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Equipment for Stereophonic Sound Reproduction — Panel Discussion
JOHN K. MILLIARD, Moderator
E. K. Carver (Eastman Kodak Co.): What tension is required over the magnetic head, especially in the two cases where a combination sprocket is used? How great is the tension on the film in that isolated portion?
John G. Frayne (Westrex Corp.): It's approximately 400 g, in the closed film loop.
Skipwith W. Athey (General Precision Laboratory}: I believe the actual tension in the film is of the order of 8 oz and the force against the head is of the order of 3 to 4 oz.
Dr. Carver: The thing that's bothering me, really, is the difficulty in running a combination sprocket, which is not suitable either for holdback or feed with a high tension.
Dr. Frayne: There's no problem if the sprocket is properly designed. In the Davis Drive, we have always recommended a sprocket-tooth base that nearly fills the sprocket hole in the film. This minimizes the "crossover" effect where the tension in the filtered film loop exceeds or becomes less than that in the external film paths. This crossover usually results in high flutter content at this point in the film running. In the Westrex CinemaScope head we use a base tooth dimension of 70 mils. We would like to use a base of 73 mils and completely fill the sprocket hole, but the film shrinkage prevents this. We refer to this type of tooth colloquially as the "fat-tooth sprocket."
Mr. Athey: I think we can confirm exactly that same effect. The sprocket form is of crucial importance. (We
This panel discussion was held on October 6, 1953, at the Society's Convention at New York.
prefer to avoid the use of a full-fitting tooth, however, because the reproducing equipment must operate properly with shrunk film, as opposed to the essentially unshrunk film with which recording equipment operates.) In the early development we did not have sprockets of the proper pitch diameter and our flutter performance was therefore not very good. However, with sprockets of optimum pitch diameter, flutter due to tooth ripple is essentially eliminated over a normal range of film shrinkage. (The most prevalent causes of the "crossover" trouble are bent reel flanges, nonuniform friction of the feed-spindle brake, and sticky firevalve rollers.) With properly adjusted equipment, the crossover phenomenon can be prevented throughout the run of the reel, and flutter from this source can be eliminated.
W. G. Hill (Ansco, Binghamton] : I'd like to clear up just one point about this fattooth sprocket. Dr. Frayne speaks of a 0.70 base. May I ask what that is? What is 0.70?
Dr. Frayne: The sprocket-tooth base is 0.070 in.
Mr. Hill: So that dimension is intended to fit the narrow way of the hole?
Dr. Frayne: Right. Lateral positioning is not critical.
Mr. Hill: May I ask, then, about how many teeth in contact do you use?
Dr. Frayne: I don't recall exactly.
Mr. Hill: It doesn't allow very much for shrinkage.
Dr. Frayne: No, but there is no problem with film with normal shrinkage. Perhaps some other members of the Panel would like to comment on this.
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March 1954 Journal of the SMPTE Vol. 62