HP Vectra VEi8 HP Vectra VEi7, VEi8 & VLi8, Technical Reference Manual (V - Page 24

Core Components and Technologies Mass-Storage Devices DVD Drives Digital Versatile Disk (DVD) Technology Digital Versatile Disc (DVD) is a medium for the distribution of from 4.7 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 CDquality 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. Diameter Thickness Track Pitch Minimum Pit Length Laser Wavelength Data Capacity (per layer) Layers DVD 120mm 0.6 mm 0.74 nanometers 0.40 nanometers 640 nm 4.7 GB 1,2,4 CD 120mm 1.2 mm 1.6 nanometers 0.834 nanometers 780 nm 0.68 GB 1 24