Abstract:
In an embodiment, a method is provided for improving signal and transducer alignment in a magnetic tape drive. The method includes writing a first track and a second track to a tape. Each track has an associated track characteristic, which may include fundamental frequency and test binary pattern, among other characteristics. The tracks are adjacent and substantially parallel to one another, and each track has a differing track characteristic value. Each track is read at multiple tracking positions to collect values corresponding to each position. An optimal offset is determined based on the collected values and the corresponding tracking positions. In another embodiment, a magnetic tape drive includes a data reader, a data writer, a processor, and a computer readable medium. The medium has stored instructions, executable by the processor, for carrying out the described method.

Description:
BACKGROUND 
   1. Field 
   Embodiments of the present invention relate to tape drives and more specifically to signal-and-transducer alignment in a tape drive. 
   2. Background Art 
   Tape drives read and write data on magnetic tapes. They commonly write data to and read data from multiple data tracks which run parallel to one another over the length of the tape. A given drive has a tape head which includes one or more data readers and data writers for respectively reading and writing the tracks. 
   With increasing track density, properly aligning the tape and the tape head becomes increasingly important. For example, as the tape moves past the tape head, lateral drift of the tape can result in the tape drive reading or writing a wrong data track—or doing so at an imprecise location. To mitigate errors resulting from lateral drift, many tape drives have servo readers on the tape head that maintain alignment with pre-written servo tracks on the tape during read and write operations. 
   However, as track density further increases, variances in the manufacture of the tape head can cause tracks to be read from, or written to, the wrong location. For example, in the fabrication of certain tape heads, servo readers and data readers may be defined in one layer of the tape head, whereas data writers may be defined in another layer. A non-ideal fabrication of the tape head may cause a layer-to-layer offset, resulting in an incorrect distance between the servo readers and the data writers. This can result in imprecise reading and writing operations. 
   SUMMARY 
   Several embodiments of the present invention take the form of a method for signal and transducer alignment in a tape drive. The method includes writing a first track and a second track to a magnetic tape. Each track has an associated track characteristic, which may include fundamental frequency and test binary pattern, among other characteristics. The tracks are adjacent and substantially parallel to one another, and each track has a differing track characteristic value. Each track is read at multiple tracking positions to collect values corresponding to each position. An optimal offset is determined based on the collected values and the corresponding tracking positions. 
   Some embodiments of the present invention take the form of a magnetic tape drive, which includes a data reader, a data writer, a processor, and a computer readable medium. The medium may have stored instructions, executable by the processor, for carrying out the method noted above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention described herein are recited with particularity in the appended claims. However, other features will become more apparent, and the embodiments may be best understood by referring to the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1A  shows a schematic diagram illustrating a tape head of a first tape drive during a read or write operation according to the prior art; 
       FIG. 1B  shows a schematic diagram illustrating a tape head of a second tape drive during a read or write operation according to the prior art; 
       FIG. 1C  shows a schematic diagram illustrating the tape head of the second tape drive reading a tape written by the first tape drive according to the prior art; 
       FIG. 2  shows a tape drive according to an embodiment of the present invention; 
       FIG. 3  shows a flow diagram illustrating a method for improving signal-and-transducer alignment in a tape drive according to an embodiment of the present invention; 
       FIG. 4  shows a schematic diagram illustrating a tape head after writing a first test track to a magnetic tape according to an embodiment of the present invention; 
       FIG. 5  shows a schematic diagram illustrating the tape head in  FIG. 4  after writing a second test track to the tape; 
       FIG. 6  shows a graph illustrating a technique for determining an optimal offset according to an embodiment of the present invention; 
       FIG. 7  shows a schematic diagram illustrating the tape head in  FIG. 5  after a write operation, in which the tape head uses the optimal offset; and 
       FIG. 8  shows a schematic diagram illustrating the tape head in  FIG. 7  after a read operation, in which the tape head does not use the optimal offset. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   This invention is not limited to the specific embodiments described below. The disclosed embodiments are merely exemplary of the invention, which may be embodied in various and alternative forms. Therefore, specific details disclosed in this section are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and for teaching a skilled artisan to employ the present invention. 
   Except in the examples, or where otherwise expressly indicated, all numerical quantities in this section are to be understood as modified by the word “about” in describing the broadest scope of the invention. Also, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” comprise plural referents unless the context clearly indicates otherwise. Unless otherwise indicated, the Figures are not necessarily to scale. 
     FIG. 1A  shows a simplified schematic diagram illustrating a first tape head  100  in accordance with the prior art. The tape head  100  is part of a first tape drive, which is not shown. The functional components of the tape head  100  are found on a bump  102 . A servo reader  106 , a data reader  108 , and a data writer  114  are coupled to the bump  102 ; the data writer  114  is located generally adjacent to the data reader  108 . Notably, the servo reader  106  and the data reader  108  are defined in a first layer of the tape head  100 ; the data writer  114  is defined in a second layer. 
