Karbosguide.com - Module 4c1a.

The Optic Media (CD-ROMs and DVD)


The contents on this page:

  • About Optic Media
  • The Compact Disk
  • The CD-ROM

    Contents on the following pages:

  • Music from CDs
  • CD-R and other variants
  • DVD
  • HD-ROM (future)
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  • Previous page


  • An introduction

    CD-ROM and DVD are optic readable media, contrary to hard disks, floppy disks and tapes, which are magnetic.

    The optic storage media are read with a very thin and very precisely aimed laser beam. They supplement the magnetic media. They have clear advantages in the areas of data density and stability: Data can be packed much more densely in optic media than in magnetic media. And they have much longer life span. It is presumed that magnetic media, such as a hard disk or DAT (digital audio tape) can maintain their data for a maximum of five years. The magnetism simply fades away in time. Conversely, the life span of optic media are counted in tens of years.

    Let us take a closer look at these disks, which are becoming increasingly popular for all types of information, education and entertainment. There are different types:

    The Compact Disk

    The compact disk (CD) was introduced by Philips and Sony in 1980 to replace LP records. It is a small plastic disk with a reflecting metal coating, usually aluminum. Myriads of tiny indentations are burned into this coating. These indentations contain the music in millions of bits. The CD is organized in tracks. Each track is assigned a number.

    The big advantage of the CD is its high quality music reproduction and total absence of back ground noise as well as a great dynamic. During operation, the software in the drive can correct errors caused by such things as finger marks on the disk. All in all, CDs are excellent music storage media.


    The CD-ROM

    The CD-ROM (Read Only Memory) came as an extension of the CD in 1984. In principle, the media and the drives are the same. The difference is in the data storage organization. In a CD-ROM, the data are stored in sectors, which can be read independently - like from a hard disk.

    The CD-ROM has become an important media in the PC world. It can hold 650/700 MB of data, and it is very inexpensive to produce. Today, there are three types of CD drives and DVD drives are on their way:

    Drive type Name The drive can
    CD-ROM Compact Disk Read Only Memory Read CD-ROM and CD-R
    CD-ROM
    multiread
    --''-- Read CD-ROM, CD-R and CD-E
    CD-R Compact Disk Recordable Read CD-ROM and CD-R. Write once on special disks named CD R
    CD-RW Compact Disk ReWritable Read CD-ROMs and CD-R. Write and re-write on special disks (CD-RW).
    DVD RAM Digital Versatile Disk Random Access Memory Reads all CD formats. Reads DVD ROM. Reads and writes DVD disks

    Let us start by look at the CD-ROM construction. To facilitate understanding, it will be easiest to compare it with other disk types, especially the hard disk. The CD-ROM is a plastic disk of 4.6" diameter.

    It is placed in a CD-ROM drive, which is like a drawer in the PC cabinet:

    When the CD-ROM disk is placed in the drive, it starts to spin the disk. It reaches operating speed in one to two seconds. Then the drive is ready to read from the disk.


    Drives and operating system

    The drive must be assigned a drive letter. That is a task for the operating system, which must be able to recognize the CD-ROM drive. That is usually no problem in Windows 95/98. However, the alphabet can be quite messy, if there are many different drives attached.

    Each drive must have its own letter. They are assigned on a first come first-serve-basis. The CD-ROM drive usually gets the first vacant letter after other existing drives, typically D, E, or F. But the letter can be changed in Windows . If you hit Win+Pause, the System box opens. Find your CD-ROM drives like here (The box is Danish, but you'll find it) :

    Highlight the drive and choose Properties. Then you can arrange the drive letters:

    I like to reserve the letters I: and J: for CD drives.

    Once the CD-ROM spins and the operating system (DOS or Windows ) has "found" the CD-ROM drive, data can be read for processing. Now the CD-ROM works like any other drive. Only, it is Read Only Memory!

    The CD-ROM holds its own file system called ISO 9660. It is not using FAT!


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    Learn more

    Read Module 5c about SCSI.

    Please read Module 6a about file systems.

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