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Sep 9, 2011

DVD: Facts

A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVD originally stood for Digital Versatile Disk, or Digital Video Disk. The acronym was dropped after DVD proved to have more uses than just storing video content. DVDs offer higher storage capacity than Compact Discs while having the same dimensions.

Pre-recorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs are known as DVD-ROM, because data can only be read and not written nor erased. Blank recordable DVDs (DVD-R and DVD+R) can be recorded once using optical disc recording technologies and supported by optical disc drives and DVD recorders and then function as a DVD-ROM. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased multiple times.

Dual-layer recording
Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store significantly more data—up to 8.54 gigabytes per disc, compared with 4.7 gigabytes for single-layer discs. Along with this, DVD-DLs have slower write speeds as compared to ordinary DVDs and when played on a DVD player a slight transition can sometimes be seen between the layers. DVD-R DL was developed for the DVD Forum by Pioneer Corporation; DVD+R DL 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 semitransparent layer. In some DVD players, the layer change can exhibit a noticeable pause, up to several seconds. This caused some viewers to worry that their dual-layer discs were damaged or defective, with the end result that studios began listing a standard message explaining the dual-layer pausing effect on all dual-layer disc packaging.
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 is now comparable to that of single-layer drives, although the blank media remain more expensive. The recording speeds reached by dual-layer media are still well below those of single-layer media.
There are two modes for dual-layer orientation. With Parallel Track Path (PTP), used on DVD-ROM, both layers start at the inside diameter (ID) and end at the outside diameter (OD) with the lead-out. With Opposite Track Path (OTP), used on many Digital Video Discs, the lower layer starts at the ID and the upper layer starts at the OD, where the other layer ends; they share one lead-in and one lead-out.

Read mechanism650 nm laser, 10.5 Mbit/s (1×)


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