| 摘要 |
PROBLEM TO BE SOLVED: To prevent electric power from being excessively consumed by accurately controlling a bias magnetic field at a focal point at a predetermined value. SOLUTION: The apparatus for controlling a magnetic field strength of a bias coil 27 at a focal point of a lens during recording on a medium includes a first controller 14 for determining a distance between the focal point of the lens and a reference point on the recording medium. A current driver 25 generates a stray magnetic field with a stray magnetic field strength at the focal point. A second controller 15 calculates a digital signal which is a function of the distance and the stray field strength. A digital/analog converter 16 converts the digital signal to a driver voltage and passes the driver voltage to the current driver, which drives the bias coil. Thus, the magnetic strength of the bias coil is controlled at a predetermined value.
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