CD/DVD Read & Write Structure

Submitted: Murtaza Mohammady, Danish, Shabir, and Samiullah

Subject: Computer Organization and Assembly language

Supervisor: Naeem shinwari

Abstract

In this seminar we will discuss about popular storage devices which named

CD/DVD they are basically optical disks generally having a capacity of data

storage. How data read and write on them. Through this short article we made our

seminar. The methodology that we use in this seminar is gathering information

from various websites that include efficient information, pictures. At the end we

discover that the discs are not only flat rounded they has different physical layers

which do specific tasks which we can’t see them

Keyword

CD (compact disc), DVD (digital versatile disc), CD/DVD physical structure,

Reading data from CD/DVD, Writing data to CD/DVD

Introduction

The main topic of our seminar is that how can we store data or read data from optical

disc. Before that we should know about these discs. CD is a quite popular storage

media. It is basically on optical disc generally having a capacity of data storage

equivalent to 700mb. While CDs are played they rotate around and a laser beam is

used to read the data. to store data on CD, they need to be burned. Through this

acritical we will discuss what this burning means and how actually data is written

on the CD. The logical structure of a compact disc (CD) or Digital Versatile disc

involves various writing techniques and the physical structure shown that how a data

can be read from these discs. CD/DVD store information in series of 0s & 1s these

1s and 0s are represented by millions of tiny bumps and flat area on the disc’s

reflective surface. How bumps and flat area store 0s and 1s. How a driver read this

data and change this 0s & 1s to humans under stable format and how a data can be

store in disc which methods is in used. In this research we want to discover these

main articles.

Scope

Optical discs which are the most popular storage devices. Has 3rd generation up to

now.

1st generation was Compact Disc (CD) which invented in 1982

2nd was Digital Versatility Disc (DVD) in 1996

3rd is Blue Ray which is the last one in 2006

And the time of riding the 4th generation is still in development with Ultra HD blue

ray. In the near future we want to discover the 4th generation characteristics, physical

structure, read & write speed, data store capacity and other efficient information

about technology.

Research Methodology

The methodology that we used for this efficient presentation is gathering

information from various websites which all of them are reliable. And observed from

experienced professoriate. This method also includes qualitative and quantitative

because the information that we gather all of them are from valid websites. This

presentation include text, pictures and a short video clip about our main title. That

make this presentation more efficient and under stable.

Data analysis and discussions

Our main discussion was about CD/DVD read & write structure which is a very

important discussion in optical media title. We discover the beauty of optical discs

that how a disc which is one of the most important storage devices store data in 0

and 1 format and read this data from this format. if we see the discs they are very

simple rounded and flat but in fact they have a complex physical structure. And made

from different layers all these layers have their own job. Thereby we can say that

optical media are storage media that hold information in digital form and that are

written and read by a laser, these media include all the various CD and DVD

variations.

Compact Disk (CD)

The first disc that could be written and read by optical means (using light as a

medium) was developed by James T Russell. In the late 1960s Russell created a

system that recorded, stored, and played audio/video data using light rather than the

traditional contact methods, which could easily damage the disks during playback.

Russell developed a photosensitive disc that stored data as 1 micron-wide dots of

light and dark. The dots were read by a laser, converted to an electrical signal, and

then to audio or visual display for playback. Russell own company manufactured the

first disk player in 1980 although the technology never reached the marketplace until

Philips and Sony developed the technology in late 1982. CD-DA was originally

developed to contain only audio information, digitized at a sample rate of 44,100

samples per second, in a range of 65,536 possible values (16 bits) with a storage

capacity of about 650 megabytes (MB).

Digital Versatile disc (DVD)

Digital versatile disc (DVD) had its beginning in 1994 when two formats super disc

(SD) and multimedia CD (MMCD) were introduced. Promoters of the competing

technologies failed to reach an agreement on a single standard until 1996 when DVD

was selected as a convergence format. DVD has in the few years since grown to

include variations efficiently. Standardization and compatibility issues aside, DVD

is well-placed to supplant CD.

