Abstract:
A system and method for encoding servo sector information in a nonvolatile data storage and retrieval system using rotating recording disks. Servo sectors are angularly-spaced approximately radial regions reserved for position reference and tracking adjustment information. Servo timing marks including means for detecting defects on the recording surface, encoded track address and sector address data, and position error signal blocks are permanently fabricated into a number of servo sectors. The present invention helps the storage system controller locate, certify, and follow any particular track and sector while maximizing the amount of disk surface area available for data storage and retrieval.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to patent application Ser. No. 08/825,651, entitled “System and Method to Compensate for Data Defects Within a Magneto-Optical Computer Memory Device,” filed on Apr. 3, 1997 and patent application Ser. No. 08/866,174, entitled “System and Method For Generating Position Error Signals Within A Magneto-Optical Computer Memory Device,” filed on Jun. 30, 1997. The subject matter of each of these related applications is incorporated herein by reference. All related applications are commonly assigned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to nonvolatile data storage systems, and more particularly to such systems having storage mechanisms including read/write heads that are indexed and precisely positioned via encoded servo sectors. 
     2. Description of the Background Art 
     Designers, manufacturers, and users of computing systems require reliable and efficient digital information storage and retrieval equipment. Conventional magnetic disk drive storage systems have been commonly used and are well known in the art. These storage systems typically use a flying magnetic read/write head to record and retrieve data from a layer of magnetic recording material on the surface of a rotating recording disk. The capacity of such a storage system is a function of the number of closely spaced concentric tracks on the recording disk that may be reliably accessed by the read/write head. Some of the recording disk surface area is used for purposes other than data storage, however. 
     Means for assuring the proper selection of a particular track by the read/write head are required for reliable data storage and retrieval. The read/write head should also be kept centered over a particular track as the recording disk rotates, to prevent accidental over-writing of data stored in neighboring tracks. Some systems use nonmagnetic guard rings between discrete tracks on the recording disk to help keep the head from skipping off-track. Gain control references should be placed at different locations on the recording disk to calibrate the electronic amplifiers used to reliably read back data signals. Time delays are also needed to allow the magnetic read/write head to demagnetize after recording data, to prevent unintentional over-writing of subsequently accessed locations. The designs created to accomplish these goals take up some of the available recording disk surface area, and thus reduce overall system capacity. 
     Various types of indexing marks and alignment indicia are also recorded on the recording disk surface for precise position reference and tracking adjustment of the read/write head. These marks and indicia are often recorded in servo sectors, which are angularly-spaced reserved portions of the recording disk surface that extend out approximately radially from the recording disk centers. Track addresses are sometimes recorded in servo sectors. Angular synchronization signals that determine the circumferential location of the magnetic head may also be recorded in servo sectors. Normal and quadrature servo blocks are often recorded in servo sectors for generation of position error signals that are used to keep the read/write head aligned. Servo sectors use recording disk surface area that could otherwise be used for data storage, however, so servo sector information should be stored as efficiently as possible. 
     Newer magneto-optical technology offers many improvements over conventional magnetic technology, particularly in terms of increased capacity. Magneto-optical storage systems also record data onto a recording material coated onto the surfaces of one or more rotating recording disks, but via different means than conventional drives. The recording material undergoes a sharp increase in magnetic susceptibility when heated beyond its Curie point, the temperature at which the magnetic properties of the recording material change from ferromagnetic to paramagnetic. A localized magnetic domain is created by heating a region of the recording material and then applying a magnetic field of a desired orientation to the heated region. When the recording material cools, the localized magnetic domain retains its magnetic orientation and again becomes far less susceptible to applied magnetic fields. 
     An optical fiber may guide an intense beam of focused laser light to heat a localized magnetic domain to be recorded or overwritten. The data stored in a particular localized magnetic domain may also be read back nondestructively by such a combined laser and optical fiber system. A low-powered, linearly polarized laser beam focused on a particular localized magnetic domain will be reflected with a Kerr rotation of the angle of polarization determined by the magnetic orientation of the localized magnetic domain. The pattern of polarization rotations read back as the low-powered laser beam moves across the recording surface thus represents the pattern of magnetic orientations previously written onto the recording surface. The overall reflectivity of a localized magnetic domain may also be determined via measurement of the relative amplitude of the reflected laser beam. 
