Abstract:
A magnetic storage system is disclosed which has one or more rotating disks. In one embodiment, the system includes a first transducer and a second transducer operatively associated with a respective first surface or second surface. Each of the first and second surfaces has a data track with a data region and an embedded servo sector. The first transducer has a data reader which includes a first MR strip and a servo reader which includes a second MR strip. The first MR strip is electrically isolated from the second MR strip. The first transducer performs a servo reading operation of the servo sector while the second a transducer performs either a data writing operation to the data region or a data reading operation from the data region. Also disclosed is a method for reading within a magnetic storage system. The storage system includes a first magnetic surface, a data reader and a servo reader. The first magnetic surface includes a first data region and a first servo sector. The data reader is not coupled to the servo reader. Servo information is read with the servo reader so that an off-track position of the data reader can be determined. After the off-track position is known, the data reader is moved to compensate for the off-track position. Once the data reader is properly positioned, the data reader retrieves user data from the first data region of the first magnetic surface.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to magnetic disk drive storage systems and, more specifically, to a method and an apparatus for using separate servo and data read elements to allow for more efficient positioning of a transducer. 
     BACKGROUND OF THE INVENTION 
     A magnetic disk drive system is a digital data storage device that stores digital information within concentric tracks on a storage disk (or platter). The storage disk is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a substantially constant rate. To write data to or read data from the disk, a magnetic transducer is positioned above a desired track of the disk while the disk is spinning. 
     Writing is performed by delivering a write signal having a variable current to a transducer while the transducer is held close to the rotating disk over the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored. 
     Reading is performed by sensing magnetic polarity transitions previously written on tracks of the rotating disk with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the magnetic signal into an analog read signal which is amplified in a preamplifier, whereafter the signal is delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that, after additional processing, can be recognized by a host computer system external to the drive. 
     The transducer contains a read element and a write element to respectively perform the functions of writing to and reading from the disk. Some transducers contain dual-purpose elements which can both write to and read from the disk, but modern transducers separate the read element from the write element for reasons explained below. 
     Portions of a standard disk drive, generally designated  100 , are illustrated in FIGS. 1A and 1B, where FIG. 1A is a top view of the disk drive  100  and FIG. 1B is a sectional side view thereof The disk drive comprises disks  104  that are rotated by a spin motor (not shown). The spin motor is mounted to a base plate (not shown). Data is stored on magnetic material which coats the two surfaces  108  of the disk  104 . An actuator arm assembly  112  is also mounted to the base plate. 
     The actuator arm assembly  112  includes a transducer  116  mounted to an actuator arm  124 . The actuator arm  124  rotates about a bearing assembly  128 . The actuator arm assembly  112  cooperates with a voice-coil motor (VCM)  132  which moves the transducer  116  relative to the disk  104 . The spin motor and voice-coil motor  132  are coupled to a number of electronic circuits mounted to a printed circuit board (not shown) which control their operation. A number of wires  136 , among other things, are used to couple the transducer  116  to the read channel (not shown in FIG.  1 A). These wires are routed from circuitry within the drive, across the actuator arm assembly  112  and to the transducer  116 . An analog read signal and an analog write signal are transported by these wires  136 . The analog read signal is amplified by a preamplifier  140  before it is further processed by other circuitry (not shown) into a digital representation of the data stored on the disk  104 . The preamplifier  140  is typically located on the actuator arm assembly  112  and positioned as close to the transducer  116  as practical so that noise may be reduced. After the preamplifier  140 , the amplified analog read signal is passed to other circuitry which may include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device, among other things. 
     As shown in FIG. 1B, each of the plurality of disks  104  has two sides, with magnetic material  108  on each of those sides. Two actuator arm assemblies  112  are provided for each disk  104 . To position the transducer  116 , the VCM  132  moves all actuator arms  124  in unison relative to their respective disks  104 . The VCM  132  makes position adjustments to the pivotally connected actuator arms  124  so that a particular transducer is centered over a data track  144  (see FIG.  1 A). As is well understood in the art, movement of each actuator arm  124  can be independently optimized for imperfections in the arcuate geometry of each data track  144  on the actuator arm&#39;s corresponding magnetic surface  108 . 
     Referring to FIG. 1A, data is stored on the disk  104  within a number of concentric data tracks  144  (or cylinders). Each data track  144  is divided into a plurality of sectors, and each sector is further divided into a data region  148  and a servo region (or servo sector)  152 . 
     Servo sectors  152  are used to, among other things, provide transducer position information so that the transducer  116  can be accurately positioned by the actuator arm  124  over the data track  144 , such that user data can be properly written onto and read from the disk  104 . The data regions  148  are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten. Because servo sectors are embedded into each data track  144  on each disk  104  between adjacent data regions  148 , the type of servo-scheme shown in FIG. 1A is known by those skilled in the art as an embedded servo scheme (or sectored servo scheme). 
