Magnetic recording/reproducing apparatus and magnetic tape

A magnetic recording/reproducing apparatus of the helical scan system employing a MR head as a reproducing magnetic head. The magnetic recording/reproducing apparatus includes a MR head 6 and a rotary drum carrying the MR head 6. In reproducing signals from the magnetic tape 7 in accordance with the helical scan system, a magnetic tape 7, having a large number of projections 7c on the tape surface and containing an electrically conductive material in its magnetic layer, is used as a recording medium.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a helical scan magnetic recording/reproducing
 apparatus and a magnetic tape used in the helical scan magnetic
 recording/reproducing apparatus.
 2. Description of the Related Art
 In a magnetic recording/reproducing apparatus employing a magnetic tape as
 a recording medium, such as a video tape recorder, an audio tape recorder
 or a data storage system for a computer, a helical scan system is used to
 raise the recording density to increase the recording capacity.
 For this type of the magnetic recording/reproducing apparatus, a demand is
 raised for further increasing the recording density and the recording
 capacity. To this end, there is proposed in the helical scan magnetic
 recording/reproducing apparatus a technique of using a magneto-resistive
 effect magnetic head (MR head) as a playback magnetic head.
 The MR head is a magnetic head employing a magneto-resistive effect element
 (MR element) in its magnetically sensitive portion. It has a sensitivity
 higher than with an inductive magnetic head and can develop a large
 playback output. Thus, by using the MR head as a playback magnetic head, a
 higher recording density and a higher recording capacity can be realized.
 However, if, in the helical scan magnetic recording/reproducing apparatus,
 the MR head is substituted for the inductive magnetic head for use as a
 playback magnetic head, a number of problems, that have not been
 experienced with the inductive magnetic head, are raised.
 Specifically, the magnetic recording/reproducing apparatus is lowered
 significantly in durability such that a practically tolerable durability
 cannot be obtained. Moreover, there may be raised such problems as failure
 to output reproduced signals (dropout) notwithstanding the high playback
 output from the MR head.
 Thus, the helical scan magnetic recording/reproducing apparatus, employing
 a MR head as a playback magnetic head, has much to be desired, such that a
 practically usable apparatus has not been realized.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a magnetic
 recording/reproducing apparatus usable as a helical scan magnetic
 recording/reproducing apparatus employing a MR head as a playback magnetic
 head.
 It is another object of the present invention to provide a magnetic tape
 which renders it possible to realize a helical scan magnetic
 recording/reproducing apparatus employing a MR head as a playback magnetic
 head.
 In one aspect, the present invention provides a magnetic
 recording/reproducing apparatus for reproducing signals by a helical scan
 system from a magnetic tape having a magnetic layer formed on a
 non-magnetic substrate thereof, including a magneto-resistive effect
 magnetic head for reproducing signals from the magnetic tape, and a rotary
 drum carrying the magneto-resistive effect magnetic head, the magnetic
 tape being such a magnetic tape having a large number of projections on
 the tape surface and containing an electrically conductive material in the
 magnetic layer.
 Since the present magnetic recording/reproducing apparatus uses a magnetic
 tape containing an electrically conductive material as a magnetic layer,
 static charges are not likely to be accumulated on the magnetic tape, so
 that the magnetic head is less likely to be produced due to static
 destruction. Also, since the present magnetic recording/reproducing
 apparatus uses a magnetic tape having a number of projections formed on
 its tape surface, the true contact area between the magnetic tape and the
 MR head is extremely small, so that it is unlikely that the sense current
 which should flow through the MR head 6 flows through the magnetic tape to
 produce electrical shorting.
 Also, in the magnetic recording/reproducing apparatus, the
 magneto-resistive effect magnetic head includes a MR device as a
 magnetically sensitive device for detecting signals from the magnetic
 tape. One of the terminals derived from the MR device is preferably
 connected to the rotary drum. If the static electricity accumulated in the
 magnetic tape is discharged, any excess current produced by the discharge
 is allowed to exit efficiently towards the rotary drum to render
 destruction of the MR device less prone to destruction otherwise caused by
 static charges.
 In another aspect, the present invention provides a magnetic tape used in a
 magnetic recording/reproducing apparatus adapted for reproducing signals
 in accordance with a helical scan system using a magneto-resistive effect
 magnetic head, including a tape-shaped non-magnetic substrate, and a
 magnetic layer formed on the non-magnetic substrate and containing an
 electrically conductive material, there being formed a large number of
 projections on a surface of the tape facing the magneto-resistive effect
 magnetic head.