     Magneto-optical storage systems should quickly and reliably locate and align to any particular storage location on the recording disk, as with existing storage systems. A scheme for accomplishing these goals that takes advantage of the unique properties of a magneto-optical storage system is needed. An efficient system for encoding servo sector information is therefore important for maximizing the amount of remaining disk surface area available for data storage and retrieval. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a system and method are disclosed for efficiently encoding servo sector information in a data storage system using rotating magneto-optical recording disks. In the preferred embodiment, servo sectors are angularly-spaced portions of the recording disk surface that extend out radially from the disk centers and include position reference and tracking adjustment information for read/write heads. The fraction of the recording disk surface occupied by servo sectors should be minimized to maximize storage capacity. 
     Servo sector patterns are quickly stamped into recording surfaces during manufacture and can simultaneously improve many aspects of prior storage systems. Automatic gain control reference blocks of conventional magnetic storage systems are rendered unnecessary because the reflectivity of the servo sectors is uniform across the recording disk surface. Write-to-read recovery time, usually needed to allow a high-intensity recording laser beam to reduce power to the level used for reading data, is also unnecessary when servo sector patterns are indelible. Servo timing marks are preferably embossed into each of the servo sectors to help synchronize data storage and retrieval with the rotation of the recording disks. The servo timing marks may also serve as part of a system to detect defects on the recording disk surfaces. Encoded track address data and sector address data, and position error signal blocks are also preferably permanently affixed into each of the servo sectors to help the read/write heads locate and follow a particular track. 
     In the preferred embodiment, servo sector information is read via reflectivity measurement, not the magnetically-induced Kerr rotation measurement used for data storage and retrieval in magneto-optical systems. Since the servo timing marks therefore do not need to be discriminated from data, error correction efficiency is roughly doubled. Surface reflectivity between distinct servo timing marks preferably certifies the recording surface as reliable for writing, eliminating the need for read-back verification delays. 
     The radial seeking speed of the read/write head is increased in the preferred embodiment by use of only a few lower-order track address bits on every sector. The full track address is rarely needed because the target track is typically selected from only a small number of frequently-scanned neighboring tracks having almost identical addresses. Higher-order track address bits may be distributed across consecutive servo sectors because they are needed only as a less frequent confirmation that the lower-order track address bits are properly wrapped, that is, successfully ramped through neighboring low-to-high and high-to-low address transitions. 
     A repeating sequence of bits distributed around the circumference of the recording disk in the preferred embodiment verifies the servo sector number kept in a counter in a disk controller. The sequence of distributed bits also identifies the phase of the distributed higher-order track address bits for proper significance assignment. A timing mark to enable precise rotation synchronization by the disk controller preferably comprises either a higher-order track address bit or its complement, to guarantee that a positive mark will always be available for detection and use. 
     The present invention thus enables the storage system to quickly and reliably locate, certify, and follow any particular track and sector while maximizing the amount of disk surface area available for data storage and retrieval. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of one embodiment of a computer system, in accordance with the present invention; 
     FIG. 2 is a diagram of one embodiment of the FIG. 1 data storage system, in accordance with the present invention; 
     FIG. 3 is a diagram of an upper surface of a recording disk, in accordance with the present invention; 
     FIG. 4 is a linearized diagram of one embodiment of a sector of the FIG. 3 recording disk, in accordance with the present invention; 
     FIG. 5A is a diagram of one embodiment of the physical layout of a servo timing mark, in accordance with the present invention; 
     FIG. 5B is a graph of track reflectivity, in accordance with the present invention; 
     FIG. 5C is a depiction of one embodiment of reflectivity signal processing into digital bits, in accordance with the present invention; 
     FIG. 6 is a logical diagram of one embodiment of a servo sector of a single track, in accordance with the present invention; 
     FIG. 7 is a table summarizing one embodiment for a Z bit distribution versus sector number, in accordance with the present invention; 
     FIG. 8 is a table summarizing one embodiment for an X bit sequence, denoting a repeating pattern used to identify circumferential phase of a sector and to detect completion of a full rotation of the recording disk, in accordance with the present invention; 
     FIG. 9 is a diagram of one embodiment for a physical layout of position error signal blocks and delay fields, in accordance with the present invention; 
     FIG. 10 is a diagram of one embodiment for a physical layout of an entire servo sector for a set of five tracks, as well as portions of neighboring data wedges, in accordance with the present invention; and 
     FIG. 11 is a flowchart of one embodiment of method steps for performing servo sector processing, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a system and method of efficiently encoding servo sector information in a data storage system using magneto-optical recording disks. 
