Abstract:
The present specification discloses a preferred a preferred method, apparatus, and system for calibrating a magnetic tape system. The magnetic tape system comprises at least one head mounted within a head drum, a magnetic tape that has a data region and a no data region, the magnetic tape being contiguous with the head, and a device for providing a relative motion between the magnetic tape and the head. A preferred embodiment of the present invention has the following. A reference track provided on the magnetic tape. The reference track is located in the no data region, at a constant distance from the data region. A processor programmed to determine the time required for the head to travel from the provided reference track to the data region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This invention relates to Provisional Application Ser. No. 60/074,770, filed Feb. 17, 1998. The contents of that application are incorporated by reference herein.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to helical scan recording systems and in particular embodiments to an apparatus, system, and method for the self calibration of helical scan read and write heads.  
           [0004]    2. Description of Related Art  
           [0005]    In helical scan magnetic tape systems, a slow moving tape is wrapped around a cylindrical head drum. The head drum is typically composed of a rotating upper drum that is attached to a stationary lower drum. At least one magnetic read/write head is embedded into the upper drum. The magnetic tape is contiguous with the upper drum and it is positioned at a slight angle to the equatorial plane of the upper drum. A capstan motor is used to transport the tape at a slow speed, relative to the upper drum, and in the opposite direction of the upper drum. Moving the tape in this manner results in a recording format in which successive tracks are written in a helical scan pattern (i.e., diagonally across the tape, from one edge of the tape to the other edge of the tape.) Each track corresponds to one field of data. The angle of the tracks are related to the geometry of the helical scan magnetic tape system, the width of the tape, and the rotation speed of the upper drum.  
           [0006]    The lower drum has a precision cut edge that protrudes from the outer surface of the lower drum. The precision cut edge can be used to guide the tape edge and to hold the tape edge in place. The upper drum has an embedded pulse generator (PG) encoder. The PG encoder produces position-related timing pulses. These PG pulses are related to the characteristics of the upper drum (e.g., the rotating speed of the upper drum, the circumference of the upper drum, etc.).  
           [0007]    The PG pulse can be used as a reference point for the read and write process. Namely, the PG pulse encoder produces a pulse signal every time the upper drum rotates one revolution. In conventional magnetic tape systems, the position of the read and write heads relative to the PG pulse is often known. That is, when the PG pulse is sensed, the read and write heads tend to be at a known position. The distance between the point at which the PG pulse is sensed and the start of the data tracks may also be known.  
           [0008]    This distance value can be used to calculate the time (TØ) required for the heads to travel from the point at which the pulse is sensed to the start of a data region. Conventional magnetic tape systems may use TØ to ensure that the read/write heads are properly aligned over the tracks. Specifically, once the PG pulse is generated and sensed, the magnetic tape system waits TØ seconds, and then begins the writing process. During the read mode (or reading process), the magnetic tape system uses a capstan motor to control the timing, such that the time required for the read head to travel from the point at which the pulse is sensed to the start of a data region is always TØ.  
           [0009]    The calculated TØ value represents the timing of a magnetic tape system when the tape position and alignment are perfectly controlled. Specifically, the tape is maintained at a constant vertical position relative to the cylindrical drum (i.e., the tape does not move up and down); the read heads are perfectly aligned with the data tracks on the tape before the reading process begins; and this alignment is maintained during the operation of the magnetic tape system.  
           [0010]    During the writing process, many factors can affect the timing of magnetic tape system, producing a relative timing that is unequal to TØ. For instance, dirt build up on the lower drum cut edge or on the capstan motor shaft may cause the vertical position of the tape to vary. When the vertical position of the tape varies, the distance between the point at which the PG pulse is sensed and the start of a data region varies. Therefore, the time required for the read heads to travel from the point at which the pulse is sensed to the start of a data region also varies during the reading process. This varying time could be unequal to TØ. Thus, using TØ can cause read errors when the vertical position of a tape varies.  
