Tracing Distortion in Sound Reproduction from Phonograph Records

Abstract

When the spherical tip of an ideal reproducer stylus slides over a warped groove surface having a sinusoidal profile, the traced curve is not exactly sinusoidal. An analysis of the harmonic content of the traced curve, similar to that given by Di Toro but avoiding his approximations, is directly applicable to reproduction from vertical-cut records. These results may be applied to reproduction from lateral-cut records by taking the original groove surface as inclined approximately 45° from the horizontal, projecting the traced curve on the horizontal and vertical planes, and adding in proper phase the guidance of the stylus tip by both sidewalls. It is shown that there is a residual vertical component of stylus motion (“pinch” effect) and complete cancellation of all even harmonics in the tracing distortion. Computation of the remaining odd harmonics indicates that, when the ideal lateral-cut reproducer characteristics include ideal “following” for vertical motion at signal frequencies, a lateral-cut record may be reproduced with 4 to 10 times less r.m.s. distortion than a similarly recorded vertical-cut record. These results are displayed for convenient reference by contours of constant distortion on a universal chart whose dimensionless coordinates characterize any recording condition, and allow the immediate specification of the maximum permissible recorded amplitude, maximum permissible predistortion of the frequency characteristic, and the required clearance angle of the recording stylus.