Physically, there are two main differences
between CD and DVD disc. DVD pits are far smaller than that of a CD. It’s only
0.44 micron in diameter where the CD pits are nearly twice as large at 0.83
micron. DVD data tracks are only 0.74 micron apart, whereas 1.6 microns
separate CD data tracks. These are the main reasons of DVD holding very large
amount of data as compared to CD.
To read the smaller pits, a DVD player's
readout beam must achieve a finer focus than a CD player's does. To do this, it
uses a read semiconductor laser that has a wave length of 635 to 650 nanometer.
In contrast, CD players use infrared laser with a longer wavelength of 780
nanometer. Also, DVD players employ a more powerful focusing lens-one having a
higher numerical aperture than the lens in a CD player. These differences,
together with the additional efficiencies of the DVD format described below,
account for the huge 4.7-gigayte 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 overall thickness-11.2
millimeter- DVDs possess two substrates that can carry information' whereas CDs
have one. A DVD 's substrates are bonded together so thet their pitted surfaces
face each other in the center of the of the disc. This setup shields the
surfaces from the damaging effects of dust particles and scratches.
In
the simplest design, the second DVD side is accessed by physically removing the
disc from the player removing the disc from the payer, turning it over
and reinserting it. Another variation-the multilayer design enables both
information surfaces to be payer from the side of the disc.
In a multilayer disc, the upper substrate is
coated with a partially reflective, partially transmissive layer. The
reflectivity of the upper layer is sufficient to enable the laser to raed the
pits in the upper substrate; its transmissivity also permits the beam to focus
on the lower subsrate and read the pits on the in that layer. When the laser
focuses on pits in the upper information layer are out of focus and so do
not interfere.(To accommodate the small but unavoidable loss of
payer back quality in this approach, a slight capacity reduction to 8.5
gigabyte necessary-which explains why
a double-sided, double-layer DVD would hold about 17
gigabytes.)
An optical glue of superior
quality must be used to bond the two subsrates,and the thickness, of the
that must be controlled with precision to avoid excessive aberrations in the
focused readout spot. The two-substrate offers advantages in addition to
increased capacity:it reduces errors caused by disc tilt and
warping. All cd are prone to warping, and when a disc's surface tilts so that
it is no longer perpendicular to the laser, reading errors can result. The
degree to which tilt degrades the readers spot is directly proportional to the
substrate's thickness. The DVD substrate is only 0.6 millimeter thick and so
benefits the overall design. The thin
substrate makes the DVD less sensitive to tilt to than the DC, which has a
substrate that is 1.2 millimeter thick. For other reasons, the DVD is less
susceptible to certain kinds of warping and tilt in the first place.
