Abstract:
Disclosed herein is a rotary head drum unit including a cylindrical drum surface and a magnetic head for recording/reproducing a signal to/from a magnetic tape running along the drum surface.. The magnetic head has a tape sliding surface adapted to come into sliding contact with the magnetic tape. The tape sliding surface of the magnetic head is positioned at a level lower than the height of the magnetic tape flying above the drum surface. The magnetic tape is brought into contact with the tape sliding surface by negative pressure. With this configuration, an increase in contact pressure of the magnetic tape sliding on the tape sliding surface of the magnetic head can be prevented and the contact pressure can be uniformed to thereby prevent a reduction and variations in magnetic recording or reproduction output.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a rotary head drum unit and a magnetic tape drive, and more particularly to a technique for improving a contact condition between a magnetic head and a magnetic tape.  
           [0002]    A rotary head drum unit is used to record a signal to a magnetic tape as a tape-like magnetic recording medium or reproduce a signal recorded on the magnetic tape. Such a rotary head drum unit is mounted in a video tape recorder, for example, and this unit is composed of a fixed drum and a rotating drum. The rotating drum is provided with a magnetic head, so that when the rotating drum is rotated in one direction, the magnetic head comes into sliding contact with the magnetic tape to thereby record or reproduce a signal.  
           [0003]    Referring to FIG. 22, there is shown a rotary head drum unit a in the related art. The rotary head drum unit a has a fixed drum b fixed to a chassis (not shown) and a rotating drum c having substantially the same outer diameter as that of the fixed drum b.  
           [0004]    The rotating drum c is fixed to a rotating shaft (not shown) rotatable relative to the fixed drum b. The fixed drum b and the rotating drum c are axially opposed to each other with a given gap defined therebetween. The rotating shaft is rotated by a motor (not shown).  
           [0005]    A plurality of recesses (which will be hereinafter referred to as “head mounting holes”) d are formed along the lower circumferential edge of the rotating drum c so as to be spaced apart from each other at given intervals in the circumferential direction. A magnetic head e is positioned in each head mounting hole d.  
           [0006]    As shown in FIGS. 22 and 24, each magnetic head e projects from the outer circumferential surface of the rotating drum c.  
           [0007]    The amount of projection of each magnetic head e from the outer circumferential surface of the rotating drum c is set larger than the flying height of a magnetic tape h above a drum surface as the cylindrical surface of the rotary head drum unit a, thereby obtaining a contact pressure of the magnetic tape h to a tape sliding surface i of the magnetic head e as will be hereinafter described.  
           [0008]    As shown in FIGS. 23 and 24, each magnetic head e is mounted on a head substrate f, and each head substrate f is fixed to the rotating drum c at an arbitrary position in such a manner that each magnetic head e projects from the corresponding head mounting hole d.  
           [0009]    The outer circumferential surface of the fixed drum b is formed with a lead guide portion g extending substantially helically for guiding the lower edge of the magnetic tape h helically wrapped around the rotary head drum unit a during running of the magnetic tape h.  
           [0010]    When tape loading is carried out, the rotary head drum unit a is rotated and the magnetic tape h wrapped a given angle around the rotary head drum unit a is run in a given direction (see FIG. 22).  
           [0011]    In the rotary head drum unit a, the rotating drum c is rotated in the condition where the magnetic tape h is running along the lead guide portion g, thereby making each magnetic head e scan the running magnetic tape h in a direction inclined a given angle with respect to the longitudinal direction of the magnetic tape h. Accordingly, the rotary head drum unit a forms a recording track extending in the direction inclined the given angle with respect to the longitudinal direction of the magnetic tape h.  
           [0012]    When the rotating drum c is rotated, air is introduced between the drum surface of the rotary head drum unit a and the magnetic tape h, so that the magnetic tape h runs at a given flying height above the drum surface of the rotary head drum unit a (see FIG. 24).  
           [0013]    This is due to the fact that an air layer is formed between the drum surface of the rotary head drum unit a during rotation and the magnetic tape h during running, and that a pressure difference between this air layer and the atmospheric air existing on the opposite side of this air layer with respect to the magnetic tape h is held constant in relation to a tape tension or the like of the magnetic tape h, thereby obtaining a given flying height of the magnetic tape h above the drum surface of the rotary head drum unit a during running of the magnetic tape h.  
           [0014]    For example, in the case of a magnetic tape drive system having such specifications that the width of the magnetic tape h is 8 mm, the diameter of the rotating drum c is 40 mm, and the running speed of the magnetic tape h relative to each magnetic head e is 10 m/sec, it is said that the flying height of the magnetic tape h above the drum surface is usually 10 μm or more.  
           [0015]    The amount of projection of each magnetic head e from the drum surface is set larger than the flying height of the magnetic tape h above the drum surface. Accordingly, when the magnetic tape h comes into sliding contact with each magnetic head e projecting from the drum surface of the rotary head drum unit a, the magnetic tape h forms a tent-shape at each magnetic head e according to the amount of projection of each magnetic head e (see FIGS. 22 and 24).  
           [0016]    That is, the magnetic tape h is urged by each magnetic head e to form a ridge along the tape sliding surface i of each magnetic head e. In this condition where the tent-shape is formed in the magnetic tape h, a signal is recorded or reproduced.  
