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Technical 
The Internal Components
Optical Technology
Optical Storage Technology
The operation of all optical storage devices depends on laser technology
a highly focused, highly controlled beam of light, either visible all
infrared. This laser beam can be used to record data on special optical
media, to enable traditional magnetic recording on special magneto-optical
media and to read pre-recorded data off optical media.
When data is recorded on optical media, a laser beam burns out any location
that stores a logical one and leaves blank any location that stores a
logical zero. The laser is controlled by electronics that translate computer
data into appropriate burn and no burn operations and where to place the
data on the disk. Generally the laser in your computer only produces enough
light to read the holes burned in the disk.
Compact Disk - Read Only Memory (CD-ROM)
Optical technology offers data integrity and storage density. Optical
media is extremely durable and reliable under normal use. You should still
store and handle your disks carefully, but because the surface of the
disk is sealed in a plastic coating it is never touched by anything other
than light and because the medium itself is almost unaffected by moisture
or temperature conditions. Magnetic read/write heads ride about 1500 times
closer to the disk surface than the heads in CD-ROM readers.
CD-ROM's are also as reliable as they are because a large amount of the
media is used to store dedicated error detection and correction checksums
that can correct automatically any misread data. Even if the read head
were to come into contact with the media surface a negligible amount of
data may be lost because of the strength of the CD media and the electronics
that are smart enough to fill in the blanks. When the laser shines on
the disk to read the data it isn't reading the very top surface of the
media were the scratches and dirt are, it looks through this by focusing
at a distance. Much the same way you can focus on something further away
and not see anything on then end of your nose.
Software distributed on CD-ROM offers advantages too. A master laser platter
is made with an electronically controlled laser that physically burns
or pits the surface of the disk as the data is being written. After this
the distribution disks can be made by pressed the old fashioned way. CD
ROM technology is the most common optical storage technology used on PCs,
particularly for software distribution and for multimedia applications
such as graphics, photographs, sound and motion video. A CD-ROM disk is
a rigid plastic platter 1.2 mm thick and 120 mm in diameter and with a
centre spindle hole that is 15 mm in diameter. Data is written to the
disk by burning pits in the recording surface. Each pit is 0.12 micrometers
deep and 0.6 micrometers in diameter. CD-ROM tracks are 0.16 micrometers
apart for a track density of 16000 tpi. A floppy disk track density is
96 tpi and a typical hard disk has a track density of a few hundred tpi.
CD-ROM data is written from the inside of the disk toward the outside.
Original data is enhanced with error correction before it is written to
the platter. Data is written one segment at a time, each sector consisting
of 2048 bytes of data surrounded by error correction information of 16
bytes in front of each sector and another 288 bytes of error code. In
addition the entire system is written again using a CRC (cyclic redundancy
check) scheme. A typical internal hard drive can find data in as little
as 7 ms and a CD-ROM typically requires above 200 ms for the same task.
Recordable CD-ROM (CD-R)
CDR technology works by using a laser source to burn information in the
platter. After the data is written in this way you can not change it.
You can read it as often as necessary and can rewrite the data to another
portion of the platter, but the information can not be modified in any
way. The disks usually are only written on one side unlike professionally
made CD-ROMs that have the data pressed on to both sides., However once
the label has been silk-screened on to one side, only the other side can
ever be used.
An important consideration is the type of metaphor used. Earlier recordable
CD-ROM drives , which evolved from WORM, or write once, read many drives,
were designed to emulate tape drives, which meant data was harder to access
and slower to retrieve and required that all data that was to be placed
on a single disk be made available at one time and only one time, because
you could not add more later, a result of the way the CD-ROMs directory
was handled. Today's recordable CD-ROMs can write data and then later
add some more.
The characteristics of a recordable CD were specified in the Orange Book
II standard in 1990 and Philips was the first to market a CD-ROM product
in 1993. It uses the same technology as WORM, changing the reflectivity
of the organic dye layer which replaces the sheet of reflective aluminium
in a normal CD disc
Rewritable CD-ROM (CD-RW)
The technology behind CD-RW is optical phase-change, an optical storage
technology in which the disk drive writes data with a laser that changes
dots on the disk between amorphous and crystalline states. An optical
head reads data by detecting the difference in reflected light from amorphous
and crystalline dots. When full a phase-change disk can be erased (or
'reformatted') using a medium-intensity pulse to restore the original
crystalline structure.
The media is generally distinguishable from CDR discs by a metallic grey
colour. A CD-RW disc's phase-change medium consists of a polycarbonate
substrate, moulded with a spiral groove for servo guidance, absolute time
information and other data, on to which a stack (usually five layers)
is deposited. The recording layer is sandwiched between dielectric layers
that draw excess heat from the phase-change layer during the writing process.
In place of the CDR disc's dye-based recording layer, CD-RW uses a crystalline
compound which, when heated to a certain temperature and cooled becomes
crystalline, but if it's heated to a higher temperature, when it
cools down again it becomes amorphous. The crystalline areas allow the
metalised layer to reflect the laser better while the non-crystalline
portion absorbs the laser beam, so it is not reflected.
