Annals of Library Science and Documentation 45,3; 1998; 77-83.

OPTICAL STORAGE MEDIA: CD-ROM

Deals with Optical Storage Media that includes:read-only optical discs, write-once-read manyoptical discs and erasable optical discs. Definesthe concept of Optical Storage and describes thephysical character of optical disc and itscharacteristics in terms of durability, high storageand portability. Read-only optical discs arediscussed in detail. Highlights the CD-ROM discs,their physical character and manufacturingprocess. Discusses the manufacturing processesthat include premastering, mastering andreplication with merits and demerits in detail.Highlights some standards of Compact Disc-Interactive (CD-I) and Compact Disc-Read-OnlyMemory Extended Architecture (CD-ROM XA)used tor accomodating multimedia information ona compact disc. Explains the characteristics ofWORM discs and erasable optical discs, theirmanufacturing process and merits and demeritsover hard discs.

The current era has acquired the name of'information era'. The new technologies haveaccelerated the research and development andlead to 'information explosion'. This in turn hasmade impact on storage, retrieval anddissemination of information. Due to thisexponential growth of information and informationsources, the print media is becoming obsolete andthe media for information storage are fastchanging in a fascinating manner.

Some of the storage media are:

Print Media (Paper, metal sheets etc.)Film Media (Microfilm, microfiche etc.)Magnetic Media (Magnetic tape, disc,drum etc.)

Electronic Media (Floppy disc, hard discs etc.)

Vol45 No 3 September 1998

Miss MEERADept. of Library & Information ScienceBanaras Hindu UniversityVaranasi-221 005

Optical Media (CD-ROM, Video disc,CD-WORM, CD-I etc.)

The above trend shows that optical media is thelatest development in the field of storage media.In this media, the data is stored on an optical disc,the surface of which is coated with magneticmaterial. Other storage medium such as tapes,floppies etc. are based on the principle ofmagnetism whereas optical media is based on theuse of light (optics), Le. laser beams (light) areused for recording and reviewing the data fromthe discs, that's why this media is termed as opticalmedia. Optical discs have high storage capacityas compared to more established magnetic media.An optical disc with 4.72 inch diameter can store600 MB information ( = 30,0000 pages of text).On a single CD-ROM disc entire text of 30 VolumeEncyclopaedia Britannica can be stored. In anoptical disc, video information, graphics, music,speech, computer programmes, textual data canco-exist on a single disc and can be accessedsimultaneously [1] [2].

OPTICAL DISCS

An optical disc is relatively inexpensive and hasa life span of about 10-50 years. Its high storagecapacity makes it appropriate not only for storingrelatively large data but also the full text of thedocuments and images, graphics and videoimages as well as sound.

Broadly optical discs can be grouped into 3categories:

1. Read-only Optical Discs2. Write-once read many Optical Discs3. Erasable Optical Discs

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READ-ONLY OPTICAL DISCS

In this type of optical discs, the information or datais recorded by the manufacturers and the end userreceives only a read only disc, that is played in aplayer in order to read the textual data, audio,video information that has been recorded on thedisc. Nothing can be written (added) or removedon this type of discs. There are different kinds ofread-only discs which have found wideapplications.

a) Video discs

These discs were developed for the distributionof pre-recorded videos. Hybrid video discs storedifferent types of information on a single discincluding text data, and high quality video. Oneside of the video disc can store or hold about 1/2hour movie pictures or programmes. Lack ofstandards and high cost of hardware has restrictedtheir adaptation.

Digital Video Interactive (DV-I)

It integrates both audio and video. With DV-I, fullmotion pictures could be stored on a CD. DV-I isexpected to store around one hour worth of motionpicture on one disc. DV-I is being developed as acomputer peripheral, rather than as a part ofconsumer electronic products.

b) Compact Discs

CDs are very popular consumer products in themarket as audio playback. They were designedto store huge amount of data and to provide highfacility of sound using a digital recordingtechnique. Since its introduction, CDs havebecome popular at an extremely rapid rate anddiscs are widely available from a number ofmanufactures. Some recognised CDs are CD-Digital Audio (CODA). CD-Read only memory(CD-ROM), CD-ROM extended Architecture (CD-ROM XA), CD-Recordable (CD-R) and CD-interactive (CD-I).

