Today’s optical discs can hold over 128 gigabytes of data. That is the equivalent of millions of typewritten pages or a medium-sized library all on a single disc. Optical discs are very common, with most software and many movies available on optical disc. In optical disc technology, a laser beam alters the surface of a plastic or metallic disc to represent data. Unlike hard disks, which use magnetic charges to represent 1s and 0s, optical discs use reflected light. The 1s and 0s are represented by flat areas called lands and bumpy areas called pits on the disc surface. The disc is read by an optical disc drive using a laser that projects a tiny beam of light on these areas. The amount of reflected light determines whether the area represents a 1 or a 0. Like hard disks, optical discs use tracks and sectors to organize and store files. Unlike the concentric tracks and wedge-shaped sectors used for hard disks, however, optical discs typically use a single track that spirals outward from the center of the disc. This single track is divided into equal-sized sectors. The most widely used optical discs are CD, DVD, and Blu-ray discs.
•Compact discs (CDs) were the first widely available optical format for PC users. Typically, CD drives store 700 MB (megabytes) of storage. Optical discs that store music are often CDs.
•Digital versatile discs (DVDs) are the standard optical discs in PCs. DVDs are very similar to CDs except that typical DVD discs can store 4.7 GB (gigabytes)—seven times the capacity of CDs. Optical discs that store movies or software are often DVDs. DVD drives and CD drives look very similar.
•Blu-ray discs (BDs) are the newest form of optical storage designed to store hi def (high definition) video, which uses the HD 720 and HD 1080 resolutions discussed in Chapter 6. The name Blu-ray comes from a special blue-colored laser used to read the discs that gives them a typical capacity of 50 GB—ten times the capacity of DVDs. Optical discs that store hi def video and the newest video games are often Blu-ray discs. Each of these optical discs has three basic formats: read only, write once, and rewritable. •Read-only (ROM for read-only memory) discs are discs that cannot be written on or erased by the user. Optical discs that you buy in a store, such as music CDs, DVD movies, and Blu-ray video games, are often read only.
•Write-once (R for recordable) discs can be written on once. After that, the disc can be read many times but cannot be written on or erased. These discs are ideal for creating permanent archives, such as CD-R, often used for storing family photos, and DVD-R, often used to store home movies.
•Rewritable (RW for rewritable or RAM for random-access memory) discs are similar to write-once discs except that the disc surface is not permanently altered when data is recorded. These changeable, portable storage options are popular for storing and sharing audio, video, and large multimedia presentations.
Some optical discs, referred to as double-sided discs, contain information on both sides of the disc and need to be flipped over to read the other side, effectively doubling the storage capacity of the disk. For example, a double-sided DVD can store 9.4 GB, or twice as much as a single-sided DVD. Another way to add capacity to an optical disc is by adding multiple recording layers. These discs store information on several layers sandwiched together on one side of the disc. For example, some Blu-ray discs have multiple layers that increase storage capacity from 50 GB to 128 GB.