Abstract:
Identification of servo tracks is provided by adding one or more additional symbols to the write elements of at least some tracks on a servo recording head. The additional symbols are ignored for timing purposes, as they are not necessarily symmetrical, but the added symbols provide identification by their alterations on the otherwise identical patterns.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to recording and reading data from magnetic storage media and, more particularly, to servo control systems that maintain the position of a magnetic head relative to tracks in magnetic tapes. 
   2. Background of the Invention 
   As the magnetic storage of information becomes more sophisticated, greater amounts of data are packed into smaller volumes of space. In storing large amounts of data on a magnetic tape, multiple “tracks” of data are stored paralleling the length of the tape. The number of tracks that can be stored on a particular width of tape depends on the sensitivity of the technology used, but it is currently possible to have hundreds of tracks on a ½ inch wide magnetic tape. 
   A major hurdle in recording and reading data from these hundreds of tracks is the lateral movement of the tape media as it traverses the read/write head. This is overcome by manufacturing the tape with recorded tracks of servo information at various lateral locations across the tape. These servo tracks provide information that allow the servo mechanisms in a tape transport to correctly position the head with respect to the data tracks. 
     FIG. 1  shows a diagram of a tape and read/write head. Tape  100  has a number of servo tracks  101 - 105 . Data is stored in bands  106 - 109  between servo tracks  101 - 105 , with each data band containing a given number of tracks, depending on the technology. In these examples, read/write head  112  contains three servo read elements  114 ,  116 ,  118  and a number of data read/write elements that are not specifically shown; read/write head  112  can be positioned over an upper portion  124  or a lower portion  126  of tape  100 . Read/write head  112  is made wider than the tape so that no matter what its position, the head  112  supports tape  100 . Servo read elements  114 ,  116 ,  118  read the information from the servo tracks; from this, the servo mechanism can calculate any movements necessary to maintain the proper position of read/write head  112  with regard to tape  100 . 
   The servo tracks, in addition to providing positioning information for the servo mechanisms, can be encoded to carry additional useful information, such as identifying the individual servo tracks and the current longitudinal position along the tape. 
   There are a number of different methods of coding information in the patterns written in the servo tracks. One very useful method is timing-based coding, as it provides a method of position sensing that is insensitive to reading speed. This method is discussed in U.S. Pat. No. 6,021,013, which is hereby incorporated by reference. In this patent, each servo track is written with a repeating cyclic sequence of two patterned lines whose separation from each other varies in a consistent manner across the width of the servo track, with periodic gaps in the pattern to serve as a starting point for the pattern. Two exemplary patterns  200  and  300  from this patent are reproduced in FIG.  2  and FIG.  3 . The time A between the detection of two dissimilar lines is compared to the time B between the detection of two similar lines. The value of A:B reveals how far off center the servo read head is at the time of reading, and thus the amount of correction necessary to properly locate the read head. Using this ratio rather than distances between symbols allows for the fact that the tape can be moving at different speeds. Thus, it is possible to maintain a position over a given band of data. 
   While the two patterns  200 ,  300  demonstrate the principles of timing-based servo patterns, it has been found that single comparisons of A and B are not enough for accurate measurements. Therefore, nested patterns, such as pattern  400  shown in  FIG. 4 , are more commonly used. Using a nested pattern of four chevrons, four measurements can be taken across a given length of tape; their averaged value of A:B is much more accurate. The detection of an expected sequence of stripes (generally, two sets of five followed by two sets of four) also provides an efficient means of error detection, as the software can immediately detect when an expected input is not found. 
   To provide the high accuracy necessary, the tape is recorded on a special servo-track machine. The first portion of the recording head contains an erase head, which erases the entire width of the tape, followed by a single-coil, multi-gap write element, capable of writing all servo tracks on a tape in one pass. Unlike normal data write elements, which are continuously powered and switch from one polarity to the opposite polarity repeatedly, the servo write element is switched on and off and normally writes in only one polarity. Additionally, the write element is patterned, so that each time it is powered, it produces an image on the tape of the patterned write element. By controlling the switching of the write element as the tape moves across the head, the pattern is repetitively written on each of the servo tracks. For a nested pattern, images of both the left and right component of a pattern are written simultaneously to maintain the accuracy. 
   Creation of the pattern shown in  FIG. 4  is demonstrated in  FIGS. 4A-J . In this example, the figures show the same section of the tape as it moves from right to left across the write head. The portion of the tape that appears white has already been erased.  FIG. 4J  is a timing pattern that shows when the write element is pulsed, while  FIGS. 4A-I  show the tape sequentially after each pulse. Looking at  FIG. 4A , the first pulse P 1  of the write element has produced an image of the pattern that is on the write element, creating the two-chevron pattern  410 - 1 .  FIG. 4B  shows the tape after pulse P 2  has written pattern  410 - 2 ,  FIG. 4C  shows it after pulse P 3  has written pattern  410 - 3 ,  FIG. 4D  shows it after pulse P 4  has written pattern  410 - 4 ,  FIG. 4E  shows it after pulse P 5  has written pattern  410 - 5 . After the initial five pulses, the write element is not pulsed again until the initial five nested chevrons  410 - 1  through  410 - 5  have all passed the write element, then four further pulses P 6 -P 9  write the pattern of four nested chevrons  410 - 6  through  410 - 7 . The process will continue until all servo patterns are written along the entire length of a tape. 
   In order for the timing based servo patterns to work properly, several factors that can adversely affect the outcome must be taken into account. First, the servo read elements must be much narrower than the servo track, so that each servo read element detects only a narrow width of the pattern created. This serves to minimize tracking errors due to false position signals. Secondly, all of the pattern lines are written using a uni-polar write current and the timing is measured only between magnetic flux transitions having the same polarity (e.g., always timing the transition at the beginning of a line). Variations in the write head or the writing process, as well as other difficulties can cause apparent shifts in the timing of transitions having opposite polarities, so opposite transitions are never compared. 
   In order to position the read head  112  of  FIG. 1  over the correct band of data  106 ,  107 ,  108 ,  109 , different patterns are used for different servo tracks. For example, if pattern  200  is used on three servo tracks  101 ,  102 ,  105  of  FIG. 1 , and pattern  300  is used on servo tracks  103  and  104 , then by reading two adjacent servo tracks and recognizing their patterns, it is possible to determine which data band is between the two servo tracks, using the following Table 1: 
   
