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Head Actuator Mechanism
Most people don't think of a CD-ROM drive as having a head actuator, in the sense that a hard disk or floppy disk drive does. In fact, however, the lens assembly does move across the CD-ROM media in a similar way to how the heads on a hard disk or floppy disk drive do.
As described in the section on the read head, only part of the whole mechanism used to read the CD-ROM actually moves. This is the lens and mirror assembly that focuses the laser energy onto the surface of the disk. The technology used to move the read head on a CD-ROM drive is in some ways a combination of those used for floppy disk drives and for hard disk drives.
Mechanically, the head moves in and out on a set of rails, much as the head of a floppy disk drive does. At one end of its travel the head is positioned on the outermost edge of the disk, and on the other end it is near the hub of the CD. However, due to the dense way the information is recorded on the CD, CD-ROM drives cannot use the simple stepper motor positioning of a floppy disk. CD-ROM media actually use a tighter density of tracks than even hard disks do!
Instead, the positioning of the head is controlled by an integrated microcontroller and servo system. This is similar to the way the actuator on a hard disk is positioned. This means that the alignment problems found on floppy drives (and much older hard disks) are not generally a concern for CD-ROM drives, and there is some tolerance for a CD that is slightly off center (but not a lot).
Like a floppy disk, the head actuator on a CD-ROM is relatively slow. The amount of time taken to move the heads from the innermost to the outermost tracks--called a full-stroke seek--is about an order of magnitude higher than it is for hard disks. This is discussed in more detail in the section on performance.