Why are CDs circular

CD-ROM: basic knowledge

1. What does CD-ROM mean?
2. What is a CD-ROM?
3. How does a CD-ROM work?
4. What is the storage capacity of a CD-ROM?
5. Technical data
6. story
7. Advantages
8. Disadvantages
9. CD-ROM server

1. What does CD-ROM mean?

CD-ROM means C.ompact D.isc - R.ead Only M.emory.
It means that the data once put on the CD can no longer be changed or overwritten. So you can't back up data like you can with a floppy disk!

2. What is a CD-ROM?

It is an optical mass storage device of the so-called compact discs family. Mass storage is a storage medium with a relatively high storage capacity, on which the data is not lost even when the power is off.
Examples: magnetic tape, floppy disk, hard disk, floppy disk, CD-ROM, DVD or MO drive.

A CD-ROM is a coated, thin, circular plastic disc about 12 cm in diameter on which data is stored. The data is stored in sectors of the same size. These sectors are placed on a single track that is laid out in a spiral. The length of this spiral track is more than 6 km!
Its designation means that it is purely a read medium, i.e. it cannot be written to more than once, like a hard drive, for example, that there is only read-in and not also write access.

Layer structure of the CD: The CD consists of different layers. The carrier material takes up most of the total thickness. Any transparent material with a refractive index of 1.55 can be used as the carrier material. Press shops mostly use polycarbonate. The spiral-shaped data track is located on a wafer-thin reflective metal layer (0.05-0.1 µm), usually an aluminum alloy. The metal layer is covered by a protective layer of label made of UV-resistant varnish.

3. How does a CD-ROM work?

With electromagnetic storage media such as hard disks or floppy disks, the information to be stored is fixed by magnetizing the medium. Since this magnetization is reversible, these media can be rewritten.
In the case of optical storage media, microscopic observation reveals a myriad of very small pits on the surface thereof. These depressions are called Pits, the spaces in between Lands, and are scanned or read with a very thin and very precisely aligned laser light beam. The information is contained in binary code in the very small pits and lands.
The success of CD systems can be attributed to the following two properties of laser technology:
1) The scanning of the CD-ROM surface by means of a laser is purely optical, so it is completely free of contact and wear, and
2) The laser beam can be focused very strongly, i.e. its diameter can be reduced so much that information can be stored in very closely spaced tracks. The result is a very high data density.

4. What is the storage capacity of a CD-ROM?

The storage capacity of a CD is around 650 Mb, which roughly corresponds to the storage capacity of one of the following carriers:

    • 460 pieces 3.5 inch floppy disks
    • 270,000 A4 pages
    • 1 to 19 hours of audio depending on quality and format
    • up to 1 hour of video depending on quality and format
    • 100 Photo CD images

1 byte = 8 bits
1 kilobyte (KB) = 1,024 bytes
1 megabyte (MB) = 1,024² = 1,048,576 bytes, i.e. around one million bytes, but exactly 1024 KB (= 210)
1 gigabyte (GB) = 1,024³ = 1,073,741,824 bytes, i.e. around one billion bytes, but exactly 1024 MB (= 230)
1 terabyte (TB) = 1,0244 = 1,099,511,627,776 bytes

5. Technical data:

The most important CD formats are:

1) CD-DA (Compact disc digital audio, Audio CD)
The technical specifications and standards for the audio CD are laid down in the "Red Book" (Sony, Philips 1982).
An audio CD essentially consists of 3 elements:
1) Lead-in: Disc Label and ToC (Tables of Content) (4mm)
2) Program area: up to 99 tracks (33mm), and
3) Lead-out: the last millimeter of the audio CD

2) CD-ROM (Compact Disc Read Only Memory)
In contrast to the audio CD, the technical standards specified in the "Yellow Book" (Sony, Philips 1985) are not sufficient for real use of the medium. Hundreds to thousands of files of different data formats have to be addressed and addressed on a CD-ROM. In order to achieve standardization in this area, since otherwise an abundance of incompatible CD-ROM and drive technologies would exist side by side, the "High Sierra Group Proposal (HSGP)" and, furthermore, ISO 9660 were adopted.

3) CD-R (Compact Disc Recordable)and CD-RW (Compact Disc Rewritable)
The rewritable optical disks use a combination of magnetic fields and laser action. The technology directly uses the optical properties of magnetic fields. Special materials are used for the realization, the direction of magnetization of which can be easily and quickly reversed. A tiny area of ​​the plate is heated by the laser for writing. At the same time, a strong magnetic field is applied from the outside. As a result, the direction of magnetization in the heated area is retained when it cools down.

other CD formats

6. story

 1979 The audio CD appears as a competing product for
classic record
1985  The CD reaches the computer area as a CD-ROM
First database on CD-ROM: BiblioFile
1989  Multimedia extension: CD-ROM / XA
1992 The Kodak Photo CD enables photo templates to be saved on CDs
1993 Platforms and game consoles from the field of entertainment electronics (3DO, ​​CD32, SegaMegadrive) and playback of videos on the PC (video CDs)
1994 Rapid development: exponential expansion of the offer
1995 Compact Disc Read Write (CD-RW), rewritable, specified in the Orange Book
1996Product announcement for the CD-RW by Sony, Philips, Hewlett-Packard, Mitsubishi and Ricoh
Blue Book CD-Extra ("enhanced CD").
A CD Extra is a multisession CD. The first session contains audio data and the second session data in CD-ROM / XA format.
 from 2000

Competition from the new storage medium DVD ("digital versatile disc"). DVD is proving to be a real breakthrough in terms of movie playback. A DVD disc, which is confusingly similar to a conventional CD-ROM, can now accommodate up to 17 GB of data instead of 650 MB (storage capacity approx. 25 times as large).

The fundamental difference between CD and DVD lies in the laser wavelength. The DVD works with short-wave light. In this way, a bit can be placed on a smaller area.

New standard from the TWG (Technical Working Group), which includes companies such as Compaq, Hewlett-Packard and IBM. DVD drives can usually read conventional CD-ROMs and audio CDs as well.


7. Advantages

  • high storage capacity
  • high material robustness: durability significantly higher than with magnetic media
  • portability
  • Standardization: clearly defined recording formats
  • simple but powerful retrieval interfaces (menu interfaces):

  • Command languages ​​with significantly simplified handling
  • Immutability of the data: is even an advantage, namely when it comes to archiving databases or when any subsequent change or deletion of the data is undesirable.
  • Inexpensive:
    Manufacturing: The manufacturing costs are low

  • Fixed annual prices: calculable costs
8. Disadvantages (compared to online research)
  • Lack of a standard for user interfaces and retrieval languages
  • low topicality (update periodicity)
  • low access speed compared to modern hard drives
  • Storage capacity restriction: Distribution over a larger number of disks
  • Single workstation application:

  • 1) Not a "multi-user"; Access to the CD-ROM by only one user;
    more than one user: dramatic slowdown in access times
    2) Site use

9. CD-ROM server or CD-ROM station

  • Solution of the disadvantages: Networking of CD-ROMs (Germany, USA)

  • Example: UltraNet Server

  • Problem:

  • 1) CD-ROM applications are pure DOS programs (making access more difficult in heterogeneous networks) that were not designed for network operation.
    2) currently no client-server applications (there is always full data traffic in the network => very heavy load)
    3) designed as a typical single workstation application
    (e.g. defined drives must be the same for all participating PCs in the network), low network security
    4) no standardization in the area of ​​retrieval programs
    5) Problems with the license
  • Examples of IT development:

  • 1) DOS interface
    2) Windows interface
    3) internet