     Referring now to FIG. 1, one embodiment of a computer system  100  is shown. The computer system of FIG. 1 preferably includes a central processing unit (CPU)  102 , a display  104 , an input device  106 , a data bus  108 , random access memory (RAM)  110 , read-only memory (ROM)  112 , and a data storage system  114 . 
     Referring now to FIG. 2, one embodiment of the FIG. 1 data storage system  114  is shown. The data storage system  114  of FIG. 2 preferably includes a recording disk  202 , a storage system controller  204 , a servo arm  206 , a servo actuator  208 , a read/write head  210 , and a rotating spindle  212 . Read/write head  210  is positioned at the end of servo arm  206  which is moved via servo actuator  208 , and transfers data between storage system controller  204  and a specific physical location on recording disk  202 . Data is preferably stored in many approximately consecutively-numbered concentric rings or “tracks”  214  on recording disk  202 ; only two tracks  214  are shown for clarity. Storage system controller  204  in the preferred embodiment may thus randomly access a specific logical location on recording disk  202  via a particular track address and a particular sector address. Tracks  214  are very closely spaced in the preferred embodiment to maximize storage capacity and economy. The mechanical precision of the movement of recording disk  202  and the movement of read/write head  210  is often far below the precision of track  214  spacing, however. Storage system controller  204  thus requires some means for precisely maintaining read/write head  210  over any track  214  and for positioning read/write head  210  quickly and accurately over other tracks  214  for subsequent storage and retrieval operations. 
     Referring now to FIG. 3, an upper surface of recording disk  202  used in data storage system  114  is shown. The upper surface of recording disk  202  of FIG. 3 preferably includes a startup zone  302 , a useable data zone  304 , arc-shaped sectors  306 , and an arc-shaped path  308  taken across recording disk  202  surface by read/write head  210 . Servo arm  206  turns around an actuator axis  310  to position read/write head  210  relative to the center of spindle  212 . Read/write head  210  thus traces out arc-shaped path  308  over the recording disk  202  surface in the preferred embodiment. Deviation of arc-shaped path  308  from a purely radial line varies by track  214  number and in the preferred embodiment is given by:        x   =       (     OD   -     n   ·   TP       )     ·     [         cos     -   1            (           B   2     -     A   2         B     )       -       cos     -   1            (           (     OD   -     n   ·   TP       )     2     +     B   2     -     A   2         2   ·     (     OD   -     n   ·   TP       )     ·   B       )         ]                              
     where x is the deviation in micrometers, n is the track  214  number, OD is the radius of outermost diameter track  214  number zero (nominally 64,600 micrometers), A is the length of servo arm  206  from actuator axis  310  to the point of read/write head  210  focus (nominally 66,789 micrometers), B is the distance from the center of spindle  212  to actuator axis  310  (nominally 76,937 micrometers), and TP is the track  214  pitch. Tracks  214  are preferably numbered sequentially from zero at the outermost edge of recording disk  202  to higher values toward the center of recording disk  202 . 
     Each recording disk  202  in the preferred embodiment is nominally 130 millimeters in diameter and is coated with a magneto-optical recording material on both the upper and lower surfaces. In the preferred embodiment, recording disks  202  rotate counterclockwise when viewed from above. There are  255  sectors  306  (numbered 0 through 254) and 55,776 concentric tracks  214  in the preferred embodiment. Each track  214  is nominally 0.71 micrometers in radial pitch in the preferred embodiment. Startup zone  302  is the approximately two millimeters of inner radial width, totaling 2,816 tracks  214  in the preferred embodiment. All data positions in startup zone  302  are positively written, that is, magnetized for Kerr rotation in the preferred embodiment, to measure the relative amplitude of the read back signal for laser power calibration and to set amplifier gains. Useable data zone  304  extends over the remaining surface of recording disk  202 , spanning 52,960 tracks  214  in the preferred embodiment, although the outermost 1,276 tracks  214  may also be used for internal drive calibration. 
     Referring now to FIG. 4, a linearized diagram of one embodiment of an exemplary sector  306  of the FIG. 3 recording disk  202 , a servo sector  402 , a data wedge  404 , a pair of neighboring numbered concentric tracks  406  and  408 , and a border  410  between startup zone  302  and useable data zone  304  are shown. Data wedge  404  preferably includes stored user data, while servo sector  402  includes address and alignment information used by storage system  114 . 
