By: John Michael Pierobon
Compact discs are everywhere. Vinyl records have been replaced by compact discs. Automobiles come with CD players. Photographs are now availabe on Picture CD. Most software is distributed on compact disc, and my nephew's favorite toy these days is a compact disc that simulates NASCAR races.
Whether it is music, pictures, software, or games, how compact discs record and play back is virtually the same. Here is how it works.
A compact disc is truly compact. It can play up to 74 minutes of stereo music or store about 650 Mbytes of data, yet it is just 1.2 millimeters thick. The bottom layer consists of an injection-molded clear polycarbonate plastic. During manufacturing, the plastic is stamped with microscopic bumps (or grooves) arranged as a very long, continuous, spiral track. Once the polycarbonate is formed, a thin, reflective layer of aluminum is applied onto the disc, to cover the bumps. On top of that goes a thin layer of acrylic to protect the aluminum. The label is then printed on the acrylic.
The spiral track is extremely compact, for it is just half a micron wide, and only 1.6 microns separates one track from the next. The bumps are only 125 nanometers high. (A micron is a millionth of a meter, and a nanometer is a billionth of a meter.)
Laser technology is used to read the bumps. A laser shines light on the bumps and a lens reads the bumps. Light reflects differently on the bumps, so light signals get read in a manner similar to Morse code, which in turn is converted into music or in another format.
A CD player has a very precise tracking mechanism which keeps the laser and lens focused on the very narrow track. This tracking mechanism moves the laser assembly linearly, but data is stored in one long spiral that starts at the center of the compact disc and moves out. As the CD player reads the data, the drive motor must precisely vary the rotation speed of the CD so the data can be read at a constant rate.
The speed at which these light signals are read is called a sampling frequency. For audio it is 44,100 Hz. A CD reader is extremely fast and accurate.
If the laser system misinterprets a bump, it can cause a computer program not to function properly. To avoid this problem, data is interleaved and recorded with error correcting codes built in.
The two most common recording formats are CD-DA for digital audio, and CD-ROM for computer data. Regardless of the format, compact discs make high technology fun.