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Rewritable DVD-ROM drives, or DVD-RAM, employs phase-change
technology with some MO features mixed in rather than the pure
optical technology of CD and DVD discs and has its roots in the PD
optical disc system. A "land groove" format allows signals to be
recorded on both the grooves formed on the disc and in the lands
between the grooves. The grooves and pre-embossed sector headers
are molded into the disc during manufacturing. The first
generation of DVD-RAM products - providing 2.6GB of reusable data
space on either side of a disc - first appeared in mid-1998.
However, these early drives will be incompatible with the
higher-capacity standard, which may use a contrast enhancement
layer and a thermal buffer layer to achieve higher density. The
specification for DVD-RAM version 2.0 - with a capacity of 4.7
billion bytes per side - was published in October 1999. Hitachi
reached the 4.7MB capacity by reducing mark size from 0.41/0.43
microns to 0.28/0.30 microns and track pitch from 0.74 microns to
0.59 microns.
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