Movie Makers (Jan-Dec 1952)

174 DeVry adds magnetic to "JAN" ELLIS W. D'ARCY, Chief Engineer, DeVry Corporation WE rather pride ourselves here at the DeVry Corporation on the development of the 16mm. sound motion picture equipment now classified as standard by the United States Armed Forces. This three-case equipment (variously designated by the several services) has come to be known as the DeVry JAN (Joint ArmyNavy) ; at the time of writing we have manufactured and delivered more than 8000 of these units to the Armed Forces. The DeVry JAN, we believe, is unique in its performance capabilities. It has been used, for example, to project a 9 by 12 foot picture at 175 feet, with a screen result equal to that projected by a carbon arc projector using conventional optics and mounted only 125 feet from the screen. It is to this equipment that we have added magnetic sound facilities, without affecting in any way its optical sound functions (see Fig. 1). There are two methods of approach in the application of magnetic sound to 16mm. motion pictures. If we may use the broad terms amateur versus professional, we feel the difference will be well identified. Our approach to the problem was very clearly defined by the professional nature and usage of the DeVry Armed Forces 16mm. equipment. Briefly, the design specifications for the conversion to magnetic sound shaped up as follows: MECHANICAL SPECIFICATIONS 1. The addition of auxiliary components for magnetic recording and reproduction must not decrease the mechanical strength of any elements of the projector. FIG. 1: Operating panel of DeVry JAN sound projector shows standard positioning of optical-magnetic sound head, with erase head separate. 2. Any auxiliary parts added to the present DeVry JAN projector must be readily accessible for parts replacement. 3. The magnetic component parts must be easily aligned and adjusted in the field by relatively inexperienced personnel. 4. The conversion to accommodate magnetic sound must be applicable to present DeVry JAN projectors now in use by the U. S. Army and Navy. 5. The conversion must have structural strength sufficient to withstand drop, shock and vibration tests as prescribed by Government specifications. 6. The addition of the magnetic sound components must not increase the flutter of the sound scanning system. ELECTRONIC SPECIFICATIONS 1. The frequency response of the magnetic sound system must at least equal that accepted as essential for 16mm. sound on film reproduction by the optical method. 2. The magnetic record-reproduce head must record and reproduce magnetic sound without exceeding the 2% distortion allowance at all frequencies in the pass band. 3. The performance of magnetic sound must equal or exceed the performance of the optical sound track with respect to spurious noises. 4. The design must permit the instantaneous reproduction of magnetic track and optical track. 5. Complete and positive erasure of the previously recorded magnetic sound track must be effected. 6. The magnetic sound system must match electrically into the amplifier supplied as a standard component of the Armed Forces equipment. 7. Switching from magnetic to optical tracks must be done simply and surely. THE RECORD-REPRODUCE HEAD The application of magnetic sound to the DeVry JAN projector involved only minor modifications of the standard sound head. Because of government specifications, the mechanical strength of the projector mechanism could not be reduced by the addition of the magnetic sound components. This factor eliminated the possibility of reducing the diameter of the shaft supporting the sound filter drum and flywheel (see Fig. 2). The shaft size was therefore determined on this basis and not on its normal functioning strength requirements. This important factor then dictated the need for a very minute type of magnetic record-reproduce head. The magnet is *4 inch in diameter and Y^ inch in length, but in spite of its tiny size it is adequately shielded to prevent hum pickup. This magnetic head generates magnetic impulses that are exact images of the desired sounds, and concentrates them in an area or gap about 0.0003 of an inch long. When the magnetically striped 16mm. film runs past this concentrated head field, the magnetic track retains a permanent magnetic image of the sound waves. POSITION OF ERASE HEAD The erase head in the specific conversion was located between the feed reel and first sprocket (see Fig. 1). The erase and record bias frequency is 112,000 cycles.