Hardware Fundamentals
© Copyright Brian Brown, 1992-1999. All rights reserved.
This material may not be reproduced in printed or electronic format without the express permission of the author.

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). 

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

Philips CD-i Home Page

• 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

Pioneer Karaoke Entertainment Systems

• 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
Philips DVD Home Page and FAQ
• 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.

Additional References on CD's
CD-R FAQ

Tape storage
Tape storage is primarily used for archival storage. The recording medium is a thin polyester tape between 0.38cm and 2.54cm wide, about .025mm thick and coated with magnetic particles.

The tape is wound on wheels of various sizes. Data is recorded sequentially, one byte after the other, from the beginning of the tape to its end. Access times for tape storage are much slower than for floppy diskettes or hard disks.The following diagram shows how data is stored on the disk. The read/write head is comprised of several tracks, 9 tracks being very common. This means nine bits are recorded across the width of the tape simultaneously.

Using nine tracks, one byte of data plus a parity bit is recorded at a time (vertically). Characters are grouped together to form a record (often called a block). Each record is separated from the next record by a blank inter-record gap (IBG). Each record is preceded by a header field. The IBG allows the software to position the tape for recording or playing at specific record numbers.

Physical markers are used to indicate the ends of the tape, and may be metallic foil, clear plastic or software markers encoded on the tape. Software encoding uses BOT (beginning of tape) and EOT (end of tape) indicators.

Write protection is provided by a groove in the reel (or notch tab in cartridge systems). If the ring is fitted, this is detected when the reel is mounted on the tape drive. This then enables writing to the tape. Removal of the ring enables write protection. 

Reel tape	Reel tape, reverse side showing red write protect ring in place.	Reel tape, reverse side showing red write protect removed, tape is now write protected.

Characteristics of tape storage
Tape units range from very small cartridge systems to large reel based systems. The following diagram shows a large reel based system which uses vacuum chambers to keep the tape correctly tensioned at all times.

The capstan is a roller which grips the tape and pulls it past the read/write heads at a constant velocity. Vacuum systems have a tape speed of about 300cm/s. Reels can hold about 730m of tape, with densities of 4Kb/cm. The transfer rates are about 320Kb/s, but access times, due to the sequential way in which data is stored, is very slow.

Tape cartridge systems used a cassette based system to hold the tape on two reels (similar to an audio cassette). The tape width varies in size, common widths are º" and 8mm.

Digital Audio Tape (DAT)
The most common form of cassette system used for large storage archival is the DAT (Digital Audio Tape) system. Data is recorded using a helical scan (similar to video recorders), diagonally across the tapes surface.

A DAT cassette is about half the size of a standard analog cassette and is 3.81 millimeters (1/8") wide.

Each cartridge can hold several gigabytes of data, and access times are very quick (somewhere between floppy and hard disk). Data transfer rates are faster than floppy (but slower than hard disk). Cartridges cost about NZ$50 each.

Digital Data Storage (DDS)
The DDS format for tape drives was developed in 1989, in order to meet the need for high-capacity, compact tape backup for network servers and multiuser computer systems.

DDS is derived from the DAT standard and has been extended to DDS-2 and DDS-3 as backup capacity requirements have increased.

DDS-1 (1989) supports 1.3 GB, DDS-2 (1991) supports 2GB. DDS-3 (1993) supports 4 GB. DDS-1 and DDS-2 use 120 minute tapes while DDS-3 uses 125 minute tape. The storage capacities mentioned above are without compression.

The DDS (Digital Data Storage) format was designed for storing computer data storage. It was originally implemented for DAT subsystems, giving a capacity of 1.3 GByte on a 60 metre tape. This was quickly expanded to 2 GByte with the introduction of 90 metre tapes.

The DDS format has been extended to include data compression. This newer format is called DDS-DC (Digital Data Storage - Data Compression), allowing up to 8 GByte of compressed data to be stored on a tape.

DDS Tour

QIC40/QIC80 and Other Standards
QIC stands for Quarter Inch Cartridge tape. 

Fig 5.14: QIC-80 cartidge which can hold 120MB of data

QIC-Wide-3010
425MB Native, 850MB Compressed

What is QIC40/QIC80

References
Memory Storage (pg 77) : Computers and Electronics, December 1982
Winchester Disk Drives (pg 133) : EDN, June 28, 1984
VideoDisc (pg 129) : ETI, May 1981
Hard Disk Drives for the masses (pg 46) : Computers and Electronics, April 1985
Optical Disks (pg 23) : Computer Design, March 1985
Principles of Optical Storage (pg 70) : Electronics and Wireless World, March 1985
Store it with Light (pg 61) : Computers and Electronics, July 1984
Tape Backup for Hard Disks (pg 80) : Computers and Electronics, September 1984
Data Storage in a Nutshell (pg 36) : Computers and Electronics, July 1983
The Floppy Disk (pg 141) : Byte, September 1984

Summary
Magnetic media has been used for a long time. It has offered large storage capacities, low cost per bit, and medium access times.

As optical storage improves, it will make inroads into the area of mass storage devices like hard disks.

The hard disk drive is divided into a number of Tracks. Each track is subdivided into a number of equal sized Sectors, each of which hold data. The minimum unit that can be read from or written to a disk unit is a sector.

As software sizes have increased, CDROM has become the preferred method of shipping Operating systems and software, as it reduces the number of diskettes, is cheaper to produce, more reliable and faster to install.

DVD promises large storage capacities which currently match those of current hard disk SCSI storage devices.

Tape cartridges, especially DAT, is used for archival purposes in backing up large amounts of data and computer systems software.