     A more detailed view of a transducer, generally designated  116 , used for reading and writing magnetic polarity transitions to a magnetic media (not shown) is illustrated in FIG.  2 . Referring to the figure, portions of the transducer  116  which face the magnetic media are shown. The part of the transducer  116  shown in this view is commonly called the air bearing surface. The transducer  116  includes a write element  200 , write gap  204 , first shield  208 , second shield  212 , read gap  216 , and magnetoresistive (MR) read element  220 . Unlike some early inductive transducers, the depicted transducer  116  has separate read and write elements. Magnetoresistive (MR) strips are commonly used in read elements because they change resistance when exposed to a magnetic field, and this change in resistance is relatively easy to sense. It should be noted that the read element  220  is used for reading both servo and data regions. It is further noted that the write element  200  typically has a width  224  which is greater than a width  228  of the MR read element  220 . For example, the width  224  of the write element  200  might be twice the width  228  of the read element  220 . The reason for this width variance is explained below. 
     As part of the writing process, a variable current is used to induce magnetic flux across the write gap  204  between the write element  200  and the first shield  208 . The write element  200  and first shield  208  act as poles for an electromagnet which induces magnetic flux across the write gap  204 . The direction of the variable current defines the direction in which the magnetic flux will be oriented across the write gap  204 . In some simple recording systems, flux polarized in one direction across the write gap  204  will record a binary “one” on the magnetic media while flux polarized in the opposite direction will record a binary “zero.” In most recording systems, a change in the direction that the flux travels across the gap  204  is interpreted as a “one” while the lack of a change is interpreted as a “zero.” As the magnetic material on the disk surface  108  (shown in FIG. 1A) travels under the transducer  116  in the direction shown by arrow  232 , a series of digital “ones” and “zeros” can be written within the data track  144  (shown in FIG.  1 A). 
     When reading, the magnetic polarity transitions, previously written onto the magnetic media, are coupled to the transducer  116  in order to recover the stored digital data. When a magnetic polarity transition in the magnetic media passes under the transducer  116 , the read element  220  will each generate a signal in response to the changing magnetic field which corresponds to a previously recorded data bit. This signal is called an analog read signal. A preamplifier  140  (shown in FIG. 1A) is used to provide low noise amplification of the analog read signal. Conversion of the analog read signal back into a digital signal is performed within a read channel, after which it is passed to an exterior environment such as a computer. During the read process, the first and second shields  208 ,  212  form a read gap  216  which serves to focus the flux for a particular magnetic polarity transition onto the read element  220  by shielding the read element  220  from other sources of magnetic flux. In other words, extraneous magnetic flux is filtered away from the read element  220  by the shields  208 ,  212 . 
     FIG. 3 shows a portions of a number of data tracks  144  drawn in a straight, rather than arcuate, fashion for ease of depiction. As is well-known, data tracks  144  on magnetic disks  104  (as depicted in FIG. 1A) are circular. Referring again to FIG. 3, each data track  144  has a centerline  300 . To accurately write data to and read data from the data region  148  of the data track  144  while the disk travels in a direction denoted by the arrow  232 , it is desirable to maintain the transducer  116  (see FIG. 1A) in a relatively fixed position with respect to a given track&#39;s centerline  300  during each of the writing and reading procedures. 
     With reference to FIGS. 1-3, to assist in controlling the position of the transducer  116  relative to the track centerline  300 , the servo region  152  contains, among other things, servo information in the form of servo patterns comprised of two or more groups of servo bursts, as is well-known in the art. First and second servo bursts  304 ,  306  (commonly referred to as A and B servo bursts, respectively) are shown in FIG. 3 for each data track  144 . Servo bursts  304 ,  306  are accurately positioned relative to the centerline  300  of each data track  144 , and are typically written on the disk  104  during the manufacturing process using a servo track writer (“STW”). The STW alternatively writes a number of A bursts  304  in a concentric circle and then a number of B bursts  306  in a concentric circle until all data tracks  144  have servo information embedded therein. The concentric circle of either A or B bursts  304 ,  306  is defined herein as a servo track. Unlike information in the data region  148 , servo bursts  304 ,  306  may not normally be overwritten or erased during operation of the disk drive  100 . 
     As the transducer  116  is positioned over the data track  144 , it reads the servo information contained in the servo regions  152  of the track, one servo region at a time. The servo information is used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer  116  and the track centerline  300 . The PES signal is provided as an input to a servo control loop which performs calculations and outputs a servo compensation signal which controls the VCM  132  to place the transducer  116  over a particular position relative to the track centerline  300 . When a write function is desired, the dual-purpose transducer  116  reads servo information from the servo region  152 , is positioned over the track centerline  300  in the manner described above, and then writes to the disk  104  when the transducer  116  is over the data region  148 . 
     As mentioned above, the read element  220 , shown in FIG. 2, reads information from both the servo region  152  and data region  148 . The dual-purpose nature of the read element  220  requires transducer designers to compromise between optimizing the read element  220  for reading servo information or for reading user data, as explained more fully below. 
     Referring to FIG. 4, the transducer  116  and a portion of the analog read channel is illustrated in block diagram form. The transducer  116  contains a write element  200  and a dual-purpose servo/data read element  220 . An analog write signal  400  is provided to the write element  200 , and an analog read signal  404  is received from the read element  220 . To provide low noise factor amplification, the analog read signal  404  is provided to a preamplifier  140 . An amplified analog read signal  408  is sent from the preamplifier  140  to a demultiplexor  412  where a servo read signal  416  and a data read signal  420  are produced under the control of a select line  424 . Typically, the preamplifier  140  is located on the actuator arm assembly  112  (see FIGS. 1A and 1B) while the demultiplexor  140  is located in the read channel. The select line  424  is provided by other circuitry within the read channel and controls the demultiplexing of the amplified read signal  408 . In this way, the dual-purpose servo/data read element  220  provides both the servo read signal  416  and the data read signal  420 . 
