Patent Publication Number: US-7719781-B2

Title: Method and apparatus for phase-shift null-burst-pattern

Description:
CLAIM OF PRIORITY 
   This application is divisional of U.S. application Ser. No. 11/421,430 filed May 31, 2006 now U.S. Pat. No. 7,457,066, which application is incorporated in its entirety herein by reference. 

   TECHNICAL FIELD 
   A disk drive is an information storage device. A disk drive includes one or more disks clamped to a rotating spindle, and at least one head for reading information representing data from and/or writing data to the surfaces of each disk. More specifically, storing data includes writing information representing data to portions of tracks on a disk. Data retrieval includes reading the information representing data from the portion of the track on which the information representing data was stored. Disk drives also include an actuator utilizing linear or rotary motion for positioning transducing head(s) over selected data tracks on the disk(s). A rotary actuator couples a slider, on which a transducing head is attached or integrally formed, to a pivot point that allows the transducing head to sweep across a surface of a rotating disk. The rotary actuator is driven by a voice coil motor. 
   Disk drive information storage devices employ a control system for controlling the position the transducing head during read operations, write operations and seeks. The control system includes a servo control system or servo loop. The function of the head positioning servo control system within the disk drive information storage device is two-fold: first, to position the read/write transducing head over a data track with sufficient accuracy to enable reading and writing of that track without error; and, second, to position the write element with sufficient accuracy not to encroach upon adjacent tracks to prevent data erosion from those tracks during writing operations to the track being followed. 
   A servo control system includes a written pattern on the surface of a disk called a servo pattern. The servo pattern is read by the transducing head. Reading the servo pattern results in positioning data or a servo signal used to determine the position of the transducing head with respect to a track on the disk. In one servo scheme, positioning data can be included in servo wedges, each including servo patterns. Information included in the servo patterns can be used to generate a position error signal (PES) that indicates the deviation of the transducing head from a desired track center. The PES is also used as feedback in the control system to provide a signal to the voice coil motor of the actuator to either maintain the position of the transducing head over a desired track centerline or to reposition the transducing head to a position over the centerline of a desired track. 
   A preamble signal is generally written ahead of a servo pattern. The preamble generally is written at a certain frequency. A phase lock loop circuit locks onto the frequency associated with the preamble so that subsequent signals can be written with a known phase relationship with the preamble. For example, servo patterns, that include several different servo bursts, are generally written so that they have a phase relationship with the preamble signal. The fact that the servo pattern is written in phase with the preamble provides needed information for the disk drive, and specifically the read channel, to properly decode the servo information and provide an accurate reading of the position of the read head or read transducer with respect to the center of the track. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures and: 
       FIG. 1  is an exploded view of a disk drive that uses example embodiments described herein. 
       FIG. 2  is a partial detailed view of a disk from the disk drive shown in  FIG. 1  that includes a first servo pattern, according to an example embodiment. 
       FIG. 3  is a schematic diagram of a disk drive and includes various electrical portions of the disk drive, according to an example embodiment. 
       FIG. 4  is a schematic diagram showing portions of the read/write path and a servo field detector of  FIG. 3 , according to an example embodiment. 
       FIG. 5  is a representation of a set of signals that include the preamble, gray code wedge number, and the A burst, the B burst, the C burst, and the D burst as the signals are actually appear on a disk after being written to the disk, in an example embodiment. 
       FIG. 6  is a signal sampled with substantially no phase error, according to an example embodiment. 
       FIG. 7  is a signal sampled with a small phase error, according to an example embodiment. 
       FIG. 8  is a signal sampled with larger phase error than found in  FIG. 7 , according to an example embodiment. 
       FIG. 9  is a representation of a set of signals that include the preamble, and a shifted A burst, a shifted B burst, the C burst, and the D burst, according to an example embodiment. 
       FIG. 10  is a flow diagram of a method, according to an example embodiment. 
       FIG. 11  is a representation of a computing system, according to an example embodiment. 
       FIG. 12  is schematic of a machine-readable media, according to an example embodiment. 
       FIG. 13  is a flow diagram of a method for shifting the phase of at least a portion of one of the servo bursts, according to an example embodiment. 
   

   The description set out herein illustrates the various embodiments of the invention and such description is not intended to be construed as limiting in any manner. 
   DETAILED DESCRIPTION 
     FIG. 1  is an exploded view of disk drive  100  that uses various embodiments of the present invention. The disk drive  100  includes a housing  102  including a housing base  104  and a housing cover  106 . The housing base  104  illustrated is a base casting, but in other embodiments a housing base  104  can comprise separate components assembled prior to, or during assembly of the disk drive  100 . A disk  120  is attached to a hub or spindle  122  that is rotated by a spindle motor. The disk  120  can be attached to the hub or spindle  122  by a clamp  121 . The disk may be rotated at a constant or varying rate ranging from less than 3,600 to more than 15,000 revolutions per minute. Higher rotational speeds are contemplated in the future. The spindle motor is connected with the housing base  104 . The disk  120  can be made of a light aluminum alloy, ceramic/glass or other suitable substrate, with magnetizable material deposited on one or both sides of the disk. The magnetic layer includes small domains of magnetization for storing data transferred through a transducing head  146 . The transducing head  146  includes a magnetic transducer adapted to read data from and write data to the disk  120 . In other embodiments, the transducing head  146  includes a separate read element and write element. For example, the separate read element can be a magneto-resistive head, also known as a MR head. It will be understood that multiple head  146  configurations can be used. 
