Distortion-Balance Tests for Motion-Picture Soundtracks

Regardless of how different two soundtrack negatives are in terms of product, process or treatment, they will contain the same amount of cross-modulation distortion if they both optimize at the same density on a given print when printed on the same printer. Sound-film technicians employ this principle intuitively when they purposely distort a negative to optimize a print. The following important points are made: (1) image spread distortion increases with amplitude and frequency and causes a shift in average level; (2) negative and print distortions are opposite in phase and tend to cancel algebraically; (3) distortion-balance tests using pulsed signals (which may be prerecorded on magnetic tape) provide a practical means of optimizing sound prints; (4) percent distortion (rather than decibels) is plotted against print density for a given negative or against negative density for a given print; (5) by examining the phase of the 400-Hz distortion component, one finds whether a print from a given negative needs more exposure or less to achieve net cancellation; (6) interpolation and extrapolation are easy because the curves of percent distortion vs density constitute a family of straight parallel lines over the practical range; and (7) optimum print density can be predicted from a knowledge of percent distortion in the negative—which means that this distortion rather than the negative density is the important factor. These tests may be evaluated with a meter, oscilloscope, densitometer or even a common loudspeaker.

Print ISSN

0036-1682

Published

1977-09

Content type

Original Research

DOI

10.5594/J06761