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Capacity |
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| There are three kinds of CD-R/RW discs classified by volume: 180MB/21min,
650MB/74min and 700MB/80min. Most prevalent of the three is the 650MB
discs that can contain up to 15,000 A4 sheets of data (a 3.5 inch
floppy disc has a capacity of 1.44MB). |
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Following calculations show how a regular CD-R disc comes to have
650MB capacity.
1 sector = 24 bytes × 98 blocks = 2,352 bytes
1 sec = 2,048 (2,352) bytes × 75 sectors = 153,600 bytes = 150 Kbytes
74 min = 150 Kbytes × 4,440 secs (60 × 74 mins) = 666,000 Kbytes =
650 Mbytes |
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Features |
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1) The lowest unit price among storage
devices
The lowest price per unit of volume among all media ever released. |
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2) Broad compatibility through standardization
Broad compatibility with new hardwares, as well as the existing CD
Players, CD-RoM drives and DVD-ROM drives |
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3) Enormous disc volume
A CD-R/RW disc can contain up to 650MB of data, which would need 325
floppy disks to store. A single CD-RW disc would be enough for almost
any kind of software. |
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4) Data preservation
Once recorded on a CD-RW disc, data can be preserved almost permanently. |
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5) Others
Convenient, secure and safe to use, as well as energy-saving and environment-friendly, |
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Life Expectancy |
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| Rewritable discs are physically classified by their color. Different
discs have a different level of life expectancy. "Blue"
CD-R (Cyaninite class) is said to last approximately 75 years while
"gold" one is durable for more than 100 years. Durability
of a CD-R disc is affected by water, sunlight and heat. However, a
CD-R or CD-RW treated well may last effectively 'forever', too - as
long as we care about it. |
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Recording Speed |
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| The following figure is approximate recording time by drive speed
(650MB) |
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Drive Speed
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Data Transfer Rate
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Approximate Recording Time
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1X
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150Kbytes/Sec
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76 mins
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2X
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300Kbytes/Sec
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38 mins
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4X
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600Kbytes/Sec
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19 mins
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6X
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900Kbytes/Sec
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12 mins
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8X
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1200Kbytes/Sec
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9 mins
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12X
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1800Kbytes/Sec
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6 mins
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Recording Principle |
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| laser beam with intensive strength is spotted onto the substrate
side of the disc and is focused into the groove.Then, the recording
layer of organic dye absorbs the beam's energy and converts it into
heat reaching 300°C inside the groove. This temperature rise causes
the plastic substrate to deform and the dye to decompose - that is,
to make holes, pits and bumps. This series of "holes" in
the dye layer are called "pits". Spaces between the pits
are called "lands". The pattern of pits and lands on a disc
is the basis of the binary system of "0" and "1".
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| Data transformed into the binary system is represented by pits of
varying lengths, from the shortest 3T to the longest 11T. Actually,
CD-R disks do not have true pits and lands. Instead, they have burned
and non-burned areas, which fulfill the same function as pits and
lands on a pressed disc. |
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Principle of Analog-to-Digital Conversion |
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| Before looking at how analog signals are converted into digital
ones, we will briefly consider the advantages of digital on which
optical media is based. The following two are most important features
of digital media. |
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1) Error detection and correction capabilities
2) Many additional services |
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| The error detection and correction capabilities built into digital
coding formats can undo the errors of up to certain magnitude and
recover the data. Interleaving is used to convert burst errors to
multiple random errors, which then can be corrected. Before the storage
or transmission of data, successive data words are scattered in a
defined order over several data frame. At the receiver the scattered
data words are rearranged in their original order. Also, to allow
detection and correction of errors additional data have to be added
to the original data. |
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Digitization & Recording |
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| The following figure shows the process of analog signals
being digitized and recorded onto CD-R/RW discs. |
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 Quantization: The process of converting analog amplitude values into
numerical values. |
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| Binary Coding: Converting the numerical values into a binary system. |
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EFM Coding: Eight to Fourteen Modulation (EFM) scheme used for encoding
each 8-bit |
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binary symbol as 14 bits. In magnetic media, a byte commonly has
8 bits. Optical media such as CD-ROM discs uses a 14-bit byte, a modification
necessary because of the way data is stored and read with lasers,
using the pits (indentations) and lands (spaces between indentations)
on the disc. In transferring from magnetic to optical media, the 8-bit
byte has to be modulated to a 14-bit byte. |
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Encoding : Recording the data based on the converted signals by
turning the laser on and off
according to the Sync. |
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