Abstract:
Method and apparatus for magnetic tape recording where the magnetic tape has data and servo tracks, and address or other positioning information is encoded on the servo track. A servo track burst pattern is provided whereby certain of the bursts define a gap between the stripes of the first and second half of the burst, each burst including a certain number of the stripes. The presence of the gap in a burst indicates a first binary state, which may be defined as a digital one, and the absence of the gap defines the second binary state, which may be defined as digital value 0. Hence by defining a set of 1s and 0s, longitudinal positioning or address information is encoded on the servo track, thereby improving servo track following of a tape drive.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to magnetic tape recording and more particularly to timing based positioning (servo) information recorded on magnetic tape and more particularly to encoding positioning information in the servo information. 
         [0002]    Storage subsystems for use with removable media, such as magnetic tape or disc drives, optical tape or disc drives, and the like, are widely used for storing information in digital form. With reference initially to  FIG. 1 , an exemplary storage subsystem  200  including a magnetic tape drive  202  and removable magnetic tape cartridge  106  is shown. Storage subsystem  200  may include, e.g., a PC server, server class machine, mainframe, desktop computer, or the like. Storage subsystems  200  may include a storage subsystem controller  201  for controlling one or more tape drives  202  contained within the storage subsystem  200  and for controlling other components of the storage subsystem  200 . 
         [0003]    The storage subsystem  200  may be coupled to a host system  210 , which transmits I/O requests to the storage subsystem  100  via a host/storage connection  112 . The host system  210  may comprise any computational device known in the art including, for example, a server class machine, a mainframe, a desktop computer, a laptop computer, a hand held computer, or a telephony device. 
         [0004]    Tape drive  202  reads and writes data to the primary storage medium, shown in  FIG. 1  as a magnetic tape medium  204  contained within a removable magnetic tape cartridge  206 . The magnetic tape medium  204  typically comprises a thin film of magnetic material, which stores the data. The tape medium  204  may be moved by the tape drive  202  between a pair of spaced apart reels and past a data transducer to record or read back information. In one type of tape drive system, one of the reels is part of the tape drive  202  while the other reel is part of the removable tape cartridge  206 . For this type of tape drive system, the reel which is a part of the tape drive  202  is commonly referred to as a take-up reel, while the reel which is a part of the tape cartridge  206  is commonly referred to as a cartridge reel. Upon insertion of the tape cartridge  206  into the tape drive  202 , the magnetic tape medium  204  on the cartridge reel is coupled to the take-up reel of the tape drive  202 . Subsequently, prior to removing the tape cartridge  106  from the tape drive  202 , the storage tape  204  is rewound onto the cartridge reel and is then uncoupled from the take-up reel. 
         [0005]    Tape drive  202  further includes a tape drive controller  203  for controlling, at least in part, data transfer operations. Tape drive controller  203  may further include or access a tape drive memory, which together may analyze and/or store historical event information. Further tape drive controller  203  may include or access logic for displaying the historical event information via a front panel of tape drive  203 , as described in greater detail below. 
         [0006]    In some tape storage subsystems, the removable tape cartridge  206  is provided with a non-volatile auxiliary memory  208  for storing data in a separate storage medium from the primary storage medium. This data is separate from and in addition to the data stored on the primary storage medium. This auxiliary memory  208  can be, for example, a solid state non-volatile memory such as an electrically erasable programmable read-only memory (EEPROM) or a flash memory which is contained in the housing for the tape cartridge  206 . Auxiliary memory  208  may further store historical event information accessible by drive  202  and/or storage subsystem  200 . 
         [0007]    A position feedback (“servo”) signal when read by a magnetic recording head in such a tape drive from timing data recorded on the magnetic recording tape generates an error signal that describes the relative motion between the head and the Lateral Tape Motion (LTM) in the tape drive. This error signal is commonly referred as the PES (Position Error Signal). Current “LTO” format (Linear Tape Open) magnetic recording tape has embedded magnetic timing stripes that are decoded by LTO tape drives to generate a linear PES signal, which is used to track the LTM that results in correct placement of data tracks on tape as defined by the tape format. (LTO is a family of industry standard formats or specifications in the magnetic tape field.) 