           [0017]    The formation of the tent-shape in the magnetic tape h means that a portion of the magnetic tape h corresponding to the tape sliding surface i of each magnetic head e is curved. Accordingly, there is a possibility that the magnetic tape h may be partially separated from a central portion of the tape sliding surface i of each magnetic head e.  
           [0018]    To cope with this, the tape sliding surface i of each magnetic head e is sometimes formed into a curved surface so as to stabilize the contact condition of the magnetic tape h to the tape sliding surface i, i.e., to uniform the contact pressure therebetween.  
           [0019]    However, the tape sliding surface i of each magnetic head e has a size of 1 mm×100 μm, for example, so that it is not necessarily easy to form the tape sliding surface i into a curved surface.  
           [0020]    Further, the tape sliding surface i may be formed into a curved surface only by predicting the curvature of the tent-shaped sliding portion of the magnetic tape h. However, the curvature of the tape sliding surface i does not always become equal to the curvature of the tent-shaped sliding portion of the magnetic tape h in actual.  
           [0021]    In particular, the tent-shape of the magnetic tape h depends greatly on the stiffness of the magnetic tape h. FIG. 25 shows a contact condition of a magnetic tape j having a high stiffness (which will be hereinafter referred to as “high-stiffness tape”) to the tape sliding surface i and a contact condition of a magnetic tape k having a low stiffness (which will be hereinafter referred to as “low-stiffness tape”) to the tape sliding surface i. In the case that the shape of the tape sliding surface i is so designed as to be adapted to the high-stiffness tape j, the low-stiffness tape k may be separated from a central portion of the tape sliding surface i. Conversely, in the case that the shape of the tape sliding surface i is so designed as to be adapted to the low-stiffness tape k, the high-stiffness tape j may come into contact with only a central portion of the tape sliding surface i.  
           [0022]    The low-stiffness tape k is so curved as to follow the shape of the projecting magnetic head e more than the high-stiffness tape j, so that the slope of the tent-shape is steeper and the curvature of a top portion of the tent-shape (a portion facing the tape sliding surface i) is therefore larger. Accordingly, in the case that the shape of the tape sliding surface i is so designed as to be adapted to the high-stiffness tape j, there arises a problem that the low-stiffness tape k does not come into contact with the tape sliding surface i (see FIG. 25), and the contact of the magnetic tape h and each magnetic head e becomes nonuniform, causing a reduction and variations in recording or reproduction output.  
           [0023]    For the above reason, the contact (contact condition) between each magnetic head e and the magnetic tape h in the initial stage of use of the rotary head drum unit a is not stable, so that “poor contact” may frequently occur. When each magnetic head e is worn after the use of the rotary head drum unit a, the contact condition of the magnetic tape h to each magnetic head e becomes good in many cases.  
           [0024]    Further, there is a case that the tape sliding surface of each magnetic head is polished so as to match the tent-shape by using a polishing tape during the manufacture of the rotary head drum unit. However, this method causes an increase in man-hour, and the stiffness of the polishing tape is not always equal to the stiffness of the actual magnetic tape. The stiffness of the actual magnetic tape differs according to the kind of the tape, the manufacturer of the tape, etc. Therefore, the above method using the polishing tape is not an actual means for solving the problem.  
           [0025]    Other means for solving the problem are described in Japanese Patent Laid-open No. Hei 11-273021 and No. 2001-297417.  
           [0026]    The solution described in the former publication is forming a recess in the vicinity of the gap on a magnetic head, and the solution described in the latter publication is forming a recess in the vicinity of a magnetic head on a drum surface.  
           [0027]    By forming such a recess, the volume of the recess is made larger than that of another portion, thereby obtaining negative pressure in this recess.  
           [0028]    Accordingly, the leading and trailing edges of the tent-shape of the magnetic tape h are attracted into the recess by negative pressure, thereby uniforming the tent-shape to suppress variations in the contact pressure of the magnetic tape to the magnetic head irrespective of a difference in stiffness between magnetic tapes.  
           [0029]    However, in each of the above publications, the magnetic head projects from the drum surface by an amount larger than the flying height of the magnetic tape to form the tent-shape of the magnetic tape. This configuration is basically similar to that shown in FIGS.  22  to  24 .  
           [0030]    With the configuration that the recess is formed on the drum surface in the vicinity of the magnetic head or on the tape sliding surface of the magnetic head, the leading and trailing edges of the tent-shape are attracted into the recess by negative pressure, resulting in positive contact of the magnetic tape to the tape sliding surface to cause an increase in the contact pressure between the magnetic tape and the tape sliding surface of the magnetic head. As a result, there is a possibility of increase in wear or damage of the magnetic head and the magnetic tape.  
           [0031]    Further, in association with a recent demand for high-capacity recording, a magnetic tape is elongated and the thickness thereof is therefore reduced. Such a thinner magnetic tape is lower in stiffness than a conventional normal magnetic tape. Such a low-stiffness magnetic tape is more susceptible to physical variations than a high-stiffness magnetic tape. That is, the contact pressure of the low-stiffness magnetic tape to the magnetic head is easily varied to result in acceleration of a reduction and variations in recording or reproduction output.  
           [0032]    Further, in response to high-speed rotation of the rotary head drum unit a required for high-speed transfer of data and broadening of the band of recording and reproduction signals, it is required to suppress variations in the contact condition of the magnetic tape to the tape sliding surface of the magnetic head, and the conditions for obtaining a good contact condition are narrowed.  