           [0011]    Read errors can also occur when one magnetic tape contains a group (or groups) of tracks written by different magnetic tape systems. Since the TØ value is generally related to the mechanics of a particular cylindrical drum, each drum may have a different TØ value due to manufacturing variations. Therefore, the magnetic tape system may be incapable of properly aligning the read heads with each group (or groups) of tracks because the magnetic tape system may only know the TØ value (and associated distance value) for tracks written by one magnetic tape system.  
           [0012]    [0012]FIG. 1 shows an exemplary tape  100  that has written data tracks  102 . The distances, Δd 1    104  and Δd 2    106 , represent the distance from the tape edge to start of a data region  108  of the tape  100 . Distance Δd 1    104  is produced by one magnetic tape system and distance Δd 2    106  is produced by a another magnetic tape system. As observed, Δd 2    106  is greater than Δd 1    104 . Consequently, the TØ value for Δd 2    106  is greater than the TØ value for Δd 1    104 . The difference between Δd 2    106  and Δd 1    104  may be caused by many factors, such as variations in manufacturing, rotating speed of the upper drum, and environmental conditions during the operation of the magnetic tape system.  
           [0013]    Minimizing the variation in distances is usually very difficult. Therefore, most conventional magnetic tape systems have calibration systems that recompute the TØ value when a read error occurs. Some calibration schemes involve control track techniques, automatic track follow (ATF) techniques, and timing tracking techniques.  
           [0014]    For the control track technique, a servo write head (embedded in the upper drum) is used to write a control track on the magnetic tape during the write mode. The control track contains a series of 30-hertz pulses. These pulses are used to synchronize the read heads, causing the read heads to pass directly over the previously written data tracks. The control track serves the same general purpose as sprocket holes in a movie film. The sprocket holes help align each frame so that a viewer sees a steady picture on the screen. However, a problem with the control track technique is that it generally requires at least four heads: a data read head for reading data; a servo read head for sensing the control track; a data write head for writing data; and a servo write head for writing the control track.  
           [0015]    Using additional servo heads during the read and write process may affect the performance of the magnetic tape system. In particular, before data is written to the tape, the servo write head writes the control track. Hence, the time required for writing data is increased. Similarly, before data is read, the servo read head senses the control track, increasing the time required for reading data. As a result, additional servo heads tend to degrade the performance of the magnetic tape system.  
           [0016]    The automatic track follow (ATF) uses four pulses to mark successive data tracks. During the read mode, the read heads sense the ATF pulses. These ATF pulses are usually very low frequency signals and they can be used to provide a position error signal (PES). Based on the PES, the ATF technique continually adjust the read heads during operation of the magnetic tape system, causing the read heads to pass directly over the written data tracks. Unfortunately, the ATF technique lacks accuracy at high track densities. The ATF pulses and data tracks occupy the same data region  112 . Hence, the ATF pulses occupy space that could be used by additional data tracks, causing density problems.  
           [0017]    In the timing tracking technique, the read heads are locked onto the data tracks using a special synchronization field within the data itself. If the relative timing of the read head, which senses the synchronization field, is known, then a capstan motor (or any tape transport mechanism) can be used to lock the read heads on the data tracks by controlling the timing (i.e., the time at which a read head passes over a portion of the tape). Since the tape edge is mechanically held against the lower drum cut edge, the position of the written data tracks relative to a read head, may vary over the length of the tape. To prevent read errors, the timing tracking technique frequently re-aligns the head with the data tracks.  
           [0018]    The timing tracking technique is more accurate than both the control track technique and the ATF technique, but it tends to require frequent calibration during the operation of the magnetic tape system. This frequent calibration slows down the process of transferring data to the magnetic tape.  
