White Paper: Recordable CD formats ( technical overview

Another attractive factor is CD-RW drives' ability to read nearly all the existing flavours of CD-ROMs and to write to...

Another attractive factor is CD-RW drives' ability to read nearly all the existing flavours of CD-ROMs and to write to inexpensive CD-R disks

When CD-Recordable (CD-R) drives were launched a few years ago, computer disk jockeys were tempted by their ability to archive 650MB of data or store 74 minutes of music on shiny disks that most PCs and stereos could read. But first-generation CD-R drives proved somewhat unreliable, and many early users found it wasn't uncommon to produce a $10 coaster rather than a usable CD.

It seems that drive manufacturers have learned from their mistakes: With today's early CD-Rewritable (CD-RW) drives, creating CDs is becoming almost as effortless as copying floppies. And this ease of use holds plenty of practical value, including the ability to harness the high-capacity disks for rewriteable mass storage, the copying of CD-ROMs, and even compiling customised audio CDs. What's more, the ability to randomly erase and rewrite data to the same disk as many as 1000 times makes CD-RW considerably more appealing than CD-R, which is a write-once format. Improvements in recording software have made archiving files as easy as dragging and dropping them onto the CD-RW drive icon in Windows Explorer.

Another attractive factor is CD-RW drives' ability to read nearly all the existing flavours of CD-ROMs and to write to inexpensive CD-R disks. Conversely, CD-RW disks can be read only by MultiRead CD-ROM drives, which do not yet have a significant installed base, and will be readable in future generations of DVD drives.

The final temptation is value: CD-RW media cost as little as $10 per disk for 650MB of storage, and the price of the drives themselves is several hundred dollars lower than that of the first CD-R units.

For this product roundup, we shopped the direct channel for CD-RW drives and found eight kits that fell into two interface camps: IDE and SCSI. The four IDE drives, which were all 2x2x6x mechanisms (capable of writing CD-R disks at 2x, burning CD-RW media at 2x, and reading CD-ROM disks at 6x), are based on Philips Electronics N.V. and Wearnes Electronics mechanisms.

Of the SCSI drives, three were manufactured by Yamaha Systems Technology and one by Ricoh Corp. The Yamaha SCSI units were based on that company's 4260 drive, which is a 4x2x6x model. Of these, two were internal drives, and the third was housed in an external case. The external 2x2x6x Ricoh unit employs that firm's MP6200s mechanism.

It's no surprise that SCSI drives command a higher price than IDE drives, but some of the disparity in price among the kits can also be attributed to the form factor of the drives (internal or external), the inclusion of a host adapter with the SCSI drives, and other accessories and software.

ZD Labs tested and benchmarked the drives using 266MHz Pentium II-based Dell Dimension XPS PCs equipped with Plextor 12/20Plex MultiRead CD-ROM drives. We used Adaptec 2940UW SCSI host adapters to test SCSI kits that did not come bundled with their own SCSI adapters. In addition to the benchmark tests, we also performed ad hoc tests using a Micron Millennia outfitted with a Pentium 166 OverDrive processor.

The CD Chronicles

Fusing the convenience and familiarity of CD-R with the advantages of rewrite capability, CD-RW represents the next logical step in the evolution of CD technology. And that adds a significant chapter to the Sony/Philips Orange Book, the spec that defines the physical layout of writeable and rewriteable CDs.

CD-RW technology supports a variety of recording modes (disk at once, track at once, multisession, linked multisession, and packet writing). It also supports disk formats such as Red Book (CD audio), Yellow Book (CD-ROM, CD-XA), Green Book (CD-i), Blue Book (enhanced CD, CD Extra), White Book (Video CD), Orange Book Parts II and III (CD-RW), CD-ROM XA (Photo CD), mixed mode, hybrid, and bootable disks.

Venturing into what was once primarily the turf of hard drives, CD-RW drives can write fixed-length data packets, thereby permitting random, direct overwriting of any area of the disk. Direct overwriting offers a significant improvement over technologies like magneto-optical, which requires a drive to make separate erase and record passes over a disk.

Current drive technology and Version 1 of the CD-R multisession specification limit CD-RW drives to a maximum write speed of 2x. Manufacturers must refine the hardware before the drives can write reliably at faster speeds. Also likely to enhance the performance of CD-RW speed is a new version of the Orange Book Part III specification, which is expected to include specifications for acceptable amounts of jitter and noise in faster media.

