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338 PRINCIPLES OF CINEMATOGRAPHY
reducing the photographic images of these sounds. This process involves the use of a 35-mm positive print which is scanned in a sound film reproducer so that the photographic variations operate either a light-valve or mirror galvanometer and so produce a new light modulation upon a 16-mm film. With such a re-recording process it is essential to reduce distortion of a photographic nature to a minimum.
The freedom from distortion and general response of any sound reproducing system is measured by its ability to maintain a constant undistorted output volume as the frequency range is increased. In the present case it is therefore possible to compare the power output from a speaker operated by a 16-mm sound track produced either by optical reduction printing or by electrical re-recording. Of particular interest is the output loss in each case as the recorded frequency is increased. The actual operations involved in the two systems are shown diagramatically in Figure 148. As previously stated, the optical reduction process involves three main stages which, if carefully controlled, will give rise to an overall loss of approximately 12db at a frequency of 6,000-cycles when using a particular recording and reproducing apparatus having scanning slits of a known width and employing a variable area type sound track. When identical recording and reproducing slits and variable area tracks are again used with the re-recording technique the overall loss in output at 6,000-cycles was found to increase to approximately 24db.
Although the difference in db loss between the two systems is proportional to the number of steps involved in each system it must not be concluded that each step is responsible for a similar degree of loss. In actual fact the second operation in the rerecording process that of producing the 35-mm positive print can be so controlled to compensate for some of the original losses inherent in the negative. However, the losses introduced by re-recording and, particularly, by contact printing from the 16-mm negative to the 16-mm positive only apply with certain types of film emulsions and, with the latest emulsions of much improved characteristics, such comparisons do not show such a considerable difference between the two processes.
Sound Lead Ahead of Picture
One final point which applies to all 16-mm films which carry both picture and sound records concerns the relative positions occupied by corresponding sounds and pictures along the film length. As is well known, that sound which is intended to synchronise with