           [0019]    [0019]FIG. 2 represents an exemplary frequent calibration scenario in accordance with the timing tracking technique. The read head senses the start of the data region  108 . The read head then performs a first read  202  on a data track  200  that has a distance Δd 1    104 . After an elapsed time, a read error occurs. The magnetic tape system performs a first calibration  206  to establish the correct TØ for the data track  200  that has a distance Δd 2 . This calibration is performed because Δd 2  is greater than Δd 1 . The correct TØ may be stored in the look-up table  204  for subsequent reads. The look-up table  204  contains track numbers and associated TØ values. Based on the information in the look-up table  204 , the magnetic tape system performs a first repositioning  208  of the heads. The second read  210  is then performed. This process of calibration and re-calibration is performed every time a read error occurs. It is conceivable that the tape could continually move up and down during the write process, causing offset written data regions. These offset written data regions are referred to as appends. Appends typically require continual re-calibration. Frequent calibration can produce poor results in audio or video playback because during the calibration, no data is played back. Therefore, the audio and/or video data may be interrupted.  
           [0020]    Thus, there is a need in the art for an improved calibration system that maintains alignment between the read heads and the data tracks during the operation of the magnetic tape system, without using additional heads, ATF pulses or frequent re-calibration.  
         SUMMARY OF THE DISCLOSURE  
         [0021]    To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the specification discloses a preferred system, apparatus, and method of calibrating for use with a magnetic tape system. The magnetic tape system comprises at least one head mounted within a head drum, a magnetic tape that has a data region and a no data region, the magnetic tape being contiguous with the head, and a device for providing a relative motion between the magnetic tape and the head.  
           [0022]    A preferred embodiment of the present invention has the following. A reference track provided on the magnetic tape. The reference track is located in the no data region, at a constant distance from the data region. A processor programmed to determine the time required for the head to travel from the embedded reference track to the data region.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0024]    [0024]FIG. 1 illustrates exemplary data tracks produced by a conventional magnetic tape system;  
         [0025]    [0025]FIG. 2 represents an exemplary frequent calibration scenario in accordance with the ATF technique;  
         [0026]    FIGS.  3 A- 3 C illustrates a conventional magnetic tape system;  
         [0027]    [0027]FIG. 4 is a graph representing the time required to travel from a point at which a reference signal is sensed to the point at which a subcode is sensed;  
         [0028]    [0028]FIG. 5 is a graph representing the relationship between the magnitude of a data signal and the time required to travel from a point at which a reference signal is sensed to the point at which a subcode is sensed;  
         [0029]    FIGS.  6 A- 6 B are graphs that illustrate how the performance characteristics of a magnetic tape system vary with track number;  
         [0030]    [0030]FIG. 7 is a graph that represents the relationship between the data signal magnitude and the sub-time count;  
         [0031]    FIGS.  8 A- 8 B represent a curve fit of the data signal magnitude for two different heads; and  
         [0032]    [0032]FIG. 9 represents and embedded reference track in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]    In the following description of preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the preferred embodiments of the present invention.  
         [0034]    Preferred embodiments of the present invention relate to apparatuses, systems, and methods for self calibrating a magnetic tape system. In particular, embodiments of the present invention maintain alignment between the read heads and the data tracks during the operation of the magnetic tape system, without using additional heads, ATF pulses or frequent re-calibration.  
         [0035]    FIGS.  3 A- 3 C illustrate a conventional magnetic tape system  300  for recording digital information on magnetic tape  310 . The magnetic tape system  300  has a tape transport mechanism  302  and a rotating cylindrical drum  304 , which rotates in a direction of travel indicated by arrow  332 .  
         [0036]    The tape transport  302  has a capstan  306  and two tape guides  308 . A capstan drive motor (not shown) rotates the capstan  306  in order to move the tape  310  in a direction of tape travel indicated by arrows  312 . As shown in FIG. 3A, the cylindrical drum  304  rotates counter-clockwise. The read and write heads enter from the bottom edge of the magnetic tape  310  and exit from the top edge of the magnetic tape  310 .  
         [0037]    The cylindrical drum  304 , is rotateable about a drum axis  314 . The cylindrical drum  304  is composed of a rotating upper drum  316  and a stationary lower drum  318 , as represented in FIG. 3B. The drum axis  314  is orthogonal to the cylindrical surface of both the upper drum  316  and the lower drum  318 . The cylindrical drum  304  also has a peripheral surface  320  which wraps around the circumference of the cylindrical drum  304 .  