           [0033]    While the rotary head drum unit a in the related art mentioned above is of an upper-drum rotating type, the above problem may be included also in lower-drum rotating type-and intermediate-drum rotating type rotary head drum units.  
         SUMMARY OF THE INVENTION  
         [0034]    It is accordingly an object of the present invention to provide a rotary head drum unit which can prevent an increase in contact pressure of a magnetic tape sliding on a tape sliding surface of a magnetic head and can suppress variations in the contact pressure, thereby preventing a reduction and variations in magnetic recording or reproduction output.  
           [0035]    In accordance with an aspect of the present invention, there is provided a rotary head drum unit including a cylindrical drum surface; and a magnetic head for recording/reproducing a signal to/from a magnetic tape running along said drum surface; said magnetic head having a tape sliding surface adapted to come into sliding contact with said magnetic tape; said tape sliding surface of said magnetic head being positioned at a level lower than the height of said magnetic tape flying above said drum surface; said magnetic tape being brought into contact with said tape sliding surface by negative pressure.  
           [0036]    In accordance with another aspect of the present invention, there is provided a magnetic tape drive including a rotary head drum unit having a cylindrical drum surface and a magnetic head for recording/reproducing a signal to/from a magnetic tape running along said drum surface, said magnetic head having a tape sliding surface adapted to come into sliding contact with said magnetic tape; and tape running means for running said magnetic tape along a given path; said tape sliding surface of said magnetic head being positioned at a level lower than the height of said magnetic tape flying above said drum surface; said magnetic tape being brought into contact with said tape sliding surface by negative pressure.  
           [0037]    According to the present invention, the magnetic tape is brought into contact with the tape sliding surface of the magnetic head by negative pressure, so that the contact pressure between the magnetic tape and the magnetic head can be suppressed to thereby reduce wear or damage of the magnetic tape and the magnetic head.  
           [0038]    Further, by preliminarily setting the contact condition of a high-stiffness magnetic tape as the magnetic tape, the compatibility in contact condition with other types of magnetic tapes (various magnetic tapes having different stiffnesses) can be easily provided to thereby support the advent of a low-stiffness magnetic tape in association with a recent tendency to reduce the thickness of a magnetic tape. Accordingly, even in the case of using a low-stiffness magnetic tape, the magnetic tape can be brought into contact with the magnetic head at a given contact pressure to thereby prevent a reduction and variations in recording or reproduction output.  
           [0039]    Further, even in the initial stage of use of the rotary head drum unit, the contact condition between the magnetic tape and the magnetic head can be maintained stable.  
           [0040]    Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    [0041]FIG. 1 is a schematic plan view of a preferred embodiment of the magnetic tape drive according to the present invention;  
         [0042]    [0042]FIG. 2 is a schematic perspective view of a rotary head drum unit according to a first preferred embodiment of the present invention;  
         [0043]    [0043]FIG. 3 is an enlarged-perspective view of a magnetic head mounted in the rotary head drum unit shown in FIG. 2;  
         [0044]    [0044]FIG. 4 is an enlarged sectional view schematically showing a condition where a magnetic tape is in contact with the magnetic head shown in FIG. 3;  
         [0045]    [0045]FIG. 5 is an enlarged plan view schematically showing a manner of running the magnetic tape at a given flying height above a drum surface;  
         [0046]    [0046]FIG. 6 is a view similar to FIG. 5, schematically showing a manner of producing negative pressure over a tape sliding surface of the magnetic head;  
         [0047]    [0047]FIG. 7 is a view similar to FIG. 5, schematically showing a condition where the magnetic tape is brought into contact with the tape sliding surface by negative pressure;  
         [0048]    [0048]FIG. 8 is an enlarged plan view schematically showing a condition where a high-stiffness magnetic tape comes into contact with the magnetic head and a condition where a low-stiffness magnetic tape comes into contact with the magnetic head;  
         [0049]    [0049]FIG. 9 is an enlarged sectional view of a rotary head drum unit according to a second preferred embodiment of the present invention, schematically showing a condition where a magnetic tape is in contact with a magnetic head;  
         [0050]    [0050]FIG. 10 is an enlarged plan view schematically showing a magnetic head according to a third preferred embodiment of the present invention;  
         [0051]    [0051]FIG. 11 is a view similar to FIG. 10, showing a modification of the third preferred embodiment;  
         [0052]    [0052]FIG. 12 is an enlarged plan view schematically showing a magnetic head according to a fourth preferred embodiment of the present invention;  
         [0053]    [0053]FIG. 13 is a view similar to FIG. 12, showing a modification of the fourth preferred embodiment;  
         [0054]    [0054]FIG. 14 is an enlarged elevational view schematically showing a magnetic head according to a fifth preferred embodiment of the present invention;  
         [0055]    [0055]FIG. 15 is an enlarged plan view of the magnetic head shown in FIG. 14;  
         [0056]    [0056]FIG. 16 is a view similar to FIG. 14, showing a modification of the fifth preferred embodiment;  
         [0057]    [0057]FIG. 17 is a view similar to FIG. 14, showing another modification of the fifth preferred embodiment;  
         [0058]    [0058]FIG. 18 is a vertically sectional view showing a contact condition of the magnetic tape to the magnetic head along its width according to the present invention;  
         [0059]    [0059]FIG. 19 is an enlarged view showing the contact condition shown in FIG. 18 for illustrating a case of increasing the width of the magnetic head;  
         [0060]    [0060]FIG. 20 is a perspective view showing a head substrate and a magnetic head having a multi-gap structure;  
         [0061]    [0061]FIG. 21 is an elevational view of the magnetic head having a multi-gap structure shown in FIG. 20;  
         [0062]    [0062]FIG. 22 is a schematic perspective view of a rotary head drum unit in the related art;  
         [0063]    [0063]FIG. 23 is an enlarged perspective view of a head substrate and a magnetic head mounted in the rotary head drum unit shown in FIG. 22;  
         [0064]    [0064]FIG. 24 is a vertically sectional view showing a contact condition of a magnetic tape to the magnetic head shown in FIG. 23; and  
         [0065]    [0065]FIG. 25 is an enlarged view schematically showing a contact condition of a low-stiffness tape and a high-stiffness tape to the magnetic head shown in FIG. 23. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0066]    Some preferred embodiments of the present invention will now be described in detail with reference to the attached drawings.  
