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Thursday, August 09 2018
What is a Compant Disc Anyways!!!!!

The compact disc is a miracle of modern technology. Here are some facts:

  • They are made principally of injection-moulded polycarbonate.
  • The diameter is 120mm.
  • They are 1.2mm thick.
  • They contain up to 680 megabytes of data. This is the equivalent of 250,000 double-sided leaves (500,000 pages) of A4 text (which would be 83 feet high and need 8 trees to make).
  • The music on a CD is imprinted in the form of pits of varying length on a spiral track 3.52 miles (5.66 kilometres) long.
  • There are approximately 16,000,000,000 pits 0.11 micro-metres deep.
  • The largest pit dimension is 3.054 microns; the smallest is 0.833 microns.
  • The width of the pits is half a micron -- which is the distance a human hair grows in two minutes and a fingernail in seven minutes. It is 700 times smaller than a pinprick.
  • The space between tracks is 1.6 micro-metres.

Read by a red-light laser beam, the CD plays from the centre to the edge, rotating at a speed varying from 400 times a minute at the beginning to 250 times a minute at the end. This is equivalent to flying round the earth one inch above the surface, up to 400 times a minute, counting every blade of grass on the way.

Your CD is read by the laser beam and makes over 44,000 arithmetical calculations every second in at least two dimensions. It is adding up columns of numbers ('digits'). But many of the numbers are missing because there are thousands of errors on the average CD. Therefore the numbers are added up laterally as well as vertically, enabling the CD-player to fill in the missing numbers by cross-checking them. This is all quite normal and is called 'error correction'.

When a disc is inserted into a CD player, the disc’s track is scanned by a low-intensity infrared laser with a 1-micrometre-diameter focal point. In order for the laser to maintain a constant scanning rate, the disc’s rotation rate decreases from 500 to 200 revolutions per minute as the light beam spirals out from the disc’s centre. (Some CD players use two additional lasers to help control the disc’s rotation and the scanning laser’s focus.) When the light beam strikes a land, it is reflected back to a photodiode, and an electrical pulse is generated. When the light beam strikes a pit, however, no electrical pulse is generated. This is because light reflected from the pit, which has a depth of approximately one-quarter the wavelength of the scanning infrared beam (0.78 micrometre), is out of phase with light reflected from the adjacent separation track, and thus the reflected light is reduced below the level necessary to activate the photodiode. Each “dark” pit on the track is interpreted (based on its length) as a sequence of 0s in binary logic, and each “bright” land is interpreted (again based on its length) as a sequence of 1s. A device known as a digital-to-analog convertor is necessary to translate—and correct for data misread because of minor surface blemishes on the disc or imperfect laser alignment—this binary information into audio signals for playback (see alsodigital-to-analog conversion). The standard CD will hold more than one hour of music.

The laser scanning method employed in compact disc playersAn infrared laser is focused onto the metallic reflective layer of the disc, where a spiral track of “pits” and “lands” represents the zeros and ones of digital signals. The return signal is converted by a photodiode sensor into a digital electric signal, which is converted to analog form for reproduction of the original recorded sound. Optical recording, introduced by Sony Corporation and Philips Electronics N.V. in 1982, allows accurate reproduction of sound over virtually the entire range of human hearing.E

Posted by: Ron AT 09:45 am   |  Permalink   |  Email
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