 With the present magnetic tape, in which the electrically conductive
 material is contained in its magnetic layer, static charges are less
 likely to be accumulated, so that it is unlikely that the MR head be
 destructed by discharge of the static charges. Also, since a number of
 projections are formed on the tape surface, the true contact area between
 the magnetic tape and the MR head is extremely small. The result is that
 there is only little risk of the sense current flowing in the magnetic
 tape instead of through the MR device to produce electrical shorting.
 With the magnetic recording/reproducing apparatus according to the present
 invention, static destruction of the MR head can be prohibited, using the
 MR head of the helical scan system, to improve durability, while reducing
 occurrences of dropout.
 With the magnetic tape of the present invention, it is possible to prevent
 static destruction of the MR head 6 of the magnetic recording/reproducing
 apparatus of the helical scan system employing a MR head as the
 reproducing magnetic head, to improve its durability, while reducing
 occurrences of dropout.
 Thus, in accordance with the present invention, a MR head that is able to
 develop a high sensitivity and a high output can be used as a reproducing
 magnetic head loaded on the magnetic recording/reproducing apparatus of
 the helical scan system, thus assuring a higher recording density and a
 larger recording capacity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to the drawings, preferred embodiments of the present invention
 will be explained in detail.
 A magnetic recording/reproducing apparatus according to the present
 invention is such a magnetic recording/reproducing apparatus employing a
 magnetic tape as a recording medium, and is used as a video tape recorder,
 an audio tape recorder, a data storage system for computer data etc. The
 magnetic recording/reproducing apparatus according to the present
 invention is a helical scan magnetic recording/reproducing apparatus
 employing a rotary drum for recording/reproduction, and uses a MR head as
 a reproducing magnetic head loaded on a rotary drum.
 FIGS. 1 and 2 show an illustrative structure of a rotary drum device loaded
 on a magnetic recording/reproducing apparatus embodying the present
 invention. FIGS. 1 and 2 are a perspective view schematically showing a
 rotary drum device land a plan view showing a magnetic tape feed unit 10
 inclusive of the rotary drum device 1.
 Referring to FIG. 1, the rotary drum device 1 includes a
 cylindrically-shaped stationary drum 2, a cylindrically-shaped rotary drum
 3, a motor 4 for running the rotary drum 3 in rotation, a pair of
 inductive magnetic heads 5a, 5b, and a pair of MR heads 6a, 6b loaded on
 the rotary drum 3.
 The stationary drum 2 is held without performing the rotation. On a lateral
 side of the stationary drum 2 is formed a reel guide unit 8 for extending
 along the running direction of a magnetic tape 7. During
 recording/reproduction, this magnetic tape 7 runs along the reel guide
 unit 8. The rotary drum 3 is arranged so that its center axis coincides
 with that of the rotary drum 3.
 The rotary drum 3 is a drum run in rotation at a pre-set rpm by the motor 4
 during recording/reproduction for the magnetic tape 7. This rotary drum 3
 is formed in a cylindrical shape of substantially the same diameter as the
 stationary drum 2 and is arranged so that its center axis coincides with
 that of the stationary drum 2. On the side of the rotary drum 3 facing the
 stationary drum 2 are loaded the paired inductive magnetic heads 5a, 5b
 and a pair of MR heads 6a, 6b.
 The inductive magnetic heads 5a, 5b are recording magnetic heads in which a
 pair of magnetic cores are joined together with a magnetic gap in-between
 and in which a coil is placed around the magnetic cores. These inductive
 magnetic heads 5a, 5b are used for recording signals on the magnetic tape
 7. The inductive magnetic heads 5a, 5b are loaded on the rotary drum 3 to
 define an angle of 180.degree. relative to the center of the rotary drum 3
 so that magnetic gap portions thereof are projected from the outer rim of
 the rotary drum 3. The inductive magnetic heads 5a, 5b are set to opposite
 azimuth angles in order to effect azimuth recording on the magnetic tape
 7.
