u00b1 1 4 track range thereby maintaining the ac gain of the servo circuits constant over the entire displacement range which may occur during track following operations the particular servo pattern employed has the advantage that the linear portion of the quadrature waveform q commences when the linear portion of the normal waveform n ends and vice versa thus by applying a dc offset to the quadrature position error signal with or without inversion of the signal the linear region of the normal signal can be extended as shown for example by the dotted waveform q 39 in fig2 now with the head displaced to position p2 its position error voltage p2 39 is still linear with respect to head displacement and the ac gain of the circuit is constant the end of the linear region of the normal error signal is readily determined by detecting when the magnitude of the normal error signal n exceeds that of the quadrature error signal q when this is the case the quadrature error signal plus a dc offset is used to effectively extend the linear region of the normal error signal n the polarity of the required quadrature and offset voltage is determined by the polarity of.