We have come a long way from stone, clay, papyrus and other modes of storing information, and now employ media that better reflect our civilization -- the age of computers and information technology.
Modern computers make use of two storage technologies -- magnetic and optical. Data storage methods relying on magnetism have predominated, with optical technology only coming on the scene in the late 1970s. MAGNETIC
All magnetic storage operates on the principle that atoms of certain elements -- iron, nickel, chromium and cobalt -- act as natural magnets. When a drum or disk is coated with a powdered alloy of one of these elements -- usually iron dioxide -- the magnetized atoms behave like microscopic bar magnets, each generating its own field.
OPTICAL
Optical or laser technology is another means of capturing information, by using a light beam to burn microscopic pits (0.12 microns deep and 0.6 microns wide) into a photosensitive disc surface. Optical systems use different types of lasers, such as small diode semiconductors, infrared or gas lasers.
While recording, a laser beam is focused on the surface of the disc and burns the data in as microscopic holes or pits. The disc is read by a laser beam focused from the back side of the disc on a reflective layer of aluminum or gold. The light is reflected off the shiny surface of the disc to a photodetector. The continuous illumination of the track will vary as the disc rotates, according to the difference in reflectivity due to the pits on the track.
Recording tracks can be either a continuous spiral, as on a phonograph record, or concentric circles sliced into sectors, as on a floppy disk. Information is read off a disc by a disc drive which reflects light off the disc surface; the drive's "read head" interprets the pits and lands according to variations in the reflected light. The disc drive converts these variations in reflected light to corresponding digital data which is usable by the computer.
There are two techniques to lay out these microscopic pits. The spiral-recording technique is called constant linear velocity (CLV), and allows for more efficient use of the disc surface because it packs the data more tightly. But it is sequential and thus can be slower for locating data.
With the sector-recording data or constant angular velocity (CAV) disc, each ring has the same amount of information on the sectors, and spreads data further apart on the outer rings, resulting in fewer data on the disc surface. However, this technique makes it quicker to locate data on a section of disc because it is not organized sequentially. Each of these methods of recording affects the software designs used for data retrieval and the time required to locate and retrieve the data from the disc.
THREE TYPES OF DISCS
There are three types of optical discs: read-only memory, write-once and erasable. Read-only memory discs cannot be changed once produced. Write-once and erasable discs are recorded directly at the workstation. Write-once discs cannot be changed once recorded, though new data could be added following the already recorded data. Erasable discs can be changed and promise greater storage densities than current magnetic storage, but with the same flexibility of allowing the users to change the information. Read-only memory and write-once discs are in actual use and available commercially, whereas erasable discs are still under development.
DVD
DVD is the storage industry's next generation optical disc. Expect them to assume a central role in computing. DVD originally meant "Digital VideoDisc," and was able to store video and audio. Today, DVD stands for "Digital VersatileDisc," because it is no longer limited only to audio or video. Words, graphics, pictures, sounds and movies can also be stored and retrieved.
DVD makes possible personal archives of gigabyte or even terabyte range, and is expected to lower the cost of data storage by a factor of 15 or more.
DVD looks the same as a standard CD-ROM but can store about eight times more data -- about 4.8 gigabytes. DVD-ROMs will eventually store up to 18 gigabytes and increase data transfer rates to 1.35 MB/second. They are priced from $300 to $500, and the good news is DVD can also play all your old CD-ROMs.
The DVD consortium (Toshiba, Matsushita, Sony, Philips, Mitsubishi, Time Warner Video, Pioneer, JVC, Hitachi and Thompson Multimedia) have reached basic agreements on the recording method to be used. Basic agreement is reached on DVD-RAM or Recordable DVDs. DVD-RAM are expected to hit the market in 1998, giving users a way to record data that can be played on high-density drives. DVD-RAM is also expected to pose a competitive challenge to rewritable CDs.
But what seems like a lot today will be insufficient for tomorrow. The computer industry is looking for new technologies to serve 21st century data storage needs -- technologies that can store more information per square inch with a faster rate of data transfer. Here are some of the most promising.
HOLOGRAPHIC STORAGE
Holographic systems offer the possibility of many trillions of bytes of information stored as "pages" of bits within a small crystal the size of a sugar cube. Lasers read, write and store data holographically in a crystal made of a photorefractive material such as lithium niobate. Aside from its capacity to store, this technology makes possible data transfer rates of about 1GBps.
Holograms are formed in the crystal by the internal pattern of electrical charges. A pattern of light and dark is produced on the crystal when two laser beams cross, causing the charges to be released in the bright areas and travel to the dark areas, where they are trapped by receptors. This selective charge migration results in regions of negative (electron-rich) and positive (electron-poor) charge.
In this way, the optical properties of the sample -- the index of refraction -- are changed to the light-and-dark pattern.
Holograms are formed in the crystal immediately without the use of chemical development. They can also be erased by simply shining uniform light onto the crystal, which evenly redistributes the charge.
While magnetic disc storage reads the data serially, a holographic system reads all stored information in each page simultaneously, giving users very fast access to all stored data.
A government/industry/university consortium co-led by IBM expects to have a demonstration system operating by 1998.
For additional information contact IBM's Mike Ross, 408/927-1283. E-mail:
ATOMIC SCALE INFORMATION STORAGE
Variations of the atomic force microscope are being developed that can read and write incredibly small data marks -- atomic scale "bits" -- on plastic discs.
Scientists at IBM's Almaden Research Center in San Jose, Calif., have demonstrated, using scanning probe microscopy -- which includes atomic force microscopy and scanning tunneling microscopy -- how to provide ultrahigh density information storage at atomic scales.
A brief laser pulse heats an atomic force microscope's (AFM) pyramid-shaped tip while it presses against the plastic disc. In the instant that it is hot, the tip sinks into the briefly softened plastic, forming a tiny, shallow indentation (bit) that can be detected later by the same AFM tip acting as a stylus. Each mark is about one-tenth the diameter of an optical storage mark.
The new method is capable of writing 30 billion bits in a single square inch of disc space at a rate of about 100,000 bits per second. Such high density is possible because the AFM tip is much smaller than even a tightly focused laser beam. Designs that write faster are being explored.
While a capacity of many trillion bytes with a data transfer rate capability of a few GBps may seem impossible or raise the question "why would anyone need so much storage?", keep in mind that the capacity of the first commercial hard disk drive from IBM -- the size of two refrigerators and containing 50 24-inch disks -- was 5MB.
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