 The MR heads 6a, 6b are playback magnetic heads having MR elements as
 magnetically sensitive elements for detecting signals from the magnetic
 tape 7. The MR heads 6a, 6b are used for reproducing signals form the
 magnetic tape 7. The MR heads 6a, 6b are loaded on the rotary drum 3 to
 define an angle of 180.degree. relative to the center of the rotary drum 3
 so that magnetic gap portions thereof are projected from the outer rim of
 the rotary drum 3. The MR heads 6a, 6b are set to opposite azimuth angles
 in order to effect azimuth recording on the magnetic tape 7.
 In the present magnetic recording/reproducing apparatus, the magnetic tape
 7 is slid along the outer surface of the rotary drum device 1 to reproduce
 signals from the magnetic tape 7.
 Referring to FIG. 2, the magnetic tape 7 is fed, during
 recording/reproduction, from a supply reel 11 through guide rolls 12, 13,
 so as to be coiled around the rotary drum device 1 for
 recording/reproduction. The magnetic tape 7, recorded or reproduced by the
 rotary drum device 1, is sent to a take-up roll 18 via guide rolls 14, 15
 and capstans 16, 17. The magnetic tape 7 is fed under a preset tension and
 a preset speed by the capstan 16, run in rotation by the capstan motor 19,
 so as to be taken up on the take-up roll 18 via guide roll 17.
 The rotary drum 3 is run in rotation by the motor 4 in the direction
 indicated by arrow A in FIG. 1. On the other hand, the magnetic tape 7 is
 fed for sliding obliquely relative to the stationary drum 2 and the rotary
 drum 3 along the reel guide unit 8 of the stationary drum 2. That is, the
 magnetic tape 7 is sent from the tape inlet side along the reel guide unit
 8 of the stationary drum 2 in sliding contact with the stationary drum 2
 and the rotary drum 3, along the tape running direction, in the direction
 indicated by arrow B in FIG. 1, so as to be then fed towards the tape
 output side, as indicated by arrow C in FIG. 1.
 The internal structure of the rotary drum device 1 is explained with
 reference to FIG. 3.
 Referring to FIG. 3, a rotary shaft 21 is inserted at the center of the
 stationary drum 2 and the rotary drum 3. The drums 2, 3 and the rotary
 shaft 21 are formed of an electrically conductive material and are
 electrically connected to one another, with the stationary drum 2 being
 grounded, although not shown.
 Within the inside of the sleeve of the stationary drum 2 are mounted a pair
 of bearings 22, 23, whereby the rotary shaft 21 is rotatably supported
 relative to the stationary drum 2. On the other hand, the rotary shaft 21
 is rotatably supported by the bearings 22, 23 relative to the stationary
 drum 2. On the other hand, the inner rim of the rotary drum 3 is formed
 with a flange 24 secured to the upper end of the rotary shaft 21. Thus,
 the rotary drum 3 is adapted to be rotated with rotation of the rotary
 shaft 21.
 Within the inside of the rotary drum device 1 is arranged a rotary
 transformer 25, which is a non-contact signal transmission device adapted
 for transmitting signals between the stationary drum 2 an the rotary drum
 3. This rotary transformer 25 includes a stator core 26 mounted on the
 stationary drum 2 and a rotor core 27 mounted on the rotary drum 3.
 The stator core 26 and the rotor core 27 are formed of a magnetic material,
 such as ferrite, and are formed by toroids centered about the rotary shaft
 21. On the stator core 26, there are concentrically arranged a pair of
 rings for signal transmission 26a, 26b, associated with the paired
 inductive magnetic heads 5a, 5b, a ring for signal transmission 26c
 associated with the paired MR heads 6a, 6b, and a ring for power
 transmission 26d for supplying power necessary for driving the paired MR
 heads 6a, 6b. On the rotor core 27, there are similarly concentrically
 arranged a pair of rings for signal transmission 27a, 27b, associated with
 the paired inductive magnetic heads 5a, 5b, a ring for signal transmission
 27c associated with the paired MR heads 6a, 6b, and a ring for power
 transmission 27d for supplying power necessary for driving the paired MR
 heads 6a, 6b.
 These rings 26a, 26b, 26c, 26d, 27a, 27b, 27c, 27d are coils toroidally
 wound about the rotary shaft 21 as center, with the rings 26a to 26d of
 the stator core 26 facing the rings 27a to 27d of the rotor core 27. The
 rotary transformer 25 is configured for transmitting signals or the power
 in a non-contact fashion between the rings 26a to 26d of the stator core
 26 and the rings 27a to 27d of the rotor core 27.
