Transient Response and Intermodulation Studies in Phonograph Reproduction
Transient response of phonograph pickups has been studied by means of an analogue circuit comprised of equivalent electrical elements driven by a square-wave generator. It was shown that a flat pickup response produces a slight transient while extended response with gradual rolloff beyond the upper response limit produces a good square wave. To test these theories and to determine the transient response of present day pickups, two new test records were recorded at CBS Laboratories. The STR-111 which contains square-wave test bands in the lateral-left-right-vertical modes; and the STR-120 which permits continuous and automatic measurement of pickup response to 50,000 cps with a General Radio Recorder type 1521A. It was determined that good square-wave response could be obtained with a pickup having extended response to 32,000 cps with little tendency to peak. Vertical tracking error between the modulation slant and the vertical pickup slant was found to produce dissymmetry in the length of the square wave, as may be predicted by the theory. An important and often overlooked effect of vertical tracking error is the generation of harmonic and intermodulation distortion. The STR-111 was provided with IM test bands in the lateral and vertical modes. The vertical bands are recorded at the newly proposed standard modulation slant of 15°. A number of pickups were measured with the STR-111 with the results indicating that some inexpensive ceramic pickups, having vertical angles close to the proposed standard, produce less IM distortion than some magnetic pickups which in general exhibit a much greater vertical angle. It is hoped that the new records will provide a measurement tool which will encourage improved practices in the design of phonograph pickups.
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