International projectionist (Oct 1931-Sept 1933)

16 INTERNATIONAL PROJECTIONIST June 1933 but low production cost to recommend them, although they did serve a useful purpose in their time in projecting at low speed. Before the advent of sound-on-film commercially, the silents were photographed and projected at 60 feet per minute, or 16 frames per second. With the coming of sound on the film it became necessary to step up the film speed in order that the higher sound frequencies might be clearly recorded. This step-up in the speeds of the camera and recording instruments also necessitated an equivalent speed increase of the projectors so that the photographic action and sound pitch would be normal. We find this speed to be 90 feet per minute, or a step-up of SO per cent. Action of Intermittent With the Geneva movement type of intermittent the pin engages the slot in the star and twirls the sprocket a quarter turn, pulling down one frame. At a projection speed of 60 feet per minute it has been calculated that the pin strikes the star a blow of 85 pounds. This sudden shock is imparted to the film through the intermittent sprocket around which film is wrapped, one full frame engaging four perforations each side of the film, or eight perforations in all. Now, if we apply the formula that power requirements are the square of the speed, then the shock of the pin contacting the star must be tremendous at 90 feet per minute. Moreover, the speed of the film past the picture aperture requires a greater gate tension at 90 feet per minute than at 60 in order that the deceleration rate be sufficiently high to stop the film instantly, leaving the picture precisely centered in the aperture to prevent "jump" of the picture on the screen. This of course presupposes the intermittent to be in good order. No tension within the limit of the film can steady the picture and prevent "overshooting" or "undershooting" the aperture if the intermittent is "sloppy." Now return to the 16 mm. film and see what may happen to the single perforation operated by a single claw intermittent under the higher speeds and gate tensions necessary in sound work. The 16 mm. film has 40 frames per foot, and when printed from an old 35 mm. negative it is projected at a speed of 16 frames per second or a linear speed of 24 feet per minute. If we have a sound track on the 16 mm. film, whether re-recorded or reductionprinted from the 35 mm., it must be projected at a speed equivalent to that from which it was made, which in this FIGURE 2 Enlarged specimen variable density recording, on left case is 24 frames per second, or a linear speed of 35 feet per minute, which is, as in the case of the 35 mm, film, a step-up in speed of 50 per cent. Film Stress Agreed that speed and power are synonymous and that speed is obtained at the expense of power, and if we again apply the formula of increasing the power with the square of the speed, we find all stresses on prime mover, gear teeth, bearings and all moving parts of the projector stepped up enormously. This can be provided for in the specification of suitable motor, metals or alloys from which the several parts shall be made; but what choice is there with regard to the film which must be subjected to the additional stresses incurred at the higher projection speeds? Surely it will not be contended that the film plays no part in it. Consider what takes place when the claw enters the perforation at full speed on the pull-down stroke of the intermittent cycle. Under the impact of the claw a certain temporary deformation of the edge of the perforation occurs. The extent of the deformation depends, first, upon the load to be moved, which in this case is the static weight of the film p^us the gate tension. Second, the speed at which it is desired to move the load; and third, the shape of the claw surface at the point where it contacts the edge of the perforation. We shall assume that we have plenty of power to drive the mechanism. Analyze briefly the statements in the previous paragraph beginning with the assumption that we have a new film with commercially perfect perforations and a new projector in which the intermittent mechanism is of the single-claw type, film properly threaded and all set to go. The first downward thrust of the claw against the edge of the perforation will cause the temporary deformation mentioned, but due to the inherent flexibility of the film base, upon release of the claw pressure a return to original form takes place almost immediately. Continued repetition of this pull-down action induces fatigue or crystallization in the film base at the point of contact with the claw and the temporary deformation becomes permanent. The fatigue or crystallization is in reality a breaking up of the molecular cohesion in the acetate base at that particular point and it is quite generally known that the acetate base is not as tough as nitro base film. In other words, its molecular grip is not so tight. Two-Claw Movement Preferred The higher the gate tension, the greater the load and the higher the speed at which the load is to be moved, the quicker do the sprocket perforations tear out. It is believed that no single claw movement, however well made, can deliver adequate efficiency under the high speeds and gate tensions of present-day practice, regardless of how the pull-down or tension is applied. Film varies slightly in thickness even when new, and wear must take place along the sprocket tracks from the shoes or rollers, but the greatest wear is due to the inexorable pressure of the projection gate and the pull-down which must ' move the film under that pressure. If that pressure be applied to but one sprocket track rather than two, then that pressure must have sufficient deceleration or breaking effect to stop the picture precisely in the aperture if we are to have a steady picture on the screen. The two-claw movement can be made to give maximum efficiency because the gate tension will be divided between the two sprocket tracks, and the pull-down force will be divided between two perforations parallel across the film and in line with the tension. Wear on the film will be more evenly distributed and the film life extended from 200 to 300 per cent. There can be no valid objection to the use of a two-claw movement. It will cost more to build, but it is better. It demands better design and more accurate construction than the single claw. Any objection to the additional weight of the reciprocating parts as a cause of vibration may be dismissed by saying: "Build it light and right and let it run". Statements that the film perforations may not be parallel are not supported by investigations to date. Details of Projector Warpage and shrinkage in the film under certain conditions does occur and varies from V2 to 21/2 per cent. This is a variable difficulty of control and must be accepted for the present. Fig. 3 shows the laboratory model