         [0038]    Heads are embedded in or mounted on the peripheral surface  320  of the upper drum  316 . In the illustrated embodiment of FIG. 3A, the peripheral surface  320  has two sets of heads embedded therein. Specifically, a first set of read heads,  326  and  328 , and second set of write heads,  322  and  324  are embedded in the peripheral surface  320 . However, the peripheral surface  320  may have any suitable number of heads, including but not limited to, multiple read and write heads, and even one read head and one write head. For simplicity of explanation, FIGS.  3 B and  3 C- 5  only show one read head  328 .  
         [0039]    The magnetic tape  310  is contiguous with both the upper drum  316  and the lower drum  318  and it is positioned at a slight angle to the peripheral surface  320  of the upper drum  316 . More specifically, the magnetic tape  310  is positioned so that helical scan data tracks  334  are generated by the magnetic tape system  300 , as shown in FIG. 3C. The data tracks  334  are written diagonally across the tape  310 , from one edge of the tape  310  to the other edge of the tape  310 . The angle of the tracks are related to the geometry of the magnetic tape system  300 , the width of the tape  310 , and the rotation speed of the upper drum  316 .  
         [0040]    The lower drum  318  has a precision cut edge  330  that protrudes from peripheral surface  320  of the lower drum  318 . The precision cut edge  330 , and other suitable guides, can be used to guide the tape edge  340  and to hold the tape edge  340  in place, as represented by FIGS. 3B and 3C.  
         [0041]    The upper drum  316  has an embedded pulse generator (PG) encoder  332 , shown in FIG. 3C. The PG encoder  332  detects rotations of the drum motor shaft (not shown), and hence of the upper drum  316 , and produces a position-related timing pulse every time the upper drum  315  rotates one revolution. These PG pulses are related to the characteristics of the upper drum (e.g., the rotating speed of the upper drum, the circumference of the upper drum, etc.).  
         [0042]    In conventional magnetic tape systems  300 , the PG pulse is typically used as a reference point for the read (and write) process. The position of the read head  328  relative to the PG pulse is often known. The time TØ required to travel between the point at which the PG pulse is sensed and the start of the data tracks may also be known. Therefore, once the PG pulse is generated and sensed, the magnetic tape system waits a required amount of time, and then begins the writing process. During the reading process, the capstan motor controls the timing, such that the time required for the read head  328  to travel from the point at which the pulse is sensed to the start of a data region is always TØ.  
         [0043]    The TØ value represents the timing of a magnetic tape system  300  when the tape position and alignment are perfectly controlled. Specifically, the magnetic tape  310  is maintained at a constant vertical position relative to the cylindrical drum  304  (i.e., the magnetic tape  310  does not move up and down); the read head  328  is perfectly aligned with the data tracks  334  on the magnetic tape  310  before the reading process begins; and this alignment is maintained during the operation of the magnetic tape system  300 .  
         [0044]    Many factors can affect the timing of a magnetic tape system  300 , such that the relative timing during the operation of the magnetic tape system  300  is unequal to TØ. Some factors are dirt build up on the precision cut edge  330 ; dirt build up on the capstan  306 ; and whether two different magnetic tape systems were used to write the data tracks  334 . Preferred embodiments of the present invention provide a calibration system that produces a relative timing, during operation of a magnetic tape system  300 , that is always equal to TØ. In particular, embodiments of the present invention employ a reference track that is located on the magnetic tape  310 .  
         [0045]    Because the reference track is located on the magnetic tape  310 , the distance between the reference track and the data tracks  334  is not related to variations in the magnetic tape system  300 . That is, the distance is not related to the ability of the precision cut edge  330  to guide the magnetic tape  310  nor is the distance related to the ability of the tape transport  302  to transport the magnetic tape  310 . Additionally, the distance is not related to whether the data tracks  334  were written by different magnetic tape systems  300 . Instead, the distance is almost solely related to the characteristics of the magnetic tape  310 . Prior to discussing the reference track, a calibration technique used in accordance with the present invention will be discussed.  