CD/DVD Types

CD: compact disc a format designed to hold audio data on a 12cm plastic disc, using

a laser to read series of pits and lands as binary data.

CD-DA: compact Disc-Digital Audio. The standard used to define the logical,

physical and data structures of data discs.

CD-R: compact Disk-Recordable. A WORM (write once-read many) format

allowing compact discs to be created using consumer hardware.

CD-ROM: compact Disc-Rewritable. A format similar to CD-R that allows for data

on the disc to be written over.

CD-RW: Compact Disc-Rewritable. A format similar to CD-R that allows for data

on the disc to be written over.

DVD: Digital versatile Disc. A format designed to store cinematic-length motion

picture on a 12cm disc. This led the creation of the disc that offered 6-10 times the

capacity of CD-ROM in the same form factor.

Enhanced CD: a format for storing CD-DA and CD-ROM data on a single dis. This

was originally impossible. But was permitted with the blue book standard in 1995.

CD/DVD Structure

A CD is flat, round, portable metal disc, made of clear polycarbonate plastic, coated

with a reflective metal, and a protective coat of clear lacquer. Data placed on the

CD-ROM in the form of small pits recorded in a spiral track starting at the center of

the CD-ROM and working to the outer edge. If the data track of a CD-ROM could

be stretched out it would be about 4.5km long. It is usually 0.75 inches in diameter

and less than one twentieth of an inch thick. A disc usually divided into Tracks,

Cylinders and Sectors. Hard disc is organized as a concentric stack of disc or (platter)

each platter has 2 surfaces and two read/write heads for each surface. Each platter

has the same No of tracks. Platter is made from aluminum, ceramic or class coated

with a magnetic material such as iron oxide.

CD/DVD Polycarbonate (plastic) layers

The polycarbonate substrate makes up most of the disc, including the area that is

read by the laser (opposite the label side on CDs). It is present on both sides of a

DVD, even a single sided disc with a label on one side. This substrate provides the

disc depth necessary to maintain laser focus on the metal and data layers. It also

gives the disc enough strength to remain flat. Anything in or on the polycarbonate

layer that interferes with the ability of the laser to focus on the data layer will result

in the misreading of data. accordingly, fingerprints, smudges can interfere with the

ability of the laser to read the data.

Data layer

As it name implies the data layer of CDs and DVDs is the layer that contain the data.

The data appear as marks or pits that either absorb light from the laser beam, or

transmit the light back to the laser photo sensor by way of the metal reflective layer.

In CDs the data and metal layer are very close to the top of the disc (label side). In

DVDs they are in the middle of the disc.

Metal (Reflective) layer

The metal layer in optical discs reflect the laser back to the laser photo sensor in the

laser head. Three type of reflective metals are typically used for this layer:

aluminum, gold, and silver or silver alloy. In double-layer DVDs silicon is

sometimes used as one of the semi reflective layer.

Figure 1

Tracks

The data is stored on concentric circle on the surfaces known as tracks.

Corresponding tracks on all platters surfaces make up a cylinder. Numbering starts

with 0 at the outmost cylinder.

Sectors

A sectors is a continuous linear stream of magnetized bits occupying a curved section

of a track. Sectors are the smallest physical storage units on a disc-each sector stores

512 bytes of data. Numbering physical sectors within a track starts with 1. Or divided

tracks in section.

Figure 2

Cylinders

Logical grouping of the same track on each disc surface in a disc unite. Or all the

tracks with the same radius are known as cylinders.

Figure 3

Figure 4

Reading of a CD/DVD

When reading a CD-ROM, a low power laser beam is focused on the rotating CD-

ROM and its reflection is viewed by the read head. When the beam reflects back

from the CD-ROM, its intensity changes as it moves from “land” to “pits”. These

variations in the laser beam are decoded as data by the CD-ROM drive. It should be

noted, that unlike hard disc which rotated at a constant angular velocity (CAV), CD-

ROMs rotate at a constant linear velocity (CLV) of about one meter per second. This

requires that the driver’s servo mechanism makes the CD-ROM turn slower as the

read head moves to the outer edge of the disc.

