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232 PRINCIPLES OF CINEMATOGRAPHY
the developer and timing necessary to produce the variable density tracks and, thirdly, the type of developer and operating time necessary to produce the combined positive 'married print' carrying both sound and picture. It has, of course, been pre-supposed that the film emulsions have also been chosen to produce the required density, contrast and resolution under these conditions.
In our earlier considerations of general sensitometry it was found that strips of film carrying a series of increasing exposure steps were produced by an instrument such as the Eastman Type 2b sensitometer. A densitometer was then used to measure the result of exposure and subsequent processing so that the combination of exposure and resultant density could be expressed graphically, as the characteristic curve of the film, by plotting the densities against the logarithm of the exposures.
In sound film recording the magnitude of the initial exposure given to the negative should be directly proportional to the volume of the original sound frequencies. The final transmission through the positive film should also vary with equal proportionality to create a high fidelity reproduction of the original sounds. As has been shown previously, transmission is closely related to density in fact, the reciprocal of transmission, when expressed as a logarithm, is the accepted measure of density and we know that density differences between neighbouring areas in a film image depend upon the contrast or gamma to which the film is processed Whilst these considerations are more easily visualised in connection with variable density recordings, it is not surprising that density and gamma values used to control any sound processing must be held between limits considerably closer than would be acceptable in picture processing.
Quite large differences in tone range may be tolerated in the positive picture image before the eye will become critical, but small departures from the correct tone balance in the sound record will result in distortions in the reproduction which the ear will immediately detect.
Any sound contains two primary characteristics, (1) Frequency and, (2) Volume. In sound film recording the frequency is obtained by changing the amount of light which strikes the film. The volume is represented by the magnitude of these changes. This is more clearly understood from Figure 100, where a simple variable area low volume track is represented at T, a simple variable area high volume track at '2', a variable density low volume track at '3' and a variable density high volume track at *4\ Referring firstly to track T, the amount of light passing to the photo-cell in the