DVD, CD, SACD, and Blu-Ray optical media are all similar. Blank disks are made from polycarbonate plastic. A reflective metal layer, usually aluminium, is applied to a polycarbonate base and protected by another, thicker layer of polycarbonate. The digital bits are created by forming pits in the metallic layer. The 'top' or label side of an optical disk is thinner than the 'bottom' - the side read by the laser. A deep scratch on the label side can damage the metallic layer permanently. The data are recorded in a spiral track, running from the inside to the outside of the disk. This is opposite to the direction in which records are played (outside to inside). The disk rotates at constant linear velocity, adjusting the rotational speed as needed. Again, this is opposite to records, which use a constant rotational speed and a variable linear velocity. The laser is focused on the track and the pits modulate the intensity of the reflected light. These variations define the edges of the data bits, which form a continuous data stream. A dual-layer disk has two metallic layers; the top layer is semi-transparent. The laser focuses on the first layer and, when it reaches the end of the track, refocuses on the second layer.

The DVD Player DVD Fact The Sony PlayStation 2 was the first video game system able to play DVDs. A DVD player is very similar to a CD player. It has a laser assembly that shines the laser beam onto the surface of the disc to read the pattern of bumps (see How CDs Work for details). The DVD player decodes the MPEG-2 encoded movie, turning it into a standard composite video signal (see How Television Works for details). The player also decodes the audio stream and sends it to a Dolby decoder, where it is amplified and sent to the speakers. The DVD player has the job of finding and reading the data stored as bumps on the DVD. Considering how small the bumps are, the DVD player has to be an exceptionally precise piece of equipment. The drive consists of three fundamental components: * A drive motor to spin the disc - The drive motor is precisely controlled to rotate between 200 and 500 rpm, depending on which track is being read. * A laser and a lens system to focus in on the bumps and read them - The light from this laser has a smaller wavelength (640 nanometers) than the light from the laser in a CD player (780 nanometers), which allows the DVD laser to focus on the smaller DVD pits. * A tracking mechanism that can move the laser assembly so the laser beam can follow the spiral track - The tracking system has to be able to move the laser at micron resolutions. Inside the DVD player, there is a good bit of computer technology involved in forming the data into understandable data blocks, and sending them either to the DAC, in the case of audio or video data, or directly to another component in digital format, in the case of digital video or data. The fundamental job of the DVD player is to focus the laser on the track of bumps. The laser can focus either on the semi-transparent reflective material behind the closest layer, or, in the case of a double-layer disc, through this layer and onto the reflective material behind the inner layer. The laser beam passes through the polycarbonate layer, bounces off the reflective layer behind it and hits an opto-electronic device, which detects changes in light. The bumps reflect light differently than the "lands," the flat areas of the disc, and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes. Research: howstuffworks.com