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11-16-2010, 06:30 AM
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I've seen this reprinted in several places online, and am not really sure what the original source is. It was probably written quite a few years ago. Still an interesting read, with some quantitative facts (more than what Wikipedia seems to offer).

Thought I'd copy it here.

Quote:
What is CD-R?

Write Once/Read Many storage (WORM) has been around since the late 1980s, and is a type of optical drive that can be written to and read from. When data is written to a WORM drive, physical marks are made on the media surface by a low-powered laser and since these marks are permanent, they cannot be erased, hence write once.

The characteristics of a recordable CD were specified in the Orange Book II standard in 1990 and Philips was first to market with a CD-R product in mid-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. In its early days, cyanine dye and its metal-stabilised derivatives were the de facto standard for CD-R media. Indeed, the Orange Book, Part II, referred to the recording characteristics of cyanine-based dyes in establishing CD-Recordable standards. Phthalocyanine dye is a newer dye that appears to be less sensitive to degradation from ordinary light such as ultraviolet (UV), fluorescence and sunshine. Azo dye has been used in other optical recording media and is now also being used in CD-R. These dyes are photosensitive organic compounds, similar to those used in making photographs. The media manufacturers use these different dyes in combination with dye thickness, reflectivity thickness and material and groove structure to fine tune their recording characteristics for a wide range of recording speeds, recording power and media longevity. To recreate some of the properties of the aluminium used in standard CDs and to protect the dye, a microscopic reflective layer - either a proprietary silvery alloy or 24-carat gold - is coated over the dye. The use of noble metal reflectors eliminates the risk of corrosion and oxidation. The CD-R media manufacturers have performed extensive media longevity studies using industry defined tests and mathematical modelling techniques, with results claiming longevity from 70 years to over 200 years. Typically, however, they will claim an estimated shelf life of between 5 and 10 years.

The colour of the CD-R disc is related to the colour of the specific dye that was used in the recording layer. This base dye colour is modified when the reflective coating (gold or silver) is added. Some of the dye-reflective coating combinations appear green, some appear blue and others appear yellow. For example, gold/green discs combine a gold reflective layer with a cyan-coloured dye, resulting in a gold appearance on the label side and a green appearance on the writing side. Taiyo Yuden produced the original cyanine dye-based gold/green CDs, which were used during the development of the Orange Book standard. Mitsui Toatsu Chemicals invented the process for gold/gold CDs. Silver/blue CD-Rs, manufactured with a process patented by Verbatim, first became widely available in 1996. Ricoh's silver/silver 'Platinum' discs, based on 'advanced phthalocyanine dye', appeared on the market in mid-1998.

The disc has a spiral track which is preformed during manufacture, onto which data is written during the recording process. This ensures that the recorder follows the same spiral pattern as a conventional CD, and has the same width of 0.6mm and pitch of 1.6mm as a conventional disc. Discs are written from the inside of the disc outward. The spiral track makes 22,188 revolutions around the CD, with roughly 600 track revolutions per millimetre.

Instead of mechanically pressing a CD with indentations, a CD-R writes data to a disc by using it's laser to physically burn pits into the organic dye. When heated beyond a critical temperature, the area 'burned' becomes opaque (or absorptive) through a chemical reaction to the heat and subsequently reflects less light than areas that have not been heated by the laser. This system is designed to mimic the way light reflects cleanly off a 'land' on a normal CD, but is scattered by a 'pit', so a CD-R disc's data is represented by burned and non-burned areas, in a similar manner to how data on a normal CD is represented by its pits and lands. Consequently, a CD-R disc can generally be used in a normal CD player as if it were a normal CD.

However, CD-R is not strictly WORM. Whilst, like WORM, it is not possible to erase data - once a location on the CD-R disc has been written to, the colour change is permanent - CD-R allows multiple write sessions to different areas of the disc. The only problem here is that only multi-session compatible CD-ROM drives can read subsequent sessions; anything recorded after the first session will be invisible to older drives.

CD-Recorders have seen a dramatic drop in price and rise in specification since the mid-1990s. By mid-1998 drives were capable of writing at quad-speed and reading at twelve-speed (denoted as '4X/12X') and were bundled with much improved CD mastering software. By the end of 1999 CD-R drive performance had doubled to 8X/24X, by which time the trend was away from pure CD-R drives and towards their more versatile CD-RW counterparts. The faster the writing speed the more susceptible a CD-R writer is to buffer underruns - the most serious of all CD recording errors. To reduce the chances of underruns CD writers are generally fitted with caches which can range from between 256KB to 2MB in size. Faster devices also allow the write process to be slowed down to two-speed or even single speed. This is particularly useful in avoiding underruns when copying poor quality CD-ROMs.

With prices down to a similar level of that of a high-speed CD-ROM drive, CD-R had finally became viable as a storage or back-up device. Indeed, it offers a number of advantages over alternative technologies.

