Abstract:
A hard disk drive device and a method and apparatus for control of the hard disk drive is provided. The hard disk drive includes disk media, a slider head, a head gimbal assembly and a control means. The disk media includes at least two layers for data storage. The slider head flies above the disk media and includes a writer and a reader, and a head gimbal assembly supports the slider head above the disk media. The control means is physically coupled to the head gimbal assembly and electrically coupled to the writer and the reader for reducing write interference from the writer when the writer is writing to the disk media while the reader is reading from the disk media, wherein write interference is reduced in one or more of a time domain and a frequency domain.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims priority to Singapore Patent Application No. 201305785-6, filed 30 Jul., 2013. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to hard disk drives. In particular, it relates to reading from a hard disk drive media while writing to the media. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Conventional hard disk drives have a media formed on a rotatable disk for storing information. The information is both servo information stored in servo sectors on the disk media and data stored in data areas between the servo sectors. A read/write head flies over the rotating disk in order to read information from both the servo sectors and the data areas and write information to the data areas. The servo information is used by the read/write head control system to determine the location of the read/write head relative to the servo sectors. However, when the read/write head is flying over a data area, there is no servo information available for servo feedback and control. 
         [0004]    In a dedicated servo system, this issue is addressed by having a magnetic dedicated servo layer in addition to the usual magnetic data layer in the disk media. This may be referred to as a buried servo layer. With the additional dedicated servo layer, much or all of the servo information is placed into the buried servo layer, leaving more space in the data layer for data areas. 
         [0005]    In conventional hard disk drives, the head either reads or writes, but cannot perform both operations simultaneously. However, in the dedicated servo implementation, in addition to increased data space, the dedicated servo layer allows reading location information and monitoring position signals from the servo layer even when the recording head is performing a data write operation. 
         [0006]    Yet, during a write operation, the write signal is necessarily coupled to the read signal, interfering with reading the servo signal while writing to data areas. The interference of the write signal is likely much stronger than the read signal and therefore poses a challenge in the recovery of useful information from the servo signal while writing to the data layer. 
         [0007]    Thus, what is needed is a method and apparatus for reducing or eliminating interference between the write signal and the read signal in order to advantageously implement the dedicated servo layer for a robust read while write operation. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure. 
       SUMMARY 
       [0008]    According to the Detailed Description, an apparatus for control of operations of a hard disk drive is provided. The apparatus for control of operations of a hard disk, drive includes a writer for writing onto a disk media of the hard disk drive and a reader for reading from the disk media of the hard disk drive. The apparatus for control of operations of a hard disk drive also includes write interference control for reducing write interference when the writer is writing to the disk media while the reader is reading from the disk media. 
         [0009]    In accordance with another aspect, a method for control of a hard disk drive is provided. The method includes writing onto a disk media of the hard disk drive and reading from the disk media of the hard disk drive. The method further includes reducing write interference when the writer is writing to the disk media while the reader is reading from the disk media, wherein write interference is reduced in one or more of a time domain and a frequency domain. 
         [0010]    In accordance with a further aspect, a hard disk drive device is provided. The hard disk drive includes disk media, a slider head, a head gimbal assembly and a control means. The disk media includes at, least two layers for data storage. The slider head flies above the disk media and includes a writer and a reader, and a head gimbal assembly supports the slider head above the disk media. The control means is physically coupled to the head gimbal assembly and electrically coupled to the writer and the reader for reducing write interference from the writer when the writer is writing to the disk media while the reader is reading from the disk media. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with a present embodiment. 
           [0012]      FIG. 1  depicts a cross-sectional view of a dedicated servo configuration for a hard disk drive system in accordance with a present embodiment. 
           [0013]      FIG. 2  depicts a block diagram of a simultaneously read/write capable pre-amplifier with write coupling suppression in accordance with the present embodiment. 
           [0014]      FIG. 3 , including  FIGS. 3A and 3B , depict graphs of frequency spectrums of a write coupling response in accordance with the present embodiment for a particular read/write head, wherein  FIG. 3A  depicts the write coupling response for a single write pulse at 150 MHz and  FIG. 3B  depicts the write coupling response for a single write pulse at 500 MHz. 
