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Tracking and Focusing
SUMMARYCD-ROM  STORAGE

Fire Controlman Volume 03-Digital Data Systems
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is off to the left, off to the right, or on track. The output of the photodiode is fed to a differential amplifier. If the  laser  is  on  track,  the  output  voltage  of  the photodiodes is equal. If the laser beam is off to the left or right, a control voltage will be generated by the differential amplifier that is fed to the track following servo.  The  amplitude  and  polarity  of  this  control voltage  determines  the  direction  and  distance  of correction needed. The  same  four  photodiodes  are  also  used  to determine the focus of the laser beam as it strikes the disc.  Figure  11-4  shows  how  the  photodiodes  will  react to detect if the disc is too close to the laser or too far away from the laser, or if the laser is in focus. Again, the output of the photodiodes is fed to an amplifier and correction of focus is made by moving the objective lens. The  output  of  these  four  photodiodes  is  also summed and contains the encoded data on the disc. It is then sent to the control section for decoding. CD  CONTROLLER The CD controller processes the signals received from the optical head, attempts to correct any errors in the data, and controls the speed of the turntable. The information from the photodiodes that is received by the controller is still encoded in eight-to-fourteen modulation (EFM) data. The  decoding  of  EFM  data  is  done  by  the microprocessor. The code addresses a ROM that contains the proper byte for the encoded data. The output of the ROM is stored in a RAM where it is checked for errors. TURNTABLE The turntable rotates the disc and is driven by a servomotor. Since the data is written in a continuous spiral, the speed of the turntable must be adjustable so that the information passes over the optical head at a constant speed. The audio CD requires a speed of 1.3 meters per second. This speed was adapted for use in computer  applications,  but  proved  to  be  extremely  slow when compared to the processing and data transfer speeds of modem computers. The 2X CD-ROM drive doubled the speed the data track passed over the optical head. The 4X, 6X, and 8X CD-ROM drives spin the disc even faster. The speed multiplication factor is based on the original speed of 1.3 meters per second. Initial speed adjustments are made when the optical head is positioned in the approximate area of the data. The header of each sector contains a synchronization pulse that is fed into a sawtooth wave generator. The sawtooth wave is fed to the turntable servomotor. The frequency of the wave is used to make fine adjustments to the turntable speed. INTERFACE SECTION The interface section provides for the transfer of data between the computer and the CD-ROM drive. Many CD-ROM drives are manufactured with the small computer systems interface (SCSI), although some proprietary interface units are available. TOPIC 3—CD-ROM APPLICATIONS Applications   that   use   CD-ROM   are   rapidly expanding  throughout  the  Navy  as  systems  are  updated and the need for reliable storage of large amounts of information increases. DATABASES AND PUBLICATIONS CD-ROMs  are  used  in  command  and  control systems, intelligence systems, and the supply system. These applications use large databases. Databases, such  as  a  part  number  cross-reference  list,  can significantly reduce the amount of paper storage space required. The CD-ROMs allow information to be quickly retrieved, cross-referenced, and displayed to the user. Many publications and instructions are also being stored on CD-ROM in an effort to reduce printing and mailing costs. As publications are updated, anew disc is made and sent to all users, who then replace the old disc. MULTIMEDIA (CD-I) APPLICATIONS Multimedia or compact disc-interactive (CD-I) applications   combine   machine   executable   code (programs), text, audio, video and graphics all on the same CD-ROM. The microprocessor in the CD-ROM drive reads the code at the beginning of each sector to determine if the information that follows is audio, video, graphics, etc. The data is then output on the appropriate channel of the CD-I drive. 11-6






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