Movie Makers (Jan-Dec 1949)

413 CONTROLLING LINE VOLTAGE How a Variac may be combined with a voltmeter to aid synchrony of film and magnetic sound GEORGE MERZ, ACL I HAVE read with interest the two articles on synchronizing sound on wire — the first by Warren Levett, ACL, in the April, 1949, issue of Movie Makers, and the latter by Austin D. Johnson in the September issue of the same year. Equally interesting is the letter by Joseph A. Wagner, ACL, in The Reader Writes column of the same issue. I agree with Mr. Wagner that line voltage conditions are a prime factor in achieving constant-speed projection, together with good synchronization between film and sound on wire or tape. To counteract the effect of a sudden drop in line voltage (which, as Mr. Wagner suggests, could be to as low as 90 volts when a motor of Y± h.p. or more starts up), the writer has had his radio repair man build for him within a small carrying case a Variac (voltage regulator) and a voltmeter. The unit receives its current from the house circuit on the input side of the Variac and on the output side dispenses controlled voltage to the recorder. FIG. 1: Control unit in closeup shows voltmeter dial at left, Variac adjustment knob at right and toggle switch for reading input or output line. FIG. 2: Operation of three units as set up here is two-man job, says au thor. Recorder and Variac are in front of projector for one-man screening Thus I am no longer at the mercy of any drop in line voltage which may momentarily slow the speed of the recorder, which in turn would slow the film if I depended upon a strobe disc on the recorder for my projector speed. I must, however, watch and, if necessary, adjust the voltmeter to see that the voltage output is maintained at the desired level — say 120 volts. With this setup, I make my recording of music or voice or sound effects (or any combination thereof) with an output of 120 volts from the Variac to the recorder. Should the line voltage drop during the process, I immediately correct it to 120 volts by adjusting the regulating knob of the Variac to keep the current constant throughout. At the same time I project the film under stroboscopic control on the projector at a constant speed of either 16 or 24 frames a second (or any other speed best suited for smooth projection) until the synchronization has been completed. From there on, whenever and wherever I project the film and play back the recording, I simply watch the pointer of the voltmeter for any deviation from the 120 volt mark and immediately correct it if it does occur. It does not matter if the line voltage is 10, 15. or 20 volts under what it should be. The Variac with voltmeter control will correct it to produce 120 volts at the recorder, for it has a range of 0 to 130 volts. The end result of this method of synchronization is that no matter where you project your films and no matter what the line voltage is, you can always have controlled synchronization between recorder and projector. In the accompanying Fig. 1, I show a closeup of the control unit. The cord entering the left hand end of the case is the power supply line. The cord at right front of the case is the regulated power supply to the recorder. The knob to the left of it is the regulating knob of the Variac. The toggle switch in the center, when flipped to the left, will give a reading on the voltmeter of the line voltage as it happens to be. (I have found line voltages in some locations where I have projected to be as low as 108 instead of 120.) When the switch is flipped to the right the meter will record the voltage in the output stage after it has been controlled by the Variac knob. In Fig. 2, I show the complete setup with the control unit and the recorder to the rear of the projector. When projecting in this setup, I usually have another person watch the voltmeter dial and regulate the Variac while I scan the projector stroboscope. When projecting alone, I assemble the two units forward of the projector so that, as I look toward the screen, I can also scan the voltmeter and, occasionally, the projector strobe.