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 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.)
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.