Patent Publication Number: US-2003223141-A1

Title: Recording/reproducting apparatus and recording/reproducing method using rotary magnetic heads

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an apparatus and method for recording/reproducing a digital information signal and, particularly, to an apparatus and method for reproducing the digital information signal in an optimum state using a magnetoresistive head (hereinafter referred to as “MR head”), which is mounted on a rotary drum.  
       [0003] 2. Description of the Related Art  
       [0004] The MR head uses the magnetic phenomenon of a recording medium, such as a magnetic tape or a magnetic disk, wherein electrical resistivity is changed due to a magnetic field signal generated by the recording medium. A sense current is supplied to the MR head to detect the resistivity change Since the resistivity change has a nonlinear characteristic with respect to an input magnetic field, a bias current, that sets an operation point of the MR head in a region having good linearity, is applied to the MR head. Currently available MR heads are designed to use the above current (hereinafter collectively referred to as “bias current”) in combination. To operate the MR head efficiently and obtain maximum reproduction output, it is necessary to optimize the bias current.  
       [0005] Various methods have been proposed for setting the bias current in a rotary head type magnetic recording/reproducing apparatus that employs the MR head and using magnetic tape as the recording medium. Japanese Patent Laid-open No. 10-177924 discloses a technique of controlling a bias current that defines an operation point of an MR head in accordance with head wear. In addition, Japanese Patent Laid-open No. 9-81909 proposes a method for setting an optimum bias current during recording/reproduction of actual data on/from a magnetic disk.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006] In a rotary head type apparatus using a magnetic tape as a recording medium, it is necessary to set a bias current value in accordance with a magnetic property specific to the tape. More specifically, it is necessary to set an optimum bias current that is suitable for the type of the tape, notably the thickness of the magnetic layer. Further, if the magnetic tape slides, a minute step (wear difference) is formed on a boundary between an MR element film and the film substrate. The minute step causes not only spacing loss in reproduction, but also a change in the optimum value of the bias current. These problems are not resolved in the above-mentioned related art literature.  
       [0007] In turn, in the method of setting the optimum bias current while running a magnetic tape, i.e., recording/reproducing actual data (verify mode), recording efficiency of the apparatus is remarkably decreased, that is., effective storage capacity is decreased since a large buffer memory capacity is undesirably consumed in order to store data which are transferred to the apparatus. If the apparatus is dedicated to data reproduction, a further disadvantage is that the effective data transfer rate is lowered, because a plurality of retries is required for minimizing errors.  
       [0008] This invention provides a recording/reproducing apparatus and a recording/reproducing method using rotary magnetic heads capable of setting an optimum bias current of an MR head in a short time. To achieve this, in the rotary magnetic head type recording/reproducing apparatus of the present invention, a magnetic tape has a test track for recording and reproducing a test signal for setting a bias current for the MR head in a predetermined region other than a region of a data track for recording the digital signal. The rotary magnetic head type recording/reproducing apparatus includes a recording circuit which supplies a recording current to the recording magnetic head; a reproducing circuit which processes a reproduction output of the MR head; a current supplying circuit which supplies a bias current to the MR head; and a current controlling circuit which reproduces the test signal from the test track on the magnetic tape and sets the bias current to be supplied to the MR head in accordance with a result of the reproduction.  
       [0009] In this aspect of the present invention, before recording or reproducing the digital signal on or from the data track of the magnetic tape, the test signal is reproduced from the test track to set the bias current to be supplied to the MR head. The test signal to be recorded on the test track is a random signal, and the error rate of the reproduction signal of the random signal is detected to set the bias current to be supplied to the MR head in such a manner that the error rate falls within a predetermined range. Alternatively, the test signal to be recorded on the test track is a low frequency signal having a period of at least 2T (T is a minimum bit period of a recording frequency), and asymmetry of a waveform of a reproduction signal of the test signal is detected to set the bias current to be supplied to the MR head in such a manner that a value of the asymmetry falls within a predetermined range.  
       [0010] To provide these benefits the magnetic tape has a test track for recording and reproducing a test signal which is used for setting a bias current value for the MR head in a predetermined region other than a region of a data track for recording the digital signal. Also the recording/reproducing method includes the steps of reproducing the test signal from the test track before recording or reproducing the digital signal on or from the data track of the magnetic tape; setting a bias current to be supplied to the MR head in accordance with a result of the reproduction of the test signal; and supplying the set bias current to the MR head to reproduce the digital signal.  
