BK> The only *DIFFERENCE* is that the DVD cannot be written at all,
BK> and the DVD-RW and the DVD-R are written with a different color
BK> laser. But they are the same when they are read.

The color of the laser is only Secondary HERE is the HOW the RW disk is
capable of being Erased and Reritten. IT is all a matter of very
precice heat up and cool down of hte Media by a Laser Beam.

***

DVD+ReWritable and how it works
With the growing trend towards multimedia documents,which require large
amounts of disc space,DVD+RW offers the ability to easily
create,share,store and access such content-rich documents using a single
disc.
In DVD+RW,data is arranged on a disc according to the industry-standard
UDF (Universal Disc Format) standardized and published by OSTA (Optical
Storage Technology Association).UDF is well suited to handling file
changes and (random) writing of small amounts of data,and facilitates
the design of multi-platform applications.
Today,UDF is increasingly used for writing to CD-RW discs instead of the
less flexible ISO9660 file system format that was developed to handle
read-only data applications.Virtually platform-independent,UDF makes DVD
systems highly compatible with CD-based data systems.Also,being designed
to handle the massive capacities of future optical storage systems,UDF
provides excellent forward compatibility.
In its original state,the recording layer of a DVD+RW disc is
polycrystalline.During writing,a focused laser beam selectively heats
areas of the phase-change material above the melting temperature
(500-700øC),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 (see Fig.1).If
the phase-change layer is heated below the melting temperature but above
the crystallization temperature (200øC) for a sufficient time (at least
longer than the minimum crystallization time),the atoms revert back to
an ordered state,i.e.the crystalline state.
The amorphous and crystalline states have different refractive
indexes,and can therefore be optically
distinguished.In the DVD+RW system,the amorphous state has a lower
reflectance than the crystalline state and, during read-out,this
produces a signal identical to that of a regular dual layer DVD-ROM
disc,making it possible to read DVD+RW discs on DVD-ROM drives and DVD
Video players.
The phase-change medium consists of a grooved polycarbonate substrate
onto which a stack (usually four layers) is sputtered (see Fig.2).The
phase-change (recording) layer is sandwiched between dielectric layers.A
commonly used phase-change material is Ag-In-Sb-Te alloy.The chemical
composition of the phase-change layer determines the minimum time of
crystallization.The disc structure (layer thickness,thermal capacities
and thermal
conductivity) determines the cooling rate during writing.Precise control
of the recording-layer composition is important to obtain the desired
recording properties.In general,low recording powers are achieved by
using thin layers.The layer thickness and refractive indexes determine
the optical properties of the phase-change medium.
The phase-change medium can be rewritten in a single pass of the focused
laser beam;this is sometimes referred to as `direct overwriting'.In the
DVD+RW system,the data is recorded on discs by means of a write
strategy,using different laser output levels (see Fig.3).This strategy
(see Fig.4a) has two parts:

a pulsed part (pulsing is necessary to write amorphous `marks'),and

a non-pulsed part,in which the strategy writes crystalline areas between
the marks.
The write strategy therefore writes new data to the disc while
simultaneously overwriting old data.Moreover, DVD+RW supports CLV (see
Fig.4a) as well as CAV (see Fig.4b).This process can be repeated several
thousand times
---
 þ SLMR 2.1a þ Typo Tom strikes agaoin