How It Works: DVD
You've seen people watching
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
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
There are currently four standards for recordable DVDs (see
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.
DVD's time has come--almost. According to analysts at International Data Corporation, DVD-ROM drives are expected to overtake CD-ROM drives as standard equipment on home and notebook PCs by 2001. But in the meantime, CD-ROM drives are still expected to outsell DVD-ROM drives 105 million to 22 million this year.
DVD-ROM drives fit into the same PC drive bays as CD-ROM drives. They have
been optional in new PCs from just about every PC maker for the past year.
DVD-ROM drive read speeds--or "X" speeds--differ from those of CD-ROM drives. A first-generation 1X DVD-ROM drive reads discs at about 1.3MB per second and reads CD-ROMs at roughly the same speed as an 8X or 9X CD-ROM drive. The 4X and faster DVD-ROM drives commonly sold today read CD-ROMs at about the same speed as a 32X CD-ROM drive. Because of the paucity of DVD-ROM apps, the biggest benefit of buying a faster DVD-ROM drive is better CD-ROM performance.
If you have a processor slower than 300 MHz, you may want to get an MPEG-2 decoder card to watch DVD movies and run multimedia apps. Video is stored on DVDs with MPEG-2 compression. For the video to run, the data must be decoded, which requires lots of processing power; too little can mean dropped frames. An MPEG-2 card, which sells for $25 to $50, takes over the decoding work for the processor, allowing the video to play smoothly.
MPEG-2 decoders also typically offer a TV-out connector, which lets you
watch DVD movies on a TV. Some new PCs include these decoder cards, while
others include a TV connection jack on their video cards. If you want to watch
full-screen movies on an underpowered notebook equipped with a DVD-ROM drive,
you might need a PC Card MPEG-2 decoder like
While DVD-ROM drives are positioned to replace CD-ROM drives, recordable
DVD faces a stiff challenge from recordable CD formats, especially CD-RW.
Drive vendors have divided into four camps, each supporting one of the four
standards for recordable DVD. DVD-R hardware is sold exclusively by
At the moment, DVD-RAM is leading the recordable race. However, only users who need storage with extremely high capacity are turning to it. Most consumers who need optical storage find that CD-RW drives fit their needs much more affordably. According to IDC analysts, recordable DVD drive sales won't pick up until more drives capable of reading recordable DVDs hit the market.