Abstract:
Time-based servopositioning systems, methods, formats, and data recording media used in association with the same, employing full amplitude recording signals to improve the available signal as media thicknesses decrease.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention concerns systems and methods for time-based servopositioning in the context of linear data recording media such as magnetic tape.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modern data storage systems use servopositioning (or “servo”) systems to guide their recording and playback components with respect to a recording medium, and thus enable high track density, which increases data storage capacity. Errors in the ability to follow the servopositioning signals on the medium can cause unacceptable reductions in storage capacity, recording/playback rates, and other parameters that are important to consumers (and thus to system manufacturers).  
           [0003]    One type of servo patterns or formats for linear magnetic tape recording systems employs so-called time-based servo techniques, examples of which are disclosed in U.S. Pat. Nos. 5,689,384; 5,930,065; and 6,021,013 (all of which are incorporated by reference in their entireties). Commercial magnetic tape drives such as the IBM model 3570 and drives known under the names “Ultrium” and “Accelis,” as described by the Linear Tape Open (LTO) consortium, use time-based servopositioning systems.  
           [0004]    The advantages of time-based servo systems include very wide dynamic range; inherent track identification; low DC centerline error; and the ability to qualify position error signal (PES) without absolute determination of the servo signal. Disadvantages include extreme sensitivity to tape speed during writing; sensitivity to high frequency speed error during reading; and poor scalability to very small track pitches.  
         SUMMARY OF THE INVENTION  
         [0005]    In general terms, the invention may be embodied in time-based servopositioning systems, methods, and formats, or in data recording media used in association with the same, and therefore this disclosure should be understood in that regard even if only an example of a particular embodiment is described in detail. Similarly, this disclosure should be understood to apply to either analog or digital signals, in accordance with principles known in the art. Thus, the terms “signal,” “data,” and the like may be used interchangeably, and should be understood to apply to either analog or digital representations of information.  
           [0006]    In the most basic embodiments of the invention, a servopositioning system for a data recording system is used in combination with a linear data recording medium, preferably magnetic recording tape. Written or recorded on the medium are servopositioning signals in which the recorded magnetic transitions have magnetization moments (M) that range between −M and +M in value, as opposed to between zero and +M or between zero and −M. Appropriate circuitry responds to the magnetization transitions and produces position error signals by sampling the time-based servo signal.  
           [0007]    One specific embodiment of the invention is a linear magnetic data recording medium, comprising a time-based servo signal in the form of at least one servo mark transition at which magnetization moment values of the signal change from a negative value to a positive value.  
           [0008]    Another specific embodiment of the invention is a servopositioning system. One portion of the system is a linear magnetic data recording medium, upon which is recorded a time-based servo signal in the form of transitions between a negative magnetization value and a positive magnetization value. Another portion of the system is any convenient circuitry that is responsive to the time-based servo signal and produces a position error signal by reading the time-based servo signal. Another portion of the system is any convenient means for reducing the position error signal by repositioning the reading head.  
           [0009]    Yet another specific embodiment of the invention is a method of writing servopositioning signals on a magnetic data recording medium. The method comprises using a primary recording head to write a time-based servo signal on the medium. The time-based servo signal is in the form of transitions between a negative magnetization value and a positive magnetization value. The time-based servo signal is read with a reading head and position error signal is generated. Then, the position error signal is reduced by repositioning the reading head. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The accompanying drawings show a particular embodiment of the invention as an example, and are not intended to limit the scope of the invention.  
         [0011]    [0011]FIG. 1 is a schematic diagram of the recorded magnetization (M) of a prior art servo writing signal, and also of servo writing signals employed in embodiments of the invention.  
         [0012]    [0012]FIG. 2 is a schematic diagram of the resultant servo signal output voltage (V s ) signal corresponding to the respective magnetization profiles of FIG. 1.  
         [0013]    [0013]FIG. 3 is a schematic diagram of the servo write current profile of a prior art recording scheme, and also of the invention.  
         [0014]    [0014]FIGS. 4 and 5 are schematic diagrams of preferred embodiments of the invention.  
         [0015]    [0015]FIG. 6 is a schematic diagram of a preferred embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]    In general terms, the invention can be embodied in an entire system of data recording and playback, including the combination of a drive and a linear recording medium; or as only the recording medium portion of such a system; or as methods for recording or playing back data in combination with the data recording medium. Thus, while the following description may occasionally focus on only one preferred aspect of an entire system (e.g., the writing of servo patterns on tape) to disclose the preferred embodiment of the invention, this is by way of example only, and not a limitation on the scope of the invention. It should be understood that the full scope of the invention includes other aspects of the system depending on the circumstances, such as combinations of the medium and drive, and methods of using such combinations or relevant portions of them.  
         [0017]    In prior art formats such as LTO, the available servo signal output created by the unipolar transitions of the time-based servo patterns are limited to half of the capability of the magnetic medium. As illustrated in the prior art portion of FIG. 1, this is because the change in magnetization moment (M) at servo mark transitions varies, from M=0 to M=+M r , where M r  is the remnant magnetic moment of the tape.  
