Linux+: Hardware Part 17 – Media – Magnetic and Optical

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  1. Jarret W. Buse

    Jarret W. Buse Well-Known Member Staff Writer

    Jul 3, 2013
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    Linux+: Hardware Part 17 – Media – Magnetic and Optical

    Before USB thumb drives, media was either magnetic or optical. Magnetic media, such as floppy disks or tape drives, use magnetism just like Hard Disk Drives (HDD). Optical media store data by using lasers to read and write data from CD, DVD and Blu-ray disks.

    Floppy diskettes were created to allow for data to be stored and removed from the computer system to be kept elsewhere or to be shared among users. Commercial diskettes were originally an eight inch format which stored 500 KB or 1.2 MB. The 5 ¼” floppy disk which came later, held 360 KB, 720 KB or 1.2 MB. Then there were 3 ½” diskettes, also called a micro-floppy, which held 720 KB, 1.44 MB or 2.88 MB.

    NOTE: The differences here in the capacity were due to the density and sides used on the diskette. The 2.88 MB 3 ½” diskette required a special disk drive which allowed Extended Density (ED) which is not covered for the Linux+ Certification.

    NOTE: It may be interesting to know that the sharing of diskettes in an office was originally termed a Sneaker-net network.

    The ‘platter’, or the circular piece of Mylar, is coated with a magnetic material. The platter is surrounded by a liner which prevents the platter from being scratched and even helps remove contaminate particles from the media. The size of the outer protective covering is a square which measures either 8”, 5 ¼” or 3 ½”.

    The ‘platter’ is used to store the data and usually had two sides. There were older drives which could only write to one side. If the other side could and was to be used, the disk had to be removed, flipped and reinserted since the drive could only handle reading and writing to one side of the disk. Later, drives had two read and write heads which were placed one above the diskette and one below. A disk which had two usable sides were termed Double Sided (DS). The original disks were termed Single Sided (SS). Of course, the Single Sided disk only stored half the amount as a Double Sided Disk.

    Another method to increase the capacity was to introduce Double Density (DD) as compared to the standard Single Density (SD). Another capacity booster was the High Density (HD) diskettes. The main difference between the SD and DD diskettes is the format type. Single Density diskettes use Frequency Modulation (FM) encoding. Double Density diskettes use the Modified Frequency Modulation (MFM) encoding. The High Density diskettes use the MFM encoding, but to achieve a higher capacity the magnetic material is made up of smaller particles. The particles can hold a stronger magnetic charge and the particle thickness is less than half that of a DD diskette.

    Diskettes are labeled as one of the following:
    • SSSD – Single Sided Single Density
    • SSDD – Single Sided Double Density
    • DSSD – Double Sided Single Density
    • DSDD – Double Sided Double Density
    • DSHD – Double Sided High Density
    NOTE: Track 0 is located on the outermost rim of a floppy diskette.

    Floppy drives are connected to a floppy controller using a floppy drive cable. There are usually three connections: one for the controller and two for floppy drives. The floppy controller can be an expansion card or built-in to the motherboard. Floppy parameters are set in the Complementary metal–oxide–semiconductor (CMOS).

    Optical drives use lasers to read or write data to disks. The three optical disks are the Compact Disc (CD), Digital Video Disc (DVD) and Blu-ray. Each of these types offers a higher capacity than magnetic diskettes as shown:

    • CD – 650 to 700 MB
    • DVD – 4.7 to 15.9 GB
    • Blu-ray – 25 to 128 GB

    All three disc types have a standard size of 120 mm in diameter with a thickness of 1.2 mm.

    The compact discs are a standard 700 MB capacity. CDs which are re-writable have a capacity of 650 MB.

    NOTE: Rewritable discs allow the data to be erased and rewritten. Depending on the manufacturer the number of rewrites is limited before another rewrite fails. The standard number of rewrites is about 1000 times.

    The Digital Video Disc (DVD) was originally used for video media for movies, but can be used to store data as well as movies. The standard type is the DVD-5 which has a capacity of 4.37 GB. The types are as follows:

    • DVD-1 – SSSL – 1.36 GB
    • DVD-2 – SSDL – 2.47 GB
    • DVD-3 – DSSL – 2.72 GB
    • DVD-4 – DSDL – 4.95 GB
    • DVD-5 – SSSL – 4.37 GB
    • DVD-9 – SSDL – 7.95 GB
    • DVD-10 – DSSL – 8.75 GB
    • DVD-14 – DSSL+DL – 12.33 GB
    • DVD-18 – DSDL – 15.90 GB

    The designation of Single Sided (SS) and Double Layer (DL), for example, are the same as with the Magnetic Media.

    The DVD laser has a wavelength of 650 nm to allow for writing to a smaller area so more data can be stored on the DVD. The capacity is increased because of the wavelength which is 780 nm for CDs.

    The Double Layer DVD is a disc which has a semitransparent layer over another. The laser uses more energy to shine through the semitransparent layer and less to only read or write the top layer.

    Double Sided DVDs do require the disk be removed, flipped and reinserted to access the second side of the disc.

    Blu-ray discs are the same as DVDs, but instead of using a standard laser, the laser is blue, so the wavelength is smaller than the 650 nm for DVDs. The Blu-ray capacity is as follows:

    • SL – 25.0 GB
    • DL – 50.0 GB
    • XL 3 layer – 100.1 GB
    • XL 4 layer – 128.0 GB

    The multi-layer discs work the same as the dual layer DVDs when reading and writing.

    The Optical Drives can be either Integrated Drive Electronics (IDE) or Small Computer Systems Interface (SCSI). The same rules apply as connecting an IDE or SCSI Hard Disk Drive (HDD).

    Optical drives, if IDE, can be configured within CMOS and can even be set as bootable on some systems.

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