Types of Optical Disk

a. Compact Disk

The most popular among all optical storage devices is the Compact Disk Read Only Memory (CD-ROM) type which are found in almost all computers. It is a non-volatile optical data storage medium using the same physical format as audio compact disk, readable by a computer with a CD-ROM drive. CD-ROM is popular for distribution of large databases, software and especially multimedia applications. The standard 12 cm diameter CD-ROM stores about 660 megabytes.

A CD-ROM disk is formed from a resin, such as polycarbonate. It consists of aluminum coated plastic, which reflects light differently for lands or pits, which are smooth or pitted areas, respectively, that are created in the stamping process.

Advantages of CD-ROM:

a. The storage capacity is enormous.
b. The storage cost is low and access time is relatively fast.
c. Data can be stored for an extended period of time.
d. It is reliable.

The major disadvantage is that CD-ROM cannot be erased and hence the disk cannot be reused. They are vulnerable to physical damages such as scratches (especially on the data surface). High temperatures can damage them and strong light sources.

b. WORM (Write Once Read Many)

WORM (Write-once read-many) is a variation of CD-ROM that allows a user to write information on each disk only once, with no subsequent erasing possibility. A low intensity
laser in the CD controller writes onto the optical disk (but only once for each bit location).
The writing process is normally slower than the reading process, and the controller and
media are more expensive than CD ROMs. The most popular size is 5.25”, which can
hold 200 to 800 MB of data.

c. Read and Write Optical disk

The users may read and write data many times on Read/Write optical disks. Usually, magneto-optical method is used to read and write data on this type of disks. Therefore they are also called magneto-optical disks. Magneto-optical disks can be erased and rewritten. The information is written into or read from the disk by means of the magnetic properties of spots on its surface. In reading, spots with different directions of magnetization give different polarization in the reflected light of a low-power laser beam. In the writing process, which erases all previous information, every spot on the disk is heated by a strong laser beam and is then cooled under a magnetic field. Thus, every spot is magnetized in one direction in other words, every spot stores 0. Then, eversing the direction of the magnetic field, only desired spots are magnetized in the opposite direction by a strong laser beam. The typical capacity is 650MB.

OPTICAL STORAGE DISK

Optical storage disk was developed from videodisc technology during the early 1980. These storage devices work on a principle similar to magnetic storage devices; however, they use light as a media to represent binary information. A very fine laser beam is projected on the reflecting surface to read data from the disk. By detecting the light intensity reflected from the surface, the information stored on the disk can be accessed. Optical disks have a greater memory capacity than most magnetic disks; the largest ones can store 1.5 gigabytes of information, which is equal to about 700,000 pages of printed material. Optical disks come in sizes ranging from 3.5 to 12 inches (30 cm). They are widely used as auxiliary memory when large memory capacity is required. Optical Disks can be found in these three forms:

Types of Optical Disk

a. Compact Disk

b. WORM (Write Once Read Many)

c. Read and Write Optical disk

 

Magnetic Drums

Magnetic drum acts as input/output medium for a computer. It has rotating cylinders, which are coated with a magnetizable material. The rotation rate of a drum can be much
higher than a disk as a result of a narrow cylindrical shape, and rotational delay is thus
reduced. The configuration of a drum is shown in Fig 8.8. The outer surface of the drum
is divided into a number of tracks. The top and bottom of the drum are not used for
storage, and the interior of the drum is not used for storage, so there is less capacity per
unit volume for a drum unit than there is for a disk unit. The transfer time for a sector on
a drum is determined by the rotational delay and the length of a sector. Since there is no
head movement, there is no seek time to consider. Nowadays, fixed head disks are
configured in a similar manner to drums with one head per track, but are considerably less expensive per stored bit than drums since the surfaces of platters are used rather than
the outside of a drum. The drums are nearly obsolete today and is being replaced by
fixed disks.