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498
0. K. KOLB
November
inflammable base are coated and afterwards slit and perforated. Large quantities of the base material can be coated quickly in this way but the slitting and perforating of the coated material quickly blunts the tools used for these operations due to the abrasive action of the iron oxide coating and resharpening of the knives and punches is necessary after a relatively small number of reels have been made.
Type B (Fig. 1) illustrates a kind of magnetic film with which this drawback has been overcome by slitting and perforating the base material before applying the coating. In this case the coating is applied only to the surface between the two rows of perforation holes. This has the further advantage that it enables the usual footage and edge numbers to be retained along the entire length of the reel.
Type C (Fig. 1) is a third kind of magnetic film where the magnetic coating extends over only approximately half the width of the film space between the perforation holes, the other half being provided
Fig. 1. Different types of magnetic film coatings.
with a layer of a white or light-colored material upon which notes, cue signs and other visible indications can be made, for example, for synchronization purposes.
Optimum Bias and Frequency Response
Magnetic sound film stock eventually became available from several sources in appreciable quantities and comparisons and measurements were made of the different manufacturers' products in order to use them in the most suitable way. It was found that the various coatings differed considerably and usually required a change in the amount of the supersonic bias in order that the best results could be obtained from each different coating.
Figure 2 illustrates graphically the output level of three different kinds of magnetic film stock plotted versus the bias current.
Curve A refers to a medium-hard coating from which the maximum