The international photographer (Jan-Dec 1937)

Eight The INTERNATIONAL PHOTOGRAPHER February, 1937 Motion Picture Sound Recording CHAPTER XXX film, [E TURN now to a consideration of the film used in the sound recording machines. To |be a thoroughly qualified sound engineer, a man must be familiar with motion picture the processes of film development and printing, and the laboratory technique surrounding the measurement of gamma, as well as being precisely trained in his own duties relating to the operation and maintenance of the sound recording equipment. In this chapter we deal with the type of film used in motion picture sound recording. This is a matter of great importance, because if the highest quality of sound is to be secured it is necessary that the film as well as every other constituent of the sound recording line-up be as perfect as is humanly possible to make it. Positive Film Used for Sound Track The negative film stock used in motion picture cameras for the photographing of scenes is not the same as the film stock commonly employed in recording machines for recording the sounds accompanying the scenes. After exposure in a motion picture camera, the negative film stock is developed and printed on positive film stock, which in turn is 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 because it has less "grain" than negative film, and it is much cheaper in cost. A positive print is made from the positive film stock used for the sound track in the same manner that a positive print is made from the neagtive film used in the camera. 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 is printed from it. The "Grain" of Film A piece of film that is exposed to light 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 composed 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" of film. Although they are not large enough in themselves to cause trouble, the specks have the odd faculty 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 shined through the film. The effect is really one form of what is known as "hallation," and is termed "diffusion hallation." Fine Grain Essential 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 lowest 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 film, which is to say the finer the grain, the better the image will be. Effect of Grain on Sound The above is particularly true of a sound track of the variable density type (such as produced by an aperture of fixed dimensions and a sound lamp of varying intensity), because in such a sound track 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 reproduction will not be cleanly interrupted by the striations in the sound track. If the grain is too coarse, 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. Because of its composition, positive film stock requires a much greater exposure than the faster negative film emulsions, which are made up entirely of silver bromide crystals in place of the silver chloride crystals. From one to ten per cent of silver iodide is often included in these emulsions. Regulation of Sound Track Exposure The greater exposure to light required by positive film stock can be provided 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. But 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. Greater exposure of the film is easily accomplish