How It Works: DVD

Digital Versatile Disc (DVD): a high-capacity optical storage medium for video and data.

• Discs hold about seven times as much information as CD-ROMs.
  
  
• May require special hardware.
  
  
• Development of recordable DVD format stunted by standards war.
  
  

You've seen people watching Austin Powers on their laptops. But with Digital Versatile Disc, aka DVD, you can do more than watch movies: You can also store gigabytes of data and run multimedia applications. DVD's 4.7GB of storage far surpasses that of CD-ROM's 650MB, though the discs are the same small size. Within the next few years, DVD-ROM drives will likely replace CD-ROM drives on most new computers.

DVD comes in two formats: DVD-Video (also known simply as DVD), which is used for movies, and DVD-Read-Only-Memory, which is used for data storage. DVD movie players, which vendors hope will supplant VCRs in home entertainment systems, can't read DVD-ROMs. But DVD-ROM drives that ship with computers can read DVDs, CDs, and DVD-ROMs.

Like CDs, DVDs store data in microscopic grooves running in a spiral around the disc. All DVD drive types use laser beams to scan these grooves: Minuscule reflective bumps (called lands) and nonreflective holes (called pits) aligned along the grooves represent the zeros and ones of digital information.

But that's where the similarities end. DVDs use smaller tracks (0.74 microns wide, compared to 1.6 microns on CDs) that allow them to store more data. The narrow tracks require special lasers--which can't read CD-ROMs, CD-Rs, CD-RWs, or audio CDs. DVD-ROM drive makers solved the problem by putting two lasers in their drives: one for DVDs, the other for CDs.

Recording-capable DVD drives let you store data on special discs called blanks. When the recording laser in these drives hits the light-sensitive material on the disc surface, tiny reflective blocks become nonreflective, like the pits on commercially pressed DVDs.

There are currently four standards for recordable DVDs (see Recordable DVDs Compared chart). DVD-Recordable is a write-once standard; first-generation DVD-Rs could hold 3.95GB of data but can now hold 4.7GB. DVD-R drives cost about $5000, so they're best suited for professional uses such as developing DVD-ROM titles.

The three other standards let you record on a disc more than once. DVD-RAMs come in cartridges and hold 2.6GB per side. The DVD Plus Rewritable, or DVD+RW, format holds 3GB, and 4.7GB versions are in the works. Pioneer's DVD-Rewritables (DVD-RW) store 4.7GB.

Among these rewritable formats, only DVD-RAM is available to consumers. It's favored to become tomorrow's replacement for both the VCR and CD-RW.