KarbosGuide.com. Module 4c1b.

The Optic Media (CD-ROMs and DVD)

Contents on this page:

  • About Optic Data Storage
  • Data read from the CD-ROM
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  • About Optic Data Storage

    The CD-ROM can be compared to a floppy drive, because the disks are removable. It can also be compared with a hard drive, because of similar data storage capacity. Actually, a CD-ROM disk can hold up to 680 MB of data. This equals the capacity of 470 floppy disks. However, the CD-ROM is neither a floppy nor a hard disk!

    While floppy and hard disks are magnetic media, the CD-ROM is an optic media. The magnetic media work in principle like an audio cassette tape player. They have a read/write head, which reads or writes magnetic impressions on the disk. The magnetic media contains myriads of microscopic magnets, which can be polarized to represent a zero or numeral one (one bit).

    In the optic readable CD-ROM, the data storage consists of millions of indentations burnt into the lacquer coated, light reflecting silver surface. The burnt dents reflect less light than the shiny surface. A weak laser beam is sent to the disk through a two-way mirror and the sensor registers the difference in light reflection from the burnt and shiny areas as zeros and ones.


    Our data consist of bits, each of which is a burnt dent or a shiny spot on the CD-ROM disk. Music CDs are designed much in the same manner. The bits are not splashed across the disk, but arranged in a pattern along the track. Without that organization, you could not read the data.

    The platters in hard disks and floppies are organized in concentric tracks. There can be hundreds of those from center to periphery:

    The CD-ROM is designed differently. It has only one track, a spiral winding its way from the center to the outer edge:

    This 5 km long spiral track holds up to 650 MB data in about 5.5 billion dots (each is one bit).

    Data read from CD-ROM

    Data is read from the CD-ROM at a certain speed. There are two principles used reading from a CD-ROM:


    Constant Linear Velocity was used in the early generations of CD-ROM drives. It implies that the data track must pass under the read head at the same rate, whether in inner or outer parts of the track. This is accomplished by varying the disk rotation speed, based on the read head's position. The closer to the center of the disk the faster the rotation speed to deliver the same constant stream of data.


    Constant Angular Velocity. It is not very smart to change the rotational speed of a CD-ROM all The time, as the CLV drives do. Therefore, in more modern and speedy drives, the CD-ROM rotates at a constant number of rounds per minute. This implies that the data transfer varies; data read from the outer parts of the CD-ROM are read at very high bit rates. Data from the inner parts are read slower.

    Let us look at a modern 40X CAV drive. It rotates constantly with a whopping 8900 RPM. This drive will deliver 6 MB per second when reading from the outer tracks. Reading from the inner tracks it only delivers 2.6 MB per second. An average will be 4.5 MB/sec.

    Problematic readings

    The CD-ROM disk has to read in random pattern. The read head must jump frequently to different parts of the disk. You can feel that. It causes pauses in the read function. That is a disadvantage of the CD-ROM media. Also the faster drives can be rather noisy.

    Within the next years the CD-ROM and DVD drives will merge into one unified drive type.

    Rotation speed and data transmission

    There are different generations of CD-ROM drives. Here you see their data.

    CD-ROM type Data transfer rate Revolutions per minute outermost - innermost track
    1X 150 KB/sec 200 - 530
    2X 300 KB/sec 400-1060
    4X 600 KB/sec 800 - 2,120
    8X 1.2 MB/sec 1,600 - 4,240
    40X CAV 2.6 - 6 MB/sec 8,900 (constant)
    40X40 multibeam 6 MB/sec 1,400 (constant)

    Personally I experience no big difference between the 24X, 32X, and 40X spin drives. However, their speedy rotation of the disk causes many physical problems, and the performance vary from drive to drive and CD-ROM to CD-ROM.

    When you see the rotation speeds, you wonder how much further this technology can be advanced. The hard disk can spin at higher speeds, because it operates in a sealed box. The CD-ROM does not, and the high rotation speed causes a lot of practical problems such as noise and vibrations.


    An interesting development in this field is the multi-beam CD-ROM drives. Instead of one laser beam, you put up seven of the kind (however, only six of them are used for data read). This TrueX/Multibeam technology from Zen Research gives 36X performance from a steady 6X CLV speed rotation.

    See www.hival.com. They produce a so-called 40X40-drives with 7 (6 data + 1 error correcting) laser beams, which read simultaneously. That yields genuine 40X performance with a transfer rate of up to 6MB per second, while the CD-ROM disk only rotates like a old 8X drive. Compaq also produces a drive on this basis.

    Music from the CD-ROM

    The PC CD-ROM drive can play regular music CDs. That is a smart "bonus". It requires three things:

  • You must have a sound card in your PC

  • The CD-ROM drive must match the MPC-3 multimedia standard (all modern CD-ROM drives do)

  • You must connect the CD-ROM drive to the sound card with the short special cable, which comes with the drive.

    The CD-ROM can easily hold sound data, which can be played directly through the sound card - without use of the short cable I mentioned. It only becomes necessary, when you want to play quality sound music. Certain games (such as Tuneland) contain both types of sound.

    S/PDIF outputs

    Some CD-ROM drives feature a S/PDIF (Sony/Philips Digital InterFace) output that can deliver a purely digital signal. This gives better sound performance and opens for new interconnectivity (i.e. with a minidisc-recorder).

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    Read Module 5c about SCSI.

    Please read Module 6a about file systems.

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