About CD
Compact disks (Compact Disc, CD ) are the media for recording digital information. In order to record music instead of a record by one of the optical disc standards, Sony and Philips developed jointly. At the present, digital information other than music, such as data for computers, can also be treated. The appearance of the compact disk is made from 12 cm in diameter or 8 cm, and the shape of a 1.2-mm-thick disk from the plastic. Although the quality of the material of a plastic is polycarbonate, there are some which use APO (amorphous polyolefin) and glass. 780-nm infrared laser is used for reading, and reflection of the laser light with which it irradiated is read. In order to reflect laser light, the compact disk has structure which piled up two or more layers of an aluminum vapor deposition film (about 80 nm in thickness and this reflect laser light with a role like a mirror), and a protection layer (about 10 micrometers in thickness) and printing films (label etc.). The very fine hollow is carved by the disk and this pattern is expressing digital information. This hollow (it will become a lug if it sees from a reading side) is called pit, and a portion without a pit is called land. Although the laser light equivalent to the portion of the land is reflected and it returns as it is, since the laser light equivalent to a portion with a pit has a reflective wave from a land, and 1/2 wave of phase difference, it interferes in it, and it is negated, suits, and becomes dark. A digital signal is read by this light and darkness, this is returned to an analog signal, and it outputs as a sound. The width of a pit is nine kinds same also as nine kinds and the distance from a pit to the next pit to 3.56 micrometers in length per 0.83 micrometer to 0.3 micrometer in 0.5 micrometer. Moreover, although the sequence of a pit is called track, this track is 1.6-micrometer width and is spirally located in a line toward the outside from the inner side. Gloss like rainbow color of CD is a structural color by the light diffracted by this track regularly located in a line interfering. For recording data as a pit sequence, a modulation method called EFM (Eight to Fourteen Modulation) is used. Moreover, although an error correction is based on CIRC (Cross-Interleaved Reed-Solomon Code), additional error detection and correction are carried out to the use allowed not a bit errors, such as data storage of a computer. In the case of a music use, the music of 99 tracks is recordable for 58 seconds in a digital PCM form for a maximum of 79 minutes. Moreover, 99 indexes (positioning sign) could be prepared all over the standard top 1 track, and although the player and disk of CD appearance which can treat an index in the first stage very much existed, it hardly spread after that. There are some which use gold instead of aluminum (called a "gold record"). As a music supply medium, CD is still in use, and the replacement to these next-generation media is not progressing even now when a Super Audio CD and DVD Audio appeared. The real enemy of CD also has the opinion that it is not these next generation (concreteness) media but a music distribution by the Internet.