         [0067]    Referring first to FIG. 1, there is shown a magnetic tape drive  1  according to a preferred embodiment of the present invention. The magnetic tape drive  1  includes a rotary head drum unit  2 , a plurality of guide pins  5  for drawing a magnetic tape  4  out of a tape cassette  3  and wrapping the drawn magnetic tape  4  around the rotary head drum unit  2  to form a given tape path, a pair of reel supports  7  for supporting a pair of reels  6  accommodated in the tape cassette  3 , a pinch roller  8 , and a capstan shaft  9  for running the magnetic tape  4  in cooperation with the pinch roller  8 .  
         [0068]    Reference numeral  10  denotes a chassis of the magnetic tape drive  1 . The rotary head drum unit  2  is arranged so that its axis is slightly inclined with respect to the chassis  10 . When the tape cassette  3  is loaded into a cassette loading portion of the magnetic tape drive  1 , the magnetic tape  4  is drawn out of the tape cassette  3  toward the rotary head drum unit  2  by the guide pins  5  and next wrapped around the rotary head drum unit  2 . At the same time, the magnetic tape  4  is sandwiched between the pinch roller  8  and the capstan shaft  9  to form a tape path.  
         [0069]    [0069]FIG. 1 schematically shows a condition where the tape cassette  3  is loaded in the magnetic tape drive  1  and the tape path is formed.  
         [0070]    After the tape path is thus formed, the rotary head drum unit  2  is rotated and the capstan shaft  9  is also rotated to thereby run the magnetic tape  4  at a constant speed.  
         [0071]    The guide pins  5 , the pinch roller  8 , and the capstan shaft  9  substantially correspond to tape running means for running a magnetic tape along a given path as defined in claim 10.  
         [0072]    As shown in FIG. 2, the rotary head drum unit  2  includes a fixed shaft  11  fixed to the chassis  10 , a rotating drum  12  rotatably supported through a bearing (not shown) to the fixed shaft  11 , and a fixed drum  13  positioned under the rotating drum  12  and fixed to the chassis  10 . The fixed drum  13  and the rotating drum  12  are axially opposed to each other with a given gap defined therebetween. A plurality of recesses (which will be hereinafter referred to as “head mounting holes”)  14  are formed along the lower circumferential edge of the rotating drum  12  so as to be spaced apart from each other at given intervals in the circumferential direction. A magnetic head  15  is positioned in each head mounting hole  14 .  
         [0073]    The outer circumferential surface of the fixed drum  13  is formed with an upward facing step surface (lead guide portion)  17  extending substantially helically for guiding the lower edge of the magnetic tape  4  helically wrapped around the rotary head drum unit  2  during running of the magnetic tape  4 .  
         [0074]    FIGS.  1  to  8  show a first preferred embodiment of the rotary head drum unit according to the present invention.  
         [0075]    As shown in FIGS. 2 and 4, the magnetic heads  15  mounted in the respective head mounting holes  14  of the rotating drum  12  in the rotary head drum unit  2  slightly project from the outer circumferential surface of the rotating drum  12  as a part of a drum surface  2 a forming the cylindrical surface of the rotary head drum unit  2 .  
         [0076]    As shown in FIGS. 3 and 4, each magnetic head  15  is mounted on a front end surface  18   a  of a head substrate  18 , and each head substrate  18  is fixed to the rotating drum  12  at an arbitrary position in such a manner that each magnetic head  15  slightly projects from the corresponding head mounting hole  14 . Since each magnetic head  15  is mounted on the front end surface  18   a  of the corresponding head substrate  18 , the dimension of the rotary head drum unit  2  in its vertical direction (axial direction) can be reduced. However, the mounting structure of each magnetic head  15  is not limited to the above structure in the present invention, but each magnetic head  15  may be mounted on the upper surface of the head substrate  18  as in the related art.  
         [0077]    As shown in FIG. 4, the amount of projection of each magnetic head  15  from the drum surface  2   a  forming the cylindrical surface of the rotary head drum unit  2  is set smaller than the flying height of the magnetic tape  4  wrapped around the rotary head drum unit  2  during running of the magnetic tape  4 .  
