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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.