PC Hardware A Beginner’s Guide

the further into the disk area, that is, the closer to the inside of the platter, the heads are

tilted slightly, which creates an azimuth problem.Azimuthmeasures the alignment of the

heads to the disk and cylinders. This is overcome on most systems by limiting how much

of the disk near the center can be used for data.

Servo systems enable the head to be positioned precisely above a specific track on a

disk, using information calledgraycode,which was written to the disk when it was manu-

factured. Gray code is a special binary notation code that identifies each track (cylinder)

and, in some systems, each sector on the disk. Through the gray code, the head positioning

system on the disk drive has the ability to place the heads directly over the cylinder it

desires. The gray code written to the disk during manufacturing cannot be overwritten in

normal use, as the area of the physical disk that it occupies is set aside.

Air Filters

It may seem odd that a sealed device like the HDA would have air filters, but it does. In

fact, most drives have two air filters, arecirculating filterand abarometricorbreather filter.

These filters are permanently sealed inside the HDA and never have to be changed. They

are designed to last the life of the drive. PC hard disk drives do not bring outside air into

the HDA and circulate it (see Figure 9-6). The purpose of the recirculating filter is to trap

any particles of media that may be scraped off the disks by the read/write heads or any

small particles that may have been trapped in the HDA during manufacturing. How

Chapter 9: Hard Disks and Floppy Disks^181

Figure 9-6. The recirculating airflow inside the HDA. Original photo courtesy of

Western Digital Corporation

Air flow