Exhibitors Herald World (Jan-Mar 1930)

40 EXHIBITORS HERALD-WORLD March I, 1930 SOUND PICTURES WOBBLES IMPEDANCE DUST By F. H. RICHARDSON BLUEBOOK SCHOOL QUESTION NO. 42.— [Another question, though I have again postponed giving an answer to a previous question in order to catch up on the replies. — F. H. R.] Are amphfying tubes connected in series or parallel? How is it that while a steady negative voltage is applied to the grid, the grid voltage is nevertheless fluctuating? In what way would you clean the studs of an amplifying tube? In certain types of equipment the amplifying tubes are held in suspension by springs. Why is this and what precautions are necessary? F JL ROM a small Nebraska city comes this query : "Dear Mr. Richardson: I want to thank both you and the HeraldWorld for the great help the Bluebook School, the Better Theatres projection department and the Bluebook have been to me. They are all far more valuable than I can tell you. "I have a bit of trouble, perhaps imaginary, but I think not, because others I have spoken to about it think they hear it, too. The sound is good, but somehow it seems to 'wobble' just the least bit, and quite fast. Can you guess what is wrong from that very poor description? I don't know how to describe it more clearly." Well, friend, the description is certainly not too good. My guess is that the sprocket which pulls your film past the sound gate is just a wee bit out of sound. Not knowing what sort of equipment you have I cannot suggest all the possible reasons, but first of all examine the sprocket very carefully— its face, I mean — using a magnifying glass, pref WOBBLES erably, to see if a bit of dirt or gum of some sort is not adhering to its face. If you find such an adherence, that would have the effect of continually slightly altering the speed of the film past the sound gate, and would produce exactly the "wobble" you describe. Should the sprocket itself be out of round — an almost inconceivable fault in modern apparatus, even of the cheaper sorts — then the effect would be the same, but it would be almost impossible to detect the fault without instruments of far greater precision than is possessed by the projectionist. However, you might do it, possibly, by fixing a thin strip of spring steel firmly in place, its end just in contact with the sprocket face. Then slowly rotate the sprocket, watching the spring to see if it binds more at one point than another as the sprocket is rotated slowly. The strip should be about two inches from its point of support to the sprocket. It should be set against the sprocket at four quartering points. That is to say, set it and test. Then with the sprocket rotated one-fourth of a revolution from where you set the spring for your first test, set the spring end again into close contact with the sprocket face, and so on. This is necessary because of the fact that if you happened to get the spring end set against the "high" point of the face the first time, it would not move as the sprocket rotated and the clearance between its face and the spring end might be so small as to be invisible to the eye at the "low" point. Of course, this test would be to determine a very small out-of-true effect. If the sprocket is out of true enough to be visible to the eye, the effect on sound would be very bad, indeed. If this "guess" as to the source of your trouble does not "click," then I am afraid you will have to describe it more clearly before I will be able to help you. I assume that you have no service man, else you would have had him locate your trouble. WATCH THE DUST CLOSELY IV^OTION picture sound projectionists owe it to themselves, to the producer, to the theatre management and to audiences to keep close watch on the matter of dust. Great care by the conscientious projectionist will be exercised in keeping atmospheric dust to the lowest possible amount in the projection room. It should be needless to repeat the fact that dust in the air gets into the projector bearing, where it combines with the oil and forms a mild grinding paste which wears mechanisms and gears very rapidly. However, that is by far the least part of the damage dust does, because it gets upon the films and just plain raises, (deleted by censor) with the sound track, besides doing serious damage to the picture itself. Dust also settles on battery tops and on the various electrical connections, where it may and very possibly will cause slight current leakage. What's that? Any such leakage would be too infinitesimal to amount to anything? Viewed merely as current loss, that is quite true. BUT while the loss in electrical energy may be very slight, oh, boy, what a lot of surface noise may be set up ! And the audience won't especially enjoy that, what? So, friends, keep that dust down. Get the boss to purchase a vacuum cleaner with wall attachment. Then USE IT on the floor every day. Once a week come a bit early and go over the walls and ceiling thoroughly. A lot of work? Well, yes, it is some work, but I guess your po'r old back won't bust entirely in two. Also, my boys, you are now receiving pretty fair pay and it is not paid you for the purpose of having you look wise. It is paid on the assumption that you will perform the duties of projection, and most emphatically keeping the projection room as free as possible from rust is one of those duties. And that's that, my boy, that's that! FROM SUNNY CUBA JL ROM the dealer in Simplex projectors at Havana, Cuba, comes this inquiry: "Due to a discussion with one of our customers, we are taking the liberty of asking for concrete, detailed information concerning the correct impedance, in ohms, of the Western Electric pickups used with Vitaphone records. Will you supply this information so that we may know whether the pickups are of high or low impedance?" The engineering department of Erpi advises me that the impedance is 500 ohms. This information was only supplied after consultation with the official data, so it may be depended upon as absolutely correct. We are glad to oblige Cuba — and at the same time supply the same information to many thousands of others who might wish to have it, not only in Cuba but also in many other countries. I believe I am well within the facts when I say that at least 30,000 projectionists and theatre managers read this department and the big one in Better Theatres, each issue. That estimate is what I have every reason to consider an understatement, rather than an overstatement.