Another class of mass storage systems applies optical technology. An example is the compact disk (CD). These disks are 12 centimeters in diameter and consists of reflective material covered with a clear protective coating. Information is recorded on them by creating variations in their reflective surfaces. This information can then be retrieved by means of a laser beam that monitors irregularities on the reflective surface of the CD as it spins.
CD technology was originally applied to audio recordings using a recording format known as CD-DA (compact disk-digital audio), and the CDs used today for computer data storage use essentially the same format. In particular, information on these CDs is stored on a single track that spirals around the CD like a groove on an old fashioned record, however, unlike old-fashioned records, the track on a CD spirals from the inside out. This track is divided into units called sectors, each with its own identifying markings and a capacity of a 2KB of data, which equates to 1/75 of a second of music in the case of audio recordings..
Note that the distance around the spiralled track is greater toward the outer-edge of the disk than at the inner portion. To maximize the capacity of a CD, information is stored at a uniform linear density over the entire spiralled track, which means that more information is stored in a loop around the outer portion of the spiral than in a loop around the inner portion. In turn, more sectors will be read in a single revolution of the disk when the laser beam is scanning the outer portion of the spiralled track than when the beam is scanning the inner portion of the track. Thus, to obtain a uniform rate of data transfer, CD-DRA players are designed to vary the rotation speed depending on the location of the laser beam. However, most CD system used for computer data storage spin at a faster, constant speed and thus most accommodate variations in data transfer rates.
As a consequence of such design decisions, CD storage systems perform best when dealing with long, continuous strings of data, as when reproducing music. In contrast, when an application requires access to items of data in a random manner, the approach used in magnetic dish storage (individual, concentric tracks divided into individually accessible sectors) outperforms the spiral approach used in CDs.
Traditional CDs have capacities in the range of the 600 to 700MB. However, newer DVDs (Digital Versatile Disks), which are constructed from multiple, semitransparent layers that serve as distinct surfaces when viewed by a precisely focused laser, provide storage capacities of several GB. Such disks are capable of storing lengthy multimedia presentations, including entire motion pictures.
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