might get with the narrower teeth." Magnetic Fields Damage Soundtracks A projector containing magnetized parts will certainly introduce some peculiar and most annoying sounds which may become a permanent part of the soundtrack. It is not difficult to de-magnetize a projector, and once demagnetized it will stay de-magnetized unless someone places a magnet in contact with it. The de-magnetizing technique was not thoroughly understood when magnetic I. A. CONVENTION EDITION July 1954 27