 On the rotary drum device 1 is mounted a motor 4 for rotationally driving
 the rotary drum 3. This motor 4 has a rotor 28 as a rotary portion and a
 stator 29 as a stationary portion. The rotor 28 is mounted on the lower
 end of the rotary shaft 21 and includes a driving magnet 30. On the other
 hand, the stator 29 is mounted on the lower end of the stationary drum 2
 and includes a driving coil 31. The rotor 28 is rotationally driven by the
 current being supplied to the driving coil 31. This rotates the rotary
 shaft 21 mounted on the rotor 28 to cause rotation of the rotary drum 3
 secured to the rotary shaft 21.
 The recording/reproduction by the above-described rotary drum device 1 is
 explained with reference to FIG. 4 schematically showing the circuit
 structure of the rotary drum device 1 and its peripheral circuit.
 For recording signals on the magnetic tape 7 using the rotary drum device
 1, the current is first fed to the driving coil 31 of the motor 4 for
 rotationally driving the rotary drum 3. While the rotary drum 3 is being
 run in rotation, recording signals from an external circuit 40 are sent to
 a recording amplifier 41, as shown in FIG. 4.
 The recording amplifier 41 amplifies the recording signals from the
 external circuit 40 to route the recording signals to the ring for signal
 transmission 26a of the stator core 26, associated with the inductive
 magnetic head 5a at a timing of recording signals by the inductive
 magnetic head 5a, while routing the recording signals to the ring for
 signal transmission 26b of the stator core 26, associated with the
 remaining inductive magnetic head 5b, at a timing of recording signals by
 the inductive magnetic head 5b.
 The paired inductive magnetic heads 5a, 5b are arranged at an angle of
 180.degree. relative to each other with respect to the center of the
 rotary drum 3, as described above. Thus, these inductive magnetic heads
 5a, 5b alternately record signals with a phase difference of 180.degree..
 The recording amplifier 41 switches between the timing of supplying
 recording signals to the inductive magnetic head 5a and the timing of
 supplying recording signals to the inductive magnetic head 5b with a phase
 difference of 180.degree..
 The recording signals, sent to the ring for signal transmission 26a of the
 stator core 26, associated with the inductive magnetic head 5a, are
 transmitted in a contact-free fashion to the ring for signal transmission
 27a of the rotor core 27. The recording signals, transmitted to the ring
 for signal transmission 27a of the rotor core 27, are sent to the
 inductive magnetic head 5a which then records the signals on the magnetic
 tape 7.
 Similarly, the recording signals, sent to the ring for signal transmission
 26b of the stator core 26 associated with the inductive magnetic head 5b,
 are transmitted in a contact-free fashion to the ring for signal
 transmission 27b of the rotor core 27. The recording signals, transmitted
 to the ring for signal transmission 27b of the rotor core 27, are sent to
 the inductive magnetic head 5b which then records the signals on the
 magnetic tape 7.
 For reproducing signals from the magnetic tape 7, using the rotary drum
 device 1, the current is first fed o the driving coil 31 of the motor 4 to
 run the rotary drum 3 in rotation. With the rotary drum 3 being run in
 rotation, the high frequency current is sent from an oscillator 42 to a
 power driver 43.
 The high frequency current from the oscillator 42 is converted by a power
 drive 43 to a pre-set ac current which is sent to the ring for signal
 transmission 26d of the stator core 26. The ac current, transmitted to the
 ring for signal transmission 27c of the rotor core 27, is rectified by a
 rectifier 44 to a dc current which is sent to a regulator 45 so as to be
 thereby set to a pre-set voltage.
 The current set to the pre-set voltage by the regulator 45 is sent as the
 sense current to the paired MR heads 6a, 6b. To these MR heads 6a, 6b is
 connected a reproducing amplifier 46 adapted for detecting the signals
 from the MR heads 6a, 6b. The current from the regulator 45 is also sent
 to this reproducing amplifier 46.
 The MR heads 6a, 6b each include a MR element whose resistance is changed
 with the magnitude of the external magnetic field. With the MR heads 6a,
 6b, the resistance value of the NR element is changed by the signal
 magnetic field from the magnetic tape 7 so that the voltage changes are
 induced in the sense current.
 The reproducing amplifier 46 detects the voltage changes to output signals
 proportionate to the voltage changes as playback signals.