CODA

In CODA, information is coded and stored on thedisc in digital form, but at playback stage this getsconverted into analog form and the audio signals

are played back using amplifiers [3] [4].

Comapct Disc - Read-Only Memory (CD-ROM)

CD-ROM is the most popular and widely acceptedoptical media. It is recorded as an exciting newtechnology for data storage and retrieval. It is anoptical disc of 120 mm with a hole of 15 mm atthe centre with thickness of 12 mm. The discshave spiral tracks divided into sectors and frames.The physical standard and data format of CD-ROMs are defined by the Yellow Books (standard).Read-only memory means that the data ispermanently stored or coded on the disc and itcan not be added to, erased, rewritten or altered.So users can only read the disc but can not writeon it, nor can erase anything written on it. Thedata is recorded in digital form using laser beams.The stored information can be read only by usinga computer and appropriate software [5].

CD-ROMXA

This format of compact discs have both audio andvideo capabilities. It was designed to accomodatemultimedia information on a compact disc. Thisstandard format allows CD-I and CD-ROM discsto be used on either type of disc player.

CD-Interactive

CD-Interactive was developed for the interactiveapplications that ran on a stand-alone player. Inthis disc, audio and video information are recordedin addition to graphics and data in an integratedmanner and used as such. In this, audio signalsare played back using amplifiers.

CD-ROM MANUFACTURING PROCESS

The manufacturing process of CD-ROM discsconsists of three steps:

i) Pre-mastering processii) Mastering processiii) Replication process

i) Pre-mastering process

This process involves the input data preparationand application software development to generatethe required pre-master CD-tape. With the help

Ann Lih Sci Doc

of database management system (DBMS) thedata is prepared in a particular format suitable forthe retrieval software. For instance, if the data istabular, then tabular data structure has to bedeveloped with the help of DBMS. In this process,first the data in required format is stored on amagnetic tape, this data is then transferred into aCD-ROM master tape which is normally a U-MATIC or EXABYTE-tape. This tape is then fedinto a minicomputer which drives the masteringprocess.

ii) Mastering Process

This process consists of transferring data fromCD-tape to a thin reflective surface of the masterdisc. The disc has a spiral track divided into anumber of sectors. The total length of the spiraltrack is about 5 km. For recording the data. ondisc "Constant Linear Velocity" (CLV) method isused. in which the linear velocity of the rotatingdisc remains constant to the read/write head dueto which the disc rotates faster when read/writehead is on the inner tracks compared with the outertracks. Resultant large data can be stored on theouter tracks.

To encode information on disc, a laser writingmachine (code cutter controlled by minicomputer)

Premastering

Disc Mastering

•

• l... ..... .

J.i1J!i¥M1flji......•...•.•.• :--:- .•.•.•..........•......

DCode Cutter

or Laser WrittingMachine

Replication

OPTICAL STORAGE MEDIA: CD-ROM

is used to produce a lanquer coating glass masterdisc. The density of the laser beam required forrecording the data is 25mw. The laser beam isswitched on and off according to the codifiedinformation on the computer tape. On receivingsignals, the laser beam aimed at blank rotatingmaster disc vaporises microscopic portion to formtiny holes called PITS. The absence of signalsleaves the surface intact. On the disc surface, thedata is stored as a series of PITS (data) andLANDS (No data) which corresponds tomagnetised and non-magnetised digital data. Thewidth of a PIT is 0.6 microns and the distancebetween two tracks is 1.6 microns. Theinformation is stored on the spiral track of discconsisting of tiny PITS in the lanquer. The glassmaster disc is then coated with nickel to obtainNickel Stamper which has all the information ofthe original glass master. This metal stamper isused in the next step to stamp out replicas.

iii) Replication Process

The raw material for this process is transparentpolycarbonate MACROLON (trade name) resion,the granules of which are dried with hot air. Themetal stamper (AI stamper for CD-audio and Nistamper for CD-ROMs), are then inserted in theinjecting moulding machine cavity into which the

