GB2195198A - Erasing head - Google Patents

Abstract

An erasing head for magnetically erasing a magnetic tape 18 movable in the direction of the arrow shown comprises a core 10 having a coil 12 for generating a DC magnetic field which passes perpendicularly through 16 the tape 18 in the direction of the arrows I and II across the gaps A and B. A keeper 20 completes the magnetic circuit. The field strength in gap B is of a reduced level compared with that in gap A and selected to bring the magnetisation of the tape back to the origin of the hysteresis loop. In a second embodiment (Fig. 3, not shown) the face of pole pieces 14, 16 are curved. In a third embodiment (Fig. 4, not shown) both the core and keeper are E Shaped. The coil 12 may be replaced by a permanent magnet. <IMAGE>

Description

SPECIFICATION Erasing magnetically recorded signals The invention relates to the erasing of magnetically recorded signals, that is, signals which have been magnetically recorded on a magnetic storage medium such as magnetic tape for example. Magnetic tape is commonly used as a storage medium in tape recorders.

Such recorders employ magnetic heads for recording signals magnetically on the tape, as the tape moves longitudinally, and other magnetic heads for reproducing the recorded signals. It is common to employ, additionally, an erase head, that is, a head which is mounted in juxtaposition with the tape and energisable for magnetically interacting with the tape so as to erase the signals recorded on the tape (prior to the recordal of further signals, for example).

According to the invention, there is provided a method of erasing signals recorded on a magnetic storage medium movable along a path lying in its major plane, comprising the step of directing a unidirectional magnetic field into the medium in a direction perpendicular to its plane, whereby to produce minimal longitudinal remanent magnetisation.

According to the invention, there is also provided magnetic erasing means for erasing signals magnetically recorded on a magnetic storage medium movable along a path lying in its major plane, comprising unidirectional magnetic field generating means positioned adjacent the said path for directing a unidirectional magnetic field into the medium substantially perpendicular to its said plane at a first position along the said path, whereby to produce minimal longitudinal remanent magnetisation.

A magnetic erase head for erasing signals magnetically recorded on magnetic storage tape and embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a cross-section through one of the heads; Figure 2 is a B-H curve of the recording medium; and Figure 3 is a crosS-section through another form of the head.

As shown in Figure 1, the head comprises an iron core 10 around which is wound an electrical energising coil 12. The core 10 has two pole pieces 14 and 16 which are of different size for a reason to be explained. The core 10 is mounted adjacent to the path of the magnetic tape 18. On the opposite side of the tape 18 is mounted a magnetic keeper 20 which completes the magnetic circuit and provides pole pieces 22 and 24.

In use, the tape 18 moves longitudinally through the gap A between the pole pieces 14 and 22 and through the gap B between the pole pieces 16 and 24, in the direction of the arrow C. For carrying out erasing, the coil 12 is energised with DC power. The energisation of the coil 12 is such as to produce a magnetic field acting in the directions of the arrows 1 and 11.

It will therefore be seen that the magnetic field generated acts perpendicularly to the tape surface.

Referring to the B-H hysteresis curve shown in Figure 2, the field within the gap A has a field strength of 5Hc, the maximum field strength. As the tape (moving in the direction of the arrow C) moves past the gap A, the field strength will fall and the flux density will reduce to Br when the field strength H has fallen to zero. The portion of the tape under consideration will now enter the gap B which is sized so that the field strength (which now of course acts in the opposite direction) has a value of about 1.5HC. This will drive the magnetisation of the tape to the point X in Fig 2.

This field strength is optimised so that, as the tape moves away from the head and the field strength falls again to zero, the magnetisation will approximately follow the path Y and will end up at the origin of the hysteresis loop.

The tape will thus have been properly erased.

Effective magnetic erasing has to ensure that the elemental magnets on the tape, by means of which magnetic recording takes place, do not become preferentially directed in one longitudinal direction along the tape. If they do, then although erasing will have taken place, an effective bias will have been stored in the tape and subsequent recording may be distorted. It is therefore necessary to ensure that the pole pieces 14,16,22,24 are properly designed and shaped to give an accurately perpendicular field within each gap A,B. Figure 3 shows a modified arrangement in which the pole faces are curved and the tape is actually in contact with them. This can help to ensure that the field is indeed accurately perpendicular to the tape.

Figure 4 shows a modified arrangement in which the core 10 is of E-form with the keeper 20 being correspondingly shaped. As before, the coil 12 is DC-energised. This time there are three gaps, A,B and D with the magnetic flux being operative in the directions shown by the arrows 1,11 and Ill. The areas of the pole pieces are arranged so as to provide a field strength of 5HC in gap A, of 2HC in gap B, and of 1 5He in gap D. As with the head of Figure 1, it is necessary to control the sizes and shapes of the gaps A,B and D and to optimise the field strengths so that the mag netisation ends up at the origin of the hysteresis loop.

