11.1 CD Writers and Media
CD writers can use one or both of these media types:
CD writers have been made in two varieties:
Just as CD writers achieved critical mass in late 1996 and early 1997, it seemed that writable CD might be stillborn as a mainstream technology. DVD in both read-only and writable forms seemed poised to take over the optical market. But squabbling among various DVD manufacturers and standards organizations led to a fragmenting of the DVD market particularly for writable DVD formats and the writable CD stepped into the breach.
Today, CD-RW drives still offer an attractive combination of low price, reasonable performance, acceptable capacity, and good (although not perfect) compatibility between various media and readers. Their big advantage over such competing niche technologies as Magneto-Optical (MO), Phase Change Dual Optical (PD), and Light Intensity Modulated Direct Overwrite (LIMDOW) drives is simple: all of those drive technologies are proprietary, or have such limited market share that they might as well be. That means that few systems can read discs produced on those drives. CD-RW drives, on the other hand, produce discs that can be read on hundreds of millions of ordinary PCs. Although each of the competing niche technologies has one or more advantages relative to CD-RW, those advantages are seldom enough to outweigh the drawbacks.
CD-RW will eventually be replaced by one of the writable DVD standards DVD-R, DVD-RW, DVD-RAM, or DVD+RW but it's still unclear which will prevail. When the marketplace sorts out a winner in the writable DVD competition, and when writable DVD discs approach the cost of CD-RW discs, then CD-RW will fade away. But we don't expect that to happen until mid-2003 at the earliest and probably not until 2004. In the interim, CD-RW will dominate the writable optical market. Even after writable DVD becomes common, CD-RW drives will sell in reasonable numbers, and discs for them will remain available for years thereafter. You needn't worry about your new CD-RW drive being obsoleted any time soon.
11.1.1 CD-Recordable (CD-R) Drives and Media
Although CD-R drives are passé, CD-R technology itself, as implemented in CD-RW drives, remains important. CD-R devolved in spirit from Write Once/Read Many (WORM) drives, which were developed in the early 1980s and were popular in data centers from the mid-80s until better means of permanent storage became available. WORM drives were so called because they used a relatively high-power writing laser to make irreversible physical changes to the disc in write-mode, and a low-power laser (or the same laser operating at lower power) to read the disc. CD-R works on the same principle.
CD-R technology is based on the Orange Book standard, which was developed in the late 1980s and has since been updated, expanded, and split to standardize support for such functions as rewritability and other developing technologies. Philips released the first CD-R drive in 1993. It was extremely expensive, wrote at only 1X, and used $50 CD-R discs made by Taiyo Yuden.
Although they must function interchangeably with standard pressed CDs in CD-ROM drives and CD players, CD-R discs have a different structure. Like pressed CDs, the label side of a CD-R disc is typically printed on a scratch-resistant and/or printable coating that resides on a base of UV-cured lacquer. The next layer is a reflective backing against which the reading laser impinges. This reflective layer may be gold, silver, or a silver alloy, depending on the brand and model of CD-R disc.
As with a pressed CD, a spiral groove is physically stamped into the backing layer at the factory. This groove makes 22,188 revolutions around a standard-length CD-R disc, with about 600 track revolutions per millimeter, and a total length of nearly 3.5 miles. Unlike the groove of a pressed CD, which has pits and lands embedded during pressing, the groove on a blank CD-R disc has no embedded pits, leaving the groove as one long, continuous land.
Whereas the next layer in a pressed CD is the protective polycarbonate layer, a CD-R disc has an extra layer between the reflective backing and the clear polycarbonate layer. This layer is an organic dye that is sensitive to light and heat, and is tuned to the 780 nm wavelength used by a CD writer laser. Although various dyes are used, which vary in color in the visible spectrum, all of them are essentially transparent at the wavelength used by the reading laser. In effect, then, a CD drive sees a blank CD-R disc as one long pristine groove all land and no pits with the dye layer providing no hindrance to light transmission and reflection.
When the CD-R is written to, the power of the writing laser is modulated to literally burn pits into the dye layer. Write power typically ranges from 4 mW to 8 mW, compared to the ~0.5 mW used to read a disc. The laser operating at write power heats the disc to 250°C, which causes a chemical reaction in the dye that renders it opaque at the wavelength used by a reading laser. When the laser of a CD reader strikes one of these burned pits, its light is absorbed and scattered, causing the reading laser to recognize that area as a pit. In contrast, unburned areas allow the reading laser to reflect cleanly from the pressed groove, which the reading laser recognizes as a land. Burned and unburned areas on a CD-R disc thereby correspond to the pits and lands (respectively) on a pressed CD.
Although the method used to make pits and lands differs between pressed CDs and CD-R discs, most modern CD-ROM drives and CD players can read CD-R discs without problems. Some older drives and players cannot, however, and the reasons for that are simple. First, the overall reflectivity of CD-R discs is lower than that of pressed discs. That means a CD-R disc absorbs more of the light used by the reading laser, which in turn requires a more sensitive optical pickup in the reader. Second, the contrast of CD-R discs is lower, which means that there is less relative difference in the amount of light reflected by pits and lands on a CD-R disc than on a pressed CD.
