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SOUND FILM PROCESSING 245
density recordings even when the entire process is maintained at the levels indicated by normal sensitometric control.
Some new system of measurement had clearly to be devised whereby these small errors could first be detected and then eliminated. In both recording systems this requirement has been met by electrical analysing processes. Very briefly these processes consist of first arranging to record two pure frequencies, one relatively low and the other relatively high. The high frequency may be termed the 'carrier wave', and this is modulated by the low frequency in a manner similar to that by which radio waves are transmitted through the ether. By using a suitable wave-form analysing equipment it is possible to measure the percentage of each frequency which is present in the final reproduction and, fortunately, it is possible so to arrange the systems that the response will only be at a minimum when the optimum processing conditions exist and the small errors, not previously detected by normal sensitometry, have been eliminated.
The wave-form analysing system applied to the variable density sound recording process is known as Tntermodulation' and that applied to the variable area process is known as 'Cross Modulation'. Since it is more convenient to discuss wave forms which are recorded in their convential shapes, we will first consider the cross modulation method applied to maintain optimum processing conditions when dealing with variable area sound tracks.
Cross Modulation
Let us first consider the growth or contraction which has been found to take place when the sound track is over or underdeveloped. The pure undistorted wave form which should theoretically be produced under perfect processing conditions is shown at 'A', Figure 107. If the film is over-developed the troughs of the waves, shown at 'X', will be filled in due to excessive image growth or spread and the processed track will appear as seen at 'B'. If the film is under-developed the peaks of the waves, shown at 'Y', will contract and the processed film will appear as seen at 'C\
Since the wave form shown at 'A' is perfectly symmetrical it is possible to measure the amount of light passing through a very thin slit at 'D', and to add to that quantity a similar measurement made at 'E'. By dividing this total by two we arrive at the average amount of light which will be transmitted and this is represented in the diagram by the height 'F'. It will be seen that point 'H' will be mid-way between the trough and the peak of the wave form, as