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Developed by the joint efforts of Philips and Sony Corporation in the early 1980s, when the digital age was taking over the stereo industry, the Compact Disc was first and foremost a high-fidelity delivery medium. The CD was initially released in the United States in 1983 (in Japan the CD got a one-year head start, officially released in 1982 according to Sony), and within five years it had replaced the vinyl phonograph record as the premiere stereophonic medium because of its wide range, lack of noise, near invulnerability to damage, and long projected life.
The CD was designed chiefly for capacity and real-time playback of music. Engineers set many of the practical aspects of the CD around the requirements of music recording. For example, they selected the 70 or so minutes of music capacity as one of the core specifications in designing the system, because a primary design goal was to fit the entire Beethoven's Ninth Symphony, without interruption, on a single disc. As originally conceived, its storage was not reliable enough for computers. And computers had little use for it. At the time, they were choking when confronted with a few megabytes. CDs wielded hundreds of them.
But as the CD rose to prominence as the primary distribution medium for prerecorded music, computer engineers began to look at the shiny medium with a covetous gleam in their eyes. They saw the digital storage provided by the disc as a repository for more megabytes than anyone had reason to use. After all, data is data (okay, data are data) regardless of whether the bytes encode a symphony or an operating system. When someone got the idea that a plastic puck that cost a buck to make and retailed for $16.99 could be filled with last year's statistics and marketed for $249, the rush was on. The Compact Disc became the CD-ROM (which stands for Compact Disc, Read-Only Memory), and megabytes came to the masses.
Soon sound became only one of the applications of the Compact Disc medium. The original name had to be extended to distinguish musical CDs from all the others. To computer people, the CD of the stereo system became the CD-DA, Compact Disc, Digital Audio.
Engineers tinkered with the storage format of the CD to stuff the discs with other kinds of data. Philips optimized the medium for interactive applications—multimedia presentations and games—to create CD-I, which stands for Compact Disc Interactive. Some even thought that compression—a lot of it—could fit video on the discs. Compact Disc-Video succeeded in squeezing video on the little discs, but not very much and not very well. Viewable video had to await another string of developments and the new generation of optical storage.
The roots of DVD go back to two competing proposals, both of which had the primary intent of storing video on a disc the same size as a CD. The original developers of the Compact Disc, Philips and Sony, backed a format they called MMCD for Multimedia Compact Disc (they owned the CD name so they took advantage of it). The other camp, led by Matsushita, Time Warner, and Toshiba, developed their own, incompatible format they called SD.
For a while, the industry appeared poised for a repeat of the Beta/VHS debacle that put two mutually incompatible videotape cassette formats on the market for nearly a decade. In September, 1995, the industry appeared to come to its senses, hammering out a single standard agreeable to both camps. To distinguish it from the earlier efforts and reflect the expanded range of possibilities afforded by the new medium, the format was rechristened with Versatile replacing the Video of the earlier proposals. Credit for developing the initial standard is generally given to an industry consortium that included Hitachi, JVC, Matsushita, Mitsubishi, Philips, Sony, Thompson, Time Warner, and Toshiba.
As with CDs, each application format for the DVD has its own subdesignation. These include DVD-Video for video applications, such as the distribution of motion pictures; DVD-Audio, as a high-quality audio disc with capabilities far beyond today's 16-bit discs; and DVD-ROM for the distribution of computer software and other data.
When it came to developing a recordable format for DVD, however, the consensus fell apart. As this is written, promoters advocate three read-only formats under the names DVD-R(A), DVD-R(G), and DVD+R. Read/write systems are equally confused with four competing standards, including DVD-Multi, DVD-RAM, DVD-RW, and DVD+RW.
Although the differences between these recordable standards is blurring—each one now boasts a full 4.7 gigabytes-per-disc capacity as well as some degree of compatibility with home DVD players—the animosity is not fading. The industry is split into two camps, the "plus" camp and the "hyphen" camp. The former, the promoters of the "plus" systems of DVD+R and DVD+RW, include Dell, Hewlett-Packard, Mitsubishi, Philips, Ricoh, Sony, and Thomson. The "hyphen" camp promotes the DVD-Multi format, which brings together DVD-R(A) and DVD-R(G) as well as DVD-RAM and DVD-RW. The DVD Forum, an organization that includes nearly all consumer electronics manufacturers (with the notable exceptions of Dell and Hewlett-Packard but including the other "plus" camp members), maintains the official standards.
The heart of both the Compact Disc and Digital Versatile Disc systems is the disc medium itself. Its design, based on a pattern of dots that can be read optically but mass-produced mechanically, makes optical storage the fastest and least expensive medium for duplicating hundreds of megabytes of data.
