Movie Makers (Jan-Dec 1953)

239 A PROJECTOR SPEED CONTROL DENIS M. NEALE With this simple electrical system, your tape sound and picture will keep in step. And you can enjoy both EVER since amateurs began using tape to provide sound for movies, they have been confronted with the problem of controlling the projector speed to keep the picture in step with the sound. First came the proposal to use a neon lamp and stroboscopic disc on the projector. This enables you to tie your projection rate to the frequency of the A.C. power line. Since your tape recorder speed also is determined largely by these frequencies, this is quite a good way of working, since changes of frequency will affect picture and sound equally. Unfortunately, other things besides current cycle influence the capstan speed in a tape recorder. As the motor is of the induction type, the speed is affected also by voltage changes. Although these speed variations are small, they can add up to produce serious errors. Then, on some recorders the capstan is driven by a spring belt which introduces a small but unpredictable degree of slip. So where accuracy is important, a better system is to put a stroboscope on the capstan spindle and illuminate it from the projector beam. The projector shutter then produces the necessary pulsation of the light, the speed of pulsation being adjusted until the stroboscope segments appear stationary (see Movie Makers, January 1952). Even this system is not perfect because stretch and shrinkage of the tape affect the speed at which the sound is reproduced. So exact synchrony can be maintained only by putting strobe marks on the tape itself, as in the Revere Synchro-Tape (see Movie Makers, August 1952). Each of these methods works well enough, but I am too lazy for them to appeal to me. When I am showing a film, I like to put my hands in my pockets, not on the projector speed control. And I like to watch the screen, not a set of dithering stripes. On the other hand, those pockets of mine do not carry the cash for a magnetic sound projector. So I looked around for a simple, inexpensive device to watch the stroboscope for me. It had to control the projector speed automatically and it had to be neat. To keep things simple, I ruled out phototubes; to cut down costs, I designed for a minimum of components. You can see from Fig. 1 that the circuit uses very few parts. The electrical components cost very little; in England (from where I write) they can be bought for the equivalent of about $2. You may foresee difficulty in fitting a rotary switch to your projector, but you can get any lathe work done at a machine shop. The device requires no modifications or attachments to your tape recorder because, like the first stroboscopic system outlined above, it relies on the recorder running at a fairly constant speed. Unlike most synchronizing devices, this one need involve no modification of the projector itself apart from a break in the motor circuit at one point. It is immaterial where you make this break so long as the current for the projector lamp does not also pass through the synchronizer. Your safesl plan therefore is to break the lead running from the speed control resistance. R, to the motor, as shown in Fig. 1. This lead carries current for the motor only. In series with the motor, then, you connect a selenium rectifier, S, and it does not matter which way around you connect it. The rectifier allows current from the power outlet to pass only half the time and so cuts down the motor speed considerably. If S is short-circuited, however — by putting the switches to "Manual" and "Fast," for example — the speed rises to the usual value determined by the setting of R. If R is set to give 19 frames per second under these conditions, then switching back to "Slow" will reduce the speed well below normal, say to 11 or 12 fps. Now suppose that a rotary switch is coupled to your projector so that it will open and close 60 times a second when the projector is running at exactly 15 frames per second. Then at this speed it will operate once for every cycle of the current supply. Whether or not it has much effect depends on the part of the current cycle at which it closes. At one extreme, the switch may close each time the rectifier passes current and open again when it blocks it. In this case the rotary switch makes little difference and the projector speed falls towards 12 frames per second. [Continued on page 243] ROTARY SWITCH FIG. 1 (left): Heart of the speed control unit is a rotating interrupter switch coupled to the film motion. Other components cost about two dollars. FIG. 2: Carbon brushes are removed to reveal the rotary switch fitted to the flywheel of author's projector. Four-sector unit will fit 60 cycle currents. -v^