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

licussion £ W. D'Arcy (De Vry Corp.} : That's the ft real bit of valid information on the s >ject I've seen. One thing which occurs time, however, is that in the case of a fcf-width magnetic stripe located either ang the outside edge or the inside edge ca photographic sound track, it wouldn't t e very long to wear the magnetic head cvn in contour to the point where it vuld lose contact with the magnetic rterial and be bearing only on the optical p -tion of the sound track. I note that you h/e five mils wear in the first 88 hours, lis may point to a consideration which b been going through our thinking, and vich has been brought up in Mr. Dimr:k's Magnetic Recording Subcommittee, a to the advisability of having half-track Hds narrower than the magnetic tracks s they would wear uniformly. This would toolve committing ourself to locating such tcks always on the same edge of the potographic sound track. What are your cnments with regard to such a plan? Mr. Ferber: With regard to using a halftck on the full-track width head, the nximum deterioration in output level yi could get would be 3 db, due to the Hf-track. The problem in general could ftft be solved, of course, by using a harder Jrlterial for the head, and that is being i: instigated. Mr. D'Arcy: No, that isn't exactly the I nt. Using a full-track head on halft cks always positioned the same, it would apear that it would take a relatively small tjie to wear a groove into the head which vuld react as, you might say, does a cDth ball in disk recording, with the s ooth or unworn part of the head bearing aainst the uncoated portion of the sound tck, or against the film base itself, and t: other half out of contact with the magpie material. " 'Mr. Ferber: That's right, but since there i magnetic coating on both sides of the sind track, shifting from one to the other vuld only cause the loss in output level. (j going from a head that is worn with a I f-track to a full-track head, you would tve that same 3-db loss. M. G. Townsley (Bell & Howe/I) : I'd like make a comment here in answer to >ars, Mr. D'Arcy, if I may. I think your |r that the magnetic material will lose contact with the head if you continue to run half-track located, say, on the outer edge of the film, is based on the assumption that the head is not worn by the film base itself. Some preliminary results from tests we have run indicate that this is not the case, and that while there is some small stepwise wear initially, it isn't very long before the film starts to wear the head down too. The head continues to wear down with a small step, but both "treads of the stair" come down together. We are currently on one test which has involved up to 300 hours of half-track operation under very comparable conditions to those reported here. No serious deterioration in either the half-track performance or in fulltrack performance using the same head has been noted. That is encouraging. Mr. D'Arcy: It should, but the fact remains that for films half-striped on the emulsion side, any very appreciable stepwear will very soon scratch the remaining photographic track so much that it becomes unusable, which is exceedingly serious considering the fundamental use possibilities for half-width magnetic striped. The point I'm making, and with this I'll withdraw, is that I believe these data certainly point out the need for special halftrack heads, and for standardization of the positioning of magnetic half-tracks on photographic sound tracks so that head wear will not cause serious magnetic performance deterioration and possible serious damage to some original photographic sound tracks. J. R. Horak (Eastman Kodak Co.): We have run 100-mil, 50-mil and 30-mil wide coating stripes of approximately 0.5-mil thickness down the centers of quarter-inch wide film strips. The strips were then run on an eighth-inch wide magnetic head. We found that the coating stripes do wear the head deeply enough so that small variations in the thickness of the coating and in its width cause changes in the coupling between the coating surface and the head, causing extreme amplitude modulation, and I mean considerably greater amplitude modulation than we would have on a smooth, well-polished head. This effect does occur, I think. Mr. Ferber: I believe we have not experienced that condition due to the force applied to the head, forcing it against the film. del Valle and Ferber; Wear of Magnetic Heads 505