   A rotary actuator  130  is pivotally mounted to the housing base  104  by a bearing  132  and sweeps an arc between an inner diameter (ID) of the disk  120  and a ramp  150  positioned near an outer diameter (OD) of the disk  120 . Attached to the housing  104  are upper and lower magnet return plates  110  and at least one magnet that together form the stationary portion of a voice coil motor (VCM)  112 . A voice coil  134  is mounted to the rotary actuator  130  and positioned in an air gap of the VCM  112 . The rotary actuator  130  pivots about the bearing  132  when current is passed through the voice coil  134  and pivots in an opposite direction when the current is reversed, allowing for control of the position of the actuator  130  and the attached transducing head  146  with respect to the disk  120 . The VCM  112  is coupled with a servo system (shown in  FIG. 4 ) that uses positioning data read by the transducing head  146  from the disk  120  to determine the position of the head  146  over one of a plurality of tracks on the disk  120 . The servo system determines an appropriate current to drive through the voice coil  134 , and drives the current through the voice coil  134  using a current driver and associated circuitry (not shown in  FIG. 1 ). 
   Each side of a disk  120  can have an associated head  146 , and the heads  146  are collectively coupled to the rotary actuator  130  such that the heads  146  pivot in unison. The invention described herein is equally applicable to devices wherein the individual heads separately move some small distance relative to the actuator. This technology is referred to as dual-stage actuation (DSA). 
   One type of servo system is an embedded servo system in which tracks on each disk surface used to store information representing data contain small segments of servo information. The servo information, in some embodiments, is stored in radial servo sectors or servo wedges shown as several narrow, somewhat curved spokes  128  substantially equally spaced around the circumference of the disk  120 . It should be noted that in actuality there may be many more servo wedges than as shown in  FIG. 1 . The servo wedges  128  are further detailed in  FIGS. 2 and 7  and in the discussions associated with those FIGs. 
   The disk  120  also includes a plurality of tracks on each disk surface. The plurality of tracks is depicted by two tracks, such as track  129  on the surface of the disk  120 . The servo wedges  128  traverse the plurality of tracks, such as track  129 , on the disk  120 . The plurality of tracks, in some embodiments, may be arranged as a set of substantially concentric circles. Data is stored in fixed sectors along a track between the embedded servo wedges  128 . The tracks on the disk  120  each include a plurality of data sectors. More specifically, a data sector is a portion of a track having a fixed block length and a fixed data storage capacity (e.g. 512 bytes of user data per data sector). The tracks toward the inside of the disk  120  are not as long as the tracks toward the periphery of the disk  110 . As a result, the tracks toward the inside of the disk  120  can not hold as many data sectors as the tracks toward the periphery of the disk  120 . Tracks that are capable of holding the same number of data sectors are grouped into a data zones. Since the density and data rates vary from data zone to data zone, the servo wedges  128  may interrupt and split up at least some of the data sectors. The servo sectors  128  are typically recorded with a servo writing apparatus at the factory (called a servo-writer), but may be written (or partially written) with the disk drive&#39;s  100  transducing head  146  in a self-servowriting operation. 
     FIG. 2  shows a portion of a disk  120  having at least one servo wedge  128 . Each servo wedge  128  includes information stored as regions of magnetization or other indicia, such as optical indicia. A servo wedge  128  can be longitudinally magnetized (for example, in the magnified portion of  FIG. 2  a servo pattern  200  includes cross-hatched blocks magnetized to the left and white spaces magnetized to the right, or vice-versa) or alternatively perpendicularly magnetized (e.g., the cross-hatched blocks are magnetized up and the white spaces are magnetized down, or vice-versa). Servo patterns  200  contained in each servo wedge  128  are read by the transducing head  146  as the surface of the spinning disk  120  passes under the transducing head  146 . The servo patterns  200  can include information identifying a data sector contained in a data field  264 . For example, the servo pattern  200  can include digital information such as a preamble  202 , a servo address mark (SAM)  204 , a track identification number  206 . The servo pattern  200  may also include a first phase burst servo pattern  210  and a second phase burst servo pattern  220  that can be used to generate a position error signal (PES) to correct deviations of the transducing head  146  with respect to the center of a track  129 . 
   In some embodiments, the servo wedge  120  will also include other information such as a wedge number. This can be a single bit to designate an index wedge (wedge #0), or the SAM may be replaced by another pattern (referred to as a servo index mark or SIM), or the wedge may contain a few low-order bits of the wedge number or a complete wedge number. 
