System Features
CD-RW devices use three different laser powers to achieve these effects in the recording layer:
•the highest, called 'Write Power', creates a non-crystalline (absorptive) state on the recording layer
•the medium, 'Erase Power', melts the recording layer and converts it to a reflective crystalline state
•the lowest, 'Read Power', does not alter the state of the recording layer, so it can be used for reading the data.
During writing, a focused 'Write Power' laser beam selectively heats areas of the phase-change material above the melting temperature (500-700oC), so all the atoms in this area can move rapidly in the liquid state. Then, if cooled sufficiently quickly, the random liquid state is 'frozen-in' and the so-called amorphous state is obtained. The amorphous version of the material shrinks, leaving a pit where the laser dot was written, resulting in a recognizable CD surface. When an 'Erase Power' laser beam heats the phase-change layer to below the melting temperature but above the crystallization temperature (200o C) for a sufficient time (at least longer than the minimum crystallization time), the atoms revert back to an ordered state (the crystalline state). Writing takes place in a single pass of the focused laser beam, sometimes referred to as 'direct overwriting' and the process can be repeated several thousand times per disc.
Once the data has been burned the amorphous areas reflect less light, enabling a 'Read Power' laser beam to detect the difference between the lands and the pits on the disc. One compromise here is that the disc reflects less light than CD-ROMs or CD-Rs and consequently CD-RW discs can only be read on CD players that support the new MultiRead specification.
CD-RW drives are dual-function, offering both CD-R and CD-RW recording, so the user can choose the best media for a particular job.
Although UDF (Universal Disc Format) allows users to drag and drop files to discs, CD-RW is still not as easy to use as a hard disk. Initially limitations in the UDF standard and associated driver software meant that when data was deleted from a CD-RW, those areas of the disc were merely marked for deletion and were not immediately accessible. A disc could be used until all its capacity was used, but then the entire disc had to be erased to reclaim its storage space using a 'sequential erase' function. In hardware terms erasing a disk is accomplished by heating up the surface to a lower temperature, but for a longer time, which returns it to the crystalline state.
Evolution of the UDF standard and developments in associated driver software have improved things considerably, making CD-RW more like hard drives or floppy disks.
Features of the DVD-ROM Drive
•Supported CD-ROM formats:
•CD-ROM Mode 1 and 2 data disc
•Photo-CD Multisession
•CD Audio disc
•Mixed mode CD-ROM disc (data and audio)
•CD-ROM XA
•CD-I
•CD-Extra
•CD-R
•CD-RW
