Compact discs (CDs) and digital video discs (DVDs) store information in the
same way. The disc's surface is marked with indentations arranged in a continuous spiral. As a laser moves across the pits, its beam is reflected back to the
player, which reads the reflections as a signal and turns it into audio, video,
graphics, or text. DVDs store more data than CDs because the pits are smaller and
arranged in tighter circles. To transmit and process the added information,
the DVD system requires more efficient compression and decompression technology.
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Although the first DVD releases will have data on one side, future discs could be
double-sided, boosting the capacity to as much as 17GB. Despite DVD's greater
capacity and enhanced compression, DVD drives and players will be able to read
audio CDs as well as multimedia CD-ROMs.
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For even more capacity, DVDs can have a second layer of pits laid over the first.
The first layer is semireflective, allowing the laser to alternately read the data
on the first layer or pass through and read the second layer. This added layer
almost doubles the disc's capacity from 4.7GB to 8.5GB.