       Calibration  
       [0046]    Before a read (or write) process begins, the magnetic tape system  300  is calibrated. Calibration involves calculating the time TØ required to travel from a point at which a reference signal is sensed to a point in a data region of the magnetic tape  310 . In many traditional magnetic tape systems, the reference signal is the system dependent PG pulse. In the preferred embodiment of the present invention, the reference signal is a recorded tape dependent reference track. Calibration is typically performed in same the manner, irrespective of the definition of the reference signal.  
         [0047]    [0047]FIG. 4 shows a graph  400  that represents the time required to travel from a point at which a reference signal is sensed to a point in the data region of the magnetic tape  310  (note, the entire width of the magnetic tape is shown in FIG. 3C and FIG. 4 only shows the data tracks,  404 ,  406 , and  408 .) The point traveled to in the data region of the magnetic tape  310  is typically the subcode  402 . The subcode  402  is a marker contained in each data track,  404 ,  406 , and  408 . It contains information about each data track  404 ,  406 , and  408 , such as the track number. The American National Standards Institute (ANSI) requires that each subcode  402  be physically positioned at a specific location within a data track. Namely, the subcode  402  must be located at a pre-specified distance (usually measured in data bits) from the starting point of the data bits contained in each data track  404 ,  406 , and  408 .  
         [0048]    The reference signal timing line  410  represents the time required to travel from the point at which the reference signal is sensed to the point at which the subcode  402  is sensed. A processor within the magnetic tape system  300  is programmed to determine the timing. The TØ value  412  represents the timing of a magnetic tape system  300  when the head  328  is sufficiently aligned with a data track. The head  328  is sufficiently aligned when the width of the head  328  equally straddles both edges of a data track. The head  328  is sufficiently aligned with data track  406  because the width of the head  328  equally straddles both edges of the data track  406 .  
         [0049]    For both data track  404  and data track  408 , the head  328  is not aligned with the data tracks. Specifically, for data track  404 , the head  328  only crosses the upper edge of the data track  404 . The resulting time required to travel from the point at which the reference signal is sensed to the point at which the subcode  402  is sensed is T 1   414 . Since T 1   414  is greater than TØ  412  a read error will occur during the operation of the magnetic tape system  300 .  
         [0050]    For data track  408 , the head  328  only crosses the lower edge of the data track  408 . The resulting time required to travel from the point at which the reference signal is sensed to the point at which the subcode  402  is sensed is T 2   416 . Since T 2   416  is less than TØ  412  a read error will occur during the operation of the magnetic tape system  300 .  
         [0051]    [0051]FIG. 5 shows a graph  500  that represents the relationship between the magnitude of a data signal and the time required to travel from a point at which a reference signal is sensed to the point at which a subcode  402  is sensed. The horizontal axis of graph  500  is the reference signal timing line  502 . The vertical axis  504  is the magnitude of the signal (also referred to as the RF envelope) measured in counts.  
         [0052]    The relative timing of data tracks  404 ,  406 , and  408  is shown on graph  500 . As observed, the head  328  senses the greatest signal magnitude for data track  406  at time TØ  412 . The magnitudes for data tracks  404  and  408  are considerably lower, at time T 1   414  and time T 2   416 , respectively.  
         [0053]    [0053]FIGS. 6A and 6B are graphs that illustrate how the performance characteristics of a magnetic tape system  300  vary with track number. FIG. 6A represents signal magnitude versus track number in counts and FIG. 6B represents relative timing versus track number in counts. This data is usually generated by moving the magnetic tape at speeds very close to the actual writing speed, without controlling the time TØ. Therefore, these curves represent the relationship between the asynchronous behavior of the read data signal amplitude and the time required to travel from the point at which the reference signal is sensed to the point at which the subcode is sensed. These curves are used for calibrating TØ.  
