The first practical device for recording and reproducing sound was developed by Thomas A. Edison in 1877. He called his device a phonograph, meaning sound writer, because of the crude, mechanically cut impressions, or "writing," it made on the surface of the recording cylinder. The sound reproduction was equally crude. Since the time of Edison's phonograph, the quest for more perfect sound recording and reproduction has led to the electric record player, stereophonic sound, tape players, and compact disc players.
Sound is a vibratory motion of particles in a medium, such as air, and it propagates as weak pressure pulsations known as acoustic waves. Any method for recording and reproducing sound utilizes the ability of these pressure waves to produce or imprint, in the physical condition or form of a certain body known as the recording medium. Subsequently, these changes can be converted back into sound waves similar to the originals. Perfectly reproduced sound waves have exactly the same component frequencies and the same relative intensities as the originals, without any losses or additions. There are four basic techniques for the audio "record-retrieval" process: mechanical, electrical, magnetic, and digital.
In the simplest mechanical recording, the air pressure waves directly actuate a very thin membrane connected to a needle. To amplify the intensity of the impact on the membrane, sound waves are let in through a horn, where the acoustic energy is concentrated on a small area. Driven by the membrane vibrations, the needle cuts a continuous groove in the moving surface of the recording medium. To reproduce the sound, a second needle traces the imparted groove, forcing the attached diaphragm to oscillate and, thus, to produce sound waves. This principle was employed by two constructively different early sound recording and reproduction instruments—Edison's phonograph (1877) and E. Berliner gramophone (1887). The phonograph used a cylindrical recording medium. The groove in the cylinder was cut vertically by the needle moving up and down. The recording medium for the gramophone was a disc with the groove cut laterally, from side to side. Both devices reproduced sound of limited volume and low quality, since the horn picked up only a small fraction of the acoustic energy passing through the air. However, the gramophone disc format, unlike its competitor, turned out to be suitable for the mass manufacturing of record copies and eventually pushed the Edison phonograph out of the market in 1929, while the gramophone was reborn as the electric record player.
In the electrical technique of recording, the acoustic waves are not directly transferred to the recording stylus. First they have to be transformed into a tiny electric current in the microphone. The strength of the current depends upon the sound intensity, and the frequency of the current corresponds to the sound pitch. After amplification, the electric signals are converted into the motion of the stylus, cutting a lateral groove in a disc. During playback, mechanical oscillations of the stylus, or needle, in the record groove are translated by the pickup into electric oscillations, which are amplified and interpreted as sound waves in a loud speaker. This innovation tremendously extended the frequency range of sound waves that could be recorded and reproduced. For over 40 years, electrical recording was continuously refined, but even very sophisticated improvements could not eliminate the limits imposed by the most vulnerable "needle-groove" part of the process. Because of the mechanical friction, sound "impressions" inevitably wore out, and the reproduction quality irreversibly degraded with each playback.
The magnetic recording process, based on the principles of electromagnetism, uses the recording medium in the form of a tape coated with magnetically sensitive particles. In this method, the electric current initiated by the sound waves in the microphone produces an electro-magnetic field which changes in accordance with the audio signals. When the tape passes through this field, the latter magnetizes the particles, called domains, making them behave as small compass needles, each aligning with the direction of the magnetic force. Moving past a receptor head during playback, domains induce electric current that can be translated into the audio signals. Introduced in the 1940s, the first tape recorders immediately won the appreciation of professionals for low-noise and wide-range-frequency characteristics of the reproduced sound. Moreover, the tape format opened opportunities for long uninterrupted recordings, which could be later easily edited or erased, allowing for reuse of the tape. In the 1960s, the tape was placed in compact cassettes, and tape recorders became versatile and reliable devices with applications far beyond just entertainment.
In the 1970s, new technologies, such as electronic digital processing and lasers, made the electrical technique obsolete. The new recording medium, however, retained the disc format. In digital sound recording, the electric signals from the microphone are converted into a digital code, or sequences of numbers. This digital code is etched into the surface of a compact 5.1 in (13 cm) diameter disc by a powerful concentrated light beam from a laser. The information from the master disc can be duplicated with absolute accuracy to any number of discs. In the playback device, called a compact disc (CD) player, the light beam of a less powerful laser reads the code etched on the disc and sends it through the long chain of transformations that finally result in the sound with a quality superior to anything previous technologies could give. The absence of mechanical friction in the reproducing process makes the lifetime of a compact disc longer than the lifetime of the technology itself.
Modern developments in digital sound recording and reproducing include the MiniDisc (MD) machines—a new generation of CD-players using smaller discs and also capable of recording.
One of the challenges for any new audio technology is remaining compatible with its predecessors. Given the current rate of audio evolution, it seems inevitable that one generation of consumers will have to deal with several technologies, each excluding the other. This would mean the unjustified waste of resources and real difficulties with preservation of the already accumulated audio information. That is why backward compatibility is the most practical and desirable feature for any future sound recording and reproduction technology.
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Elena V. Ryzhov