METHOD FOR RECORDING INFORMATION ON MAGNETIC TAPE AND HEAD FOR RECORDING INFORMATION ON AND REPRODUCING INFORMATION FROM MAGNETIC TAPE

Abstract: Method for recording information on magnetic tape, and a head for recording information on, and reproducing information from magnetic tape. The essence of the invention is that there are two airgaps (3, 4) in the direction of movement of the magnetic tape, provide with separate magnetic circuits (1, 2), and there is a magnetic coupling between the two airgaps (3, 4) of different width. During recording, a magnetic bias field is induced in the first airgap, whereas in the second one a magnetic field, varying at the frequency of the information to be recorded, is induced. By way of using the proposed method and head the quality of recordings, particularly of the tape-recorded recordings may be significantly improved, retaining the quality of playback, and the manufacturing process of the head is simpler than that of the known heads. Implementing the method and the head, the electronic circuits for recording and playback may be simplified.

Description: METHOD FOR RECORDING INFORMATION ON MAGNETIC TAPE AND HEAD FOR RECORDING INFORMATION ON AND REPRODUCING INFORMATION PROM MAGNETIC TAPE The invention relates to a method and a head for recording information on and reproducing information from magnetic tape. According to the proposed method a magnetic bias (pre agnetizing) field and a magnetic field of variable frequency, corresponding to the information to be recorded is induced in the airgap, in front of which the magne¬ tic tape is moving. The head as invented is provided with a magnetic circuit, made of laminar material, furnished with coil, and an airgap in the said magnetic circuit. This head is suitable for realizing the proposed method. The method according to the inven¬ tion provides efficient separation of the recording and playback/retrieval functions in information storages on magnetic tape, thus particularly tape recorders and in tape drives for computers, ensuring recording of outstanding quality. • A tape coated with a magnetic layer is moving in front of a recording/playback head in information storages on magnetic tape designed for the recording and retrieval of information. The recording and playback heads include a magnetic circuit provided with a coil; the magnetic circuit is made of laminar material with a suitable airgap in it. During recording the magnetic tape is uniformly premagnetized by a high-frequency field, its non-linearity is largely eliminated then the magnetic field is changed in the airgap according to *_>- ___,;_. i information to be recorded. The stray portion of the variable magnetic field penetrates the magnetic coating of the tape moving in front of the airgap, reversing its magnetization. During playback the information recorded in the magnetic coating of the tape modulates the airgap's magnetic field and the variations produced may be restored into the required information, using a suitable electronic circuit (playback of a sound recording). The recording and playback functions are fully separated in professional equipments, whereas in home systems demanding relative¬ ly less care this function is performed usually by a single recording/playback head (combined head). The most important specifications of magnetic tape storages are: frequency response, signal-to-noise ratio and linearity (distortion). Apart from these the mechanical construction (steady operation of the tape transport mechanism, accuracy of tape positioning) also plays a very important role. Linearity depends, first of all, on the magnetic coating of the tape, which is outside the scope of this invention. For storing information, it is utilized the phenomenon of remanent magnetization (noted by B r ), according to which the maximum amplitude of the signal permanently recorded on the tape will be equal to the remanence magnetization of the tape. Using high-frequency premagnetization (bias filed) during recording, the latter will be a minor source of noise, therefore the signal-to-noise ratio is very favourable from this point of view. In order to increase this ratio, the remanent magnetic field recorded on the tape should be increased. Examining the operation of magnetic informa- tion storages from the point of view of playback it can be concluded that in order to obtain good quality of playback, the thickness of the magnetic coating should be decreased, because in doing so, the material of the coating can be better magnetized at lower magnetic flux density. This, however, decreases the flux produced at playback as a consequence of which a tape of higher B r value should be used. For this very purpose were introduced tapes, coated with hard magnetic foil, which may be considered as to be better that the traditional tapes only from the point of view of the flatness of frequency response but the signal-to-noise ratio. Good quality of playback can be provided, in addition, by establishing the narrowest possible airgap in the playback head (less than 2 um). This, however, conduces to the loss of a significant portion of the magnetic flux through short-circuiting in the airgap, and the decrease of the signal-to-noise ratio cannot be balanced by an improved electronic circuit. It is not recommended consequently, to choose