           [0015]      FIG. 4  is a graph illustrating a frequency spectrum of a readback signal containing dual-frequency servo information with interference from a pseudorandom write signal coupled inside. 
           [0016]      FIG. 5  illustrates the readback signal of  FIG. 4  after passing through a low pass band filter in accordance with the present embodiment. 
           [0017]      FIG. 6  illustrates the readback signal of  FIG. 4  after passing through a bandpass filter and a bandstop filter in accordance with the present embodiment. 
           [0018]      FIG. 7  illustrates the position error signal (PES) transfer curve as read from the dedicated servo layer and after using the combination of filters used to generate the filtered readback signal in  FIG. 6 . 
           [0019]      FIG. 8  illustrates the concept of using “elementary building blocks” of coupled write responses from rising and falling edge of the write signal in the time domain in accordance with the present embodiment. 
           [0020]      FIG. 9  illustrates time domain write interference suppression for a 150 ns write pulse. 
           [0021]      FIG. 10  illustrates time domain write interference suppression for a 30 ns write pulse. 
           [0022]      FIG. 11  illustrates an approach for predicting a write coupling response by using a reference head gimbal assembly (HGA) signal or reference trace signal wherein a target useful readback signal is recovered by subtracting a predicted write coupling response. 
           [0023]      FIG. 12 , including  FIGS. 12A and 12B , illustrates a bottom planar view and a side planar view of a reference trace/reader approach and implementation. 
           [0024]      FIG. 13  illustrates using an unloaded HGA&#39;s response to write interference as a reference for write interference suppression/cancellation for another loaded HGA of the same type. 
           [0025]      FIG. 14  illustrates canceling/suppressing write coupling/interference during write operation by subtracting the write coupling/interference signal from the raw readback signal where the readback signal is a single frequency pattern written on the disk. 
           [0026]      FIG. 15  illustrates canceling/suppressing write coupling/interference during a write operation by subtracting the write coupling/interference signal from the raw readback signal where the readback signal is a pseudorandom pattern written on the disk. 
           [0027]      FIG. 16 , including  FIGS. 16A and 16B , illustrates a bottom planar view and a side planar cross-section view of reference reader in accordance with a first embodiment. 
           [0028]      FIG. 17 , including  FIGS. 17A and 17B , illustrates a block diagram and trace line implementation of typical trace lines from the preamplifier to the head/slider and trace lines from the preamplifier to the head/slider in accordance with the first embodiment. 
           [0029]      FIG. 18 , including  FIGS. 18A and 18B , illustrates a bottom planar view and a side planar cross-section view of reference reader in accordance with a second embodiment. 
           [0030]    And  FIG. 19  illustrates a block diagram and trace line implementation of trace lines from the preamplifier to the head/slider of the second embodiment. 
       
    
    
       [0031]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale. For example, the dimensions of some of the elements in the illustrations, block diagrams or flowcharts may be exaggerated in respect to other elements to help to improve understanding of the present embodiments. 
       DETAILED DESCRIPTION 
       [0032]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. It is the intent of present embodiments to present methods and apparatus for write interference suppression during reading while writing in a hard disk drive environment. 
         [0033]    In hard disk drives (HDDs), an actuator arm rotates about its pivot and the disk media is rotated below the actuator arm in order to locate the slider head (mounted at one end of the actuator arm to allow it to “fly” over the disk) over a particular location on a particular track for writing information to and/or reading information from such location. Thus, it is necessary for the actuator arm to “know” the particular location it is over at any one time. This “knowledge” is gained by reading servo information from the disk media. 
         [0034]    In accordance with a present embodiment, a dedicated servo layer is located below the data magnetic layer within the disk media. Referring to  FIG. 1 , a cross-sectional view  100  shows the configuration of a dual layer media configuration with a dedicated servo layer in accordance with the present embodiment. A slider head  110  flies over a disk media  120  for reading servo location information and reading and writing data therefrom. The slider head  110  includes a reader  112  such as a read element, a heater  114  for thermal fly height control, a writer  116  including such as write pole and writing coil, and a slider overcoat  118 . In accordance with the present embodiment, the disk media  120  includes a top overcoat and lubrication layer  122 , a data layer  124  for magnetically reading data from and writing data to the disk media  120 , a first intermediate layer  126 , a dedicated servo layer  128 , a second intermediate layer  130  and a substrate underlayer (SUL)  132 . A magnetic spacing  140  defines the distance between the reader  112  and the writer  116  and the data recording layer  124 . The width of the magnetic spacing  140  is dependent on a fly height  150  between an underside of the slider head  110  and a top surface of the disk media  120 . Finally, a head keeper spacing  160  is the distance from the top of the SUL  132  and the underside of the slider head  110 . 