         [0078]    As shown in FIGS. 3 and 4, each magnetic head  15  has a substantially flat front end surface (which will be hereinafter referred to as “tape sliding surface”)  16  on which the magnetic tape  4  slides during running. The tape sliding surface  16  is substantially square or slightly oblong as viewed in elevation. For example, the tape sliding surface  16  is formed in the shape of a rectangle having a size of about 1 mm×1.2 mm. Thus, the head width W (the dimension of the tape sliding surface  16  along the axis of rotation of the rotary head drum unit  2 ) is larger than the head width w of the magnetic head e in the related art (see FIG. 23).  
         [0079]    Each magnetic head  15  is not limited in its type. For example, a so-called wire wound type head, an MR (Magneto Resistive) head, or a GMR (Giant Magneto Resistive) head may be used.  
         [0080]    When the tape loading is carried out, the rotary head drum unit  2  is rotated, and the magnetic tape  4  wrapped a given angle around the rotary head drum unit  2  is run in a given direction (see FIG. 1).  
         [0081]    During running of the magnetic tape  4 , each magnetic head  15  forms a track extending in a direction inclined with respect to the running direction of the magnetic tape  4  to thereby record a signal to the magnetic tape  4 , or scans a recorded track to thereby reproduce a signal from the magnetic tape  4  (see FIG. 2).  
         [0082]    When the rotating drum  12  is rotated, air is introduced between the drum surface  2   a  of the rotary head drum unit  2  and the magnetic tape  4 , so that the magnetic tape  4  runs at a given flying height above the drum surface  2   a  of the rotary head drum unit  2  (see FIG. 5).  
         [0083]    This is due to the fact that an air layer is formed between the drum surface  2   a  of the rotary head drum unit  2  during rotation and the magnetic tape  4  during running, and that a pressure difference between this air layer and the atmospheric air existing on the opposite side of this air layer with respect to the magnetic tape  4  is held constant in relation to a tape tension or the like of the magnetic tape  4 , thereby obtaining a given flying height of the magnetic tape  4  above the drum surface  2   a  of the rotary head drum unit  2  during running of the magnetic tape  4 .  
         [0084]    When the magnetic tape  4  approaches each magnetic head  15  having the substantially flat tape sliding surface  16  as shown in FIG. 6, the air layer formed between the drum surface  2   a  and the magnetic tape  4  is forced away by a front edge  16   a  of the tape sliding surface  16  (a leading edge in the rotational direction of the rotary head drum unit  2 , that is, a leading edge first coming into contact with or close to the magnetic tape  4 ). Accordingly, the amount of air flowing into the gap between the tape sliding surface  16  and the magnetic tape  4  is reduced and the air pressure in this gap is therefore reduced. That is, negative pressure is produced between the magnetic tape  4  and the tape sliding surface  16  to attract the magnetic tape  4  to the tape sliding surface  16 . As a result, the magnetic tape  4  comes into contact with the tape sliding surface  16  as shown in FIG. 7.  
         [0085]    Such a reduction in the amount of air flowing into the gap between the magnetic tape  4  and the tape sliding surface  16  is determined by the width of each magnetic head  15 , i.e., the width W of the tape sliding surface  16 .  
         [0086]    That is, by increasing the width of each magnetic head  15 , the amount of air forced away by the front edge  16 a of the tape sliding surface  16  is increased when the magnetic head  15  is moved. Accordingly, the amount of air flowing away from the opposite side edges (upper and lower side edges) of the tape sliding surface  16  is increased, so that negative pressure is produced over the tape sliding surface  16 , especially at a vertically central portion thereof, thereby increasing a force of attracting the magnetic tape  4  to the tape sliding surface  16 .  
         [0087]    Thus, the contact of the magnetic tape  4  and the tape sliding surface  16  is caused by negative pressure produced therebetween. Accordingly, as shown in FIG. 8, a high-stiffness magnetic tape  4   a  comes into contact with a central portion of the tape sliding surface  16 , whereas a low-stiffness magnetic tape  4   b  comes into contact with the substantially entire area of the tape sliding surface  16 .  
         [0088]    By designing each magnetic head  15  so that the contact pressure between the high-stiffness magnetic tape  4   a  and the tape sliding surface  16  becomes a predetermined pressure, the low-stiffness magnetic tape  4   b  can be brought into contact with the tape sliding surface  16  at a pressure higher than the predetermined pressure. Accordingly, the degree of freedom of design can be increased.  
         [0089]    That is, by preliminarily setting the contact condition of the high-stiffness magnetic tape  4   a  as the magnetic tape  4 , the compatibility in contact condition with other types of magnetic tapes can be easily provided to have a merit in reducing the stiffness of the magnetic tape  4  in association with a reduction in thickness of the magnetic tape  4  as a trend.  
         [0090]    In the rotary head drum unit  2  according to the first preferred embodiment, the magnetic tape  4  can be brought into contact with the tape sliding surface  16  of each magnetic head  15  at a predetermined pressure or higher irrespective of whether the magnetic tape  4  is the high-stiffness magnetic tape  4   a  or the low-stiffness magnetic tape  4   b , thereby preventing a reduction and variations in recording or reproduction output.  
         [0091]    Further, even in the initial stage of use of the rotary head drum unit  2 , the contact condition between the magnetic tape  4  and each magnetic head  15  can be maintained stable.  
