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cordance with the CinemaScope proposed standards for magnetic sound reproducing equipment. The heads are shielded against stray magnetic fields by a mu-metal case and shield. The heads contact the film at a point midway between the two drums, thus maintaining a symmetrical relationship of film wrap about the gap regardless of variations in film compliance. Means are provided for adjusting the heads as a unit to insure correct azimuth, centering of the gap and track position. An amplifier assembly, consisting of four preamplifiers mounted in a metal box, has been designed to provide a signal output from the four magnetic heads at a nominal level of —12 dbm for the stereophonic tracks and —18 dbm for the effects track. The frequency response characteristic of the amplifiers is shown in Fig. 7. This characteristic, together with that resulting from scanning with heads having 0.5-mil gaps, gives an overall reproducing characteristic which is in accordance with the standard proposed by the Motion Picture Research Council. The resulting overall frequency characteristic is flat from 50 to 8000 cycles/sec, within the limits of the proposed standard.
The auxiliary soundhead described in this paper offers a simple and relatively inexpensive means of reproducing CinemaScope stereophonic sound films with a high degree of fidelity. When mounted on a projector in good operating condition, low-frequency flutter components are not greater than 0.03%, while the total root-mean-square value of flutter is within the 0.15% value which is the generally accepted requirement for theater equipment. The guiding of the film at the sprocket reduces film weave to a minimum which is particularly desirable with the use of narrow tracks. Maintenance requirements have been reduced to a minimum. There is no contact with the film in the picture area, no lubrication is required, the mechanism stabilizes well within the allotted starting time and complete facilities are provided for alignment should occasion arise for the ultimate replacement of the magnetic head.
References
1. Loren L. Ryder and Bruce H. Denny, "Magnetic sound track placement," Jour. SMPTE, 58: 119-136, Feb. 1952.
2. C. C. Davis, "An improved film-drive mechanism," Jour. SMPE, 46: 454-464, June 1946.
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March 1954 Journal of the SMPTE VoL 62