the width of the airgap below 1 yum, because otherwise the quality deteriorates to a significant degree. To sum up: good playback quality is provided by using a playback head of narrow airgap, high initial permeability and low coercive force. This latter requirement is necessary to eliminate distortions and retroactions. It is not recommended to choose the playback head's airgap width below 1 am and over .2 yum. The recording head's material should be cha¬ racterized by high magnetic saturation since it must provide magnetization of the tape's magnetic layer practically up to saturation in a relatively large distance from the airgap. The saturation induction of the magnetic coating is at least 0,15 to 0,2 T and it may be as high as 1 T for metal tapes. The recording head should have an airgap not narrower than 3 . /urn, because otherwise only a very narrow layer will be positioned into the optimum workpoint in front of the airgap, where magnetization to the suitable depth is guaranteed. Similarly, the airgap should not be wider than 5 /um» because otherwise the gradient of the stray magnetic field will be too small in front of the airgap, the magnetic material of the tape will demagnetized to a degree when information is lost. In order to satisfy the two contradictory requirements mentioned above two separate heads are used in professional equipments, i.e. a recording head and a playback head. The recording head includes a coil, fed by a high frequency current and by a current of lower, variable frequency, superimposed onto the former. In the foregoing a reference was made to the fact, known also from the special literature that the magnitude of the recorded signal is decisively influenced by the gradient ©H ' . / _ x of the stray magnetic field of the airgap, where x denotes the direction of movement of the tape. This phenomenon is manifest mainly at frequencies over 10 kHz. The increase of the gradient results in the increase of the remanence magnetism in the information carrying stratum of the tape. From phenomenological point of view this may be interpreted in such a way that the high-frequency field has also a demagnetising effect, and. the shorter the distance at which the stray magnetic field's intensity falls below a value determined by the coercive force, the less intensive is the demagnetizing effect. In this respect the geometry of the magnetic field of single-airgap magnetic heads 15 far from optimum. Considering both the characteristics of the magnetic materials and geometry, the single recording/play- Q back heads of cheaper tape-recorders carry the implication of a compromise, even in comparison with the formerly mentioned. The generally accepted 3 ... 3,5 yum width of the airgap does not comply with the optimum values for the recording and playback airgaps, and the penalty of this compromise choice is paid in the deterioration of the recording/playback quality. There were attempts to decrease the demagnetizing effect, and one of the results was the introduction of the so-called cross-field technique, involving a separate head, pressed onto the reverse side of the tape, producing the high-frequency premagnetizing field. This method does not increase the gradient of the stray magnetic field, but adjusting the overlapping of the bias field and the variable frequency field, the strength of which is proportional to the useful signal, the demagnetizing effect can be moderated to some degree. Approximately 15 $> increase in the signal's magnitude was achieved by this method. Synchronized operation of the two heads requires a complicated and expensive mechanical system, and it explains why this technique did not become widely popular. The basis of this invention is the recognition, that establishing two airgaps in close magnetic coupling with each other and separated from each other by a wall of high magnetic saturation, an advantageous interaction evolves between the magnetic fields of the two airgaps. The distance between the two airgaps, that is,, the thickness of the wall is in the 5 ••• 25 yum range, ensuring partly the correct adjustment of the overlapping of the high- -frequency bias field and the variable frequency field, independently from any external influences, partly the increase of the premagnetizing field*s gradient OH _-_./Ox through a conterphase high-frequency field of different amplitude, generated in the other airgap. To achieve the objective, a method was developed, according to which the magnetic tape moves in front of the airgaps, in which the bias and the variable frequency fields are generated (the latter varies at a frequency corresponding to the information to be recorded), and in compliance with the invention there are two airgaps along the direction of movement of the tape, the second one being narrower than the first one; the bias (premagnetizing) field is induced in the first airgap, the variable frequency magnetic field - in the second airgap, and a magnetic coupling is established between the two airgaps. It may prove advantageous, if the bias field, in counter phase with the one in the first airgap and at an attenuated amplitude, is superimposed onto the field in the second airgap, and if necessary, the variable frequency field is superimposed on the one induced in the first gap. In this case the control signals of