   The bump  102  is positioned relative to a magnetic tape  118 . As shown, the tape  118  has a pre-written servo track  120 , along with two data tracks  122 , 124  written by the first tape drive; each track is generally parallel to the others. For illustrative simplicity, only these tracks are shown; the tape  118  may of course have other servo tracks and data tracks written to it. Notably, the tape drive may control the speed at which the tape  118  moves past the head  100  (along  126 ) and the lateral offset of the tape  118  (along  112 ). 
   The servo reader  106  may align with the servo track  120  along two positions,  119  or  121 . The position of the servo reader  106  corresponds to the region of the tape  118  that is either written to, if the tape drive is in a write operation, or read from, if it is in a read operation. For example, if the servo reader  106  is aligned with position  119 , data will either be written to or read from the region corresponding to track  122 . When the servo reader  106  is aligned with position  121 , as illustrated, the tape drive will either read to, or write data from, the region corresponding to track  124 . 
   As noted, the servo reader  106  and the data reader  108  are defined in a first tape-head layer; the data writer  114  is defined in a second layer. Due to manufacturing variations, the first layer and the second layer are offset relative to one another. As shown, the data writer  114  is offset low, so the first track  122  is written at a non-nominal distance x 1  from the servo track  120 . This offset carries over to each track, so the data writer  114  also writes the second track  124  low relative to the nominal position. As a result, the data reader  108  is offset high relative to the center  125  of the second data track  124 . This offset causes the data reader  108  to read the second data track  124  imprecisely during a subsequent read operation, possibly result in a higher incidence of read errors, especially at higher track densities. 
     FIG. 1B  shows a simplified schematic diagram illustrating a second tape head  100 ′ in accordance with the prior art. The tape head  100 ′ is part of a second tape drive, which is not shown. The second tape head  100 ′ is structurally similar to the tape drive  100  previously shown and described. The tape head  100 ′ is positioned relative to a magnetic tape  118 ′. The tape  118 ′ has a previously written servo track  120 ′, along with two data tracks  122 ′, 124 ′ written by the second tape drive. Due to a layer-to-layer offset in the tape head  100 ′, the data tracks  122 ′,  124 ′ are offset relative to the nominal position; the first track  122 ′ is written at a distance x 2  from the servo reader  120 ′. Notably, this distance is different from the write distance x 1  in the first tape head  100 . Due to this offset, the data reader  108 ′ is offset relative to the center  125 ′ of the second data track  124 ′. This may result in more read errors, especially at higher track densities. 
     FIG. 1C  shows a schematic diagram illustrating the second tape head  100 ′ reading the tape  118  previously written by the first tape head  100  according to the prior art. As mentioned, the first-head writer  114  and the second-head writer  114 ′ are each non-nominally offset from the respective servo reader due to tape-drive manufacturing variation. However, the offsets are different in each tape drive (e.g., x 1  for head  100  and x 2  for head  100 ′), due to different layer-to-layer offsets in each tape head. As a result, both the data reader  108 ′ and the data writer  110 ′ are offset relative to the center  125  of the second data track  124 . Moreover, the data writer  114 ′ is partially aligned with the first data track  122 . As a result, during a write operation, the data writer  114 ′ may undesirably overwrite part of the first data track  122 . In this manner, when multiple tape drives have non-ideal tape-head variances, data loss can occur when a tape is swapped from one drive to the other. 
     FIG. 2  shows a tape drive  200  according to an embodiment of the present invention. The tape drive  200  houses a processor  202 , a storage medium  204 , a controller  206 , and a tape head  400 ; each are electrically coupled to one another. The tape drive  400  is configured to receive magnetic tape  418 , which may, for example, be rolled on a spindle (not shown) mounted within the housing of the tape drive  200 . The processor  202  (e.g., a microprocessor) is configured to receive instructions from the storage medium  204  and provide instructions to the controller  206  (e.g., one or more motors). The controller  206  is configured to functionally operate the speed and offset of the received tape  418 , along with components in the tape head  400 , based on instructions received from the processor  202 . 
   Several embodiments of the present invention takes the form of a method to compensating for some of the errors that cause tracks to be misplaced during write operations.  FIG. 3  shows a flowchart  300  illustrating one such method. Typically, servo tracks are placed on magnetic tapes during the manufacture process. In at least one embodiment, these pre-written tracks are used to calibrate a given tape drive in order to compensate for tape-drive non-idealities (e.g., a layer-to-layer offset). The following disclosure refers primarily to manufacturing non-idealities in the form of tape-head layer-to-layer offsets. However, embodiments of the present invention may also be applicable to mitigating the effects of other manufacturing non-idealities, for example, a non-nominal data writer width. To explain each step of the flowchart  300 , reference is made to the tape drive  200  shown in  FIG. 2  and to the tape head  400  shown in  FIGS. 4 ,  5 , and  7 . 
   In step  302 , the tape drive  200  writes a first test track  422  to a tape  418 .  FIG. 4  shows a schematic view of the tape head  400  upon writing the first test track  422 . The tape head  400  attempts to write the test track  422  at a nominal location from the servo track  420  (e.g., 20 um) with a nominal track width (e.g., 11 um); however, a layer-to-layer offset in the tape head  400  may cause the actual write location of the track  422  to vary from this nominal location. Note the imprecise location of the data reader  408  relative to the center  423  of the first test track  422 . The first track  422  is written without an attempt at correcting the layer-to-layer offset. 
