Stylus, spherical

The most common stylus for low-cost players is cone-shaped with a spherical point having a radius of 0.6-0.7 mil (15-18 microns). While placing the stylus at an optimum position halfway up the groove walls, its contact span is too large to follow short wavelengths. When the span equals a half-wavelength it will trace an amplitude of up to 0.2 of that half-wavelength. The contact span is 1.4 times the tip radius, so for a 0.7 mil radius it is 0.98 mil. Frequencies corresponding to this half-wavelength are 9.8 KHz for the start, and 48 KHz for the end of the disc. Higher frequencies can be traced but at lower amplitudes. As the disc cutter is triangular, the groove width is narrower at the sides of the recorded wave than at the top and bottom. Thus the stylus is squeezed into spurious vertical motion twice for each cycle, so producing second harmonic distortion. An average of 6.4% harmonic distortion is produced at 8 KHz with 5 cm/sec recorded velocity. This is called pinch effect.