A basic overview of CDROMS
Introduction
Mass storage devices have long since been established as an important resource to make available electronic data of any kind. Compact disc-read only memory (CD-ROM) marks the beginning of a new era during which the emphasis is likely to switch from magnetic storage to optical technology.
Re - writable magnetic media such as floppy disk and tape are familiar to almost all users of technology. Both removable and compact, floppy disk provides a vehicle for distributing programs and data.
The often non-removable hard disk supports random access for large amounts of data. Magnetic tape provides a cost effective means storing large amounts of data where random access is not required for example, data back up for archive purposes and for submission of CD-ROM data to replication companies. Used to back up hard disks, tape streamers offer a safety net in the event of catastrophic system failure. Subsequent data recovery simply involves reading the tape contents back to a functioning hard disk.
A read only digital storage medium (DSM), CD-ROM supports up to about 660 Mbytes data storage capacity in standard form. It offers numerous features unrivalled by current magnetic media, including vast storage capacity, compactness of design, reliability and durability.
Attributes such as these make CD-ROM ideal for the distribution of computer software, courseware, maps, databases, reference material, material, multimedia and more.
The growing CD-ROM user base will result i less specialized publications and it is not unreasonable to assume that low cost books and magazines journals will appear in significant numbers in the foreseeable future.
CD-ROM:
The standard 12-cm-diameter CD-ROM supports up to about 660 Mbytes (692,060,000) bytes data capacity. A single disc is equivalent to between approximately 400 1.44 Mbytes floppy disks or 1,500 360 Kbytes floppy disks. 8-cm-diameter CD-ROMs exist, but are not cost effective for the moment due to the non-existence of an appropriate standard.
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Cross section
CD-ROM DISC CONSTRUCTION
A 12-cm-CD-ROM can store up to 250,000 A4 pages of text or approximately 100,000,000 words. Note, these methods of quoting data capacity are rather vague and not likely to satisfy many people.
Like audio CD, a CD-ROM disc physically consists of a metallic disc bonded to a polycarbonate base. This is coated with a transparent, protective lacquer (refer to fig 1). a track spiraling from its center measures some three miles long, And is arranged at a density of 16,000 tracks per inch.
DISTRIBUTION MEDIUM:
The emergence of CD-ROM highlights more clearly the many problems that exist when floppy disk is used as distribution medium.
Equating to approximately 1,400 360 Kbytes floppy disks in standard form, CD-ROM epitomizes compactness of design. It is reliable, virtually immune to physical damage, and impervious to data corruption through the presence of electromagnetic fields. CD-ROM has potential to become a universal distribution medium used for everything from computer software to multimedia.
COMPUTER SOFTWARE DISTRIBUTION:
The first commercial CD-ROM software distribution disc from a major producer was Microsoft office. Farallon computing released lesser-known programs on CD-ROM prior to this, as Microsoft itself. Also PC SIG released the worlds first shareware compilation on CD-ROM in the US
Cost effectiveness of using CD-ROM naturally hinges upon the number of floppy disks(i.e. size of program and related data), and scale of distribution. How ever, CD-ROM need not to be chosen purely for reasons of economy, because its basic convenience will often justify higher distribution costs.
This basic convenience emanates from the fact that it helps smooth over the often tedious process of software installation, and eliminates the need to insert and reinsert floppy disks. In addition their physical properties dispense with the return of corrupt or damaged floppy disks, benefiting users and vendors alike.
Its most significant advantage is that of vast data storage capacity, of which the most sophisticated of current commercial programs, would consume but a small portion. Spare storage space remaining can be used for program demonstrations. Eventually program documentation, training material and advertising could also be accommodated.
Using (conventional) data compression methods a single CD-ROM can yield more than GByte of programs and data phethean’s PD library, for instance, comprises around one Gbyte of programs on a single disc. Effects of CD-ROM as a distribution of commercial programs available. When swapping between applications stored on different discs we are faced with a problem one solution to which lies are using magazine drives, daisy chained drives or a CD-ROM network. Alternatively, larger disc data storage capacities will emerge, in which case it might be advantageous for software producers to join forces and store programs on common discs.