•••••~!I\\Microcomputer

Compact disc

Minicomputer Spiral Tracksof disc

Glass Masterdisc

Fig. 1 CD-ROM Manufacturing Process

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Miss MEERA

polycarbonate granules are injected under highpressure and heated upto 300°C. The temperatureis lowered and the polycarbonate with desiredPITS formed on it are taken to the sputting cavityfor vacuum metalisation. The metal vapoursprovide the reflective AI or Ni coating. To protectthe CD from contamination and surface damage,clear lanquer coating is applied by spin coatingprocess. After further cleaning, the label printingis done on the back of the disc giving informationabout the contents [6] [7].

CD-ROM eventually has become a new tool forstorage, retrieval and distribution of vast data orinformation. CD-ROM discs have spiral trackswhich are divided into a number of sectors. Eachsector or block on CD-ROM disc actually containssome 98 CD-frames, each of 24 bytes to give atotal sector capacity of 2352 bytes of which 2048bytes are always for user data and remaining 304bytes are used for the codification purpose. Thecodes are appended to the front and back of theuser data. These codes consist of header,synchronisation, error detection and errorcorrection bytes and the bytes that drives need tolocate data on disc, as illustrated below:

Synchronisation field 12 bytesHeader field 04 bytesUser data - 2048 bytesAuxiliary data 288 bytes

TOTAL - 2352 I:)'tes

To Read The Data

For reading the data from the disc, a lower density(5 mw) laser beam is used. The data is stored onthe surface as a series of PITS and LANDS ofvariable length. A PIT is a small dent (0.6 micron)and LAND is the surface between PITS. Inoperation, when laser beam of lower densityfocuses on the rotating surface, the PITS scatterthe laser beam and the LANDS reflect it. Thedirection and amount of reflected light changesas the surface changes from LANDS to a PITSand vice versa. These changes in the light aredetected by the reading head (photo-detector) andgive CD-drive to read data. The CD-ROM readinghead does not get close to the disc as in the case

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of magnetic read/write head. Hence the chanceof the head crashing into the disc is virtually zero[8].

Comparison with Magnetic Disc

The comparison of a CD-ROM Disc with a harddisc [8] is given below:

CD-ROM Hard Disc

Capacity 600 Mb 10-40 MbNumber of tracks 18,000 612Average Seek 500 ms 100 msRotational Speed 300-600 rpm 3600 rpmAverage latency 100 ms 8.3 msTransfer rate 150 mb/ 96 kb/

sec sec

Application and Use of CD-ROMs

The enormous data capacity of CD-ROM makesit ideal for a wide range of applications [9] [10]from publishing to database as given below:

(i) as archives i.e. for the storage of archivalmaterial

(ii) for distribution, provides a cost effectivemedium for distributing large quantity of data.They are portable and can be easily posted;

(iii) as reference material

Publishing large amount of data likeencyclopaedias and other voluminousreference information is one of the primeapplications of CD-ROMS. Some voluminousreference information can be accessed byonline systems too.

(iv) as an interactive medium e.g. used for arcodegames and education. Now CD-ROM hasbeen used as an alternative method forpublishing the training manuals, compounddocuments having bit mapped images withgraphics and documents having informationabout games and courses.

Other areas where CD-ROM is being used are asfollows:

Ann Lib Sci Doc

MedicalLaw

for creating drugs databasefor publishing governmentrules, regulationsfor publishing corporatefinancial informationand budgetsfor publishing parts inventoryfor publishing the drawinginformation

Financial

ManufacturingAerospace

Advantages of CD-ROM Discs

CD-ROM discs have a lot of advantages [11] [12].

