Projection engineering (Jan 1932-Mar 1933)

Page 14 PROJECTION ENGINEERING A re-recording machine for sou nd fi ms By J. J. Kuhn: A MODERN sound-picture film as "released" usually bears little resemblance to the films initially taken of the production. It is rather a modified composite built from sections of the original films with the addition of various accompanying sounds such as rain, thunder, trains, revolver shots, or orchestral accompaniments. Not only are these other effects added, but the volume levels of the various sections are adjusted to make the whole fit properly together. The building up of a complete sound picture for release is thus an art in itself, and requires apparatus and a technique distinct from those of the original taking. In the construction of the final "released" picture, sections of the sound tracks of preliminary films, corrected for level, are transferred electrically and in the desired order onto a new film. This requires a machine to convert the variations of the sound track of a film into equivalent electri jBell Laboratories Record, March, 1932. *Special Products Engineer, Bell Telephone Labs., Inc. Fig. 1. On the rear of the new re-recorder are the photoelectric cell cabinet, the gear box, and the spring-damped flywheel of the sound sprocket. cal currents to be used not by loudspeakers but for the recording of a new sound track. The machine that does this, known as a re-recorder, thus plays an essential part in the making of the final form of the picture. Such a piece of apparatus, embodying all the refinements and improvements that a wide but rapidly acquired experience has made possible, has recently been developed by the Bell Laboratories and is being offered to the industry by the Electrical Research Products, Inc. Requirements placed on it are more severe than those demanded of a reproducer since in the process of re-recording, of which there may be several successive steps, the variations introduced by -the apparatus are cumulative. The reproduction must be as nearly perfect as is commercially possible. Front and rear views of the new machine are shown at the head of this article and in Fig. 1. Mechanically there are three major sect i o n s — a film magazine above, a sound reproducing head beneath it. and a driving motor at one end, all mounted on a substantial cast iron base. The film magazine compartment has space for two reels of either one or two thousand feet capacity. One is the supply reel and the other the takeup reel. Beneath it is the re producing head which includes a sound lamp with the necessary optical system for focusing the light on the sound track, and a photoelectric cell for receiving the modulated light beam. To the left of the reproducing head, when viewed from the front, is the driving motor which is arranged to be synchronized with the recording machine with which the re-recorder is being used. Details of the reproducing head are shown in Fig. 3. The drive shaft from the motor enters a gear box, which is integral with the head but faces the opposite direction, and through suitable gears drives two sprockets within the head. One, in the upper left-hand corner, is a combination pull down and hold back, and the other is the sound sprocket, which draws the film across the light beam. These are the only driven sprockets in the compartment. The film, coming from the supply reel of the film magazine, is held against the pull down sprocket by a double pressure . roller, then passes around an idler roller directly beneath it, and then around a drag roller which maintains a constant tension on the film as it passes up through the sound aperture. From here it passes over the sound sprocket, by the other side of the pull down sprocket, and then before passing back to the film magazine runs over the roller of the signalling device. This device, evident in both Figs. 3 and 4, gives an alarm if the film should break. Like most of the other rollers in the compartment, the drag roller is of stainless steel and accurately made to insure that it runs true and with constant tension. Flanges on each side guide the film along the proper path, and tension is maintained by friction discs bearing against the sides. Tension may be controlled by adjust