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228 PRINCIPLES OF CINEMATOGRAPHY
one particular developing solution, one particular temperature and a known state of agitation). As the developing time increases the resolving power drops rapidly until, at 8 minutes, it has fallen to 40 lines to the millimeter.
Resolving Powers as high as 150 lines per mm. are now quite common but it is important to realise that these results, so necessary to the final sound quality, can only be obtained if the equipment and processing conditions are very accurately controlled to maintain a good average negative density, correct image contrast and accurate processing time and solution activity.
Variable Area versus Variable Density
A very natural question to ask at this point is 'which is the better system to use, Variable Area or Variable Density?' From the foregoing very brief outline of the growth of sound film recording processes it should be obvious that improvements made to one system are usually not possible with the other system until further research work has been carried out. It is therefore not surprising that for many years the scales were first weighted in favour of variable area and then, as some improvement was made to the other system, they turned in favour of variable density.
As has been shown, both systems are well able to record frequencies up to 9,000 cycles per second, both are now provided with means to reduce the ground noise as and when silent or quiet passages occur in the sound tracks, both are capable of providing 'push-pull' types of sound tracks for use with modern amplifiers and both are equally flexible in the methods employed to make duplicate copies, in editing and re-recording.
Perhaps the most amazing feature of sound recording as a whole is that recent developments in the moulding compounds and waxes used in the disc recording process have made possible the faithful reproduction on discs of a frequency range equal to and in isolated cases, even beyond that of the photographic methods ! However, in case some projectionists may be shuddering at the possibility of returning to the 16-inch disc, it should be added that no system of ground noise reduction, such as squeezing the grooves or raising the pick-up arm from the disc has yet been developed and, from this point of view, both the density and area film systems still have advantages over the latest disc recordings. On the other hand one must admit the possibility of magnetic recordings in the future and the very high signal-to-noise ratios already associated with them.