CDs and DVDs: Operation and Variety
CD is short for compact disc. DVD initially stood for digital video disc, then digital versatile disc, but today the term DVD is often used without referring to a specific set of words. Both CDs and DVDs are optical media, meaning media that use light technology (more specifically,laser light) for data retrieval. A disc drive focuses a laser light beam into the CD or DVD to “read” the bits (data) in the disc. The drive can also “write” bits by focusing the laser beam into recordable CDs or DVDs. The laser reads and writes data starting from the center of the disc and proceeding in a spiral direction toward the outer edge. A pre-groove is stamped in all blank recordable and rewritable CDs and DVDs to guide the laser as it writes.Optical discs are differentially identified to designate specific features such as recordability, rewritability, and accessibility. For example, CD-R, DVD-R, and DVD+R discs are dye-based recordable (write-once) discs—i.e., recordable but not erasable. CD-RW, DVD-RW, and DVD+RW discs are phase-change based, recordable, (rewritable) discs, or discs that permit the erasing of earlier information and the recording of new material in the same location on the disc. DVD-RAM discs are phase-change based, recordable (rewritable) discs formatted for random access,much like a computer hard drive. CD-ROM and DVD-ROM discs are pressed and molded,nonrecordable,read-only discs.
Structure
CDs and DVDs consist of the same basic materials and layers but are manufactured differently. A DVD is actually like two thin CDs glued together. A CD is read from and written to (by laser) on one side only; a DVD can be read from or written to on one or both sides, depending on how the disc was manufactured. Recordable DVDs (DVD-R, DVD-RW, DVD-RAM) can be manufactured with one recording
layer on each side. Prerecorded DVDs (DVD-ROM) can be manufactured with one or two recorded layers on each side.
Polycarbonate (Plastic) Substrate Layer
The polycarbonate substrate makes up most of the disc, including the area that is read by the laser (opposite the label side on CDs). It is present on both sides of a DVD, even a “single-sided” disc with a label on one side. This substrate provides the disc depth necessary to maintain laser focus on the metal and data layers. It also gives the disc enough strength to remain flat. Anything in or on the polycarbonate
layer that interferes with the ability of the laser to focus on the data layer will result in the misreading of data. Accordingly, fingerprints,smudges,or scratches,as well as such substances as dirt, dust, solvents, and excessive moisture (which polycarbonate will absorb), can interfere with the ability of the laser to read the data. Contact of any foreign material with the polycarbonate substrate layer should be avoided.
The dye-based (R discs) and the phase-changing film layers (RW discs) both hold data by allowing or blocking light transfer through the data layer.The laser-affected “written”) areas of the data layer absorb the “reading” laser beam as it is emitted from the laser to the metal layer and reflected back to the laser photosensor.The light and dark areas give reflectivity effects that are similar to the interference
effect of the “pressed” and molded data in the metal/substrate layer in ROM discs. The reflection, whether the result of dye, film, or pressed effects,is represented digitally as ones and zeros by the firmware in the disc drive as the laser reads the disc.
Data Layer in ROM Discs
ROM discs are commercially available or made-to-order prerecorded discs, also called “replicated” discs. Examples of CD-ROMs include the Audio-CD, Video-CD, CD-i, and CD+G, as well as any number of CDs used in computer applications. Among DVD-ROMs are the DVD-Video, DVD-Audio, and any of various DVDs used in games and computer applications.The data in CD-ROM or DVD-ROM discs are not actually in a separate layer. A molding machine uses a stamper to impress the pits (depressions) and lands (surface), which form the data, into the polycarbonate substrate surface. Metal is then sputtered or condensed onto the molded substrate to form a “reflective data layer.” The reflective metal layer in ROM discs can also be considered the data layer because the metal is integrated with the pits and lands in the polycarbonate.The metal layer in ROM discs is usually aluminum. For double-sided DVD-ROM discs, the semi-reflective layer is gold, silver alloy, or silicon.
