HOW IT WORKS Thomas Edison's breakthrough in sound recording was creating a mechanism to store an analog wave mechanically. He did this by having a diaphragm control a needle, which then scratched an analog signal onto a tinfoil cylinder. In its simplest form, when you made a sound, the diaphragm vibrated, causing the needle to vibrate and those vibrations were scratched onto the tin. In playback, the needle moves over those scratches or grooves, creating a vibration that causes the diaphragm to vibrate and re-create the sound. The gramophone's major improvement was the use of records with a flat groove, making mass production relatively easy. The next step was to record the music in the form of an analog wave on tape, giving a higher quality of reproduction. When moving to the manufacturing process, that master tape was used to stimulate a diamond tipped stylus which then cut a groove in a "lacquer". This lacquer was taken to a plant where it was coated with silver. This coating was then peeled off to create a 'metal master' which is now a negative image of what was on the lacquer. Several "mothers" are made from that which are intermediate masters. From these mothers, "stampers" are made which are used in the press to create the finished record. Converting Analog to Digital In a CD, the goal is to give perfect reproduction. This is done by converting the analog wave into a stream of numbers and recording the numbers instead of the wave. This conversion is done by an analog-to-digital-converter. To play back the music, the stream of numbers is converted back to an analog wave by a digital-to-analog converter. The analog wave thus produced is amplified and sent to the speakers to produce the sound. This digital approach allows the reproduction to be very precise because of the number of times the analog wave can be sampled (the 'sampling rate'). This rate of approximately 44,000 samples per second gives an output that so closely matches the original wave that to most of us, the sound is "perfect". What it is made of<br> The CD itself is a fairly simple piece of plastic about 1.2 millimeters thick. Most of the CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single continuous, spiral track of data, circling from the inside of the disc to the outside. The Manufacturing Process The process begins by making the "glass master". To do this, a glass plate about 300 mm in diameter is lapped flat and polished. The plate is coated with photoresist and exposed. The photoresist is developed. A layer of metal (typically silver over a nickel flash) is evaporated over the photoresist. The master is then checked for accuracy by playing the disk. The master is then subject to an electroforming process. In this electrochemical process, additional metal is deposited on the silver layer. When the metal is thick enough (typically a few mm's) the metal layer is separated from the glass master. This results in a metal negative impression of the disk -- called a father. The electroplating process is then repeated on the father. This typically generates 3-6 positive metal impressions from the father before the quality of the father degrades unacceptably. These impressions are called "mothers". The electroplating process is repeated again on the mothers. Each mother typically makes 3-6 negative metal impressions called sons or stampers. The sons are suitable as molds for injection molding. Polycarbonate is used to injection mold the CD disks. Once the disks are molded, a metal layer is used to coat the disks. Aluminum, gold, copper and silver are all reflective enough to be optically acceptable. Gold is typically too expensive and copper has a peculiar appearance. Thus, aluminum and silver are the most commonly used metals. Following metal deposition, a thin plastic layer (1-30 microns) is spin-coated on over the metal. This can be a nitrocellulose layer suitable for air drying, or an acrylic plastic that is cured in UV light. Finally, the logo and other information is silk screened on the top. Further Information: This section was compiled from many sources, the most important of which was How Stuff Works where you will find many graphs and further explanations.