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DVD-RAM (DVD–Random Access Memory) is a disc specification presented in 1996 by the DVD Forum, which specifies rewritable DVD-RAM media and the appropriate DVD writers. DVD-RAM media have been used in computers as well as camcorders and personal video recorders since 1998.

Introduction
Currently there are three competing technologies for rewritable DVDs: DVD-RAM, DVD+RW and DVD-RW. DVD-RAM is considered a highly reliable format, as the discs have built-in error control and a defect management system. Therefore, DVD-RAM is perceived to be better than the other DVD technologies for traditional computer usage tasks such as general data storage, backup and archival, though the Mt. Rainier standard for DVD+RW lessens this somewhat. Curiously, DVD-RAM has a larger presence in camcorders and set-top boxes than in computers, although the DVD-RAM's popularity in these devices can be explained by the fact that it is very easily written to and erased, which for example allows extensive in camera editing.
The on-disc structure of DVD-RAMs is closely related to hard disk and floppy disk technology, as it stores data in concentric tracks. DVD-RAMs can be accessed just like a hard or floppy disk and usually without any special software. DVD-RWs and DVD+RWs, on the other hand, store data in one long spiral track and require special packet reading/writing software to read and write data discs. It is a common misconception that DVD-RAM uses magneto-optical (MO) technologies: DVD-RAM is a pure phase change medium, similar to CD-RW or DVD-RW.
Specification

Since the Internationale Funkausstellung Berlin 2003 the specification is being marketed by the RAM Promotion Group (RAMPRG), built by Hitachi, Toshiba, Maxell, LG Electronics, Matsushita/Panasonic, Samsung and Teac.
The specification distinguishes between
* DVD-RAM version 1.0, recording speed ?
o Single-sided, one layer discs with a capacity of 2.58 GB
o Double-sided one layer discs with a capacity of 5.16 GB
* DVD-RAM version 2.0, recording speed 2x
o Single-sided, one layer discs with a capacity of 4.7 GB
o Double-sided one layer discs with a capacity of 9.4 GB
* DVD-RAM version 2.1/Revision 1.0, recording speed 3x
* DVD-RAM version 2.2/Revision 2.0, recording speed 5x
* DVD-RAM version ?/Revision 3.0, recording speed 6x max
* DVD-RAM version ?/Revision 4.0, recording speed 8x max
* DVD-RAM version ?/Revision 5.0, recording speed 12x max
* DVD-RAM version ?/Revision 6.0, recording speed 16x max
Physically smaller, 80 mm in diameter, DVD-RAM discs also exist with a capacity of 1.46 GB for a single-sided disc, but they are uncommon. DVD-RAMs were originally solely sold in cartridges; recent DVD recorders however also work with no-cartridge discs – some devices even do not support cartridges anymore. A cartridge disc is about 50% more expensive than a disc without a cartridge.

Compatibility
Many operating systems like Mac OS (Mac OS 8.6 or later), Linux and Microsoft Windows XP support DVD-RAM operation directly, while earlier versions of Windows require device drivers or the program InCD. The optical drives shipped with most Apple Macintosh computers do not support DVD-RAM operation, but a third party DVD-RAM-compatible drive can be connected and used directly with Mac OS.
Windows XP can only write directly to FAT32 formatted DVD-RAM discs. For UDF formatted discs, which are considered faster, compatible device drivers or software such as InCD or DLA are required. This is a non-issue with Linux however, which allows the use of virtually any file system of the multitude that ship with the operating system, including UDF. Mac OS can read and write HFS, HFS Plus, FAT and UDF formatted DVD-RAM discs directly. It is possible to use the ext3 file system on a DVD-RAM disc, but this filesystem was designed for use on hard drives, as a result using this file system on a DVD-RAM is very slow and prone to corruption. So while it is possible to use any filesystem one likes, only very few perform well on DVD-RAM. This is because some file systems frequently over-write data on the disc and the table of contents is contained at the start of the disc.
Many DVD standalone players and recorders do not support DVD-RAM, especially older or cheaper versions. However, within the 'RAMPRO' (DVD RAM promotion) group are a number of well-known manufacturers of standalone players and recorders that DO support DVD-RAM. Panasonic for instance have an excellent range of players and recorders which make full use of the advantages of DVD-RAM. There are also a number of video cameras that use DVD-RAM as the recording media.
Advantages of DVD-RAM
- Long life — without physical damage, data is retained for 30 years minimum.
- Can be rewritten over 100,000 times (DVD±RW can be rewritten approx. 1,000 times). Faster DVD-RAMs support fewer rewrites (3x speed: 100,000, 5x speed: 10,000), but still more than DVD±RW. (Remember, these are theoretical numbers. In practice they could be smaller depending on the drive, the treatment of the disc and the file system.)
- Reliable writing of discs. Verification done in hardware by the drive, so post-write verification by software is unnecessary.
- Disc defect management safeguards data.
- Write speed twice as fast (when verification not enabled) as DVD±RW. (Note: 12x DVD-RAM spec = 16 DVD±RW spec. As of 7/24/06, DVD±RW write speed was at 8x (DVD±RW specifications) while DVD-RAM is at 12x (DVD-RAM speed specification).)
- DVD-burning software may not be required — discs can be used and accessed like a removable hard disk. Mac OS (8.6 or later) supports DVD-RAM directly. Windows XP supports DVD-RAM directly only for FAT32-formatted discs. Device drivers or other software are needed for earlier versions of Windows or if one wants to use the decidedly better UDF format rather than FAT32.
- Easier to use than other DVD technology.
- Very fast access of smaller files on the disc.
- 2 KB disc block size wastes less space when writing small files.
- Finalization not necessary.
- Media available with or without protective cartridges.
- In video recorders, DVD-RAM can be written to and watched at the same time, much like TiVo.
- Supports time slip recording and recording without border in/out writing.
Disadvantages of DVD-RAM
- Less compatibility than DVD+RW and DVD-RW
- 12x media is not readily available in the USA and will not be available in the US and Europe in the near future due to low market demand. 16x media may not be available anywhere except manufacturers' R&D laboratories.
- DVD-RAM media is more expensive than other DVD types. 12x media is expected to cost almost $9 per disc, at least initially, but this price will fall as production ramps up.
- DVD-RAM writing will be slower than DVD+RW and DVD-RW until 12x DVD-RAM media becomes available. If write verification is enabled, the writing speed is decreased by about half. Using FAT32 instead of UDF may also slow DVD-RAM performance.


Wikipedia information about DVD-RAM
. This article is licensed under the GNU Free Documentation License . It uses material from the Wikipedia article 'DVD-RAM'







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