   The magnified portion of  FIG. 2  illustrates one example servo pattern  200 . The servo pattern shown is a null-burst pattern. The null-burst servo pattern  200  includes an A burst  210 , a B burst  220 , a C burst  230  and a D burst  240 . The phase of the A burst is 180 degrees out of phase with the B burst. The A burst and the B burst are adjacent one another, and the border between them is on the centerline of a track. The phase of the C burst is 180 degrees out of phase with the D burst. The C burst and the D burst are adjacent one another and the border between them is on the edge of a track. When a read head is passing over the center of a track, the A burst and the B burst will be null or zero because the adjacent servo patterns will cancel. When the read head is off center, the signal will have a varying amplitude and phase. The phase can be detected through a demodulation scheme. The amplitude can be detected through peak detection. The amplitude can also be determined using a demodulation scheme. Given the phase and the amplitude, the location of the read head from the center track of the disk can be determined. Similarly, the same demodulation can be done with respect to the C burst and the D burst. The C burst and the D burst information can be used as further information regarding the position of the read head with respect to the center of the track or with respect to the border between a first track and a second track. The information from the C burst and the D burst can be used to confirm the position of the read head or may, in some instances, provide information necessary to determine if the read head is on one side (above in  FIG. 2 ) or on the other side (below in  FIG. 2 ) the centerline of the track. The magnified portion of  FIG. 2  illustrates one example null pattern having an A burst  210 , a B burst  220 , a C burst  230 , and a D burst  240  written in phase relationship with respect to the preamble  202 . It should be noted that in  FIG. 2 , the signals forming the A burst  210 , the B burst  220 , the C burst  230 , and the D burst  240  written in phase relationship with respect to the preamble  202  are shown in a simplified manner for the sake of illustration. 
   The disk drive  100  not only includes many mechanical features and a disk with a servo pattern thereon, but also includes various electronics for reading signals from the disk  120  and writing information representing data to the disk  120 .  FIG. 3  is a schematic diagram of a disk drive  100  that more fully details some of example electronic portions of the disk drive  100 , according to an example embodiment. Referring to  FIG. 3 , the disk drive device  302  is shown as including a head disk assembly (HDA)  306 , a hard disk controller (HDC)  308 , a read/write channel  313 , a microprocessor  310 , a motor driver  322  and a buffer  324 . The read/write channel  313  is shown as including a read/write path  312  and a servo demodulator  304 . The read/write path  312 , which can be used to read and write user data and servo data, may include front end circuitry useful for servo demodulation. The read/write path  312  may also be used for writing servo information in self-servowriting. It should be noted that the disk drive  100  also includes other components, which are not shown because they are not necessary to explain the example embodiments. 
   The HDA  306  includes one or more disks  120  upon which data and servo information can be written to, or read from, by transducers or transducing heads  146 . The voice coil motor (VCM)  112  moves an actuator  130  to position the transducing heads  146  on the disks  110 . The motor driver  322  drives the VCM  112  and the spindle motor (SM)  316 . More specifically, the microprocessor  310 , using the motor driver  322 , controls the VCM  112  and the actuator  130  to accurately position the heads  146  over the tracks (described with reference to  FIGS. 1-3 ) so that reliable reading and writing of data can be achieved. The servo fields  128 , discussed above in the description of  FIGS. 1-2 , and further detailed below, are used for servo control to keep the heads  146  on track and to assist with identifying proper locations on the disks  120  where data is written to or read from. When reading a servo wedge  128 , the transducing heads  146  act as sensors that detect the position information in the servo wedges  128 , to provide feedback for proper positioning of the transducing heads  146 . 
   The servo demodulator  304  is shown as including a servo phase locked loop (PLL)  326 , a servo automatic gain control (AGC)  328 , a servo field detector  330  and register space  332 . The servo PLL  326 , in general, is a control loop that is used to provide frequency and phase control for the one or more timing or clock circuits (not shown in  FIG. 3 ), within the servo demodulator  304 . For example, the servo PLL  326  can provide timing signals to the read/write path  312 . The servo AGC  328 , which includes (or drives) a variable gain amplifier, is used to keep the output of the read/write path  312  at a substantially constant level when servo wedges  128  on one of the disks  120  are being read. The servo field detector  330  is used to detect and/or demodulate the various subfields of the servo wedges  128 , including the SAM  204 , the track number  206 , the first phase servo burst  210 , and the second phase servo burst  220 . The microprocessor  310  is used to perform various servo demodulation functions (e.g., decisions, comparisons, characterization and the like), and can be thought of as being part of the servo demodulator  304 . In the alternative, the servo demodulator  304  can have its own microprocessor. 