Pits and lands

The CD-ROM holds digital data. the data is stored in the form of lands and pits.

Lands are flat area on the metal surface pits are depressions or hollows.

Figure 5

Reading Speed

Reading speed is measured in KB/s. reading speed determines the transfer of data in

one second. 1* or 1-speed which gives a data transfer rate of 150 KB/s in the most

common data format. Maximum speed available today is 52*.

Writing of CD/DVD

It would be good to know a bit about how data is recorded on to optical disc before

we dive into a comparison of recording modes. Optical disc is made up of a

polycarbonate material with a layer of encoding material on top and reflective

surfaces. Data is written in a spiral fashion from center to periphery of the disc which

creates bumps into the encoding surfaces or changes the optical properties of the

surface. Whenever there are bumps the surfaces is non- reflective and non-modified

surface stays reflective. As the data is binary the bumps present 0 and the reflective

areas represent 1s. the data form the disc is read by reflecting light from the surface

using a laser diode. As the disk spins and data is read spirally from center to

perimeter 1s are recorded where light is reflected and 0s are recorded where it isn’t.

by controlling setting of the optical drive recording software you can modify the way

in which data is written on the disk which bring us to the recording modes.

The store data on a CD they need to be burned. The burning process of a CD is

nothing but creating a pattern of pits and lands over the polycarbonate layer. But

since the data must be accurately encoded on such a small scale. The burning process

must be extremely précised. A CD burner is used to write the data on a CD. It

incorporates a moving laser quite similar to a CD player which known as “Write

laser”. The write laser which is more powerful than the Read laser has the capability

to alter the surface of CD instead of just bouncing the laser light off. During burning

process as per the data the write laser bounces the light beam over the CD surface

and create a series of pits on it. When you play the CD, the read laser bounces the

light beams on the surface and detects the pits and lands. Each change between pit

to lands or vice versa is translated as 0 and no change is translated as one. These

binary values from the actual data. the logical structure of a CD or DVD involves

various writing techniques and the logical organization of data within a file system.

Writing to a CD or DVD can be done using any of the following writing strategies.

Track-at-once is the most common form of CD recording for user created data disc.

Disc-at-once is the most common way to create audio discs and DVD. Incremental

recording or packet writing is used with drag-and-drop writing software.

Figure 6

Track at once

When you choose Track at once mode while burning an audio CD, the audio tracks

are burned onto the disc discontinuously. After one track is written by the laser it is

programmed to stop, to write a ‘Run out’ block. The next track is separated from the

first track by ‘link’ and ‘Run in’ blocks. This of recording mode is used when

burning a multisession audio disc. In this type of recording mode both audio data

and other type of data can be recorded. Due to the space taken up by run out, link

and run in blocks in between in track at one recording mode the full storage capacity

of a disc is not utilized.

Disc at once

As opposed to track at once recording disc at once mode of recording directly burns

all the track without the laser taking any breaks. Hence this recoding mode can use

the full storage capacity of the disc. When creating a master disc for duplication this

is the recording mode used. This mode should be used when finalizing a disc in one

writing session.

Packet writing

Packet writing is a technology that enables the writing of data to a recordable CD-R

incrementally. In multiple small blocks of data rather than in a single block or in

block defined by track. The other two common methods. Packet writing software

makes it possible for the user to save data to a CD-R or CD-RW in the same way as

it would be saved to the hard drive or a floppy. Not all CD recorders support packet

writing.

Conclusion

In this seminar we talked about the importance of optical discs the functionality of

those disc and how the data where stored and read from these discs. In the end we

can say that CDs are type of optical storage. Which means the data can be read and

write through laser. It holds data through physical marks. It is played in a CD-player,

and read in different speeds. File system is required to write data on CD by different

methods. CDs are compact storage devices that can be moved from one place to

another easily.

Refrenceses

http://www.sciencedirect.com/science/article/pii

http://www.techopedia.com/definition

http://www.globalspec.com/reference

http://sites.google.com/site/phsicsofcdsanddvds/the-physical-cd-dvd-structure

http://www.investopedia.com/terms/c/certificateofdeposit.asp