Section 11.1. CD Writers and Media

   

11.1 CD Writers and Media

CD writers can use one or both of these media types:

CD-R discs

CD-R discs record data permanently. Data written to a CD-R disc cannot subsequently be deleted, which may be an advantage or a drawback, depending on how you use the drive. If you partially fill a CD-R disc, you can add more data to it during a later session, but once that disc is full, no more data can be written to it. CD-R discs are cheap half a dollar each in bulk and are a cost-effective means to archive data or to transfer large amounts of data to someone else. CD-R discs can be read in all but the oldest CD-ROM drives, and in most consumer CD players made in the last two or three years. CD-R discs may be written to in audio or various data formats, and can be read by any CD-R, CD-RW, or MultiRead-compatible CD-ROM drive.

MultiRead and MultiRead2 are OSTA standards for CD-ROM and DVD-ROM drives and players. A MultiRead-compatible device can read pressed CDs (CD-DA and CD-ROM), CD-R discs, and CD-RW discs, and can read any disc written using fixed- or variable-length packets. MultiRead2 extends the specification to include DVD-ROM and 2.6 GB DVD-RAM devices. All MultiRead-compatible devices are also multisession-compatible, but the converse is not true. For more information, see http://www.osta.org/specs/multiread.htm.

CD-RW discs

CD-RW discs allow data to be erased. In fact, CD-RW was originally designated CD-Erasable (CD-E), but marketing folks decided that "erasable" had negative connotations. A CD-RW disc can be used repeatedly, deleting old data to make room for new. Current CD-RW discs can be written and re-written at least 10,000 times. CD-RW discs are a bit more expensive than CD-R discs $1 to $2 each as of June 2002 but their reusability makes this price difference trivial. Because CD-RW discs are less reflective than CD-R discs and much less reflective than standard pressed CDs, only recent CD-ROM drives can read them. Few consumer CD players older than two years or so can read CD-RW discs.

CD writers have been made in two varieties:

CD-R drives

These drives can write CD-R discs but not CD-RW discs, and can read standard CDs, CD-R discs, and (usually) CD-RW discs. CD-R drives are usually used in "batch mode" to copy standard audio and data CDs. They can also be used with packet-writing software, although their lack of rewritability makes them better suited to archiving data than backing it up. CD-R drives remained popular through about mid-2000 because they cost less than CD-RW drives. By late 2000, however, the price differential had all but disappeared, and with it CD-R drives. Nowadays, all new CD writers are CD-RW drives.

CD-RW drives

These drives can write CD-R and CD-RW discs, and can read any CD disc. A common use for the RW functionality of CD-RW drives is packet-writing software, such as Ahead InCD or Roxio DirectCD, that allows the CD-RW drive to appear as a Windows drive letter and to function much like an enormous floppy diskette. For example, you can drag files over to the CD-RW drive icon and drop them to back up those files to the CD-RW disc, where they can also be readily retrieved using Explorer. When a disc is full, you can simply erase some or all existing files to make room for new ones.

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.

First-generation DVD-ROM drives use a 650 nm laser, which is so badly matched to the reflective characteristics of CD-R dyes that these drives may have trouble reading CD-R discs. Later DVD-ROM drives use a dual-wavelength laser, which can read CD-R discs without difficulty.

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:

Drive cost

When CD-RW drives began shipping, they sold at a substantial premium $250 or more over the price of CD-R drives with comparable speed and features. Nowadays, that premium has disappeared, and all CD writers support both CD-R and CD-RW.

Media cost

CD-RW discs originally sold for 10 times the price of CD-R discs, which is to say $20 per disc at a time when CD-R discs were selling for $2 each. Although early CD-RW discs could be reused at least 1,000 times (and later versions 10,000 times), this relatively high cost per disc resulted in sticker shock for many potential users. Nowadays, CD-RW discs sell for a dollar or two each in bulk, not much more than CD-R discs.

Speed

The first CD-RW drives supported only 1X rewriting at a time when 4X CD-R drives were common, 6X CD-R drives were available, and 8X CD-R drives were on the near horizon. At those speeds, filling a CD-R disc took 10 or 20 minutes versus the 1.25 hours needed to fill a CD-RW disc, so CD-RW was used only by very patient people. The speed disparity remains, albeit in greatly reduced proportion. Typical CD-RW drives now write at 16X to 40X and rewrite at 10X to 12X. With such a drive, writing a full CD-R disc takes 3 to 7 minutes, versus perhaps 8 to 10 minutes to fill a CD-RW disc, a minor difference for most users.