These are:

a) High Storage Capacity

CD-ROM discs have tremendous storage capacity(600 Mb or 570 billion alphanumeric characters)as compared to magnetic storage media.

b) Multimedia Capacity

CD-ROM discs have multimedia capacity asdifferent types of data viz. conventional text andnumeric data with audio, computer graphics, videoor diqited photographic images can coexist on asingle CD-ROM disc. These data can be accessedsimultaneously which is not possible with otherstorage media [13].

c) Enhanced Retrieval Capabilities

It offers powerful method of information retrievalsuch as free text, controlled vocabulary searchingand combining of concepts through boolean logic.Some CDs provide menu-driven access for thenovice and command-driven for the experiencedsearchers.

d) Easy and Unlimited Access

CD-ROM products provide an easy and unlimitedaccess to the stored information as and whenneeded. The disc can be used repeatedly andallow unlimited access to the information similarto books.

e) PCs and CD-ROM

CD-ROM is a powerful peripheral for personal

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OPTICAL STORAGE MEDIA: CD-ROM

computers and with the use of CD-drives, PCscan be transformed into a very powerfulinformation resource. A convenient way ofdatabase searching can be obtained by combiningPCs and CD-ROMs.

f) Controlled Cost

The cost of manufacturing discs and theirdistribution is low as compared to the amount ofdata it stores. The expensive access to largemainframe systems can be diminished by usingCD-ROM data base on PCs. The cost of CD-ROMs are independent of the amount of searchingtime.

g) Portability and Durability

CD-ROMs being small in size, are easily portableand have high durability i.e. their storage life isabout 10-50 years.

h) Psychological Advantage

Searching on CD-ROM has a definitepsychological advantage because the users arefree from time pressure, they can try variousstrategies to obtain optimum results and there isno financial penulty for commiting mistakes.

i) Data Protection

On CD-ROMs it is possible to maintain theintegrity, purity and permanance of data sincethere is no chance of accidental data erasure,head crash and any wear and tear because theRlW head does not touch the surface of the disc.

j) Easy Storage

Like a floppy it can be removed from the drivegiving it mobility.

Disadvantages of CD-ROMs

Besides a lot of advantages, the CD-ROMs havefollowing disadvantages.

a) Limited searching

Its main disadvantage is longer access time ascompared to hard discs and online searching.

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Response rate is also very slow. Only one usercan access a CD-ROM at one time so others haveto wait for their turn [14].

b) Read-Only Device

Since it is a read-only device (i.e. data is storedpermanently), the currency of information can notbe maintained.

c) Lack of Standardisation

It is a major barrier of CD-ROM. Standardisationis essential for the flexibility. Throughstandardisation, hardware and softwareincapabilities and differences in operatingprocedures can be minimised.

WRITE ONCE READ MANY (WORM) OPTICALDISCS

As we know that CD-ROM stores informationpermanently, we can not erase the data writtenon it and can not re-record anything. The data orinformation on disc are coded by themanufacturers. Therefore, for inhouse archivingof data, CD-WORM was invented. It is alsosometimes referred to as Direct Read After Write(DRAW). It is an optical (compact) disc on whichuser can record the data once and read as manytimes as desired. CD-WORM discs are used forinhouse archiving of data that an organisation maywish to store and consult data but not amend.Once the data is stored, a user can not erase thewritten data or re-record on it.

The manufacturing process of CD-WORM isalmost similar to CD-ROM disc production andwriting data. CD-WORM disc's tracks areconcentric circles having a number of sectors,instead of spiral tracks of CD-ROM discs. Thelaser beam used for writing data is of modestdensity forming PITS or bubbles on the surfaceof the disc. Here also digital data is recorded inthe form of PITS and LANDS. For reading thePITS, the density of the laser beam is less thanthat used for writing.

It can be used for storing permanent data,information and files for maintaining records.While read-only devices are valuable forpublishing in computer readable form, write once

X2

devices are suitable for preservation of valuabledocuments in compact storage medium. IBM hasdeveloped 200 MB WORM for its PS/2 system.WORM disc has high storage capacity, reliabilityand takes less time in searching as tracks rotatewith constant speed. Standardisation has been aproblem with WORM disc too but once thestandards are established, WORM discs may beused on mainframes to provide multi-user accessto a published database. Publishers may alsodevelop image databases on WORM disc andthen distribute on CD-ROM and thus CD-ROMand CD-WORM may complement one another.Its main draw back is great access time comparedto that of hard discs.