Data Layer in R Discs
The recordable, write-once optical disc (CD-R, DVD-R, DVD+R) has its data-recording layer sandwiched between the polycarbonate strate and the metal layer. This layer is an organic dye. The dyes used in CDs and DVDs are the same basic types; those used in DVDs, however, are patented by the manufacturer, and the disc color does not indicate the type of dye used. The dyes in both CDs and DVDs are photosensitive. Bits (marks) are written to the dye by a chemical change caused by the laser light beam. This
dye degrades over time, eventually making the disc unreadable.The data layer in CD-R discs consists of one of three basic dye types, each yielding a different disc color depending on the type of dye and the type of reflective metal used in the disc. Even on a plain, unlabelled disc, the label side can be a different color from the reading side. If the label side of a recordable disc does not have a printable surface, a label attached, or some other protective layer, it will have the color of the metal used (silver or gold).
Data Layer in RW and RAM discs
The data-recording layer of the rewritable optical disc (CD-RW, DVD-RW, DVD+RW, DVD-RAM) also lies between the polycarbonDye ate substrate and the metal reflective layer. This is a phase-changing metal alloy film. A laser beam writes bits (marks) to the film by heating the film above the melting temperature in the areas selected for bits. The rapid cooling enabled by the dielectric layers on both sides of the phase-changing film causes these bit or mark areas to remain in the amorphous state caused by melting. By heating the phase-changing film to a specific temperature above the crystalline temperature but below the melting temperature, the film can revert back to the crystalline state, thereby erasing previous bits. The writing and erasing processes can be done together in a single pass when rewriting a disc.
Metal (Reflective) Layer
The metal layer in optical discs reflects the laser beam back to the laser
photosensor in the laser head. Three types of reflective metals are typically used for this layer: aluminum, gold, and silver or silver alloy.In “double-layer” DVDs, silicon is sometimes used as one of the semireflective layers.
Metal Layer in RW, ROM, and RAM Discs
RW, ROM, and RAM discs (CD-RW, CD-ROM, DVD-RW, DVD+RW, DVD-ROM, DVD-RAM) use aluminum for the reflective layer, mainly because it is inexpensive and easy to apply. Aluminum oxidizes on exposure to oxygen from the environment or to moisture that has penetrated the disc. In some earlier CDs, poor sealing allowed oxygen to come into contact with the aluminum metal layer, causing the aluminum to oxidize. Oxidation of the aluminum diminishes its reflectivity, making the disc unreadable by the laser, and is sometimes referred to as disc “rot.” It is the primary cause of ROM disc degradation from environmental influences. Not so, however, for RW and RAM disc degradation; the phase-changing film in these discs normally degrades at a faster rate than the aluminum in the disc oxidizes.
Metal Layer in R Discs
In R discs (CD-R, DVD-R, DVD+R), gold, silver, or a silver alloy is used for the reflective layer. Silver is slightly more reflective than gold but can lose reflectivity with corrosion on exposure to adverse environmental conditions. Silver corrodes through reaction with sulfur dioxide, an environmental pollutant that can migrate through the disc with moisture. Gold is noncorrosive, very stable, and longer lasting, but it is also expensive. Either metal should outlast the dye. Aluminum is not used with these discs because it can react with the dye in the recording (data) layer.
Metal Layers in Double-Layer DVD-ROM Discs
DVD-ROMs can be manufactured with two reflective metal layers that allow the laser to read data from both layers using one side of the disc. These “double layered” DVDs provide up to four times the capacity for content (video, audio, computer applications) as do “single-layered” DVDs. The laser beam must pass through a semi-reflective metal layer to read data from a fully reflective layer. The outer metal layer (silicon, gold, or silver alloy) is semi-reflective; that is, it reflects back some of the laser beam and allows some of it to pass through to a fully reflective layer (aluminum) and then reflect back. Both parts are thus reflected to, and detected by, the photosensor in the laser head, which focuses on one layer at a time.
These are a few points I had made note of from a few good ebooks.Not much detailed..but I couln't find more time to refine this..
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