 The paired MR heads 6a, 6b are arranged at an angle of 180.degree. relative
 to each other with respect to the center of the rotary drum 3, as
 described above. Thus, these MR heads 6a, 6b alternately record signals
 with a phase difference of 180.degree.. The playback amplifier 46 switches
 between the timing of outputting the playback signals from the MR head 6a
 and the timing of outputting the playback signals from the MR head 6b with
 a phase difference of 180.degree..
 The playback signals from the reproducing amplifier 46 are sent to the ring
 for signal transmission 27c of the rotor core 27 and thence transmitted in
 a contact-free fashion to the ring for signal transmission 26c of the
 stator core 26. The playback signals transmitted to the ring for signal
 transmission 26c of the stator core 26 are amplified by a reproducing
 amplifier 47 and thence supplied to a correction circuit 48. The playback
 signals are corrected in a pre-set manner by the correction circuit 48 so
 as to be outputted to the external circuit 40.
 Meanwhile, in the circuit configuration shown in FIG. 4, the paired
 inductive magnetic heads 5a, 5b, paired MR heads 6a, 6b, rectifier 44,
 regulator 45 and the reproducing amplifier 46 are loaded on the rotary
 drum 3 so as to be rotated with the rotary drum 3. The recording amplifier
 41, oscillator 42, power drive 43, reproducing amplifier 47 an the
 correction circuit 48 are arranged on a stationary portion of the rotary
 drum device 1 or are arranged as an external circuit separate from the
 rotary drum device 1.
 The MR heads 6a, 6b, loaded on the rotary drum 3, are explained in detail
 with reference to FIG. 5. The MR heads 6a, 6b are constructed similarly to
 each other except that the azimuth angles are opposite to each other.
 Therefore, in the following description, these MR heads 6a, 6b are
 referred to collectively as a MR head 6.
 The MR head 6 is a read-only magnetic head loaded on the rotary drum 3 and
 which is adapted to detect signals from the magnetic tape 7 by exploiting
 the magneto-resistive effect. In general, the MR head has a sensitivity
 and a playback output larger than those of the inductive magnetic head
 which performs recording/reproduction by exploiting the electromagnetic
 induction. Thus, the MR head is suited to high density recording. Thus, a
 higher recording density can be realized by using the MR head 6 as the
 reproducing magnetic head.
 This MR head 6 has a pair of magnetic shields 51, 52, formed of a soft
 magnetic material, such as N--Zn ferrites, and a substantially rectangular
 MR element unit 54, sandwiched between paired magnetic shields 51, 52 via
 an insulator 53, as shown in FIG. 5. From both ends of the MR element unit
 54 are derived a pair of terminals, via which the sense current can be
 supplied to the MR element unit 54.
 The MR element unit 54 is made up of a MR device exhibiting
 magneto-resistive effect a soft adjacent layer (SAL) film and an
 insulating film interposed between the MR element and the SAL film. The MR
 device 6, SAL film and the insulating films are layered together. The MR
 device is formed of a soft magnetic material, such as Ni--Fe, whose
 resistance value is changed with the strength of the external magnetic
 field. The SAL film is used to apply the bias magnetic field in accordance
 with the so-called bias system, and is made up of a magnetic material of
 low coercivity and high magnetic permeability, such as Permalloy. The
 insulating film is used to insulate the MR device and the SAL film from
 each other to prohibit electrical current division loss and is formed of
 an insulating material, such as Ta.
 This MR element unit 54 is substantially rectangular in profile and is
 sandwiched between a pair of magnetic shields 51, 52, with an insulator 53
 in-between, so that the MR element unit 54 has its one side exposed to a
 magnetic tape sliding surface 55. Specifically, the MR element unit 54 is
 sandwiched between the paired magnetic shields 51, 52, with the insulator
 53 in-between, so that its short-axis direction is substantially
 perpendicular to the magnetic tape sliding direction and so that its
 long-axis will be substantially perpendicular to the magnetic tape sliding
 direction.
 The magnetic tape sliding surface 55 of the MR head 6 is ground to a
 cylindrical surface, along the sliding direction of the magnetic tape 7,
 so that the MR element unit 54 has its lateral side exposed to the
 magnetic tape sliding surface 55. The magnetic tape sliding surface 55 is
 also ground to a cylindrical surface along the sliding direction of the
 magnetic tape 7. Thus, the MR head 6 is protruded most significantly at
 the MR element unit 54 or its near-by portion. By having the MR element
 unit 54 or its near-by portion protruded most significantly, the MR
 element unit 54 can be improved in its abutting characteristics with
 respect to the magnetic tape 7.
