1. Field of the Invention
The present invention relates to a modulated magnetic field generation circuit used in magnetic field modulation method magneto-optical memory apparatuses
2. Description of the Prior Art
Various data overwriting methods have been proposed to increase the data recording speed of magneto-optical memory apparatuses. One of these methods is the magnetic field modulation method (JP-B-60-48806). Data is recorded in this magnetic field modulation method by modulating the magnetic field applied from an external source according to the data being recorded. The magnetized direction of the recording film in the area heated by laser beam irradiation is modified, and data is recorded. The external magnetic field is generated using a magnetic head comprising a coil and core, or using a floating magnetic head (U.S. Pat. No. 5,020,041). A magnetic field modulation circuit having a constant current switching circuit is used to modulate the external magnetic field generated by the magnetic head according to the data. This magnetic field modulation circuit functions to change the current direction through the coil of the magnetic head, and composes two or four transistors or other switching devices.
Several problems are presented by this conventional magnetic field modulation circuit. When a switching operation is effected to change the direction of the magnetic head current, the current through the magnetic head will be changed with a delay determined by a time constant R/L (where L is the coil inductance and R is the DC resistance). Thus, it has been difficult to obtain a high frequency change of the electric current direction at the magnetic head.
Two methods have been used to decrease the current reversing time (A) reducing the magnetic head inductance, or (B) increasing the DC resistance of the magnetic head.
It is difficult to use method (A), however, because it is necessary to apply a magnetic field of sufficient strength to the magneto-optical storage medium. With method (B), it is necessary to increase the power supply voltage, and circuit power consumption becomes extremely high. These factors make it extremely difficult to reduce the size and power consumption of magneto-optical disk apparatuses using the magnetic field modulation method.