sound recording

sound recording, process of converting the acoustic energy of sound into some form in which it can be permanently stored and reproduced at any time.

In 1855 the inventor Leon Scott constructed a device called a phonautograph that recorded tracings of the vibrations of sound. Thomas Edison, starting about 1877, made great improvements in mechanical sound recording and was the first inventor to achieve the actually audible reproduction of recorded sound. The greatest advances, however, were made after the adoption in 1925 of electromechanical systems using electronic amplifiers (see record player). Generally (since the 1920s), in recording, the sound waves impinge on a microphone or set of microphones and are converted into an electrical signal that is used to make a recording. Originally, the signal was recorded directly on a master phonograph disk. Later, magnetic recording tape and then digital computer files were used to record the signal. Both magnetic tape and computer file recordings can be edited if desired.

The development of tape recording ended the cutting of a master phonograph disk directly during a recording session. Now a master disk, or original, for a phonograph record is created from a tape or computer file recording. A disk of soft acetate composition coated on an aluminum base is placed on a rotating turntable. The recording is played back and controls a stylus that cuts a spiral groove starting from the outer edge and moving to the inner edge of the disk. For monophonic sound the stylus vibrates from side to side as it cuts the groove. For stereophonic sound the stylus vibrates vertically, as well as from side to side, recording one sound channel in the left wall of the groove and one in the right.

In a series of steps the master is used to make a metal stamper that presses the groove into commercial records. In order to play a phonograph record, a stylus, or needle, is placed in the disk's groove while it is in motion on a turntable. The vibrations of the stylus cause the transducer to which it is attached to produce a varying voltage. This voltage is amplified and fed into a loudspeaker.

In magnetic tape systems the varying electrical voltage is converted in a small electromagnet, called a head, into a varying magnetic field that causes magnetic particles embedded in the tape to become aligned in varying degrees as the tape passes through the magnetic field. On playback, the magnetic tape moves past the head, generating a varying voltage in the coil of the head, which is boosted in an amplifier and converted to sound by a loudspeaker. (See also tape recorder.)

Compact discs (CDs), first introduced commercially in the early 1980s, employ laser technology to inscribe and “read” digital information in a way that avoids actual physical contact between the disc and any type of stylus. The optical properties of the disc's tracks are measured by a sensor and converted to digital signals and then to sound. CDs have the advantage of minimal wear and a greater possible dynamic range. Digital versatile disc (DVD) technology led to the introduction of the DVD-Audio format in 1999. Using a similar optical technology, DVD offers greater storage capacity and even more accurate sound reproduction, but DVD-Audio did not supersede CDs in the marketplace.

Recorded music and other sound may also be stored on and played from computer disks and solid-state storage using several different computer-program file formats. The recording may be stored in a format that is uncompressed or compressed. If the recording is compressed, the compression may be lossless, which allows for the the full reconstruction of the recording when it is decompressed, or lossy, which approximates the original recording to produce an even smaller file size but necessarily results in some loss of audio quality. Lossy compression formats, such as MP3, may offer a number of different degrees of compression, allowing for larger or smaller file sizes with better or worse audio quality.

Motion-picture film soundtracks are called optical recordings. The sound to be recorded is converted into an electrical signal that is used to modulate the intensity of a beam of light. This modulated beam exposes moving film to make a recording of the sound. Reproduction is effected by shining a steady beam of light through the developed film that is the sound track. As the film moves across the light beam, some of the light passes through it into a photocell, the amplified output of which activates a loudspeaker.

See L. Baert et al., Digital Audio and Compact Disc Technology (1995); F. Jorgenson, The Complete Handbook of Magnetic Recording (1995); J. Borwick, ed., Sound Recording Practice (4th ed. 1996); G. Alkin, Sound Recording and Reproduction (3d ed. 1997); R. E. Runstern and D. M. Huber, Modern Recording Techniques (4th ed. 1997); B. Bartlett and J. Bartlett, Practical Recording Techniques (2d ed. 1998).

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