         [0018]    As the physical thickness of magnetic layers on tapes decreases (to improve high recording density response, for example), the available servo signal reduces linearly with thickness. The position error signal (PES) of the servo system is, to first order, proportional to the system signal to noise ratio (SNR). Thus, as the servo signal is reduced with decreasing tape thickness, so is the SNR.  
         [0019]    To increase the available servo signal, as shown in the other portion of FIG. 1, the invention relies on the recording of the servo transitions at levels between negative and positive magnetization levels, preferably between M=−M r  and M=+M r . This effectively doubles the available servo signal, and thus improves the SNR and the PES. It is preferred, but not necessary, that the absolute value of the negative and positive magnetization values be the same, and so this will be described below with the understanding that it is not a limitation on the scope of the invention.  
         [0020]    It is possible to implement such a so-called “−M r  to +M r ” recording scheme by maintaining a continuous DC write current sufficient to record to the medium at M=−M r , and then reverse the polarity of the current to record servo transitions. This approach is simple in concept but has a severe practical constraint.  
         [0021]    Conventional servo writing heads are designed to have low inductance, so that they have fast rise times for improved performance. Therefore, they employ coils that have relatively few turns and thus require several amperes of current to generate suitable recording signals. Significant amounts of heat are generated if the circuit continuously sustains several amperes in such a fast switching environment.  
         [0022]    [0022]FIG. 3 compares the write current profiles of a typical prior art “0 to M r ” approach (again, on the left side of the Figure) and the “−M r  to +M r ” approach of the invention (on the right side of the Figure). In the typical prior art 0 to M r  scheme, the write driver duty cycle is less than 10% for an LTO-type recording pattern. The heat dissipation need is on the order of only a few tens of watts. To implement the continuous DC approach just described, the write driver circuit duty cycle increases to 100%, and thus the circuit must dissipate several tens of watts of power in an area of only approximately one square centimeter.  
         [0023]    There are two preferred ways to implement the “−M r  to +M r ” time-based servo writing scheme of the invention and avoid this constraint (although systems which do not avoid the constraint are within the full scope of the invention). The first, as illustrated schematically in FIG. 4, is to employ an independent secondary write head  10  to record a DC signal  11  on the tape  12  in addition to the servo transitions that are written by the conventional primary servo write head  14 . The additional secondary head  10  is situated in-line and upstream of the primary (unipolar) head  14 , as indicated by the arrow  15  showing the direction of motion of the tape  12  relative to the heads  10 ,  14 . This ensures that the secondary head  10  magnetizes the tape  12  before the primary head  14 .  
         [0024]    As illustrated in FIG. 6, the secondary write head  10  preferably has a track width as wide as the target servo track width defined by the servo marks  101  and  102 . It is preferred, but not required, for the orientation of the secondary head gap to be parallel to the bisector of the included angle θ that the components of the time-based servo pattern make with each other. In less preferred embodiments, deviations from the bisector angle are acceptable.  
         [0025]    In the generic servo format illustrated in FIG. 6, servo marks  101  and  102  are located at arbitrary angles to the track direction  103 . In the LTO format, servo marks  101  and  102  are arranged symmetrically about the line perpendicular to track direction  103 , each at an angle of six degrees from the perpendicular, and thus the included angle is θ=12° and the bisector is zero degrees. However, as FIG. 6 illustrates, the invention is not limited to the LTO format, or even to other formats having symmetric patterns.  
         [0026]    Typically, the tape  12  is previously erased, such as by an AC signal (not shown) to produce fully erased portion  16 . A DC current  11  having proper polarity and magnitude (−I w ) passes through the winding of the secondary head  10  to saturate the erased portion  16  of tape  12  to the M=−M r  level, creating portion  17 . Next, the primary head  14  records the servo transitions at the M=+M r  level. The result is a magnetization which varies between M=−M r  and M=+M r  on the servo recorded portion  18  of tape  12 .  
         [0027]    In this embodiment, the magnetomotive force (“mmf”) output of the primary head  14  is the same as in the prior art “0 to +M r ” scheme. Thus, the write driver circuit of this embodiment of the invention does not require any additional heat dissipation than does the write driver circuit of the prior art.  
         [0028]    [0028]FIG. 5 shows another embodiment of the invention. This embodiment can be described as a passive method because it does not require an additional, or active, DC current signal  11  to place the recording medium  12  at the M=−M r  magnetization level prior to writing the servo pattern. This embodiment is similar to the one illustrated in FIG. 4, but in this embodiment the DC magnetomotive force of the secondary head  10  is generated by a permanent magnet  20  embedded in the head structure. One possible embodiment would employ a single segment of a conventional in-line degaussing magnet, modified to have a slotted surface that defines the servo band position.  
         [0029]    Any means for reducing the position error signal by repositioning the reading head is suitable for use with the invention.  
         [0030]    The servo signal level can be controlled by modulating the DC current described above, or by controlling how much AC current is used to write the servo pulses. Independently, all embodiments of the invention can be adapted to address the transition from +M r  to −M r  by simply reversing polarity in the heads from what is shown in the Figures and described above. Any such variation is considered equivalent to the invention for the purpose of assessing the scope of the following claims.