     Referring once again to FIGS. 1-3, the geometric relationship of data tracks  144  and transducer  116  is explained. The data region  148  has a width  312  (see FIG. 3) in the radial direction of the disk  104  generally equal to the width  224  (see FIG. 2) of the write element  200 . As an artifact of the write process, erase bands  316  (see FIG. 3) are created between each data track  144 . The erase bands  316  are considered wasted space since they cannot store user data. To reliably position the read element  220  over the centerline  300  and avoid the erase bands  316 , one skilled in the art can appreciate the desirability of having an read element width  228  (see FIG. 2) smaller than the data region width  312 . In other words, the smaller the read element width  228  with respect to the data region width  312  (see FIG.  3 ), the more the likely the read element  220  will be positioned over a portion of the data region  148  with a magnetic signal of sufficient amplitude to reliably read the user data. 
     As shown in FIG. 1A, circular data tracks  144  extend from an inner usable radius of the disk (“inner diameter” or “ID”)  156  to an outer usable radius of the disk (“outer diameter” or “OD”)  160 . The servo regions  152  for each data track  144  are generally aligned radially, and the servo regions are generally the same size regardless of the radial positioning of the servo region  152 . A servo burst width  320  (see FIG. 3) is approximately equal to the data region width  312  plus an erase band width  324 . As can be appreciated, toward the distal dimensions of the disk the data regions  148  are physically longer because the servo regions  152  remain the same size as the circumference of the data track  144  grows larger. 
     Along a given radius of the disk there is a ratio between the total number of A and B servo bursts  304 ,  306  and the data tracks  144 . In FIG. 3, each data track  144  requires half of the A burst and half the B burst to accurately position the transducer  116  over the track centerline  300 . Accordingly, the ratio of A and B servo burst  304 ,  306  to data tracks  144  for this configuration is one to one. 
     With reference to FIG. 5, a block diagram of the read electronics for a conventional two disk (i.e., four surface) drive system is shown. Each surface  108  (see FIG. 1B) of the two disks  104  has a corresponding transducer  504 ,  508 ,  512 ,  516  which reads both servo and user data from that surface  108 . A read element  220  (see FIG. 2) in a first transducer  504  produces a first analog read signal  520  which is amplified in a first preamplifier  536 , whereupon a first amplified analog read signal  552  is produced. Similarly, a second, third and fourth preamplifiers  540 ,  544 ,  548  respectively amplify each of a second, third and fourth analog read signals  524 ,  528 ,  532  to produce corresponding second, third and fourth amplified analog read signals  556 ,  560 ,  564 . In this way, the amplified read signal  552 ,  556 ,  560 ,  564  is produced for each disk surface  108  (see FIG.  1 A). 
     In conventional disk drives  100  with embedded servo sectors, only one disk surface  108  is read from at a time. This means only one of the first, second, third, and fourth amplified analog read signals  552 ,  556 ,  560 ,  564  requires decoding to read from the single disk surface  108 . This allows multiplexing  572  of the first, second, third, and fourth amplified analog read signals  552 ,  556 ,  560 ,  564  into a single read channel  580 . Based upon the select inputs  568 , the multiplexer  572  routes one of the first, second, third, and fourth amplified analog read signals  552 ,  556 ,  560 ,  564  to a selected analog read signal path  576 . The read channel  580  takes the signal coupled to the selected analog read signal path  576  and processes the signal to produce either a digital representation of the user data or the servo information. In this way, each of the four disk surfaces  108  is read by a single read channel  580 . 
     As those skilled in the art can appreciate, when the read element width  228  is less than the servo burst width  320 , conventional position error techniques will produce PES signals with a non-linear response. The non-linear response makes it difficult to determine the centerline  300  of the data track  144 . This problem in the prior art is best illustrated with examples. 
     FIGS. 6A-C show three different radial positions for the read element  220  with respect to servo region  152  and also show the respective A and B burst signals produced by the read element  220  at each radial position. In the depicted examples, the read element width  228  is less than the servo burst width  320 . To determine off-track position, the A burst signal produced over the A servo burst pattern  304  is compared to the B burst signal produced over the B servo burst pattern  306  as the disk  104  rotates in the direction of the arrow  232 . The first example in FIG. 6A depicts the read element  220  straddling the division between A and B servo bursts  304 ,  306  which produces the A and B burst signals shown. In the second example shown in FIG. 6B, the read element  220  is at a radial position toward the top edge of the A servo burst  304  which produces the A and B burst signals shown. The resulting A and B burst signals from the second example in FIG. 6B should be contrasted with the A and B burst signals produced in the third example shown in FIG.  6 C. Although the read element  220  is at a radial position closer to the centerline  300  of the data track  144  in FIG. 6C, the A and B burst signals produced are indistinguishable from that of FIG.  6 B. 
