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© Copyright Brian Brown, 1992-2001. All rights reserved.

Floppy Drives
The floppy drive uses a thin circular ceramic disk for data storage. The disk is coated with magnetic particles and is flexible (hence the term floppy).

The disk rotates at 360rpm. A read/write head makes physical contact with the disk surface. Data is recorded as a series of tracks subdivided into sectors. Typical values for an IBM-PC compatible are

stepper motor. The stepper motor moves a small amount for each pulse applied to it. A mechanical switch detects when the read/write head is in the outermost position (track 00).

Fig 5.5: 5.25" Floppy Drive

The two main methods of identifying the beginning sector of a track are

• Hard Sectored Floppy
  In hard sectored, the start of each sector is identified by a physical hole. A light is shone through the hole and picked up on the other side by a detector. As the disk rotates, this causes a sequence of pulses to occur as the sector ID holes allow the light to pass through to the detector. The holes are called index holes. This method was used in the Apple II computer.
• Soft Sectored Floppy
  In soft sectored, only one index hole is used, and each sector is preceded with data bytes which identify the sector number. This format is used in IBM-PC compatible computers.
  

Fig 5.6: Floppy Drive Cleaning Kit

The image on the left shows a floppy diskette cleaning system. It consists of a floppy disk containing a cleaning disk and some cleaning fluid. The fluid is placed on the floppy disk and then the disk is inserted. Any foriegn material on the read/write heads are transferred to the disk. The cleaning disk can only be used a limited number of times.

Hard Disks
Hard disk drives use a thicker non-flexible disk. The disk is made from aluminium or ceramic material, and coated in magnetic particles. The disk rotates at a much higher speed (3600rpm). In addition, multiple disks are stacked on top of each other (with a gap in between), with a read/write head allocated for each surface (top and bottom).

Storage capacities range from 10MB (first PC disk used in XT) to several gigabytes.

• Exchangeable Disks
  The disks are provided as removable cartridges which can be removed and inserted into the drive. The drive unit contains the motor drive, electronics and read/write heads. When the cartridge is inserted, the read/write heads are positioned to make contact with the disk platters inside the cartridge pack.

  This allows a flexible method of data storage. As each disk pack becomes full, it can be removed and a new pack inserted to handle the next lot of transactions etc.

• Fixed Disks
  Fixed disks are permanent disks which cannot be removed. Data is archived off the disk onto another storage medium in order to free up space for new data.

Optical Disks
Optical disks use laser technology and special crystalline ceramics (amorphous crystal) to implement data storage. They are similar to an audio compact disk (CD).

Optical disks for PC's store 660MB per disk. They have an access time of about 300ms (little slower than floppy).

The laser beam is shone onto the surface of the disk. Data is stored as a sequence of surface variations called lands (flat surface) and pits (bumps or holes). The light is scattered by the pits and reflected by the lands. These two variations encode the binary 0's and 1's.

A light sensitive diode picks up the reflected laser light from the surface of the disk, and converts the light to digital data.

Data is stored on a single spiral track. The rotational speed of the CD is altered so that the data on the track is always read at the same speed. This is called Constant Linear Velocity.

• WORM (write once read many)
  This CD is used to store permanent data, suitable for archiving. It can be written to once, though some CD writers support what is known as Multiple Sessions, which allows CD's to be written to more than once. About 640MB of data can be written to the CD, as some space is reserved for timing and other information. In addition, for multiple session CD's, each session written has an overhead of approximately 20MB per session.

  
  Fig 5.7: HP SureStore CD-Writer: © HP

• Single Speed/Double Speed/Quad Speed and Hex Speed CDROM
  CDROM's first produced adhered to a set speed of 150KB/s. This is known as single speed CDROM. With the advent of multi-media and graphics, high speeds of data transfer is required. The following table lists the currently available speeds and data transfer rates for each.

  Speed	Transfer Rate
  x1	150KB/s
  x2	300KB/s
  x4	600KB/s
  x8	1200KB/s

• CDROM Interfaces
  CDROM's can be connected to a computer system using the following interfaces
  • SCSI
  • IDE
  • proprietary

  The type of CDROM must have a matching controller type. For example, It is not possible to connect a SCSI CDROM using an IDE interface.

• CD-I
  CD-Interactive is a format which combines multi-media graphics, sound and video on a CDROM. Especially designed for interactive use, it is suited to presentations, training and games.