The flat disc shape also offers a distinct advantage. Machines can mold copies of discs by stamping them between dies instead of filling a three-dimensional mold with a casting liquid. This stamping process has a long history. It has been used for over a century in duplicating recorded music—first with shellac and clay to copy Emil Berliner's first phonograph records, then with vinyl for old-fashioned record albums. Duplicating CDs and DVDs requires extra precision and a few extra steps, but it remains essentially the same stamping process.
The disc-duplicating process begins with a disc master. A mastering machine equipped with a high-powered laser blasts the pits in a blank disc—the recording master—to make an original mechanical recording. Then the master is made into a mold called a stamper. A negative copy is electroplated onto the master, then separated from it, leaving the master unscathed. One master can make many duplicate molds, each of which is then mounted in a stamping machine. The machine heats the mold and injects a glob of plastic into it. After giving the plastic a chance to cool, the stamping machine ejects the disc and takes another gulp of plastic.
In making a CD or DVD, another machine takes the newly stamped disc and aluminizes it so that it has a shiny, mirror-like finish. To protect the shine, the disc is laminated with a clear plastic cover that guards the mechanical pattern from chemical and physical abuse (oxidation and scratches). Finally, another machine silk-screens a label on the disc. It is packaged, shrink-wrapped, and sent off to a warehouse or store.
The DVD process differs in that a disc can have multiple layers. In the current process, a separate master is made for each layer. The layers are stamped out separately, each only half as thick as a complete disc. The two complete layers are then fastened together with a special transparent glue.
In theory, a disc could be any size, and setting a standard is essentially an exercise in pragmatism. Size is related to playing time. The bigger the disc, the more data it holds, all else being equal. If you want to store a lot of data, a big disc has its allure. On the other hand, a platter the size of a wading pool would win favor with no one but plastics manufacturers. You can also increase the capacity of a disk by shrinking the size of every stored bit of digital code, but the practical capabilities of technology limit how small you can make a bit. In trying to craft a standard, engineers have to balance the convenience of small size, the maximum practical storage density, and a target for the amount of information they need to store.
In the late 1970s when Philips and Sony were developing the CD, the maximum practical storage density of the then-current technology was about 150 megabytes per square inch. The arbitrary design goal of about 70 minutes per disc side (enough room for Beethoven's Ninth) dictated about 650MB at the data rate selected (which itself was a tradeoff between data requirements and sound quality). The result was that the design engineers found a 120-millimeter (that's about 4.6 inches) platter to be their ideal compromise. A nice, round 100 millimeters was just too small for Beethoven.
For portable applications, the engineers came up with a smaller form factor for discs, 80 mm (about 3.1 inches). Once plated and given its protective plastic coating, either size of CD is about 1.2 mm (about 0.05 inch) thick.
For DVDs, the same sizes have been retained. This expedient allows the same equipment used to make CDs with only minor modification to stamp out DVDs. In addition, the same drives that read and write can also use CDs, whether in your home entertainment system or inside your computer.
The DVD medium differs from conventional CDs in that discs can use both sides for recording data and have multiple layers on each side. (Although two-sided CDs are possible, they have been commercially produced only rarely.) As noted earlier, current technology fabricates each DVD in two pieces, each 0.6 mm thick, which are later cemented together. Cemented back-to-back, the disc gains two sides. Cemented so the face of one butts the back of the other, the disc gains two layers. The latter configuration allows you to play both recorded surfaces without flipping the disc over. To shift between layers, the DVD player needs only to refocus its laser beam. The process is fast—it takes only milliseconds—and, with adequate buffering, can be completely invisible to your computer and its software. There's no pause in the data streaming from the drive.
Under the DVD standards, eight possible types of disc are currently defined, depending on the size, number of sides, and number of layers. Table 18.1 summarizes the various DVD disc types and their storage capacities.
Almost all discs made thus far conform to the DVD-5 standard. This format was tailored to the needs of the motion picture and videocassette industry. It allows a standard Hollywood-style movie to fit on a single disc. Unlike videocassettes, however, the movie will have digital quality images and sound—and not just stereo sound but full eight-channel surround. In that the cost of duplicating DVDs is a fraction of that of videocassettes, the software industry will be urging you into the new medium as fast as it can. In that you should easily be able to see and hear the difference, you shouldn't need too much encouragement.