     Although FIG. 4 is a linearized diagram, for simplicity, it is important to note that the actual shape of any servo sector  402  in the preferred embodiment is determined by the equation given above. Servo sectors  402  are preferably not recorded on the surface of recording disk  202  by read/write head  210  as in some magnetic storage systems, but instead are indelibly stamped into recording disk  202  surface during manufacture. Information stored in servo sectors  402  thus cannot be overwritten by data storage system  114 . In practice, the deviation described by the equation given above is used to delay the mechanism used to produce the master servo sector patterns to be stamped into recording disks  202  during manufacture. Two master servo sector patterns are needed in the preferred embodiment, one for upper surfaces and one for lower surfaces of recording disks  202 . 
     The precise shape of each arc-shaped path  308  taken by read/write head  210  should exactly align with the pattern of servo sectors  402  embossed into each recording disk  202  surface. As servo arm  206  turns around actuator axis  310 , read/write head  210  should move over the surface of recording disk  202  and precisely circumferentially match up with the beginning of each servo sector  402 . 
     Referring now to FIG. 5A, one embodiment of the physical layout of a servo timing mark  502  is shown. Servo timing mark  502  is preferably the first type of information read from servo sector  402 . Servo timing marks  502  denote the beginning of servo sector  402  and the end of data wedge  404  in the preferred embodiment. Each servo sector  402  of each track  214  preferably includes a servo sector pattern, comprising a number of positions  504  which may be blank or which may include a full-width elliptical flat-bottom quarter-wavelength pit  506 . These pits  506  appear optically dark (are of low overall reflectivity) due to destructive interference, and are preferably patterned into each track  214  of each servo sector  402  during manufacture. Each pit  506  in the preferred embodiment is 0.59 micrometers in radial length and 0.35 micrometers in circumferential width, and denotes one bit of information. Pits  506  cannot be altered by the laser beam and magnetic field used for data storage, so they are indelible after manufacture. In the preferred embodiment, there are 132,600 pit positions  504  per complete rotation of recording disk  202 , with  38  pit positions  504  on each servo sector  402  and  520  pit positions  504  in each data wedge  404  between servo sectors  402 . The reflectivity of the pits  506  is uniform across the surface of the recording disk  202  in the preferred embodiment, so automatic gain control references are not required on every servo sector  402  as they typically are with existing magnetic storage systems. This space may thus be used to increase storage capacity available to the user. The write-to-read recovery time usually needed to allow the recording mechanism to turn off is not needed when servo sectors  402  following data wedges  404  are indelible. This further increases storage capacity and also increases system speed. 
     Referring now to FIG. 5B, a graph of track reflectivity is shown. Servo sector information is read via dips  508  in reflectivity measurement due to servo sector pits  506  as recording disk  202  rotates. Note that servo sector pits  506  are not read via the magnetically-induced Kerr rotation measurement used for data storage and retrieval in the preferred embodiment. There is thus no need to discriminate servo timing marks  502  from recorded data because each is read via a different mechanism. Since servo timing marks  502  are known quantities, versus uncertain data, the error correction efficiency is roughly doubled. 
     Referring now to FIG. 5C, a depiction of one embodiment of reflectivity signal processing into digital bits is shown. Storage system controller  204  processes the reflectivity signals into digital bits  510  of encoded information. Servo timing mark  502  is preferably a simple four-bit pattern identifying the beginning of the servo sector  402 . Servo timing mark  502  is specifically defined by two pits  506  separated by precisely two blank positions  504  in the preferred embodiment. The placement of two blank positions  504  between two pits  506  in servo timing mark  502  equates to a “1001” digital pattern, which reduces the chance of contamination-induced errors. 
     Referring now to FIG. 6, a logical diagram one embodiment for a servo sector  402  of a track  214  is shown. Servo sector  402  of FIG. 6 preferably includes a qualifying piece  602  of the preceding data wedge, a servo timing mark  604  as described above, encoded track address and sector address fields  606 ,  608 , and  610 , and position error signal blocks  614 ,  618 ,  622 , and  626  that are separated by delay fields  612 ,  616 ,  620 ,  624 , and  628 . The servo timing marks are preferably also used to certify sectors by detecting defects on the magneto-optical recording surface through reflectivity measurement, in one example of the utilization of a priori information to increase storage system efficiency. 
