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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
Numerous Optical Disks
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
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.