         [0054]    In FIG. 6A, the graph  600  displays the performance of two different heads, head A  610  and head B  612 . Data from head A  610  is represented by a square and data from head B  612  is represented by an “x.” The vertical axis  606  is the signal magnitude measured in counts. The horizontal axis  604  is the track number. The data follows a sine wave, with both head A  610  and head B  612  sensing the greatest signal magnitude at track number 0 and track number 10.  
         [0055]    In FIG. 6B, the graph  602  also displays the performance of head A  610  and head B  612 . The vertical axis  608  is relative timing measured in counts. A count is related to the internal clock of the magnetic tape system  300 . For example, if the magnetic tape system  300  has an internal clock that is incremented every 100 nanoseconds, than each individual count represents 100 nanoseconds. The horizontal axis  604  is the track number. Both head A  610  and head B  612  achieve the smallest timing at track number 6 and again at track number 15.  
         [0056]    [0056]FIG. 7 is a graph  700  that represents the relationship between the data signal magnitude and the relative timing. The graph  700  is a combination of graph  600  and graph  602 . In particular, the vertical axis  606  represents the signal magnitude and the horizontal axis  608  represents the timing. The performance of both head A  610  and head B  612  is shown. For head A  610 , the greatest signal magnitude is approximately 74 counts at time 2825 counts. For head B  612 , the greatest signal magnitude is approximately 80 counts at 2825 counts.  
         [0057]    FIGS.  8 A- 8 B represent a curve fit of the data signal magnitude for head A  612  and head B  610 . During calibration, a computer simulation of the magnetic tape system  300  uses a curve fit to determine the greatest signal magnitude. In particular, the computer simulation calculates the derivative at each point on the curve and then identifies the time at which the derivative is equal to zero. The time at which the derivative is zero corresponds to the maximum point on the curve, and thus, to the greatest signal magnitude. For head A  610 , the greatest signal magnitude is approximately 74 counts at time 2825 counts. For head B  612 , the greatest signal magnitude is approximately 80 counts at 2825 counts.  
       Reference Track  
       [0058]    In preferred embodiments of the present invention, the reference track is recorded on the magnetic tape  310 . As shown in FIG. 9, an exemplary magnetic tape  310  has an upper no data region  802 , a data region  804 , and a lower no data region  806 . The recorded reference track  810  can be located within either no data region,  802  or  806 . Locating the recorded reference track  810  in this manner prevents density related problems because the recorded reference track  810  does not occupy the data region  804 .  
         [0059]    The magnetic tape system  300  is calibrated using the recorded reference track  810  (also referred to as an embedded stripe). TØ is fixed, and thus, the distance Δd 3    812  is fixed. During the read process, the read head  328  senses the recorded reference track and the magnetic tape system  300  controls the speed of the capstan motor, such that the time required for the read head  328  to travel from the point at which the reference track is sensed to the subcode is always TØ.  
         [0060]    TØ is related to the characteristics of the magnetic tape  310 , and not the characteristics of a particular magnetic tape system  300 . Therefore, during the writing process, incorrect appends are not generated. Moreover, the read head  328  is always sufficiently aligned with the data tracks  814  and no re-calibration is required.  
         [0061]    In the preferred embodiments of the present invention, calibration is typically only required once per magnetic tape system  300 . However, a subsequent calibration may be required if the magnetic tape  310  is affected by environmental factors or excessive wearing. Such affects may cause the magnetic tape  310  to shrink or stretch, thus modifying TØ and Δd 3    812 . But even in this worst case scenario, calibration is performed before the read and write process begins, and not during the operation of the magnetic tape system  300 .  
         [0062]    As described above, the reference track is recorded on the magnetic tape  300 . However, in other embodiments the reference track could be composed of other forms of detectable markings.  
       Conclusion  
       [0063]    This concludes the description of the preferred embodiment of the invention. The present specification discloses apparatuses, systems, and methods for self calibrating a magnetic tape system. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.