A CD-R (Compact Disc-Recordable) is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once, Read Many optical media (though the whole disk does not have to be entirely written in the same session) and retains a high level of compatibility with standard CD readers (unlike CD-RW which can be rewritten but has much lower compatibility and the discs are considerably more expensive).
Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. Known as CD-Erasable (CD-E) during its development, CD-RW was introduced in 1997, and was preceded by the never officially released CD-MO in 1988.
There are several competing DVD Formats:
DVD-ROM: These are pressed similarly to CDs. The reflective surface is silver or gold colored. They can be single-sided/single-layered, single-sided/double-layered, double-sided/single-layered, or double-sided/double-layered. As of 2004, new double-sided discs have become increasingly rare.
DVD-D: a new self-destructing disposable DVD format. Like the EZ-D, it is sold in an airtight package, and begins to destroy itself by oxidation after several hours.
DVD Plus: combines both DVD and CD technologies by providing the CD layer and a DVD layer. Not to be confused with the DVD+ formats below.
Recordable formats, supported by the DVD Forum
DVD-R for Authoring: a special-purpose DVD-R used to record DVD masters, which can then be duplicated to pressed DVDs by a duplication plant. They require a special DVD-R recorder, and are not often used nowadays since many duplicators can now accept ordinary DVD-R masters.
DVD-R (strictly DVD-R for General): can record up to 4.5 GB in a similar fashion to a CD-R disc. Once recorded and finalized it can be played by most DVD-ROM players.
DVD-RW: can record up to 4.7 GB in a similar fashion to a CD-RW disc.
DVD-R DL: a derivate of DVD-R that uses double-layer recordable discs to store up to 8.5 GB of data.
DVD-RAM (current specification is version 2.1): requires a special unit to play 4.7GB or 9.4GB recorded discs (DVD-RAM disc are typically housed in a cartridge). 2.6GB discs can be removed from their caddy and used in DVD-ROM drives. i Top capacity is 9.4GB (4.7GB/side)...
Recordable formats, supported by the DVD+RW Alliance
DVD+R: can record up to 4.7 GB single-layered/single-sided DVD+R disc, at up to 16x speed. Like DVD-R you can record only once.
DVD+RW: can record up to 4.7 GB at up to 16x speed. Since it is rewritable it can be overwritten several times. It does not need special "pre-pits" or finalization to be played in a DVD player.
DVD+R DL: a derivate of DVD+R that uses dual-layer recordable discs to store up to 8.5 GB of data.
Dual Layer recording allows DVD-R and DVD+R discs to store significantly more data, up to 8.5 Gigabytes per disc, compared with 4.7 Gigabytes for single-layer discs. DVD-R DL (dual layer — see figure) was developed for the DVD Forum by Pioneer Corporation, DVD+R DL (double layer — see figure) was developed for the DVD+RW Alliance by Philips and Mitsubishi Kagaku Media (MKM).
A Dual Layer disc differs from its usual DVD counterpart by employing a second physical layer within the disc itself. The drive with Dual Layer capability accesses the second layer by shining the laser through the first semi-transparent layer. The layer change mechanism in some DVD players can show a noticeable pause, as long as two seconds by some accounts. More than a few viewers have worried that their dual layer discs were damaged or defective.
DVD recordable discs supporting this technology are backward compatible with some existing DVD players and DVD-ROM drives. Many current DVD recorders support dual-layer technology, and the price point is comparable to that of single-layer drives, though the blank media remains significantly more expensive.
HD DVD, High Density DVD, or High-Definition DVD is a high-density optical disc format designed for the storage of data and high-definition video.
HD DVD has a single-layer capacity of 15 GB and a dual-layer capacity of 30 GB. There is also a double-sided hybrid format which contains standard DVD-Video format video on one side, playable in regular DVD players, and HD DVD video on the other side for playback in high definition on HD DVD players. JVC has developed a similar hybrid disc for the Blu-ray format. These hybrid discs make retail marketing and shelf space management easier. This also removes some confusion from DVD buyers since they can now buy a disc compatible with any DVD/HD DVD player in their house. The HD DVD format also can be applied to current red laser DVDs in 5, 9, 15 and 18 GB capacities which offers a lower-cost option for distributors.
A Blu-ray Disc is a high-density optical disc format for the storage of digital media, including high-definition video.
The name Blu-ray Disc is derived from the blue-violet laser used to read and write this type of disc. Because of this shorter wavelength (405 nm), substantially more data can be stored on a Blu-ray Disc than on the common DVD format, which uses a red, 650 nm laser. Blu-ray Disc can store 25 GB on each layer, as opposed to a DVD's 4.7 GB. Several manufacturers have released single layer and dual layer (50 GB) recordable BDs and rewritable discs.
Blu-ray Disc is similar to PDD, another optical disc format developed by Sony (which has been available since 2004) but offering higher data transfer speeds. PDD was not intended for home video use and was aimed at business data archiving and backup. Blu-ray Disc is currently in a format war with rival format HD DVD.
About 9 hours of high-definition (HD) video can be stored on a 50 GB disc.
About 23 hours of standard-definition (SD) video can be stored on a 50 GB disc.
On average, a single-layer disc can hold a High Definition feature of 135 minutes using MPEG-2, with additional room for 2 hours of bonus material in standard definition quality. A double-layer disc will extend this number up to 3 hours in HD quality and 9 hours of SD bonus material.
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