Recording method for an optical information recording medium

Abstract: The present invention relates to a recording and reproducing method and apparatus for an optical disk (1) of the phase transition type that can be overwritten. According to this invention, the capacity of repeated rewriting of the optical disk (1) is expanded by recording either non-inverted write data (11a) or inverted write data (11b) at random. The deterioration due to the repeated rewriting at a start point and an end point of the recording is suppressed by recording a synchronizing signal before the write data and/or dummy data after the write data in the way such that thermal stress is less likely to occur. A record end position can be determined precisely by detecting a stop pulse to stop recording.

Description: The present invention relates to an optical disk drive for a disk that permits overwriting of a sector of the disk. As the above optical disk (optical information recording media), a phase transition type system using a chalcogenide as a recording thin film material is known well. This recording thin film is a photo-sensitive layer formed on a substrate, which transfers into a crystal state or an amorphous state by irradiation with a light beam such as a laser beam. The recording layer of the overwritable optical disk of a phase transition type can record data usually by using a crystal state for a non-recorded state and an amorphous state for a recorded state. The amorphous state is generated by irradiating a laser beam to melt the recording layer followed by rapid cooling down thereof. The crystal state is generated for erasing the recorded data by irradiating a laser beam at a lower power to raise the temperature of the recording layer. One of the merits of the phase transition type system is that only one laser beam is required and that the disk can be overwritten easily. If the laser beam power is modulated between two levels, i.e., a recording level and an erasing level, according to the write data, erasing old data and recording new data can be performed simultaneously by irradiating the modulated laser beam at the track of recorded data (JP-A-56-145530). The phase transition type optical disk having the above merits is widely used for recording (i.e., writing) and reproducing (i.e.,reading) document files, picture image files and other data files. The overwritable optical disk usually has a guide track in a spiral or in circles that is detected optically, for high density recording and for necessity of dispersed recording. The optical disk drive (i.e., optical disk reader/writer) irradiates a laser beam focused in a diameter of less than 1 µm to the recording layer on the guide track of the optical disk for writing or reading of data. It is common to divide a track into sectors to record variable length data effectively. Each sector that has usually a memory capacity of 512 bytes or 1024 bytes including the sector ID area including a track address and a sector address, and a recording area for writing and reading of data. An address portion that indicates a physical address of a sector is preformatted at the manufacturing stage. Usually, a recording format for data recorded in the recording area includes a synchronizing signal (i.e., VFO) portion for drawing in of PLL (Phase Locked Loop), a data head indicating mark (i.e. DM) added to the head of the write data as a kind of synchronizing signal, a modulated data portion and a resynchronizing signal for word synchronizing. In the process of data recording, an address of a target sector ID is read and the data is written into the recording area of the target sector after detecting the address. There are two recording methods, i.e., a pulse position modulation and a pulse width modulation. In the pulse position modulation, mark positions are detected for reading of data . On the other hand, both ends of marks are detected in the pulse width modulation. The pulse width modulation has an advantage in record density. The recording method of the phase transition type optical disk in the prior art is explained below, referring Fig. 4 and 5. Fig. 4 illustrates a block diagram of a reader/writer (optical disk drive) in the prior art. Fig. 5 shows write data, a laser power and a recording state of the optical disk for explaining the write/read operation. As shown in Fig. 4, a system controller 4 connected to a host computer outputs the write information as a binary signal. This write information is provided with error correction information, then encoded in a encoder 7a with e.g., 1-7 RLL. A composer 8 adds a synchronizing signal (VFO) to each data block to be written into each sector so as to generate write data 11a. A laser power controller 12 controls the laser housed in a optical head 3 to modulate the intensity of the laser according to the write data 11a. The system controller 4 also controls a spindle motor 15 to rotate the optical disk. If the strong laser beam (at laser power Pp) focused by the optical head 3 is irradiated to the recording layer of the optical disk 1 to raise the temperature of the recording layer above its melting point, the spot irradiated by the laser beam is melted and cooled rapidly, and assumes the amorphous state as a recorded mark 20. On the other hand, if the laser beam (at laser power Pb) is focused and irradiated to raise the temperature of the recording layer above the crystalization temperature but below the melting point, the recording layer at the irradiated spot assumes the crystal state. Data recording is performed in the above mentioned way using a difference between the crystal state and the amorphous state. Data reproducing(reading) from the optical disk is performed using the difference of the optical character of the recording layer between the crystal state and the amorphous state. A weak laser beam (at laser power Pr) is focused and irradiated to the optical disk and a change of a reflected beam is detected as a read RF signal 14 of the recorded data. Then, the signal is converted to a binary signal in a read signal processor 13; it is further processed for decoding and error correction to be a desired read information 6. A similar weak laser beam is irradiated to the optical disk for reading address information when the beam scans the address portion between sectors 18 