Projection engineering (Sept 1929-Nov 1930)

Page 26 Projection Engineering, April, 1930 imm 323ME REPULSION MOTOR pnnsi i CONTROL CIRCUIT speed of 33% r.p.m. so that each record can play through a whole reel. By using two projectors alternately a continuous program can be run through in the same way as in the silent picture. The end of each reel overlaps the beginning of the next and the new reel is brought in with increasing volume, or "faded in," as it is called, while the old one is faded out, keeping the total volume the same so that the audience is unaware that the change is being made. Film Run at Constant Speed An important point in reproduction is that the film must be run at a constant speed of 90 feet per minute past the sound attachment. That is the speed at which it was taken. A program can no longer be speeded up nor retarded at will by the operator, as was the case with silent pictures. Any changes from standard speed changes the pitch and causes distortion, therefore the film must be run at constant standard speed. The great majority of moving picture projectors are run by a-c. motors, although in some cases d-c. motors are used. The method of speed control in the case of a d-c. motor would be in the same way, or in a similar manner, as for the speed control of the a-c. driving motor described above. The speed control system used for a-c. motors will be described as applied to repulsion motors as these motors are the most widely used for this purpose, being well adapted for fine speed regulation and control. The control circuit is very similar, and in many cases exactly like the control circuit described above and shown in Fig. 2. Therefore we may FIG. 3 Repulsion motor and associated reactor. J consider the motor circuit and the d-c. output of the control circuit. Referring to Fig. 3, the control circuit is shown as lumped in a box with two output terminals. The armature of the repulsion motor is in series with the two outer windings of a threewinding reactor, while the d-c. output of the control circuit flows through the center winding. The a-c. flux circulates around the two outer legs and the d-c. flux flows from the center leg and returns around the two outer legs. In this manner the saturation of the two outer legs, and therefore the impedance of the windings on them to a-c, is dependent upon the d-c. flux, that is, to the d-c. output flowing in the center winding. In this case also, a small generator whose frequency is 720 cycles at 1200 r.p.m. is coupled to the motor. A change in the speed of the motor will, as in the former case, cause a change in the d-c. output of the control circuit. As explained, this will change the impedance of the two outer windings, and therefore change the impedance in series with the armature of the repulsion motor. This change of impedance will bring the motor back to stable speed, namely 1200 r.p.m. The feedback circuit is employed here also, in order that hunting will not occur. The governing action is not made too sensitive and compensation for static fluctuations is made as before by means of the feedback circuit. We have seen the dependence of the processes of sound recording and reproduction upon electrical engineering developments. Many kinds of apparatus developed for other purposes have been borrowed or adapted and studies having other objects in view have aided the commercial application of sound to motion pictures. Almost innumerable researches and studies carried on in the communication field have influenced the design of sound picture apparatus and the manner in which it is used. The sound picture has revolutionized the work of the scenario writer, of the director and of the actor. It has become necessary to thoroughly plan and rehearse each scene. Actors must memorize their lines and directors are no longer of the megaphone wielding type, but must remain silent during action. They must broaden their artistic and dramatic efforts to include this new technical branch. Sound engineers, with more electrical engineering experience than motion picture experience, have found it necessary to adjust themselves to new environments. To the motion picture producer, nothing is impossible. The entertainment value of a picture depends upon novelty and creative ability, and each picture reveals some limitations of the recording system which must be surmounted. The sound equipment must be adapted to every conceivable use. Due to the characteristic methods employed in the motion-picture industry and the financial resources and backing which this industry enjoys, the development of sound motion pictures is sure to be highly successful. Time has proved that the talking motion picture is not a novelty or a passing fancy, but that it is here to stay. The reception accorded to the talking picture by the public has been very enthusiastic and today all big pictures are "talkies." In the development of this new industry the electrical engineer is certain to have an important place. Acknowledgment The author wishes to acknowledge the very helpful material obtained from the following papers: "The Electrical Engineering of Sound Picture Systems," K. F. Morgan and T. E. Shea, A.I.E.E. Journal, October, 1929. "Sound Recording with a Light Valve," D. MacKenzie, Bell System Technical Journal, January, 1929. "Synchronization and Speed Control," H. M. Stoller, Bell System Technical Journal, January, 1929. "A Sound Projector System for use in Motion Picture Theaters'," E. 0. Scriven, Bell System Technical Journal, January, 1929. KODAK NO. 1 George Eastman invented the short, sharp trade name "Kodak." His No. 1 Kodak put on the market in the year 1881, was a box 6% x 8% inches containing a film roll which would take 100 circular pictures 2y2 inches in diameter. When all the hundred pictures had been taken the owner of the Kodak returned the machine to the factory for reloading. The Kodak sold for $25.00, and reloading cost $10.00 S. M. P. E. The Spring Convention of the Society of Motion Picture Engineers will be held at the Wardman Park Hotel, Washing ton, D. C, May 5-8, 1930. The Convention program in eludes a number of highly im portant technical papers and various attractive entertainment features. METROPOLITAN MOTION PICTURE CLUB At the March 28 meeting of the Metropolitan Motion Picture Club, New York, there was a discussion devoted to Spring filming with particular reference to filters and panchromatic film. Future meetings of the club will be held on April 25 and May 27, at the rooms of the Camera Club, 121 West 68th Street, New York.