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[ The PC Guide | Systems and Components Reference Guide | CD-ROM Drives | CD-ROM Drive Construction and Operation ]

Optical "Head" Assembly

The middle two letters in "CD-ROM" stand for "read only", so it shouldn't be any surprise that standard CD-ROM drives are read only devices, and cannot be written to. (Newer variants of CD-ROMs, CD-R and CD-RW drives, break this long-standing rule of this type of device.) Most people know this anyway because CD-ROMs use the same basic technologies that CD audio players have for many years, and everyone knows that these devices can only play back, not record.

The reason that the word "head" is in quotes is that CD-ROM drives do not use a read head in the conventional sense the way a floppy disk or hard disk does. It isn't just that the head cannot record, it really isn't a single solid head that moves over the surface of CD-ROM media, reading it. The head is a lens--sometimes called a pickup-- that moves from the inside to the outside of the surface of the CD-ROM disk, accessing different parts of the disk as it spins. This is just like how a hard disk or floppy disk head works, but the CD-ROM lens is only one part of an assembly of components that together, read the information off the surface of the disk.

Here's how the CD-ROM works, in a nutshell (I'm not going to go into the gory details of how the laser beams are manipulated within the drive because that can get complicated--there are in fact several slightly different ways that the internals work):

  1. A beam of light energy is emitted from an infrared laser diode and aimed toward a reflecting mirror. The mirror is part of the head assembly, which moves linearly along the surface of the disk.
  2. The light reflects off the mirror and through a focusing lens, and shines onto a specific point on the disk.
  3. A certain amount of light is reflected back from the disk. The amount of light reflected depends on which part of the disk the beam strikes: each position on the disk is encoded as a one or a zero based on the presence or absence of "pits" in the surface of the disk. This is discussed in more detail in the section on CD-ROM media.
  4. A series of collectors, mirrors and lenses accumulates and focuses the reflected light from the surface of the disk and sends it toward a photodetector.
  5. The photodetector transforms the light energy into electrical energy. The strength of the signal is dependent on how much light was reflected from the disk.

Most of these components are fixed in place; only the head assembly containing the mirror and read lens moves. This makes for a relatively simplified design. CD-ROMs are of course single-sided media, and the drive therefore has only one "head" to go with this single data surface. Details on how data is encoded onto and decoded from the CD-ROM disk are included in the section on CD-ROM media.

Since the read head on a CD-ROM is optical, it avoids many of the problems associated with magnetic heads. There is no contact with the media as with floppy disks so there is no wear or dirt buildup problem. There is no intricate close-to-contact flying height as with a hard disk so there is no concern about head crashes and the like. However, since the mechanism uses light, it is important that the path used by the laser beam be unobstructed. Dirt on the media can cause problems for CD-ROMs, and over time dust can also accumulate on the focus lens of the read head, causing errors as well. See this section on media care for more.

Next: Head Actuator Mechanism

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