In order to achieve these effects in the recording layer, the CD-Rewritable
recorder use three different laser powers. During writing, a focused 'Write
Power' laser beam selectively heats areas of the phase-change material
above the melting temperature, so all the atoms in this area can move
rapidly in the liquid state. Then, if cooled sufficiently quickly, the
random liquid state is 'frozen-in' and the so-called amorphous state is
obtained. The amorphous version of the material shrinks, leaving a pit
where the laser dot was written, resulting in a recognisable CD surface.
When an 'Erase Power' laser beam heats the phase-change layer to below
the melting temperature but above the crystallisation temperature for
long enough, the atoms revert back to a crystalline state. Writing takes
place in a single pass of the focused laser beam, which is referred to
as 'direct overwriting' and can be repeated several thousand times per
disc. Once the data has been burned the amorphous areas reflect less light,
enabling a 'Read Power' laser beam to detect the difference between the
lands and the pits on the disk. The recorded tracks on a CD-RW disc are
read in the same way as regular CD tracks by detecting transitions between
low and high reflectance, and measuring the length of the periods between
the transitions. The only difference is that the reflectance is lower
than for regular CDs. CD-RW discs cannot be read by many older CD-ROM
drives or CD players.
ISO 9660 is a data format designed by the International Standards
Organisation in 1984. It's the accepted cross-platform protocol for filenames
and directory structures. Filenames are restricted to uppercase letters,
numbers and the underscore character. Directory names can be a maximum
of only eight characters (with no extension) and can only be eight sub-directories
deep. The standard can be ignored under Windows 95 - but older CD-ROM
drives may not be able to handle the resulting 'non-standard' discs. Every
CD has a TOC (Table Of Contents) which carries track information.
The ISO 9660 file format standards didn't cope well with adding data in
small increments. Writing multiple sessions to a disc results in about
13 Mb of disc space being wasted for every session, and the original standard
limits the number of tracks that can be put on a disc to 99. These limitations
were subsequently addressed by the OSTA's (Optical Storage Technology
Association) ISO 13346 Universal Disc Format (UDF) standard. This operating-system
independent standard for storing data on optical media, including CDR,
CD-RW and DVD devices, uses a redesigned directory structure which allows
a drive to be written to efficiently a file at a time.
Digital Versatile Disk (DVD)
At first glance, a DVD disc can easily be mistaken for a CD: both are
plastic discs 120 mm in diameter and 1.2 mm thick and both rely on lasers
to read data stored in pits in a spiral track. And whilst it can be said
that the similarities end there, it's also true that DVD's sevenfold increase
in data capacity over the CD has been largely achieved by tightening up
the tolerances throughout the predecessor system.
The tracks are placed closer together, thereby allowing more tracks per
disc. The DVD track pitch is reduced to 0.74 micron, less than half of
CD's 1.6 micron. The pits, in which the data is stored, are also a lot
smaller, allowing more pits per track. The minimum pit length of a single
layer DVD is 0.4 micron as compared to 0.834 micron for a CD. With the
number of pits having a direct bearing on capacity levels, DVD's reduced
track pitch and pit size alone give DVD ROM discs four times the storage
capacity of CDs. The packing of as many pits as possible onto a disc is,
however, the simple part and DVD's real technological breakthrough was
with its laser. Smaller pits mean that the laser has to produce a smaller
spot, and DVD achieves this by reducing the laser's wavelength from the
780 nanometers infrared light of a standard CD, to 635 nm or 650 nm red
light.
The DVD specification allows information to be scanned from more than
one layer of a DVD simply by changing the focus of the read laser. Instead
of using an opaque reflective layer, it's possible to use a translucent
layer with an opaque reflective layer behind carrying more data. This
doesn't quite double the capacity because the second layer can't be quite
as dense as the single layer, but it does enable a single disc to deliver
8.5 GB of data without having to be removed from the drive and turned
over. An interesting feature of DVD is that the discs' second data layer
can be read from the inside of the disc out, as well as from the outside
in. In standard-density CDs, the information is always stored first near
the hub of the disc. The same will be true for single- and dual-layer
DVD, but the second layer of each disc can contain data recorded 'backwards',
or in a reverse spiral track. With this feature, it takes only an instant
to refocus a lens from one reflective layer to another. On the other hand,
a single-layer CD that stores all data in a single spiral track takes
longer to relocate the optical pickup to another location or file on the
same surface.
DVD allows for allows for double-sided discs. To facilitate the focusing
of the laser on the smaller pits, manufacturers used a thinner plastic
substrate than that used by a CD-ROM, thereby reducing the depth of the
layer of plastic the laser has to travel through to reach the pits. This
reduction resulted in discs that were 0.6 mm thick, half the thickness
of a CD-ROM. The thinner discs were too thin to remain flat and withstand
handling, so manufacturers bonded two discs back to back, resulting in
discs that are 1.2 mm thick which doubles the potential storage capacity
of a disc.
DVD has made the structure of the data put on the disc more efficient.
When CD was developed, it was necessary to build in cumbersome error correction
systems to guarantee the discs would play. When bits are being used for
error detection they are not being used to carry useful data, so DVD's
more efficient and effective error correction code (ECC) leaves more room
for real data.
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