     As can be appreciated, the non-linearity in the A and B burst signals at different radial offsets demonstrated in FIGS. 6A-C makes off-track position determination difficult. This problem is solved by having the read element width  228  be greater than or equal to the servo burst width  320 . However, enlarging the read element width  228  makes the positioning of the read element  220  while avoiding the erase bands  316  (see FIG. 3) more difficult, as explained above. 
     With reference to FIG. 7, a quadrature servo burst pattern is depicted with an arrow  232  defining the direction the disk rotates. To solve the non-linear response problems when the read element width  228  (see FIG. 2) is smaller than the servo burst width  320  additional servo bursts patterns are added by staggering four groups of servo bursts  304 ,  306 ,  704 ,  708  in a manner commonly called quadrature servo burst patterns  712 . Additional third and fourth servo bursts  704 ,  708  (commonly called C and D servo bursts, respectively) are added to the A and B servo bursts  304 ,  306 . In this way, the read element width  228  can be reduced to half the servo burst width  320  and still avoid nonlinear responses in the off-track detection. There is a ratio between the total number of A, B, C, and D servo bursts  304 ,  306 ,  704 ,  708  along a given radius and data tracks  144  of two to one for quadrature servo burst configurations. However, while adding more servo bursts  304 ,  306 ,  704 ,  708  per data track  144  may allow for linear off-track position determinations and more accurate read operations, the writing of the quadrature servo patterns  712  increases the drive production time and equipment costs needed for additional servo track writers while decreasing manufacturing throughput. Additionally, consuming space on the surface  108  of the magnetic disk  104  with additional servo bursts leaves less space available for user data. 
     Other conventional systems have attempted to solve the nonlinear off-track position problem by using the write element to read servo bursts which allows dedication of the read element for reading user data. In other words, the write element serves a dual role as a data writer and a servo reader. Since the read element is dedicated to the reading of user data, the width of the read element can be made optimally small. Unfortunately, conventional write elements do not efficiently perform read functions which makes this solution impractical. 
     As noted in the above discussion, the width of the write element generally defines the width of the data track  144 , and the width  320  (see FIG. 3) of the servo bursts in systems, which only use the A and B bursts  304 ,  306 , is generally equal to the width of the data track  144  plus the width  324  of the erase band  316 . As can be appreciated, the dual purpose data writer and servo reader will experience a non-linear response to the servo bursts since the width  224  of the write element is necessarily smaller than the width  320  of the servo burst. 
     With reference to FIG. 8, the use of a center-tapped reader  802  has been suggested to bifurcate a single MR strip  818  into a user data read element and a servo read element. A first conductor  806  supplies a first current  822  to the MR strip  818  and a third conductor  814  supplies a second current  826  to the MR strip  818 , whereupon a second conductor  810  returns a third current  834 . The third current  834  returns a sum of the first and second currents  822 ,  826  and is common to the bias path of the data read element and the servo read element. It should be noted that the bias currents in the data read element and servo read element necessarily flow in opposite directions. 
     Center-tapped read elements suffer from poor servo read sensitivity since the placement of the second conductor  810  on the MR read element  818  creates a dead spot. As can be appreciated by those skilled in the art, a metallic conductor  810  in a central portion of the MR read element  818  serves to make the read element insensitive to the magnetic signal stored on the data track  144 . In other words, the affected portion of the MR element  818  is not sensitive to magnetic fields incident upon the dead spot. 
     Center-tapped MR read elements are also undesirable because proper biasing of the bifurcated reader  802  is difficult. As noted above, the first and second currents  822 ,  826  flow in opposite directions because of the common center conductor  810 . Those skilled in the art can appreciate that magnetic biasing is aligned with the current flow such that magnetic biasing of each half of the MR strip  818  should also be in opposite directions. However, it is difficult to bias permanent magnets, which provide the magnetic biasing, in opposite directions during manufacture. Accordingly, there is a need to provide separate data and servo read elements which do not experience dead spots in a configuration which allows for proper magnetic biasing. 
     In summary, it would be desirable to develop a transducer positioning system which: (1) optimizes the transducer for reading user data; (2) also optimizes the transducer for reading servo information while minimizing the number of servo bursts; and, (3) avoids the deficiencies of center-tapped MR read elements, including their dead spots and biasing problems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to develop a transducer positioning system which: (1) optimizes the transducer for reading user data; (2) also optimizes the transducer for reading servo information while minimizing the number of servo bursts; and, (3) avoids the deficiencies of center-tapped MR read elements, including their dead spots and biasing problems. 
     A magnetic storage system is disclosed which has one or more rotating disks. In one embodiment, the system includes a first transducer and a second transducer operatively associated with a respective first surface or second surface. Each of the first and second surfaces has a data track with a data region and an embedded servo sector. The first transducer has a data reader which includes a first MR strip and a servo reader which includes a second MR strip. The first MR strip is electrically isolated from the second MR strip. The first transducer performs a servo reading operation of the servo sector while the second transducer performs either: (1) a data writing operation to the data region, or (2) a data reading operation from the data region. 
     Also disclosed is a method for reading within a magnetic storage system. The storage system includes a first magnetic surface, a data reader and a servo reader. The first magnetic surface includes a first data region and a first servo sector. The data reader is not coupled to the servo reader. Servo information is read with the servo reader so that an off-track position of the data reader can be determined. After the off-track position is known, the data reader is moved to compensate for the off-track position. Once the data reader is properly positioned, the data reader retrieves user data from the first data region of the first magnetic surface. 