The DVD format allows your disc drive to read the two layers in either of two ways, determined when the disc is recorded. Parallel tracking path (PTP) tracks are read in parallel. Your drive reads both layers at nearly the same time, in parallel. In practice, the two tracks may contain the same material in different formats—one might hold a standard aspect-ratio image and the other a widescreen image—allowing you to switch between them instantly. Opposite tracking path (OTP) tracks are read sequentially. The drive reads one layer to its end, then reverses and reads the second layer in the opposite direction. To minimize changeover time when the read head reverses direction, the drive starts reading the bottom layer from the inside out and then switches to reading the inside layer from the outside in. Discs that use OTP tracks are sometimes describe as reverse-spiral dual-layer (RSDL) discs. Some disc producers prefer them because they offer a longer (nearly) continuous playing time.
The first DVDs were mostly two-sided, single-layer DVD-10 discs, usually a motion picture in full-frame format on one side and widescreen on the other. One reason was that drives capable of reading two-layer discs required more development, but all DVD drives can handle two-sided discs, providing you physically flip the disc to access the other side.
A modern DVD drive distinguishes the separate layers of a multiple-layer disc by selective focus. To read the bottom layer, it simple focuses on its pits. To read the upper layer, it focuses through the semitransparent lower layer to the upper layer. The separation between the two layers, about 20 to 70 microns, is sufficient for each layer to have a distinct-enough focus for them to be distinguished. The pattern in the layer that is out of focus blurs together so that individual bits blur together and don't register.
Four-layer DVDs, which have two layers of data on each of their two sides, require a not-quite-perfected manufacturing process that writes two layers simultaneously on each of the two halves that are cemented to make the completed disc.
The recording materials used by read-only, recordable, and rewritable optical disc systems are quite different because each system operates on different physical principles. Read-only media depend on fast, mechanical reproduction of discs. Recordable media require a one-use material with a long life. Rewritable media require a recording material that can be cycled between states so that it can be erased and reused.
Every Compact Disc is a three-layer sandwich. The bulk of the disc is a transparent polycarbonate plastic substrate called the carrier. It is made from the material injected into the stamping press—the machine smashes a warm glob of raw plastic flat and embosses the spiral pattern of pits encoding the digital data into one of its surfaces. The aluminum coating that makes the disc reflective (except for the data pits) is vapor-plated on the polycarbonate substrate to a thickness of about one-tenth micron. To protect the aluminum from oxidation (which would darken it and make the pattern of pits difficult or impossible to read), it is protected with a final layer of lacquer about 200 microns thick.
The laser in a disc drive reads from the reflective side of the disc. That is, the laser reads through the thick carrier to the aluminized layer. The label of the disc gets silk-screened onto the thin protective lacquer coating. Note that the coating is about one thousand times thinner than the carrier on the reflective side. As a result, a compact disc is more vulnerable to damage on the label side. Although a scratch here may not interfere with the reading of the disc, it may allow air to come into contact with the aluminize layer and oxidize it, thus ruining the disc.
The construction of multilayer DVDs is similar, except the polycarbonate carrier is only half as thick. Double-sided and double-layer discs cement two half-thickness discs together. DVD-5 discs (single-sided, single-layer) cement a dummy half-thickness disc to the back of the one active layer. In any case, in the center of the disk, the protective plastic layer is replaced with a transparent plastic cement, which seals the aluminize layer on the lower half of the disc and binds the upper half to it.
Compared to vinyl phonograph records or magnetic discs, CDs and DVDs offer a storage medium that is long-lived and immune to most abuse. The protective clear plastic layer resists physical tortures (in fact, Compact Discs and one-sided DVDs are more vulnerable to scratches on their label side than the side that is scanned with the playback laser). The data pits are sealed within layers of the disc itself and are never touched by anything other than a light beam. These discs never wear out and acquire errors only when you abuse them purposely or carelessly (for example, by scratching them against one another when not storing them in their plastic jewel boxes). Although error correction prevents errors from showing up in the data, a bad scratch can prevent a disc from being read at all. The smaller features used by the DVD system for storing data are more vulnerable (one scratch can wipe out more stored data), but the DVD system has a more robust error-correction system that more than compensates.
Discs used in CD recorders differ in two ways from those used by conventional CD players—besides being blank when they leave the factory. CD-R discs require a recordable surface, something that the laser in the CD recorder can alter to write data. This surface takes the form of an extra layer of dye on the CD-R disc. Recordable CDs also have a formatting spiral permanently stamped into each disc.
As with other CDs, a recordable disc has a protective bottom layer or carrier of clear polycarbonate plastic that gives the disc its strength. A thin reflective layer is plated on the polycarbonate to deflect the CD beam back so that it can be detected by the drive. Between this reflective layer and the normal protective top lacquer layer of the disc, a CD-R disc has a special dye layer. The dye is photoreactive and changes its reflectivity in response to the high-power mode of the CD recorder's laser. Figure 18.1 shows a cross-section of a typical CD-R disc.