   In step  304 , the tape head  400  writes a second test track  424  to the tape  418 .  FIG. 5  shows a schematic view of the tape head  400  after it writes the second test track  424 . As shown, the second track  424  is written adjacent and substantially parallel to the first track  422 . As with the first track  422 , the tape head  400  attempts to write the second track  424  at a nominal location from the servo track  420  (e.g., 40 um); again, the layer-to-layer offset in the tape head  400  may cause the actual write location to vary from this nominal location. The second track  424 , like the first track  422 , is written without an attempt at correcting the tape-head layer-to-layer offset. Notably, the data reader  408  is offset relative to the center  425  of the second test track  424 . 
   Each test track  422 , 424  has a number of track characteristics, such as a fundamental frequency and a binary pattern. For example, the first track  422  may have a fundamental frequency of 10.6 MHz and a binary pattern of “1010,” and the second track  424  may have a fundamental frequency of 7.075 MHz and a binary pattern of “100100.” Although in this example the tracks  422 , 424  have differing fundamental frequencies and binary patterns from one another, each track may instead differ by a single track characteristic, for example, fundamental frequency alone. Notably, any suitable coding system (i.e., binary, trinary, etc.) may be used to encode the test tracks  422 , 424 . 
   In step  306 , each written test track  422 , 424  is read at multiple offsets from the respective nominal read location. More specifically, the controller  206  offsets the tape  418 , such that the servo track  420  is offset from the servo reader  406  by a specified distance (e.g., one micron). In the region of each track  422 , 424 , the magnitude of the track&#39;s distinguishing characteristic(s) is assessed by reading at this offset; for example, the amplitude of the given track&#39;s fundamental frequency may be read at a one micron offset. This process is then repeated at other offset distances (i.e., 2 um, 3 um, etc.). From these iterations, two sets of amplitude values result—a first set, corresponding to the data values collected measuring the value of the distinguishing characteristic of the first test track  422 , and a second set, corresponding to the values collected measuring the value of the distinguishing characteristic of the second test track  424 . 
   In step  308 , an optimal offset is determined from the two amplitude-value sets.  FIG. 6  shows a graph  600  illustrating a technique for determining the optimal offset according to an embodiment of the present invention. The horizontal axis  602  of the graph  600  corresponds to offset distance (d). A 0-um offset distance  606  represents the nominal track position; each distance greater than 0-um represents a different offset between the servo track  420  and the servo reader  406 . The vertical axis  604  of the graph  600  represents amplitude ratio (AR). The amplitude ratio is a function of offset distance. In an embodiment, each amplitude-ratio value is the ratio of the first-amplitude-set value to the second-amplitude-set value for the given offset distance. This ratio can be mathematically expressed as follows:
 
 AR ( d )= A   1,d   /A   2,d ; where:
 
   AR(d) is the amplitude ratio as a function of an offset distance, d; 
   A 1,d  is the amplitude value in the first-amplitude set for the given offset distance, d; and 
   A 2,d  is the amplitude value in the second-amplitude set for the given offset distance, d. 
   The amplitude ratios and the corresponding offset distances define a curve  608 , which has a maximum amplitude value (AR max )  610  at a corresponding offset distance (d max )  612 . This distance  612  is selected as the optimal offset distance. In at least one embodiment, the optimal offset distance is stored to the storage medium  204  for later use by the tape drive  200 . 
   The tape drive may use the determined, stored optimal offset distance during either subsequent write operations or read operations.  FIG. 7  shows a schematic view of the tape head  400  using the optimal offset during a write operation according to an embodiment of the present invention. During write operations, the servo reader  406  does not track either position  419 , 421  of the servo track  420 . Instead, the tape-drive controller  208  laterally shifts the tape  418 , offsetting the respective position  421  of the servo track  420  from the servo reader  406  by the optimal offset distance (d max ). As a result, the data track  430 , written during the previous write operation, is also offset relative to the previously-written test track  424  by the optimal offset distance. 
     FIG. 8  shows a schematic view of the tape head  400  during a read operation. Here, the tape drive  200  does not use the optimal offset distance during read operations. Accordingly, the servo reader  406  aligns with the respective position  421  of the servo track  420 . Due to correct placement of the data track  430 , the data reader  408  properly aligns with the center  431  of the written data track  430 . In this way, the tape drive  200  can use the optimal offset distance to compensate for a layer-to-layer offset, among other manufacturing non-idealities, in the tape head  400 . 
   One or more of the steps shown in  FIG. 3  may be performed during manufacturing of the tape drive  200 , effectively allowing for a one-time calibration of the tape drive  200 . Notably, the optimal offset distance may be stored to the storage medium  204  for subsequent retrieval by the tape drive  200 . Moreover, the tape drive  200  may have instructions stored to the storage medium  204  for performing the steps shown in  FIG. 3  after the tape drive has been manufactured. Such instructions may be useful for performing periodic diagnostic checks to ensure a proper signal-to-transducer alignment. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.