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IMPLICATIONS OF PERFORMANCE REQUIREMENTS 183
test may be made in accordance with American Standard Z22.511946 " Method of Making Intermodulation Test on Variable-Density 16-Mm Sound Motion Picture Prints." The tones used for the intermodulation test are 60 cps and 1000 cps.
In the case of variable-area sound track, the major distortion introduced by improper processing is envelope distortion. A somewhat similar testing method called the cross-modulation test is used ; different test frequencies have been selected due to the difference in the character of the distortion produced. This test may be made in accordance with American Standard Z22.52-1946 "Method of Making Cross-Modulation Tests on VariableArea 16-Mm Sound Motion Picture Prints." The test frequencies selected for variable-area testing are 400 cps and 4000 cps.
The data collected on intermodulation distortion tests and on crossmodulation distortion tests for 16-mm prints are too sketchy as yet to establish good correlation with the more customary listening tests, or to determine the relative amounts of subjective distortion when measurements by both methods yield the same numerical values. Such correlations must of necessity be established if we are to continue that desirable practice of altering the form from variable-area to variable-density or vice versa by re-recording* — as was very common during World War II. The practice, which has a number of operating advantages, will no doubt become more common in the near future, since Maurer, RCA, and Western Electric market equipment capable of turning out either form. It is also possible that the manufacturers of laboratory sound printers will market machines capable not only of copying the form of the sound track supplied, but also, by the simple shifting of a lever or similar adjustment, to alter the form in printing from variable-area to variable-density and vice versa. Such printers have already been described in some detail in patents issued and applied for.
Over-all distortion characteristics do not receive the attention they deserve. Such characteristics often will indicate a strong preference for one particular method of accomplishing a specific result over alternate methods. A typical example would be the re-recording of a negative
* Re-recording presumes a high-quality film phonograph that translates a highquality print or other sound-positive record into electrical currents suitable for electrically actuating an input channel of a recording equipment (replacing a microphone) for the purpose of obtaining a duplicate sound record, usually modified in responsefrequency characteristic, of the high-quality print or record.