2182480-A is referenced by 14 patents.
Apparatus for retrieving data recorded in the form of a pattern of surface deformations in the surface of a medium, such as an electrically conductive disc 2, comprises a probe 1 positioned to enable an electron tunnelling current 8 to be generated, when a small voltage is applied between the probe 1 and disc 2, a tracking means employing piezo-electric devices 10 and 11 and servo means employing a piezo- electric device 3 for controlling the separation of the probe 1 and disc 2 so that the electron tunnelling current 8 is maintained substantially constant, electrical signals generated as a result of operation of the said servo means, but lying in a frequency band substantially higher than that to which said servo means responds, being processed by the amplifiers 4 and 5 and indicative of the recorded data.; Probe height may be controlled by two piezo-electric crystals (13, 14) (Fig. 2) respectively responsive to lower and higher frequency signals. In (Fig. 3), current through hot wire (27) attached to hinged, turntable supporting, arm (25) effects movement of the arm relative to the probe, a feedback loop assuring control.