     In the FIG. 6 embodiment, storage system controller  204  uses a group of five pit positions  504  in a qualifying piece  602  from data wedge  404  immediately preceding current servo sector  402  to further assure detection of contamination-induced errors. The reflectivity of these five positions  504  is preferably used to detect surface defects, but not to read any user data written onto those positions  504  via magnetic orientation. User data is read only via Kerr rotation measurement in the preferred embodiment. If the measured reflectivity of the last five positions  504  on a given data wedge  404  is not within an acceptable range, the current sector servo  402  and data wedge  404  as well as those preceding and following the present location are disqualified for storage. Reflectivity monitoring of both the data wedge qualifying piece  602  and the two positions  504  in servo timing mark  502  eliminates the need for read-after-write verification delays, further increasing the speed of the preferred embodiment of the system. Conversely, the presence of the valid data wedge qualifying piece  602  confirms the identity of servo timing mark  502 . In the preferred embodiment, the servo sectors use only 7.25% of the total number of available pit positions to minimize overhead. Utilization of a priori information, specifically the knowledge that servo timing mark  502  should be preceded by data wedge qualifying piece  602 , thus simultaneously increases system speed and decreases overhead. 
     Encoded track address data is also placed into servo sector  402 . In the preferred embodiment, there are no guard rings between neighboring tracks  406  and  408  because such guard rings decrease the overall capacity and radial head speed of the storage system  114 . Guard rings between tracks  214  are rendered unnecessary if read/write head  210  can be radially positioned quickly and precisely. In the preferred embodiment, sixteen bits of digital information are needed to uniquely address each track  214 . However, only some of the low-order bits of a track address are needed on every servo sector  402  because immediately neighboring tracks  406  and  408  have track addresses that differ by only one value. Similarly, relatively close tracks  214  should have addresses that differ by only a few values. The speed of storage system  114  is preferably increased by using only enough lower-order track address bits on every servo sector  402  to correctly select from a small number of frequently scanned neighboring tracks  214  that are known to have almost identical addresses. The number of track address bits needed on each servo sector  402  depends on the highest seek speed desired, and also depends on read/write head  210  switching speed if data is stored on two different recording disk  202  surfaces accessed with two different read/write heads  210 . Higher-order track address bits distributed across consecutive servo sectors  402  are thus needed only as a less frequent confirmation that the lower-order track address bits are properly wrapped. Higher-order track address bits may thus preferably be read less frequently without degrading overall storage system performance. Distribution of higher-order track address bits across consecutive servo sectors  402  is another example of the use of a priori information, specifically the knowledge that nearby tracks have similar track addresses, to increase system speed and storage efficiency. 
     The preferred embodiment of this invention Gray-encodes the high-order byte and the low-order byte of the track address separately. A Gray code is a sequence of binary numbers having only one bit change from one number to the next. The encoded lower byte of the track address is referred to as bits Y 7 -Y 0  and is preferably stored in every servo sector  402  in field  608 . The Gray-encoded higher byte of the track address is preferably not stored in every servo sector  402 , but is instead distributed over eight consecutive servo sectors  402 , one bit at a time in field  610 . This is efficient because large changes in the track address do not occur very frequently, so there is no need to store the high-order byte of the track address in every servo sector  402 . Each encoded higher-order track address bit stored in a given servo sector  402  is referred to as the Z bit. 
     Referring now to FIG. 7, a table summarizing one embodiment for a Z bit distribution is shown. Any particular sector address may be thought of as being a sum of a number of eights and some remainder from zero to seven; the modulo function returns such a remainder. The pattern of Z bits for a given track  214  will repeat in modulo  8  as recording disk  202  turns from sector zero through sector  247 , and will then partially repeat for the remaining sectors numbered  248  through  254  in the preferred embodiment. As recording disk  202  rotates, sequential reading of eight Z bits will yield the bits of the high-order byte of the track address. However, the higher-order track address bits should be properly synchronized with the phase of the sectors, which is the sector address modulo  8  (the remainder after division by 8), so the eight Z bits read will be assigned to their proper significance in the high-order byte of the track address. Means for accomplishing this efficiently in the preferred embodiment are given in the discussion of sector address information immediately below, and in FIG.  8 . The complement of the Z bit is preferably always written in each servo sector  402  just before the Z bit, in field  610  of FIG. 6, for reasons that will be subsequently described in the discussion of position error signal blocks of FIG.  9 . 
