Movie Makers (Jan-Dec 1951)

Record Details:

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322 More on FIG. 1: General view of RCA "400" magnetic projector shows four major features added to "400" Senior optical projector. WITH the recent public introduction of a new 16mm. magnetic sound projector, the Radio Corporation of America had the privilege of presenting another first in 16mm. recording and reproduction. Ever since iron-oxide-coated tapes became an accepted medium for sound recording, the possibility of applying the same material to 16mm. motion picture film has been evident. Work on the development and design of equipment to handle this film has been going on for several years in the laboratories of the RCA Engineering Products Department in Camden, N. J. The problem of applying the narrow stripe of magnetic material to 16mm. acetate stock has not been simple; hence the relatively late appearance of magnetic sound on film. However, a commercially acceptable stripe of magnetic oxide can now be coated on the edge of 16mm. film economically. Moreover, the striping can be placed on the film either before or after it has been used for picture taking and even if it already has an optical (or photographic) sound track. With this film, the new RCA "400" magnetic sound projector makes available one of the greatest advances in the movie field since the advent of sound on film. The new equipment is basically RCA's familiar "400" Senior-type projector, specially engineered to accommodate the component parts required for recording and reproducing magnetic sound without altering in any way the characteristic simplicity of its threading. The newly added features greatly enhance the utility of the projector by endowing it with four separate functions (see Fig. 1). It reproduces optical sound; it erases and records magnetic sound: it reproduces magnetic sound; and finally, it can be used as a public address system. Any one of these four functions can be chosen simply by turning one or both of two knobs: one to select the amplifier operation desired, and one to adjust the projector for reproduction of either optical or magnetic sound. Recording level is checked by a glow-lamp indicator on the upper portion of the amplifier panel. For recording and reproducing magnetic sound, a very small record-playback combination head has been mounted inside the sound drum (see Fig. 2). To obtain good physical contact between film and head, consistent with low head wear and low film deformation, this head is MAGNETIC SOUND G. A. DEL VALLE, Engineering Section, RCA-Victor Division, Camden, N. J. mounted on the free end of a pivoted arm. The arm is secured to a bracket through a rotatable eccentric pivot, and the bracket is secured to the main frame of the projector. The free end of the arm is shaped to fit freely around the shaft of the constant speed drum of the projector. The head on the arm is positioned, with respect to the drum, to contact the inside of the loop of film as it is being pulled around the drum, one edge of the film projecting beyond the rear edge of the drum. Rotation of the eccentric pivot connecting the arm to the bracket provides a longitudinal adjustment of the arm which carries the head. This adjustment is used to position the gap in the head correctly with respect to the magnetic track on the film. The record-playback head is moved into position for operation by an arm actuated by the track selector ("MAG-OPT") knob, which is mounted on the projector's main frame between the lower sprocket and the sound optical bracket (see Fig. 2). The presence of the head in the projector amplifier does not necessitate any change from the standard method of threading the film through the projector. Also, the location of the recordplay head inside the sound drum offered several advantages over any other location, primarily because it permits constancy of film motion, and also because it makes possible the same 26-frame spacing from sound to picture that is standard for optical tracks. The erase head and two guide rollers are mounted on the main frame of the projector to the right of the upper sprocket (see Figs. 3 and 4). While it is necessary to thread the film between these two guide rollers only when using the erase head, it is simpler, and therefore best, to do so at all times, rather than adopt two dissimilar methods of threading. For recording and erasing, the erase head must be pushed downward until it automatically locks in position for threading (see Fig. 3). The film from the upper reel is threaded first between the two guide rollers, next between the erase head and a nylon shoe mounted under the head, and thence through the projector in the conventional manner. To render the projector as foolproof as possible against accidental erasure of the magnetic sound track, intentional interference between the rewind lever and the erase head has been provided. When a recording is completed, the erase head must be moved out of the way before the rewind lever can be moved to rewind the film for playback (see Fig. 4). This automatically puts the erase head out of the film threading path. Besides this precaution, it is also necessary, in order to erase, to turn the amplifier-function selector switch (see Fig. 1)