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The optical storage device that most of us are familiar with is the compact disc (CD). A CD can store huge amounts of digital information (783 MB) on a very small surface. The design that makes this possible is a simple one. The CD surface is a mirror covered with billions of tiny bumps that are arranged in a long, tightly wound spiral. The CD player reads the bumps with a precise laser and interprets the information as bits of data.
The spiral of bumps on a CD starts in the center. CD tracks are so small that they have to be measured in microns(millionths of a meter). Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about 1.2 millimeters thick. During manufacturing, this plastic is impressed with the microscopic bumps that make up the long, spiral track. A thin, reflective aluminum layer is then coated on the top of the disc, covering the bumps. The tricky part of CD technology is reading all the tiny bumps correctly, in the right order and at the right speed. To do all of this, the CD player has to be exceptionally precise when it focuses the laser on the track of bumps.
When you play a CD, the laser beam passes through the CD's polycarbonate layer, reflects off the aluminum layer and hits an optoelectronic device that detects changes in light. The bumps reflect light differently than the flat parts of the aluminum layer, which are called lands.The optoelectronic sensor detects these changes in reflectivity, and the electronics in the CD-player drive interpret the changes as data bits.
CD-R works by replacing the aluminum layer in a normal CD with an organic dye compound. This compound is normally reflective, but when the laser focuses on a spot and heats it to a certain temperature, it " burns" the dye, causing it to darken. When you want to retrieve the data you wrote to the CD-R, the laser moves back over the disc and thinks that each burnt spot is a bump. The problem with this approach is that you can only write data to a CD-R once. After the dye has been burned in a spot, it cannot be changed back.
CD-RW fixes this problem by using phase change, which relies on a very special mixture of antimony, indium, silver and tellurium. This particular compound has an amazing property. When heated to one temperature, it crystallizes as it cools and becomes very reflective; when heated to another, higher temperature, the compound does not crystallize when it cools and so becomes dull in appearance.
CD-RW drives have three laser settings to make use of this property:
• Read- The normal setting that reflects light to the optoelectronic sensor
• Erase- The laser set to the temperature needed to crystallize the compound
• Write- The laser set to the temperature needed to de-crystallize the compound
Other optical devices that deviate from the CD standard, such as DVD, employ approaches comparable to CD-R and CD-RW.
DVD, the Digital Versatile Disc, is a high capacity CD-size disc for video, multimedia, games and audio applications. DVD started in 1994 as two competing formats, Super Disc (SD) and Multimedia CD (MMCD). Capacities for the read-only disc range from 4.7GB to 17.1GB. The high quality of video and audio has helped DVD-Video to compete very effectively with VHS.
DVD-Video, which was launched in 1997 in the USA, has become the most successful of all the DVD formats, as it has proved to be an ideal vehicle for distributing video content from the movie industry. It can store a full-length movie in high quality video with surround sound audio on a disc the same size as a CD.
DVD-ROMis beginning to replace the CD-ROM and provide a new high capacity disc format for the computer industry. New PCs are now provided with DVD drives instead of CD drives. The entertainment industry has developed new games consoles (e.g. Sony's PS2 and Microsoft's X-Box) which incorporate DVD-ROM drives for more sophisticated and realistic games applications.
DVD-Audio, which was launched in 2000, is slowly gathering momentum to become the format for very high quality, surround sound music, offering the music industry new revenue opportunities.
Recordable formats such as DVD-RAM, DVD-RWand DVD-Rare now being extensively used in PCs for computer backup and short runs of DVDs and in standalone products such as video recorders and camcorders.The main features of the DVD formats are:
• Backwards compatibility with current CD media. All DVD hardware will play audio CDs and CD-ROMs and most will play CD-Rs and CD-RWs.
• Physical dimensions are identical to the compact disc but each disc comprises two 0.6 mm thick substrates, bonded together.
• Single-layer/dual-layer and single/double sided options are available.
• Up to 4.7 GB read-only capacity per layer, 8.5 GB per side maximum.
• Designed from the outset for video, audio and multimedia, not just audio.
• All formats use a common file system (UDF).
• Digital and analogue copy protection for DVD-Video and DVD-Audio built into the standard.
• Recordable and re-writable versions are part of the family.
Although identical in appearance, DVDs and CDs differ in a number of key physical parameters. To meet the requirements for 133 minutes of high quality video on one side of a single disc requires the use of a thinner (0.6 mm) substrate, two of which are bonded together to form a disc that is thick enough for general use. The use of a sandwich of two substrates allows a range of formats from one layer to four and one or two sides, giving capacities from 4.7 GB to as much as 17.1 GB.
All types of DVD discs (DVD-Video, DVD-ROM and DVD-Audio) contain data in files. These files are accessed using a file system common to all DVD discs. For compatibility with recordable and re-writable versions and with existing operating systems, including Windows9x and later the UDF Bridge Format was chosen.
DVD-ROM is essentially the pre-recorded DVD physical and logical format used for DVD-Video, DVD-Audio and a range of other applications, particularly general computer and multimedia applications, for which it can provide at least 7 times the capacity of a CD-ROM. Applications can include MPEG-2 video, as used on DVD-Video discs, to give added realism to games and richer content for multimedia applications. DVD-ROM drives will also play CD-ROM and CD audio discs and are forecast to replace CD-ROM drives within the next few years. They are already available as add-on drives and built into new PCs.
Copy protection for DVD is only for video and audio content and comprises both digital and analogue methods for preventing users from making perfect copies of the source material.
UNIT 8 “Modems”
1. Read and translate the following international words:
To communicate; to concentrate; to electrify; standard; technique; to manipulate; diagram.
II. Read the text and find English equivalents for the following Russian words and word combinations:
Обрабатывать информацию; двойной язык; связка единиц и нулей; более общий термин; произвольно; в обратном направлении; единственный недостаток.