The simplest CDs are a CD-ROM with the following principle of recording information. Binary codes are recorded on a polycarbonate substrate in the form of depressions (holes) and flat areas located inside the concentric or spiral tracks of the disk. This relief is applied to the disc when it is manufactured in the factory by mechanical means. Hence the main disadvantage of CD-ROM is the inability to record new information on them. CD-ROM CD-ROMs have the following advantages: low cost, relatively large capacity - 600-800 MB, as well as their reliability and durability (theoretically up to 100 years).

The simplest CDs are a CD-ROM with the following principle of recording information

A significant shortage of CD-ROMs - the impossibility of recording did not have WORM CDs, which are also called CD-R. On compact discs of this type, you can write information only once, and reading can be done any number of times. Recording on CD-R, produced by the action of a high-temperature laser beam on a special photosensitive layer (green cyanine or yellow-orange ptalocyanine), which seems to burn out under this influence.

There are disk drives that allow you to perform multiple dubbing to CDs. This requires CD-RWs. Externally, CD-RW does not differ from CD-R and CD-ROM. But the principle of recording information on CD-RW CDs is completely different. On the metal (or other) basis is the working layer of a special material (silver, indium, antimony and tellurium alloy), which under the influence of the laser beam changes its state, passes from the crystalline state to amorphous or vice versa. Moreover, such a transition from one state to another can be performed many times. This process also provides the ability to repeatedly rewrite information on CD-RW CDs. Sites with a crystalline and amorphous state reflect light differently. This provides the ability to read information written on CDs in the manner described above!

You can mention the CD-ROMs DDCD (Duble Density CD) developed in 2000, which provide the ability to record on standard CDs of 1.3 GB of data.

Much greater opportunities for recording information are provided by DVD-ROMs (Digital Versatile Disk - digital versatile disk). The recording of information on DVD is performed on several layers, which are placed on the same working surface. The volume of disks reaches 17-27 Gbytes.

It should also be mentioned the very development of the company Installation 3D-FMD-ROM (Fluorescent Multilayer Disk), in which it is proposed to place up to 100 working layers with a total capacity of 150 GB on the working surface of a five-inch disk.