   One or more registers (e.g., in register space  332 ) can be used to store appropriate servo AGC values (e.g., gain values, filter coefficients, filter accumulation paths, etc.) for when the read/write path  312  is reading servo data, and one or more registers can be used to store appropriate values (e.g., gain values, filter coefficients, filter accumulation paths, etc.) for when the read/write path  312  is reading user data. A control signal can be used to select the appropriate registers according to the current mode of the read/write path  312 . The servo AGC value(s) that are stored can be dynamically updated. For example, the stored servo AGC value(s) for use when the read/write path  312  is reading servo data can be updated each time an additional servo wedge  128  is read. In this manner, the servo AGC value(s) determined for a most recently read servo wedge  128  can be the starting servo AGC value(s) when the next servo wedge  128  is read. 
   The read/write path  312  includes the electronic circuits used in the process of writing and reading information to and from disks  120 . The microprocessor  310  can perform servo control algorithms, and thus, may be referred to as a servo controller. Alternatively, a separate microprocessor or digital signal processor (not shown) can perform servo control functions. 
     FIG. 4  is a schematic diagram showing portions of the read/write path  312  and the servo field detector  330  of  FIG. 3 , according to an example embodiment. Since the example embodiments relate to reading the servo bursts and processing the signals resulting from reading the servo bursts, the read portions of the read/write path  312  will now be further detailed. The read portion of path  312  is shown as including a variable gain amplifier (VGA)  412 , which receives signals from transducing heads  146 , or more likely from a pre-amplifier (not shown) driven by a signal received from transducing heads  146 . In some embodiments, the VGA  412  may be external to the read/write path  312 . During servo reading, the VGA  412  is at least partially controlled by the servo AGC  328 . Additional amplifiers, such as buffer amplifiers and/or one or more additional VGAs may also be present. The read/write path  312  is also shown as including an analog filter/equalizer  414 , a flash analog-to-digital (A/D) converter  416 , a finite impulse response (FIR) filter  418  and a decoder  420 . Alternatively, the FIR filter  418  can be upstream of the A/D converter  416 , and FIR filtering can be performed using analog circuitry. 
   During servo reading, magnetic flux transitions sensed by the selected transducing head  146  are may be preamplified before being provided to the VGA  412 , which controls amplification of an analog signal stream. The amplified analog signal stream is then provided to the analog filter/equalizer  414 , which can be programmed to be optimized for the data transfer rate of the servo information being read by one of heads  146 . The equalized analog signal is then subjected to sampling and quantization by the high speed flash A/D  416  which generates raw digital samples that are provided to the FIR filter  418 . Timing for sampling can be provided by the servo PLL  326 , as shown. Alternatively, sampling maybe performed asynchronously, e.g., using an asynchronous clock (in which case, most features of the present invention are still useful). The FIR filter  418  filters and conditions the raw digital samples before passing filtered digital samples to the decoder  420 . The decoder  420  decodes the digital sample stream and outputs a binary signal. The servo PLL  326  can also provide other timing signals that are necessary for the path  312  and portions of the servo demodulator  304  to operate properly. 
   The binary signal output is provided to the servo field detector  330 , and more specifically to a SAM detector  432  and a track number detector  434  of the servo field detector  330 . The output of the FIR filter  418  is provided to a burst demodulator  436 . Alternatively, the output of the flash A/D  416  can be provided to the burst demodulator  436 . The SAM detector  432  searches for a SAM using, for example, pattern recognition logic that recognizes the SAM pattern within the binary stream. The SAM detector  432  can allow some fault or error tolerance, so that a SAM pattern will be detected even if one or more of the bits in the binary stream do not exactly match the SAM pattern. As a consequence, should minor errors occur in reading or writing the SAM patterns, it may still be possible to fully demodulate the information contained in the servo wedge  138 . The track number detector  434  performs decoding of gray codes (if necessary) and detects track numbers. The burst demodulator  436  measures burst amplitudes and/or phases. 
   The read channel  313  reads the first servo burst  210  and the second servo burst  220 . The servo signals, as read by the transducing head  146 , are less than perfect sine waves. The signal stream of sine waves are sent through the VGA  412  and the analog filter/equalizer  414 , which is programmed to be optimized for the data transfer rate of the servo information. The signal stream of sine waves are sampled at a selected frequency which corresponds to a sampling time, T. In the example embodiment, the servo signals are sampled at a rate of four samples per sine wave cycle. In the read channel, the flash analog-to-digital (A/D) converter  416  is used to sample the servo signals as read by the transducing head  146  ( FIGS. 1 and 2 ). The flash analog-to-digital (A/D) converter  416  is capable of sampling millions of samples per second. Each servo burst contains multiple cycles. As shown in  FIG. 2 , the first servo pattern  210  includes three cycles and the second servo burst contains three cycles. Therefore, sampling the first burst  210  yields 12 numbers. Similarly, sampling the second burst  220  yields another 12 numbers. These samples are then sent to the FIR filter  418  which filters and conditions the raw digital samples before passing filtered digital samples to the decoder  420  and to the servo burst detector  436 . 
