CD-ROM Drvie

disc, the conversion from coded information back to a byte is accomplished from a look-up table. This table is in a ROM on the disc drive. Three additional bits are added to each 14-bit code to provide separation and low-frequency suppression. TOPIC 2—CD-ROM DRIVES Although still relatively new, CD-ROM drives are becoming popular as a tertiary storage media device. CD-ROM drives vary by manufacturers in the method the data is read from the disk and the laser system used, but the basic operation is similar. In this section, we cover  the  common  components  and  operation  of CD-ROM drives. The  basic  components  of  the CD-ROM drive are the following: l l l l Optical head Turntable Computer interface section Microprocessor based control system.

 

Figure 11-3 shows a basic block diagram of a CD-ROM drive. OPTICAL HEAD The optical head contains the circuitry to read the data from the disc. This unit usually consists of four main subassemblies; (1) the laser, used to generate a light beam; (2) a lens system, to focus the laser beam on the disc and to direct the reflected light to the photodetector; (3) a series of servomotors that controls the position of the laser and lenses to ensure proper tracking  and  focus;  and  (4)  a  photodetector,  that evaluates the reflected light and converts the light to electrical   impulses. Laser The laser in a CD-ROM drive is generally a small injection  laser  diode  that  emits  light  in  the  infrared band. An injection laser is energized by injecting it with an  electric  current  across  a  semiconductor  junction. Injection laser diodes are the smallest laser light source. They are highly efficient and mass produced.