The CD-R medium records information by burning the dye layer in the disc. By increasing the power of the laser in the drive from 4 to 11 milliwatts, its beam heats the dye layer to about 250 degrees (Celsius). At this temperature, the dye-layer melts and the carrier expands to take its place, creating a nonreflective pit within the disc.
Colored Substrate CD-Rs, sometimes simply called Color CD-Rs, dye the polycarbonate substrate a pleasing tint. The most popular colors include black, blue, orange, purple, and red. The dye has absolutely no effect on the digital code that's written to the disc, so it does not affect the data or music stored on a CD.
The color of the substrate should not be confused with the color of the recording medium. Three different compounds are commonly used for photoreactive dyes used by CD-R discs, and each of these dyes has a characteristic color. These include green, gold, and blue.
Some manufacturers use multiple layers of dyes on their discs, sometimes even using two different dyes. The multiple-layer CD-R discs are often described as green-green, gold-gold, or green-gold, depending on the colors of the various layers.
Additionally, the reflective layers of recordable CDs also vary in color. They may be silver or gold, which subtly alters the appearance of the dye. The basic reflective layer looks silver because it is made from aluminum. Gold discs have actual gold added to the mix to increase the lifespan of the medium, which also makes the discs look better—golden.
With current CD drives, there is no functional difference between the different CD-R colors, be they a result of tinting the substrate, the dye, or the reflective layer—all appear the same hue to the monochromatic laser of a CD drive that glows at a wavelength of 780 nanometers. All the current CD-R materials reliably yield approximately the same degree of detectable optical change. In the past, however, early CD-ROM readers had varying sensitivities to the materials used in CD-R discs and would reliably read one color but not another. Consequently, lore about one color or kind of disc being better overall than the others arose. The major differences in discs result from mechanical tolerances (how well the manufacturer maintains the perfect spiral of the track) than they do from the color of the disc. Poorly performing discs are simply badly made, regardless of color.
No matter the dye used, recordable CD media are not as durable as commercially stamped CDs. They require a greater degree of care. They are photosensitive, so you should not expose them to direct sunlight or other strong light sources. The risk of damage increases with exposure. The label side of recordable CDs is often protected only by a thin lacquer coating. This coating is susceptible to damage from solvents such as acetone (finger nail polish remover) and alcohol. Many felt-tip markers use such solvents for their inks, so you should never use them for marking on recordable CDs. The primary culprits are so-called permanent markers, which you can usually identify by the strong aroma of their solvents. Most fine-point pen-style markers use aqueous inks, which are generally safe on CD surfaces. Do not use ballpoint pens, fountain pens, pencils, or other sharp-tipped markers on recordable CDs because they may scratch through the lacquer surface and damage the data medium.
The safest means of labeling a recordable CD is using a label specifically made for the recordable CD medium. Using other labels is not recommended because they may contain solvents that will attack the lacquer surface of the CD. Larger labels may also unbalance the disc and make reading it difficult for some CD players. In any case, once you put a label on a recordable CD, do not attempt to remove it. Peeling off the label likely will tear off the protective lacquer and damage the data medium.
The CD-RW system is based on phase-change media. That is, the reflective layer in the disc is made from a material that changes in reflectivity depending on whether it is in an amorphous or crystalline state. The most common medium is an alloy of antimony, indium, silver, and tellurium, which has an overall silver color.
In its crystalline state, the medium has a reflectivity of about 15 to 25 percent. In its amorphous state, the reflectivity falls a few percent—enough to be reliably detected by the laser-based disc-reading system.
A blank disc has all its reflective medium in its crystalline state. To record data, the drive increases laser power to between 8 and 15 milliwatts and heats the medium to above its 500 to 700-degree (Celsius) melting point. The operating is straightforward and equivalent to the CD-R writing process, except for laser power.
CD-RW discs typically use a fine metal alloy for their phase-change medium, coating it on the substrate in 8 to 12 layers. Some sources state that the phase-change media are more stable than the one-use CD-R media and therefore can preserve your data for decades longer.
Erasing the CD-RW disc complicates the recording process. To completely erase a disc and restore it to its original crystalline state, the disc must be annealed. The reflective layer is heated to about 200 degrees Celsius and held at that temperature while the material recrystallizes. The first CD-RW systems required that you entirely erase a disc to reuse it, a time-consuming process. Although the heating and recrystallization took only a tiny fraction of a second for each spot of data, erasing an entire disc required about 37 minutes. Most modern drives use on-the-fly erasing, selectively annealing small areas of the disc with the laser at moderate power as they need to be reused. The annealed areas may be rewritten with higher laser power.
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