   A Discrete Fourier series Transform (DFT) is done on the resultant sample series from the transducing head  146  passing over the A burst  210 , the B burst  220 , the C burst  230 , and the D burst  240 . The end result of the DFT on the resultant sample series includes a real part and an imaginary part. The real part and the imaginary part of the DFT can be used to characterize of the samples in terms of amplitude and phase shift. The real and imaginary parts are combined to determine the amplitude of the signal by squaring both the imaginary part and the real part, summing the two, and then taking the square root of the sum. The phase or angle of the first of A burst  210  can be determined by computing an arc tangent of the ratio of the real and imaginary parts. Determining the phase shift between the A burst  210  and the B burst  220  can be accomplished by determining the difference between the angle associated with the A burst  210  and the angle associated with the B burst  220 . Given the phase shift between the A burst  210  and the B burst  220  and the specific track number  206  (see  FIG. 2 ), the position of the transducing head  146  on the specific track can be determined. If the read head or transducing head is over the centerline of the track, the signals cancel since they are 180 degrees out of phase with one another. If the position of the transducing head  146  or read head is not on the center of the desired track, the microprocessor  310  delivers a signal to the motor driver  322  which passes current through the VCM  318  to bring the transducing head over the track center. The signal is related to a position error signal which indicates the distance of the transducing head  146  is from the centerline the track. A similar calculation is done using the C burst  230  and the D burst. 
   In the example embodiment, the sampling rate is four samples per sine wave cycle. According to Nyquist theory, one can reject up to the Nyquist frequency of the signal by doing a DFT on a set of signal samples. The Nyquist frequency is half of the sampling rate. In the example embodiment, the Nyquist frequency is two since the sample rate per sine wave cycle is four. As a result, given a sample rate of four samples per sine wave cycle, the first harmonic can be demodulated and the second harmonic can be rejected. 
     FIG. 5  is a representation of a set of signals that include the preamble  502 , and the A burst  510 , the B burst  520 , the C burst  530 , and the D burst  540  as the signals are actually appear on a disk after being written to the disk, in an example embodiment. As shown, the burst signals A, B, C and D do not appear to be neat rectangular bars as depicted in  FIG. 2 . Rather the A burst  510 , the B burst  520 , the C burst  530 , and the D burst  540  are actually crescent-shaped or moon-shaped after being written into the servo wedge  128  on the disk  120  of the disk drive  100 . In addition, the signals making up the preamble  502  also actually appear as a series of crescent shapes. The transitions appear as crescent-shaped when written perpendicularly with respect to the surface of the disk  120 . In other words, in disk drives that write transitions to a magnetic layer of the disk  120  using perpendicular or vertical magnetic recording, the transitions appear at the surface as crescent shapes. It should be understood that this is only one example of how transitions appear on the surface of a disk  120 . It should be understood that this may also occur when writing transitions that are horizontal with respect to the major surface of the disk  120 . It should also be understood that the crescent-shaped transitions shown in  FIG. 5  are somewhat exaggerated for the sake of illustration. The transitions also are represented as crescents curved in one direction. It should be noted that the crescents can be curved the other way or that the transitions are not even crescent shaped. The crescents are representative of transitions that are not substantially vertical with respect to a horizontal track. In other words, the crescents merely represent a transition that may include an in phase portion and an out of phase portion. 
   Now looking more closely at  FIG. 5 , it can be demonstrated that the crescent shape, or moon shape, of the various signals written can introduce phase errors that depend upon the path which a read head required transducing head  142  will take as it passes over the various burst signals. For example, if the read head passes over the center of track zero as depicted by dotted line  550 , the phase of the A burst  510 , and the phase of the B burst  520  will be out of phase with respect to the phase of the preamble signals  502 . When starting at the left hand side of  FIG. 5  and proceeding along path  550  to the right hand side, the preamble signals  502  are all written in phase with the A burst signal or set of signals  510 , the B burst signals  520 , and the C burst signals  530 . The preamble signals  502 , the A burst signals  510 , the B burst signals  520 , and the C burst signals  530  are along the path  550  and will be read by the transducer when the transducer passes over the path. Again, all the signals written are essentially crescent shaped. Therefore, as the head flies over the preamble signal  502 , even though the A burst signals  510 , and the B burst signals  520  have been actually written in phase, the crescent-shape of the signals are the portion of the A burst and the B burst that is along the path  550 . As a result, only the end of the A burst signal, such as end  512  and the end of the B burst signal, such as end  522  will be passed over by the read head portion of the transducing head. The ends will be slightly out of phase with respect to the center of the preamble signal  502 . It should also be noted that the path  550  will also pass through or over the center of the crescent shaped signals forming the C burst pattern  530 . Therefore, the signals associated with the C burst pattern  530  will always be in phase with the signals, such as signal  502  that forms the preamble portion of the servo wedge. 