         [0092]    Further, since the contact of each magnetic head  15  and the magnetic tape  4  is effected by using negative pressure produced therebetween, the contact pressure of the magnetic tape  4  to the magnetic head  15  can be suppressed to thereby reduce wear or damage of the magnetic head  15  and the magnetic tape  4 .  
         [0093]    The tape sliding surface  16  of each magnetic head is a “substantially flat” surface which means not only a flat surface, but also a slightly curved surface. When the tape sliding surface  16  is a flat surface, it is clear that negative pressure is produced over the tape sliding surface  16 . However, even when the tape sliding surface  16  is a slightly curved surface, negative pressure is produced over the tape sliding surface  16 . Strictly speaking, negative pressure is produced over the tape sliding surface  16  provided that the curvature of the tape sliding surface  16  is slightly smaller than that of the magnetic tape  4  wrapped around the rotary head drum unit  2  at a given height (i.e., the tape sliding surface  16  is nearly flat). The curvature of the tape sliding surface  16  is a curvature enough to produce negative pressure allowing the attraction of the magnetic tape  4  to the tape sliding surface  16 .  
         [0094]    In the case that the tape sliding surface  16  is a flat surface, each magnetic head  15  can be easily manufactured, whereas in the case that the tape sliding surface  16  is a slightly curved surface, the contact condition between the magnetic tape  4  and the tape sliding surface  16  can be improved. Thus, it can be said that whether the tape sliding surface  16  is to be formed into a flat surface or a slightly curved surface is within the scope of a design matter.  
         [0095]    As mentioned above, the size (the width W) of each magnetic head  15  in a direction perpendicular to the running direction of the magnetic tape  4  is larger than the size (the width w) of the conventional magnetic head e (see FIG. 23), so that negative pressure enough to attract the magnetic tape  4  to the tape sliding surface  16  can be produced over the tape sliding surface  16 . By controlling the size (width) of the front edge  16   a  of the tape sliding surface  16 , negative pressure to be produced can be controlled to thereby control the contact condition between the magnetic tape  4  and the tape sliding surface  16 .  
         [0096]    In association with this structure of each magnetic head  15 , each head mounting hole  14  of the rotary head drum unit  2  is larger in size than the conventional one (head mounting hole d shown in FIG. 22).  
         [0097]    [0097]FIG. 9 shows a second preferred embodiment of the rotary head drum unit according to the present invention. The second preferred embodiment is different from the first preferred embodiment in that the tape sliding surface of each magnetic head is retracted from the drum surface. Accordingly, the following description of the second preferred embodiment will be focused on only this different point, and the description of the other same parts denoted by the same reference numerals in the first embodiment will be omitted herein.  
         [0098]    Reference numeral  2 A denotes a rotary head drum unit according to the second preferred embodiment. The rotary head drum unit  2 A includes a magnetic head  15 A provided in the head mounting hole  14 . The magnetic head  15 A has a tape sliding surface  16 A retracted from the drum surface  2   a.    
         [0099]    When the rotary head drum unit  2 A is rotated, the magnetic tape  4  runs at a given flying height above the drum surface  2   a  as similar to the first preferred embodiment.  
         [0100]    The air layer formed between the magnetic tape  4  and the drum surface  2   a  is increased in sectional area at a position corresponding to the head mounting hole  14 , causing a reduction in pressure of the air layer at this position.  
         [0101]    The magnetic tape  4  is wrapped around the rotary head drum unit  2 A under a given tension, so that the magnetic tape  4  runs linearly under this tension so as to be retracted into the head mounting hole  14 .  
         [0102]    Accordingly, a portion of the magnetic tape  4  corresponding to the head mounting hole  14  is attracted into the head mounting hole  14  to come into contact with the tape sliding surface  16  of the magnetic head  15 A.  
         [0103]    The rotary head drum unit  2 A according to the second preferred embodiment can exhibit an effect similar to that of the first preferred embodiment. That is, the magnetic tape  4  can be brought into contact with the magnetic head  15 A by using negative pressure produced therebetween. Accordingly, a stable contact condition between the magnetic tape  4  and the magnetic head  15 A at a predetermined pressure or higher can be obtained irrespective of whether the magnetic tape  4  is a high-stiffness magnetic tape  4 a or a low-stiffness magnetic tape  4 b, thereby preventing a reduction and variations in recording or reproduction output. Further, since the contact between the magnetic tape  4  and the magnetic head  15 A is effected by using negative pressure produced therebetween, the contact pressure of the magnetic tape  4  to the magnetic head  15 A can be suppressed to thereby reduce wear or damage of the magnetic head  15 A and the magnetic tape  4 .  
         [0104]    While the contact pressure of the magnetic tape  4  to the magnetic head  15  or  15 A is determined by negative pressure produced therebetween, there is a case that negative pressure may be insufficient. By increasing negative pressure to increase the contact pressure between the magnetic tape  4  and the magnetic head  15  or  15 A, this contact pressure may be controlled more easily.  
         [0105]    As means for increasing negative pressure, the following preferred embodiments and modifications according to the present invention have been made, and by adopting any of these preferred embodiments and modifications individually or in combination, the contact pressure between the magnetic tape  4  and the magnetic head  15  or  15 A can be more easily set to a desired pressure.  
         [0106]    Negative pressure can be increased by the following means.  
         [0107]    (1) Forming an angle of elevation in mounting the magnetic head.  