PUBLISHING MEDIUM:
In retrospect the belief that information technology would lead to the paperless office in the eighties is now slightly embarrassing when we consider that it led to even greater consumption of paper. It is now clear that optical storage techniques will be highly influential in molding the paperless office. Rewritable optical storage devices lay at the heart of electronic filing systems such as canon’s canofile, a desktop electronic filing system.
With CD-ROM we are now faced with the equally debatable prospect of a paperless library. At more basic level, greater flexibility than paper is achieved, where information can be accessed randomly and instantly. However, CD-ROM is not without disadvantages. Many users are unable to read from domestic television screens (using CDTV, for instance) for more than an hour or so. Some comfort is gained from the fact that the majority of CD-ROM drives are installed in systems connected to monitors. Although these cannot always to be taken as significant improvement because they fail to emulate paper sufficiently well.
High specification computer monitors offer one solution, but these are fat from wide spread at present for reasons of cost. However the comparatively poor domestic television of today is likely to be superseded by highly definition television (HDTV). The first impact of this will be on multimedia and interactive video systems.
Portability is an important prerequisite for any medium, which is to take over from paper. An affordable book sized CD-ROM reader
simply has to materialize. More than just affordable it has to be reasonably sophisticated and incorporate an appropriately sized display, which is both sharp and fast.
Returning to conventional CD-ROM, text and graphics are available in digital form early on in the modern publishing process, providing the necessary prelude to developing a CD-ROM. There is usually a clause ì¥ÁG
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l-screen) frame of video consuming around 1,000 Kbytes, the problem of data storage capacity can be appreciated. There is also a more fundamental problem in that the average data transfer rate between a standard CD-ROM drive and computer is far to slow at 171 Kbytes/sec using a mode 2 disc. The solution to supporting motion video on CD-ROM lies in image compression of which numerous techniques and technologies exist. The image compression algorithm used in Intel div for example operates by storing only those parts of a frame, which are different from a previous one. Compression ratios achieved therefore depend upon the type of sequence being compressed.
DATABASES:
Databases have become synonymous with computer technology, where in recent years i9nformation on the printed page has been transcribed to magnetic storage media. A further migration to optical media now looks inevitable, and the creation of many new databases will almost certainly involve a decision concerning whether or not the end product should reside on CD-ROM.
Many useful on line databases are in the gbyte range. These can sometimes be stored on CD-ROM by daisy chaining typically four to eight drives from which it is then possible to access data randomly as from a single drive. The number of drives, which can be daisy chained, varies according to manufacturer. An alternative to daisy chaining is to use a magazine drive such as the pioneer6-disk drive, which is capable of reading six discs simultaneously. Optical networks also provide a means of accessing multiple CD-ROM drives.
Database protection remains an issue for many publishers and notion of installing a CD-ROM on site increases the threat of illegal copying or unlicensed access. Encryption methods provide one solution to this, requiring the user’ computer to be fitted with a decryption peripheral. These are very much like dangles, i.e., hardware devices which prevent access to programs.
Product pricing is often a complex problem for data bas publishers and traditional methods of charging users are usually perceived as unfair. Flat payment for a complete database installation or yearly subscription fee, are two methods which are likely to leave either publisher or user dissatisfied.
A fairer costing system naturally takes into account the amount of service provided to the user. Return on investment (ROI) owned by personal library software inc., is hardware/software system which monitors each access to a database calculation a fee commensurate with usage.
CD-ROM DRIVES:
Though little appears to separate drives on the surface, deeper analysis reveals many subtle differences that govern overall performance. Buffer size helps improve speed of operation by permitting the drive to search for and read data without waiting for the computer to accept all outgoing data. The buffer is simply and area of RAM( random access memory) which acts as a temporary store between drive and computer. Buffer sizes of 64 Kbytes are common.
Average access time, a measure of how long it takes the laser head to locate and begin reading an appropriate region of disc, is measured in milli seconds and tends to vary from 1500 ms down to 300 ms. Integral to access time, latency represents time taken for the read head to find the appropriate CD-ROM data block during random access. This tends to increase with turns of track farthest from the disc'’ center. Typically, latency may vary from between from50 to 80 ms for innermost turns, and approximate 150 ms for outermost turns.
