The compact disc is an audio data carrier with optical characteristics. It is a circular plastic construction with a hole in the center. The music is recorded and further read by means of laser technology.
Initially, these discs were designed to store audio in digital format, but later they were used as a storage medium for any files in the form of binary information. Then, devices with the ability to not only read audio files, but also to record and re-record them began to appear.
The method of recording information by laser method to disks appeared even before the creation of computers. The Soviet specialists A. Prokhorov and N. Basov actively worked on the creation of this technology.
It was they who created the so-called “cold” lasers, which later began to be used not only in the creation of CDs, but also in the production of various computer equipment. In 1964, Soviet scientists became the honored winners of the Nobel Prize.
The first CD on laser technology was created in 1980 by Philips and Sony specialists. A unique method of signal coding (pulse-code modulation method) was used to create the CD. The first band to be released on CD was “Abba” (their collection of songs “The Visitors” was recorded).
According to Philips, over the past 25 years, about 200 billion CDs have been sold. Today, a large number of people prefer to listen to and download music files online, but in 2007, CD sales accounted for about 70 percent of all music sales.
As for Ukraine, in the 1990s, the main part of the income in the show business came from the sale of music albums of pop singers on CDs.
Alternative version of the appearance of music carriers
There is a theory that CDs were created not by two well-known companies, but by an American physicist J. Russell, who in 1971 demonstrated to the world his created storage device.
Some believe that the scientist did it for personal use to prevent needle-player scratches of vinyl records. Only a few years later was this invention independently approved by Philips and Sony.
The data carrier is a polycarbonate substrate with a thickness of 1.2 mm and a product diameter of 120 mm. The substrate is covered with a thin layer of metal, which can be aluminum, less often – gold or silver. The metal layer is protected by a lacquer, the surface of which has graphic information about the content of the medium.
By reading the information through the substrate, its structure can be protected. Unique technical features increase the system’s resistance to interference, dust particles and scratches.
On the outer part of the system there is a 0.2 mm high ring-shaped projection. This part prevents the disc from touching the surface if it is in a horizontal position (e.g. on a table). In the center of the device there is a round hole with the size of 15 mm.
Thanks to the error correction system with the help of a special code, small scratches and defects on the surface do not affect the quality of information reading. If faulty signal parameters appear, they are replaced by approximate values, which are formed by interpolation of correct information. It is worth noting that Philips is entitled to a sign called “Compact disc digital audio”, which is the logo of the CD format.
Manufacturers produce read-only media and single- or multiple-writer devices that have their own letter markings (CD-R and CD-RW, respectively).
Files are read by means of a 780 nm laser beam created by a semiconductor laser. The peculiarities of reading information with the help of a laser for different types of carriers consist in recording the transformation of the degree of intensity of reflected light.
Some manufacturers produce so-called cleaning discs, on the outer surface of which is placed a special cleaning mechanism to clean the sensor lens.
There are also various shaped discs, which are designed mainly for commercial use. It is necessary to note that such products are not intended for use in computer drives as at high speed of rotation they can burst and damage the device. The standard diameter of the products is 120 mm, but there are also smaller models, which have a small capacity (only 21 minutes of sound).
Copy protection features
The specifics of CD creation and operation do not provide any protection against unauthorized copying and distribution of information. However, since 2002, many recording companies have started to develop CDs that are protected against unauthorized copying of data.
The essence of the techniques is to intentionally misrepresent the information on the disc in such a way that the music is played only on the domestic music centre, not on the computer. As a result, these discs may not be readable on all household devices, and software has also become available to copy even the protected device.
At the moment, specialists from the recording industry are not leaving attempts to create high-quality protected discs.