         [0108]    (2) Deviating the magnetic head in the head mounting hole along a tangent line to the rotary head drum unit.  
         [0109]    (3) Providing a projection at the front edge of the magnetic head in the rotational direction of the rotary head drum unit.  
         [0110]    These means will now be described in sequence.  
         [0111]    (1) Forming an angle of elevation in mounting the magnetic head.  
         [0112]    [0112]FIG. 10 shows a third preferred embodiment of the rotary head drum unit according to the present invention. Reference numeral  15 B denotes a magnetic head according to the third preferred embodiment. The magnetic head  15 B projects from the drum surface  2   a  of the rotary head drum unit  2  so as to be tilted at a negative angle of elevation.  
         [0113]    The magnetic head  15  according to the first preferred embodiment is mounted so that the tape sliding surface  16  is parallel to a tangent line to the drum surface  2   a  at a point corresponding to the center of the tape sliding surface  16 . In contrast thereto, the magnetic head  15 B according to the third preferred embodiment is tilted at a negative angle of elevation so that the amount of projection of the front edge  16   a  of the tape sliding surface  16  from the drum surface  2   a  is greater than that of the rear edge  16 b of the tape sliding surface  16 .  
         [0114]    With this configuration, the air flowing into the gap between the magnetic tape  4  and the tape sliding surface  16  at a position behind the front edge  16 a can be further reduced in amount to thereby further reduce the air pressure in this gap. Accordingly, negative pressure greater than that in the first preferred embodiment can be produced between the magnetic tape  4  and the magnetic head  15 B.  
         [0115]    By controlling the above angle of elevation of the magnetic head  15 B, negative pressure to be produced can be controlled to thereby control the contact pressure between the magnetic head  15 B and the magnetic tape  4 .  
         [0116]    [0116]FIG. 11 shows a modification of the third preferred embodiment. Reference numeral  15 C denotes a magnetic head according this modification. The magnetic head  15 C is tilted at a negative angle of elevation so that the tape sliding surface  16  is retracted from the drum surface  2   a  of the rotary head drum unit  2 .  
         [0117]    With this configuration, negative pressure greater than that in the second preferred embodiment can be produced between the magnetic tape  4  and the magnetic head  15 C.  
         [0118]    By controlling the above angle of elevation of the magnetic head  15 C as in the case of the magnetic head  15 B, negative pressure to be produced can be controlled to thereby control the contact pressure between the magnetic head  15 C and the magnetic tape  4 .  
         [0119]    (2) Deviating the magnetic head in the head mounting hole along a tangent line to the rotary head drum unit.  
         [0120]    [0120]FIG. 12 shows a fourth preferred embodiment of the rotary head drum unit according to the present invention. Reference numeral  15 D denotes a magnetic head according to the fourth preferred embodiment. The magnetic head  15 D projects from the drum surface  2   a  of the rotary head drum unit  2  so as to be deviated along a tangent line to the drum surface  2   a  of the rotary head drum unit  2  at a point corresponding to the center of the tape sliding surface  16  in a forward direction of rotation of the rotary head drum unit  2 .  
         [0121]    The magnetic head  15  according to the first preferred embodiment is mounted so that the center line perpendicular to the tape sliding surface  16  extends in a radial direction of the rotary head drum unit  2 . In contrast thereto, the magnetic head  15 D according to the fourth preferred embodiment is deviated along the tangent line to the drum surface  2   a  of the rotary head drum unit  2  in a forward direction of rotation of the rotary head drum unit  2 .  
         [0122]    With this configuration, the amount of projection of the front edge  16   a  of the tape sliding surface  16  of the magnetic head  15 D from the drum surface  2   a  becomes greater than that of the rear edge  16   b , thereby obtaining the same effect as that in the previous case of forming a negative angle of elevation. Accordingly, the air flowing into the gap between the magnetic tape  4  and the tape sliding surface  16  at a position behind the front edge  16   a  can be further reduced in amount to thereby further reduce the air pressure at this gap. As a result, negative pressure greater than that in the first preferred embodiment can be produced between the magnetic tape  4  and the magnetic head  15 D.  
         [0123]    By controlling the amount of deviation (offset amount) of the magnetic head  15 D along the tangent line, negative pressure to be produced can be controlled to thereby control the contact pressure between the magnetic head  15 D and the magnetic tape  4 .  
         [0124]    [0124]FIG. 13 shows a modification of the fourth preferred embodiment. Reference numeral  15 E denotes a magnetic head according to this modification. The magnetic head  15 E is deviated along the tangent line to the drum surface  2   a  of the rotary head drum unit  2  in the forward direction of rotation of the rotary head drum unit  2  so that the tape sliding surface  16  is retracted from the drum surface  2   a  of the rotary head drum unit  2 .  
         [0125]    With this configuration, negative pressure greater than that in the second preferred embodiment can be produced between the magnetic tape  4  and the magnetic head  15 E.  
         [0126]    By controlling the offset amount of the magnetic head  15 E as in the case of the magnetic head  15 D, negative pressure to be produced can be controlled to thereby control the contact pressure between the magnetic head  15 E and the magnetic tape  4 .  
         [0127]    (3) Providing a projection at the front edge of the magnetic head in the rotational direction of the rotary head drum unit.  
