CD-ROM and DVD-ROM

CD-ROM (compact disk read-only memory) is an optical data storage system that uses a disk coated with a thin layer of metal. In writing data, a laser etches billions of tiny pits in the metal. The data is encoded in the pattern of pits and spaces between them (called “lands”). Unlike the case with a magnetic hard or floppy disk, the data is written in a single spiral track that begins at the center of the disk. The CD-ROM drive uses another laser to read the encoded data (which is read from the other side as “bumps” rather than pits). The drive slows down as the detector (reading head) moves toward the outer edge of the disk. This maintains a constant linear velocity and allows for all sectors to be the same size. This system was adapted from the one used for 

the audio CDs that largely supplanted phonograph records during the 1980s.

A CD can hold about 650 MB of data. By the early 1990s, the CD had become inexpensive and ubiquitous, and it has now largely replaced the floppy disk as the medium of soft-ware distribution. The relatively large capacity meant that one CD could replace multiple floppies for a distribution of products such as Microsoft Windows or Word, and it also made it practical to give users access to the entire text of encyclopedias and other reference works. Further, the CD was essential for the delivery of multimedia (graphics, video, and sound) to the desktop, since such applications require far more storage than is available on 1.44-MB floppy disks. CD drives declined in price from several hundred dollars to about $50, while their speeds have increased by a factor of 30 or more, allowing them to keep up with games and other software that needs to read data quickly from the disk.

Recordable CDs

In the late 1990s, a new consumer technology enabled users to create their own CDs with data or audio tracks. The cheapest kind, CD-R (Compact Disk Recordable) uses a layer of a dyed material and a thin gold layer to reflect the laser beam. Data is recorded by a laser beam hitting the dye layer in precise locations and marking it (in one of several ways, depending on technology). The lengths of marked (“striped”) track and unmarked track together encode the data.

A more versatile alternative is the CD-RW (Compact Disk, Readable/Writeable), which can be recorded on, erased, and re-recorded many times. These disks have a layer made from a mixture of such materials as silver, anti-mony, and rare earths such as indium and tellurium. The mixture forms many tiny crystals. To record data, an infra-red laser beam is directed at pinpoint spots on the layer. The heat from the beam melts the crystals in the target spot into an amorphous mass. Because the amorphous state has lower reflectivity than the original crystals, the reading laser can distinguish the marked “pits” from the surround-ing lands. Because of a special property of the material, a beam with a heat level lower than the recording beam can reheat the amorphous material to a point at which it will, upon cooling, revert to its original crystal form. This per-mits repeated erasing and re-recording.

DVD-ROM

The DVD (alternatively, Digital Video Disc or Digital Ver-satile Disc) is similar to a CD, but uses laser light with a shorter wavelength. This means that the size of the pits and lands will be considerably smaller, which in turns means that much more data can be stored on the same size disk. A DVD disk typically stores up to 4.7 GB of data, equivalent to about six CDs. This capacity can be doubled by using both sides of the disk.

The high capacity of DVD-ROMs (and their record-able equivalent, DVD-RAMs) makes them useful for stor-ing feature-length movies or videos, very large games and multimedia programs, or large illustrated encyclopedias. The development of high-definition television (HDTV) standards spurred the introduction of higher capacity DVD formats. The competition between Sony’s Blu-Ray and HD-DVD (backed by Toshiba and Microsoft, among others) was resolved by 2008 in favor of the former. Blu-Ray offers high capacity (25GB for single layer discs, 50GB for dual layer).