greater a forcing function is generated tending to send the spot toward the periphery of the disc if the received signal is greater the spot is directed to the center of the disc as the spot follows the spiral track properly the differential output tends towards the null for this example it is assumed that an appropriate mechanism drives the rotatable element 24 so that the arm moves in the radial direction at a predetermined rate the output of the integrator would then provide a correcting signal tending to correct the rate at which the arm is moving toward the center alternatively if the arm is to be driven entirely by the output of the integrator the convention observed is substantially immaterial if the bias signal being greater urges the spot toward the center of the disc then the spot will follow the track on the 34 inner 34 edge on the other hand if a greater bias signal drives the spot toward the periphery then the spot will follow the outer edge of the track in either case the error signal when integrated will provide an appropriate forcing function to the arm driver circuits so that the arm generally follows the track in fig5 there is illustrated the prior art optical detector electronics utilized and shown as fig1 in the previously issued gregg et al u s pat no 3 530 258 assigned to the assignee of the present invention for convenience the same reference numbers are used in gregg et al and herein a pair of photo detectors 96 98 are employed which in combination provide an additive information signal and when differenced an error signal which controls servo elements that redirect the reading elements as applied to the present.