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 (500-700Â°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 (200Â°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. On this occasion the reality matched the hype and the first DVD+RW reached the market in late 2001.
By early the following year product from five different manufacturers was available, perhaps indicating that companies had more faith in the long term survival of DVD+RW than the rival DVD-RW format. Of these, Sony found themselves in the interesting position of supporting both formats – DVD-RW through its ongoing membership of the DVD Forum and DVD+RW as a result of its participation in the DVD+RW Alliance.
All the drives on the market in early 2002 used CLV to deliver a maximum 2.4x write speed for DVD+RW media – equivalent to 3.32 MBps – and CAV to allow CD-ROMs to be read at up to 32-speed. Using the X rating system – which has become extremely cumbersome in this age of multi-format drives, especially since there’s a 9:1 ratio in the actual transfer rates between DVD and CD ratings – the other performance characteristics were: 8x (DVD-ROM/+RW) reading, 12x (CD) writing and 10x (CD) re-writing.
Which of the rival formats will dominate in the long term remains unclear. The addition of DVD-R capabilities allows DVD-RAM drives to record cross- compatible DVDs. However, its reliance on cartridge-based rewritable media makes the format a great deal more useful for data archival applications than as a consumer device. By early 2002 the momentum appeared to be with the DVD-RW format. However, despite its proponents’ early claims regarding the format’s superior compatibility, the fact that DVD+RW media is less reflective than DVD-R media – and therefore less compatible with some DVD Players and DVD-ROM drives – is a potential drawback. However, a solution – in the shape of higher reflectivity DVD+RW media – was already under development.
The uncertainty about whether one or other of the rival formats would win out in the end was reinforced by reports that Sony was developing a drive that supported both DVD-RW and DVD+RW formats. The industry waited with interest to see if this would ever make it to market, or if the company would eventually abandon it as an unnecessary hedging strategy.
- History of DVD development and birth of the DVD Forum
- DVD Formats
- DVDs – digital versatile disks – how they’re made and how they work
- DVD OSTA
- DVD File Systems
- CDR-RW Compatibility Issues
- DVD Encoding
- DVD Content Protection
- Regional codes for DVDs
- DVD DivX Codec
- DVD Recordable Formats
- DVD-R – write once recordable DVDs
- DVD Multi-Writers