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Optical Memory: External Memory

The first successful non-erasable compact disk (CD) based on optical technology was introduced sometime in 1983 by Sony and Philips companies, designed to hold up to 75 minutes of data providing a total of about 3 x 104 bits (3 gigabits) of storage using digital representation of 16-bit samples of analog sound signals taken at a rate of 44,100 samples per second for achieving high-quality sound recording and reproduction. Subsequently, low-cost, higher-capacity, and read-only storage media suitable for computer usage referred to as CD-ROM have been developed. Over time, this technology, however, got continuously improved and gradually became highly mature, resulting in the introduction of a variety of optical disk system with different characteristics, almost at regular intervals. A brief summary of the different products of this category has been presented in Table 4.3. We will now examine all these varieties of popular optical disks one after the other.

Compact Disk (CD) Technology

Computer-usable CD-ROM, similar to the audio CD, has used an optical technology in which a sharply focused coherent laser light source is made incident on the surface of the rotating disk. The surface is arranged with physical indentations along the tracks of the disk. These indentations reflect the focused beam incident on them towards a photodetector (optical sensor), which subsequently detects the stored binary patterns. Coherent light used here consists of synchronized waves of the same wavelength. If a coherent beam is combined with another beam of the same kind, and the two beams are in phase, then the intensity of the resultant beam will be high, giving a brighter light. Similarly, if the waves of the two beams are out of phase (180 degrees), they will cancel each other, thereby causing the resultant beam to be of feeble intensity. These characteristics are sensed by a photo-detector that eventually distinguishes a brighter spot (first case) from a relatively darker one (second case).

A cross-sectional view of a small portion of a CD-ROM is depicted in Figure 4.9. The bottom layer of a CD-ROM is normally a polycarbonate plastic that functions as a clear glass base. The surface of the plastic is programmed to record digital information (either music

TABLE 4.3

Numerous Optical Disks

CD: Compact disk of early days of 120 mm in diameter was simply a non-erasable disk and capable of storing only digitized audio information of more than 60 minutes of uninterrupted audio playing time on one side of it.

CD-ROM: Compact disk Read-Only Memory is a non-erasable one-time recorded disk of 120 mm diameter with standard speed and format used for storing computer-related data. A CD meant for 75 minutes can store information of about 680 Mbytes. The speed of the CD drive is expressed in terms of X (i.e. say 40X) that influences the data-transfer rate.

CD-R: This compact disk called CD Recordable (WORM) allows to write only once subsequently after production (original writing). The disk controller of CD-R although is somewhat expensive than that of a CD-ROM, but allows to read the disk as usual. The resulting disk after writing can also be read by an ordinary CD-ROM drive.

CD-RW: This compact disk called CD Rewritable allows the user to erase and rewrite the disk multiple times. Compared with CD-ROM and CD-R, it is much better while considered for use as a secondary storage. In fact, it often competes even with the magnetic hard disk for many reasons including itscost-wise high storage capacity, easy removability, and high portability, and as such, they are found to be extremely suitable for archival storage of information.

DVD: This disk called the Digital Versatile Disk, a forerunner of Video CD (VCD), can store digitized, compressed representation of mostly video information, as well as bulk volume of other digital data, currently as much as seven times of a CD-ROM. The physical size of the DVD, however, is the same as the CD. DVDs of both 80 mm and 120mm diameter are used, with a double-sided capacity of up to 17 Gbytes. The basic DVD is, however, a read-only, i.e. DVD-ROM, and is one-sided. The data-transfer rate in generic DVD is relatively much higher than the other optical disks because of its higher storage density.

DVD-R: This disk called the DVD Recordable is only one-sided, and can be written only once subsequently after production (similar to CD-R) using a DVD Recorder, and then can function as a DVD-ROM.

DVD-RW: This DVD Rewritable may be a two-layered and two-sided one, and is similar to a DVD-ROM, but the user is allowed to erase and rewrite the disk multiple times.

HD-DVD: This high-definition DVD (HD-DVD) using a disk format different from conventional DVDs provides even more storage capacity compared with traditional DVDs, thereby enabling to store high-definition videos. Higher storage capacity has been realized by way of achieving higher bit density, and that became possible by using a shorter wavelength laser beam of 405 nm in the blue-violet range. The single-layer/single-sided HD DVD scheme can store 15 GB, and a dual layer HD DVD can store up to 30GB. However, HD-DVD was not able to survive for long and gradually went out of the market in the competition with Blu-ray DVD.