 For reproducing signals from the magnetic tape 7, using the above-described
 MR head 6, the magnetic tape 7 is slid along the MR element unit 54, as
 indicated in FIG. 6, in which an arrow schematically indicates the manner
 of magnetization of the magnetic tape 7.
 With the magnetic tape 7 this slid against the MR element unit 54, the
 sense current is fed to the MR element unit 54, via terminals 54a, 54b
 connected to both ends of the MR element unit 54, to detect changes in
 voltage of the sense current. Specifically, a pre-set voltage VC is
 applied at the terminal 54a connected to one end of the MR element unit
 54. The terminal 54, connected to the opposite end of the MR element unit
 54, is connected to the rotary drum 3. the rotary drum 3 is electrically
 connected via rotary shaft 21 to the stationary drum 2, while the
 stationary drum 2 is grounded, in a manner not shown. Thus, the terminal
 54b, connected to the MR element unit 54, is grounded via the rotary drum
 3, rotary shaft 21 and the stationary drum 2.
 If, as the magnetic tape 7 is slid, the sense current is sent to the MR
 element unit 54, the resistance value of the MR device formed in the MR
 element unit 54 is changed to produce voltage changes in the sense
 current. By detecting the voltage change in the sense current, the signal
 magnetic field from the magnetic tape 7 is detected to reproduce signals
 recorded on the magnetic tape 7.
 In the MR head 6 used in the present invention, it suffices if the MR
 device formed in the MR element unit 54 is a device exhibiting the
 magneto-resistive effect. For example, a so-called giant magneto-resistive
 effect device (GNR device), exhibiting a higher magneto-resistive effect
 by layering plural thin films, may be used. The technique of applying the
 bias magnetic field across the MR device need not be the SAL bias system,
 since a variety of techniques, such as permanent magnet bias system, a
 shunt current bias system, a self-bias system, an exchange bias system, a
 barber pole system, a split device system or a servo bias system, may be
 used. The giant magneto-resistive effect and the various bias systems are
 explained in detail in, for example, "Magneto-resistive Head--Its
 Fundamentals and Application, translated by K. Hayash", published by
 Maruzen Co. Ltd.
 The magnetic tape 7, used in the magnetic recording/reproducing apparatus
 having the rotary drum device 1 as described above will be hereinafter
 explained.
 The magnetic tape 7 includes a non-magnetic substrate 7a, comprised of a
 plastics film wound in a tape, and a magnetic layer 7b formed thereon, as
 shown in FIG. 7.
 The magnetic layer 7b of the magnetic tape 7 contains an electrically
 conductive material so that its surface resistance to current conduction
 is on the order of 1-k.OMEGA. or less. In general, a material having the
 surface resistance to current conduction not higher than approximately 100
 k.OMEGA. is termed an electrically conductive material. With an
 electrically conductive material, having a lower surface resistance to
 current conduction, static charges are scarcely produced on its surface.
 As a material for the magnetic layer 7b, a metal magnetic material is
 preferred. Specifically, the magnetic layer 7b is formed by depositing a
 ferromagnetic metal material, such as Co, CO--Cr, Co--Ni, Co--Fe--Ni or
 Co--Ni--Cr by deposition techniques such as vacuum vapor deposition,
 sputtering or ion plating, on the non-magnetic substrate 7a. This gives
 the magnetic layer 7b containing an electrically conductive material and
 which has a sufficient small surface resistance to current conduction.
 On the surface of the magnetic tape are formed a large number of
 micro-sized projections 7c. For forming these numerous micro-sized
 projections 7c, a large number of micro-sized projections are previously
 formed on the surface of the nonmagnetic substrate 7a and the magnetic
 layer 7b is then formed thereon. This gives a large number of micro-sized
 projections 7c on the surface of the magnetic tape 7, that is on the
 surface of the magnetic layer 7b.