         [0035]    While conventional HDDs have servo information embedded in servo sectors between data areas in a single data recording layer, the separate magnetic layer  128  dedicated for servo information provides continuous servo information. In this manner, position error signal (PES) and other location determining information can be continuously read by the reader  112  on the slider head  110  as it “flies” over the disk media  120 . 
         [0036]    The reader  112  reads a readback signal for both data reading and reading of servo information. In a conventional HDD, the servo information is readily obtained via the readback signal when the reader flies over the servo sectors. It is not necessary for this readback process to occur during a write process as the two processes will never occur simultaneously. In addition, the write signal would provide substantial interference with the readback signal during the write process. 
         [0037]    Yet, in the dedicated servo system such as the configuration shown in the view  100 , it is necessary to recover the servo signal and servo information even during the write process. Thus, it is an object of the present embodiment to provide a method to suppress, reduce, or remove the write coupling signal to be able to perform meaningful servo layer  128  readback while writing to the data layer  124 . 
         [0038]    Referring to  FIG. 2 ; a block diagram  200  depicts a schematic for write coupling, suppression in a simultaneously read/write (R/W) or read-while-write (RWW) enabled preamplifier (Preamp)  202  in accordance with the present embodiment. The RWW interference between a writing signal  206  and a readback signal  208  is represented by coupling  204 . This coupling  204  can take place between traces on the flex (connecting the Preamp  202  to the slider head  110 ) or in the slider head  110  ( FIG. 1 ). The resulting interfered readback signal  208  is read by the Preamp  202 . 
         [0039]    A normal preamp  210  generates the write signal  206  by receiving write signals WDX, WDY at a positive emitter-coupled logic (PECL) write input buffer  212 . When over the appropriate region of the data layer  124  on the disk media  120 , the signal are provided from the buffer  212  to drivers  214  for providing the write signal  206  to drive the writing coil  116 . In accordance with the present embodiment, the RWW preamp  202  includes additional components and circuitry as described herein that provides means to remove, suppress, or reduce the write signal  206  coupling to the read (or readback) signal  208  during the write process. Two approaches to suppress the write signal  206  coupling to the read signal  208  during the write process are presented in accordance with the present embodiment: a frequency domain approach and a time domain approach. 
         [0040]    In regards to the time domain approach, the read signal  208  is compensated in the time domain by predicting the write coupling/interference signal in the time domain. The write coupling/interference signal in the time domain is predicted by obtaining the response (write coupling) of a single write pulse. Since a write signal is composed of a combination of such write pulses in the time domain, it is possible to obtain a predicted response by combining individual responses from each write pulse making up the write signal. Thus, a time domain write interference suppression module  216  is coupled to the write signal  206  for operation when the writer is writing to the disk media  120 . The time domain write interference suppression module  216  includes a reference signal generating means  218  and a signal delay means  220 . The resulting generated reference write signal from the signal delay means  220  is subtracted from the read signal  208  via a mixer  222 . The reference write signal is generated by the reference signal generating means  218  in response to the write pulses on the write signal  206  and modeled responses of write pulses previously obtained. The reference write signal is appropriately delayed by the signal delay means  220  before injection to the read signal  208  at the mixer  222  in order to suppress any write interference when the writer  116  is writing to the disk media  120  while the reader  112  is reading from the disk media  120 . 