         [0008]    LTO specifies a ½ tape width. It is intended for large amounts of data storage. There are typically  384  to  896  tape tracks, and the tape drive has 8 or 16 write elements. The tracks occur in groups, with four data bands interspersed between five servo (positioning) bands. The tape drive read/write heads straddle the two servo bands that border the data band being written or read. Usually the servo tracks are written onto the tape when the LTO tape cartridge is manufactured. The servo mechanism in the tape drive constantly moves the read/write head to keep it on the data track. The head includes special sensors that monitor (read) the servo tracks, to provide the read/write head positioning. LTO tapes are housed in cartridges having a specified form factor. 
         [0009]    The LTO format has a series of  18  timing stripes all with ±6 degrees of azimuth angle written in a specific format, having a set of A, B, C and D stripes. The LTO format specifies the accuracy of the servo writing by specifying critical physical dimensions that will result in precise PES decoding to measure RHP (Relative Head Position). 
         [0010]    As described in the LTO format specifications, the PES is defined as the ratiometric timing difference between the sets of A, B, C and D stripes as shown below. Since the format defines the A to C and C to A distance as 100 μm±0.25 μm over 7.2 mm of longitudinal distance, this uncertainty results in a calculation error which limits the performance of the tape drive&#39;s servo tracking system. 
         [0011]    LTO drives and tape are typically used for recording backup data in computer systems, but not so limited. There are several versions of the LTO standard. The cartridge which houses the tape also has a particular form factor defined by the standard. Various current LTO standards are referred to as LTO- 1  through LTO- 5 . LTO in one version specifies four wide data bands sandwiched between five narrow servo bands or tracks, as referred to above. The data bands are numbered 3, 1, 0, 2 across the tape, and are recorded individually in numeric order. The head unit straddles the two servo bands that border the data band that is being written or read. The servo bands (tracks) are used as explained above to keep the head assembly precisely aligned with the data band currently being read or written to. Typically the magnetic servo tracks are written on the tape in the factory, when the tape cartridge is manufactured. Note that the above information pertaining to LTO is merely illustrative. The present invention is not limited to the LTO or any other particular magnetic tape recording standard, but is useful in conjunction therewith in certain embodiments. 
         [0012]    Albrecht et al. U.S. Pat. No. 5,930,065, incorporated herein by reference in its entirety, discloses in magnetic tape recording that a way to maximize recording capacity is to maximize the number of parallel tracks on the magnetic tape. The typical way of maximizing the number of tracks is to employ servo systems, also known as positioning systems, which provide track following and allow the tracks to be spaced closely. An example of track following is provision of groups of pre-recorded parallel longitudinal servo tracks that lie between groups of longitudinal data tracks carrying the recorded data so that one or more servo heads (magnetic recording read heads) may read the servo information. An accompanying track following servo subsystem adjusts the lateral position of the magnetic head (or of the tape) to maintain the servo heads centered over the corresponding servo tracks. Since the servo heads are spaced a well defined distance from the respective data read/write magnetic heads, centering of the servo heads results in the data heads being centered over the respective data tracks. 
         [0013]    The servo patterns are, like other data recorded on a magnetic tape, a set of magnetic flux transitions recorded on the tape. The servo patterns are typically recorded at non-parallel angles such that the timing of the servo transitions read from the servo pattern at any point on the pattern varies continuously as the servo head is moved across the width of the servo pattern. For instance, a pattern may include straight transitions essentially perpendicular to the length of the track alternating with sloped or slanted transitions. Thus, the relative timing of transitions read by a servo read head varies linearly depending on the lateral position of the servo read head. 
         [0014]    Although determination of the lateral position of a head with respect to the width (latitude) of a tape may be readily accomplished by such servo systems, generally there has not been a good way of determining longitudinal (length) position of the tape relative to the read/write heads. Rough estimates of longitudinal position of a tape may be made by counting the number of rotations of an idle guide wheel or of a motor or reel of the tape drive. More accurate longitudinal position information relative to data records may be based on detection of the actual data records. These methods are generally not a 100% successful. 
