Database Reference
In-Depth Information
1.5.2 DVDs
The digital versatile discs can hold anywhere from 4-16 GB of data currently,
depending on whether they are single or double sided and single or dou-
ble layered. Pitch between tracks for DVD discs is 0.73
m. Access rates
are determined by the speed of the drive used, but can achieve over 20
MB/s on writes for a 16x drive with double-layered discs. Archival life of
the media is similar to CD in that a disc is expected to last 5-7 years be-
fore data may become unreadable. Speed, capacity, and archival life are still
only achieving what the oldest and least capable magnetic storage devices can
deliver.
μ
1.5.3 Blu-Ray and HD-DVD
Blu-ray disk and high-definition DVD (HD-DVD) are the newest multilayer
optical storage mediums and are currently capable of holding 25 or 50 GB
of data per Blu-ray disc, depending on whether they are single or dual layer,
and 15 GB per HD-DVD disc. Blu-ray has been locked in a multiyear battle
with HD-DVD for the digital video market, but the battle has largely been
won by Blu-ray at this point in time. The much higher storage capacity of
this technology is largely enabled by the availability of solid-state blue lasers
(hence the Blu-ray name). The much shorter wavelength of blue light can
be focused to a much smaller point size on the disk medium. The specifi-
cation and design for this technology includes plans to expand a single disk
to 100-200 GB by simply defining new tracks. Current drives are capable of
1x-16x speeds providing anywhere from 6 MB/s to 16 MB/s. The specifi-
cation allows for up to 35 MB/s. These are substantial improvements over
previous generation optical storage, but still not competitive with magnetic
storage.
1.5.4 Future Trends
Traditional optical storage technologies have fundamental limits imposed by
the physics of far-field diffraction. This means that the limits of the amount
of data stored on a two-dimensional disc are determined by the wavelength
of light used in the optical drive. In order to increase the amount of data
on the disc, new optics with smaller wavelengths (blue and ultraviolet light)
are necessary, and there are physical limitations on how small optics can get.
Holographic storage promises solutions that reduce or eliminate the far-field
diffraction limitations by using several different techniques capable of using
the entire volume of media available. However, holographic storage continues
to struggle with bringing a product to market, so it is not clear that the
physical limitations of optical media will be overcome.
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