    
    
     Other objects, features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a diagrammatic representation of a top view of portions of a conventional magnetic disk drive system where some of the many tracks on the surface of the disk are accentuated; 
     FIG. 1B is a diagrammatic representation of a side view of the drive in FIG. 1A showing two disks, each of the disks having two surfaces and an actuator arm corresponding to each disk surface; 
     FIG. 2 is a diagrammatic representation of an air-bearing view of a conventional transducer which has separate read and write elements, wherein the illustrated portion of the transducer faces the magnetic disk; 
     FIG. 3 is a diagram illustrating a conventional section of a surface of a magnetic disk, shown as having straight, rather than curved, data tracks for ease of depiction; 
     FIG. 4 is a block diagram of a portion of a conventional analog read channel which is interfaced to a transducer; 
     FIG. 5 is a block diagram of the read electronics for a conventional four transducer disk drive; 
     FIG. 6A is a diagram and bar graph which shows the conventional relationship between read element position and PES signal output from the shown read element for a position near track centerline; 
     FIG. 6B is a diagram and bar graph which shows the conventional relationship between read element position and PES signal output from the shown read element for a position far above track centerline; 
     FIG. 6C is a diagram and bar graph which shows the conventional relationship between read element position and PES signal output from the shown read element for a position above track centerline; 
     FIG. 7 is a diagram illustrating a conventional section of a surface of a magnetic disk with a quadrature servo sector pattern, shown as having straight, rather than curved, data tracks for ease of depiction; 
     FIG. 8 is a diagrammatic representation of a perspective view showing a conventional center-tapped data reader; 
     FIG. 9 is a diagrammatic representation of a perspective view showing the servo and data reader portions of an embodiment of the transducer of the present invention; 
     FIG. 10 is a diagrammatic representation of an air-bearing view of the transducer of FIG. 9, wherein the illustrated portion of the transducer normally faces the magnetic disk; 
     FIG. 11 is a block diagram of an interconnection between the read elements and a portion of an analog read channel which utilizes four wires; 
     FIG. 12 is a block diagram of an interconnection between the read elements and a portion of an analog read channel which utilizes three wires; 
     FIG. 13 is a block diagram of a portion of an analog read channel which is interfaced to a transducer with separate data and servo readers; 
     FIG. 14 is a block diagram of the read electronics for a four transducer disk drive which has separate data and servo readers for each transducer; 
     FIG. 15 is a diagram illustrating a section of a disk surface with wide servo bursts and narrow data tracks, shown as having straight, rather than curved, data tracks for ease of depiction; 
     FIG. 16 is a block diagram of the read electronics for a four transducer disk drive which can simultaneously process both user data and servo information; and 
     FIG. 17 is a diagram illustrating a portion of four separate disk surfaces which have staggered servo sectors. 
    
    
     DETAILED DESCRIPTION 
     While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail, a number of embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
     As will be described in detail below, having a servo reader separate from an user data reader allows optimizing of each for their respective tasks. The servo reader can be made optimally wide, while the user data reader can be made optimally narrow. Additionally, the separation of the readers allows avoiding the dead spots and biasing problems found in the conventional center-tapped and dual-purpose readers. 
     Referring first to FIG. 9, a perspective view of a data reader  900  and servo reader  904  is depicted. The data reader  900  and servo reader  904  are preferably arranged side-by-side so that additional manufacturing steps are not necessary to create both readers  900 ,  904 , as will be described more fully below. The magnetic media travels beneath the servo and data readers  900 ,  904  in the direction shown by the arrow  232 . However, alternative embodiments may have the servo and data readers  900 ,  904  arranged in other configurations such as in-line with the direction of the magnetic media denoted by the arrow  232 , for example. 
     The data reader  900  is positioned over the data region  148  (see FIG. 3) of the data track  144  in order to read the user data stored magnetically on the disk surface  104 . Included in the data reader  900  is a first conductor  908 , a first permanent magnet  912 , a first magnetoresistive (MR) strip  916 , a second permanent magnet  920 , and a second conductor  924 . As is well known in the art, MR strips change their resistance when exposed to a magnetic field. To take advantage of the magnetoresistive effect, a constant first bias current  930  (the direction represented by the arrow) is supplied to the first conductor  908 , flows through the first MR strip  916  and returns from the second conductor  924 . In accordance with Ohm&#39;s Law (V=I×R), the voltage across the first MR strip  916  will vary in proportion to the resistance change in the MR strip  916 . 
     The first and second permanent magnets  912 ,  920  are magnetically polarized in the same direction during manufacture so that they set a permanent bias. The resistance response of a MR strip when exposed to the permanent bias has linear and non-linear portions. As those skilled in the art can appreciate, a linear response is preferred over a non-linear response. To achieve the linear response based upon a preselected bias current, the MR strip is preferably biased into the linear region with a soft adjacent layer (SAL), spin valve (i.e., giant MR) or exchange bias layer. These techniques magnetically bias the MR strip into a linear portion of the resistance response. 