         [0015]    Hence Albrecht discloses superimposing servo longitudinal data information on the servo tracks. This longitudinal data information includes, e.g., longitudinal addressing or tachometer information. The Albrecht servo information is recorded in magnetic flux transition patterns defining at least one longitudinal servo track. A servo burst pattern of at least two repeated pairs of non-parallel magnetic flux transitions is provided at least one of which transitions of each pair is slanted or otherwise continuously longitudinally variable across the width of the servo track. Moreover, at least two transitions of the repeated pairs are shifted longitudinally with respect to other of the transitions of the repeated pairs, the shifted transitions comprising the superimposed addressing data information. 
         [0016]    To better understand this, present  FIG. 2 , identical to  FIG. 2  of Albrecht, shows a timing based servo system  10  as used in tape drive  202  of  FIG. 1  and that reads a magnetic tape servo pattern and reads data superimposed in the servo pattern. System  10  is typically part of a conventional magnetic tape drive  201  that accepts a tape cartridge  206  (not shown in  FIG. 2 ) and is connected to a host processor  210  (not shown in  FIG. 2 ). The tape cartridge  206  conventionally is a housing containing a length of magnetic tape  20  in  FIG. 2 , only a short length of which is shown for purposes of simplicity. Such tape drives typically include drive motors for rotating the wheels of the cartridge to move the tape  20  across a head assembly  24 , not shown in detail. Head assembly  24  includes a relatively narrow servo read head  26  that detects a servo pattern recorded in a servo track  27  of the tape  20 . A data head  28  of the head assembly is typically larger than the servo head and is positioned over a data track region  29  of the tape containing multiple data tracks (not shown) for reading data recorded in a data track or for writing data in a data track. 
         [0017]    For simplicity,  FIG. 2  shows only a single servo read head and single data head. Most tape systems have multiple parallel servo tracks, multiple servo read heads, and multiple data read and write heads, all of which are conventional. The servo track centerline  30  is indicated as extending along the length of tape  20 . Servo read head  26  is relatively narrow and has a width substantially less than the width of the servo track  27 . The tape  20  is moved longitudinally across the tape head assembly  24  so that the servo track  27  is moved linearly with respect to the servo head  26 . When such movement occurs, the servo pattern magnetic flux transitions are detected by the servo read head  26  to generate an analog servo read head signal that is provided via a servo signal line  34  to a signal decoder  36 . Signal decoder  36  processes the servo read head signal and generates a position signal is transmitted via position signal line  38  to a servo controller  40 . Servo controller  40  generates a servo control signal and provides it on control line  42  to a servo positioning mechanism, which is part of head assembly  24 . The servo positioning mechanism (not shown) responds to the control signal from the servo controller  40  by moving the assembly, including servo head  26  laterally with respect to the servo track centerline  30  to reach the desired servo track or to maintain the servo head  26  center with respect to the servo track centerline  30 . 
         [0018]      FIG. 4  illustrates somewhat similarly to what is shown in  FIG. 2  a portion of a servo track which is conventional, that is not including the shifted flux transitions of the servo track of Albrecht. In this case, the servo track  27  defines a centerline  30 , the same as in  FIG. 2 . Here the servo patterns are depicted as sets of slanted lines, also referred to as “stripes” and which are in a physical sense magnetic flux transitions recorded on the tape. In this case according to the above described well known LTO tape standards, these flux transitions for the servo track are organized in bursts, referred to as bursts A, B, C, and D. 
         [0019]    In  FIG. 4 , each servo burst includes four or five stripes. Each stripe in fact is defined by two (one positive and one negative) magnetic flux transitions. In this case bursts A and B each have five stripes, whereas bursts C and D only have four stripes. As shown, the stripes lie at an angle to a hypothetical line perpendicular to servo track centerline  30 . This deviation from the perpendicular is referred to as the azimuth angle and is defined in the appropriate tape standard. As shown, the tape is in this case divided into frames F, each frame length being approximately  200  micrometers long. Hence  FIG. 3  shows a conventional LTO type servo track. 
