[carteman]

1. The audio frequency disc
Now the audio carrier most possibly found in the library is the disc with close veins and slow turning speed (such as 12 inches, 33 turns/minute or 7 inches, 45 turns/minute), it is usually inhibited on the PVC material or on the insect gum disc that turns 78 turns per minute.
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2. The magnetic medium
The magnetic tape (video tape, soft dish of computer, etc.) is usually glued of the magnetic layer of chrome or the magnetic layer of the oxidized iron and the glue agent easily changes because of the water resolving and oxidation. The information saved in the tape is as magnetic grain and it is very easy to be lost if the magnetic material is damaged.
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3. optical media
The laser CD was born in 1978 and is usually made of about 30cm glass or plastic disc. More than 10 millions of information spots are engraved in the surface of the CD and they are read through the laser beam projected to the surface. The laser projected to the surface will reflect and the reflecting ray is converted into the traditional imitate signal.
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