the airgaps require synchronization. Also to achieve the objective, a head, particular¬ ly suited for use in magnetic tape information storages was produced, having magnetic circuits and airgaps in the magnetic circuit, and in compliance with the invention there are two magnetically coupled, essentially parallel airgaps of different width, established in separate magnetic circuits and a wall, made of a material of high magnetic saturation, between the airgaps. This head is particularly suited for the embodiments of the described method. There is a wall, preferably made of amorphous metal, between the two airgaps, established, if necessary, with parallel walls. The magnetic circuits may be laminated in parallel with the airgap. It was found to be particularly advantageous, if the proportion, of width of the two " airgaps is between 1,5 and 8, and the wall is not thicker than the width of the wider airgap multiplied by 3,5. The signal level at the reproduction of the useful signal recorded on tape by means of the combined head, complying with the invention, and applying the method, may be increased by 30 ... 50 . £, and according to the theoretical calculations, up to 75 increase may be achieved. At the same time, information storages of high reliability and high fidelity of reproduction may be designed, using simpler electronic circuits, than the well-known ones. In the following the object of the invention is discussed in detail, purely by way of example, with reference to a preferred embodiment illustrated in the accompanying drawings, wherein Fig. 1 cross-sectional views of the head, according to the invention and Fig. 2 curves of comparative measurements. When realising the method, according to the invention, a magnetic tape is loaded into the mechanical unit of the tape-recorder, and it is transported at a constant speed for the purposes of recording or playback. According to the well-known method, one unit of an electronic circuit supplies the audio-frequency current, the other unit - the high-frequency current. The audio-frequency current corresponds to the information to be recorded. There are two airgaps along the path of the magnetic tape in close fit with the tape, the first airgap in the direction of movement of the tape being wider than the second one. There are magnetic circuits established around the airgaps, the coils of which are connected to the audio-frequency and high-frequency current sources. In the first airgap, which is the wider of the two, the high-frequency current induces a magnetic bias field, whereas the audio-frequency current induces - in the second, narrower airgap a magnetic field of frequency, varying according to the information to be recorded. Magnetic coupling is provided between the two airgaps, for example by means of a wall of suitable material and thickness between them. It was found to be advantageous, if the coil of the second airgap is also supplied from the high-frequency source, at a reduced amplitude of the current. It may be expedient for multiplayback or other acoustic operations to supply simultaneously the coil of the first airgap from the audio-frequency source, selecting usually an amplitude unlike to the one in the second airgap. In such cases (increasing the recording intensity, deleting certain frequencies, multiplayback and so on) a suitable circuit is inserted between the coils of the two airgaps and the current sources - for example a delay circuit - to provide synchronization. This layout is suitable for the reproduction of sound recordings and other information, in which case the high-frequency current is not necessary, only the narrower airgap operates. The head as invented includes magnetic circuits 1 and 2, fitting the tape 6 moving in front of them (Fig. 1). The magnetic circuits are of laminar construction. Magnetic circuit 1 consists of coil In and airgap 3, magnetic circuit 2 consists of coil !« and airgap 4. Ai gaps 3 and 4 are of different width. There is a wall 5 between airgaps 3 and 4, and magnetic circuits 1 and 2, respectively, the material of which is characterized by high magnetic saturation, its permeability is at least 2000. The width of airgap 3 is usually 3 ... 8 yum, airgap 4 is 1 ••; 2 yum wide. In respect of the wider airgap 3 it was found that good quality of recording may be achieved at airgap width up to 7 yum instead of 3 ••• 5 characterizing the traditional solutions, even under the most severe requirements. The width of wall 5 is usually 5 ... 25 yum. Magnetic coupling is provided between the two airgaps in such arrangement at 0,3 ... 