DVD-ROM DRIVES
The up and coming drive of the year 2000 may be the DVD-ROM. The DVD, also known as the digital versatile disc, represented an agreement among the world's leading consumer electronics and entertainment companies on a new standard for CD-based pre recorded entertainment and information. It is similar to the CD-ROM, yet is able to contain a whopping 8.5 GB, or 8500MB, compared to a CD, which contains 650 MB. This capacity marks a vast improvement.
Optical data storage
is a method of storing digital information (1's and 0's) by using light to
read the information. Analog information is converted into digital information,
which is then encoded onto the disc from the inside edge out. Digital data
are encoded by means of pits on the recording layer of the disc. The encoding
is done using a technique referred to as EFM, eight-to-fourteen modulation,
in CDs and EFMPlus, eight-to-sixteen modulation, in DVDs. The pits and the
separations between pits, called lands, vary in length to
represent the digital
information stored in the disc. The pits are
arranged
in a track that forms a spiral pattern on the recording layer
of the disc. The disc revolves in a circular motion inside the player,
while an optical head laser slowly moves outward and remains
focused on the pits. The laser beam is reflected back to a detector
when it hits the lands, and it is scattered away from the detector by the
pits. The transition between a pit and a land corresponds to a "1" in the
digital bit stream.
The increased
storage capacity was the result of three advances in technology.
First Advance
The first was the use of smaller pits and lans on the disc surface. Pits (indentions)
and lans (smooth spots) represent the bits - 1's and 0's-of digital information.
The smallest pits on a DVD are about 0.4 microns (0.0001575 inches) wide,
whereas CD pits have a minimum width of 0.83 microns. The pits are arranged
in tracks that are spaced much closer on a DVD (0.74 microns) than on a CD
(1.6 microns).
Second Advance
The
second advance was the development of small lasers that emit the very-short-wavelength
beams required to read that information.The smaller pit size on a DVD means
the laser beam used in a DVD player must have a shorter wavelength than that
in a CD player. But this requirement allows for dual compatibility: DVD players
are designed to play DVD discs as well as CDs. Unfortunately, this compatibility
is backwards and so CD players will not play DVDs.With
such small pit areas, the number of possible errors in reading the disc is
limitless. As a result, error correction storage has seen rapid development.
In CD technology, there is error correction coding (ECC) on the disc as a
means of correcting problems when they occur. The ECC algorithms are configured
to detect and perfectly correct errors in the data. This same method is used
for error correction in DVD technology.
The greater amount of information stored on a DVD video is chiefly due to the smaller pit size and tighter track spacing, but another factor comes into play--namely, compression. The Moving Picture Experts Group has come out with an advanced video compression algorithm referred to as MPEG2. This development is based on temporal compression that compares each frame of video to the following frame and only stores the changes in the scene. It stores a single frame in a delay so as to continually put out the background of a scene and have more room to store the changes in the scene. MPEG2 also uses the discrete cosine transform, which takes images and finds their frequency and then compresses any repetitive frequencies.
Third Advance
The third
advance was progress in data compression. Video compression allows more room
to store additional viewing options, a big part of what makes DVD such an
incredible advancement in technology. Allowing for the full theater experience,
DVD-video comes in Dolby Digital (AC-3) Surround Sound [Dolby Digital (AC-3)
is a trademark of Dolby Laboratories Licensing Corporation]. Other options
include the choice of wide (16:9) or full-screen (4:3) viewing, sound tracks
in different languages and closed-captioned viewing. DVD video can also let
the viewer decide the rating, R or PG, thanks to preprogrammed scene cuts
on the disc itself. Viewers can also opt to watch an uncut version of the
film or see out takes from the original. All of these selections can be chosen
at the start of the film. The DVD then makes seamless skips from one scene
to the next in the arrangement the viewer has chosen. The director and the
producer decide upon the amount and type of options available, so each individual
DVD may have some, all or none of these options.
At this point in time, consumer DVD-ROM-RS is not available on the market, but several companies have scheduled them for release in the very new future. Also, in order to read a DVD-ROM, a special de-encoding hard is required. The enquiry: Does anyone need so much space? Won't people be satisfied with ZIP disks or simple CD-Rs?
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