A magnetic head reads data from reference zones on tracks at differing ra of a magnetic memory disc rotating at constant angular velocity. Each track includes at least one reference zone, each of which includes a plurality of initial cells. Each cell has magnetic flux variations, which when read by the head result in a first waveform that includes undulations enabling the head to be properly servo controlled to a position over the center of the track and a further undulation that is distinguishable from the head positioning undulations. The initial cells are followed by plural binary bit representing data cells each having magnetic flux variations which when read by the head result in a second waveform including a data representing undulation that is generally the same amplitude as the further undulations. The undulations are susceptible to having varying amplitudes as the head is positioned at different positions of the disc. The peak amplitude of the further undulations, for one reference zone, is detected. In response to the detected peak amplitude of the further undulations a threshold for the data representing undulations is set and maintained while the second waveforms for the cells of the reference zone are read by the head. The set threshold and amplitude of the second waveforms are compared to enable the binary value of each data cell to be determined. A feedback look controls the gain of an amplifier that derives the signal to be detected for peak amplitude, except immediately after a change is ordered for a transition to read data from heads on opposite faces of the disc, at which time the gain is maximized.