         [0041]    In regards to the frequency domain approach, the servo information in the readback signal  208  being read from the dedicated servo layer  128  has a different frequency range as compared to frequency ranges of data signals. Similarly, it has been determined that the write coupled signal in the readback signal  208  has the write coupling or interference signal with dominant frequency components in a different frequency range as compared to the servo information. Since the two signals are predominantly in different frequency ranges, in accordance with the present embodiment the write coupling signal is suppressed by frequency filtering while leaving the desired servo signal largely intact. Thus, a frequency domain write interference suppression module  224  includes one or more filters  226 ,  228  that can be coupled to the readback signal  208  by a filter selector  230  for filtering out write interference from the readback signal  208  when the writer  116  is writing to the disk media  120  while the reader  112  is reading from the disk media  120 . In this manner, the frequency domain approach utilizes a single filter  226  or a combination of filters  226 ,  228  to suppress the write coupling and allow recovery of the servo signal. The filtered readback signal  232  (e.g., the recovered servo signal) is provided to a differential amplifier  234  of the preamplifier  210  and then to a read output buffer  236  for provision of readback signals RDX and RDY. 
         [0042]      FIG. 3 , including  FIGS. 3A and 3B , depicts graphs  300 , 350  of frequency spectrums of a write coupling response in accordance with the present embodiment for a particular read/write head. More particularly,  FIG. 3A  depicts the graph  300  of the write coupling response for a single write pulse at 150 MHz where a signal&#39;s frequency is plotted along an x-axis  302  and the signal&#39;s amplitude is plotted along the y-axis  304 .  FIG. 3B  depicts the graph  350  of the write coupling response for a single write pulse at 500 MHz where the signal&#39;s frequency is plotted along an x-axis  352  and the signal&#39;s amplitude is plotted along the y-axis  354 . The range of the servo signal is typically within 0 to 100 MHz. It can be observed from the graphs  300 ,  350  that there are few dominant frequency components in the typical range of the servo signals. 
         [0043]    Referring to  FIG. 4 , a graph  400  with frequency plotted along an x-axis  402  and amplitude plotted along a y-axis  404  shows the frequency spectrum of a readback signal containing dual-frequency servo information  410 ,  420  interfered or coupled with a 150 MHz pseudorandom write signal. The two peaks of the dual frequency servo  410 ,  420  are clearly seen above the background noise from the write interference or coupling. 
         [0044]    Referring to  FIG. 5 , reference numeral  520  denotes a time domain signal corresponding to the frequency spectrum in graph  400  ( FIG. 4 .). Using a single low pass filter  226  ( FIG. 2 ) removes portions of the readback signal of  FIG. 4  higher than 90 MHz to obtain a filtered signal  530 . As one skilled in the art will observe, the high frequency components are completely removed from the filtered signal  530 , leaving behind the frequency range useful for reading the servo signal. 
         [0045]      FIG. 6  shows an illustration  600  including a first graph  610  having time plotted along the x-axis  612  and amplitude plotted along the y-axis  614  and a second graph  620  having frequency plotted along the x-axis  622  and amplitude plotted along the y-axis  624 . In accordance with the present embodiment, the time domain graph  610  and the frequency domain graph  620  depict use of a combination of a bandpass filter  226  with a bandstop filter  228  to filter the readback signal of  FIG. 4  having the servo signal being coupled with or interfered with the 150 MHz pseudorandom write signal as shown by reference numeral  615 . The recovered signal after filters  226 ,  228  is denoted by reference numeral  616  in  FIG. 6 , the combined servo signal  630 ,  640  in time domain. Both the time domain waveform graph  610  and the frequency domain graph  620  show that the dual frequency servo signal  630 ,  640  is cleanly recovered, while the write coupling signal is removed or effectively suppressed. 
         [0046]    Referring to  FIG. 7 , a graph  700  having crosstrack position plotted along the x-axis  702  and position error signal (PES) plotted along the y-axis  704  shows the position error signal (PES) transfer curve after using the combination of filters  226 ,  228  used for filtering to obtain the signal in the graph  620 . The PES information is critical for servo feedback and control and, in accordance with the present embodiment, can be cleanly recovered even, during a writing process where the write signal couples into the readback signal. A first PES transfer curve  706  shows the PES with a 150 MHz pseudorandom write interference signal, a second PES transfer curve  708  shows the PES with a 150 MHz single tone write interference signal, and a third PES transfer curve  710  shows the PES with no write interference signal. In regards to the first PES transfer curve  706 , portions  712  and  714  show the effects of some of the pseudorandom signal inside the pas band of the filter  226  which still do not affect the recovery of the PES signal. 
