The motion picture projectionist (Nov 1931-Jan 1933)

26 Motion Picture Projectionist December, 1931 Outline of Sound Recording By George Dobson "j" Having completed in our November issue his discussion of the principles and the mechanism involved in the recording of sound on disc, the author now takes up the subject of film recording. Both the variable density and the variable area methods are described. Consideration is also given to the subject of ground noise reduction.— The Editor. Part IV TURNING now to the film recording methods, ■ it is well to note that the details of these methods are largely determined by manner of reproducing sound from the film. This, as is well known to motion picture operators, is to shine a very thin ray of light (approximately .001 inch thick) upon the film and impress whatever part of the ray passes through the film upon a photo-electric cell. When the sound track on the film varies in density, the amount of light Fig. 12 passing through will change. This change will vary the flow of electricity through the photo-electric cell and produce the varying "current necssary to control the amplifiers and consequently the current which operates the loud speakers. Another method is to vary the amount of light passing through the film by use of a zigzag pattern. The two types of sound track are shown in Figure 12. Variable Density Recording For many reasons the variable density film has been more widely used than the variable area film. It has certain advantages, such as relative freedom from scratch troubles, but it requires a little more care in the processing of the films. In Figure 13 will be seen a schematic of the arrangement used to produce a variable density sound track on the negative film using a light valve. To the left there is a lamp, whose light is focused by means of the condensing lens upon a narrow slit. This slit must be so arranged that its final image (produced upon the film by means of the objective lens) is no wider than the ray of light used in reproduction. Practically, owing to halation and similar effects, it is desirable that the image be somewhat narrower in width than this, and, as a matter of practice, it is frequently made only half as wide (that is .0005 inch). The amount of light reaching the negative film is controlled by varying the width of the slit. In the flashing lamp method, the light valve is replaced by a slit of fixed width and the illumination of the film is varied by changing the current through the lamp and therefore the amount of light it emits. Owing to the difficulty in getting high illumination from flashing lamps, the negatives produced by the two methods differ appreciably in density; however, this is of no serious importance, since by proper handling in the chemical processing the final results, while not the same in appearance, will both give satisfactory sound. Details of Light Valve It may be of interest to look at the light valve itself, or rather at the light valve yoke, which is shown in Figure 14. It will be perceived that there are two narrow ribbons stretched across a small hole in a projection on the yoke. This projection is a pole face, which matches with a corresponding pole face on an electromagnet mounted between the two sets of lenses. The magnetic circuit of the electromagnet is completed through the frame of the light valve yoke, thus producing a very intense magnetic field between the two pole pieces. Thus, when a current flows through the ribbons, they will tend to move in the magnetic field and are so arranged that they will both move either closer together or further apart, depending on the direction of the current. The light from a lamp is focused by the condensing lens upon these two ribbons. Therefore, when they move closer together, less light passes between them and when they move further apart more light passes between them. This varying light is again focused by the objective lens so that when it reaches the moving film a thin ray of intense light comes t Commercial Engineering Dept., Electrical Research Products, Inc. Fig. 13 to exact focus on the emulsion. The variations in this light will, of course, affect the emulsion just as the variations of light from different objects affect the ordinary photographic film and there will be produced upon the moving film a pattern of light and dark which looks very much like a ladder with varying sizes of rungs and spaces. Reproduction of Record The negative thus produced is developed and printed upon a positive. The positive, when passed in front of a similar thin ray of light, will vary the amount of light which it transmits to the photo-electric cell so that the photo-electric cell will receive variations in light corresponding exactly to those originally produced by the motions of the ribbons of thelight valve. As already noted, when the flashing lamp is used, an aperture which would correspond to the light valve remains constant in width and the variations in light are obtained by controlling the amount of current in the flashing lamp. In order that theremay be produced on the negative an average grayness corresponding to the grayness produced when no current is passing through the light valve, it is necessary that the flashing lamp have an average current which increases and decreases in accordance with the sound waves. However, many difficulties are encountered in getting the same average exposure with the flashing lamp as with the light valve. Therefore, while satisfactory results are obtained, the handling of the two types of negative and two types of positive must be somewhat different. Variable Area Recording In producing the variable area film a similar thin ray of light is required which, however, moves to and froacross the sound track. When no sound is being recorded half of the track is exposed and the other half not exposed. When sound is recorded the excursions of the ray will vary with the loudness of the sound being recorded. Fig. 15 shows diagrammatically how this is done. The moving element which produces the motion of the ray of light is very similar in form to the oscillograph element which has been used for many years in making pictures of relatively high frequency currents. The ribbons or wires of this element are also placed in a strong magnetic field, but when a current passes through them they move in such a way as to rotate the small mirror attached to them. Since the variable area film re