| The first CD-ROMs (Read Only Memory) were produced in 1986. The name of the CD-ROM pointed to the fact that the new CD-ROM was designed only for reading already stored information. But you cannot erase it or write something new. Capacity of the first CD-ROM was 680 MB - in those years, that amount seemed enormous. The volume of a typical hard
disk drive (HDD) was several times smaller. There were no programs that could completely fill a CD. Today the situation has changed: the capacity of hard drives in modern PCs in the tens of gigabytes, many
types of multimedia applications will not fit on one CD-ROM. However, the popularity of the CD is still great. In its structure,
disc resembles a puff pie. The first layer - the main is made of plastic (polycarbonate), the second - reflecting is made of metal (aluminum, gold, silver), a third - security is made from the polyacrylate. Drawings, descriptions and other
types of decoration are often applied over lacquer. The basic layer contains useful information encoded in applying it microscopic indentations called pit. The reference
disk is made of very pure glass and covered with a plastic film. Powerful recording laser with controlled burns in the film pits that contain coded information. Then produced a metal matrix by means of which the serial stamped CDs. Reflective layer CD-ROM is used for optical readout. The basic principle of reading is that the laser beam is directed on the surface of the CD spinning at high speed. Reading data from a CD by a special drive. The drive can be internal (built-in system unit) and external (as a separate device). A typical actuator consists of an electronics board, the spindle motor of the optical pickup head and the system boot
disk. Engine serves to bring the disk to rotate at a constant or a variable linear speed, as well as for its inhibition. The system of the optical head includes an infrared semiconductor laser focus system of rays, light detector and pre-amplifier and a system of moving the head. Important characteristics of CD-drives are the access time, which indicates how quickly find the information on
disk and a read speed of data after the file is found. The last is usually measured in units equal to the speed reading information from an audio CD (about 150 kilobytes per second). However, if the reliability of the information read is deteriorating, for example due to poor quality of the
disc, the speed is automatically reduced. |