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16-MM SOUND-FILM PROJECTION 379
However, during that part of the cycle when the film is in motion, the covering sections 'Dx' and 'E^ do not assist the intermittent in any way. The 'pull-down' must always take place within angle 'F', position '3', Figure 162, and must not commence before or continue beyond the boundaries of this angle. It should now be apparent that the effective dark and light periods on the screen will only be equal when the opaque sections of the shutter are slightly larger than the clear sections. In the example under consideration each opaque blade should be increased by an angle equal to 'Dx' if the dark and light periods are to be equal.
These considerations have been applied to a practical example and are shown in diagram 'B', Figure 162, in which a double bladed shutter such as would be employed in a sound projector to obtain a 48 cycle flicker frequency is seen to be symmetrically divided into four equal areas of 90° each. Since the closing and opening of the aperture must be of equal duration, these operations are performed by areas 'HOK', 'LOP, 'JON' and 'MOH' and, in this case, are all assumed to occupy an angle of 10° each. The periods when the aperture is completely open will therefore be equal to angles 'JOH' which, together produce a total 'open' period of 180°. The periods of partial opening or closing will therefore all be equal to angle 'HOK' and, since the change from 'open' to 'closed' occurs four times, these periods will effectively increase the showing time by twice the angle 'HOK', that is, by 20°. The total effective showing time is therefore 200°. The periods during which the aperture is completely masked will be equal to angles 'HOL' and 'NOM' which, together produce a total 'closed' period of 140° but, to this amount must be added the remaining two 'change-over' periods which amount to 20°. The total effective period during which the screen is dark is therefore equal to 160° of the total cycle.
Obviously, a total effective 'open' period of 200° and a total effective 'closed' period of 160° will not produce a balanced flicker frequency of equal periods of light and dark on the screen. This frequency can be balanced by increasing angles 'KOL' and 'HOM' each by 10° and, at the same time, decreasing both angles 'HOJ' by equal amounts. Under these new conditions the total effective 'open' and 'closed' periods are balanced at 180° each, whilst the completely 'open' periods are each 80°, the four 'changeover' periods are each 10° and the completely closed periods are each 80°.
It will be realised that the need to make this correction to double-shutter blades such as that shown at *B', Figure 162,