Movie Makers (Jan-Dec 1952)

206 SICNAL HOLE MADE IN FILM FIG. 1: Secret of Synchcro-Tape system is stroboscope pattern imprinted directly on back of tape. BCPLECTCD LIGHT IMPROVED SYNCHRONY WITH MAGNETIC TAPE FIG. 2: In operation, tape recorder is positioned in front of projector so that intermittent shutter light scans moving strobe pattern. LATE this past June, at a press party held in New York City, the Revere Camera Company announced an im"portant new aid in achieving accurate synchrony between amateur motion pictures and sound scores recorded on magnetic tape. That aid is a unique new tape product called Synchro-Tape. On its face, or recording side, Synchro-Tape seems to be nothing more than a standard % inch wide magnetic recording medium — which is exactly what it is. On its back or base side, however, Synchro-Tape carries a repetitive pattern of vertical black lines (see Fig. 1) — and in these black lines lies the secret of its very real importance. Movie Makers is proud to state that the basic idea now commercially embodied in Synchro-Tape was first advanced by a contributor to these pages in January, 1952. BACKGROUND OF THE PROBLEM To appreciate the importance of Synchro-Tape in amateur film scoring, it is first necessary to understand clearly the difficulties which it solves. Magnetic tape scoring, naturally, is no new thing in the amateur film world. Ambitious amateurs, without respect to 8 or 16mm. film widths, have been using this magnetic medium with their movies almost since its inception. Music, narrative and sound effects were poured through countless microphones, in synchrony (during the recording process, at least) with a monitoring projection of the picture. But on playback these carefully timed accompaniments tended to get out of step. PROJECTOR SPEED VARIES The magnetic recorder, we were told, ran at a constant speed — 3% or 7]/i> inches of tape per second. But projector speeds had a distressing tendency to vary. As the motor heated up it would run faster, even under a fixed speed setting. And line voltages varied from day to day, hour to hour and place to place. A recording made in perfect synchrony with your projector on Saturday afternoon might be out by two to twenty seconds that night, as the voltage dropped under increased load. Or a recording you could play back perfectly in your own home might be disastrously out of sync at a friend's house in the neighboring town. Some system was necessary to compare and correlate the fixed speed of the tape recorder with the varying speed of the film projector. THE STROBE DISC SYSTEM That system, as it developed, was the application of the familiar stroboscopic disc control to these two machines. Its principles and operation were outlined in Synchronizing Sound On Tape, an article prompted by Herman E. Dow, ACL, which appeared in Movie Makers last January. His application of a strobe disc to tape recording was based on a still earlier method used with wire recorders by Warren A. Levett, ACL, and outlined by him in April, 1949. Central in the Levett-Dow system was the mounting of a suitably patterned strobe disc on some unit of the recorder which revolved at a fixed speed. On a tape recorder (as Mr. Dow illustrated) this proved to be the capstan. Once equipped with such a disc, the tape recorder was then positioned directly in front of the projector (as in Fig. 2), so that spill light from the projector's beam fell across the revolving strobe pattern. This light beam, although it seemed steady, was actually intermittent in character, since its path to the screen was being regularly interrupted by the projector's shutter. The mechanical rate of these interruptions was fixed (on modern projectors) at three light cutoffs for each frame of film passed. But the temporal rate of these cutoffs (their number per unit of time) clearly would vary directly with the projector's operating speed. Thus, if one's strobe disc was patterned for, say, an 18 frame-per-second projector speed, the strobe segments would seem to stand still when scanned under the intermittent light from a projector operating at that speed. But, should the projector speed vary, even as little as a frame per second, the strobe pattern would instantly begin to "creep" — and a suitable adjustment in projector speed could be made. OTHER SOURCES OF ERROR Under this Levett-Dow system, the synchrony of amateur films scored with magnetic tape improved markedly. Largely done away with were all sources of error traceable