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what is the difference between cd & dvd ?
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The Differences Between CD and DVD
There are two essemtail phsical differences between CD and
DVD disc.First, thesmallest DVD pits are only 0.44 micron in
diameter; the equivalent CD pits are nearly twice as large,
or 0.83 micron wide.And DVD data tracks are only 0.74 micron
apart, wherea 1.6 microns separate CD data tracks. So
although a DVD is the same size as a CD,its data spiral is
upward of 11 kilometers long-more than twice the length of a
CD's data spiral. To read the smallar pits, a DVD player's
readout beam must achieve afiner focus than a CD player's
does. Inorder to do this, it uses a readsemiconductor laser
that has a wave length of 635 to 650 nanometer. In contrast,
CD playrs use infrared laser with a longer wavelength of
780 nanometer. Also, DVD palyers employ a more powerful
focusing lens-one having a higher numericl aperture than the
lens in a CD payer.The se differences, together with the
additional efficiencies of the DVD format descrobed below,
account for the huge 4.7-gigayte capacity of each DVD
information layer.
A DVD 's capacity can be douled to 9.4 gigabytes-and nearly
doubled agaain to about 17 gigabytes- by two more
innovations.Although DVDs and CDs have the same overall
thickness-11.2 millimeter- DVDs possess two subtrates 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 danaging effects of dust
particles and scratches.
In the simplest design, the second DVD side is accessed by
physically removig the disc from the palyer removing the
disc from the payer, tnrnig it over and reinserting it. An
other variation-the mulilayer designenables both information
surfaaces to be payer from the side of the disc.
In a mulilayer disc, the upper subsrate is coated with a
partially reflective, partially trasmissive layer. The
reflectivity of the upper layer is sussfficient 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 infformation layer are
out of focus and so do not interfere.(To accommodate the
small but unaviodable loss of payerback quality in this
approach, a slight capacity reducction to 8.5 gigabyter
inescessary-which expalains why a double-sided, double-layer
DVD would hold about 17 gigabytes.)
Spec CD DVD
Diameter 12cm 12cm
Thickness 1.2mm 1.2mm(.6mm*2)
Numerical aperture .60 .38 to .45
Readout wavelength 780nm 650 or 635nm
track pitch 1.6 micrometers .74 micrometers
Pit length .822 to 3.560 micrometers .400 to 1.866
mirometers .440 to 2.054 micrometers
Capacity (per side) 650MB 4.7 GB (single layer), 8.5 GB (
daul layers)
Scanning speed 1.2 to 1.4 m/s 3.49m/s 3.84m/s
Rotational speed (1*) 200 to 500 rpm 570 to 1600 rpm
Reflectivity 70% minimum 45 to 85%
Modulation 8/14(8/17 with merge bits) 8/16
User data rate (1*) 1.41 Mbps 11.08 Mbps
Error correction CIRC RS-PC
Error correction overhead 23/34% 13%
Format overhead 252% 136%
An optical glue of superior quality must be used to bond
the two susrates,and the thickness, of the that must be
comtrolled with precision to avoid axecessive 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 awhen 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 reades spot is
directly proportional to the substrate's thickness.The DVD
susstrate is only 0.6 millimeter thick and so benefits the
overall dessign.The thin substrate makes the DVD less
senstive 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.
For instance, sudden changes in temperatures or humidity can
cause swelling or shrinkage in the DVD's plastic subsrates.
But because of the DVD's symmetric construction,changes in
one layer tend to countrerct those in the other,reduing the
overall effect of environmental chnges and minimizing the
resulting amount of tilt.
Because consumers have already invested agood amount of
money in their CD audio and CD-ROM collections, it was
considered a top design priority thet DVD palyrs read
axisting CDs as well as new disc.To make DVD payers with
this ability required specific optical design features. The
simplest design is to mount two lenses in a single optical
head-one optimized for a 1.2 millimeter-thick substrate and
another for a0.6 millimetre-thick one -and then to switch
mechanincally 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.Theelement at
its center.The light passing throght the outer annulus of
the lens is unaffected by the hologram and focuses to a spot
size in the plane small enough to be suitable for
DVDreadout.About one thid of the readout beam incident onthe
centralpart is focused by both the the and the hohgram to
aspot suitable for reading pits on the thicker CD.
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