The first compact disc was developed by Philips and Sony and introduced as CD-Digital Audio (CD-DA) in 1982. Since then the compact disc has come a long way. It dominates the market and has helped the music and gaming industry prosper to heights not even dreamed of 20 years ago. It is the preferred mode of data transfer for these two key industries.
It has an ultimately very basic structure to it. Just plain bumps and pits etched onto the surface of the disc.
Through this simple design a very complex reading system has developed. The
size of a standard CD is a mere 12 centimeters or 4.75 inches in diameter. It
has a hole in the middle, which is conveniently sized for a person's finger to
go through and handle the CD. As big as it is in diameter, a disc would not be
called a disc if it were not flat. The CD is only 1.2 mm thick.
It is composed of a polycarbonate plastic substrate (main body), one or more thin reflective metal layers (usually aluminum), and a lacquer coating. There are 3 distinct areas on a CD, a lead-in area, which contains the table of contents, a program area, which contains the audio data, and a lead-out area, which contains nothing at all.
There are 2 different types of CDs, which hold different amounts of information. One type can hold 650 megabytes (74 minutes of audio) and is very common, and the other can hold 700 megabytes (80 minutes) of data. The only significant difference is the data storage capacity. Data on the CD is organized into sectors (the smallest possible separately addressable block) of information and the sectors are further broken down into logical blocks (smaller segments within a sector that can only be accessed by the logical block number and are identified by the header bytes that hold the address information. There are a variety of logical block sizes, 512, 1024, and 2048, which might be used on different CDs, but the logical block size cannot exceed the sector size). The information is stored in frames of 1/75-second length.