Storage

All about CD-R and CD-RW

CD-R and CD-RW technologies use a combination of metal and dye to fool CD-ROMs into thinking they're reading commercially prepared CDs. In this Daily Drill Down, Faithe Wempen shows you how to troubleshoot your CD writing.


This Daily Drill Down is part of a continuing series that illuminates various computer hardware subsystems. I have already tackled the IDE interface ("All about IDE") and CD-ROM formats ("All about CD-ROM standards"); in this Daily Drill Down, you’ll learn how writable and rewritable CD-ROM drives work, how they have evolved, and how you can get the best performance when creating your own CD-R and CD-RW discs.

What’s the difference between CD-R and CD-RW?
CD-R stands for CD Recordable. CD-R discs can be written to only once (except in the case of multisession writing, which I’ll explain later). They are cheap (under $1 each) and work best when recording data that will not change. CD-RW stands for CD Rewritable. CD-RW discs can be written to many times, and they function somewhat like a hard disk or floppy. CD-RW is a good choice when you need to store small amounts of data over time, such as a daily backup of a few critical files. Early recordable CD-ROM drives were CD-R only, but most drives sold today support both CD-R and CD-RW writing.

Can all CD-ROM drives read CD-R and CD-RW discs?
Some very old CD-ROM drives can’t handle CD-R and CD-RW, or can’t handle them consistently. (A drive might be able to read one CD-R disc but not another seemingly identical one, for example.) But most CD-ROM drives, including all the drives manufactured in the last several years, treat a CD-R or CD-RW disc just like any other CD. Almost all audio-only CD players (like in your home stereo system) can handle CD-R discs with no problem, so you can make your own music CDs and play them almost anywhere that regular CDs will play.

How does a CD-ROM drive read discs?
To understand how CD-R/CD-RW drives write to a disc and how regular CD-ROM drives can read those discs, you need to grasp the basics of how a CD-ROM drive reads data from a CD. A CD-ROM stores data in binary form, as do all computer media. The CD’s surface (beneath a smooth, clear protective coating) contains aluminum film that reflects light. Data is stored on the CD with a pattern of pits in the aluminum and unpitted areas called land. The CD-ROM drive reads the data by shining a laser onto the disc and measuring the amount of light that gets bounced back from it. Areas with pits reflect the light less strongly than unpitted areas. When the laser detects a transition from a pitted to an unpitted area, it sends information about that transition to the PC as binary data. As you can imagine, such a system is far from 100 percent perfect, so the CD-ROM standard has error correction built into the read process.

How does a CD-R drive write to a blank CD?
When you make your own CDs, you don’t have the manufacturing equipment to create the pitted aluminum layer that exists on a mass-produced CD. Therefore, the recording process must be different. The recording process for CD-R is made possible by the fact that CD-ROM readers don’t actually touch the surface of the disc—they only look at it. A home-recorded CD need not actually have the pits and land areas of a normal CD, as long as it appears to have them.

Recordable CDs are physically different than mass-produced ones. They are coated with metal and then overlaid with photosensitive organic dye. The dye layer reflects back to a CD-ROM drive just as a blank CD would (that is, all land). During recording, a laser heats the metal and the dye layers in certain spots so that they change their reflectivity to resemble a pit on an aluminum-pitted CD. When a drive reads the CD, the CD appears to have the normal pit and land areas of a commercially produced CD, even though there are not actually any pits.

What is multisession recording?
When you write data to a CD-R, you permanently change the areas on which it is written. In the early days of CD-R, however, many people found that they had far less than the 650 MB limit to store on a CD, so most of the CD’s space ended up being wasted. Because CD-R discs were so expensive, multisession recording was devised to help take advantage of the wasted space.

With multisession-capable recording software, you can reuse a partially used CD-R by creating a new session on the remaining blank space of the disc. When you do so, however, the previous sessions become inaccessible. Only the last session on the disc can be read. This might be useful if you back up a small number of files every day and you don’t need the previous day’s backups after you have made today’s copy. You could use the same CD several times and always have access to the most recent copies.

