Compact Discs (VD), Digital Discs (CD), and CD-ROMs for computers belong to this category. A write-once optical disc means that the user can write in one time and read it many times, but it cannot be erased. The data to be modified can only be recorded in the blank space on the disc, so it is also called a write-once optical disc. It is suitable for large database systems that do not need to be modified. Eraseable optical discs are divided into magneto-optical discs, phase-change type and organic pigment type. The first-generation machine for erasable optical discs was a rewrite type, and the second-generation machine was a rewrite type. Magneto-optical storage, which has always occupied an important position in erasable optical discs, has undergone fruitful development and research in recent years, and various manufacturers have begun to establish production systems. Among existing memories, the recording density of magneto-optical disks is the highest, which can achieve 108 bits/cm2. From the current published magneto-optical disk single-sided capacity, the diameter is 300 mm, and the capacity can reach 1000-4000MB; the diameter is 130 mm (5.25 inches), and the capacity is 300MB; 86 mm (3.5 inches), the capacity is 100MB. Therefore, in terms of recording density and storage capacity, magneto-optical disks have greatly surpassed magnetic disks. However, the high-speed performance of magneto-optical disks is far lower than that of magnetic disks. Magneto-optical disks are mainly used in image databases, moving image databases, production of cartoons, plate-making systems, digital audio disks, and data storage in computers.
