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

nterstate down in Texas, when they led off mmediately following the opening of The louse of Wax here in New York. After a onsiderable period of time had elapsed, /e had the same RCA service engineer, /ho had made his measurements on orignal installation, return to the theater with he same equipment and make another set f tests jointly with Interstate engineers, tfter what we estimate to be approxiaately 6,000,000 ft of film travel, we found hat lowering of the output level was, from . practical viewpoint, negligible; and the /ear was not visibly apparent. That's the mly specific case that we know of where /e feel the results were accurate enough by irtue of having the same personnel, the ame test equipment, the same units and m identical check made. Frankly, the esults have surprised us. We didn't expect hat we would be able to run 6,000,000 ft •f film without having more difficulty than ve have experienced in that particular heater. Dr. Frayne: How much of a factor might he tension on the films be, in this case? Wallace V. Wolfe (RCA Victor Div., Hollyuood, Calif.}: It is significant in that connecion that this is a soft-loop type of machine, fhe pressure on the head would probably )e somewhat lower than would be the case n the tight-loop machine. But exactly vhat it measures I don't know. Dr. Frayne: Is that a full-coated film or he CinemaScope film we're talking about? Mr. Wolfe: That's a full-coated film. Mr. Heacock: This is a typical double-film ystem with separate sound coating. One >ther comment: we made a number of nagnetic reproducer heads to have them ivailable for field maintenance of our mtton-on soundheads. I don't believe ve've had to use any of these reserves. Mr. Wolfe: As to wear of heads in studio equipment, we find these things happening. Fhe inductance of the head does go down is it wears; this is an obvious fact, and lignificant for these reasons. The frequency characteristic is a function of the Dias current ; and the bias current, in turn, s a function of the inductance of the head. Consequently, as the head wears, the frequency characteristic changes. The frequency characteristic can be brought oack by appropriately changing the bias current. [See: Kurt Singer and Michael Rettinger, "Correction of frequency response variations caused by magnetic head wear," Jour. SMPTE, 61: 1-7, July 1953.] Skipivith W. Athey (General Precision Laboratory): The basic problem that Mr. Heacock described, and which, I believe, is the major concern as far as CinemaScope and double-film systems are concerned, is how much wear the theater gets out of the film on playback rather than the recording situation. I don't think many of us can get too concerned about the cost of the manufacture and operation of the recording equipment, because so few of us can see those dollars slipping away; but if the theater owner has to replace heads at a great rate, then that is of great concern. I have not had enough experience with wear on CinemaScope film to say what the effects of wear are. Am I correct that we would normally expect an increase in output with wear because of the reduction of the inductance of the head? This is for playback only. And am I not correct that unless resonance in the amplifier is changed in a major way we should expect an increase rather than a decrease in highfrequency response? W. W. Wetzel (Minnesota Mining and Mfg. Co.): It's my opinion that the only two things which affect frequency response are bias effect on record and the gap effect on playback. Now all heads are constructed with a certain 20 to 25 mils of magnetic material which can wear away before the gap starts to widen and I don't believe we expect a change in frequency response resulting from that wear until it begins to get down into the V-shaped lower portion of the head. Then the gap widens. You do lose frequency response quite rapidly. Mr. Athey: May we expect some lowfrequency effects as wear changes the effective gap? Dr. Wetzel: I think that they wouldn't be adverse, since you're increasing the length of gap. What you'd be afraid of doing is getting those funny little humps. Mr. Athey: Or moving them too violently. Dr. Wetzel: That's right. Mr. Athey: Because they're certain to be there anyway. Dr. Wetzel: That's right. Mr. Athey: Some experience can be reported for the General Precision Laboratory Magnetic Head Wear Discussion 239