     The first MR strip  916  has a width  928  which is optimized for reading the data region  148  (see FIG. 3) of the data track  144 . The first MR strip width  928  is smaller than the width of a data track  312  so that it is more likely the MR strip  916  will be positioned over the data track  144  while reading therefrom. As those skilled in the art can appreciate, it is desirous to limit the magnetic fields incident on the first MR strip  916  to the field produced from the data region  148  of the data track  144 . To that end, first and second permanent magnets  912 ,  920  are positioned on each side of the first MR strip  916  to “pin down” the strip  916  to restrict sensitivity to the defined trackwidth. In this way, the magnetic noise from other tracks is reduced and/or eliminated so that the resistance of first MR strip  916  is only affected by the desired data track  144 . 
     The servo reader  904  is optimized to read servo sectors  152  (see FIG.  3 ). Included in the servo reader  904  is a third conductor  932 , a third permanent magnet  936 , a second MR strip (or data read element)  940 , a fourth permanent magnet  944 , and a fourth conductor  948 . The second MR strip  940  has a width  952  which is sized based upon a width  320  (see FIG. 3) of servo bursts  304 ,  306 . In a servo sector system with two servo bursts  304 ,  306 , the second MR strip width  952  is typically sized to have a width just greater than the servo burst width  320 , while in a quadrature servo sector system (see FIG. 7) the second MR strip width  952  is optimally sized to have a width just greater than half the servo burst width  320 . As is understood by those skilled in the art, proper sizing of the second MR strip width  952  with respect to the servo burst width  320  avoids non-linear off-track PES signal response. 
     To operate effectively, the second MR strip must be electrically and magnetically biased. A constant second bias current  950  (the direction represented by an arrow) flows into the third conductor  932 , through the second MR strip (or servo read element)  940  and out the fourth conductor  948 . This second bias current  950  creates a voltage drop across the second MR strip  940  as the resistance therein changes as a result of changes in the incident magnetic field. The third and fourth permanent magnets  936 ,  944  serve to “pin-down” or restrict the sensitivity of the MR strip  940  to the defined track width. 
     The side-by-side configuration of the separate data and servo readers  900 ,  904  makes manufacture of the transducer simpler. Modem lithography techniques build the transducer in layers. Since each layer has the same composition and similar structures, those skilled in the art can appreciate that no additional masking or deposition steps are required to produce two side-by-side readers  900 ,  904 . 
     Manufacturing techniques magnetically polarize the first, second, third, and fourth permanent magnets  912 ,  920 ,  936 ,  944  in a single step. This is performed by subjecting the first through fourth magnets  912 ,  920 ,  936 ,  944  to a large magnetic field so that they are polarized therefrom. Unfortunately, conventional techniques do not allow the first through fourth permanent magnets  912 ,  920 ,  936 ,  944  to be selectively polarized in different directions. As a result of the uniform polarization of this embodiment, the bias current must flow through each of the first and second MR strips  916 ,  940  in the same direction to properly achieve the magnetic and electric biasing. 
     The servo reader  904  is separated from the data reader  900  to provide better biasing without dead spots associated with systems which teach a single center-tapped MR strip  818 , as shown in FIG.  8 . As can be appreciated by those skilled in the art, the use of a single MR strip  818  with first and third conductors  806 ,  814  attached at the ends and a second conductor  810  connected to a point therebetween will create a dead spot because the second conductor  810  causes the current to avoid a portion of the MR strip  818 . Without current in that portion of the MR strip  818 , there is no sensitivity to changes in the incident magnetic field. Additionally, since the first, second, third, and fourth permanent magnets  912 ,  920 ,  936 ,  944  are magnetically biased in the same direction during manufacture, the first and second bias currents  930 ,  950  in the first and second MR strips  916 ,  940  also flow in the same direction. Unfortunately, the center tapped read elements  802  require a first and second bias currents  822 ,  826  to flow in opposite directions through each half of the bifurcated MR strip  818  because of the common center conductor  810 . As explained above, the current flow in opposite directions  822 ,  826  makes magnetic biasing difficult because any permanent magnets should also be magnetically biased in opposite directions, although conventional manufacturing techniques generally require biasing the permanent magnets in a single direction. 
     With reference to FIG. 10, a detailed air-bearing view of a transducer, generally designated  1000 , with separate user data and servo read elements  900 ,  904  is illustrated. Included in the transducer  1000  are the data reader  900 , the servo reader  904 , a data writer  1004 , a first shield  1008 , and a second shield  1012 . To write information to the disk surface  108  (see FIG.  1 A), a magnetic field is induced between the data writer  1004  and the first shield  1008 . Magnetic material on the disk surface  108  is magnetically polarized as a result of this magnetic field. In this way, data is stored in the polarized magnetic material. 
     The embodiment in FIGS. 9 and 10 has a first and a second conductor  908 ,  924  for the data read element  916 , and a third and fourth conductor  932 ,  948  for the servo read element  940  so that there are a total of four conductors. Referring to FIG. 11, the four conductors  908 ,  924 ,  932 ,  948  would have four wires running from a portion of the transducer  1120  to a portion of the read channel  1116 . The servo read element  940  is supplied a first signal  1100  and returns a second signal  1104  and the data read element  916  is supplied a third signal  1108  and returns a fourth signal  1112 . It should be noted however, that the portion of the transducer  1120  never reads both servo information and user data at the same time. 
