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DVD audio and DVD video are different formats. DVD audio discs and players are relatively rare right now, but they will become more common, and the difference in sound quality should be noticeable. In order to take advantage of higher-quality DVD audio discs, you will need a DVD player with a 192kHz/24-bit digital-to-analog converter (DAC). Most DVD players have only a 96kHz/24-bit digital-to-analog converter.
DVD audio recordings can provide far better sound quality than CDs.DVD audio discs can hold 74 minutes of music at their highest quality level, 192kHz/24-bit audio. By lowering either the sampling rate or the accuracy, DVDs can be made to hold more music. A DVD audio disc can store up to two hours of 6-channel, better than CD quality, 96kHz/24-bit music. Lower the specifications further and a DVD audio disc can hold almost 7 hours of CD-quality audio.
Reading a DVD
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 motoris 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.
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.
The hardest part of reading a DVD is keeping the laser beam centered on the data track. This centering is the job of the tracking system. As the DVD is played, the tracking system has to move the laser continually outward. As the laser moves outward from the center of the disc, the bumps move past the laser at an increasing speed. This happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving. So, as the laser moves outward, the spindle motor must slow the spinning of the DVD so that the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.
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