 Among the methods of forming a large number of micro-sized projections on
 the surface of the non-magnetic substrate 7a, there are a method of
 dispersing fillers of a pre-set size in the starting material for the
 non-magnetic substrate 7a and aggregating the filler to a pre-set density
 at the time of manufacturing the non-magnetic substrate 7a to relieve it
 on the surface of the non-magnetic substrate 7a to provided an irregular
 surface of the non-magnetic substrate 7a, and a method of dispersing fine
 particles of a pre-set particle size on the non-magnetic substrate 7a to
 fix it such as with a binder resin.
 Meanwhile, it is unnecessary for the magnetic tape 7 of the present
 invention to be constituted solely by the non-magnetic substrate 7a and
 the magnetic layer 7b. That is, the magnetic layer 7b may be formed on an
 undercoat previously formed on the non-magnetic substrate 7a, or a
 backcoat layer may be formed on the back side of the non-magnetic
 substrate 7a. If the surface resistance to current conduction is
 sufficiently small, the magnetic layer 7b need not be limited to the
 above-described structure. For example, the magnetic layer 7b may be of a
 dual structure for improving the electromagnetic conversion
 characteristics.
 Next, signal reproduction from the magnetic tape 7 by the MR head 6 loaded
 on the rotary drum device 1 is explained in further detail.
 The MR head has already been commercialized as a hard disc drive etc. In
 the conventional hard disc drive, a MR head is loaded on a floating slider
 and signal reproduction is performed with the MR head being floated above
 the magnetic disc. If the MR head 6 is slid on the magnetic tape 7, a
 number of problems, not encountered in conventional hard disc drive, are
 met. The present invention is aimed at obviating the problem caused on
 sliding the MR head 6 on the magnetic tape 7.
 For reproducing signals from the magnetic tape 7 by the MR head 6, the MR
 head 6 is slid obliquely relative to the magnetic tape 7, as described
 above. At this time, the MR element unit 54 is positioned at the distal
 end of the MR head 6 and is slid in perpetual contact with the magnetic
 tape 7.
 The MR device formed on the MR element unit 54 is electrically conductive
 and is destructed if excess current flows therethrough. If, when the MR
 head 6 is in contact with the magnetic tape 7, static charges are
 accumulated on the magnetic tape 7 such that the static charges are
 discharged, the MR device is destructed by so-called static destruction.
 Thus, in the rotary drum device 1 embodying the present invention, the
 terminal 54b, connected to the MR element unit 54, is grounded via the
 rotary drum 3, rotary shaft 21 and the stationary drum 2. If, with the
 rotary drum device 1, static electricity is discharged, any excess current
 produced on discharge is allowed to exit efficiently via the rotary drum
 3, rotary shaft 21 and the stationary drum 2. This renders difficult the
 destruction of the MR device by static destruction.
 The present invention also employs the magnetic tape 7 containing the
 electrically conductive material as the magnetic layer 7b as the recording
 medium. The magnetic tape 7, containing the electrically conductive
 material in its magnetic layer 7b, is less susceptible to discharging of
 static electricity because the magnetic tape 7 can hardly accumulate the
 static electricity. Thus, according to the present invention, destruction
 of the MR head 6 is not liable to be produced.
 However, if the magnetic tape 7 containing the electrically conductive
 material in its magnetic layer 7b, the sense current, which should flow
 through the MR device of the MR element unit 54, flows in the magnetic
 tape 7, thus possibly producing electrical shorting. Specifically, the
 sense current flowing through the magnetic tape 7 tends to produce
 shorting across the terminals 54a and 54b connected to the MR element unit
 54, that across the terminal 54a or 54b connected to the MR element unit
 54 and the MR device formed in the MR element unit 54 or that across the
 MR devoice and the SAL film. Such shorting leads to the absence of an
 output of the playback signals to produce so-called dropout, thus, the
 present invention uses the magnetic tape 7 having plural projections 7c on
 its surface as a recording medium. If the magnetic tape 7 having numerous
 projections 7c on its surface is used, the contact state between the
 magnetic tape 7 and the MR head 6 is shown in FIG. 8. that is, the
 magnetic tape 7 and the MR head 6 are contacted only at the micro-sized
 projections 7c so that the real contact area between the magnetic tape 7
 and the MR head 6 is extremely small. The aforementioned shorting then is
 not likely to be produced. That is, according to the present invention,
 the real contact area between the magnetic tape 7 and the MR head 6 is
 reduced to an extremely small value, by the provision of the numerous
 micro-sized projections 7c, thereby prohibiting the occurrence of the
 shorting.