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SIGNAL-TO-NOISE RATIO 185
low-frequency delay and the high-frequency delay becomes apparent when compared with the 400-cps value. In commercial equipment of reputedly good manufacture, the ratio between the 30-cps value and the 400-cps value may be as high as 20 to 1.
For the present there is very little that can be suggested for the routine measurement and control of phase distortion and for correction of major defects in present-day 16-mm recording systems other than to suggest a more intensive stud\r of the problem by all concerned. In the past, phase distortion and its control has been very important in longdistance telephone transmission where appreciable time delay occurs. Although the maximum transmission time delay for 16-mm recording is but a small fraction of that encountered in a cross-country telephone line, the importance of phase distortion in the high-quality recording and reproduction of 16-mm sound films cannot be ignored, because of the subjective importance of transient and other sounds of sharp attack characteristics. The difference between a high-quality system with low phase distortion and an ordinary system is obvious to even a casual untrained observer when transient sounds such as dancing taps are recorded. Unfortunately there are no simple standard methods available for measuring phase distortion and for correcting it ; very little analysis of film processing as a source has been made although it has become more common in recent years for equipment manufacturers to investigate and measure phase distortion in their amplifiers. Much progress should be made toward improving sound recording and reproduction in 16-mm sound-films as a result of the more intensive efforts to analyze and measure phase distortion that will be made in the years to come.
Signal-to-Noise Ratio
The signal-to-noise ratio of a system is another important over-all characteristic. It should be obvious that the best signal-to-noise ratio available is that of the original event being recorded, since each subsequent step in the process reduces the signal-to-noise ratio because noise is added. This ratio of the final result (the sound projected into the auditorium from the release print) is usually very much smaller than that of the original event. The noise in the auditorium itself reduces the
subjectively localized in space at points distant from the mid-range frequencies ; such reproduction with excessive phase shift is quite harsh despite the fact that the measured distortion of the recording and of the reproducing systems may be quite low. (From Offenhauser and Israel, "A Study of the Advantages of Quasi-Binaural Reproduction Systems," 1940, unpublished.)