The heads shown in Figures 1, 3 and 4 have a number of significant advantages. By arranging for the erasing fields to act perpen dicularly to the tape, it is relatively easy to ensure random orientation of the magnetic ele ments in the tape after the erasing has taken place, thus avoiding distortion during subsequent recording. As explained, it is necessary to control accurately the sizes and shapes of the gaps between the opposed pole pieces.

Because DC energisation is used, the energisation circuitry can be simple and in fact there may be-a readily available source of DC in the circuitry of a conventional tape recorder for this purpose. The heads themselves can be inexpensive. It is not necessary to use ferrite or laminations. Heating of the coil or core is minimal. Any externally radiated field is of course DC, causing minimal interference.

Because the DC field used for the erasing acts perpendicularly to the tape, it is relatively easy to ensure that erasing takes place with random orientation of the elemental magnets of the tape, thus avoiding introduction of bias into the tape; the disadvantageous introduction of such bias, which may occur with erasing arrangements using a DC field acting longitudinally of the tape, is thus avoided.

It is of course necessary to provide the keeper 20 on the opposite side of the tape to the core 10 and this may make the type of head difficult to use in certain circumstances.

In other applications, however, this can readily be done. Our co-pending patent application filed on the same day as this application, (Mathisen, Macara & Co reference "Cassette Lacing") discloses an arrangement in which a magnetic recording tape has a path particularly suited to providing access to both sides of the tape and to enabling a keeper to be accurately moved into juxtaposition with the core, the tape running between them.

It is not necessary for the core to be' magnetically energised by means of an electrical coil; a permanent magnet could be used instead.

Claims (21)

1. A method of erasing signals recorded on a magnetic storage medium movable along a path lying in its major plane, comprising the step of directing a unidirectional magnetic field into the medium in a direction perpendicular to its plane, whereby to produce minimal longitudinal remanent magnetisation.

2. A method according to claim 1, in which the field passes in the said perpendicular direction through the medium at a first position in its said path of movement and then passes through the medium in the opposite direction but at a second position downstream (in the' direction of movement of the medium) of the first position.

3. A method according to claim 2, in which the field strength at the said second position is less than the field strength in the said first direction and selected so as to tend to return the magnetic state of the medium to the origin of the B-H curve applicable thereto.

4. A method according to any preceding claim, in which the field is produced by an electrically energised coil.

5. A method according to any one of claims 1 to 3, in which the field is produced by a permanent magnet.

6. Magnetic erasing means for erasing signals magnetically recorded on a magnetic storage medium movable along a path lying in its major plane, comprising unidirectional magnetic field generating means positioned adjacent the said path for directing a unidirectional magnetic field into the medium substantially perpendicular to its said plane at a first position along the said path, whereby to produce minimal longitudinal remanent magnetisation.

7. Erasing means according to claim 6, including further means for directing a unidirectional field into the medium substantially perpendicular to its said plane at a second position along its said path.

8. Erasing means according to claim 7, in which the said further means comprises means on the opposite side of the plane of the medium to the said magnetic field generating means for receiving the said field thereon and directing it through the medium in the opposite direction at the said second position.

9. Erasing means according to claim 8, in which the second position is downstream, in the direction of movement of the medium of the first position, and in which the field strength of the field at the second position is less by a predetermined amount than the field strength at the said first position and selected so as to tend to return the magnetic state of the medium to the origin of the B-H curve applicable- thereto.

10. Erasing means according to any one of claims 6 to 9, in which the unidirectional magnetic field generating means comprises a magnetic core having pole faces respectively positioned at the said first and second positions and the said further means is magnetic keeper means facing and on the opposite side of the tape to the said pole faces.

11. Erasing means according to claim, including means for directing the magnetic field through the magnetic storage medium in a direction perpendicular to its plane at a third position downstream of the said second position in the direction of movement of the medium and in the same direction through the medium as at the said first position.

12. Erasing means according to claim 11, in which the field strength at the said third position is lower by a predetermined amount than the field strength at the said second position and selected so as to tend to return the magnetic state of the medium to the origin of the B-H curve applicable thereto.

13. Erasing means according to claim 11 or 12, in which the unidirectional magnetic field generating means comprises a magnetic core which is E-shaped to provide pole faces at each of the three positions, and in which the further means comprises magnetic keeper means facing and on the opposite side of the tape to faces all the said pole faces.

14. Erasing means according to any one of claims 6 to 13, in which the or each said field is generated by an electrically energisable coil.

15. Erasing means according to any one of claims 6 to 13, in which the or each said field is generated by a permanent magnet.

16. A magnetic tape erasing head, substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

17. A magnetic erasing head, substantially as described with reference to Figure 3 of the accompanying drawings.

18. A magnetic erasing head, substantially as described with reference to Figure 4 of the accompanying drawings.

19. A method of magnetic erasing, substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

20. A method of magnetic erasing, substantially as described with reference to Figure 3 of the accompanying drawings.

21. A method of magnetic erasing, substantially as described with reference to Figure 4 of the accompanying drawings.