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16-MM SOUND-FILM PROJECTION 371
trolled by other factors such as the distance between the centre about which the shutter rotates and the picture aperture, that is, the diameter of the shutter, and the relative position of the aperture, the angle through which the eccentric claw pin rotates to completely engage the claw teeth, etc. In short, the final design of the shutter blade is controlled by the path traced out by the claw teeth.
If the claw shown at 'A', Figure 160, and the shutter shown at 'B' were fitted to a projector mechanism the duration of the 'pulldown' period during which the screen would be dark would be equal to the duration of the period in which the picture was projected and both would take place at a frequency of 16 or 24 cycles per second the rates at which silent or sound films are projected. Although such an arrangement would have faithfully reproduced the conditions under which the film was recorded, it would not lead to a screen picture free from flicker. The screen picture will only be relatively free from flicker when the frequency at which the still pictures are shown is raised to approximately 48 interruptions per second. It is important to realise that it is not necessary to change the pictures at this higher frequency indeed it is most essential that the rate of picture change should be equal to that which took place in the camera but merely to interrupt the showing of the individual frames at a higher frequency than that at which they are moved forward. To say that pictures will only appear perfectly free from flicker when shown at such a rate that 48 interruptions occur each second is not necessarily true since the appearance of flicker is also largely dependent upon the brightness of the screen. However, with any of the normal screen brightnesses now employed flicker will be below the visible threshold when such a frequency is employed.
The requirements which must be met in designing a projector intermittent mechanism and shutter blade are therefore dependent upon the type of film which is to be projected. For example, if silent pictures are to be projected the film must be moved forward at a rate of 16-pictures every second and, at the same time, the screen images must be interrupted at a rate of 48 times every second. If, on the other hand, the projector is intended for use with sound films, the film must be moved forward at a rate of 24 pictures every second whilst the frequency at which the screen images are interrupted must still be maintained at 48 every second.
In the first example, when silent films are being projected, any given frame or picture must be brought into the projecting aperture whilst the shutter is closed and must then be projected