METHOD OF RECORDING ON MAGNETOOPTICAL DISC

Abstract: A magnetooptical disc is initialized (a) by reversing the direction of magnetization at fixed intervals (2, 3, 4..., 2n, 2n+1,...) and the direction of external magnetic field (H ex ) applied to it is reversed for each overwrite recording (c, e) by exposure to laser light.

Description: Background of the Invention This invention relates to a method of recording on a magnetooptical disc which allows recording, reproduction and erase. Currently, optical discs of read-only type are nearly in the stage of being practically applicable and those of add-on type are becoming available commercially. All these optical discs have large capacities and non-contact characteristics and they are easy to exchange. Magnetooptical discs have all these advantageous characteristics and have the additional advantage that data can be rewritten easily. Thus, they can be used not only where optical discs could be used before but potentially also where floppy discs and magnetic discs have been employed. When a magnetooptical disc is used as a code data memory, however, it used to be considered a problem that data could not be rewritten (overwrite) as easily as on a floppy disc or a magnetic disc. For this reason, a common practice has been, when it is desired to enter new data where old data already exists, to erase (initialise) the area first and then to write new data. This means that it takes twice as long in time as if use is made of a device with overwrite capability. Although it has been known (Applied Optics Vol. 23, 15th November 1984, article entitled "Digital Magneto-optic Disc Drive"), as an overwrite method on a magneto-optical disc, to modulate the magnetic write field while keeping the incident laser power constant on the recording medium, the inductance of the coil for generating the magnetic field tends to become too large by this method and since high-speed modulation of the magnetic field is difficult, the speed of data transmission becomes limited. Summary of the Invention The present invention aims to alleviate the aforementioned drawbacks of the conventional method of recording on a magneto-optical disc. In accordance with one aspect of the invention there is provided a method of recording information on a magneto-optical disc by exposure of laser light and application of an external magnetic field, said disc having sequentially defined positional intervals, said method characterised in that the steps of: initialising said disc by establishing an initial disc condition wherein the direction of magnetisation in said disc is reversed from one to the next of said sequentially defined positional intervals, and reversing the direction of said external magnetic field for each overwrite recording. In accordance with a second aspect of the invention there is provided a method of recording information on a magneto-optical disc by exposure to laser light in the presence of an externally applied magnetic field, the disc having sequentially defined bit locations, characterized by the steps of: dividing the sequentially defined bit locations into two discrete sets of non-adjacent regularly spaced bit locations, the bit locations of the two sets being interspaced one with the other; and recording data on a first of the discrete sets of bit locations while subjecting the second set of bit locations to clearing conditions, the first and second sets being switched for the next recording of data. Brief Description of the Drawings The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an embodiment of the present invention. In the drawings: Fig. 1 is a schematic drawing of a magnetooptical disc device, and Fig. 2 shows the magnetization conditions of the bits and recording pulses. Detailed Description of the Invention An example of recording method on a magnetooptical disc embodying the present invention is explained below initially by way of Fig. 1 which shows the structure of a magnetooptical disc device. Numeral 4 therein indicates a magnetooptical disc with a recording medium which includes a thin film of a rare earth-transition metal alloy and numeral 6 indicates a motor which provides rotary motion to the disc 4. Beams emitted from a laser 1 pass through a converging lens 2 and an objective lens 3 and are focused at a designated position on the disc 4 to heat a spot. Numeral 5 indicates a magnet to provide an external magnetic field to the heated spot for causing magnetization reversals. With this magnetooptical disc device, recording is effected, as by the usual method, by maintaining a constant magnetic field while modulating the laser power. The direction of the magnetic field usually does not change between the time of erasing and the time of writing. According to the method of the present invention, by contrast, the direction of the magnetic field is reversed for each writing. Fig 2. shows the conditions of the bits on the magnetooptical disc and the timing of recording pulses. As shown in Fig. 2(a), initialization of the bits according to the present invention means establishing a condition where the direction of magnetization is reversed at fixed spatial intervals. If address numbers are assigned in ascending order sequentially to the bit positions as shown in Fig. 2(a), it may be stated that magnetization is in upward direction at the 2nth positions and in the downward direction at the (2n+1)st positions, n being an integer. Fig. 2(b) shows the write pulse of data supplied to the laser when it is desired to write "1101011....". At this moment, the laser beams are made incident with the external magnetic field H ex in the downward direction. At the (2n+1)st positions, the write pulse is generated with the laser power on and the desired data is written at the positions inbetween. Let us assume, for example, that it is desired to write the aforementioned data "1101011..." by using the 4th, 6th, 8th,... bits. According to the method of the present invention, the laser power is on if the data is 0 and the laser power is off if the data is 1. Thus, the laser power is on in this example at the 8th and 12th positions but is off at the 4th, 6th, 10th, 14th,... positions as shown in Fig. 2(b). Since the coercive force H c of the recording medium is reduced where temperature is raised by the incident laser beam, the direction of magnetization becomes affected by the aforementioned downward external magnetic field H ex , the conditions of the bits become as shown in Fig. 2(c). One can see immediately that Fig. 2(c) represents the desired data written in the return-to-zero format. If the next data to be written is "11110100...". a write pulse as shown in Fig. 2(d) may be applied to the disc in the condition of Fig. 2(c) with the direction of the external magnetic field H ex reversed from the previous time of writing and hence pointing upwards. As can be noted readily, the write pulse shown in Fig. 2(d) is on at the 2nth positions and at those of the (2n+1)st positions corresponding to data 0. It is off at those of the (2n+1)st positions corresponding to data 1. The resultant conditions of the bits are shown in Fig. 2(e) which represents the desired data in the return-to-zero format and the data previously written at 2nth positions is erased. The procedure above can be summarized as follows: (1) Magnetization is reversed successively at spatial intervals. (2) At the (2m+1)st time of writing, magnetic field points upward and data is written at the 2nth positions while the bits at the (2n+1)st positions are erased. (3) At the (2m)th time of writing, magnetic field points downward and data is written at the (2n+1)st positions while the bits at the 2nth positions are erased. (4) If data is 0, power is on. If data is 1, power is off. (5) Reproduction pulses are treated as return-to-zero pulses. The method described above requires that the positions of the bits are accurately detected when write pulses are generated. These positions may be detected, for example, by providing timing marks before the data recording section. Track numbers and sector marks may be utilized as timing mark . These markers may be provided with information regarding the direction of magnetic field at the time of next writing. It is also an effective method to form a bit for timing on the tracks on the disc. Timing may be obtained by reading previous information before writing by using a multi-beam laser. When a multi-beam laser is used, a read beam may be added after a write beam such that the condition of a bit can be checked immediately after information is recorded. In such a case, erase, recording and reproduction can be carried out at once such that data can be processed much faster than by an ordinary magnetic disc when verify is required. In summary, the present invention makes high-speed data overwrite possible even with a magnetooptical disc and hence has the effect of making magnetooptical discs useful in more areas of application. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description.