         [0128]    [0128]FIGS. 14 and 15 show a fifth preferred embodiment of the rotary head drum unit according to the present invention. Reference numeral  15 F denotes a magnetic head according to the fifth preferred embodiment. The magnetic head  15 F has a configuration that a projection  19  is formed at the front edge  16   a  of the tape sliding surface  16  in the rotational direction of the rotary head drum unit  2 .  
         [0129]    In each of the magnetic heads  15  and  15 A according to the first and second preferred embodiments, the entirety of the tape sliding surface  16  is substantially flat. In contrast thereto, the tape sliding surface  16  of the magnetic head  15 F according to the fifth preferred embodiment has the projection  19  extending along the front edge  16   a  over the length thereof (over the width W of the tape sliding surface  16 ).  
         [0130]    With this configuration, the amount of projection of the front edge  16   a  of the tape sliding surface  16  of the magnetic head  15 F from the drum surface  2   a  becomes greater than that of the rear edge  16   b , thereby obtaining the same effect as that in the previous case of forming a negative angle of elevation. Accordingly, the air flowing into the gap between the magnetic tape  4  and the tape sliding surface  16  at a position behind the front edge  16 a can be further reduced in amount to thereby further reduce the air pressure at this gap. As a result, negative pressure greater than that in the first preferred embodiment can be produced between the magnetic tape  4  and the magnetic head  15 F.  
         [0131]    By suitably adjusting the height of the projection  19  of the magnetic head  15 F, negative pressure to be produced can be controlled to thereby control the contact pressure between the magnetic head  15 F and the magnetic tape  4 .  
         [0132]    While the projection  19  extends over the length of the front edge  16   a  (over the width W of the tape sliding surface  16 ) in this preferred embodiment, the present invention is not limited to this configuration, but the projection  19  may be formed at only a central portion of the front edge  16   a  provided that a given negative pressure can be obtained.  
         [0133]    [0133]FIG. 16 shows a modification of the magnetic head  15 F according to the fifth preferred embodiment. Reference numeral  15 G denotes a magnetic head according to this modification. A substantially U-shaped projection  19 A opening to the rear edge  16   b  as viewed in elevation is formed at the front edge  16   a  of the tape sliding surface  16  of the magnetic head  15 G. More specifically, the projection  19 A is composed of a vertical portion extending along the front edge  16   a  over the length thereof and a pair of horizontal portions extending along the upper and lower side edges of the tape sliding surface  16  from the opposite ends of the vertical portion.  
         [0134]    With this configuration, the air forced away by the front edge  16   a  and the projection  19 A is hard to flow into the gap between the magnetic tape  4  and the tape sliding surface  16  from its upper and lower side edges. Accordingly, negative pressure to be produced between the tape sliding surface  16  and the magnetic tape  4  can be further increased.  
         [0135]    [0135]FIG. 17 shows another modification of the magnetic head  15 F according to the fifth preferred embodiment. Reference numeral  15 H denotes a magnetic head according to this modification. The front edge  16   a  of the tape sliding surface  16  of the magnetic head  15 H is arcuate as viewed in elevation, and a substantially C-shaped projection  19 B opening to the rear edge  16   b  as viewed in elevation is formed at the arcuate front edge  16   a . More specifically, the projection  19 B is composed of an arcuate portion extending along the arcuate front edge  16   a  over the length thereof and a pair of horizontal portions extending along the upper and lower side edges of the tape sliding surface  16  from the opposite ends of the arcuate portion.  
         [0136]    With this configuration, the air forced away by the front edge  16   a  and the projection  19 B can smoothly flow away to thereby suppress the air flowing into the gap between the magnetic tape  4  and the tape sliding surface  16  from its upper and lower side edges. Accordingly, negative pressure to be produced between the tape sliding surface  16  and the magnetic tape  4  can be further increased.  
         [0137]    Each of the magnetic heads  15 F,  15 G, and  15 H according to the fifth preferred embodiment and its modifications mentioned above may project from the drum surface  2   a  of the rotary head drum unit  2  or the tape sliding surface  16  may be retracted from the drum surface  2   a . In each case, negative pressure to be produced over the tape sliding surface  16  can be increased.  
         [0138]    In the rotary head drum unit  2  according to the present invention, it is possible to improve the contact condition between the magnetic tape  4  and the magnetic head  15  not only in the running direction of the magnetic tape  4 , but also along the width of the magnetic tape  4 .  
         [0139]    By enlarging the size of the magnetic head  15  along the width of the magnetic tape  4 , the amount of air flowing into the gap between the magnetic tape  4  and the tape sliding surface  16  at its vertically central portion can be reduced, so that the size of a portion of the magnetic tape  4  coming into contact with the tape sliding surface  16  due to negative pressure can be enlarged in the vertical direction of the tape sliding surface  16  (along the width thereof) (see FIGS. 18 and 19). As a result, two or more gaps G can be formed in the single magnetic head  15 , thus realizing a so-called multi-gap structure (see FIGS. 20 and 21).  
         [0140]    The above-mentioned methods for forming a negative angle of elevation and providing a projection at the front edge may be applied not only to a rotary head drum unit as mentioned above, but also to a fixed head (magnetic head in a so-called linear tape system).  
         [0141]    It should be noted that the specific shapes and structures of parts in the above preferred embodiments and their modifications are merely illustrative in embodying the present invention and that the technical scope of the present invention is not limited thereto.