     
       
             
             
             
           
         
             
                 
             
             
               Upper servo pattern 
               Lower servo pattern 
               Data band 
             
             
                 
             
           
           
             
               200 
               200 
               106 
             
             
               200 
               300 
               107 
             
             
               300 
               300 
               108 
             
             
               300 
               200 
               109 
             
             
                 
             
           
        
       
     
   
   Within the servo tracks, additional information, such as the longitudinal location, can be encoded by varying specific portions of the pattern, such as the distance between successive groupings of the pattern. Because servo tracks are typically recorded at a much lower density than the data tracks, they can be read at high speeds. This ability to read longitudinal data at high speeds makes searches much more efficient. 
   While the beauty of a timing-based servo system lies in the fact that it utilizes comparisons rather than individual symbols, this very concept also confines it. Since meaning is only carried in comparative relationships, the density of information is necessarily low. It would be desirable to carry additional information in the timing pattern without destroying the timing pattern itself. 
   One solution that has been disclosed is U.S. Pat. No. 6,169,640. This patent discloses using, for example, four servo tracks on which the timing patterns are identical, but not synchronized with each other. Rather, as seen in  FIG. 15 , the two inner tracks  1502  and  1503  are offset a given amount from the two outer tracks  1501  and  1504 . The result is that each adjacent pair of servo tracks exhibits a different timing relationship as compared to the other pairs of servo tracks, providing identification of the servo tracks by this offset. However, this scheme comes with its own drawback. Since the servo tracks are not synchronized with each other, the software must be concerned with phase differences between the servo tracks when creating a position error signal. It would be beneficial to identify the tracks without losing synchronicity. 
   SUMMARY OF THE INVENTION 
   Rather than the simple nested patterns currently used, the present invention adds additional symbols to the pattern of at least some of the tracks written by the servo write element. The patterns still contain only the two azimuthally inclined symbols of the prior art, but the servo patterns created are no longer symmetrical. The lack of symmetry means that not all symbols in the pattern will be used for timing purposes; however, they can be used to identify the servo track, and hence the data bands between the servo tracks. In a preferred embodiment, spaces between the symbols separate the symbols into groups, the number of symbols in each group defines the coding for the tracks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  shows a tape and a read/write head positioned to read the tape. 
       FIG. 2  shows one pattern for a timing based servo track. 
       FIG. 3  shows an alternate pattern for a timing based servo track. 
       FIG. 4  shows an alternate nested pattern for a timing based servo track. 
       FIGS. 4A-I  show the sequence as a nested pattern is created, and  FIG. 4J  shows the timing pattern for pulsing the write element to create this pattern. 
       FIGS. 5A-I  show writing a first servo track according to a first embodiment of the invention and  FIG. 5J  is the timing pattern. 
       FIGS. 6A-I  show writing a second servo track according to a first embodiment of the invention, while  FIG. 6J  again shows the timing pattern. 
       FIGS. 7A-C  show writing a first servo track according to a second embodiment of the invention, with the process shown respectively after one pulse, five pulses, and nine pulses.  FIG. 7D  shows the timing pattern. 
       FIGS. 8A-C  show writing a second servo track according to a second embodiment of the invention, with the process shown respectively after one pulse, five pulses, and nine pulses.  FIG. 8D  is the timing pattern. 
       FIGS. 9A-C  show writing a first servo track according to a third embodiment of the invention;  FIG. 9D  is a corresponding timing pattern. 
