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PRINCIPLES OF CINEMATOGRAPHY
(although fifteen frames later than frame No. 20) is only two frames away from frame No. 19. This may be summarised as follows: Let it be assumed that all the even numbered frames have been extracted from a normal film and have been printed onto one reel, leaving spaces between each printed frame and into which the odd numbered frames would normally be placed. Now assume that all the odd numbered frames from the original film have been extracted and printed onto a second reel so that spaces exist between each printed frame and into which the even numbered frames would normally have been printed. Now assume that these two new reels have been moved with respect to each other, by a distance of fifteen frames, and have then been recombined onto one support.
It should be noted that each mechanism in the twin-head projector must move the film forward by a distance equal to two frames since, in the example given, the lower head will project frames 1, 3, 5, 7, etc., whilst the upper head will project frames 2, 4, 6, 8, etc. Also it will be necessary to make provision to tilt one head with respect to the other to ensure that both pictures are accurately aligned on the screen. Obviously, this re-arrangement could only be carried out with a specially designed printing machine and would only be possible on the final positive copies of a production although, once the required arrangement had been achieved, it could then be copied on any of the existing types of printing machines.
Should the film become broken it would be necessary to remove either two or four frames when making a join to ensure the alternating odd and even frame sequence were maintained. Whether the reduction in flicker and the increase in screen brightness, obtainable with such a system, would justify its installation throughout the Industry is very
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15 FRAMES ADVANCE
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Fig. 179.— Interlaced printing for flickerless projection.