Since CD-R discs have become cheaper in recent years, the popularity of multisession recording has waned. Although most recording software supports it, few consumers go to the trouble anymore.

What do the different colors of CD-R signify?
Different CD-R manufacturers have devised different combinations of metals and dyes. Any of them will work fine for almost any task, but certain colors and combinations are especially advantageous for certain applications.

The following information comes from my all-time favorite hardware reference guide, Scott Mueller’s Upgrading and Repairing PCs:
  • Green dye, gold metal: This is the standard type of CD-R and the first to be developed. It has a rated lifespan of 10 years and uses cyanine dye, which can be more forgiving of disc-write and disc-read variations than some other dyes. This results in a CD that will likely play well in any CD player. Manufacturers include Imation, 3M, Memorex, Kodak, BASF, and TDK.
  • Gold dye, gold metal: The gold dye used here is phthalocyanine. It is a more sensitive CD with less tolerance for power variations and might be less likely to work in a wide variety of drives. Manufacturers include Mitsui, Kodak, Maxell, and Ricoh.
  • Blue dye, silver metal: The blue dye is azo. This combination has similar properties to the green-gold combo but is rated to last much longer: 100 years. These discs are great for long-term data storage.

Which CD-Rs should I buy?
It all depends on what type of data you plan to burn, how much money you have to spend, how critical the data is, how tolerant you are of the occasional CD ruined due to errors in the recording process, and how long you plan to keep the CDs.

My experience has been that cheaper CD-R discs, such as generic or store-brand, tend to result in more errors during recording. I might spend $20 for a huge spindle of generic CDs and discard 10 percent due to recording problems. After figuring in my lost time in waiting for the recordings, it’s not much of a bargain. These errors are usually caused by manufacturing defects in the CDs, inferior scratch-resistant coating, or scratches from being stored against other discs on the spindle.

If you have a high-speed recording drive, make sure the discs you buy will support the speed at which you want to record. Almost any disc will record reliably at up to 4X, but if you want to record at higher speeds, such as 6X or 8X, you will want discs that state clearly on the package that they are compatible with that speed. I recently bought some Memorex CD-Rs that were rated at 16X at an ordinary office supply store, so the speed you need should not be difficult to find. If the discs do not specifically indicate a speed limit on the packaging, I normally assume that they can support no more than 4X.

The discs you choose will also depend on how you plan to label them. If you have a CD printer that accepts CD-R discs and prints directly onto them, make sure you get discs that have a blank face or only a minimal amount of text on them. If you plan to slap a sticker on them, such as one created with a CD labeling kit, the writing on the face shouldn’t matter much.

If you don’t need jewel cases for each CD, you can save some money by buying bulk CDs on a spindle. I have had a few problems in the past with dust getting in between the stored discs and scratching them, however, so if you do go with a spindle, make sure you keep a dust cover on the spindle at all times, especially if you work in a dusty environment.

Another consideration is disc capacity. Standard CD-Rs have a 650-MB capacity (74 minutes of audio), but discs are also available that have a 700-MB capacity (80 minutes of audio). These high-capacity discs are more expensive, and very old CD-R drives might not be able to use them—or might not be able to use the excess capacity. However, if you’re copying an extra-long audio CD (within your legal rights to do so, of course), or trying to fit just a little more data onto a CD, they can be a real lifesaver.

What’s the difference in media between CD-R and CD-RW?
The CD-RW medium is physically different from a CD-R disc. It’s more complex and costs more to manufacture, which is why CD-RW discs cost so much more. Earlier, you learned how data is burned into a CD-R by heating the dye and metal to change how it reflects in certain spots on the disc. That change is permanent.

A CD-RW disc, in contrast, does not have the traditional dye-and-metal coating. Instead, it is coated with a metal alloy (containing silver, indium, antimony, and tellurium, in case you’re curious) with reflective properties that change depending on the temperature to which you heat it.

