CertiGuide to A+ (Core Hardware)  9  Chapter 0110:  Removable Media

Do “The Twist” 1 The X-Factor: CD-ROM Performance

Due to their small capacity, poor performance, and generally being a pain in the butt, the industry needed a removable storage option that could eventually replace floppy disks. It seemed the best option available was to take the existing technology that had been developed by Philips and Sony, called the Compact Disk (CD), and enhance it by allowing it to store computer data and be accessed by a PC. Few changes were needed, like some additional error correction code technology, and the Compact Disk-Read Only Media or CD-ROM was born.

The physical makeup of the CD-ROM is similar to that of a hard disk drive. There's a spindle motor that spins the CD, and a 'read head' that reads the data off of the disk. The reason 'read head' is in quotes, is that it is quite different technology than what is used in a hard disk drive. Instead of using an electromagnetic method of reading and writing data to the device, a CD-ROM drive uses an optical read head. This optical head assembly is made up of an infrared laser, a mirror, and a focusing lens. A typical read from a CD-ROM drive happens like this (see Figure 72):

• The infrared laser fires of beam of light on to the reflecting mirror. The mirror is part of the head assembly and moves with the laser and focusing lens from the inside to the outside of the CD.
  
  
• The mirror reflects light through the focusing lens and on to a point on the CD.
  
  
• Some portion of the light is reflected back from the disk. Depending on the amount of light reflected back defines whether it is read as a 1 or 0. Data is encoded onto the CD using a series of 'pits' and 'lands.' A pit is read as a 1 and a land is read as a 0.
  
  
• A series of mirrors and lenses focuses the reflected light into a photo-detector.
  
  
• The photo-detector converts the light into electrical energy and sends it to the controller as binary data.
  

  Figure 72: Internal components of a CD-ROM

  
  

This method has significant advantages over both floppy disks and hard drives for data access. Because the head itself does not come close to the surface of the disk, dirt build-up on the head assembly will not damage the CD. And, because the head is not lying fractions of a millimeter from the CD, there's no chance of a head crash. However any defects, scratches, or fingerprints on the surface of the CD can cause misreading of the data.

CD-ROMs also share some other interesting attributes with hard drives. Like a hard drive, a CD has tracks. However they are not laid out in concentric circles, but sequentially in a spiral that begins at the center of the disk and moves out to the edge of the disk, like a vinyl record.

Do “The Twist” 1 The X-Factor: CD-ROM Performance