Dvd Drives - HP Vectra VEi7 Technical Reference Manual

Mass-Storage Devices

DVD Drives

Digital Versatile Disk
Digital Versatile Disc (DVD) is a medium for the distribution of from 4.7
(DVD) Technology to 17 GB of digital data on a 120-mm (4.75 inch) disc. This huge volume
of data (CD-ROMs can store 680 MB) can be used to store up to nine
hours of studio quality video and multi-channel surround-sound audio,
highly interactive multimedia computer programs, 30 hours of CD-
quality audio, or anything else that can be represented as digital data.
A DVD looks like a CD-ROM: it is a silvery disc, 4.75 inches in diameter,
with a hole in the center. Like a CD, data is recorded on the disc in a
spiral trail of tiny pits, and the discs are read using a laser beam. The
DVD's larger capacity is achieved by making the pits smaller and the
spiral tighter, and by recording the data in as many as four layers, two
on each side of the disc.
To read these tightly packed discs, lasers that produce a shorter
wavelength beam of light are required, as are more accurate aiming and
focusing mechanisms. In fact, the focusing mechanism is the
technology that allows data to be recorded on two layers. To read the
second layer, the reader simply focuses the laser a little deeper into the
disc, where the second layer of data is recorded.
Not only are two layer discs possible, but so are double-sided discs.
The availability of four layers is what gives DVD its 17 gigabyte
capacity.
24
Core Components and Technologies
Diameter
Thickness
Track Pitch
Minimum Pit Length
Laser Wavelength
Data Capacity (per layer)
Layers
DVD
120mm 120mm
0.6 mm 1.2 mm
0.74 nanometers 1.6 nanometers
0.40 nanometers 0.834 nanometers
640 nm 780 nm
4.7 GB 0.68 GB
1,2,4 1
CD