A CD-RW drive has a laser that has three different power settings. The high setting heats the alloy to around 600 degrees Celsius, at which temperature it liquefies. When it solidifies again, it has lost its reflective properties. This imitates a pit. The same spot can be reheated to a lower temperature (around 200 degrees Celsius), causing it to revert back to its original reflectivity, imitating a land area. That’s how it rewrites an area. The lowest power setting is used to read the data without changing it.

Can all drives read CD-RW discs?
CD-RW discs have less reflectivity contrast than regular discs, so older CD-ROM drives might not be able to reliably read them. However, almost all regular CD-ROM drives manufactured today, including all multiread drives, will have no trouble with them.

What is packet writing?
Packet writing is a software feature that enables you to transfer files individually, or a few at a time, to a CD-R or CD-RW disc, rather than writing the entire disc at once. Most CD-RW drives come with packet-writing software. Adaptec’s EasyCD software, for example, has a program called DirectCD. Packet-writing software enables a CD-RW disc to function more or less like a floppy or hard disc, with selective writing and erasing.

Packet writing works by creating a virtual allocation table (VAT) each time data is written to the CD. It contains information about the files just written, plus all the information from the previous VAT on the disc, forming a complete table of contents.

There are some limitations to packet-writing technology, however. Audio CDs cannot be packet written, and your CD-RW or CD-R drive must support packet writing. The resulting packet-written CDs require the computer on which they are read to have Universal Disc Format (UDF) reader software and a compatible operating system (such as a 32-bit version of Windows). Packet writing and UDF are much bigger topics than can be covered here; see the list of resources at the end of this Daily Drill Down for more information.

How can I avoid ruining discs when recording?
This is the million-dollar question for most CD-R users. Almost everyone who has ever experimented with CD-R recording has experienced the dreaded “buffer underrun” error, which means your recording has failed and the disc is trashed.

Because a CD-R disc can be written to only once, in a single pass (except in special cases like multisession and packet writing), recording success depends on the CD-R drive being fed a steady, uninterrupted stream of data at exactly the right pace. The drive contains a small buffer (usually 512K or more) in which data from the PC waits to be recorded. If that buffer empties and no data is forthcoming from the PC, the writing laser is idle and a buffer underrun error occurs, thereby ruining the disc.

Other problems can occasionally occur to ruin a disc, but by far the buffer underrun error is the most common. Here, then, are some ways to prevent the buffer underrun error from occurring:
  • Don’t use the PC for anything else while recording. Don’t check your e-mail. Don’t play solitaire. Don’t even move the mouse.
  • Turn off your screen saver before recording so it doesn’t kick in and compete with the recorder for system resources.
  • Disable all programs running in the background before recording, such as your antivirus program, for the same reason as above.
  • If possible, turn on DMA support for the CD-RW drive. This allows it to operate with less CPU interaction.
  • Buy a CD-RW drive with a large buffer (1 MB or more).
  • If you have an IDE drive, place it on its own IDE subsystem (that is, a ribbon cable all by itself).
  • If you are copying a CD from a CD-ROM drive to CD-R, and both are IDE, make sure each drive is on its own IDE subsystem.
  • If your writing software supports it, do a test write first.
  • Use a lower recording speed than the maximum that your drive is capable of.
  • Defragment your hard disk and run ScanDisk (or other disk-checking software) before making the CD.
  • If you are making a CD from data on your hard disk, turn off the power management feature for your hard disk before recording.
  • If your writing software supports it, create a disc image first and then create the CD from the image.

Conclusion
CD-R and CD-RW are amazing, if delicate, data storage technologies. With their large capacities and long data life, using these technologies could solve most of your storage needs, providing you understand the technologies and how to troubleshoot them.
For more information about optical data storage and technology, I suggest the following resources:
TechProGuild
Books
Manufacturers and associations
Articles

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