Compatibility issues exist between high-speed CD-RW discs and earlier CD-RW drives. High-speed CD-RW discs can be written at 4X, 8X, 10X, or CAV (4X through 10X variable) in current high-speed CD-RW drives, but cannot be written by older 1X, 2X, and 4X CD-RW drives. Also, most 1X and 2X drives and some 4X drives may have trouble reading 10X media. The most recent CD-RW media support 12X rewrites, and are compatible only with 10X and 12X drives.

Buggy software

Although premastering software (like EasyCD or Nero Burning ROM) works fine with CD-RW discs, it imposes batch-mode limitations. That is, you can use premastering software to dupe a CD to a CD-RW disc, or to batch-copy a selection of files from your hard drive to a CD-RW disc, but premastering software doesn't provide drive letter access. With drive letter access, the CD-RW disc appears as just another Windows volume, allowing you to use drag-and-drop to manipulate files. But drive letter access requires a different class of software, called packet-writing software. Until recently, all packet-writing software we tried caused compatibility problems and system crashes. In fact, Adaptec (now Roxio) DirectCD was so bad particularly when running under Windows 2000 that we banished it permanently from our systems. Recent versions of InCD (from Ahead Software, the Nero Burning ROM folks) seem usable, although we've been burned so often by packet-writing software that we're still leery of it as a class. Don't let that discourage you if you want to use packet writing, though. Many of our readers report excellent results with InCD.

Convenience

To support packet-writing, CD-RW discs must first be formatted, a process that took more than an hour on early CD-RW drives. That's less a problem now, because modern CD-RW drives require only a few minutes to do a full format on a CD-RW disc. In fact, preformatted CD-RW discs are available, but only for DirectCD, which we refuse to use. Formatting is a one-time operation and can be done in the background, so format times are seldom a problem.

There are three ways to prepare a CD-RW disc for use. For use with premastering software, it needn't be formatted at all. You can use it just as you would a CD-R disc. For use with packet-writing software, the CD-RW disc requires a full format before the first use (done either by you or by the factory). Finally, if the disc has already been formatted and had data written to it that you want to delete, you can do a quick format, that is analogous to a quick format on a floppy diskette. A quick format requires only a few seconds, and "zeros out" the disc without actually erasing the data previously written to it. As with quick formatting a floppy, quick formatting a CD-RW disc is insecure. Although the disc appears empty after the quick format, the data can still be retrieved by anyone with a sector editor. To do a secure erase on a CD-RW disc, choose the full format option.

Confusion

Even knowledgeable people are confused about the relationship of premastering versus packet-writing software to CD-R versus CD-RW discs. Many people believe premastering software (like Easy CD or Nero) can be used only with CD-R discs, and packet-writing software (like DirectCD or InCD) only with CD-RW discs. That's simply not true. You can use premastering software with CD-RW discs (e.g., to duplicate a CD-ROM disc to a CD-RW disc), just as you can use packet-writing software for drive letter access to a CD-R disc.

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:

Write power

The highest power level. Heats the recording layer to high temperature (typically, between 500°C and 700°C), called the melting temperature. When melted, the molecules that form the recording layer are extremely mobile, which allows them to assume an amorphous (noncrystalline) state as they cool. When amorphous, molecules are relatively nonreflective, and correspond to pits on a pressed CD. Write power is modulated during writing of a CD-RW disc to form pits as needed to represent data being written.

Erase power

A medium power level. Heats the recording layer to moderate temperature (typically, 200°C), called the crystallization temperature. At this temperature, the molecules that form the recording layer have only limited mobility insufficient to assume the fully unordered amorphous state, but sufficient to allow them to arrange themselves in a rigidly ordered crystalline state. When crystalline, molecules are relatively reflective, and correspond to lands on a pressed CD. Erase power may be applied unmodulated to return an entire CD-RW disc (or portions of it, such as a track or a file) to a formatted, blank state. Alternatively, erase power may be applied modulated to form lands as necessary to represent data being written.

Read power

A low power level. Does not significantly heat the recording layer, but provides sufficient illumination to allow the current state of each bit to be read. Read power is applied unmodulated. The resulting varying reflectance caused by the virtual pits and lands on the CD-RW disc surface modulate the light, which is then detected and decoded by the read circuitry of the drive.

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).

When using a CD-RW disc with packet-writing software, which is described later in the chapter, how you can erase that disc depends on whether you have written to it using fixed-length packets or variable-length packets. In the former case, you can erase individual files, which is called random erase. In the latter case, the only option is to erase the entire disc, which is called serial erase or sequential erase, and in effect means reformatting the disc.

       


    PC Hardware in a Nutshell
    PC Hardware in a Nutshell, 3rd Edition
    ISBN: 059600513X
    EAN: 2147483647
    Year: 2002
    Pages: 246

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