       FIGS. 10A-C  show writing a second servo track according to a third embodiment of the invention;  FIG. 10D  is a corresponding timing pattern. 
       FIG. 11  shows a tape encoded with two servo patterns according to the third embodiment above. 
       FIG. 12A  shows a simplified diagram of a servo writer system that can implement the innovative system of servo tracks;  FIG. 12B  shows a simplified diagram of a tape drive system that can use the innovative system of servo tracks. 
       FIG. 13  gives a flow for creating the servo tracks on a tape. 
       FIG. 14  gives a flow for using the tape. 
       FIG. 15  shows a prior art solution to identifying the servo tracks. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the following discussion of the servo patterns, a grid is shown above many of the patterns to mark positions along the tape. These grids are, of course, not present on the tape, but are shown for ease of understanding only. 
   In  FIGS. 5A-I  and  6 A-I, we see two separate tracks on a tape as they are written with their respective patterns  500  and  600 . The patterns of the write element extend across the entire width of the servo track. The first figure for each track is thus the image that will be created after the first pulse of the write element. In this embodiment, the servo track  500 , seen in  FIG. 5A , produces right-inclined lines at positions  1  and  6  and a left-inclined line at position  13 , while the servo track  600 , seen in  FIG. 6A , produces a right-inclined line at position  1  and left-inclined lines at positions  8  and  13 . 
   In  FIGS. 5B and 6B , the write element has pulsed again one position later on the tape, forming a second image of the write element, displaced from the first by one position. In the following  FIGS. 5C-I  and  6 C-I, the servo pattern is shown at each step as it is created using the timing diagram of  FIGS. 5J and 6J , with a first set of five pulses followed by a longer movement of the tape, then a second set of four pulses, just as in the previous example. Looking at the groupings of the symbols and the number of symbols in each group, the first track would decode as 10, 5, 4, 4, 4, while the second track would decode as 5, 10, 4, 4, 4. 
   An alternate embodiment of the servo track coding is shown in  FIGS. 7A-C  and  8 A-C on servo tracks  700  and  800  respectively, following the timing patterns reproduced as  FIGS. 7D and 8D . As seen in  FIGS. 7A and 8A , only track  700  contains a third symbol in its patterned write element.  FIGS. 7B and 8B  show the partial patterns after five pulses;  FIG. 7C and 8C  show the tracks  700  and  800  after the tape is advanced and a further four pulses. In this embodiment, track  700  decodes as 10, 5, 4, 4, 4 and track  800  decodes as 5, 5, 4, 4. 
   In a further alternate embodiment, the coding of a track lies not in the number of symbols in each group, but in how closely the transitions are spaced.  FIGS. 9A and 10A  show embodiments of the write element patterns for servo tracks  900  and  1000  respectively. Both tracks  900  and  1000  contain a right-inclined line at positions  1  and  11  and a left-inclined line at position  24 , but only track  1000  has a symbol at position  12 . The timing for this embodiment, shown in  FIGS. 9D and 10D , is slightly different from the previous versions, as the tape is allowed to move two positions between adjacent pulsing of the erase heads. In  FIGS. 9B and B , we see the pattern on tracks  900  and  1000  created by the first five pulses,  FIGS. 9C and 10C  show the pattern on tracks  900  and  1000  after advancing the tape and an additional four pulses. As a consequence of the additional symbol, the second track contains a much higher frequency of symbols in some portions than does the first track. This higher frequency can be detected by the servo hardware and used as identification. 
   In  FIG. 11 , we see a tape with five servo tracks  1101 ,  1102 ,  1103 ,  1104 , and  1105  and four data bands  1106 ,  1107 .  1108 , and  1109  containing an unknown number of data tracks. Tracks  1101 ,  1102 , and  1105  contain the pattern shown in track  900  of  FIG. 9C , while tracks  1103  and  1104  contain the pattern shown in track  1000  of FIG.  10 C. Position of the head laterally across the tape can be determined by reading any two adjacent servo tracks and using the following Table 2: 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
             
