Movietone Bulletin (November 1928)

ments of synchronized sound reproduction; and in the case of the synchronous A.C. motor, there are the objections that it does not have a very good starting torque, and that speed regulation, such as is required for running silent pictures, could not be obtained. Evidently, therefore, the problem is one which requires new methods for its solution. Before describing the operation of the motor control box it is necessary to explain one important special electrical feature on which its operation depends. This is what is-called a “tuned” or “resonant” circuit. This means a circuit which will permit alternating current of a certain frequency to pass fairly readily, but which offers a high resistance to alternating currents of any other frequencies. The frequency which such a circuit is designed to pass is called the “cuned” or “resonant” or “critical” frequency. Perhaps this phenomenon will appear less mysterious if it is stated that exactly the same thing happens in mechanical engineering and can be illustrated very simply. Take a piece of rubber band and, holding it by one end, hang on the other end some object heavy enough to stretch it moderately. The weight will bounce up and down at a regular rate and a touch will be enough to keep it going. But if you try to move it faster or slower than this natural rate, you will find that the rubber band gives you very little assistance and you will have to do most of the work yourself. In other words, this is a mechanical arrangement which readily permits back-and-forth movement at a certain frequency, but offers considerable resistance to movements at other frequencies, just as we said previously was the case with a “tuned” circuit. It has a mechanical “resonance” effect. Any combination of mass and elasticity will act the same way—for instance, this is the reason why the prongs of a tuning fork display their property of vibrating at a constant pitch. Now in electrical engineering a choke coil or inductance has the same effect on an electric circuit that a weight, (or more properly a mass, such as a flywheel), has on a mechanical device, and a condenser plays the same part electrically that a rubber band, spring, or other elastic member does mechanically. An alternating current is nothing but a back-andforth movement of electricity in a circuit; and, therefore, by using a circuit consisting of a choke coil and a condenser, it is possible to create resonance effects, such as were just mentioned; fundamentally, that is all there is to a “tuned” circuit. In providing speed control for the motor, the first thing necessary is to have some means by which the motor can, so to speak, signal to the control box to show at what speed it is running. This is done very effectively by simply building into the motor a small A.C. generator, on the same shaft and within the same housing. This generator produces alternating current whenever the motor is running, of course, and the frequency of this alternating current will always be exactly proportional to the speed of the motor at each moment. Thus the A.C. generator acts as a pilot or speed indicator. When the motor is running at the standard speed of 1200 RPM, the generator is producing alternating current at a frequency of 1720 cycles per second—below 1200 RPM of the motor, the generator frequency is below 1720 cycles per second, and vice versa. (Concluded in next issue.) Keep Sound Gate Clean pee for the benefit of both old and new movietone projectionists, we caution you concerning the sound gate, just alittle accumulation of dirt upon the face of which may, and in all human probability will seriously injure or ruin a lot of sound band, which of course means a lot of film. Before you thread up, make it your invariable practice to run a finger over the surface of the sound gate with which the sound band comes into contact, and if the slightest roughness is felt, remove it by rubbing with either a moistened finger or cloth. Never scrape a deposit off. If you do you will roughen the polished metal and add to the likelihood of further deposit. A moistened finger or cloth is the only right way to remove such deposits. FLICKERS | Get the sound regulated for movietone before the show starts. When you experiment with the fader while the show is running the audience thinks every singer is trying to yodel. The _ constitution guarantees everybody life, liberty and the pursuit of happiness. Traffic laws are to keep people from pursuing happiness at a rate of eighty or ninety miles an hour. Back in-the-Bible-times-it rained forty days and forty nights to make a flood. A projectionist can give the bearings one in ten seconds by over-oiling. —— All the important men around an office are minute men. When they are out the stenographers always tell you they are expected back any minute. Thanksgiving Day is coming. That is the day on which you are supposed to eat turkey instead of talk it. There are sprocket idlers in your projection mechanism. They are the only kind of idlers supposed to be around a booth. Some women do not like a man with a past. All women like a man with a present. Many a young man points with pride to the house that Jack built. Father’s jack. They used to say that actions speak louder than words. But that was before movietone was invented. A merchant and an acrobat are a great deal alike. Both depend to a great extent upon the turnover. Whatever your problem, do the best you can. If that isn’t good enough, do better. Movietone talks like a human being. But unlike a human being it doesn’t talk to itself. Si