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magnetic recording[mag′ned·ik ri′kȯrd·iŋ]
a system for the recording and playing back of information—in recording, by varying the remanent magnetism of a medium or its parts in conformity with the signals of the information recorded, and in playback, by its reverse conversion, so that information signals are produced that correspond to the variations indicated.
Magnetic recording is used to record sound (in tape recorders and dictaphones) or an image and its audio accompaniment (in video tape recorders). It is also used in measuring, monitoring, and computing signals (precision recording).
In magnetic recording (see Figure 1), electric signals delivered to the input channel (for example, the amplifier of a tape recorder) undergo amplification and various conversions in order to obtain the required recording quality. The last stage in the channel is the magnetic recording head. The magnetic field of the head, which is spread over the working gap, is proportional to the strength of the current in the head’s coil. The field affects a moving medium and, by magnetizing sections of the medium in conformity with the signals being recorded, produces a magnetic track. The medium may be a ferromagnetic body that is readily magnetized and remains magnetized for a long time; it may take the form of a string (magnetic wire), a tape (magnetic tape), a disk, a drum, or a sheet. The signal record (that is, the medium now carrying the magnetic track), as it moves, comes into contact with the working gap in the core of the magnetic playback head, which is analogous in construction to a recording head, and excites a magnetic flux proportional to the magnetization of the various sections of the track. The flux variations cause the appearance (in the coil of the head) of an electromotive force that contains the recorded information.
In the playback channel, in addition to the head, there are devices for amplifying the signals and making conversions the reverse of those in the recording channel. A recording is erased either by demagnetizing the medium or by uniformly magnetizing it to saturation. This may be done in a special device where the entire recording can be erased at once. During the recording itself, it may be done by means of an erasing head located in front of the recording head, with respect to the motion of the medium. (In this case, a specified value of AC or DC current is passed through the coil of the erasing head.)
The higher the recording speed, the better the quality of the recording. To record electric oscillations with audio frequencies between 30 hertz (Hz) and 16 kilohertz (kHz), a tape speed of 9.5 cm/sec is adequate. To record signals in a frequency range of up to 10 or 15 megahertz (MHz) on a video tape recorder, the translational velocity of the rotating head relative to the tape is increased to nearly 50 m/sec. Parallel recording tracks are placed on the medium to increase the density of a magnetic recording.
There are several methods of magnetic recording, differing in the direction of magnetization of the medium, in the kinds of signal conversion in the recording and playback channels, and, sometimes, in extra current delivered to the coil of the recording head (either AC or DC magnetic biasing) to achieve almost complete proportionality between the magnetization of the medium and the value of the signal current. For example, in tape recorders, the magnetic biasing of the medium is accomplished during recording by a current with a frequency between 40 and 200 kHz (high-frequency magnetic biasing). The recording process becomes an anhysteretic magnetization of the medium by the field of the signals recorded, and the distortion associated with the curvature of the usual (hysteretic) characteristics of a ferromagnetic material is eliminated.
Among the advantages of magnetic recording are the simplicity of the apparatus, the fact that the recording is immediately ready for use, the durability of the recorded signals, and the possibility of using the recording medium repeatedly. Among the disadvantages are the invisibility of the recording, which, in some cases (for example, with the sound track of a motion picture) makes it difficult to edit the recorded signal; distortion of the information because of relatively high noise from magnetic and mechanical irregularities in the medium; and the magnetic transfer effect.
Magnetically recorded signals are copied either by rerecording (sometimes at a higher speed) or by contact printing in a thermal or magnetic field. The major trend in the development of magnetic recording is the improvement of mediums to increase density and fidelity.
REFERENCESFizicheskie osnovy magnitnoi zvukozapisi. Moscow, 1970.
Tekhnika magnitnoi videozapisi. Moscow, 1970.
V. G. KOROL’KOV