“Many of today's personal computers are equipped with optical drives for storing and retrieving data on compact discs (CDs) or digital versatile disks (DVDs) that can be removed from the drive. A CD is a silvery plastic platter on which a laser records data as a sequence of tiny pits in a spiral track on one side of the disk. One CD can hold 680 MB of data. A DVD uses smaller pits packed in a tighter spiral. Some Blu-ray disks (high-density optical disks read with a blue-violet laser) can hold multiple layers of data-for a total capacity of 200GB, sufficient storage for many hours of studio-quality video and multi-channel audio.”