             
               Upper servo pattern 
               Lower servo pattern 
               Data band 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               900 
               900 
               1106 
             
             
               900 
               1000 
               1107 
             
             
               1000 
               1000 
               1108 
             
             
               1000 
               900 
               1109 
             
             
                 
             
           
        
       
     
   
     FIG. 12A  is a simplified version of a servo writer that can implement the innovative system of servo tracks, while  FIG. 13  gives a flow for creating the tape. These figures will now be discussed in terms of the overall flow. The tape runs between reel  1205  and reel  1204 . Tensioners  1206  help keep a constant tension on the tape for smoother operation. Erase head  1215  continuously erases the tape, followed by write head  1214 , which is pulsed to write the pattern, and read head  1216 , which verifies that the pattern was correctly written. Controller  1232  controls the movement of the reels  1204  and  1205 , as well as the erase head  1215 , write head  1214  and read head  1216 . In writing servo tracks to a tape, the tape moves from reel  1205  to reel  1204  as all servo tracks are written in one pass (step  1310 ). As the tape moves, it is first erased (step  1320 ), then the write head is pulsed repeatedly, each time creating one copy of each servo track&#39;s pattern. The writing of this repetitive pattern creates the servo track(step  1330 ). On the hardware side, pattern generator  1212  processes the given pattern and controls the pulse generator  1210  so that the write head  1214  is driven to create the desired pattern. Once the signal is written, it is verified by reading the signal, amplifying the signal in preamp  1220  and sending it to the pattern verifier electronics  1222 . Once the entire tape has been processed, the tape is ready to use for data storage. 
     FIG. 12B  shows a tape drive and tape that is being used for data storage, while  FIG. 14  shows a flow chart for the tracking mechanisms. The tape is connected so that it runs from a first reel  1265  to a second reel  1266 , passing across read/write head  1264  (step  1410 ). Tape drive controller  1282  controls the movement of reels  1265  and  1266 . The controller  1282  contains write channel  1283  and read channel  1284 , used to control the activity of read/write head  1264 , servo channel  1285  that carries signals from the servo read heads, and head positioner  1286 , which controls the servo mechanisms (not specifically shown) by which the lateral movement of the head is controlled. Whether the tape is being written to or read from, the servo tracks are read and their patterns sensed (step  1420 ). The times between detection of like symbols is measured, as are the times between detection of mated pairs of unlike symbols; average times are calculated (step  1430 ). From the calculations, corrections are figured and sent to the servo mechanisms to act upon (step  1440 ). 
   The simple innovation disclosed in this application provides the possibility of increasing the efficiency of information stored in the servo tracks by allowing the track identification to be stored in additional data other than the timing portion. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments were chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
   For instance, while tapes were disclosed having only specific combinations of two different servo patterns, any number or combination of servo patterns can be utilized. 
   Additionally, the use of patterned write heads has been disclosed for writing the servo patterns, but other methods can be devised that do not require the write heads to be patterned. 
   It is further noted that servo tracks can also be referred to as servo bands; the meaning is the same.