The vertical tracking angle problem in stereophonic record reproduction

At the time of the widespread introduction of stereophonic records in 1957, two cutter systems became available-one embodying a 23° vertical recording angle and the other a 0° angle. In 1961 the Engineering Committee of the RIAA recommended a 15° vertical tracking angle standard for stereophonic pickups. In the intervening years, however, the matter of vertical angles appears to have been neglected by most pickup makers in as much as a range of angles of 0° to over 40° has been reported, in the extreme cases audible distortion being noticed. A new factor has been added by recent CBS Laboratories' discovery that, because of lacquer and stylus springback, the actual slant of the recorded wave is considerably smaller than the recording angle. With 23° cutter normally used, for example, the actual modulation angle is near 0°. A special mounting and special recording stylus have been developed for providing the cutter with an added inclination of 14° which produces the desired 15° effective modulation slant. Modulation slants can be measured by microscopic measurements of dissymmetry of square-wave traces, by measurements of the shift of optical patterns, by measurement of distortion with the forward and reverse pickup orientation on a test record, and by placing the pickup in a normal and an inclined playing position. This latter method of measurement also yields the effective value of pickup tracking angle including the effect of any existing longitudinal elasticity of the stylus mounting.