       [0011] These and other benefits, features and advantages of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a diagram showing a tape format for recording and reproducing in a rotary head type recording/reproducing apparatus according to the present invention;  
     [0013]FIG. 2 is a diagram showing the arrangement of recording heads and reproducing MR heads mounted on a rotary drum of the rotary head type recording/reproducing apparatus according to the present invention;  
     [0014]FIG. 3 is a block diagram showing a circuit configuration of the rotary head type recording/reproducing apparatus according to the present invention;  
     [0015]FIG. 4 is a characteristic diagram showing a relationship between an MR head resistivity and an optimum bias current;  
     [0016]FIG. 5 is a characteristic diagram showing a relationship between the MR head element height and an optimum bias current;  
     [0017]FIG. 6 is a characteristic diagram showing a relationship between a bias current and an error rate;  
     [0018]FIG. 7 is a characteristic diagram showing a relationship between a bias current and asymmetry of a reproduction waveform;  
     [0019]FIG. 8 is a diagram showing an example of a reproduction signal waveform of an MR head;  
     [0020]FIG. 9 is a flowchart showing operations of setting a bias current for the MR head at the time of recording according to the present invention; and  
     [0021]FIG. 10 is a flowchart showing operations of setting a bias current for the MR head at the time of reproduction according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0022] Embodiments of the present invention will be described in detail below. FIG. 1 is a schematic diagram showing a tape pattern of a magnetic tape that is used for recording and reproducing data in a rotary-magnetic-head-type apparatus according to the present invention. A magnetic tape  1  is provided with data recording tracks  2  and test tracks  3 . Test tracks  3  are located between data recording tracks  2  and a BOT (Beginning Of Tape) position  5 . A region occupied by data recording tracks  2  is referred to as an actual data area  6 , and a region occupied by test tracks  3  is referred to as a rehearsal area  7 .  
     [0023] Although four test tracks are shown in FIG. 1, the number of test tracks is not limited, except that having two or more test tracks is advantageous. When recording or reproduction is finished, the tape is unloaded after being rewound to BOT position  5 . Therefore, it is possible to immediately scan rehearsal area  7 , which is near BOT position  5 , of the tape inserted into the apparatus.  
     [0024]FIG. 2 is a schematic diagram showing a construction in which heads are disposed on a drum of a rotary-magnetic-head-type apparatus according to the present invention. Recording heads  9  and  10  as well as reproducing MR heads  11  and  12  are mounted on a rotary drum  8 .  
     [0025] The recording pattern shown in FIG. 1 is recorded simultaneously by recording heads  9  and  10  and reproduced by reproducing heads  11  and  12 . The number of recording heads and reproducing heads is not limited. There may be more than one of each.  
     [0026]FIG. 3 shows a circuit configuration for performing recording and reproducing in the rotary-magnetic-head-type apparatus according to a preferred embodiment of the present invention. A power supply circuit  13 , current circuits  14  and  15 , respectively, for supplying a bias current to the MR heads  11  and  12 , a recording amplifier  16 , a reproducing amplifier  17 , and a current controller  23  are mounted on the side of rotary drum  8 . On a fixed side, a power signal generating circuit  19 , a recording circuit  20 , a reproducing circuit  21 , a bias current controlling circuit  22 , and a memory  24  are provided. A rotary transformer  18  transmits a signal between the rotating side and the fixed side.  
     [0027] Recording and reproducing operations of the rotary-magnetic-head-type apparatus is described below with reference to FIG. 3. At the time of recording, a recording signal is transmitted from recording circuit  20  disposed on the fixed side to the rotating side via rotary transformer  18 . The signal is then amplified by recording amplifier  16  in rotary drum  8  and is recorded on a tape by recording heads  9  and  10 . In this embodiment of the invention, a voltage is supplied from power supply circuit  13  to recording amplifier  16 , reproduction amplifier  17 , and current circuits  14  and  15  on the rotary side. A rectangular (or sinusoidal) wave signal voltage is applied from power signal generating circuit  19 , which is disposed on the fixed side to power supply circuit  13  via rotary transformer  18 , and power supply circuit  13  regulates the voltage to supply the regulated voltage.  
     [0028] At the time of reproduction, signals reproduced by the MR heads  11  and  12  are amplified by reproduction amplifier  17  and transmitted to reproduction circuit  21  via rotary transformer  18 . In this embodiment of the invention, the bias currents are supplied from current circuits  14  and  15  to MR heads  11  and  12 , and current controller  23  controls a magnitude of each of the bias currents. The bias currents generated by current circuits  14  and  15  are so controlled as to achieve a maximum reproduction output. Memory  24  stores the data of the bias current achieved at this time, so that the bias currents may also be set by reading out the data from memory  24 .  
     [0029] According to the present embodiment, it is possible to detect and set the optimum bias currents of the plurality of MR heads that are mounted on the rotary drum immediately after the magnetic tape is inserted. Therefore, it is possible to omit the process of setting the optimum bias currents at the same time as detecting errors by the verify mode, which has been performed in the conventional technique, in the case where data are transferred from a host. Further, since it is unnecessary to rewrite the transferred data on another region of the tape, efficient data writing is achieved without consuming the storage capacity of the apparatus.  
     [0030] The relationship between an MR head characteristic and an optimum bias current is shown in each of FIGS.  4  to  8 . A relationship between an element resistivity of the MR head and a bias current optimum for minimizing an error rate of data is shown in FIG. 4. It is necessary to set, or reduce, the bias current in accordance with an increase in the resistivity of the MR head.  
     [0031] The relationship between a height of the MR head, i.e., a height of the MR element, and an optimum bias current value is shown in FIG. 5. With a reduction in the MR element height, the resistivity is increased while the optimum bias current value is decreased. In other words, since the height of the MR element is decreased with progress in MR head wear, the optimum bias current must be decreased with the progress in MR head wear.  