Blu-Ray DVD: It is also a high-definition DVD (HD-DVD), similar in many respects with HD-DVD, but uses a disk format different from conventional HD-DVD. It provides even more storage capacity compared with traditional HD-DVDs. In a single-layer/single-sided disk, the storage capacity is 25GB and a dual layer disk can hold up to 50GB. Blu-ray disk, however, had a competitive challenge from the existing HD-DVD, and eventually won in the format war of 2006-2008. Consequently, the Blu-ray scheme achieved market dominance, and became the de facto successor to the DVD format.

FIGURE 4.9

Cross-sectional view of a CD and its operation.

or computer data) by indenting (imprinting) the surface of the polycarbonate with a series of microscopic pits. The unindented parts (i.e. the areas between the pits) that remain on the surface are called lands. This indenting is first accomplished with a finely focused, high-intensity laser to create a programmed master disk. This master is then subsequently used to make a die to stamp out copies of this master onto other polycarbonates. The entire pitted surface of the copy is then coated with a thin layer of a highly reflective surface, usually aluminium or gold. This shiny sensitive surface is then protected against dust and scratches by a top coat of clear acrylic (lacquer). Finally, a label can be silkscreened to stamp onto the acrylic. The total thickness of the disk, however, comes to approximately

1.2 mm, most of which is contributed by the polycarbonate plastics. The other layers are extremely thin. Figure 4.10 exhibits a layered view of a CD-ROM.

Information is retrieved from a CD or CD-ROM by a low-powered laser that is housed in an optical disk player or in a CD-ROM drive unit. The laser source and the photo-detector are positioned below a polycarbonate disk. From the laser side, the pits actually appear as bumps with a convex surface with respect to the land. The disk is rotated with the help of a motor, and the beam that is emitted travels through the plastic, and is incident on the shining aluminium layer, and then reflects off the layer, and travels back towards the photo-detector.

When the disk rotates, the laser beam scans across the disk, and comes upon different situations. The intensity of the reflected laser light changes, resulting a low intensity when it falls on a pit having somewhat of a relatively convex surface, since the light here scatters. In contrast, when the laser beam falls on a land which is a relatively smooth surface, the reflected light does have a higher intensity. This change between pits and lands is detected by the photo-detector and is converted into a corresponding digital signal. A different situation arises when the beam moves through the edge where the pit changes to the land, i.e. a transition from a pit to a land and vice-versa. The beginning or end of a pit represents a 1; when no change in elevation occurs between intervals, a 0 is recorded. This pattern is, however, not a direct representation of the stored data. The CDs actually use a complex encoding scheme to represent error-free data. Each byte of data is represented by a 14-bit code, so that it can provide adequate error detection capability. (As the pits are very small, it is difficult to implement all of the pits perfectly, and as such, physical imperfections cannot be avoided that may cause to inject some errors in data while stored. For the audio/video CD, these errors in data have no perceptible impact on the reproduced sound or image. Flowever, such errors are not acceptable in computer applications, and it is thus

FIGURE 4.10

A layered view of CD-ROM (capacity 682 MB).

necessary to use additional bits to provide adequate error-checking and correcting capabilities to ensure the integrity of the stored data. CD used in computer applications are equipped with such capability). We restrict ourselves here from entering into any further discussion on the details of this code.

Unlike concentric tracks in the magnetic disk, the CD contains just one single physical spiral track, beginning near the centre and spiralling out to the outer edge of the disk, to realize greater capacity. But, it is customary to refer to each such circular path spanning 360 degrees as a separate track, which is analogous to the terminology used for the magnetic disk. The pits are, however, arranged along such tracks. The CD is 120 mm in diameter with a 15 mm hole in the centre. Data are stored on tracks that cover the area from a 25-mm radius to a 58-mm radius. The space between the tracks is 1.6 pm, or even less nowadays. Pits are 0.5 pm wide and 0.8-3 pm long. The track density is about

6,000 tracks/cm (more than 15,000 tracks on a disk) which is much higher than the density achievable in magnetic disks that usually ranges from 800 to about 2,400 tracks/cm.

 
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