A Variable Speed Turntable and Its Use in the Calibration of Disk Reproducing Pickups

The frequency response of a disk reproducing pickup, when measured by means of a conventional variable-frequency test disk is a function of the dimensional and physical properties of the disk, as well as of the pickup itself. This is because the effects of elastic deformation of the record material and of finite stylus tip size vary with the physical wavelength of the undulations of the recorded groove, and hence with frequency. If variations of test frequency are instead produced by variations of the rotational speed of a constant frequency disk, the above-mentioned effects are constant and do not affect the response characteristic of the pickup. A test procedure embodying this latter method is discussed, along with experimental results which have been obtained. A variable speed turntable well suited to this method of calibration is described. It covers a very wide continuous range of speeds, with excellent speed stability and low flutter.