The Read Head
The part of the head assembly that moves across the CD-ROM is thehead actuator, also called
theread head, which consists of the read lens and a mirror. The technology used to move a
CD-ROM drive’s read head is much like that used for floppy disk and hard disk drives.
The read head is guided over the disc on a set of rails that, at one end, positions the
head on the outermost edge of the disc and, on the other end, stops it near the CD’s hub
ring. The mechanism used to control the positioning of the CD read head over the disc is
an integrated microcontroller and servo system (small motors used to move the head).Constant Linear Velocity (CLV) and Constant Angular Velocity (CAV)
The CD-ROM drive uses a spindle motor to rotate the disc so that it can be read. Unlike
thespinningmediainaharddiskorfloppydiskdrive,adiscinaCD-ROMdrivedoesnot
spin at a constant speed. The speed at which the disc spins varies depending on the part
of the disc that is being read.
On a hard disk drive, the disk spins at the same speed regardless of where the
read/write heads may be. Using a constant spin speed for the media is calledconstant
angular velocity (CAV)because every spin of the media takes the same amount of time at
all times. On the hard disk and floppy disk, the disk’s inside tracks are much shorter than
those on the outside of the disk. When the disk’s heads are on the outside tracks, they(^210) PC Hardware: A Beginner’s Guide
Figure 10-5. The head assembly, including the read head, of a CD-ROM drive