     Sector address information is more easily managed than track address information because there are usually far fewer sectors  306  than tracks  214 , and because there are usually fewer mechanical disturbances that may lead to sector address errors. A simple 8-bit counter in storage system controller  204  can preferably monitor the sector address as recording disk  202  rotates. However, it is possible that a stream of data could be recorded onto more than one recording disk  202  surface, requiring storage system  114  to switch to another read/write head  210  in midstream. Some of the lower-order sector address information may therefore be stored in a manner similar to that of the higher-order track address bits for verification of the sector number. 
     Referring now to FIG. 8, a repeating sequence of bits (preferably 00010111) placed around the circumference of every surface of every recording disk  202  with one bit per consecutive servo sector  402  in field  606  is shown. Each of these bits is referred to as the X bit in a particular servo sector  402 . The interruption of the usual sequence after sector number  254  in the preferred embodiment is used as an index mark denoting the beginning of the circumference of the recording disk  202  with sector zero. This serves to verify that a full rotation of recording disk  202  has occurred. As recording disk  202  rotates, three sequential X bits can identify a phase of sector  306  even in the absence of a sector address counter. The phase of sector  306  is needed to calculate the high byte of the track address properly, as described above. There is an uncertainty of one servo sector  402  period when switching read/write heads  210 , however, so four sequential X bits instead of three are used to identify the phase of sector  306  reliably in the preferred embodiment. This sequence verifies the sector number stored in the counter in storage system controller  204  and quickly identifies the phase of the distributed higher-order track address bits for proper significance assignment. 
     Referring now to FIG. 9, one embodiment for position error signal blocks, the final components of servo sector  402 , are shown. For purposes of illustration, two neighboring tracks  902  and  904  are depicted in FIG. 9; the first,  902 , is the target track to which read/write head  210  is to be moved, and the next,  904 , is a neighboring track which shares some of the embossed pits. The complement of the Z bit is preferably written on every servo sector  402  so that a positive mark will always be available in field  610  of FIG.  6 . The blank position  906  (which is in field  612  of FIG. 6) is used to precisely separate the beginning of the position error signal blocks from the end of the Z bit or its complement, in the preferred embodiment. The elapsed time between the reading of the Z bit or its complement and the beginning of the position error signal blocks is used by storage system controller  204  to compensate for variations in the rotational speed of recording disk  202  and to synchronize the spacing of pit positions  504 . Use of the a priori knowledge that the Z bit or its complement provide a definite mark in field  610  of FIG. 6 further verifies that servo sector  402 , versus a sequence of reflectivity variations due merely to contamination, is read. 
     In the preferred embodiment, and referring to target track  902 , a block  908  of four pits  506  is placed above or radially away from spindle  212 , and then a similar block  910  is placed below or radially toward spindle  212 , off the centerline of each track  214 . A subsequent block  912  of four pits  506  is preferably placed directly on the centerline of the target track  902 , and then a final block  914  is placed directly off the centerline of the target track  902 , completing a quadrature pattern. The relative reflectivity of the position error signal blocks is preferably used to precisely control radial positioning of read/write head  210  over the centerline of a track  214 . Blank positions  504  placed between the position error signal blocks  908 ,  910 ,  912  and  914  and directly after the last position error signal block  914  are preferably used to create discharge delays for analog integrators used in position sensing circuitry. 
     Referring now to FIG. 10, a diagram of one embodiment for the physical layout of an entire servo sector  402  for five neighboring tracks  214  is shown. In FIG. 10, solid marks indicate positions with pits  506 , open marks indicate blank pit positions  504 , and shaded marks indicate positions  504  that include recorded user data. Although the recording surface mastering equipment is capable of radially overlapping pits  506  to enhance their detection, such overlapping is omitted here for clarity. 
     Suppose that sector number  234  is shown in FIG. 10, with track number 5,678 (00010110 00101110 binary) depicted by  1002 , and track number 5,679 (00010110 00101111 binary) depicted by  1004 . Field  1006  represents data wedge  404  preceding servo sector  402 . The last five positions  504  of data wedge  404  are preferably monitored for reflectivity deviations indicating a surface defect. Field  1008  includes servo timing mark  502 , with the “1001” pattern denoting the beginning of servo sector  402  and further assuring surface quality. Field  1010  includes the X bit, which is part of the 00010111 pattern preferably repeated over the circumference of the recording surface. The X bit for sector  234  is 0, the third digit in the pattern. The phase of the sector is preferably determined from the subsequently read X bits of consecutively following sectors  306 , in this case 010. 
