How It Works: CD-R, CD-RW

Go for a Spin

Nearly every computer sold in the past ten years has some kind of optical drive that reads CDs. And CD-R, CD-RW, and CD-ROM drives can be found in almost any store that sells peripherals. Prices range from $40 to more than $500: Basic CD-ROM readers from companies including Hi-Val and Iomega cost the least, while the fastest CD-RW drives from such vendors as Hewlett-Packard and Plextor cost the most. Average CD-R drives fall in the middle of that price range, though they are becoming rare as CD-RW drives push them out of the market.

As for the discs themselves, CD-Rs from vendors including Verbatim and Memorex cost about $1 to $2 each; CD-RW blanks cost about $5 to $10 each, depending on the brand and where you purchase them. One cautionary note: If you have a CD-R or CD-RW drive that records at 8X or 12X speeds, you need to buy specially rated discs with fast-acting dyes.

Besides cost, the main differentiating factors between drives are read speed and, for CD-R and CD-RW drives, write speed. CD read speeds are usually rated as a multiple of how much faster a given drive reads data than the first generation (1X models) of drives, which could read discs at a sluggish 150 kilobytes per second. For example, a drive advertised as "32X CD-ROM" reads data at up to 32 times the speed of first-generation drives.

You'll sometimes see a range of read speeds, "12X-32X," for example. The reason is that grooves toward the center of a CD-ROM have less linear velocity relative to a drive's fixed read head than grooves on the outer edge. More data passes under the head per revolution on the longer outer tracks than on the shorter inner tracks. Older CD-ROM drives compensated for this by varying the rotation speed so that data passed under the head at a constant rate. Today's CD-ROM drives keep the disc spinning at the same rate (slowing down only if the disc is dirty or damaged). That means they read data at the inner part of the disc more slowly than data at the outer edge--which accounts for the range of read speeds in the drive specs. Be sure to note the minimum speed as well as the more-often-advertised maximum speed.

With a CD-R or CD-RW drive, you'll also want to know how fast the drive burns, or writes: That speed makes a big difference in the time it takes to record data. The fastest CD-R drive available can record data at 12X. CD-RW drives often have three sets of "X" speeds, listing, in order, the recording speed for CD-Rs, the recording speed for CD-RWs, and the read speed (6X/4X/24X).

Keep in mind that you might not need the fastest drive. If you own one with a minimum speed of 8X to 12X, and you use it only occasionally--to install software or to look up something on a reference disc, for example--the benefit of getting data a few milliseconds faster doesn't outweigh the cost of adding a top-of-the-line drive.

While recordable CDs are dominant now, a threat is on the horizon: DVD, which has taken its first steps into the recordable drive era. DVDs--which look similar to CDs but can't be read by conventional CD-ROM or CD-R drives--can store as much as 4.7GB of data on each side, or 135 minutes of digital video. DVD-ROM drives range in price from $70 to $200.

Analysts predict DVD will gradually supplant CD formats--although the time line for this transition keeps getting longer, as industry squabbles about standards and worries about copy protection have given CD technology some breathing room. DVD-Recordable drives are extremely expensive and made for professional applications only.

The only rewritable DVD format currently available is DVD-RAM. DVD-RAM drives can store 2.6GB on each side of a disc, which comes in a protective cartridge, although drives that record 4.7GB per side are due next year. The competing DVD+RW standard has not made much headway, and the outlook for it remains grim.

Additional Information

• The Recordable CD FAQ
  
  
• CD Information Center
  
  
• CD Speed drive-testing software
  
  

Andrew Brandt is an associate editor for PC World.