Digital Topography

Like other CD media, CD-RW storage is based on the concept of "pits" and "lands." In CD-ROM disks, these are microscopic structures that are physically imprinted on one layer of the disk when it is created. As the disk is spun in a CD-ROM drive, the lands reflect the laser beam and the pits scatter it, sending a stream of Morse-code-like pulses to the drive's optical sensor.

CD-R disks simulate physical pits and lands via an optical technique in which dark spots (the pits) are precisely burned into an organic dye layer sandwiched in the disk. When a CD player's laser beam illuminates the disk, it cannot penetrate these dark pits. Conversely, the areas between the pits (the lands) allow beams to pass through the dye layer and reflect off the disk's gold reflective layer. Once the dye has been burned, it cannot be altered; thus, CD-R is a write-once medium.

The structure of a CD-RW disk is similar to that of a CD-R disk. As with CD-Rs, the disk blank is polycarbonate, but the recording layer consists of three parts, rather than the single dye layer found in CD-R disks. In CD-RW, a layer of a metallic phase-change alloy is sandwiched between two dielectric layers that draw excess heat from the phase-change layer during the writing process.

During the writing process, the drive changes the duration and intensity of its laser to create or erase "pits" in the disk's phase-change layer. When this layer is heated to approximately 600 degrees Celsius, then cooled quickly, the material changes from a crystalline to an amorphous state. When this spot is reheated to approximately 200 degrees Celsius and cooled more gradually, its phase reverts back to the crystalline state, which allows more light to reflect from the disk's aluminized layer below.

While a CD-RW disk looks and behaves much like its CD-R or CD-ROM counterparts, it exhibits much lower reflectivity. And that means that the signal is too weak for today's conventional CD-ROM and CD-R drives to read. CD-RW drives as well as MultiRead CD-ROM and newer DVD drives have a simple circuit called an Automatic Gain Control that amplifies this weaker signal and enables the devices to read disks with low reflectance.

Packet Power

Though advances in engineering have brought CD-RW drives to the market, the technology wouldn't be half as useful if not for several notable software achievements. Chief among them is packet writing, which lets the CD-RW drive write small portions of data to a disk over a period of time, rather than in one session.

Packet writing, however, isn't entirely compatible with the logical file system that was developed for CD-ROM disks. That's because this standard, referred to as ISO 9660, must know exactly which files will be written during a session to generate the Path Tables and Primary Volume Descriptors, which point to the physical location of files on the disk.

A new file system called the Universal Disk Format (UDF) 1.5 promises to help locate files by assigning each file on the disk a virtual address. Under UDF, even if you overwrite a file, its virtual address remains the same. At the end of each packet-writing session, UDF writes a Virtual Allocation Table (VAT) to the disk that describes the physical locations of each file. Each newly created VAT includes data from the previous VAT, thereby letting UDF locate all the files you've ever written to the disk.

The downside? UDF isn't supported by current operating systems, which means that you can only share a packet-written disk with another PC that is running packet-writing software--or provide other means for that computer to read the disk. One solution is to embed a UDF driver on each disk. A second alternative, the route chosen by Adaptec for its DirectCD CD-creation software, is to "close" the disk by writing an ISO 9660 Level 3 session around the packet-written data. In closing a disk, the software inventories the disk's contents and writes ISO 9660-compatible directory information to the disk. After completion, the disk can be read by machines that support ISO 9660 Level 3 disks.

Fortunately, by the time you read this, Adaptec plans to have released a free UDF driver that will enable Windows 95 and NT 4.0 machines to read native-format DirectCD disks.

Each drive in this roundup included Adaptec pre-mastering software of one flavor or another, and all were capable of packet writing, although not all of the kit vendors included the necessary software to perform this useful feat. Only one kit, the HP SureStore CD-Writer Plus 7200e, included software that delivered recording options beyond those provided by Easy CD Creator/Pro and DirectCD.

Considering the improvements in software and usability and the huge installed base of CD-ROM playback devices, CD-RW could become the data-archiving device of choice for the next few years.

( ZDnet June 1999

Compiled by Rachel Hodgkins


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