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Основы CD-ROM

Дуэйн Маркис

SIGCAT, г. Аннаполис, США


The Basics of CD-ROM

Mr. Duane Marquis

SIGCAT, Annapolis, USA

What is CD-ROM

Compact Disc Read Only Memory. A compact disc format that is used to hold text, graphics, and hi-fi stereo sound. The disc is almost the same as the music CD, but uses different tracks for data. The music CD player cannot play CD-ROM discs, but CD-ROM players may be able to play music CD discs and have jacks for connection to an amplifier and/or earphones. A CD-ROM player is cabled to and controlled by a card that is plugged into one of the PCs expansion slots.

History of CD-ROM

Sony of Japan and Philips of the Netherlands developed the compact disc (CD), a small hard plastic disc that could be economically produced in large quantities. CD-Audio was the first application for this new technology. CD-Audio was very successful, due in large part to the producers adherence to a set of specifications that later became standards. The adherence to these standards lead to the worldwide growth of the CD-Audio industry. When CD-ROM was designed, the industry recognized the need for standards and developed ISO 9660 as the base standard for all CD-ROMs.

Physical Attributes

Physically, the standard compact disc is a disc made of clear polycarbonate plastic, coated with a reflective metal, and a protective coat of clear lacquer. Data is 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.

Structure and Capacity

The standard CD-ROM can hold up to 74 minutes (about 680 Megabytes) of data, uncompressed. This is roughly equivalent to 300,000 typewritten pages. Along with the data, error detection and correction codes are also recorded on the disc. This accounts for the incredibility low error rate when reading CD-ROMs.

How CD-ROMs are Made

Since CD-ROMs are replicated in large quantities, the process requires producing a "master" disc. To produce the glass master, the encoder's high power laser beam burns the pits onto a glass disc that has been coated with photoresist. Once "burned", the glass disc is then coated with an ultrathin metal coating (usually a nickel alloy). Finally, it is used to produce the metal stampers that are fitted into the CD-ROM replication machines to press the final CD-ROMs. After stamping the CD-ROMs, they are coated with the reflective layer (usually aluminum), the protective lacquer, and finally the label is printed on it.

Reading Data from a CD-ROM

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, it's 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 discs which rotate 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 drives servo mechanism makes the CD-ROM turn slower as the read head moves to the outer edge of the disc.

CD-ROM Formats

ISO-9660

A widely used file format for CD-ROM. The ISO 9660 (formerly High Sierra) standard defines a directory structure which has been accepted by the International Standards Organization. This standard, supported by Microsoft in the MS-DOS Extensions, allows ISO 9660 formatted CD-ROM discs to be read like a DOS write-protected hard disk. Formatting a CD-ROM to this standard will allow CD-ROM interchange on any platform that supports the ISO 9660 standard.

CD-I (Interactive)

A compact disc format that stores audio, still video pictures, and animated graphics and full motion video as well. CD-I provides up to 144 minutes of CD-quality stereo sound, up to 9.5 hours of AM-radio-quality stereo, or up to 19 hours of single channel (mono) audio. Developed by Philips and Sony, CD-I is designed for home and business use with CD-I players connected to TV's and personal computers starting in the early 1990s. CD-I discs feature interactive games and education as well as reference works and movies. CD- I includes an operating system standard as well as a proprietary chipset for decompressing video images. CD-I discs require a CD-I player and will not play on a CD-ROM player.

CD-ROM-XA (Extended Architecture)

A version of CD-ROM which allows for the inclusion of various grades of medium to low-fidelity audio (see ADPCM) to be played concurrently while viewing data. Announced by Philips, Sony, and Microsoft in August 1988, CD-ROM/XA allows for data (text and pictures) to be viewed and narrated at the same time. It also functions as the audio bridge between CD-ROM and CD-I. CD-ROM/XA plays on a standard CD-ROM player, but requires a CD-ROM/XA controller card in the personal computer to decode the interleaved audio information and play it back.

Photo CD

A development from Kodak. Photographs and/or 35mm slides, can be scanned, digitized, and recorded on the Photo CD and then plays back through CD-ROM/XA, Photo CD Player, CD-I Players, or Photo CD compatible drives and displayed on a TV set or computer monitor. Each disc can contain 100 photos. The photos can also be printed out on regular photographic print paper by a special Kodak machine for high resolution prints. In order to accommodate the different resolutions available for playback or printing, the format contains the picture in 5 different resolutions. It is also used in professional markets to store or archive photographs.

Multimedia

Term used to describe the use of more than one medium in a program or system. For example, use of audio, video, graphics, animation, computer data, etc., together. Historically, video has been considered separate from audio only (CDs, records, tapes) and computers separate from video and audio. Multimedia means the joining of any two or more of these. Multimedia computers playback high quality sound and video as well as text and graphics. Multimedia itself combines multiple forms of media in the communication of information between users and machines.

Steps in a CD-ROM Project

  1. Design of the Application
  2. A good CD-ROM application will improve the way things are currently done, provide the needed information in a value-added structure, as well as offer ease of use when used with today's computers. Their application designer must select the most appropriate collection of data and software features that will help the users obtain the desired results. A user interface that makes using the CD-ROM application a non-threatening experience is required. Obviously, a balance of these aspects is most necessary, because fancy or powerful software with substandard data is just as undesirable as a fine collection of data with an ineffective software package.