In combination, the general usefulness of CD-R, inexpensive drives and media, and increased performance and reliability mean that CD-R will remain a mainstream technology for several years to come. But CD-R is not a perfect technology.
Read-compatibility of CD-R discs in older drives remains an issue, although such problems are decreasingly common as older, incompatible CD-ROM and DVD-ROM drives age and are retired from service. Unlike pressed CDs, whose aluminum backing layer has nearly level reflectivity across and beyond the visible light spectrum, the reflectivity of CD-R discs is optimized for the 780 nm wavelength used by standard CD-ROM drives. This increases the likelihood that any CD-ROM drive will be able to read the CD-R disc, which has much lower reflectivity than a standard CD. But it may cause problems when trying to read CD-R discs in very old CD-ROM drives, which were not calibrated to read CD-R discs, and in first-generation DVD-ROM drives.
The major advantage of CD-R, its write-once nature, is at the same time its major drawback. For some applications, such as archiving data, the immutability of CD-R is desirable. For others, such as doing daily backups or exchanging data between non-networked users, the permanence of CD-R and the requirement to close a disc before that disc can be read in ordinary CD-ROM drives wastes resources. With blank CD-R discs selling for less than $0.50 each in bulk, the cost in dollars and cents is minor, but juggling numerous partially full CD-R discs can be vexing.
For people who wanted to use a writable CD like a gigantic floppy diskette, something else was needed. That something is called CD-Rewritable.
11.1.2 CD-Rewritable (CD-RW) Drives and Media
A CD-RW drive is a dual-purpose device. When writing CD-R discs, it works just like an ordinary CD-R drive. But if instead you use CD-RW discs and the proper software, a CD-RW drive can erase an old file and write a new file in its place, functioning almost (but not quite) like an enormous floppy drive.
CD-RW is an extension of CD-R technology, initially championed by Mitsubishi Chemical, a major maker of CD-R media, and a group of CD-R drive manufacturers including Hewlett-Packard, Sony, Philips, and Ricoh. CD-RW drives and discs started shipping in mid-1997, just as CD-R seemed poised to become a mainstream technology. Rewritability was considered such a huge advantage that for a time it appeared that CD-R would disappear, killed by CD-RW. That turned out not to be the case. Relative to CD-R, CD-RW had several problems initially, most of which are no longer an issue with the latest drives, media, and software:
11.1.3 CD-RW Technical Details
If you want to know more about the technical details of how CD-RW drives and discs work, read on. Otherwise, you can safely skip to the next section.
CD-RW discs use optical phase change technology, which is similar to that used by magneto-optical (MO) drives, but does not use magnetism to aid the phase change. CD-RW discs are constructed similarly to CD-R discs, except for the recording layer, which is dramatically different. Like CD-R discs, CD-RW discs include a preformed pristine spiral groove, which provides servo (tracking) information, absolute time in pregroove (ATIP) timing data, and so on. But rather than the simple dye layer used by CD-R discs, CD-RW discs use a recording layer sandwiched between two dielectric layers, which absorb and dissipate excess heat generated during writes.
The recording layer comprises a crystalline compound of Silver (chemical symbol Ag), Indium (In), Antimony (Sb), and Tellurium (Te), and so is often referred to as the AgInSbTe layer. Together, these chemical elements form an exotic mix with a very special characteristic: when heated to a specific temperature and then cooled, the compound forms a crystalline matrix, but when heated to a higher temperature and then cooled, the compound assumes an amorphous (disordered) form. The reflectivity of the crystalline form lands, at 25% or so, is much lower than the equivalent on a pressed CD or CD-R disc (>70%). The reflectivity of the amorphous form pits, at 15% or so, is also much lower than that on a pressed CD or CD-R disc (~25%). These lower reflectances, in combination with lower contrast (>3:1 for pressed CDs and CD-R discs versus about 1.6:1 for CD-RW discs) means that CD-RW discs are much more likely to generate read errors or be unreadable in all but very recent CD-ROM drives and CD players.
CD-RW drives use three laser power settings to write and read data:
CD-RW does not use erasure in the traditional sense. Instead, new data is simply written over old data in one pass, which is called direct over-write (DOW). During a write, the laser modulates between write power and erase power, selectively heating the small domains on the track that will become pits and lands. An area that is to be a pit has write power focused on it, heating it instantaneously to between 500°C and 700°C, from which the dielectric layers quickly cool it to the amorphous (nonreflective) state. An area that is to be a land has erase power focused on it, heating it to about 200°C, from which it cools to the crystalline (reflective) state. Once the disc has been written, the read laser in a MultiRead-compatible device can discriminate between the less reflective amorphous areas (pits) and the more reflective crystalline areas (lands).