of the optical disk during the recording process. However, it is known that repeated recording in a sector of the optical disk of the phase transition type may generate a deterioration that is unique to the phase transition type optical disk. This deterioration causes a reading error. The area of this type of deterioration usually spreads according to the number of times recordings is repeated. Three main patterns of the deterioration are as below: (1) a defect of the recording layer is generated in a record start portion of a chain of recording areas due to the repeated recording; the defect spreads backward (direction of laser scanning on the disk); (2) a defect of the recording layer is generated in a record end portion of a chain of recording areas due to the repeated recording; the defect spreads forward (opposite direction of laser scanning on the disk); (3) a defect of the recording layer is generated in an area where the same pattern mark train is recorded; the defect spreads forward and backward. Usually rewriting of the optical disk is performed by sectors. Therefore, the whole sector is rewritten even if the recorded data is only changed partly. Especially, a TOC (Table Of Content) area and a directory area of the disk are often written by similar data repeatedly. The above deterioration pattern (3) occurs in such areas. It is understood that these three patterns of deterioration are all due to a slow migration of the material that forms the recording layer in the laser scanning direction or the opposite direction. However, what drives the recording layer material to migrate during laser irradiation is not known. Some driving forces are conceivable such as surface tension due to a thermal gradient in the recording layer during the laser irradiation or a deformation of the layers making up the optical disk due to a thermal load. If the recording layer has deteriorated, the necessary reflection of the laser beam corresponding to the recorded or non-recorded state of the optical disk cannot be obtained. Some recording methods are proposed to solve these problems so that the performance of rewriting improves. For example, as a solution of the above deterioration pattern (3), there is an optical disk drive that can suppress the deterioration by altering the start position of the VFO recording at every recording (JP-A-63-229625). As a solution of the above deterioration pattern (2), there is an optical disk drive that can suppress the effect of deterioration of the waveform that spreads forward (direction toward the record start point) from the record end point by recording a fixed length of dummy data (JP-A-2-297724). In the above writing methods of the prior art, the start position of the VFO writing is altered at every writing, or the fixed length of dummy data is added to the end of the data block to suppress the deterioration of the read data due to the repeated recording. Therefore a dummy data area is necessarily added to the record area of the data block. This means that the recording capacity (byte number) is substantially decreased. The object of this invention is to provide a recording/reproducing method and apparatus that can suppress the deterioration of the overwritable phase transition type optical disk due to the repeated recording so that the number of possible overwriting is increased. In order to achieve this object, a first aspect of the recording method of the present invention comprises steps of deciding at random to invert or not to invert write data that has a predetermined block length; inverting the write data if the decision is to invert the write data; and recording either the non-inverted write data or the inverted write data into the photo-sensitive record layer. By this method, a concentration of the deterioration in a certain spot is avoided even if similar write data are overwritten repeatedly into the same area such as the TOC area. As a result, the deterioration due to the repeated rewriting is suppressed. The recording apparatus according to the present invention includes devices for performing the above recording methods. This invention will be described in detail by following embodiments referring figures, where Fig.1 is a block diagram illustrating the optical disk drive of a first embodiment according to the present invention; Fig.2 is a flow chart of the operation performed by the optical disk drive illustrated in Fig.1 for rewriting a sector of the optical disk; Fig.3 is a timing chart for the write data, the inverted write data, the laser power and the written state of the optical disk in the optical disk drive illustrated in Fig.1; Fig. 4 is a block diagram illustrating an optical disk drive of the prior art; Fig. 5 is a timing chart for the write data, the laser power and the written state of the optical disk in the optical disk drive illustrated in Fig.14; First embodiment A block diagram of an optical disk drive or an optical disk reader/writer (i.e. recording apparatus) of a first embodiment of the present invention is illustrated in Fig.1. A flow chart of the process to rewrite a sector performed by the reader/writer is illustrated in Fig.2. In step 201, the reader/writer detects ID (address data) of the sector to be rewritten. In step 202, a system controller 4 that is connected to a host computer outputs write information 5 in the form of binary data. This write information 5 is provided with information for error correction in step 203 and it is encoded in step 204. In step 205, each data block to be written in a sector is provided with a synchronizing signal (VF0) and other signals, so that write data 11a is made in a composer 8; the synchronizing signal is generated in a synchronizing signal generator 2. The above operation of this reader/writer is similar to the prior art; the following operation is different. As shown in Fig.1, the write data 11a is divided into two paths; one path connects to a selector 9 directly; another connects to a selector 9 via an inverter 10 that makes inverted write data 11b (step 207 in Fig.2). The selector 9 is triggered by detection of the address data of the sector to be rewritten. It selects the non-inverted write data or the inverted write data at random, and holds the selected data until the data is written into the sector. Therefore it is determined by the