   Thus, even though the A burst signals  510 , and the B burst signals  520  are written so that they are in phase with the preamble signals  502 , their shape will put them slightly out of phase because the end of the crescent will be what is read by the transducing head  146  as it passes over the center of track zero along path  550 . It should also be noted that any of the signals that have the same horizontal position as the preamble signals  502  will always be in phase presuming, of course, that substantially the same geometric shape or crescent shape will be made whenever a signal is written by a particular right head. Thus, for example, along path  550  the C burst signal  530  will always be substantially in phase with the preamble signal  502  provided that the same shape of signal is reproduced consistently by the write head and also provided that the C burst signals  530  was initially written in phase with the preamble  502 . 
   Now looking at path  552 , which is along the border or midway between the center lines of track zero and track one, and moving from the left to the right, the curved ends of the preamble signals  502 , such as end  503  and end  504 , will be read as the preamble signal  502 . Since the ends  503 ,  504  of all the preamble signals, such as signal  502 , are read as the preamble signals along path  522 , the phase lock loop will also lock slightly out of phase from the center of the crescent shape which represents the original position or desired position of the preamble signals  502 . As a result, as the transducing head  146  moves from left to right and passes through the middle of the B burst signals  520 , the B burst signals will be slightly out of phase with respect to the tails  503 ,  504  of the preamble signal  502 . As the transducing head  146  proceeds further, it encounters the tails of the C burst signals  530  and the tails of the D burst signals  540 . These probably will be substantially in phase provided that these third and fourth bursts  530 ,  540 , respectively, were originally written in phase with the preamble signals such as  502 . 
   When the phase of a burst is effectively shifted such as by the different shapes formed by writing of the various burst signals  510 ,  520 ,  530 ,  540  it can cause errors or difficulties when demodulating of the signals. These difficulties or slight errors induced by the shape of the various signals as written will cause an error in the position error signal. This, in turn, will result in a miscorrection of the position of the transducing head  146  with respect to the center of the track under certain conditions. 
     FIG. 6  is a signal sample of a substantially no phase error according to an example embodiment.  FIG. 7  is a signal sample with a small phase error according to another example embodiment.  FIG. 8  is a signal sample sampled with a larger phase error than found in  FIG. 7  according to yet another example embodiment. Now referring to  FIGS. 6 ,  7 , and  8 , the ideal sampled signal of  FIG. 6  will be compared with the less than ideal conditions induced by various sized phase errors as shown in  FIGS. 7 and 8 .  FIG. 6  shows a signal  600  which is sampled four times every 360° or at every 90°. The first sample will be taken at approximately 45°, the second sample will be taken at approximately 135°, the third sample is taken at approximately 225°, and the last sample is taken at approximately 315°. The end result is that if the bursts A, B, C, and D are in phase, then the signal will be sampled at points  610 ,  612 ,  614 , and  616 . The portion of the sinusoidal signal that is used for the samples  610  and  612  is substantially linear. Similarly, the portion of the sinusoidal signal sampled at points  614  and  616  is also substantially linear. This allows for a substantially coherent written pattern. A coherent burst signal is a signal written with substantially the same phase as the preamble or the exact opposite phase (which means 180 degrees out of phase) with the preamble. 
   Shifting the phase slightly, as shown in  FIG. 7 , results in the first sampling point  710  having a slightly higher value and then the second sampling point  712  has a slightly lower value. Similarly the third sampling point  714  has a slightly higher value and the fourth sampling point  716  has a substantially higher value. However, the sample points or the samples fall on the substantially linear portions of the sine waves. As long as the samples remain in the substantially linear region of the sine waves, and both get multiplied and, added, as they normally do during signal processing, the samples approximately cancel. Therefore slight changes in phase do not result in substantial errors in the position errors signal. 
     FIG. 8  shows a signal sampled with a larger phase error than that found in  FIG. 7 . Now the sample points  810 ,  812 ,  814 , and  816  are outside the linear regions of the sine waves. In essence the phase shifting of the signal, combined with the timing of the sampling points staying the same, results in sample points, or at least some of the sample points such as  812  and  816  falling outside the linear regions of the sine wave read from the servo patterns. This larger phase error can produce, or contribute to errors during the signal demodulation process that ultimately produce an errant position error signal. 
     FIG. 9  is a representation of a set of signals that include a preamble, and an A burst, the B burst, the C burst, and the D burst where the bursts A and B are phase shifted to account for the shape of the transition as actually written to the disk, according to an example embodiment. It should be noted that knowing that a slight phase change has little or no effect for the PES signal, as depicted by  FIG. 7  above, a slight phase change can be accommodated for by writing a null pattern with one of the sets of phase bursts shifted with respect to the preamble and with respect to the other phase burst. In other words, because of the shape of the written signals the A and B bursts are written slightly out of phase with respect to the preamble and the C and D bursts. Writing the A and B bursts slightly out of phase compensates for that phase shift due to the shape of the written signals and the position of the transducing head as it passes over a particular path and lessens or substantially removes errors in the position error signal. 
