Movie Makers (Jan-Dec 1952)

18 SYNCHRONIZING SOUND ON TAPE Photographs for MOVIE MAKERS by HERMAN E. DOW, ACL Single-second accuracy is easy with this adaptation of the Levett sound system to tape units. The capstan does the trick FIG. 1: General setup for using this synchro-sound system places recorder in front of and just below path of projector light beam, the flicker from which is used to scan stroboscopic disc. The projector must be one with variable speed. FIGS. 2-3: Closeup views, from the side and front, show positioning of strobe disc on tape capstan of Revere recorder. IN APRIL, 1949, under the heading Synchronizing Sound On Wire, we presented in these pages an outline of what apparently was then a new system for matching movies and magnetic wire. Warren A. Levett, ACL, was the author of this discussion; and his methods have come since then to be referred to as the ''Levett synchro-sound system." Briefly for now (there will be more detail later ) , here's how it worked : THE LEVETT WIRE SYSTEM Central in the Levett system was a suitable stroboscopic disc, which was mounted on the takeup spool of the magnetic recorder and, in operation, was scanned under the intermittent light of the projector's beam. Essential to the success of this setup was the fact that the takeup spool on the wire recorder revolved at a fixed speed, while the speed of the projector — and therefore the periodicity of its light flicker — could be varied as needed. The system was simple. It offered accurate synchrony within a one-second tolerance. And it was widely adopted. But only by those movie makers using magnetic wire recorders. For on the face of it, the Levett system seemed unavailable to those using magnetic tape units, since the revolutions-per-minute of their takeup spools varied with the volume of tape on the spools. If, therefore, the Levett method was to be adapted for scanning tape tracks, some other unit of the recorder would have to be employed as the strobe disc mount. And, of course, this unit must revolve at a fixed speed. ADAPTATION TO TAPE Movie Makers, in response to repeated inquiries by our readers, has been on the lookout for just such an adaptation of the Levett system. We are now pleased to present it herewith — based on the data and the excellent illustrations of Herman E. Dow, ACL. Neither we nor Mr. Dow make any claim that his methods are the first of their kind in the magnetic tape field. Because of their essential simplicity, it seems likely that others have made similar adaptations. But this we can say: Mr. Dow has been the first to share his system with other amateurs. And now the system . . . As will be seen in his pictures, the tape recorder used by Mr. Dow is a Revere. However, his adaptation of the Levett system to this unit can be used with equal success on other makes — with, of course, slight variations necessitated by design. For, the adaptation depends solely on the strobe disc mount. And the mount chosen is the capstan which, at a fixed speed, advances the tape through the sound-scanning unit. ESSENTIALS OF THE SYSTEM Now, for the benefit of those who weren't around (or perhaps even interested) at the time of the original Levett lesson, let us double back for a moment and see how his system works. Essentially it is based on the well known practice of scanning a suitable repetitive pattern (the strobe disc) under an intermittent light source. Generally — as in the use of strobe patterns to time the speed of a projector or turntable — the periodicity of the light pulsations (120