ERASABLE OPTICAL DISCS

Data can be recorded, read, erased, re-recordedon erasable optical discs, so that they can be usedin application where magnetic discs are currentlyin use and can serve as a secondary memory ofcomputer. The tracks of the disc are concentriccircles which are divided into a number of sectors.The track rotates with constant speed to makequick access.

The manufacturing process is similar to that ofCD-ROM disc production using a slightly differentsystem for writing and reading the information.The disc surface is coated with magnetic propertyat room temperature. A different magneto-opticalsystem is used for writing and reading data. Inthis system, both the principle of magnetism andoptics are involved. For recording the data onconcentric track of disc surface, moving withconstant speed, a laser beam together withmagnetic field is used. To create magnetic field,the read/write head of laser writing rnachinecontains a current coil which produces aperpendicular magnetic field on disc when acurrent is passed through it. At room temperaturethe created magnetic field can not magnetise anyspot on disc. In writing process, first the laser beamis focussed to heat at the spot which isimmediately magnetized by the applied magneticfield by current coil and a tiny vertical magnet iscreated on the place of spot. This tiny magnetrepresents the data. For reading the written datapolarised laser beam is used (due to presence ofmagnetic field, laser beam is polarised). When apolarised laser beam is focussed on tiny magnet

Ann Lih Sci Doc

it is reflected from the tiny magnet due to whichplane of polarisation rotates by a few degree. Thischange in plane of polarisation is read by theoptical system. The density of laser beam usedfor reading is less than that used for writing. Forremoving the data from the disc, the data or spotto be erased (or entire data) is heated with laserbeam in the absence of magnetic field (i.e. verticalmagnetic field is turned off). The tiny magnetdisappears in this condition.

Its advantage over hard disc are:

i) Very high storage capacity. A 5.25 inch opticaldisc can store about 650 mbyte of data;

ii) An optical disc can be removed from the drive;

iii) It has a long life;

iv) It is more reliable.

The drawback is its longer access time comparedto that of hard discs [15, 16].

REFERENCES

1. NAIR(R Raman). Computer application tolibrary and information services. 1992. EssEss publication; New Delhi. p. 32, 60.

2. PANDEY (S K Sharma). Librarycomputerisation: theory and practice. 1993.ESS ESS publication; New Delhi, p. 46-47.

3. 8ALARAMA. Establishment of CD-ROMbased information system - plan andprospects. LiD Sci Slant Doc Inf Sci. 25, 4;1988; 224-260.

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OPTICAL STORAGE MEDIA: CD-ROM

4. ROWLEY (Jannifer E). Computers forlibraries. Library Association; London, p. 162-178.

5. RASHID (Abdul). Application of CD-ROMtechnology. INICAE. 14, 1; 1995; 15-33.

6. CHAKRAVARTI (A K), VASANTHA (8),SHROFF (N) and KRISHNAN. Technologywatch CD-ROM. Ann Lib Sci Doc. 42, 9; 1995;27-34.

7. PAHWA (Ash). CD-ROM~ the largest littlewonder. Oataquest. 1998; 61.

8. Ibid. 60-61.

9. FOX (E A). Optical discs and CD-ROM. AnnRev Inf Sci Techn. 23; 1988: 85-124.

10. PAHWA (ASH). Op. cit. p. 61.

11. SIDDIQUE (Moid A). Can CD-ROM replaceon line? INICAE. 10, 1; 1991; 40-48.

12. RAM (8). Computer findamentals:architecture and organisation. 1994. WileyEastern; New Delhi. p. 296-97.

13. SHERMAN (Chris). The CD-ROM Handbook,1988. McGRAW Hill; New York, p. 19.

14. ANDERS (Vicki) and JACKSON (Kathy M).Online Vs CD-ROM: the impact of CD-ROMdatabase upon a large online searchingprogram. Online. 12,6; 1988; 24-32.

15. ROWLEY (Jannifer E). Op Cit. 162-78.

16. RAM (8). Op cit. 97-98.

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