   Now turning to  FIG. 9 , it can be seen that the preamble  902 , as well as the A burst  910 , the B burst  920 , the C burst  930 , and the D burst  940  all appear as crescent shapes as written on the surface of the disk  120  (see  FIG. 1 ). The A burst  910  and the B burst  920  have been shifted in phase so that the tails, and all the other portions of the A and B bursts, are now more closely in phase with the center portions of the preamble. Thus, when the head flies through the center of the preamble signals  902 , the A and B bursts,  910  and  920  respectively, will now produce signals that are more substantially in phase with the preamble and also in phase with the C servo burst  930  that follows when the transducing head  146  (see  FIG. 1 ) flies over the center of the track. It should be noted that the A and B bursts are phase shifted by a distance d which is depicted in  FIG. 9 . The distance d is selected so that the phase shift compensates for the shape of the signal as written on the disk when the read head is in a position where it is necessary to have an accurate position error signal. This avoids a miscorrection in the position of the transducing head through the control mechanism show in  FIG. 3 . It should also be noted that the amount of phase shift introduced by writing the A burst  910  and the B burst  920  out of phase introduces errors when the transducing head  146  (see  FIG. 1 ) is flying on one path and lessens errors when the transducing head  146  (see  FIG. 1 ) is flying along another path. Therefore, the amount of phase shift introduced must provide benefits which outweigh any potential errors introduced by the phase shift. 
   A media  120  includes a plurality of tracks, a preamble portion  902  including a set of signals, a first servo burst or C burst  930  having a first plurality of signals written substantially in phase with the preamble portion, and a second servo burst or A burst  910  written out of phase with the preamble portion  902  and the first servo burst or C burst  930 . The amount of phase shift between the second servo burst or A burst  910  and the preamble portion is selected to compensate for an attribute of the signals associated with the second servo burst or A burst  910 , as written to the media  120 . In one embodiment, the attribute of the second servo burst or A burst  910  is a shape of the signals as written to the media. In one embodiment, the shape of the signals written to the media is a crescent shape. In still another embodiment, the first servo burst or C burst  930  and the second servo burst or A burst  910  are written in a null burst pattern. The media  120  can also include a third servo burst or B burst  920  which is written substantially 180 degrees out of phase with the second servo burst or A burst  910 . In some embodiments, the first servo burst or C burst  930 , the second servo burst or A burst  910  and the preamble portion  902  are written with perpendicular transitions. 
   A disk drive  100  includes a disk, a transducing head  146  to read information from the disk  120 , and a read channel  313  to read information from the disk  120  including the information associated with the first servo burst or C burst  930  and the second servo burst or A burst  910 . The disk  120  further includes a preamble portion  902  including a set of signals, a first servo burst or C burst  930  having a first plurality of signals written substantially in phase with the preamble portion  902 , and a second servo burst or A burst  910  written out of phase with the preamble portion  902  and the first servo burst or C burst  930 . The amount of phase shift between the second servo burst or A burst  910  and the preamble portion  902  is selected to compensate for an attribute of the signals associated with the second servo burst or A burst  910  as written to the disk  120  of the disk drive  100 . In one embodiment, the attribute of the second servo burst or A burst  910  is a shape of the signals as written to the disk  120  of the disk drive  100 . The disk drive  100  can also include a third servo burst or B burst  920  which is written substantially 180 degrees out of phase with the second servo burst or A burst  910 . In one embodiment, the first servo burst or C burst  930 , the second servo burst or A burst  910  and the preamble portion  902  are written with perpendicular transitions. 
     FIG. 10  is a flow diagram of a method  1000 , according to an example embodiment. The method  1000  includes writing a preamble  1010 , writing a first burst signal that is in phase with the preamble  1012 , and writing a second burst signal that is out of phase with the preamble and the first burst signal  1014 . Writing the second burst includes shifting the amount the second burst is out of phase by an amount to reduce incoherence of a sampled signal generated by the second burst. 
   In another embodiment, the second burst is not shifted on the media but is rather shifted using an instruction set, such as instruction set  2062 . In still other embodiments, the instruction set is executed by a machine such as a computer. Now turning to both  FIGS. 11 and 12 , a computer  2000  and an instruction set, such as instruction set  2062 , will be further detailed. 
   A block diagram of a computer system that executes programming for performing the above algorithm is shown in  FIG. 11 . A general computing device in the form of a computer  2010 , may include a processing unit  2002 , memory  2004 , removable storage  2012 , and non-removable storage  2014 . Memory  2004  may include volatile memory  2006  and non volatile memory  2008 . Computer  2010  may include any type of information handling system in any type of computing environment that includes any type of computer-readable media, such as volatile memory  2006  and non volatile memory  2008 , removable storage  2012  and non-removable storage  2014 . Computer storage includes random access memory (RAM), read only memory (ROM), erasable programmable read-only memory (EPROM) &amp; electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, compact disc read-only memory (CD ROM), Digital Versatile Disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium capable of storing computer-readable instructions. Computer  2010  may include or have access to a computing environment that includes input  2016 , output  2018 , and a communication connection  2020 . The computer may operate in a networked environment using a communication connection to connect to one or more remote computers. The remote computer may include a personal computer (PC), server, router, network PC, a peer device or other common network node, or the like. The communication connection may include a Local Area Network (LAN), a Wide Area Network (WAN) or other networks. A microprocessor or controller associated with the disk drive  100  (see  FIG. 1 ) is also such a computer system. A set of instructions for shifting phase is generally located in an instruction set associated with the disk drive  100  that is generally termed firmware. The microprocessor or controller associated with a disk drive executes the firmware or computer readable instructions to shift the phase of the servo bursts, in one embodiment. 