  Access to the information is controlled via a simple point and click menu interface, which people find easy to use and navigate. This explains it use in public type kiosks and sales presentations.

  
  Fig 5.8: Philips CD-I Player: © Philips

• Photo CD
  Introduced in 1992 by Kodak Eastman, the Photo CD is a CDROM which holds up to 100 high resolution images (2048 x 3072 pixels) captured from 35mm film.

  A Photo CD Master Disc storages images up to five different levels of resolution, from small thumbnail sizes of 128x192 pixels to 2048x3072 pixels. This is 16 times larger than standard TV resolutions, and four times the resolution currently being considered for HDTV (High Definition TV).

  Users can take their negatives to a Kodak licensed photo processing shop and have their pictures transferred onto Photo CD. Most CDROM drives sold in personal computers are Photo CD compatible. The entire Photo CD does not need to be filled at once, photos can be added onto the Photo CD as required.

  
  Fig 5.8: Kodak Photo CD Master Disc: © Kodak

  
  Fig 5.9: All 100 images on a Photo CD shown as thumbnails: © Kodak

• CD+G
  CD Plus Graphics, which contains audio songs and graphics, commonly used for karaoke, which displays the lyrics of a song with graphics enabling users to sing-a-long. This is popular as contests in night clubs and entertainment centers.

  
  Fig 5.10: Pioneer Karaoke System: © Pioneer

• Digital Video Data (DVD) CDROM
  The history of DVD

  1980	Sony and Philips create a standard for Compact Disc Digital Audio
  1982	Sony introduces the first CD player, and the first CD arrives... Billy Joel's 52nd Street
  1984	Sony introduces the portable CD player and the first car CD player
  1985	Sony and Philips create a standard for Compact Disc Read Only Memory CDROM
  1989	Sony and Philips create a standard for Compact Disc Interactive (CD-I)
  1990	Sony and Philips introduce CDROM-XA and CD-R (Recordable CD)
  1994	CDROM firmly established as a medium for personal computers
  1995	CD-Erasable and CD-Plus (enhanced music) are announced

  Differences between DVD and CDROM

  • standard DVD holds 4.7GB per layer, dual layer single sided DVD holds 8.5GB on a single side
  • error correction is more robust than CDROM
  • every DVD is a bonded disc, composed of two 0.6mm substrates joined together
  • smaller pits are used and tracks are closer together than CDROM
  • DVD uses MPEG2 compression for high quality full screen pictures
  • disk is 4-3/4"
  • a single layer DVD can hold a two hour 13 minute movie, with full digital sound in three languages
  • dual layer single sided DVD can hold a movie greater than 4 hours long
  • DVD-ROM drives have a much faster transfer rate than CDROM drives
  • DVD-ROM drives will read and play existing CDROM's and CDA disks
• ISO-9660
  Level 1 ISO-9660 defines names to be the familiar 8+3 convention that MS-DOS users have suffered through for many years: eight characters for the name, a period, followed by three characters for the file type, all in upper case. Many special characters (space, hyphen, tilde, equals, and plus) are forbidden. There's also a file version number, separated from the name by a semicolon, but it's often ignored.

  Level 2 ISO-9660 allows longer filenames and deeper directory structures (32 levels instead of 8), but isn't usable on some systems, notably MS-DOS.

• Rock Ridge
  The Rock Ridge extensions to ISO-9660 define a way for UNIX-isms like long mixed-case filenames and symbolic links to be supported.

  Because it's still an ISO-9660 filesystem, the files can still be read by machines that don't support Rock Ridge; they just won't see the long forms of the names. Rock Ridge is supported by UNIX systems and the Mac. DOS and Windows currently don't support it.

• HFS
  HFS is the Hierarchical File System, used by the Macintosh. This is used in place of the ISO-9660 filesystem, making the disc unusable on systems that don't support HFS.

  At present, the only systems that can read HFS CD-ROMS are Macs, Amigas (with AmiCDROM, available from ftp://ftp.cdrom.com/pub/aminet/disk/cdrom/), and the Apple IIgs, and SGI machines running Irix (they appear as AppleDouble format).

• Joliet
  Microsoft, being Microsoft, created their own standard called "Joliet". This is currently supported by Win95 and WinNT. It's useful when doing backups from Win95 onto a CD-R, because the disc is still readable as ISO-9660 but shows the long filenames under Win95. The spec can be found at http://www.ms4music.com/devl/dvjoliet.htm.

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© Copyright Brian Brown, 1992-2001. All rights reserved.