Until the 1980s most popular music had come into homes in the form of records. But traditional sound recording had been done in analog, a continuous waveform which is theoretically capable of producing an exact replica of the sound recorded, but which loses fidelity when it is sonically "compressed" onto a record. In addition, analog records are prone to high levels of noise from dust or scratches in the record groove, which are reproduced by the needle as sound. Records also lose their quality of sound over years of use, as the needle gradually wears the playing surface away. With digital recording, instantaneous "samples" of an analog wave are taken at set intervals; the resulting information is stored as binary code (a system of 1s and 0s). Such binary code is then replayed from a disc using a laser beam, which never touches the playing surface and is less prone to transform most slight imperfections into sound.
The first to suggest the digital approach for information storage was the French mathematician, Jean Joseph Fourier (1768-1830). Fourier postulated that if samples of a continuous wave (such as a musical passage or voice) could be taken, the original wave could be precisely reproduced as long as a sufficient number of samples was obtained. Acoustic technicians worked to determine how many samples would be necessary to duplicate a waveform to the ear. In 1928, Harry Nyquist (1889-1976), a Bell Laboratories mathematician working on formulas for the improvement of telegraph signaling, found that maximizing the accuracy of such signals required twice as many samples as the length of the actual wave involved. In other words, each wave had to be sampled at least twice to obtain accurate reproduction. Applying the principle to sound, a sampling rate of at least 40,000 times per second should then produce flawless sound, since a human can hear no higher than a frequency of 20 kilohertz (20,000 hz). Not long after, a second Bell Laboratories researcher, mathematician Claude Shannon (b. 1916), developed a new science, called information theory, that would provide an additional theoretical background for digital data storage. In 1948, Shannon clarified how units of information carried along communication lines can be measured, transmitted, and tested, and he suggested ways to reduce or eliminate errors that distort the flow of information. Using his work, two Massachusetts Institute of Technology researchers (Irving Reed and Gustave Solomon) were able to develop procedures to correct information errors, which were later used in compact disc players.
Even though the conditions required to store information digitally were understood by this time, the devices capable of carrying out such storage were yet to be invented, and many technological advances had to take place before compact discs could become a reality. For example, computer and electronics technology had to be developed capable of handling the large number of samples involved in digital recording. The introduction of the laser in 1960 by Theodore Maiman would make the compact disc player a reality. Engineers at a number of audio firms realized that a laser beam could be used to retrieve digital data from a recording without physically touching it, and set about developing a practical optical disk.
A number of digital recording and playing systems came out of this effort. First, a system had to be devised to record data in a digital form. NHK and the Sony Corporation were the first to develop working digital recorders in the late 1960s, and within a few years a number of recording studios were using digital recorders, which store sounds from a recording session more faithfully than analog equipment does. Producing an economical digital player took longer, and it was not until 1978 that Magnavox introduced the first digital compact disc player for the consumer. This player, called the "videodisc" or "laserdisc," was for movies rather than for music. Around this time, nearly a dozen companies began to unveil competing digital music systems. Recalling the marketing nightmare associated with the conflicting audiocassette and eight-track systems, as well as the VHS and Beta video tape disaster, which confused consumers and cost companies millions, Philips and Sony Corporation began a joint effort to create a standard, superior digital playback system which they could license to any other company wishing to produce a CD player. Within about a year, they had succeeded, and beginning in the early 1980, CD players came onto the market in earnest.
The system adopted by Sony and Philips works as follows: digital impulses coded from music, pictures or voice are embossed as peaks and valleys on a hard plastic disk. The plastic is coated with a thin microscopic layer of metal such as gold or aluminum. The metal is then coated with a resinous protective layer. The finished disk, called a CD (compact disc), is about 4.7 inches (12 cm) in diameter. In a compact disc player, a small laser beam shines upon the peaks and valleys on the metalized portion of the disc while the disc spins. A mirror or prism between the laser and the disc picks up light reflected from the disc and bounces it onto a photosensitive diode, which transduces the impulses into electrical current. The current is then converted into an analog waveform for playback through stereo speakers.
Compact disc technology continued to develop throughout the 1980s and 1990s. In 1988, the first rewritable compact disc (CD-R) was introduced, though it could only be rewritten once. This technology was not commonly employed in compact disc players. The year 1992 saw the introduction of SonyÕs MiniDisc system. MiniDiscs were miniaturized versions of compact discs, and had similarly small players. They were intended to be the compact disc equivalent of Sony's walkman, and had early erasing and recording technology, but remained an anomaly through 1998. In 1996, an erasable compact disc, called the CD-E, became available. CD-Es can be read, erased, and rewritten repeatedly in special compact disc players, which became available that same year, yet can still be played on standard compact disc players. CD-E discs have the same dimension as compact discs, but have extra layers of different materials. They read and write data using the technology of phase-change. In 1997, Paras Prassad, a chemist at SUNY-Buffalo, announced his discovery of a technology that allows one thousand times more data to be squeezed on to one disc, by layering the data and using a precise laser to read it. Despite these innovations, experts predict that compact disc and compact disc players will be eventually replaced by DVD technology.
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