Consistent
with the bickering that had dogged DVD since its inception, the
DVD-RAM specification was a compromise between two different proposals
by the principal protagonists - the Hitachi, Matsushita Electric
and Toshiba grouping and the Sony/Philips alliance - but with primary
reliance on that put forward by the former.
The degree to which Sony and Philips were unhappy about this became clear in
the summer of 1997 when, together with Hewlett-Packard, they broke away from
the agreed format to develop Phase-change Rewritable, referred to as DVD+RW.
The format is based on DVD and CD-RW technology, and is incompatible with the
DVD-RAM standard which had been agreed only three months previously. While they
have not chosen to drop out of the DVD Forum, the DVD+RW camp have submitted
a modified form of their original specification to the European Computer Manufacturers'
Association (ECMA) for adoption as a standard. The format is not supported by
the DVD Forum.
It was DVD-RAM's reliance on a caddy, making it look like a large floppy disc,
that generated the loudest criticism from DVD+RW's supporters; they say that
the DVD-RAM approach would force future DVD-ROM to have to be modified to take
caddies and discs. A single-sided DVD-RAM can be removed from its caddy to play
in any DVD-ROM drive, but disc manufacturers say that the DVD-RAM disc cannot
be reliably replaced for further recording. The DVD+RW consortium further claimed
that the cartridge requirement of DVD-RAM could lead to larger carriage mechanisms,
thus limiting the technology's use in laptops or small enclosures. The companies
sticking with the DVD Forum (Matsushita, Hitachi and Toshiba), on the other hand,
claimed the DVD-RAM cartridge improved reliability, especially for double-sided
media and believed that the costs and difficulties of making DVD-ROM drives physically
compatible with DVD-RAM were overstated.
DVD+RW has much in common with the rival DVD-RW technology, using phase-change
media and offering a user experience similar to using CD-RW drives. Users may
record a bare disc or use a protective caddy or cartridge for the media. This
is in contrast to DVD-RAM drives which require cartridge-based media. With the
DVD+RW format, discs can be recorded in either CLV format for sequential video
access or CAV format for random access. 'Linking loss' is a consequence of having
to suspend and resume a constant bit rate writing process, with the result that
the disc is incompatible with read-only devices like DVD-Video players and DVD-ROM
drives. 'Lossless linking' is a technique developed specifically for DVD+RW that
- through its support for variable bit rate recording (VBR) - allows video applications
to suspend and continue the writing process without any consequent linking loss.
To do this, it is necessary to write any data block in the correct position with
high accuracy (within I micron). For this purpose a disc's groove is mastered
with a high wobble frequency which ensures that the writing can be started and
stopped at an accurately defined position. Together with the option of no defect
management, this feature allows DVD+RW discs to be written in a way that maximises
compatibility with existing DVD players and drives.
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 (500-700°C), 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.
If the phase-change layer is heated below the melting temperature but above the
crystallisation temperature (200°C) for a sufficient time - at least longer than
the minimum crystallisation 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. The former is moulded with a spiral
groove for servo guidance, address information and other data. The phase-change
(recording) layer is sandwiched between dielectric layers that draw excess heat
from the recording layer. 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 crystallisation.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.
Arguably, DVD+RW's principal advantage over DVD-RW is in the area of compatibility.
Its proponents claim it is the only rewritable DVD technology that offers seamless
media exchange between consumer electronics and personal computing environments
and that the format was compatible with most of the installed base of 35 million+
DVD-Video players and DVD-ROM drives at the end of 1999. Recordings made with
a DVD Video Recorder on DVD+RW discs - 4 hours record/playback time per side
- can be played on DVD-Video players as well as on PCs with a DVD-ROM drive and
MPEG-2 video decoding capability. Moreover, DVD+RW offers the possibility to
combine digital video and digital data in a single file system as required for
multimedia recording applications.
Hewlett-Packard was first to announce a DVD+RW drive - promising its 3.0GB DVD
Writer 3100i drive would reach the US market by the autumn of 1999. However,
at about the time the device was expected to reach the market HP announced that
it was instead will focusing on a 4.7GB version for release in 12 to 18 months,
claiming there would be little sense in releasing sooner because at the time
DVD-ROM drives were unable to read either 4.7GB media or 3.0GB DVD+RW discs.
This compatibility issue was subsequently addressed in early 2000.
In March 2001, the DVD+RW Alliance announced that a complete offering of DVD+RW
products - data drives and DVD+RW video recorders - were expected to be available
from member companies before the end of the year. That promise came a step closer
a few months later, with HP's announcement of it's DVD-writer dvd100i drive -
slated for availability in the USA in the autumn of 2001. Designed to be compatible
with the majority of DVD players on the market, the HP drive will be capable
of reading CD-ROMs and DVD discs at speeds of 32x and 8x respectively, and rewrite
to CD-R, CD-RW and DVD+RW media at speeds of 12x, 10x and 2.4x respectively.
In a move that will up the ante in terms of competition with its rival formats,
both the drive and the disc prices have been pitched beneath those of manufacturers
using the DVD-RW or DVD-RAM formats. Source:
pctechguide.com
DVD+RW Standard
DVD+RW has been standardized by 'DVD+RW Alliance' that
consists of eight leading companies including those recently promoted
CD-R/RW: Sony (Japan), Dell Computer (USA), Thomson Multimedia (France),
Hewlett-Packard (USA), Philips (Netherlands), Mitsubishi Chemical (Japan),
Yamaha (Japan), and Ricoh (Japan). With extensive experience in popularizing
CD-ROM/-R/RW, the Alliance has taken a serious consideration for user
comfort in operation, including compatibility with existing drives/players
and supporting Random Access suitable for data storage on PC. Providing
a High storage capacity is also an important consideration of the Alliance.
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