Explain the working of DVD-ROM with the help of block diagram. 4m Jun2008
CD-ROM and DVD-ROM
Optical disks use Laser Disk Technology, which is the latest, and the most promising technology for high capacity secondary storage. The advent of the compact disk digital audio system, a non-erasable optical disk, paved the way for the development of a new low-cost storage technology. In optical storage devices the information is written using a laser beam. We will discuss here the use of some optical disks such as CD-ROM and DVD-ROM devices that are now becoming increasingly popular in various computer applications.
DVD-ROM (Digital Versatile Disk Read Only Memory):
DVD-ROM uses the same principle as a CD-ROM for reading and writing. However, a smaller wavelength laser beam is used. The total capacity of DVD-ROM is 8.5GB. In double-sided DVD-ROM two such disks are stuck back to back which allows recording on both sides. This requires the disk to be reversed to read the reverse side. With both side recording and with each side storing 8.5GB the total capacity is 17GB.
In both CD-ROMs and DVD-ROMs, the density of data stored is constant throughout the spiral track. In order to obtain a constant readout rate the disk must rotate faster, near the center and slower at the outer tracks to maintain a constant linear velocity (CLV) between the head and the CD-ROM/DVD-ROM platter. Thus CLV disks are rotated at variable speed. Compare it with the mechamism of constant angular velocity (CAV) in which disk is rotated at a constant speed. Thus, in CAV the density of information storage on outside sectors is low.
The main advantage of having CAV is that individual blocks of data can be accessed at semi-random mode. Thus the head can be moved from its current location to a desired track and one waits for the specific sector to spin under it.
The main disadvantage of CAV disk is that a lot of storage space is wasted, since the longer outer tracks are storing the data only equal to that of the shorter innermost track. Because of this disadvantage, the CAV method is not recommended for use on CD ROMs and DVD-ROMs.
What is the significance of FAT? What are the limitations of FAT 16? 4
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