CD-ROM data blocks provide a means of storing information logically to yield a data structure, which is both manageable and readable. A less important area of difference concerns the maximum data transfer rate is achieved through burst transfer (or burst mode), where the drive seizes the computer processor time completely. However, it cannot be sustained for reasonable periods of time.
CD-ROM DRIVE INSTALLATION:
Almost all-desktop computers are designed with a view to future expansion, able to accommodate extra floppy disk drives, hard drives, fax cards, network cards, and many other popular peripherals. IBM PC XT, AT’s and PS/2s, for example include a number of expansion slots intended to accommodate such add-ons. CD-ROM drives can be either internal or external.
Other than saving desk space and often reducing the number of main leads, there are no advantages in using internal drives. Lack of space or an insufficient power supply can force the need for an external or portable drive. Indeed a number of desktop computers on the market support little more than two 3.5 inch drive bays, as it seems that the advent of surface mount technology has encouraged some manufacturers to opt for yet smaller cases than ever before.
In so far as the PC is concerned, problems of small cases could be overcome using CD-ROM cards, which like hard cards would embody a drive mechanism and controller as a single module, however, standard 12 cm CD-ROM drives would be too large for many machines. Internal or external, the connection of CD-ROM drive requires and appropriates interface, of which there are basically two kinds; proprietary bus interface and SCSI.
The simplest interfaces are indigenous to particular drives, and include a proprietary bus connection. Proprietary bus interfaces are indigenous to different manufacturers, resulting in zero interchangeability between different drives. For example, you cannot simply swap between Hitachi and NEC drives.
SCSI implementations tend to vary in performance in terms of speed of input and output operations. For example comes controllers consist of the most basic bi-directional bus offering low transfer speed and in many instances they barely meet official SCSI requirements. However the main advantage of a SCSI drive is interchangeability where for example it can disconnected from an IBM PC or compatible machine and simply added to an Apple Macintosh. For this to be so, you will need appropriate drivers and interface adapters. After physically adding a CD-ROM drive, it is necessary to indicate its presence by running an installation program.
CD-ROM OPERATION:
The under lying technology of CD-ROM is markedly similar to that used in Phillips first optical videodisc system demonstrated in Eindhoven as early as 1972. This research and development led to laser vision. CD-ROM is a reflective light system, where laser light is shone against turns of track, which are encoded with digital data using pits and areas of land. Pits scatter the laser light, while areas of land produce reflected light. Reflected light is diverted to a photo detector that produces a series of electrical pulses corresponding to encoded data.
A CD-ROM disc rotates at a speed to provide a constant average read rate of around 75 block/sec. Because data is embedded in the track or tracks of CD-ROM at a uniform density, the disc must spin faster when the read head converges on its center (and vice-versa). This is quite unlike traditional vinyl audio discs, which rotate at a constant angular velocity (CAV). The required constant linear velocity (CLV) is achieved by coordinating the position of the read head with speed of rotation using a simple feed back system.
The reason why standard drives operates in CLV mode rests firmly with the manner in which discs are encoded. It would have been equally possible to vary distance between pits on a CD-ROM and so adopt a CAV system. There is a reasonable argument to suggest that CAV would have eliminated the need to vary motor speed and so reduce drive prices. It would also have reduced access times.
Like all random-access storage devices, encoded information has to be organized into manageable data blocks so as it may be located and retrieved. Irrespective of whether or not a disc uses one track or up to 99, CD-ROM data blocks of 2.353 bytes used. The amount of user data that CD-ROM data blocks provide is a function of mode of operation.
The addition of error detection and correction codes makes mode1 discs more suited data highly sensitive to error, such as computer software. In multimedia, mode 2 is commonly used for the storage of video and audio data, as its accuracy is not so critical.
Like audio CD each second of playback results in 75 blocks read irrespective of mode. It should be pointed out that data transfer rate and user data transfer rates are quite different. Data sheets and drive specifications regularly opt for the former when they mean the latter.
