Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1929 Page 13 erly mixed, are amplified sufficiently to operate whichever type of recording device is used. In general two separate film records are made, one for the sound and one for the photography. This permits a different technique of development for the two films, which is very desirable. Synchronization is accomplished by an interlocking electrical drive system, which contains mechanical filters in the drive of the sound film to maintain a constant and uniform rate of travel past the exposure slit. It is customary in both disc or photographic recording to make two identical sound records and subsequently to choose the better for printing the released positive films or preparing the playing records. The optical sound record, as printed on the films sent out to the theaters, takes the form of a strip about oneeighth of an inch wide along one side of the picture. The picture and the sound strip are printed separately on the positive film, the space occupied by one being shielded from the light while printing the other. Precautions Great care must be taken in selecting both the positive and negative raw film stock to be used in sound picture work. Any irregularities in the transparency of the film or emulsion will, of course, generate unwelcome "background" noises in the final reproduc tion. While the eye can barely detect a two per cent change in film density, sudden irregularities of only one-tenth of one per cent, will give rise to an audible background noise. The Photoelectric Cell In reproducing the optical sound record in the theater we employ another device used in the art of television, namely, the photoelectric cell. When subjected to a steady polarizing voltage the photoelectric cell allows a current to flow through it which is proportional to the intensity of light falling upon the cell. A narrow transverse slit is interposed between the cell and a constant source of light. This slit extends across the portion of the film carrying the sound record so that the intensity of the light passing at any instant into the photoelectric cell depends either upon the relative density of the banded film back of the slit or upon the width of the "cut-off" portion of the saw-tooth film record at the point in question. Thus if the film is drawn uniformly at the original speed across the slit we will obtain a pulsating current in the photoelectric cell circuit which will be a reproduction of the current in the studio microphones. Mechanical Filters When projecting a motion picture the film is advanced intermittently at the ra^e of sixteen frames per second. each • frame being stationary for the brief instant during which light is passing through it to the screen. Obviously, then, the synchronized picture and sound record can not be adjacent on the film. In practice they are spaced about fifteen inches apart along the film, thereby allowing for a "loop" to take up the intermittent slack between them. Mechanical filters are used in the drive to insure an extreme uniformity of motion past the photoelectric cell slit. The electrical impulses obtained from the photoelectric cell are extremely small in amplitude. A twostage resistance-coupled amplifier is ordinarily necessary to bring them up to an energy level comparable with that obtained directly from the electromagnetic pickup used in the disc method of reproducing. On account of the high impedance of the photoelectric cell circuit, it is desirable to build this amplifier into the same container which holds the photoelectric cell. The output of the amplifier, at low impedance, is then carried to the fader and, from this point on, the same speech amplifying system used with the disc method is employed. Likewise, the same technique of fader operation monitoring, and control of the out-put panel. In conclusion we wish to acknowledge our indebtedness to the Bell Laboratories Record for certain data used in this article. The Projectionist' s New Role Knowledge of Sound Equipment Is Important Consideration THE "Film Daily" sums up the case of sound pictures when it says : "The destiny of sound pictures, insofar as the public is concerned, rests weightily in the lap of the man in the theatre booth. The projectionist can make or break the show and therefore, his importance in the exhibition scheme of things should be realized." This should go on further and stale that the projectionist himself should realize the importance of his position and equip himself to get the mosl oul of the pictures which he is running on the apparatus at his disposal. But more essential than even this is his ability to be in a position to meet emergencies when they arise. This does not mean that the projectionist should be a qualified electrical-acoustic engineer, but he should be familiar with most of the fundamental principles in order to obtain the best results. But even more important than that the projectionisl should have the ability to think and think fast in an emergency, as hardly anything ever happens that cannot be *Oeneral Manager, Projectionist Sound Institute. By F. A. Jewell* handled in a temporary fashion to keep the show going until an engineer or a serviceman can be called in to rectify the trouble permanently. It seems from observation that this new sound game has the average projectionist baffled and when something happens, he becomes rattled in his frenzied efforts to do something and as a rule docs more harm than good. Just a Switch Unthrown A typical example of this follows. It is an actual occurrence, which cost the theatre a considerable amount and could have easily been avoided if the operator had used his head and not become excited. When the evening crew relieved the afternoon force, everything seemed to be in perfect working order. The right-hand machine was running on the first reel of the feature! and the sound was <>. K., but when reel two was faded over to the left -hand machine, there was no sound. By this time the afternoon shil'l had lel'i and the evening crew tried to Locate the I rouble. The exciter litrlit was lil and burning at the proper brilliancy. The tubes in the amplifiers wore all going. No loose connections could be found and all the other usual examinations were made, but with no results. All of this required time and the longer it went the more excited the operator became. Added to this was the manager's ravings while the audience hissed, clapped and stamped. Then as a last resort, the operator began to experiment by changing tubes, photoelectric cells and resistances in the coupling between the photoelectric cell and the amplifier and anything else that, he could think of. The rest of the story is too sad to relate as it requires bul little Imagination to realize what actually happened. When the sound engineer arrived, il was too late to save the show for that nighl so the best he could do was to undo all the damage thai the operator had done by bis experiments. Then an Investigation was made to determine the original cause of ••no sound" and Ibis is what was found. Previous to running the feature, a silent comedy bad lieen run and the music was furnished by a non-synchronous device thai played standard (Continual on ixifir 20)