     [0032]FIG. 6 shows the relationship between error rate of the MR head and a bias current value is shown in FIG. 6. The error rate exhibits a U-shaped characteristic with respect to the bias current, and there is an optimum region as indicated in the diagram. Such characteristic is due to waveform distortion that changes depending on the bias current, and the error rate rises when the distortion is expanded.  
     [0033] However, if a step (wear difference) is formed between the MR element and the substrate, the above settings are insufficient because the formation of the step causes the U-shaped characteristic shown in FIG. 6 to shift to a region of relatively high bias current. Further, if a tape magnetic layer is too thick, a magnetic field to be retained by the medium becomes so strong that the MR element tends to be saturated, thereby raising the error rate. As a result, the U-shaped characteristic shifts to a region where the bias current is relatively low.  
     [0034] An asymmetric relationship between a bias current and an output waveform is shown in FIG. 7. The linear relationship is achieved because the error rate of FIG. 6 depends on asymmetry (a type of distortion) of a reproduction waveform. Although one straight line (approximation) is shown in FIG. 7, a point on which the asymmetry is 0% exists in each of the MR resistivities.  
     [0035] The asymmetry is explained below with reference to FIG. 8. The asymmetry is defined by a ratio of difference between an output value in a positive direction and an output value in a negative direction of a reproduction pulse, to a sum of the output value in the positive direction and the output value in the negative direction of the reproduction pulse. A definition expression of the asymmetry is shown below as Expression (1).  
     Asymmetry=[((B−A)/(B+A))]×100(%)  (1)  
     [0036] If the output values in positive and negative directions are the same, the asymmetry becomes 0% since A=B. The distortion of the waveform is expanded with an increase in magnitude of the asymmetry, thereby raising the error rate.  
     [0037] In the case of optimizing the error rate by using the asymmetry thus detected, a low frequency signal having a period of at least 2T or less (T is a minimum bit period of recording frequency) is suitably used as a test recording signal. In turn, in the case of directly detecting the error rate, a random signal is suitably used as the test-recording signal. A frequency component of a signal to be recorded on a tape depends on a recording modulation mode, and an output at the 2T signal frequency has considerable influence on the error rate in a partial response mode or like modes.  
     [0038]FIGS. 9 and 10 are flow charts for setting a bias current of an MR head of the present invention in accordance with respective states of the head immediately before recording and reproduction. An operation of recording data on a virgin tape is shown in FIG. 9. An initial value of the bias current is the (ex-factory initial value or a bias current value updated when the apparatus is used) is stored in memory  24  in the recording/reproducing apparatus. Since MR heads in general have variations in element characteristic and height, the bias current is set at the time of assembly. Before recording/reproducing actual data, a test signal is simultaneously recorded and reproduced on and from a rehearsal area of the tape in order to detect an error rate. At this point, the bias current value that has been stored in the memory  24  is read out, so that the MR head is operated at the bias current value. If the error rate is equal to or lower than a threshold value defined in the apparatus, the error rate is set as a normal state, and then the actual data is moved to a data area on the tape to be recorded thereon. However, if the error rate at the initial bias current value is higher than the threshold value, the bias current is changed to detect an error rate by the simultaneous recording and reproduction procedure again. A bias current that achieves an error rate equal to or lower than the threshold value is detected by repeating the above operation. The thus-obtained bias current value is stored in the memory  24  as a new current value. After that, the actual data is moved to the data area on the tape to be recorded thereon. In addition, a waveform asymmetry value (%) may be used in place of the error rate as an index for setting the bias current value.  
     [0039] An operation of reproducing data that has been recorded on a tape is shown in FIG. 10. Bias currents for various types of tapes are set in memory  24 . A backward-compatible low-recording-density tape has a relatively thick magnetic layer and, therefore, it is necessary to set the bias current as low as possible in order to suppress the distortion by limiting a magnetic flux reproduced by the MR head. Accordingly, bias current values for various types of tapes are stored in the memory  24 . However, the characteristic of the MR head immediately before reproduction is susceptible to change due to head wear, for example, so that an effective bias current may be different from the set bias current value in some cases. Also in such cases, an optimum value can be detected by reproducing the rehearsal area of the tape inserted into the apparatus. The bias current values stored in memory  24  vary from one another depending on the types of tapes.  
     [0040] In the present invention, the test track is recorded on the BOT position; however, the rehearsal area may be located near the BOT position if a recording apparatus unloads without failure at the tape-end position. Alternatively, the rehearsal area may be located at any predetermined portion of the tape other than the tape-end, the BOT position, and the data track. In this case, it is unnecessary to rewind the tape to the BOT position, so that rapid switching over from the test operations to the actual reproduction operations is realized.  
     [0041] According to the present invention as described above, an optimum bias current value of each of a plurality of MR heads mounted on a rotary drum is detected and set immediately after insertion of a magnetic tape. Therefore, the present invention can provide a recording/reproducing apparatus which is always in the best condition, highly reliable, and excellent in operability.  
     [0042] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and the range of equivalency of the claims are therefore intended to be embraced therein.