How It Works: CD-R, CD-RW

CD-Recordable and CD-Rewritable, or CD-R and CD-RW: optical media for storing audio, video, and other types of data.

• Widely adopted standards
  
  
• High capacity, low cost
  
  
• Lower capacity than DVD
  
  

PC users have an embarrassment of options these days when it comes to storage: from DAT to Zip, with digital linear tape, floppies, Jaz, Orb, SyQuest, and Travan in between. But, despite these many options, the versatile, inexpensive, and durable compact disc--in its CD-ROM, CD-Recordable, and CD-Rewritable forms--continues to thrive. Some kind of CD drive is standard equipment on most PCs sold today. And with the latest challenger to the throne, Digital Versatile Disc, mired in standards disputes, CD drives are likely to remain the high-capacity drive of choice for most users for years to come.

Most CDs can store up to 650MB of data. While standard CD-ROMs can't be "written to" by home users, CD-Rs can be written to once, and CD-RWs can be written to hundreds of times. CD-RW drives offer the added benefit of being able to write to CD-Rs as well.

All three disc types store information in grooves that are 1.6 microns wide (a micron is one one-thousandth of a millimeter; the average human hair is about 50 microns wide). Unlike magnetic storage media, such as hard disks, which store data as polarized particles on a magnetic surface, CD-Rs and CD-RWs store data as microscopic reflective and nonreflective spots along the grooves. A drive reads the disc by shining a laser onto its surface and noting how the light reflects (or doesn't) off these spots in the grooves.

Mass-produced audio CDs and CD-ROMs are stamped by small presses that create tiny bumps (called lands) and holes (known as pits) in the grooves. Pits reflect light differently than lands along a disc's aluminum or gold surface, and the laser tells the difference by measuring the brightness of the reflection. CD-Rs and CD-RWs contain light-sensitive dyes or chemicals embedded beneath layers of protective plastic. When the high-intensity recording laser hits these light-sensitive materials, they become reflective (or not).

CD-ROM, CD-R, CD-RW, and even DVD drives can all read just about any disc type. The exceptions: A few early CD-ROM and some first-generation CD-R drives cannot read rewritable discs. There's less contrast between the reflective and the matte spots on the surface of blank CD-RWs, and the early, less sophisticated laser read-heads can't distinguish between the two. Those drives will simply reject a CD-RW.