For the technically minded...
CD-ROM discs are like the compact discs bought in music stores :12 cm in diameter and 1.2 mm thick. The data is encoded in a shiny, metallic film in the form of tiny pits and lands which can be of different lengths but are of standard depth and width: 0.12 and 0.6 microns. When a laser beam passes over the lands, most of the light is reflected back, but pits scatter the light and only a fraction comes back. In this way, the laser beam reads these miniature pits on a three mile long spiral track and decodes them into binary 'ones' and Zeros. When one subtracts the capacity needed for the internal operation of this technology, about 600 megabytes of usable memory remains. Access to this data is relatively easy compared to more powerful systems because CD-ROM discs can be read in just over one second. Even if it took ten seconds to read an entire encyclopedia, one could hardly complain.
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| Format: | News Item biblioteca |
| Language: | English |
| Published: |
Technical Centre for Agricultural and Rural Cooperation
1987
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| Online Access: | https://hdl.handle.net/10568/44632 http://collections.infocollections.org/ukedu/en/d/Jcta09e/ |
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| Summary: | CD-ROM discs are like the compact discs bought in music stores :12 cm in diameter and 1.2 mm thick. The data is encoded in a shiny, metallic film in the form of tiny pits and lands which can be of different lengths but are of standard depth and width: 0.12 and 0.6 microns. When a laser beam passes over the lands, most of the light is reflected back, but pits scatter the light and only a fraction comes back. In this way, the laser beam reads these miniature pits on a three mile long spiral track and decodes them into binary 'ones' and Zeros.
When one subtracts the capacity needed for the internal operation of this technology, about 600 megabytes of usable memory remains. Access to this data is relatively easy compared to more powerful systems because CD-ROM discs can be read in just over one second. Even if it took ten seconds to read an entire encyclopedia, one could hardly complain. |
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