The perspectives in the development of the optical carriers market are unambiguous. Most of the companies that specialise in IT market analysis agrees that in the nearest few years the DVD drives will push the CD-ROM readers out [
www.allcom.pl].
DVD (Digital Disc or Digital Video Disc) - digital carrier of the turn of XX and XXI centuries. Despite of the same dimensions as the usual CD, this disc may contain up to 17 GB of data (two-layer and two-sided disc). Similar to the CDs the DVD found a wide application in informaticsas a capacious various data carrier (DVD-ROM, DVD-RAM), as a digital carrier of high quality video materials (DVD-Video) and a music disc format (DVD-Audio) [www.pcworld.pl].
There are four discordant formats of DVDs on the market:
- DVD-R one time record, may be readed by standard DVD-ROM drives
- DVD-RAM multiple record, current DVD-ROM drives are not able to read those discs
- DVD RW multiple record, increased the capacity and the attainments
- DVD-RAM, current DVD-ROM drives do not read those discs
- DVD-R/W multiple record, these discs may be readed in DVD-ROM drives
The result is that a disc recorded in one format cannot be readed in other. According to the experts this situation will change in the following few years [www.allcom.pl,
www.pcworld.pl].
On the DVD carrier similar to the older CDs data are recorded on one of the spiral paths. Individual data bits have the form of tiny dinges on the surface of the disc (so called pits). When a laser happens upon the surface between the dinges its ray is reflexed. However when a ray hits upon the ding (pit) it will obtain such an angle that after the reflexion it will not get back to the phototransistor. Individual spaces on the disc are interpreted as "1" or "0". In order to get greater capacity the construction of a DVD has been changed. Ordinary DVD differs from the CD with the packing density but it may consist of two sticked together discs, from wich each enables a both side data recording.
Due to the use of much smaller data structures than in the CDs, one side od the DVD may contain up to 7 times more data. Minimal length of the pit space may be now 0.4 mm, not 0.83 as in the CDs. Paths on the disc are closer to eachother - the distance had declined from 1.6 to 0.74 mm. To maintain the possibility of a correct reading when the packing density is so big, the laser of the DVD reader must have smaller wave length. In the CD drives a laser with a wave length of 780 nm is used and in the DVD this length is 640 nm [www.allcom.pl].
For a data record on the DVD-RAM/RW discs a Land-and-Groove-Recording technology is used. On the contrary to the CDs that use only grooves in the carrier DVD-RAM discs have primary defined paths which enables presise laser guiding during the recording. This new technology enables also the use of convex parts of the carrier (land). As the producers of the DVD-RAM discs assure that these discs may be rerecorded up to 100 times. Due to the necessity of ensuring high precision of the recording, most of the recorded DVDs is places on special casings (cartridges), which disables their reading in a ordinary reader [www.pcworld.pl].