Presented byRamchandra Regmi

Roll No. IT/12/096th semester

Sub:- Multimedia Technologies

IntroductionIn term of construction and basic

components, CD-ROMs are rather similar in most regard to other devices that use circular, spinning media.

The main different is way to store the information is recorded on the media, and the way that it is read from the media as well.

Standard hard disk use the magnetic principles to read and write data, CD use optical principles for this purpose.

Merits and Demerits Merits 1)Ease of use and durability2)Random access Capability as compared to

tapes.3)High storage Volumes.4)Data is not affected by electrical/magnetic

fields.5)Does not require maintenance or special

handling methods.6)Permanence of data is ideal for distributing of

error free software as well as music, movies, etc.

Demerits1)Access is slower than hard discs.2)Presence of dirt on the R/W head or scratches on

the media may affect efficiency.3)Normal CD-ROMs do not have write capability.

Cont..

Working Principle of Compact disc

Fig:- working principle of CD-ROM

Cont..

Consists of a Polycarbonate substance 120 mm in diameter and 1.2 mm in thickness. The Polycarbonate layer contains microscopic pits, each pit is 100nm in depth and 500nm in width.

The space between two pits is called lands.The polycarbonate substrate is covered by

reflective aluminum or gold to increase reflectivity and reflective surface is protected by a layer of lacquer to prevent oxidation.

The head is a lens sometimes called a pickup that moves from the inside to the outside of the surface of CD-ROM disk, accessing different parts of disk as it spins.

Cont..

A beam of light energy is emitted from an infrared laser diode and aimed toward a reflecting mirror.

A certain amount of light is reflected back from the aluminum layer behind the substrate layer, the amount of light is reflected depends on which part of the disk the beam strikes.

When the laser hits a land, it reflects cleanly off the aluminum coating, but when it hits a pit much of the light is diffused. The reflected light falls on a photo-detector that can sense the presence of land or pit by intensity of the light falling on it.

Cont..

Fig:- Logical Recording Format

A transition from a land to pit or pit to a land is interpreted as a ‘1’ and absence of transition is interpreted as a ‘0’.

Pre-recorded CDs available in the market are referred to as stamped CDs.

CAV and CLVIn Magnetic hard disk, the recording surface

is divided into concentric tracks and sectors, each sector contains 512 bytes.

To eliminate this wastage of space in (B) a CD, the tracks are not concentric but spiral outwards, so that the sectors are equal in area.

Fig:- Sectors on a magnetic disc and CD

Cont..

In a disk as it rotate at a constant Angular Velocity(e.g. 3600 rpm) the linear velocity of a point nearer to the center of the disk(e.g. in area A) is slower than the velocity of point nearer to the outer periphery of the disk(e.g in area B), because a point inside area A has to traverse a smaller distance than a point inside area B, within same time.

Hence, data bits(pulses representing) moves slowly past the R/W head in a than the bits in B.

However, the length of A being smaller than the length of B, this balanced out and R/W head take the same amount of time reading 512 bytes from A as well as B. This scheme is called constant angular velocity(CLV).

Cont.…

When the head is positioned near the center, the speed of the disc becomes faster and when the head is positioned near the periphery, the rotation speed become slower, so as keep the linear velocity of the data bits flowing past the R/W head constant.

Since, size of the sectors is the same, this implies that the time taken to read 512 bytes from A and B will be the same. This scheme is called constant Linear Velocity(CLV).

X Rated Speed

The actual data transfer speeds will depend on the various overhead involved:-

1)Speed change time :- Amount of time required to change the speed of the spindle motor as the head moves radially across the disc.

2)Seek time- Time taken to move the head to specific part of thee disc for doing a read operation.

3)Latency- Amount of time taken for the correct data block to come underneath the head.