Movie Makers (Jan-Dec 1949)

412 STROBOSCOPES ARE EASY Simple formulas and basic drawings will create strobe discs for projector or sound system LAURENCE E. BATY, ACL PERHAPS the handiest way to time a projector, or to synchronize it with dual turntables or a magnetic wire recorder, is to use a stroboscopic disc. Basically, this consists of a specially patterned disc of alternate black and white segments which is attached to some regularly revolving part of the instrument being timed. When this revolving disc is viewed under a properly pulsating light and seems to stand still, the instrument (projector or sound source) will be operating at the predetermined speed desired. In practice, these discs may be attached to one's projector on the hub of either the feed or takeup sprocket — if these are free for such attachment — or on the face of the hand testing knob, if the hubs are not free. (Attachment to one's wire recorder is made on the face of the takeup spool, as already ably outlined — April, 1949 — in Synchronizing Sound On Wire. ) The League's consulting service has complete data on these features of projector design and can advise whether your projector will accept the sprocket or testing knob disc — and what its segment pattern should be. said that I would need a neon glow lamp with which to scan the intended disc. We settled on a }4 watt unit in a suitable socket. I was now ready to design the disc itself. FORMULA NO. 1 The general formula for computing the number of black segments of any strobe disc is as follows: i F In equals r The values in this formula are: N equals the number of black segments; F equals the frequency of the light source in flashes per second; r equals the number of revolutions per second of the unit on which the disc is mounted. This general formula can be used in computing the design of a strobe disc to be mounted on a projector sprocket. N would then equal 120 (the number of flashes per second of 60 cycle current) divided by 2 (the number of revolutions per second of the 8 frame sprocket at 16 fps speed) for an obvious answer of 60 black segments. DISC ON PROJECTOR HUB These patterns are determined through the use of simple formulas, one for each of the three discs mentioned— sprocket or testing knob on the projector, takeup spool on the wire recorder. Use of the formulas is quite easy, while the actual creation of the indicated disc is almost as routine. It will be the purpose of this discussion to show you how to do both. Let us take an example. I wanted my projector to run at exactly 16 frames a second. There was room to attach a stroboscopic disc to the hub of the lower sprocket, and I found that this sprocket had 8 teeth on its circumference. This meant, of course, that it would pass 8 frames of film with each revolution. My electrical dealer told me that the frequency of our alternating current was the standard of 60 cycles a second, and FORMULA NO. 2 However, when timing a projector shaft with a neon glow lamp, this general formula is better expressed as follows: AT 1 2 x f x t IN equals s The values in this formula are: N equals the number of black segments; f equals the frequency of the alternating current in cycles per second; t equals the number of teeth on the sprocket; s equals the projection speed desired in frames per second. Substituting the known values in formula No. 2, we get the following result: 2 x 60 x 8 N equals tt N equals 60 black segments [Continued on page 426] FIG. 1: A simple method of laying out strobe disc design. One shown is 15 segment line-type for 8 frame sprocket projector. FIG. 2: The 12 segment, fan-shaped design above also serves an 8 frame sprocket. It results from dividing 5 into original 60.