Abstract: A controller for a disk drive measures track misregistration, extracts non-repeatable runout from measured track misregistration, determines one or more components of non-repeatable runout at various radial locations of a disk, and compares the determined components to known values at different altitudes to determine an operating altitude of a disk drive.

Abstract: The optical disc apparatus comprises a convergence lens 12, a light detection device 14 for outputting a first and a second detection signals after detecting reflected optical beam area by area, an actuator 13 for shifting the convergence lens 12, a gain balance circuit 30 for changing a gain balance of the first and the second detection signals, an addition circuit 123 for outputting the addition signal AS after adding the first detection signal to the second detection signal in response to the shift in lens position and the change in gain balance made by the gain balance circuit 30 and the actuator 13, and an addition signal measurement unit 361 and a gain balance adjustment unit 362 operable to obtain the addition signal AS from the addition circuit 123, specify the gain balance that equalizes the detection sensitivities in the respective areas of the light detection device 14 based on the obtained result, and control the gain balance circuit 30 that realizes the determined gain balance.