Discs.net

Compact Discs (CDs)

At first, compact discs were
meant for audio only...it wasn't
until they had become popular
that the computer industry saw
their potential to store data.

Compact Discs are the first optical discs used to store digital data. They first hit the market in 1982, used purely to store audio (the first album released for compact discs was The Visitors by ABBA)...and quickly became the industry standard for audio playback. Later, as the technology developed, the digital world came to see it as a way to store actual data, as opposed to just sounds. CD-ROMs rolled out in 1985, and recordable compact discs (CDR) were introduced in 1990.

The compact disc (like a record) contains a tracks. The 'track' is a single line, starting from the center of the disc and spiraling out (on a record, it starts at the edge and works its way inward). The track contains a series of pits (small dimples in the surface) and lands (parts that have no dimples in them) put into the aluminum surface of the CD. The pits are about 500nm wide and 100nm deep. A nanometer (nm) is one millionth of a millimeter. The area between each track is about 1600nm.

A red laser (using a wavelength of 780 nm) looks at the tracks, and looks for the changes between pits and lands...much like the way a record needle used to run along the track and sense the varying bumps, which it used to read the sounds. What makes the CD different, is that it doesn't look for different kinds of pits. It simply looks for a change from pit to land. If it sees a change, it counts that piece of data as 'on'. If it doesn't see a change, it counts it as 'off'. The on's and off's (or I's and O's) are then translated into data.

The different types of compact discs include:

CD-ROM
Uses the same technology to read audio CDs, but able to read information to be used by computers. It quickly became the standard way sell data for computers.

Recordable CD (CD-R)
CD-Rs are CDs that a computer or CD recorder is able to write data to. A blank track is imprinted on the disc when it is manufactured. It's coated with a certain type of dye...one that will change colors when a certain type of light shines on it. The CD writer uses a special laser to change the color of the dye in a series, so that normal CD players see the dye color changes the same way they would see the pits and lands of normal CDs. CR-Rs have a shelf life (as the dye begins to deteriorate) of 20-100 years, depending on the quality of the CD-R and storage conditions. Once a CD-R has been burned ('burn' is the common term used to describe the act of creating a CD), it is final. While information can be added to it, the parts that have been burned, are not alterable.

ReWriteable CD (CD-RW)
In a CD-RW, the pits and lands are made of a metallic alloy instead of a dye. CD-RW lasers use heat instead of light (as used by CD-Rs) to write to the CD. So usually, writing to a CD-RW takes longer than writing to a CD-R. CD-RWs also won't last as long, as the metallic alloy deteriorates faster than the dye...especially if used more frequently. CD-RWs can be written on and erased again and again.

Video CD (VCD)
A VCD is supposed to have about the same picture quality as a VHS tape. It can be played in some DVD players, and on most computers. The resolution for a VCD is 352 X 240

Super Video CD (SVCD)
SVCDs are much like the VDCs, but have a higher resolution.

CD +Graphics (CD+G)
Commonly used in karaoke, CD+G's use the same technology as normal audio CDs. It contains the music, along with graphics. CD+G's will generally play in normal CD players...but obviously without the graphics.

CD +Extended Graphics (CD+EG or CD+XG)
Very much the same as CD+Gs, but the graphics data is slightly improved.

Super Audio CD (SACD)
Made to give a much higher fidelity sound, the SACD was never widely published.

CD-MIDI
Made to store MIDI music on CDs, but uses the same technology as audio CDs.

Photo CD
The Photo CD (or Picture CD) was designed by Kodak, and is simply a format for CDs to hold pictures. Some DVD players will play Photo CDs, and all computers will provided they have the right software.

Click on the next arrow to find out how DVDs are different from CDs, or