1. Field of the Invention
The invention relates in general to a method for generating a sub-beam add signal (SBAD) value, and more particularly to a method for reducing the difference between the read SBAD value and the write SBAD value.
2. Description of the Related Art
Living in modern times when science and technology advance rapidly and the applications of pictures, music, movies and computer software are gaining a greater popularity, high capacity disc such as CD, CD-R, CD-RW, DVD, DVD-R and DVD-RW have become an important software carriage to modern people in their everyday lives. Besides, the optical disc drives for reading/writing the optical disc are therefore become an essential peripheral of a personal computer.
As shown in FIG. 1A, a partial diagram showing the status after a conventional disc is formatted is shown. In the conventional method of formatting a disc 70, the optical disc drive would format the optical disc 70 as a lead-in region 71, a data packet write area 72 and a lead-out region 73, wherein the data packet write area 72 has a plurality of tracks for storing data with each and every of the tracks has a plurality of blocks disposed thereon. However, when parts of the tracks on the data packet write area 72 have defects such as scratches or fingerprints, the optical disc drive is unable to detect any defects on the optical disc 70 according to the above formatting method. It is very likely that the optical disc drive would have data written on defective blocks, causing incomplete data writing and largely affecting the quality of data writing.
To avoid the data being written on defective tracks, a “Mount Rainier” disc formatting method is provided. When the optical disc drive formats a disc according to the Mount Rainier method, the optical disc drive would first of all detect whether any defects exist on the optical disc. During the detecting process, the optical pick-up head of the optical disc drive would emit a Laser light having three or five beams onto the optical disc. Take the five-beam Laser light for example, the five beams comprise a main-beam and four sub-beams, wherein two sub-beams and another two sub-beams are symmetrically positioned at the two sides of the main-beam. The five beams are emitted onto the optical disc and form a corresponding main-spot and four corresponding sub-spots thereon. The optical pick-up head receives and senses the reflected beams of the four sub-beams reflected from the optical disc to generate a corresponding sub-beam add signal (SBAD).
As shown in FIGS. 1B-1C, the optical disc drive has a defect threshold range, whose range is −200 mV˜400 mV for instance, and whether the spot at which the Laser light is emitted has defects or not is determined according to the comparison between the defect threshold range and the wave pattern of the SBAD. When the optical disc has defects such as scratches or fingerprints, the optical disc drive would generate a downward-projected SBAD 75 or an upward-projected SBAD 76, wherein each and every spot of the downward-projected SBAD 75 or the upward-projected SBAD 76 corresponds to an SBAD value. In FIG. 1B, the downward-projected SBAD 75 has a lowest point A, corresponding to a minimum SBAD value, wherein the minimum SBAD value is smaller than −200 mV. In FIG. 1C, the upward-projected SBAD 76 has a highest point B, corresponding to a maximum SBAD value, wherein the maximum SBAD value is larger than the 400 mV. Therefore, the optical disc drive would determine whether the SBAD value falls within the defect threshold range so as to conclude whether the spot at which the Laser light is emitted has defects or not. The block with a defect is regarded as a bad block.
As shown in FIG. 1D, after the optical disc drive complete defect detection of the optical disc 80, the optical disc drive would format the optical disc 80 as a lead-in region 81, a data packet write area 82, a data replacement area 83 and a lead-out region 84. The optical disc drive creates a main defect table 85 in the lead-in region 81, and further backups the main defect table 85 in the lead-out region 84 to record the address or the time domain of a bad block 86 on the optical disc 80. Unlike the conventional method which would have data written onto a bad block 86 with defects disposed thereon, the optical disc drive would not do the same. According to the main defect table 85, the optical disc drive would have data written onto the data replacement area 83 and records the data's writing position on the data replacement area 83. Consequently, the completeness and reliability of data can be maintained.
During the process of having data written onto the optical disc according to the Mount Rainier method, the optical disc drive adopts a sequential write method and divides the data into N data packets. The optical disc drive has the N data packets written onto the optical disc during the N read status and N write status with every read status followed by a write status. During the N read status, the optical disc drive uses a read laser power to read the optical disc, uses a stand-by status to perform focusing servo control and holding servo, and then waits for next data writing. During the N write status, the optical disc drive uses a write laser power to have the N data packets written onto the optical disc. After reading the optical disc at the ith read status, the optical disc drive is immediately switched to the ith write status from the ith read status and has the ith data packet written onto the optical disc, wherein i ranges from 1 to N. Such write method is also called “packet write method”. Besides, when the optical disc drive is respectively at the read status and the write status, the optical pick-up head still receives the reflected beams of the four sub-beams reflected from the optical disc to generate a corresponding read SBAD signal and a corresponding write SBAD value.
As shown in FIG. 1E, when switched from a read status to a subsequent write status, the optical disc drive would vibrate under high speed rotation because the writing power of the laser light is higher than the reading power of the laser light, causing the SBAD level to upsurge at least 400 mV, resulting in a write SBAD value higher than the read SBAD value. For example, the write SBAD value and the read SBAD are respectively equal to 2.2 V and 1.7 V. When the optical disc drive compares the write SBAD value with the defect threshold range, the write SBAD value would fall outside the range of the defect threshold range, causing the optical disc drive to mis-determine that defects exist on the optical disc and result in an erroneous detection of defects. Therefore, the optical disc drive would have each and every of the data packets written onto data replacement area of the optical disc. Due to a small capacity, the data replacement area will soon be full loaded, causing write failure to occur.