         [0020]    In Albrecht as described above, longitudinal positioning data is encoded into the servo bursts. In order to encode the information, two stripes in a burst are shifted longitudinally (longitudinal here refers to the direction of tape travel as indicated by the servo track centerline  30 ) with respect to other of the stripes in that burst. The shifting defines the superimposed addressing data, see Albrecht  FIG. 5 . Briefly, in order to encode a digital data bit indicated by 1, one shifts the second stripe in either the A burst or the B burst to the left by 0.25 micrometers. One also shifts the fourth stripe in the A burst or the B burst to the right 0.25 micrometers. The shifted stripes are indicated by the diamond patterns in present  FIG. 4 . (This shifting of course is compared to their nominal positions.) 
         [0021]    Similarly to encode a digital value 0, Albrecht shifts the second stripe in the A burst or B burst to the right 0.25 micrometers and shifts the fourth stripe in the A burst or the B burst to the left 0.25 millimeters. In either case, this opens up gaps in terms of the nominal spacing of the stripes in a burst and these gaps define the digital data states 1, as indicated above. Note the selection of the A and B bursts here is arbitrary. 
         [0022]    This approach has significant drawbacks as recognized by the present inventor. If the stripes are placed closer together than their nominal spacing, the maximum linear density of the servo patterns is the new minimum distance between any two adjacent stripes. For instance in the LTO standard, typical spacing between stripes is 5×10 −6  meters. But given the above shifting in Albrecht, the minimum distance between any two stripes is only 4.75×10 −6  meters for the encoded data. This places a limit on the maximum linear density that can be realized with a given servo pattern. It also may cause what is referred to as inter-symbol interference (ISI) in the individual stripes on the tape, in other words making it harder to read such data. It is expected that future tape drives will have increased linear density of the servo pattern. In that case, the Albrecht approach of  FIG. 4  becomes even less useful. 
       SUMMARY 
       [0023]    In accordance with the present invention, a system and method for carrying encoded data indicating longitudinal information on a magnetic tape using the servo pattern are provided which differ from and improve upon that referred to above. In the present encoding scheme, servo stripe shifting is also used. However in this case the shifting of the stripes occurs in, e.g., “even” bursts C and/or D. Moreover those stripes are shifted so that the stripes are never any closer than the nominal stripe spacing defined by the overall servo pattern. This reduces the ISI problem. In this case the shifting is in the C and D bursts, which in this case contain an even number (four) of stripes as shown in  FIG. 4 . The gap between stripes in the center of the burst due to the shifting, in one embodiment, is less than or equal to two times the stripe spacing as defined by the overall (nominal) density of the servo pattern. Of course, this gap increases the duration of the burst by an equal amount. Another way to express this is that the first, second and third stripes of a C or D burst are shifted away from the burst center by an amount less than or equal to the nominal stripe spacing, and similarly the fourth, fifth and sixth stripes in that burst are shifted away from the burst center (in the opposite direction) by the same amount. Hence the overall burst length is increased by the total of the two (opposing) shifts. However the shift in any case is only used to indicate the 1 bit. The absence of any shift indicates a zero bit (or vice versa). 
         [0024]    This approach has several advantages. First, the minimum gap between stripes to encode data is always increased, to ensure ease of detection. Since the servo pattern is made up of bursts that contain either odd (long burst) or even (short burst) numbers of stripes, by using only the even bursts to carry data, one can make the gap equal to the width of two stripes without increasing the length of the burst (compared to the odd bursts) on the tape in the longitudinal direction. Hence stripes are never moved closer together (shifted) for encoding purposes, but only moved (shifted) apart. Moreover by shifting both halves of any particular burst in opposite directions, any two adjacent stripes in the burst can be used to detect the encoded bit. 
         [0025]    To put this another way, the so-called even bursts, that is the bursts having an even number of stripes such as bursts C and D in  FIG. 4 , are effectively split in half and each half is shifted away from the other to encode the first digital value, such that the resulting gap in the center of the burst is LESS THAN OR equal to two times the stripe spacing defined by the overall linear density of the servo pattern. Thus the longitudinal length on the tape of these even bursts carrying the data, even with the shifted stripes, is LESS THAN OR EQUAL TO the length of the A and B (odd) bursts. Of course this is because the A and B odd bursts here have five stripes versus four. Further, detection of the stripes is relatively easy since instead of only detecting the gap, one can detect the encoded data by observing the relative locations of any combination of two stripes where the first stripe is taken from the first half of the burst, and the second stripe is taken from the second half of the burst. Alternatively, an average of the stripe positions in the two halves can be used to identify the gap. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0026]      FIG. 1  shows a storage subsystem in the prior art and in which the present invention may be implemented. 
