2. Introduction
In 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.
3. Merits and Demerits
Merits
1) Ease of use and durability
2)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.
4. Demerits
1) 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..
6. 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.
7. 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.
8. 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.
9. CAV and CLV
In 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
10. 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).
11. 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).
12. 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.