CD-ROM ADDRESSING AND USER DATA CAPACITY:
CD-ROM addressing is carried out using measurements of time and data blocks read. Minute’s seconds and blocks provide enough information to locate any item of information. Not these are inherited directly from CD-DA. For example considering a one-hour CD-ROM, absolute addressing would take the following form:
Minutes (M): 0 – 59
Seconds (S) : 0 – 59
Blocks (B): 0 – 74
a track beginning mid way through the CD-ROM for instance is addressed 29:29:37(M:S:B).
User data capacity = disc length (seconds) * blocks read/second *usable block data.
User data capacity, besides being a function of user data block size also depends upon disc playtime. Once again it is important to recognize the difference between data capacity and user data capacity of which the former will be always be significantly larger, and even more so using a mode 1 disc. Such mixed mode discs are necessary where both software (retrieval system, for example) and audio and video are stored on the same disc.
CD-ROM STANDARDS:
Virtually all commercially successful technologies are products of workable standards able to unite manufacturers and yield a user base sharing a common bond. Two notable examples include video cassette players and audio CD players, both of which rapidly gained worldwide acceptance.
Often the path to standardization is strewn with obstacles as manufacturers commit themselves to particular technologies or approaches. Sony for instance is well known as having once backed the Beta Max VCR only to be squeezed out of the market by VHS players.
Left to industry, resultant standardization need not be the best. For developer it is not necessary to understand the intricate detail of every (emerging) standard, but merely to be made aware of their existence, through which the necessary or largest market for a product can be addressed, which today means ISO 9660.
Created by Philips and Sony, the yellow book is an early standard dealing with the physical format of discs. The latest addition to the CD continuum is the Electronic Book (EB) format.
MS-DOS CD-ROM EXTENSIONS:
An additional device driver and an interface module developed to meet the demands of CD-ROM are known as the ms-dos CD-ROM extensions. The CD-ROM extensions enable exploitation of the full CD-ROM data capacity, as opposed to 32 Mbytes chunks.
Prior to the CD-ROM extensions the situation was all rather chaotic, with each drive manufacturer supplying programs that would swap individual 323 Mbytes chunks. In one sense CD-ROM actually being used like a number of small mass storage devices, negating its fundamental advantage. Commercial solutions to this problem did emerge, however. One of which came from TMs Inc., by way of laser dos. This permitted access to the full data capacity, and also supported conventional DOS commands such DIR.
DEVICE DRIVER COMMANDS:
For developers of device drivers and readers who refuse to adhere to the ‘black box’ concept the following passages briefly run through the device driver commands. To anyone writing a device driver, perhaps one of the first decisions, which need to be addressed, is which mode of operation to use.
READ LONG PREFETCH is intended to optimize access time by estimating which block is most likely to be read. In response to these estimates, the device driver positions the read head appropriately or caches relevant sectors.
IOCTL calls take care of the many mundane tasks that controls a CD-ROM drive. They allow the application program to send control strings to the device driver. PLAY and STOP PLAY commands are relevant to audio tracks. The SEEK command simply positions the read head over appropriate disc locations. All MS-DOS device drivers require DEVICE OPEN and DEVICE CLOSE commands, without they become incompatible and inoperable.
CD-ROM NETWORKS:
Local area networks present a cost effective multi-user environment in which computer resources and applications can be used to best effect. Based around a reasonably powerful file server, diskless workstations (devoid of mass storage devices) or conventional computers provide multi user access to applications.
CD-ROM networks offer many of the same advantages of conventional LANs. A multi-user environment is created, and computer resources are put to optimum use.
At the heart of the network lies and optical server, which dynamically distributes requested information from CD-ROM, drives to LAN users. The optical server is connected directly to CD-ROM drives, with some installations totaling 64 drives per server.
There are several commercially available CD-ROM network packages, many of which are software oriented. These are intended for installation on a LAN ready in place and run along side the accompanying networking software.