           [0027]      FIG. 2  shows a servo pattern reading system in the prior art. 
           [0028]      FIG. 3  shows detail of the  FIG. 1  signal decoder  36  in the prior art. 
           [0029]      FIG. 4  shows a method of encoding longitudinal position information on a servo track in the prior art. 
           [0030]      FIGS. 5A and 5B  show in the accordance with the invention encoding data referring to longitudinal positioning on a servo track. 
           [0031]      FIG. 6  shows a servo writer in the prior art and in which the present invention can be implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 5A  shows how in accordance with the invention longitudinal position information is encoded on a servo pattern. In this case, the bursts are designated (as in  FIG. 4 ), A, B, C, and D.  FIG. 5A  shows encoding a first digital state (e.g., binary value 0), which as indicated above, includes no shift in any stripes. Hence, in  FIG. 5A  all the stripes in the C and D bursts (the only bursts of interest here) are in their nominal position. In this case, the C bursts have an even number of stripes (eight), as do the D bursts. The A bursts have an odd number of stripes, equal to the even number of stripes plus one (nine), as do the B bursts. The particular number of stripes is not limiting since various tape standards typically specify different numbers of stripes depending on the standard in the servo tracks. Here,  FIG. 5A  shows that some bursts have an even number of stripes, which are the C and D bursts, and the other bursts have odd numbers of stripes, which are the A and B bursts. Note that moreover the stripes can have different shapes. While they are shown as being straight lines at an azimuth angle from the vertical here (per the LTO standard), they could adopt the diamond or chevron shapes typical in the field. Moreover in  FIG. 5A , the servo pattern is imposed purely for purposes of illustration on a grid. The tape movement is in the longitudinal direction. Thus  FIG. 5A  shows the “no shift in any stripes” state which is pointed out above indicates in one embodiment encoding a 0 in terms of position information. 
         [0033]    In contrast,  FIG. 5B  shows the other digital data (binary) state in accordance with the invention which encodes in one embodiment the digital value 1 in terms of position information by shifting each half of certain bursts away from the burst center. In this case, the “even” bursts C and D are subject to the shifting which is more convenient since they have one less stripe then do the A and B bursts. As indicated in  FIG. 5B , the shifting causes a gap in the center of certain of the C and D bursts. In this case the C and D bursts do carry the digital value 1. The A and B bursts have their stripes in the nominal position since having an odd number of stripes, one does not apply the shifting to these bursts and they are not used to encode data. This is because the even bursts having one less stripe may have the shift applied to them without increasing the overall width of any particular burst to be any greater than that of an A or B burst. This limits the overall length (duration) of even the shifted bursts to the nominal burst length of the (longer) “odd” A and B bursts in this embodiment. 
         [0034]    Hence as pointed out above, by using the even bursts to carry the encoded 1 or 0 digital data values, one can make the gap in these separated or shifted bursts equal to the width of two stripes without increasing the length of the burst on the tape compared to that of a (long) A or B burst. Also with regard to the shifted bursts, the stripes are never moved closer together unlike Albrecht, but only moved apart. This reduces the effects of ISI. Moreover since all the stripes in each shifted burst are actually moved from their nominal positions, any two stripes in a burst, one taken from the first half of the burst and one taken from the second half of the burst, may be used to detect the encoded bit (or its absence). This is not the case with Albrecht. 
         [0035]    In Albrecht, no matter how many stripes are in the bursts, there are only two chances of detecting the change in spacing between the relative stripe shifts, i.e. for the 4455 pattern the distance between the second and fourth stripes of burst A or B will signify a one or zero. With the present encoding scheme, if (N=number of stripes in the even burst) one has N chances to detect the encoded bit. As example, if there are six stripes in the even burst, one has three chances in each C or D burst to detect the encoded bit. Hence the present approach also provides more reliable bit detection. 