CD-Rs generally come in 63- or 74-minute formats holding up to 550MB or 650MB of data respectively and provide a cheap bulk storage medium, working out at about 1p per megabyte. Furthermore, the ubiquity of CD-ROM drives means that discs will be readable on many PCs, a fact that also makes CD-R an excellent medium for transferring large files. Unlike tape, CD-R is a random-access device, which makes it fast to get at archive material and discs are also more durable than tape cartridges and can't be wiped by coming into contact with, say a magnetic field. Finally, just about any form of data can be stored on a CD-ROM, it being possible to mix video, Photo-CD images, graphics, sound and conventional data on a single disc.

The CD-R format has not been free of compatibility issues however. Unlike ordinary CDs, the reflective surface of a CD-R (CD-Recordable) is made to exactly match the 780nm laser of an ordinary CD-ROM drive. Put a CD-R in a first generation DVD-ROM drive and it won't reflect enough 650nm light for the drive to read the data. Subsequent, dual-wavelength head devices solved this problem. Also, some CD-ROM drives' lasers, especially older ones, may not be calibrated to read recordable CDs.

However, CD-R's real disadvantage is that the writing process is permanent. The media can't be erased and written to again. Only by leaving a session 'open' - that is, not recording on the entire CD and running the risk of it not playing on all players - can data be incrementally added to a disc. This, of course, is not the most ideal of backup solutions and wastes resources. Consequently, after months of research and development, Philips and Sony announced another standard of CD: the CD-Rewritable (CD-RW).

What is CD-RW?

Just as CD-R appeared to be on the verge of becoming a consumer product, the launch of CD-Rewritable CD-ROM, or CD-RW, in mid-1997 posed a serious threat to its future and provided further competition to the various superfloppy alternatives.

The result of a collaboration between Hewlett-Packard, Mitsubishi Chemical Corporation, Philips, Ricoh and Sony, CD-RW allows a user to record over old redundant data or to delete individual files. Known as Orange Book III, CD-RW's specifications ensure compatibility within the family of CD products, as well as forward compatibility with DVD-ROM.

The technology behind CD-RW is optical phase-change, which in its own right is nothing radical. However, the technology used in CD-Rewritable does not incorporate any magnetic field like the phase-change technology used with MO technology. The media themselves are generally distinguishable from CD-R discs by their metallic grey colour and have the same basic structure as a CD-R disc but with significant detail differences. 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 CD-R disc's dye-based recording layer, CD-RW commonly uses a crystalline compound made up of a mix of silver, indium, antimony and tellurium. This rather exotic mix has a very special property: when it's heated to one temperature and cooled it 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:
  • the highest laser power, which is called 'Write Power', creates a non-crystalline (absorptive) state on the recording layer
  • the middle power, also known as 'Erase Power', melts the recording layer and converts it to a reflective crystalline state
  • the lowest power, which is 'Read Power', does not alter the state of the recording layer, so it can be used for reading the data.
During writing, a focused 'Write Power' laser beam selectively heats areas of the phase-change material above the melting temperature (500-700 oC), 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 (200 oC) for a sufficient time (at least longer than the minimum crystallisation time), the atoms revert back to an ordered state (i.e. the crystalline state). Writing takes place in a single pass of the focused laser beam; this is sometimes referred to as 'direct overwriting' and the process 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. One compromise here is that the disc reflects less light than CD-ROMs or CD-Rs and consequently CD-RW discs can only be read on CD players that support the new MultiRead specification. Even DVD-ROM drives, which themselves use the UDF file format, need a dual-wavelength head to read CD-RW.

CD-RW drives are dual-function, offering both CD-R and CD-RW recording, so the user can choose which recordable media is going to be the best for a particular job. By mid-1998 devices were capable of reading at 6-speed, writing both CD-R and CD-RW media at 4-speed. By the end of that year read performance had been increased to 16-speed - a level of performance at which the need for a dedicated, fast CD-ROM drive for everyday access to disc-based data was debatable. By late 2000 the best drives were capable of writing CD-R/CD-RW media at 10/12-speed and of reading CD-ROMs at 32-speed.

Although UDF allows users to drag and drop files to discs, CD-RW isn't quite as easy to use as a hard disk. Initially limitations in the UDF standard and associated driver software meant that when data was deleted from a CD-RW, those areas of the disc were merely marked for deletion and were not immediately accessible. A disc could be used until all its capacity was used, but then the entire disc had to be erased to reclaim its storage space using a 'sequential erase' function. In hardware terms erasing a disk is accomplished by heating up the surface to a lower temperature, but for a longer time, which returns it to the crystalline state.

Evolution of the UDF standard and developments in associated driver software have improved things considerably, making CD-RW behave more like, but still not quite in identical fashion to, hard drives or floppy disks.

Unconfirmed-but-often-credited source may be pctechguide.com

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