Claims: A method of recording information on a magnetooptical disc by exposure to laser light and application of an external magnetic field, said disc having sequentially defined positions, said method being characterised by the steps of: initialising said disc by establishing an initial disc condition wherein the direction of magnetisation in said disc is reversed from one to the next of said sequentially defined positions, and reversing the direction of said external magnetic field for each overwrite recording. A method according to claim 1 wherein information is recorded at the 2nth of said positions in one recording operation and at the (2n-1)th of said positions in the next recording operation, where n is an integer. A method according to claim 1 or claim 2 wherein said laser light is a pulse with power on at those of said positions where data 0 is written and off at those of said positions where data 1 is written. A method of recording information on a magnetooptical disc by exposure to laser light in the presence of an externally applied magnetic field, the disc having sequentially defined bit locations, characterized by the steps of: dividing the sequentially defined bit locations into two discrete sets of non-adjacent regularly spaced bit locations, the bit locations of the two sets being interspaced one with the other; and recording data on a first of the discrete sets of bit locations while subjecting the second set of bit locations to clearing conditions, the first and second sets being switched for the next recording of data.

Inventors: AKIRA, TAKAHASHI, KENJI, OHTA, MICHINOBU, MIEDA, TOMOYUKI, MIYAKE, YOSHITERU, MURAKAMI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1986
Main classification: G11B13
IPC codes: G11B11, G11B5