         [0036]    For an LTO format tape an exemplary amount of the actual stripe shift in terms of longitudinal direction, in other words the gap width, is approximately 5×10 −6  meters. This is not limiting. 
         [0037]    The tape drive servo system to read the servo patterns of  FIGS. 5A and 5B  would be very similar to that shown in  FIG. 2  using signal decoder  36  in a servo reader  10 . Present  FIG. 3  is the same as  FIG. 11  of Albrecht, and shows the signal decoder  36  of  FIG. 2  in a block diagram in more detail. Hence, this is a data decoding system or decoder. The analog output signal from the servo head is provided on input line  34  from  FIG. 2  to peak detection channel  70 , which provides output signals of the position and negative peaks of the servo transitions to servo position error signal (PES) generation circuit  71 . PES circuitry  71  also provides signals indicating the various gaps between the stripes. PES circuit  71  counts the stripes to establish the longitudinal position of the servo head with respect to the stripes and provides one of four signals at each gap. The signals indicate the number of stripes counted and whether they are typically chevrons or diamond patterns, which are variance of the stripes. The resulting output signals are provided on line  73  to bit detection and synchronization logic  75 . 
         [0038]    The output signals of peak detection channel  70  are also supplied to bit detection and synchronization logic  75 , which decodes the detected positive peaks of the stripe transitions based on the intervals between the peaks to decode the encoded data bits. The resulting bits as detected are supplied to format decoder  77  to be formatted into digital words and the result in data streams supplied to the tape drive controller microprocessor (not shown) over interface  78 . Note that this merely illustrative of a particular type of data decoder. 
         [0039]    The only modification to the  FIG. 3  apparatus to carry out the present method would be in the bit detection and synchronization logic  75 , since instead of detecting the type of gap shown in Albrecht in  FIG. 4 , one instead detects the type of gap shown in  FIG. 5B . Moreover the detected absence of a gap here would indicate the value 0. Such a modification to the logic for bit detection  75  described in Albrecht would be routine and easily accomplished by one of ordinary skill in the art. 
         [0040]    Also needed is a suitable servo writer to write the patterns of  FIGS. 5A and 5B . Again, this would be accomplished with routine modifications (in light of this disclosure) to servo writers of the well known type. Present  FIG. 6  corresponds to  FIG. 28  of Albrecht showing a conventional “writing generator”, also known as a servo writer, for writing servo patterns. Servo write head  402  writes the servo pattern on a tape  504  with the servo pattern itself as illustrated just above  504 . In this case the servo patterns are shown as being chevrons rather than straight stripes, but of course this is not limiting. Tape  504  is conventionally moved between the reels (not shown) of a tape drive. Controller  432  and encoder  433  together are a pattern generator. The encoded data, that is the 1s and 0s to be written, is loaded from the encoder  433  to a shift register  435  under control of controller  432  and shifted to pulse generator  518 . The shift register  435  represents the timing of the supply of pulses by the pulse generator  518  to cause the write head  402  to write the corresponding flux transitions on tape  402 . Thus rather than a regularly repeating stripe pattern, which does not carry any encoded information, the shift register data controls the timing of the pulse generator so as to shift the flux transitions to superimpose the desired encoded data as explained above on the servo pattern. The tachometer counter  437  is incremented by the shift register  435  and supplied to encoder  433  to track the tape position. 
         [0041]    In one embodiment, the I/O binary data encoded in the servo bursts conforms to what is referred to in the field as LPOS words (numbers), each word spanning  36  servo frames with one binary digit per servo frame. The LPOS word value increments by one along the length of the tape every  36  servo frames for the full length of the tape. Each LPOS word also includes a sync mark, and is of course 36 bits long. This is merely illustrative. 
         [0042]    The storage system  200  of  FIG. 1 , which includes magnetic tape drive  202 , is readily modified to include the present servo reader for reading the servo patterns on magnetic tape medium  204  in accordance with this invention as described above. 
         [0043]    This disclosure is illustrative and not limiting. Further modifications and improvements will be apparent to those skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.