High-Frequency Intermodulation Testing of Stereo Phonograph Pickups
A notable lack of instrumentation and techniques exists for measuring the performance of stereophonic phonograph pickups in the frequency range from approximately 7.5 to 20 kHz. While test records now permit convenient and accurate measurement of frequency response in this frequency range, such a measurement is not sufficient for evaluating the total performance of a pickup. Harmonic distortion measurements are not trustworthy, either because most pickups do not produce a usable signal at the frequencies at which harmonic components are generated or because the frequency response is highly irregular in the high-frequency range. The only test that shows promise of yielding significant results at high frequencies is the CCIF intermodulation test in which two closely-spaced tones of equal magnitude are recorded. In playing back this recorded signal any improper tracing or tracking or other nonlinear behavior in the pickup will be indicated by a tone at a frequency equal to the difference between the frequencies of the primary tones. A number of tests have been made using a test record on which the primary CCIF tones were 400 Hz apart and glided from a center frequency of 2 kHz to 20 kHz in synchronism with a chart recorder that plotted the magnitude of the difference tone as a function of frequency. Results that are discussed show the usefulness of this type of test in studying such factors as side thrust, pickup mechanical impedance, stylus-tip size, groove-wall deformation, and vertical tracking angle, all at high frequencies.
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