  3. Data Preparation
  4. This step is generally the most time-consuming, expensive, and problem-ridden of the entire process – especially for first-time applications. Data used for CD-ROM applications come from all sources: paper, microfiche, files from word processors, paste-up graphics, rasher and OCR scan files, databases, spreadsheets, mainframes, and others. Too often, however, the desired files are in varied and mostly incompatible formats – especially graphics files. Therefore, to make them useable in an application, the files have to be "converted" to the appropriate format for the chosen software. Often, the data comes from different sources, with different or incompatible structures or formats, so that it becomes preferable to "re-key" the text or re-scan the graphics, because the conversion process would be a much more costly process. Currently, the problem of multiple formats is being addressed by conversion programs, rather than by standardization. Until standardization of formats progresses, data preparation for large collections will remain a time-consuming and costly undertaking.

  5. CD-ROM Disc Image
  6. The disc image, also known as program image, refers to the complete collection of data files that make up the application and will be placed on the CD-ROM. The files are placed in the desired order, taking into consideration the software's patterns of access to the data, the need to join or separate document types, or the need to access and use series of files together. Attention is also given to the treatment of graphics files, especially if the application involves large numbers of them. Once all the files are in the desired order, they are treated as a set or "image".

  7. Simulation
  8. This next step involves use of the application software and the data (in the CD-ROM image), as if the whole were already the final CD-ROM application. Understandably, this step is more important for first time products. Previously, simulation took place at the CD- ROM developer's site, using the CD-ROM "publisher" hardware and software. But, today, many applications can be simulated in-house, because most of the newer desktop CD- ROM "publishing" packages have the capability to do simulation. They require, obviously, a PC with large storage capacity to accommodate the CD-ROM program image as well as the index tables and other look-up tables that are created for the search and retrieve functions. It is at this stage that the features, operation, output, and "feel" of the CD-ROM application must be adjusted, improved, or redone. Obviously, review or redesign of the application at later stages would be more costly.

    Today, with desktop CD-recordable hardware at affordable levels, testing of applications has become much less complicated. In fact, when the application needs to be tested at various sites, sufficient copies can be economically produced in one-off CD- ROMs, and be tested in depth in the user field, before deciding on mastering and mass replication.

  9. Premastering
  10. This is a term that is used somewhat inconsistently in CD-ROM circles. Here we will consider premastering to mean the conversion of the program image data into fully encoded CD-ROM blocks . This encoding is usually referred to as ISO-level encoding. After the encoding, the premastered CD-ROM disc image is usually transferred to an appropriate medium – so it can be transported to the mastering plant and be used for mastering. Various tape formats are used to transfer the premastered image files to the mastering plants. Today , however, one-off CD-ROMs are also being used as a transfer medium.

  11. Mastering
  12. This step involves creating a master disc and the stampers used to replicate the CD- ROMs. The premastered disc image undergoes what is known as the CD-level encoding, or final encoding. Basically, this involves taking the coded 8-bit data and merging it with other coding that adds error detection and correction codes to produce the 14-bit CD-ROM byte. The fully encoded data string is used to create the glass master. Finally, the glass master is used to produce the metal stampers that are fitted into the CD-ROM replication machines.

    Recently, with the advent of the one-off desktop CD-ROM publishing hardware, this entire process can be emulated in-house. The one-off CD-ROM can be sent to the mastering plant as a source for the glass-mastering and replication.

    For applications that do not need to be replicated in large numbers, such as archival sets, the one-off CD-ROM solution is a more practical and economical option.

  13. Replication
  14. This is the mass-production stage. The injection molding machines are fitted with the stampers produced from the glass master. Then pure melted polycarbonate plastic is injected into the stamper molds and, as the polycarbonate cools down, the pattern of pits and flat spaces are transferred to it. This process is fast, usually producing over 300 CD- ROMs per hour. Once replicated, the clear plastic CD-ROM receives a reflective metallic coating, is covered with a clear coat of lacquer, and then fast-dried. The CD-ROMs are next tested for quality. The final steps include labeling, packing in jewel boxes or other CD- ROM containers, shrink-wrapping, and shipping.

Benefits

  1. Data Access
  2. Unlike tape that must be accessed sequentially, CD-ROM can be accessed randomly. Random access reduces the time required to retrieve data. It also allows retrieval software to quickly retrieve related data items across the entire database.

  3. Costs
  4. 	
                                               Paper           CD-ROM
    
    
      Production        220,000 pages         $3,300             $2
                          6,000 pages         $    90            $2 
       
      Shipping          220,000 pages         $1,390             $0.85
                          6,000 pages         $    38            $0.85
    
    
    
  5. Longevity and Integrity
  6. Testing suggests that a properly produced CD-ROM can last more than 100 years. CD- ROM is a read-only medium, which insures the integrity of the data for the life of the CD- ROM.

Conclusions

Compared to other storage disks or diskettes, a CD-ROM is a very rugged, long- lasting, secure medium for storing large amounts of read-only information. CD-ROM is the most cost effective way to produce and distribute large quantities of data.


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