When a pair of tones (400 Hz and 4 kHz) is reproduced by a phonograph, mechanical nonlinear processes cause the 4-kHz tone to be frequency modulated at the 400-Hz frequency deviation using the CBS STR-112 test record is described and it is excplained how "real" and "imaginary" components of FM can be computed from these data. A mathematical analysis is presented which shows that tracking-angle errors produce only real FM, while tracing errors produce only imaginary FM. This makes it possible to deduce tracking angles and tracing radii from data on real and imaginary FM. Data are presented which show that precise values for tracking angles can be obtained in this way. In contrast, the values for tracing radii deduced from imaginary FM data differed significantly from values determined by optical means. It is conjectured that this discrepancy was the result of small longitudinal vibrations of the reproducing stylus caused by frictional drag forces.
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