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Compact disc : Compact diskA compact disc or CD is an optical disc originally used to store music, in the form of digital audio, but now also used for data. Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner aluminium layer which is protected by a film of lacquer. The lacquer can be printed with a label. CDs are available in a range of sizes but the most commonly available is 120 mm in diameter. A 120 mm disc can store about 74 minutes of music or about 650 megabytes of data. Discs that can store about 700 megabytes (80 minutes of music) have become more common. There are also less common 90, 99 and 100 minute discs, but they are not compatible with all CD writers or readers. The format of the disk, known as the 'Red Book' standard, was laid out by the Dutch electronics company Philips, who own the rights to the licensing of the 'CD' logo that appears on the disk. In broad terms the format is a two channel (left and right, for stereo) 16-bit PCM encoding at 44.1 kHz. Reed-Solomon error correction allows the CD to be scratched without degradation of the contents.The information on a standard CD is encoded as a spiral track of "pits" moulded into the top of the polycarbonate layer. Each pit is approximately 125 nm deep by 500 nm wide, and varies from 850 nm to 3.5 μm long. The spacing between the tracks is 1.5 μm. A CD is read by shining light from a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer, and monitoring the light reflected by the aluminium coating. The light from the laser forms a spot of approximately 1.7 μm diameter on the metal surface. Since the CD is read through the bottom of the disc, each "pit" appears as an elevated "bump" to the reading light beam. The areas without bumps are known as "land". Light striking the "land" areas is reflected normally and detected by a photodiode. Light striking a bump, however, undergoes destructive interference with light reflecting from the land surrounding the bump and no light is reflected. This occurs because the height of each bump is one quarter of the wavelength of the laser light (in the polycarbonate medium), leading to a half-wavelength phase difference in light reflecting from the land to that of light reflecting from the bump.
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Attempts are currently (early 2002) being made by record companies to market so called 'copy-protected' compact disks. These rely on deliberate errors being introduced into the data recorded on the disk. The intent is that the error-correction in a music player will enable music to be played as normal, while computer CD-ROM drives will fail with errors (though not all current drives fail, and copying software is being adapted to cope with these damaged data tracks.) Philips have stated that such disks, which are written with deliberately degraded data on them which fails the Red Book specification, will not be permitted to have the CD logo on their packaging; and it seems likely that Philips' new models of CD recorders will be designed to be able to record from these 'protected' disks.
Compact discs cannot be easily recorded, as they are manufactured by etching a glass plate and using that plate to press metal. However there are also CD-R discs, which can be recorded by a laser beam using a CD-R writer (most often on a computer, though standalone units are also available), and can be played on (most) compact disc players. The recording on a CD-R is permanent - the medium cannot record more than once - so the process is also called "burning" a CD. CD-RW on the other hand is a medium that allows multiple recordings on the same disc over and over again. Many CD-Audio players cannot read CD-RW discs, but more standalone DVD players can read CD-RW than only CD-R discs. These issues are moot, however, when referring to drives installed in computers, as all but the oldest CD-ROM and DVD-ROM drives can read (if not write) CD-R and CD-RWs.
The term EP is used for both a CD and a vinyl record of intermediate play-time.
time of the revolution now mentioned, will best ascertain the foundation
change in the political form of the state.html">state, they did not carry their
institutions of Numa Pompilius, the second of their kings, according to
placed in the hands of the aristocracy. By this wise policy a restraint
government, and a decided superiority given to the Senate both in the
afterwards indeed diminished by the creation of Tribunes of the people.html">people; a
dissensions, and who at last abused their authority to such a degree,
state who could purchase their friendship. In general, however, the
interests of the multitude, rather than those of individuals, they did
of the public; and when the occasional commotions subsided, there
usurpation.
In every government, an object of the last importance to the peace and
community is enlarged by propagation, or the accession of a multitude of
dissolution of manners to which a crowded and extensive capital has a
state of the Republic. In the year of the City 312, two magistrates were
value of their estates; and soon after, they were invested with the
inflicting public censure for any licentiousness of conduct, or violation
restrain the people within the bounds of good order and obedience to the
a strong security against those vices which operate most effectually
become too weak to restrain the audacious efforts of ambitious and
check to treasonable designs, had been lately violated by the usurpations
predominated over the consciences of men. The shame of public censure
at that time, informs us, that venality universally prevailed amongst the
.
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