Inside the CD-R

Recordable CDs–CD-Rs and

CD-RWs–are

more than just another standard for CD-ROMs. These define new physical media and ways of

recording on them, while continuing to use the existing CD-ROM formats.

CD-Rs

Media and encoding

Conventional CD-ROMs are created by physically

etching data on it in the form of pits and lands, using specialized equipment. The pits

and lands are placed on a spiral groove in the substrate, 1.6 microns apart. A CD-ROM

drive reads pits and lands. CD-R technology faced the difficulty of emulating these pits

and lands onto the disk, without using specialized pressing equipment.

CD-R’s addresses this problem by doing

away with physically etching pits and lands. Like the conventional CD-ROM disk, the CD-R

is also made up of a polycarbonate substrate. It comes stamped with a spiral groove which

is used as the data path during recording. On the top of this polycarbonate substrate, a

special photosensitive dye layer is deposited, and on the top of that a metal reflective

layer–gold or silver alloy is the preferred metal. The whole thing is then encased in

a protective plastic layer.

The photosensitive dye layer emulates the pits

and lands of a regular CD-ROM disk. When the laser of the recorder is turned on, this dye

heats up rapidly and changes its chemical composition. As a result, the area reflects less

light than the unburnt area, thus resembling a pit (diffuses the laser). The unburnt area

resembles a land (reflects light cleanly). The created CD-R can thus be used in standard

CD-ROM drives or CD players.

Writing on CD-R is irreversible, and the data

once stored cannot be removed. Many of the later recorders, however, allow you to append

more data in the unused space, if any, on the disk, in subsequent sessions. Such disks are

called multi-session disks.

CD-R media are generally 74 minutes in length,

and as in a regular 74 minute CD-ROM disk, has a capacity of 650 MB.

CD-R drives

To burn or write onto a CD-R, you need special

drives. Though these drives can read CD-ROMs, they are different from the conventional

CD-ROM drives in the intensity of the laser used. Most of these drives support a large

number of formats to enable writing and reading of a wide variety of CDs. The writing

speed is always lower than its read speed. Since the disk moves at a constant speed as it

writes, it must have the data available in a smooth flow. Any interruption in the spin of

the disk or flow of the data will stop the write process, ruining the disk.

This can be taken care of in two ways: Use of

a substantial amount of memory buffer in the drive or the creation of an image file on the

hard disk. The buffer is supplied with the data that is to be burnt, which in turn is

supplied to the writing process. If the buffer runs short of data during the burning

process, the familiar "buffer under-run error" is caused.

Creating the image file is one of the most

reliable ways of ensuring a constant flow of data to the laser head. Instead of copying

the data to be recorded from its original locations, an exact image of the disk to be

created is stored in a single large file on the hard disk, and then transferred to the

disk. This reduces the chances of interruption while writing, but needs a good chunk of

disk space.

SCSI has been the interface of choice for CD-R

driver, though several IDE/ATAPI drives are also available. The high, sustained throughput

in SCSI allows the flow of data to the drive more easily.

CD-R software

Most CD-R drives ship with their own software.

The software controls the creation of the disk and allows the user to compose his disk.

Several third-party software are available. Adaptec is the pioneer in this field, and its

Easy CD Creator Pro is the most popular for the PC. Toast, also from Adaptec, is used for

CD Creation on Mac.

CD-RWs

Rewritable CDs or CD-RWs can be used like any

conventional media, allowing files to be written onto them right from within applications,

like your word processor or spreadsheet. Also, you can add to or delete files that have

been written to the disk.

Media and encoding

CD-RW media are formed in the same basic way

as the CD-R. However, the recording layer of photosensitive dye is replaced with a

phase-changing compound. This compound changes its state when heated and retains it even

after the heat source is removed. It can later be returned to its original state. What

happens is that when heated, this compound crystallizes. But when heated further and

cooled, it again becomes non-crystalline. It reflects more light in the crystal state

acting as a land, and a pit otherwise. By using two different laser settings, it’s

possible to bring the compound back to its native state, thus allowing you to rewrite to

the disk. But once "closed", you cannot add or delete data from it.

The major drawback of these disks is that

these can’t be read by the older CD-ROM drives and audio players—the pits and

lands created here are not similar to the ones created on a CD-ROM or a CD-R. To read a

CD-RW, the drive has to be CD-RW compatible, which many of the newer ones are. Audio CD

players still can’t read these.

When a CD-RW disk is formatted, the recording

compound is completely crystallized. Writing involves de-crystallization of the

crystallized compound wherever required to form the pits. The laser head is set to throw

the laser beam at the required temperature for this process, and the disk is

"locked". The disk can be ejected by the software only after the head is brought

back to its default state.

The process of burning a CD-RW disk is much

slower than for a CD-R, and most drives available today can write on a CD-RW disk at 2x

speed only.

Declining prices, compatibility with several

formats and increasing application areas is paving the way for CD Writers and ReWriters to

become a mass-market peripheral.