Exhibitors Herald and Moving Picture World (Jul-Sep 1928)

September 29, 1928 EXHIBITORS HERALD and MOVING PICTURE WORLD 19 BETTER PROJECTION mtm* THIS article of my series on sound equipment will be devoted to the pick-up. It will probably prove more interesting than the articles which have preceded it. In it I shall explain just how the sound is "picked up," both by the sound-on-the-film and the record methods. First, let us examine into the sound-on-thefilm, as exemplified by the Alovietone process. It will prove to be a most interesting subject to my readers. As you all now know. Movietone utilizes a strip of the film about one-tenth of an inch wide, located on the right hand side of the film as it is threaded into the projector, for the recording of sound. This strip carries a portion of the picture in silent pictures, hence the sound-on-the-film picture has approximately one tenth of an inch less width than has the silent one, though its height remains the same. The strip upon which the sound is recorded is called the sound band. Upon this band, running crosswise thereof, appear the photographic impressions of any and all sounds reaching the microphone to which the camera taking the scene is attached. The sounds are in the form of lines of various width and photographic density, and in various shadings of density between them. Broadly speaking, the wider the lines the lower the pitch of the sound. Conversely the more narrow the lines the higher the pitch. For example, a trombone would make a series of wide, dark lines, as also, in varying degree, would a base singer. A tenor horn or a soprano singer would make a record consisting of narrow lines which because of their narrowness would not appear to be so photographically dense as the wider ones. The greater the contrast in density between the light and dark lines, the louder the sound. The problem of reproduction then is to transform these photographic representations of sound into electric current which will operate the diaphragm of a loud speaker in exactly the same manner that it was vibrated by the sound which cause the photographic record. I think you will all agree that the man who tackled that proposition certainly had a healthy nerve. It certainly was SOME considerable job. Let us now take a look — and see how it was — and is — done. The motion picture projector pick-up optical system consists of a straight coil filament light source, a condenser, a "slit," a projection lens, the sound band upon the film and a photoelectric cell, the arrangement of which is diagrammatically shown in Figure 1. The condenser, the slit and the projection lens are all contained in a metallic casing, which I shall, for the sake of convenience, refer to in this article as the slit assembly. This cell is sealed in fixed position so that the projectionist cannot alter its position. This is for good and sufficient reasons which will be fully explained further on. The exciting lamp is located in front of the condenser, which latter is a piano convex condenser about 5^-inch in diameter. It is for the purpose of concentrating the light in a "spot" upon the "slit," and in the side of the slit assembly is an opening so that the projectionist can adjust the slit spot to cover the slit properly. The "slit" is a most important item — a most By F. H. RICHARDSON In this issue, Mr. Richardson, technical editor of Better Theatres, presents the fourth of his series of articles on sound equipment. This month he discusses — The Pick-Up important one indeed. Its office is to project a straight, exceedingly thin, perfectly horizontal line of light which will illuminte each line of the sound band separately, without at the same time illuminating any portion of any other line or any space between lines, as they pass through the light line at the rate of eighteen inches of film per second. Let us examine into this matter a bit. As has been said, the lines of the sound band run crosswise of the film and are exactly at right angles to its length. For convenience I shall hereinafter refer to this as "exactly horizontal." It is readily seen that if the thin line of light to which I have referred is to only illuminate one line — no, I don't like that for some of the lines are much wider than the light line. Let's try again. It is readily seen that if the light line is to meet the coming edge of each sound line squarely, and thus illuminate no portion of the space between lines when it is on a line, it must be precisely horizontal to, or square with the lines themselves. * * * All right ! Now let's look at the slit. It is formed by two thin knife edges placed in exactlj' horizontal position. Remember how I use the word "horizontal" — to indicate its position with relation to the sound line on the film. The whole projector might be leaning a bit to one side, and thus throw the slit out of exact true horizontal, but that is not my meaning. The slit is placed so that it will be exactly square with the sound line. You know what I mean, even if it be not very well put. These knife edges are spaced one and one-half (I5/2) thousandths of an inch apart. The slit is of sufficient length so that its image when focused at the sound band by the objective lens is a little less than one-tenth of an inch wide. I cannot recall, nor do I seem to have any record of its exact length up here at South Lyme. You will see from this that we are projecting to the slit objective lens a horizontal line of light which starts out at one and one-half thousandths of an inch thick, increases in width in proportion to the diameter of the condenser and its distance from the slit (just as the light beam does in regular projection using piano convex condensers), passes through the objective and is focused at the film band with a reduction of one and a half to one, so that at the film band it is actually less than one and one-half thousandths of an inch thick and somewhat less wide than the film sound band. When it is understood with what infinitesimal measurements we are dealing, it is not w^mmm diflicult to also understand that this is _ no "monkey-wrench-and-cold-chisel" proposition, but one calling for great care and extreme exactitude of adjustments. I have told you that the slit assembly is placed in position and sealed there so that the projectionist cannot move it. From what has already been said you can understand that extreme accuracy is necessary in its placement. Were it removed by the projectionist and set with even the least bit of error, trouble would certainly ensue. If it be rotated ever so sHghtly from its former position, then the image of the slit would no longer be exactly square with the lines on the sound band, with result that two or more tonal effects would be set into action simultaneously, which of course would mar the effect. If the displacement be bad enough it would utterly ruin it. I think you Avill all agree with me when I say that the projectionist is not equipped to effect such a microscopical adjustment, and anyhow we would have no means of knowing when it was made exactly right. But that is only half the story. I have said that the image of the slit projected forward must be in sharp focus at the film band. If the slit assembly be moved forward or backward from its correct position it will of course throw the slit image out of focus, cause it to be larger, and to therefore cover more space than it should, with result that the sound will be blurred. I think I have said quite sufficient to justify the sealing of the lens assembly cell into place. The slit is very well protected indeed — so well that it seems impossible that dirt or oil can get to it.: However, since even the finest watch case ever made permits dirt to enter, we perhaps have no right to assume it will not in time also get into this cell. If you suspect that to be the case, advise your service engineer and let him attend to the matter. We have now considered everything up to the film. Let us go behind it and see what is happening. We have seen that the thin, brilliant line of light projected forward strikes the film, or to use correct optical terminology, is incident upon the sound band, in such a way that the rapidly passing lines upon the sound band are illuminated one at a time. This is evenly and exactly true, even though the action is almost inconceivably rapid. We know that the varying density of the lines and spaces between them represents the varying intensity, if I may use the term, of the sounds which made them. We also know that the sound band will absorb light in exact proportion to its density at any point, hence it follows that in the beam of light beyond the film we have light densities precisely representing the sound densities photographically impressed upon the film sound band. The problem now is to transform them into electric current of precisely equal values. We now arrive at the last element of thp pick-up system, namely the photo-electric cell. though just why that particular name was tacked to it I am free to say I don't quit? understand. It is a glass tube of peculiar shape, illustrated in Figure 3. As nearly as Correspondence for F. H. Richardson, editor of this department, should he addressed to South Lyme, Conn., Box 100.