     Referring to FIG. 12, a block diagram of an embodiment which reduces the number of wires between the portion of the transducer  1120  and the portion of the read channel  1116  from four to three. To reduce the number of wires running from the portion of the transducer  1120 , one of the conductors from each read element  916 ,  940  is connected together to reduce the number of wires to three, so that a fifth wire  1200  serves the function of the second and fourth wires  1104 ,  1112  (see FIG.  11 ). Since the transducer portion  1120  never reads both user data and servo information at the same time, the three conductors can be switched by the multiplexer  1114  to enable either the data read element  916  or servo read element  940  at the appropriate times. 
     Referring to FIG. 13, an embodiment of the transducer  1000  and a portion of the analog read channel is illustrated in block diagram form. The transducer  1000  contains the data write element  1004 , the data read element  916  and the servo read element  940 . An analog write signal  1304  is provided to the data writer  1004 . In contrast, a data read signal  1102  and a servo read signal  1106  are received from the transducer  1000 . The data read signal  1102  and servo read signal  1106  are combined into a single read signal  1320  by a multiplexer  1114  under the control of a select line  1324 . When the transducer  1000  is positioned over the data region  148  (see FIG.  3 ), the data read signal  1106  is selected. However, while over the servo region  152 , the servo read signal  1102  is selected. In this way, the servo and data read signals  1102 ,  1106  can are multiplexed  1114  so that a conventional preamplifier  140  and demultiplexer  412  (see FIG. 4) can be used. By comparing FIGS. 4 and 13 one can appreciate only an additional multiplexer  1114  is needed in FIG. 13 to interface the transducer  1000  with a conventional preamplifier  140  and demultiplexer  412 . The use of a conventional preamplifier  140  and demultiplexer  412  eases integration of the separate read elements  916 ,  940  into conventional read channels. 
     With reference to FIG. 14, a block diagram of an embodiment of the read electronics for a two disk (i.e., four surface) drive system  100  (see FIG. 1B) is shown. Each surface  108  of the two disks  104  has a corresponding first, second, third, and fourth transducer  1404 ,  1408 ,  1412 ,  1416  which each contain separate data and servo read elements  916 ,  940  (see FIG. 9) to respectively read user data and servo information from the respective disk surface  108 . The first transducer  1404  produces servo and data read signals  1436 ,  1440  which are multiplexed together and amplified in a first preamplifier  1420  to produce a combined first read signal  1468 . If chosen by the select lines  1424  of a multiplexer  1428 , the combined first read signal  1468  is further multiplexed and passed to a read channel  1432 . The read channel  1432  processes and demultiplexes the user data and servo information for the selected transducer. In the same way, each of the second, third or fourth servo read signals  1444 ,  1452 ,  1460  and second, third or fourth data read signals  1448 ,  1456 ,  1464  are respectively multiplexed and amplified in their second, third or fourth preamplifiers  1424 ,  1428 ,  1432 , whereupon the selected combined second, third or fourth read signal  1472 ,  1476 ,  1480  passes through the multiplexer  1428  and is processed by the read channel  1432 . 
     A portion of a disk surface  108  (see FIG. 1A) is illustrated in FIG. 15 which shows an embodiment of the servo sectoring scheme. Although only nine data tracks  144  are shown to simplify the drawing, those skilled in the art know a disk surface  108  has many more data tracks  144 . The first MR strip  916  and second MR strip  940  of the transducer  1000  are superimposed upon the disk surface  108  to generally indicate relative dimensioning. As can be seen, the first MR strip  916  has a width  928  which is less than a width  312  of the data region  148 . The second MR strip  940  has a width  952  which is just greater than a width  1504  of either servo burst  1508 ,  1512  (i.e., A burst or B burst). 
     Positioning the first MR strip  916  relative to the data track  144  requires different algorithms than conventional systems. In conventional two servo burst systems (see FIG.  3 ), the servo burst width  320  is generally equal to the data track width  312  plus an erase band width  324 . However, in the depicted embodiment, the servo burst width  1504  is greater than or equal to three data track widths  312  and three erase band widths  324 . The first MR strip  916  is radially offset from the second MR strip  940  by approximately one track width  312  plus two erase band widths  324 . To position the first MR strip  916  over the desired data track, a predetermined ratio between the A burst signal and B burst signal in the position error signal (PES) is needed. For example, positioning over track  4  would require the ratio between the A burst signal and B burst signal of two to one while track  5  would require three to zero and track  6  would require one to two. Modifying the PES algorithms may require new firmware in the drive and/or other modifications. It should be noted however, other embodiments could use both the data and servo readers  900 ,  904  in combination with appropriate off-track algorithms to position the transducer  1000 . 
     The embodiment in FIG. 15 has a ratio of total A and B servo bursts  1508 ,  1512  to data tracks  144  along a given radius of one to three. As can be appreciated, having less servo bursts is desirable because laying down less servo bursts during manufacturing reduces the time needed for the servo track writer (“STW”) to format the disk surface  108  (see FIG.  1 B). Having less servo bursts also reduces the equipment needed to format the drives in a timely manner during manufacture. Although it is recognized that conventional STW may not be capable of writing the wide servo bursts  1508 ,  1512 , a conventional STW could write each wide servo burst  1508 ,  1512  as a series of conventional servo bursts  304 ,  306  (see FIG. 3) which are “stitched” together to form a contiguous wide servo burst  1508 ,  1512  as will be understood by those skilled in the art. 