0,5 coupling coefficient. Magnetic circuits 1 and 2 consists of four-five laminae of high magnetic induction. Airgaps 3 and 4 are suitably surrounded by the laminae in such a way, that their plane is parallel with the longitudinal axis of the airgap. Magnetic circuits 1 and 2 are preferably encased from the airgaps' side in a material of high magnetic saturation of 20 yum thickness. Wall 5, the internal layer of the magnetic circuits and the laminae may be produced, for example, of amorphous strip metal of high magnetic saturation and permeability at least 2000. The requirements may be satisfied by a relativelly cheap amorphous metal, containing iron and boron. (Amorphous metals are produced in the well-known way, by the extremely rapid chilling of the melt, therefore it does not crystallize, but conserves electric features, cha¬ racteristic of the metals. It's use in magnetic heads is widely known, single-airgap combined heads are made of it, because it multiplies the abrasive resistance of the surfaces being in direct contact with the tape.) The head, described herein, provides signal level U = 3 mV according to the control measurements performed at f = 10 kHz by means of a recording mechanism and using a standard test tape (Fig. 2, curve 8, where curve 7 illustrates the measurement result of an AKAI head), whereas the signal level of a commercial head was 2 Y at 12 kHz. The amplitude of the output signal of the head according to the invention was equal to 1,1 mV and that of the commercial head - 0,8 mV. From this follows, that the improvement in the signal level is at least 30 • According to the measurements performed OMPI the invented head provides a better frequency response up to 25 kHz, then the commercial one. The production of the head contrived is extremely simple. The commercial combined heads consist of 50 ... 100 leminae of permalloy, the processing of which is very cumbersome: they are pressed to a form, corresponding to the shape of the parts of the combined head, then they are heated at a temperature specified accurately around 800 °C, and held at + 5 °C tolerance during 3 ... 8 hours, under a protective atmosphere. This heat treatment is a vital process of the preparation, but as a consequence of it, the material becomes very delicate. Then the laminae are fitted together, polished and assembled to form a combined head. In contrast to this, amorphous metals can be cut easily, even by means of simple scissors. Heat treatment is relatively simple, actually it is simply holding at a temperature 250 ... 350 for about half an hour, without protective atmosphere. An additional advantage is that the material remains flexible even after heat treatment, it is not susceptible to deforma tion, the parts keep true to shape and are exclusively wear-resistant. Heat-treatment generally improves the magne features of the amorphous metal. The quality of the recordings, made by the head according to the invention, is better than the quality achieved by the best known traditional heads, whereas the narrower airgap provides identical playback quality. The head, complying with the invention, provides conditions for the significant increasing of the signal level at recording. Outstanding recording/playback features are provided through the use of electronic circuits of simpler than the currently used design. O PI

Claims: Claims : 1. A method for recording information on magnetic tape, wherein a magnetic tape is transported in front of an airgap, a premagne izing field and a variable magnetic field of frequency, corresponding to the information to be recorded, is induced in the airgap, c h a r a c t e r i z e d in that said pre¬ magnetizing field being induced in the first airgap established in the direction of movement of the magnetic tape, and in a second airgap, magnetically coupled to the first one and narrower than the first one, a magnetic field of variable frequency is induced. 2. The method as claimed in Claim 1, c h a ¬ r a c t e r i z e d in that said premagnetizing field being superimposed in counterphase to said magnetic field of variable frequency, at a magnitude lower than the one in the first airgap. 3. The method as claimed in Claim 2, c h a ¬ r a c t e r i z e d in that said magnetic field of variable frequency being preferably superimposed onto the premagnetizing field in the first airgap at a magnitude differing from the one in the second airgap and said magnetic field of variable frequency in both airgaps is induced in a synchronized fashion, taking into consideration the tape's movement. 4. A head for recording information on and reproducing information from magnetic tape, particularly for the realization of the method as claimed in any of Claims 1-3, provided with a magnetic circuit of laminar material furnishied with and an airgap in the magnetic circuit, c h a r a c t e r i z e d in that said head comprises two magnetically coupled, essentially parallel airgaps (3, 4) of different width, established in separate magnetic circuits (1, 2) and a wall (5), made of a material - of high magnetic saturation, being between the airgaps. 5. The head as claimed in Claim 4, c h a ¬ r a c t e r i z e d in that the walls of said airgaps (3, 4) are parallel with each other. 6. The head as claimed in claim 4 or 5, c h a r a c t e r i z e d in that the plane of the laminae of said magnetic circuits (1, 2) being parallel with the plane of the airgaps. 7. The head as claimed in any of Claims 4 to 6, c h a r a c t e r i z e d in that the material of said wall (5) is amorphous metal. 8. The head as claimed in any of Claims 4 to 7, c h a r a c t e r i z e d in that the proportion of width of the said two airgaps (3, 4) is 1,5 ... 8, preferably 2 ... 3. 9. The head as claimed in any of Claims 4 to 8, c h a r a c t e r i z e d in that said wall (5) being at most 3,5 times wider than the wider airgap (3).

Inventors: TICHY GEZA, VASS LASZLO, TICHY, GEZA, VASS, LASZLO
Applicants: KOEZPONTI VALTO HITELBANK, KOEZPONTI VALTO- ES HITELBANK RT
Application year: 1982
Main classification: G11B5
IPC codes: G11B5