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

rate so that at the end of 900,000 ft it averaged 11 mils, but with a spread of about 5 mils between individual heads. The BK-1544 Head has a depth of about 20 mils at the gap, thus leaving an average thickness of 9 mils of mu-metal remaining at the gap. The 8000-cycle response dropped by an average of about 3 db from the initial value at the end of 300,000 ft, then rose to equal the starting point at 600,000 ft and then dropped about 1£ db below the starting value at the end of 900,000 ft. [Film to head contact wear has not appeared to present a problem up to the present time. The inductance of the heads appears to diminish from the original value. It would seem that the head under test would continue to give reasonably satisfactory service for at least another million feet.] Dr. Wetzel: I might add some rough calculations that are becoming apparent during this discussion. Let's take the 3,000,000 to 5,000,000 ft, which is approximately correct for the wide-head wear. Kurt Singer, Dr. Frayne, quite a number of people in the industry, appear to agree with that value. That means somewhere between 500 and 1000 hr. of actual machine operation. Let's assume that you have two projectors operating, two heads to wear. You're switching between these and you're operating over a period of 12 hr. That means 6 hr per day of actual head wear. Seven days a week means 42 hr per week and in 10 weeks it seems to me you might begin to expect a little trouble. Certainly I would expect it in 20 weeks. Dr. Frayne: Along the same lines, I hope these optimistic predictions about head wear will not deter Minnesota Mining or other companies from their program of developing ferrite heads. It is well to remember that in standard photographic sound tracks the optical system was good for an indefinite operating life. Once in a while a lamp will burn out but a lamp replacement is relatively cheap. Compared to this, a 4-track magnetic head is going to cost — well, anywhere from $75 to $125. This presents a very serious economic problem to the theater owner and I am sure we all hope that somebody will come up with a successful ferrite head that will have an extremely long life. Mr. Seeley: I wholeheartedly agree; but, in the meantime, we have to continue using the heads we have and are currently receiving in the field. We in the service companies are very much concerned over this matter. Most of the remarks have related to visible effects of wear. Of more interest are performance effects. I have assumed that wear would possibly produce a slow increase in mid-frequency level and in addition some slight decrease in reproduced high-frequency level due to increase in resonance frequency resulting from reduction of inductance. There will also probably be effects resulting from changes in the relation of the film to the gap. John K. Hilliard (A/tec Lansing Corp.): Most of the magnetic heads are designed for a 30-ohm circuit. It would appear that since the head inductance is decreasing, there could be a tendency to have a resonance at a high frequency because of the leakage inductance in the input transformer. This is caused by the fact that as you lower the impedance of the generator or driving transformer, the effect of the leakage inductance becomes more of a factor and would vary with the type of input transformer used. In some cases this will be of only theoretical importance, but with input transformers having a very high leakage value, it may cause a marked peak in the high-frequency end. Co!. Richard H. Ranger (Rangertone, Inc.): 1 can't give you any information on magnetic film, but I think the reaction would be quite similar on j-in. tape. We get anywhere from 15,000 to 20,000 hr on a head, but the thing that disturbs you when it does wear is dropouts. In other words, you actually lose contact, and once you do that, there is nothing but to get a new head or refinish it. The frequency changes are, generally speaking, immaterial. Magnetic Head Wear Discussion 241