     Other embodiments of the invention could use separate data and servo readers  900 ,  904  and have a ratio of the total number of servo bursts  1508 ,  1512  to data tracks  144  along a given radius of one to one or less. For example, a quadrature servo sectoring system which has a ratio of two to one. Under these circumstances, the first MR strip  916  would still be not as wide as the second MR strip  940  so that each reader  900 ,  904  could be optimized for its respective task. In this case, more traditional PES algorithms, known to those skilled in the art, could be used to position the transducer  1000  over the desired data track  144 . 
     With reference to FIGS. 16 and 17, the circuitry which processes an analog read signal produced by the data and servo readers  900 ,  904  is further optimized by separating the data read channel from the servo read channel. The separate servo and data read channels allow for processing user data and servo information at the same time which allows for circumferentially staggering the servo sectors on each disk surface. For example, if the first disk surface has servo sectors placed radially every eight degrees (i.e., 0°, 8°, 16° . . . 352°) the second surface has servo sectors offset two degrees (i.e., 2°, 10°, 18° . . . 354°) from the first surface. By staggering the servo sectors in this way, the flying-blind time is reduced since radial position of the transducers is corrected more often which allows for improved shock tolerance and/or reduction of the number of servo sectors. 
     With reference to FIG. 16, further benefit of separate data and servo readers is possible when a first, second, third, and fourth servo read signals  1440 ,  1448 ,  1456 ,  1464  are kept separate from a first, second, third, and fourth data read signals  1436 ,  1444 ,  1452 ,  1460  throughout the read channel. In other words, the multiplexing  1114  (see FIG. 13) and demultiplexing  412  (see FIG. 4) of the servo and data read signals does not take place in this embodiment. 
     The first through fourth servo read signals  1440 ,  1448 ,  1456 ,  1464  are kept separate from their respective first through fourth data read signals  1436 ,  1444 ,  1452 ,  1460  throughout the servo and data read circuitry. The first transducer  1404  produces the first servo read signal  1440  and the first data read signal  1436  which are each amplified in a first dual channel preamplifier  1604  to respectively produce a first amplified servo signal  1636  and a first amplified data signal  1620 . If a first select input  1652  connects the first amplified data signal  1620  to a data read channel  1668  through a first multiplexer  1660 , the first amplified data signal  1620  is processed to determine the data read by the first transducer  1404 . Similarly, if a second select input  1656  connects the first amplified servo signal  1636  to a servo read channel  1672  through a second multiplexer  1664 , the first amplified servo signal  1636  is processed to determine off-track position error from the first transducer  1404 . 
     By manipulating the first and second select lines  1652 ,  1656  any of the first, second, third, or fourth servo read signals  1440 ,  1448 ,  1456 ,  1464  or first, second, third, or fourth data read signals  1436 ,  1444 ,  1452 ,  1460  can be chosen for processing by their respective servo or data read channel  1672 ,  1668 . For example, the first servo read signal  1440  could be used to position the first through fourth transducers  1404 ,  1408 ,  1412 ,  1416  while the fourth transducer  1416  is used to read user data. Additionally, any one of the first through fourth transducers  1404 ,  1408 ,  1412 ,  1416  can be used to position the first through fourth transducers  1404 ,  1408 ,  1412 ,  1416  while another transducer writes user data. The separation of the data and servo read channels allows mixing and matching signals from the first through fourth transducers  1404 ,  1408 ,  1412 ,  1416  according to need. 
     Referring next to FIG. 17, a six data track segment of four different disk surfaces is shown, in straight rather than arcuate fashion, for ease of depiction. Each of the first, second, third, and fourth disk segments  1704 ,  1708 ,  1712 ,  1716  shows the data tracks  144  lying between a 0° and 8° slice of the disk. In this example, each disk surface has servo sectors  1516  circumferentially spaced every 8°. Further, each disk surface  108  is aligned on the spindle with a 2° offset between adjacent disk surfaces  108  so that no two disks have a servo sector  1516  the same circumferential position. 
     By staggering the servo sectors, the first through fourth transducers  1404 ,  1408 ,  1412 ,  1416  (see FIG. 16) can correct position four times more frequently. This is made possible by having separate data and servo readers  900 ,  904  which have separate data and servo channels (see FIG.  16 ). For example, while writing to the fourth segment  1716 , the servo information  1516  from the first through third segments  1704 ,  1708 ,  1712  can be analyzed to provide position correction to all transducers. As an alternative to correcting position more often, the number of servo sectors  1516  could be reduced, or a compromise between less servo sectors  1516  and more positioning correction could be made. 
     Although the discussion of the invention has generally been limited to bias currents which flow in the same direction, other embodiments could flow the bias currents in opposite directions. For example, an advanced magnetoresistive (AMR) device with no off-axis anisotropy may allow bias currents to flow in opposite directions. Additionally, other embodiments could avoid the “hard bias” of the permanent magnets by using exchange bias or exchange pinning in combination with an AMR device. 
     It will be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof For example, the invention could be used with magnetic tape drives. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not intended to be limited to the details given herein.