     Field  1012  includes the lower-order byte of the track address. The Gray-encoded high-order track address byte comprising bits Y 15 -Y 8  is 00010101. The separately Gray-encoded lower-order track address byte comprising bits Y 7 -Y 0  is 00111001 for track 5,678. The Z bit for this sector includes bit Y 10 =1 of the higher-order byte of the track address; either the sector number in the counter of storage system controller  204  or the phase of sector  306  from the pattern of X bits may be used to determine that the correct Z bit is Y 10 , and not another value. The Z bit and its complement are preferably placed in field  1014 . Field  1016  is a blank position  504  between the Z bit or its complement and the beginning of the position error signal blocks. The synchronization of pit positions  504  and the true rotational speed of recording disk  202  are determined by the spacing from the positive mark in field  1014  and the beginning of the position error signal block in field  1018 . 
     Fields  1018 ,  1022 ,  1026 , and  1030  preferably include the blocks of four pits  506  that generate position error signals. The first two position error signal blocks are halfway off the track centerline in radially opposite directions, and the next two are directly on or directly off the track centerline. This pattern alternates for the next track  1004 , but the information needed to precisely center read/write head  210  over track  1004  centerline is obtained in the same manner. As recording disk  202  rotates in the preferred embodiment, storage system controller  204  receives position error signals as the position error signal blocks pass beneath read/write head  210 . The radial position information initially received from the position error signal blocks in servo sectors  402  of start-up zone  302  is preferably used to estimate the rotationally repeating radial drift of all tracks  214 , due to imprecision in centering recording disk  202  on spindle  212 . 
     Fields  1020 ,  1024 , and  1028  are integrator delay fields that are preferably two bits in length. Field  1032  is an integrator delay field that is one bit in length and is the final field in the servo sector  402  in the preferred embodiment. Field  1034  is the data wedge  404  to which servo sector  402  has guided and aligned read/write head  210  and preferably helped to certify against surface contamination. 
     Referring now to FIG. 11, a flowchart of one embodiment of method steps for implementing a servo sector  402  processing scheme is shown, in accordance with the present invention. Initially, in step  1102 , storage system controller  204  monitors the reflectivity of pit positions  504  pas sing under read/write head  210 . In step  1004 , when the qualified servo timing mark  502  pattern (preferably 000001001) determines the beginning of servo sector  402 , then in step  1006  the process of decoding servo sector  402  data begins. In step  1108 , storage system controller  204  reads the X bit and then the bits Y 7 -Y 0  are read and decoded to obtain the low-order byte of the track address. 
     In step  1110 , if the low-order byte of the track address matches that of the target track  1002 , then read/write head  210  is very probably in the right location. Subsequently read sectors  306  will provide the higher-order track address bits to completely verify read/write head  210  location. If the low-order byte of the address is not correct, then read/write head  210  is in the wrong location and servo actuator  208  responsively moves read/write head  210  toward the correct location. 
     In step  1112 , once the correct track  1002  is located, storage system controller  204  reads the complement of the Z bit and the Z bit to provide a positive mark. A synchronization timer (not shown) in storage system controller  204  is preferably started by the positive mark. In step  1114 , storage system controller  204  stops the synchronization timer at the beginning of the first position error signal, and computes the true rotational speed of recording disk  202  used for logic synchronization. 
     In step  1116 , storage system controller  204  reads the remaining position error signal blocks. In step  1118 , storage system controller  204  computes and applies the proper correction current to servo actuator  208  to put read/write head  210  precisely over the center of target track  1002 . In step  1120 , storage system controller  204  reads user data, or simultaneously writes and reads user data (avoiding write-to-read recovery delays of conventional magnetic disk drives) until the end of data wedge  404  is reached, at which point the FIG. 11 process returns to step  1102  and the next servo sector  402  begins. 
     While the invention has been described with reference to a specific embodiment, the description is intended for purposes of illustration only and should not be construed in a limiting sense. Various modifications of and changes to the disclosed embodiment, as well as other embodiments of the invention, will be apparent to those of ordinary skill in the art, and may be made without departing from the true spirit of the invention. It is therefore contemplated that the language of the following claims will cover any such modifications or embodiments that fall within the true scope of the invention.