Movie Makers (Jan-Dec 1949)

135 SYNCHRONIZING SOUND ON WIRE Photographs by Warren A. Levett, ACL One amateur's simple system for keeping recorder and projector in split-second step. The flicker does it WARREN A. LEVETT, ACL EVER since its introduction, in approximately 1936, the double turntable, disc system of sound accompaniment has been tops in amateur movie scoring. It has served ambitious amateur filmers well — for there existed no other system half so good to compete with it. But today this competition does exist. It is found in the magnetic wire (or tape) recorder, now offered widely in almost a dozen different makes. More and more amateurs are turning to the magnetic system for an effective, almost theatrical, accompaniment to their films. And, as they do, they soon find that their chief operational problem is that of maintaining accurate synchrony between the recorder and projector. Varying systems of electrical coupling already have been offered. The system about to be described requires no electro or mechanical coupling of any kind. It depends on a simple stroboscopic control, and it will hold your film and sound in synchrony within a fraction of one second. Before considering it in detail, however, let us see in general how the magnetic recorder may serve your films. FUNCTIONS OF THE RECORDER Firstly and mainly, it will provide musical accompaniment that may be synchronized to your pictures with a minimum of effort. Secondly, you can add narration on the wire with the musical background. This, too, can be timed perfectly to individual scenes, with little more difficulty than in timing the music. Basically, the secret of synchronization lies in varying the speed of the projector so that it is constant with respect to the recorder. PROJECTOR VARIES MORE Most recorders are powered with governed induction motors, rather than with synchronous motors. Their speed is, therefore, nearly constant, although not exactly so. Fluctuations in line voltage will affect the projector speed to a far greater extent than that of the recorder — which speed, in fact, cannot be controlled at all. Therefore, we realize immediately that if synchrony is to be maintained, it must be done by regulating the speed of the projector to the speed of the recorder. Further, by this method of control it is possible to use your prepared wire with your film on another recorder of the same make, at, say, your movie club or at the home of a friend. This is not possible by the usual 60 cycle stroboscopic method of projector control, in which the projection speed is held to a fixed number of frames per second. Instead of this fixed synchrony, the projector must be free to vary its speed in step with the slighter variations of the recorder. It is, therefore, on the recorder that the strobe disc is placed. FIG. 1: Projector and recorder, positioned as above, make synchronization possible from projector flicker over disc. FIG. 2: Detail of setup with Filmo projector is author's design of right-side speed control. Flexible auto radio cable is inset in speed knob and leads to operating side. So much for the type of synchronization. But how is it done? Obviously, the light flashes to illuminate the strobe disc on the recorder must be created by and emanate from the projector. The shutter of my projector (and of most other projectors) interrupts the light beam three times per frame — twice while the film is motionless in the gate, and once while the next frame is moving into the gate. I determined to use the flicker from the projection light beam to light the strobe disc, and I soon found it entirely practical. By positioning the projector and the recorder as shown in the illustration, the light beam from the projector passes directly over the strobe disc, giving off enough stray light to illuminate the disc. DESIGNING THE DISC The number of segments in the strobe disc was determined from the formula R x N equals F x 3, where R is the number of revolutions per second of the disc, N is the number of segments, and F is the number of frames per second passing through the projector. The takeup spool of the Webster wire recorder which I use makes about 114 revolutions per minute. Thus, when N equals 26, F equals 16.4 N equals 27, F equals 17.1 N equals 28, F equals 17.7 I found that for flickerless projection, a frame speed of 17.7 per second was desirable; hence the disc I designed has 28 black segments. [Continued on page 157]