Projection engineering (Jan-Dec 1931)

Page 24 The use of positive film in sound recording By Charles Felstead* CONTRARY to general opinion, the film used in film sound recording machines is not of the same type as that employed in motion picture cameras. Negative film stock is used in the cameras for photographing pictures. After exposure this negatime film is developed and printed on positive film stock, which is likewise developed after being exposed to the printing light. But in the film recording machines employed for recording sound in a track near the edge of a strip of motion picture film, positive film is used. There are two reasons for this : positive film has less "grain" than negative film, and also it is less expensive. A positive print is made from the sound track in the same manner that a positive print is made from the negative picture film. Paradoxically, to differentiate between the positive film stock and the positive print, the positive film that is exposed in the film recording machines is called the "sound negative," because the final positive print is printed from it. Grain A piece of film that is exposed and developed has on it an image of the objects which reflected light against it and exposed it; but that image is not smooth and homogeneous ; it is com ' Sound Engineer, Universal Pictures Corpora posed of a multitude of minute specks that cannot be seen individually under even the most powerful magnifying glass. These specks constitute what is known as the "grain" in film. Although they are not large enough in themselves to cause trouble, the specks have the odd property of so reflecting light that they appear to collect in groups and form larger specks. These specks are crystals of solid silver formed on the celluloid film base, and as such they are covered with a multitude of little reflecting faces, or facets. A light shining on the crystals is reflected from one to another, back and forth, and cannot get through the film at that point because of the reflection and re-reflection of the light beam. This multiplies the apparent size of each speck many fold, and produces the optical illusion of a much larger black spot when light is thus directed through the film. The effect is really one form of what is known as "hallation," and is termed "diffusion hallation." As far as possible, the grain of the developed and fixed emulsion should be maintained extremely fine — or rather, the reflecting quality of the crystal facets should be reduced to the minimal amount by treating them chemically in the fixing solution to dull them — in order that the grain will not be evident in the film. Naturally, the more homogeneous and smooth the image on the An Explanation of What is Meant by "Grain" in Film, and Its Effects. PROJECTION ENGINEERING film, which is to say the finer the grain, the better the image will be. This is particularly true of a sound track of the variable density type, because in it the exposure lines representing the high frequency sounds are very narrow and very close together. If these lines are not smoothly and evenly recorded by the film, the reproduced sound will not be a faithful copy of the sound that was recorded, because the light beam used in the reproduction will not be cleanly interrupted by the striations in the sound track. If the grain is coarse enough, the reproduced sound may even have noise in it from that source. Positive film stock is not used in motion picture cameras because it is treated with a "slow" emulsion, made up mostly of silver chloride crystals with silver bromide crystals sometimes added carried in suspension in a solution of gelatine, which means that it requires a much greater exposure than the faster negative film emulsions, in which silver bromide crystals are used entirely in place of the silver chloride. From one to ten per cent of silver iodide is often included in these emulsions. This greater exposure can be accomplished by slowing down the rate of speed at which the film moves through the camera or recording machine, or by increasing the brightness of the light that causes the exposure of the film. The result in either case is practically the same. Since the speed of movement of the film is kept at precisely ninety feet a minute in the cameras and recording machines in sound picture work, this speeding up is not possible. But greater exposure of the film is easily accomplished in a film recording machine by increasing the intensity of the light source producing the exposure. However, it is not practicable to increase the light causing the exposure of the film in motion picture cameras because the illumination used on sets is already as intense as convenience and economy will permit. SPLIT MAT SHOTS IN SOUND RECORDING A paper on the above subject presented by L. E. Clark, at the recent S. M. P. E. meeting at Hollywood, discusses the technical problems which arise in motion picture studios and which must be solved quickly, cheaply and with the utmost simplicity. All engineering work must be done with an eye to production. As a result, the methods and equipment developed in the studios are frequently peculiar and highly unorthodox when viewed alongside scientific developments produced in the laboratory. The paper describes the problems which arose in one particular studio, as a result of the use of double exposure photography, and the methods, more practical than scientific, which were developed to meet these needs.