   Computer-readable instructions stored on a machine-readable medium are executable by the processing unit  2002  of the computer  2010 . A hard drive, CD-ROM, and RAM are some examples of articles including a machine-readable medium. For example, a computer program  2025  executed to shift the phase of one of the servo bursts so as to compensate for the shape of the signals forming the servo burst. The computer program may also be termed firmware associated with the disk drive  100 . In some embodiments, a copy of the computer program  2025  can also be stored on the disk  120  of the disk drive  100 . 
     FIG. 12  is a schematic diagram that shows a machine readable medium  2060  and an instruction set  2062  associated with the machine readable medium  2060 , according to an example embodiment. The machine-readable medium  2060  provides instructions  2062  that, when executed by a machine, such as a computer, cause the machine to perform operations that include reading information from a magnetized portion of the media that includes a preamble portion including a set of signals, a first servo burst having a first plurality of signals written substantially in phase with the preamble portion, and a second servo burst written substantially in phase with the preamble and the first servo portion. The instructions  2062  further cause the machine to shift the phase of at least a portion of the second servo burst with respect to the preamble. Shifting the phase, in one embodiment, includes shifting the phase of at least a portion of the second servo burst with respect to the preamble by an amount to improve coherence of a sampled read back signal from the second servo burst. In another embodiment, shifting the phase includes shifting the phase of substantially the entire second servo burst with respect to the preamble. Shifting the phase can also include shifting the phase of substantially the entire second servo burst with respect to the preamble by an amount to improve coherence of a sampled read back signal from the second servo burst. 
   This other embodiment can be implemented in a disk drive having a media written as shown in  FIG. 5 . Now referring both to  FIGS. 1 and 5 , the drive  100  includes a disk  120 , a transducing head  146  to read information from the disk  120 , and a read channel  313  to read information from the disk  120  including the information associated with the first servo burst  530  and the second servo burst  510 . The disk  120  further includes a preamble portion  502  including a set of signals, a first servo burst  530  having a first plurality of signals written substantially in phase with the preamble portion  502 , and a second servo burst  510  written substantially in phase with the preamble  502  and the first servo portion  530 . The disk drive  100  also includes a phase shifter  590  to shift the phase of at least a portion of the second servo burst  510  with respect to the preamble  502 . In one embodiment, the phase shifter  590  is associated with the read channel  313  of the disk drive  100 . The phase shifter  590  shifts the phase of substantially the entire second servo burst  510 . The phase shifter  590  includes a filter for filtering the second servo burst to effectively shift the phase of the second servo burst  510 . In one embodiment, the preamble  502 , the first servo burst  530  and the second servo burst  510  are written with transitions substantially perpendicular to a major surface of the disk  120 . In still another embodiment, the phase shifter  590  operates on an instruction set  2062  to effectively shift the phase of the second servo burst  510 . 
     FIG. 13  is a flow chart showing a method  1300  for shifting the phase of at least a portion of one of the servo bursts. The method  1300  includes reading information from a magnetized portion of the media that includes a preamble portion including a set of signals, a first servo burst having a first plurality of signals written substantially in phase with the preamble portion, and a second servo burst written substantially in phase with the preamble and the first servo portion  1310 . The method  1300  also includes shifting the phase of at least a portion of the second servo burst with respect to the preamble  1312 . Shifting the phase, in one embodiment, includes shifting the phase of at least a portion of the second servo burst with respect to the preamble by an amount to improve coherence of a sampled read back signal from the second servo burst. In another embodiment, shifting the phase includes shifting the phase of substantially the entire second servo burst with respect to the preamble. Shifting the phase can also include shifting the phase of substantially the entire second servo burst with respect to the preamble by an amount to improve coherence of a sampled read back signal from the second servo burst. The phase can be shifted by way of filters or tap weights in various filters. In another embodiment that samples, a discrete fourier transform includes a real part and an imaginary part. A band limited sample signal can be effectively phase shifted using a particular linear combination of the real part and the imaginary part of the discrete fourier transform to yield the real and imaginary parts of a phase shifted sampled signal. 
   The foregoing description of the specific embodiments reveals the general nature of the invention sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the generic concept, and therefore such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. 
   It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents and variations as fall within the spirit and broad scope of the appended claims.