Devices for Eliminating WOW in the Reproduction of Commercial Disk Records

Abstract

With modern record reproducing systems, a degree of fidelity of reproduction is attainable in which the wow inherent in commercial records is the limiting factor in the enjoyment of the reproduced music. In the manufacturing steps which lead to the stamper, the mechanical center of the record is lost, and is later re-introduced. In the re-centering, errors of 0.005 to 0.01 inch are typical, and occasional records have centering errors as large as 0.02 inch. At the inner grooves, a centering error of 0.01 inch produces a peak-to-peak frequency modulation of one percent, whereas 0.3 percent is detectable in a free field, and 0.03 percent is detectable in a room. Three different devices have been constructed whose purpose is to reduce the effect of the centering errors on the reproduction. All three employ the amplitude of the lateral oscillation of the reproducing stylus as the measure of the error to be corrected Two of them introduce a motion of the stylus along the groove which is equal in amplitude to, and in phase quadrature with, the lateral oscillation. The third device involves two servo mechanisms which center the record on the turn-table during the first few seconds of reproduction. The advantages and limitations of the three devices will be described.