selector 9 in step 206 whether the write data is inverted or not before writing. If a reader/writer that accesses sectors of a optical disk (i.e. optical information recording medium) at random is used, selecting the non-inverted write data or the inverted write data alternately by the selector 9 can generate substantially the same situation as selecting at random. In this case there is no need to provide means such as a random number generator. An output of the selector 9, i.e. either non-inverted write data 11a or inverted write data 11b, is given to a laser power controller 12, that performs laser intensity modulation driving a laser housed in a optical head 3 (step 208). Thus the data string is written into the sector of the optical disk. Fig.3 shows an example of a time chart that includes write data, laser power and written state. Laser power when the inverted write data is selected by the selector 9 has an opposite phase to that when the write data 11a is selected, concerning the level of Pp and Pb. Consequently, amorphous written marks 20 are formed in the opposite phase to each other on the sector of the optical disk. Therefore, each position in the sector has substantially the same probability of being formed with a written mark, even if the same information is written repeatedly in a sector. Thus damage due to the repeated rewriting in the particular position does not occur. A concrete example will be explained below. The substrate of the optical disk whose diameter is 130 mm was made of polycarbonate. This polycarbonate substrate was preformatted with uneven phase pits as address information and guide grooves for writing were formed in a sector area. A reflection layer, a photosensitive layer and a protection layer were formed on the substrate. Then, a protecting plate was bonded thereto. The reflection layer was formed with Al; the protection layer was formed with ZnS-SiO 2 ; the photosensitive layer was formed with Te-Sb-Ge. The optical disk made by the above process was rotated at a linear speed of 5 meters per second. A laser beam that has a 680 nm wave length was focused on the disk for writing by using a lens that has the numerical aperture (NA) of 0.6. The laser power for reading and writing was adjusted to Pp=11mW, Pb=4mW and Pr=1mW. Pulse width modulation (PWM) with 1-7 RLL (Run Length Limited) was used for write data modulation. The shortest mark length and the shortest mark distance were set both at 0.6 µm. After rewriting the same information into the same sector repeatedly using the above conditions, the jitter value was measured. The measurement was performed in five different numbers of rewriting times, i.e. one, ten, a hundred, a thousand and ten thousand. The information written in a sector had 2970 bytes. Table 1 shows a comparison of jitter values in this example (example 1), in another example that will be explained later, and in the prior art. In this table, σ represents standard deviation of jitter values on zero cross point of read data signal; Tw represents the window width of the detecting system. Jitter (σ/Tw) Times of rewriting 1 10 100 1,000 10,000 Prior art 6.4 7.3 7.4 8.5 12.8 Example 1 6.2 7.1 7.0 7.4 8.3 Other example 6.2 7.2 7.1 7.2 7.6 As seen in Table 1, the jitter value in the prior art increases as the number of times of rewriting increases. However, the increasing rate of the jitter value in example 1 by this invention is much lower than in the prior art. This means that deterioration of the read data signal due to repeated rewriting is suppressed in this embodiment of the invention. If a method disclosed in JP-A-2-94113 that varies a write start point at random, and the method of the above embodiment of this invention are combined, deterioration of the photosensitive layer may further be suppressed, since the probability of forming a boundary between the area melted/hardened repeatedly and the non-melted area may be substantially uniform over the whole writing area. Such a configuration can be obtained by adding a delay circuit that can control the delay time by steps next to the composer 8, and changing the delay time at random in each sector rewriting. Jitter values of the "other example" in Table 1 are measured in this configuration. It is clear that the increasing rate of the jitter value in the other example is lower than in example 1. In this embodiment, written mark distance is not varied when write data are inverted, since pulse width modulation is used. Therefore, one decoding method can be used for both non-inverted write data and inverted write data, so the decoder circuit does not need to be complicated.

Claims: A recording method for an optical information recording medium in which a light beam is irradiated to change a state of a photo-sensitive record layer corresponding to binary data, characterised by deciding at random to invert or not to invert write data (11a) that has a predetermined block length; inverting the write data (11a) if the decision is to invert the write data (11a); and recording either the non-inverted write data or the inverted write data (11b) into the photo-sensitive record layer. The recording method for an optical information recording medium according to claim 1, wherein the binary data is written with a pulse width modulation. The recording method for an optical information recording medium according to claim 1, wherein a start point of recording in a sector of the recording medium is altered at random. A recording apparatus for an optical information recording medium in which a light beam is irradiated to change a state of a photo-sensitive record layer corresponding to a binary data, comprising means (7a) for encoding write information of predetermined block length into write data, characterised by means for inverting the write data (11a) to generate an inverted write data (11b); means for selecting either the non-inverted write data (11a) or the inverted write data (11b) at random; and means for controlling the light irradiation energy according to an output of the selecting means (9).

Inventors: AKAHIRA NOBUO, NAGATA KEN ICHI, NARUMI KENJI, NISHIUCHI KENICHI, OHNO EIJI, AKAHIRA, NOBUO, NAGATA, KEN'ICHI, NARUMI, KENJI, NISHIUCHI, KENICHI, OHNO, EIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1995
Main classification: G11B7
IPC codes: G11B7, G11B27, G11B20