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SOUND FILM PROCESSING 241
portionality this must represent the exposure through the valve when it is opened to twice its normal width assuming no reverse bias is obtainable and, as we have seen in Figure 103, such a point is 6db above the normal working point. So, if we move down the recorder curve to the left of point 'A' by a distance equal to 6db, we shall arrive at the maximum density and exposure we can permit at the unbiased working exposure of the valve.
We have now answered all three questions set at the beginning of this discussion and are in a position to forecast the shape of the final over-all characteristic curve for the entire system, as it would be obtained by normal sensitometric control. However, we must first list the following factors which will control this forecast :
(1) We shall start off with a 2b sensitometric control strip on the negative film.
(2) We know this must now be modified by the recorder characteristic.
(3) When this negative is printed the printing machine will have a gamma or contrast factor of its own and must be allowed for.
(4) The positive film itself will be developed to a certain 2b sensitometric control gamma.
(5) The film when projected will be modified by the projection factor.
The system of sensitometric control is therefore most easily visualised by examining a specific set of conditions. Such an example is given in Figure 105. In the lower right-hand quadrant 'A' is shown the starting point the 2b sensitometric curve after it has been modified by the recorder characteristic. In the lower left hand quadrant 'B' is plotted the characteristics of the printing machine thereby indicating the change in contrast due to the optics of the printer this usually produces a 'printer gamma' of the order of 0-90-0-95. In the upper left-hand quadrant 'C is the 2b characteristic curve of the positive film onto which the sound record was printed. The measurements of density in this quadrant are increased by the factor 1 • 25 to allow for the projector optical system. The curve in the right-hand upper quadrant *D' is only produced by tracing out a series of reference points through curves A, B and C as suggested by the 'arrowed' line in the figure and, where they intersect in 'D', plotting the over-all characteristic curve for the entire system.
This final curve 'D' is of positive film densities multiplied by
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