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Follow UsDVD and CD formats share the same basic optical storage
technology–microscopic pits formed on the surface of the plastic disc when the
material is injected into the mold, which represents information. The pitted side of the
disc is then coated with a thin layer of aluminum followed
in the case of the CD by a layer of protective lacquer and a label. To read the data, the
player shines a small spot of laser light through the disc substrate on to the die data
layer as the disc rotates.
The intensity of the light reflected from the disc’s surface
varies according to the presence or absence of pits along the die information track. A pit
reflects much less light than the flat part of a track. A photo-detector and other
components inside the player translate this variation into 0s and 1s, representing the
stored information.
There are two essential physical differences between CD and DVD
discs.
First, the smallest DVD pits are only 0.4 micron in diameter; the
equivalent CD pits are twice as large or 0.83 micron wide. Moreover, DVD data tracks are
only 0.74 micron apart, where as 1.6 microns separate CD data tracks. Therefore, although
DVD is the same size as a CD, its data spiral is upward of 11 km long–more than twice
the die length of a CD’s data spiral.
The read-out beam of a DVD must achieve a finer focus than a CD
player does. In order to do this, it uses a red semiconductor laser that has a wavelength
of 635 to 650 nanometer. In contrast, CD players use infrared lasers with a
longer wavelength of 780 nanometer. In addition, DVD players employ a more powerful
focussing length–one having a higher numerical aperture than the lens in a CD player.
These differences together with the additional efficiencies of DVD format described below
account for the huge 4.7gigabyte capacity of each DVD information layer.
A DVD’s capacity can be doubled to 9.4 gigabytes and nearly
doubled again to about 17 gigabytes, by two more innovations. Although DVDs and CDs
have the same thickness, 1.2 millimeters, DVDs posses two substrates that can carry
information, where as CDs have one. DVDs substrates are bounded together so that the
pitted surface face each other in the center of the disc. This setup shields the
surface from the damaging effects of dust particles and scratches. In the simplest DVD
design, the second side is accessed by physically removing the disc from the player,
turning it over and reinserting it.
Multi-layer design
Another variation–the
multi-layer design–enables both information surfaces to be played from the same side.
In a multi-layer player disc, the upper substrate is coated with a psartially reflective
layer. The reflectivity of the upper layer is sufficient to enable the laser beam to read
the pits in the upper substrates’. It also permits the laser beam to focus on the
lower substrate and read the pits in that layer. When the laser focuses on pits in the
lower information layer, the pits in the upper layer are out of focus so that they
don’t interfere. (To accommodate the small but unavoidable loss of playback quality
in this approach a slight capacity reduction to 8.5 gigabytes is necessary which explains
why a double-sided, double-layer DVD would hold about 17 gigabytes.)
Optical glue of superior quality must be used to bond the two
substrates, and the thickness of that bond must be controlled with precision to avoid
excessive aberration in the focused readout spot.
increased capacity: it reduces errors caused by disc tilt and warping. All compact disks
are prone to warping, and when a disc’s surface tilts so that it’s no longer
perpendicular to the laser beam, reading errors can occur. The degree to which tilt
degrades the read out, is directly proportional to the substrate’s thickness. The DVD
substrate is only 0. 6 millimeter thick and so benefits the overall design. This
thin substrate makes the DVD less sensitive to tilt than the CD, which has a substrate
that is 1.2 millimeters thick. For other reasons, the DVD is less susceptible to certain
kinds of warping and tilt in the first place.
For instance, sudden changes in temperature or humidity can cause
swelling or shrinkage in DVD’s plastic substrates. But because of DVD’s
symmetric construction, changes in one layer tend to counter act the changes in the other,
reducing the over all effect of environmental changes and minimizing the tilt.
Because consumers have already invested a good amount of money in
their CD audio and CD-ROM collections, it was considered a top design priority that DVD
players read existing CDs as well as new discs. To make DVD players with this ability, a
specific optical design feature is required. The simplest design is to mount two lenses in
a single optical head–one optimized for a 1.2 mm thick substrate and another for a
0.6 mm substrate–and then to switch mechanically from one to the other as needed.
A more elegant solution that emerged uses a single molded optic with
a holographic element at its center. The light passing through the outer annulus of
the lens is unaffected by the hologram and focuses to a reduced spot size in the plane,
small enough to be suitable for DVD readout. About one third of the read-out beam incident
on the central part is focused by both the lens and the hologram to a spot suitable for
reading pits on the thicker CD.
More bits
Besides having more pits than CDs
do, DVD also packs more information into those pits, thanks to the improvement in two
aspects of format coding efficiency. Whatever the original form of the
information–data, text, image, audio, video–the digital 0s and 1s that directly
represent the content, called user bits must be protected from the effects of errors
introduced during playback. These errors arise from such factors as dust, scratches,
corrosion. Error correcting and control techniques (ECC) minimize such problems by way of
special algorithms that compute additional data bits to be stored along the user data.
These additional bits, though essential, reduce the capacity available for actual content.
Nevertheless, the DVD ECC is extremely powerful. It can, for
example, correct an error burst up to 2,000 bytes long, which corresponds to about 4 mm in
length along the track. In DVD format, the ECC data account for about 13 percent of the
disc’s capacity. The improved efficiency of the DVD ECC, at no cost in
error-correcting abilities, results in large part from the increased computing power of
today’s silicon. Such power was not available when the CD format was designed.
During recording, the combined user data and ECC data must be
converted into so called modulation code bits. These are the actual streams of binary bits
represented by the pits in the disc surface. This step is necessary to control the range
of pit sizes required to represent the data, an important aspect of assuring reliable data
detection and tracking during playback. The CD format coding method transforms eight user
bits into 17 modulation code bits.
16 modulation code bits, while preserving the benefits inherent in the original CD method.
Because fewer modulation code bits are needed to represent user data bits, DVD can hold
more of them at once. This feature leads to an efficiency improvement of about 6 percent
over the CD format.
Read/write versions of the DVD drive should be appearing soon, and
both the write-once DVD-R and erasable DVD-RAM promise to be much more capable and useful
than the CD-R and the erasable CD-E formats. Until now, optical recording systems have
relied prominently on magneto-optical technology. However, for the DVD-RAM, the recording
medium is more likely to be "phase-change materials". In this scenario, a thin,
ultra-fine grained polycrystalline film is deposited on the surface of the RAM substrate.
To store each bit, an intense but fleeting recording pulse from the laser melts a
sub-micron size region of the film. Because such a small spot of material cools very
quickly, the molten region cannot re-crystallize. Rather it remains frozen in a
disordered, amorphous state that happens to reflect much less light than the crystalline
phase does. This difference in reflection means that a low-intensity reflection beam, can
decode the data.
The DVD format was conceived with extensions in the mind. For
example, development of reliable short wavelength lasers emitting green or blue light can
perhaps double data density again. Derivatives of the DVD technology may someday shell out
50 GB or more on one platter. Essentially a small library on a single disc.