Projection engineering (Sept 1929-Nov 1930)

Page 18 Projection Engineering, July, 1930 arrangements, is also illustrated. This, or some adaptation of it, is the arrangement which has been adopted in nearly all sound-on-film systems of projection. The slit is moved away from its exposed position near the film, sealed within a lens system, made considerably wider and longer, say 0.004 by 0.4 inches, and then optically reduced to a light rectangle 0.001 by 0.1 inches on the film. With proper design of the optical elements the results are equivalent to the closeup slit system. Synchronism Synchronization is maintained in the Movietone, as well as in the other types of sound-on-film systems, by setting up the film with the proper loops, so that when a given picture is at the picture aperture the appropriate portion of the sound track will be at the light aperture in the sound reproducing section of the projector. An error of one or two frames is allowable, but beyond this the defect in synchronism becomes noticeable to observers. The proper separation of the picture and sound elements is taken care of in the printing of the film, the sound preceding the picture, since the sound head is below and a given point on the film reaches it after it has passed the picture aperture, by 19 picture frames or approximately 14.5 inches, so that both scene and sound are projected simultaneously. It can be seen that the main difference between Vitaphone and Movietone lies in the pickup apparatus used at the projector. In theatres where it is desired to use productions made by both methods the projector is equipped with both turntable and sound attachment unit. The apparatus is known as the dual sound projector system and is employed in nearly all of the larger existing installations. Apparatus is being manufactured in sufficient sizes to make it adaptable to all classes of theatres. The RCA Photophone System The RCA Photophone is the result of research and development in the laboratories of the General Electric Company. It resembles the Movietone, so far as the film itself is concerned, a difference being that the Photophone marks the sound track with an irregular divisional line, which charts the sound and thus leaves part of the sound track transparent. The difference can easily be seen by comparing illustrations of the two types of films. (Fig. 3.) It will be recognized that this form of recording is very much similar to that of an oscillograph. Studio Equipment The Photophone Studio apparatus used in the recording process consists of camera, sound recorder, an amplifier, a sound collector, and the necessary power supply devices. Standard cameras are used, except that a syn Fig. 3. A. Movietone. B. RCA Photophone. chronous motor is substituted for the hand-crank and a device is added to mark the starting and stopping points on the film. The optical system is somewhat similar to that used in the reproducing device. It consists of a concentrated filament lamp and a system of lenses that produce an image of a slit on the film. The variation of exposed area on the film is obtained by means of an electromagnetic galvanometer, similar to that used in oscillographs. The moving system consists of a current-carrying strip and a small mirror, so designed that the response is practically uniform over the entire range of speech and music. Each microphone used is connected to its own two-stage amplifier, and all are connected to a power amplifier which is capable of fine control. Only the microphones and cameras are located in the studio, the remainder of the apparatus being in a separate room. The method of recording used by Photophone is somewhat different from that used by Movietone. In recording, the Photophone system uses two films of different kinds in two different machines, one a standard motion-picture camera similar to that used in photographing silent motion pictures, and the other a sound recorder. The camera uses negative film, while the sound recorder uses a positive film. The aperture mask is used on the camera, as described in the Movietone process, so that the width of the opening in the camera is 0.125 inch smaller than in the silent pictures. The film is developed as usual, except that a narrow track 0.125 inch wide is left blank between the picture and the sprocket holes in the film. The dimensions of the film are shown in Fig. 4. As the picture is being taken a sound recorder is in operation ■ — in exact synchronism with the camera — and records the sound on the positive film which is of standard width. This sound recorder consists of two main parts, an oscillating galvanometer, and an optical system, as well as the motor-driven mechanism which supplies the motive power for carrying the film through the machine. The current from the microphone is amplified through the several stages of amplification which were already mentioned, then combined with the output of other microphones which has been niiiiiii iiiiiiiiiiiiiiiiiiiiiiiniiiiiiiin Fig. 4. Dimensional characteristics RCA Photophone positive sound film. Diniiiimniiiiimiiiiiiiiiiimiiiiiiiiiiiiii ii7J O U EMULSION SIDE UP .3471" - GUIDING--" EDGE h < -FILM DIMENSIONS .800" -PROJECTOR DIMENSIONS