The compact disc digital audio format or music CD was the first CD format that was introduced into the marketplace in 1982. The goal was for this digital audio CD to replace the audio cassette and vinyl LP albums. After a slow start, it gained momentum in the marketplace and the audio compact disc became the main method for the distribution of audio.
The digital audio CD is a read-only format or in other words the information is placed on the disc at the time that the disc is manufactured. Therefore, these discs are purchased with the information already on them and the user can only play or read the discs. No additional information can be added to this type of disc and no information can be erased. However, you can produce discs with all the same elements as audio CDs using the CD-R and CD-RW formats although these are not read-only discs.
The physical structure of the compact disc digital audio format is the same as all other read-only compact disks. The majority of the 1.2 mm disc thickness is a very clear polycarbonate base. Molded into the top of the polycarbonate plastic are very tiny indents or pits that are laid out in a spiral pattern that starts from the inner portion of the CD and ends on the other portion. The audio CD plays from the inside out, from the portion just outside of the hub to the outer portion of the disc. If you follow the spiral, you get millions of alternating areas of indents or pits and non-indents or non-pits. It is these pits, which are of varying width, that are responsible for generating the digital “1”s and “0”s of the digital information that get decoded to produce the audio information.
In order for the digital audio CD to be readable, a metal layer made of aluminum or aluminum alloy is deposited onto the top of the polycarbonate plastic and pits. When laser light reads the digital audio CD it does so through the base or non-label side of the disc. The laser light then hits a pit or non-pit area and either gets reflected back to the detector of the player or reader to create a signal or no signal if it does not get reflected back. This light signal is interpreted as a digital 1 or 0 and the digital code for the information is created.
The metal reflective layer is a very thin layer and susceptible to chemical oxidation or corrosion and physical damage. Any kind of disruption of this layer will lead to errors or an unplayable digital audio CD. Therefore, a protective layer is applied over the metal. Finally, with all compact disc digital audio formats, a label is silk screened on top of the protective layer. The music CD is then placed in a jewel case with various liner notes with lyrics, pictures, and information about the artists, etc.
Other Information about the Audio CD
Many like the digital audio compact disc because there is quick and random access to the different tracks on the disc. The compact disc digital audio format also provides great sound, although some prefer the sound from the old analog LP vinyl albums. The audio information that ends up on an audio CD conforms to a standard called the Red Book – the standard for audio compact disks. This standard indicates that sampling of an analog audio signal occurs at 44.1 KHz, which is roughly twice the amount that is audible by humans. The bit size of each sample is 16 bit, which allows for 65,536 different sounds. The Red Book standard also provides many other specifications that a digital audio CD needs to have such as maximum playing time, maximum number of tracks, error correction, etc. Red book audio CDs can be played in audio CD players and computer drives. Note that you can also create audio CDs on recordable and erasable CD media. These CDs can be made to conform to Red Book and play in all players or can consist of different formats of audio files than what the Red Book specifies. This latter disc with audio information on it will not play in stand alone audio CD players as these are only meant to recognized Red Book standard audio CDs.