RETRIEVAL SYSTEMS
CD-ROM data can only be made available through an appropriate retrieval system of which many well known implementations exist. A retrieval system takes the form of a program distributed on the disc itself and is most often installed to hard disk for the benefit of increased speed of operation. It can also be memory resident, loaded from disc each time the title is invoked. Proprietary systems designed exclusively for a particular application or title are also relatively common.
The combination of an exceptionally large amount of information, long access time and slow data transfer rate, renders retrieval system design of critical importance. Often providing a means of navigation its suitability to the task in hand is just as important as the CD-ROM data itself.
As one would expect, retrieval systems are specific to applications, where the underlying design features vary considerably. At the simplest level, negotiating a telephone directory on CD-ROM merely requires the entry of simple search criteria. Wildcards or Bookman logic may also play a role. Such systems are termed largest systems and are useful for database applications.
Used in hypertext and hypermedia information structures, navigational systems are quite different. These enable the user to browse and navigate repeatedly building upon searches in order to pursue specific paths.
DATA STORAGE:
Though the finality of a CD-ROM pressing means that developers are unlikely to get second chance to re organize data it also means that encoded data will not be relocated following use.
Because CD-ROM drives are products of standardization and therefore markedly similar, efforts to improve the performance of a program or application invariably lie in optimizing the data structure and retrieval software design. However the developer cannot always be expected to understand the precise way in which such structures are developed and it is therefore important that the relevant replication company is willing pass on appropriate information and to give advice on software to be used in conjunction with data.
ORIGINS OF CD-ROM
All too often a potted history of CD-ROM begins with its announcement in October 1983. Leading up to this climax a great deal of research and development took place. Its line of development is traceable because one company, namely Philips, can be accredited with many of the widely recognized milestones.
Initially there was no long-term plan to speak of but just a drive to find an efficient means of storing still or moving images on disc. Associated projects instantly became known as videodisc experiments. The term had nothing whatever to do with optical storage techniques but simply referred to the fact that images were being stored on disc.
In the early 60’s Decca and Telfunken had some success with short recordings using vinyl discs and spiral grooves. As anticipated the high speed of rotation required led to rapid wear of both disc and stylus.
With the transition of the laser (light amplification by stimulated emission of radiation) from being an awkward and bulky gas filled tube to a low powered and more compact light emitting diode in the fall of 1962, experiments with optically encoded discs began. First transparent discs were tried, including floppy disc system, which was to prove ineffective. The commercial launch of laser vision was in 1978 (US) and in 1982(Europe).
Laser vision was originally intended as a consumer product for playing films/videos. Coupled with the paucity of films available on laser vision disc, the fact that programs could not be recorded seemed to outweigh the advantages of durable discs and improved picture and sound quality. In terms of technology laser vision can be considered as a rather large CD-ROM drive where data is stored in analogue form.
CD-ROM was announced as an information storage medium with the standard physical format issued.
Conclusion:
The CD-ROM technology has brought a new life to audio and video world. Their capacity of storing large amount of data has brought a significant change in storing data as well as programs. As the advent of new technology is springing upon the need for storing data off line has become a great necessity, the CD-ROM can be relied upon as they are easily portable, secure and can be maintained easily. CD-ROM technology has brought new energy to multimedia. As the technology has grown presently in the market we have rewritable CDs. Now-a -days CD drives have become an essential peripheral for every computer.
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v INTRODUCTION
v CD-ROM
v DISTRIBUTION MEDIUM
· COMPUTER SOFTWARE DISTRIBUTION
· PUBLISHING MEDIUM
· MULTIMEDIA
· DATABASES
v CD-ROM DRIVES
· CD-ROM DRIVE INSTALLATION
v CD-ROM OPERATION
· CD-ROM ADDRESSING AND USER DATA CAPACITY
v CD-ROM STANDARDS
v MS-DOS CD-ROM EXTENTIONS
· DEVICE DRIVER COMMANDS
v CD-ROM NETWORKS
v RETRIEVEL SYSTEMS
· DATA STORAGE
v ORIGINS OF CD-ROM
v CONCLUSION
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I am Nishanth Singamala, a Software Engineer from India. 
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