Movie Makers (Jan-Dec 1952)

19 per second, for example, from a 60 cycle current) remains the fixed quantity; the revolutions of the strobe disc are then varied to bring it into step with tms factor. But in the Levett system these values are exactly reversed— and of necessity. The known revolutions of some part of the recorder — upon which the strobe disc is mounted — become the fixed quantity. And, under adjustment as needed, the speed and therefore the light flicker of the projector becomes the variable under which the disc is scanned to a standstill. The projector flicker, of course, is created by the number of times per frame of film the shutter interrupts the light beam. Putting these facts to work is far easier than outlining them. The essential setup calls for a suitable strobe disc on the recorder, across which there will fall enough light from the projector to scan it. This setup is seen clearly in Fig. 1 of the Dow illustrations. Specifically, Mr. Dow determined on the use of an 18 segment disc (black and white each) to monitor an 18 frames-persecond projection speed. THE STROBE-DISC FORMULA But how were these determinations arrived at? Well, quite simply enough: by working from known factors in a formula to find the unknown. Again, that formula was first stated in the Levett discussion of wire synchrony. But it will apply writh equal ease and accuracy to all makes of tape units. The formula follows: R x N equals F x S where R is the number of revolutions per second of the disc; N is the number of segments; F is the number of frames per second of projection speed, and S is the number of shutter interruptions in passing a frame of film. The one totally unknown factor here will be N, the number of segments in a suitable disc, which is what we want to determine. Among the known or determinable factors are the following: S, which on all modern projectors is generally 3 shutter interruptions per frame of film; F, which we may set in a preliminary test at 16 fps; and the value R — the number of revolutions per second of the disc. This figure — which will, of course, vary from make to make of recorder — is arrived at by dividing the number of revolutions per minute of the capstan by 60. With the Revere the capstan speed on the model tested was 180 rpm — making R equal 3 revolutions per second. Inserting these values into our formula, we arrive at the following : 3 x N equals 16 x 3 3N equals 48 N equals 16 This result would indicate a strobe disc design of 16 segments to monitor a 16 fps projection speed. There is also to be noted an obvious but only coincidental equality of segment pattern and projection speed created by the revolutions per minute of the Revere recorder capstan. Noting this equality, and probably wishing to avoid screen flicker, Mr. Dow apparently raised his strobe pattern to 18 to monitor an 18 fps projector speed. MAKING FORMULA FIT Again coincidentally, the working out of the formula based on a Revere recorder is a neat and singularly easy operation. Amateurs using other makes of tape recorders should not be discouraged if at first their answers do not come out as easily. They should remember that there will be available two fixed and unchangeable FIG. 4: Front tape guard has now been removed and the disc on its wooden mount is ready for positioning on capstan top. FIG. 5: With tape guard still removed, disc and mount are seen affixed by suitable cement to the exposed tape capstan. FIG. 6: To provide clearance for strobe disc mount, a notch is cut in tape guard, which is now ready for reinstallation. values: S, which is likely to be 3 but can in any case be determined by observation of the projector shutter, and R which can be determined by querying the recorder manufacturer concerning the capstan revolutions per minute. They should remember also that the value N (number of segments) must be a whole number, but that the value F (fps of projection speed) need not be. Thus, if a trial run of, say, 18 frames per second in the formula indicates a fractional number of segments in the disc pattern, the nearest whole number may then be selected as the value N. Now inserting it in the formula, you may arrive at a projection speed of, say, 17.6 fps — which will serve quite satisfactorily. THE PRACTICAL APPLICATION So much, then, for the basic operation of the Levett synchro-sound system. In adapt [Continued on page 28]