Compact Disk (CD) [3]
- Introduction
- CD Drive: basic design
- Compact disk (CD): basic design
- Data encoding
- Making CDs
- Spin rates and Data transfer rates
- Interfaces
- Multi-beam technology
- White Papers on compact disk technology
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CD: Basic design
The figures below illustrate the structure and operating principles of the compact disk. The CD is 12 cm in diameter, 1.2 mm thick, has a center hole 15 mm in diameter, and spins at a constant linear velocity (CLV) or constant angular velocity (CAV). Unlike the hard disk or floppy disk, there is only one track on the optical disk and all data are stored in a spiral of about 2 billion small shallow pits on the surface. There are about 20,000 windings on a CD - all part of the same track. This translates into about 16,000 tracks per inch (TPI) of track density and an areal density of 1 Mb/mm2. The total length of the track on a CD is almost 3 miles (~4.5 km).
Low-magnification (x 32) image of a CD showing an edge of the the data zone.
A transparent polycarbonate (PC) polymeric substrate (layer) has the pits molded onto its surface. These pits are the coded data and carry the information. The areas in between the pits, which are 0.9 mm (microns) to 3.3 mm long, are called "lands". The substrate layer is covered with a thin reflective layer of metal (aluminum) and with a protective layer of lacquer. On top of the CD sits the label layer.
Summarizing, the compact disk consists of:- The label
- The protective layer
- The reflective layer
- The substrate layer.
A laser beam of approximately 780 nm wave length is focused on the data side of the disk into a spot of about 1 micron in diameter. The laser moves in the radial direction over the fast spinning disk and scans the data track for the intensity of the reflected light.
[To see an animated version of this schematic, go to the following page (you will need an SVG plug-in from Adobe.]
The data pits are about 0.12 microns (120 nanometers) deep and about 0.6 microns wide. The distance between the neighboring windings of the track is about 1.6 microns. The laser beam scattering occurs when it scans the pits, which translates into a slight drop in intensity of the reflected beam.
The change in reflected light intensity occurs every time the laser spot moves from the pit onto the land and vice versa. The high-frequency modulated signal produced by these changes in light intensity represents the data stored on the CD.
The reason for the high reliability of the CD is good protection of the data from damage both inside and outside CD drive. Outside, the data layer is protected by tough 1.2 mm thick layer of polycarbonate on one side and 10-20 microns of a protective lacquer layer on the other side. Small scratches on the surface of CD do not directly erase the data, but just create additional areas of light scattering. This can confuse the drive's electronic, which is also much less sensitive to radial scratches than to the circumferential ones. Gentle polishing of the scratch can (in many cases) make the CD readable again. But, this is rarely necessary thanks to the large size of the laser spot on the surface of the PC layer - about 1 mm. This large spot diameter "integrates" the signal over the large area making the system much less sensitive to dirt and scratches on the disk surface.
Inside the drive, the disk and the drive's optics are separated by a distance of about 1 mm, making mechanical interaction and "crashes", even with wavy disks and imperfect clamping almost impossible.