         [0047]    Turning next to the time domain approach, this approach compensates the readback signal  208  in the time domain by predicting the write coupling/interference signal in the time domain. One way to predict the write coupling/interference signal in the time domain is by obtaining the response (write coupling) of a single write pulse. Considering that a write signal is composed of a combination of such write pulses in the time domain, it is possible to obtain a predicted response by combining individual responses from each write pulse making up the write signal. 
         [0048]      FIG. 8  illustrates the concept of using “elementary building blocks” whereby the write coupling is composed of rising edge and falling edge responses. A time domain input waveform  810  depicts that a single pulse is composed of a rising edge followed by a falling edge. A first time domain graph  820  having time plotted along an x-axis  822  and amplitude plotted along a y-axis  824  shows a rising edge response  826  of a write coupling signal  828 . A second time domain graph  830  having time plotted along an x-axis  832  and amplitude plotted along a y-axis  834  shows a rising edge response  836  of a write coupling signal  838 . As the write coupling is composed of rising edge and falling edge responses, a time domain response waveform  840  can be generated by combining the rising edge response  826  and the falling edge response  836  as “building blocks”. 
         [0049]      FIGS. 9 and 10  show the time domain write interference suppression for a 150 ns write pulse and a 30 ns write pulse, respectively. Referring to  FIG. 9 , an illustration  900  depicts a first graph  910  having time plotted on the x-axis  912  and amplitude plotted on the y-axis  914 . An actual step response  918  of the 150 ns write pulse  916  is shown overlapping a synthesized coupled response  919 , and the synthesized coupled response  919  shows good agreement to the actual step response  918 . A second graph  920  has time plotted on the x-axis  922  and amplitude plotted on the y-axis  924 . An original write interference  928  is shown with a suppressed write interference signal  926  generated by subtracting a predicted response from the actual write interference. The write coupled signal in the readback signal consists of responses from the rising and falling edge of a single write pulse. By predicting the expected write coupling from the rising and falling edge responses and subtracting it from the readback signal during writing, the write coupling may be suppressed. This is also shown in  FIG. 10 , where an illustration  1000  depicts a first graph  1010  having time plotted on the x-axis  1012  and amplitude plotted on the y-axis  1014 . An actual step response  1018  of the 30 ns write pulse  1016  is shown and overlapping with a synthesized step response  1019 , and good agreement is observed between the actual step response  1018  and the synthesized step response  1019 . A second graph  1020  has time plotted on the x-axis  1022  and amplitude plotted on the y-axis  1024 . An original write interference  1028  is shown with a suppressed write interference signal  1026  generated by subtracting a predicted response from the actual write interference 
         [0050]    Besides using a response from a single write pulse to predict the interference or write coupling, in accordance with the present embodiment a reference head gimbal assembly (HGA) or reference component/circuitry approach may be used. If the reference HGA is used, the reference HGA is manufactured to specifications and similar to the actual HGA in use. Thus, the response from the reference HGA is similar to that of the actual HGA. In this manner, the reference response from the reference HGA can be used to predict, and therefore subtract, the write coupling in the actual readback signal. In a preamp implementation, this can be in the form of a separate set of Read/Write lines (e.g., trace lines) to the reference HGA or a reference component/circuitry that has the same frequency response as the HGA in use. 
         [0051]    Referring to  FIG. 11 , an illustration  1100  depicts this approach. To predict the write coupling response, a TX REF    1120  similar to a write signal TX 1    1110  sent to a loaded HGA  1102  is sent to a reference unloaded HGA  1104  from a preamp  1106 . The responses RX REF    1122  are readback and recorded. Subsequently, this recorded response RX REF    1122  is subtracted at mixer  1132  in a time domain synchronized manner (i.e., using an optional delay line  1130  for time domain synchronicity) from a received response RX 1    1112  when the actual write signal TX 1    1110  is sent out to form the write interference suppressed signal  1134 . Thus, the readback signal during the write process will have a write coupling response that is suppressed or subtracted away by the recorded reference signal. In some cases, the predicted coupling response can be used to subtract the interference signal in real-time without needing to first record the predicted coupling response. 
         [0052]    Instead of using a reference HGA which requires significant additional space in a HDD to accommodate, it is possible, as introduced above, to use a reference trace/reader approach. 
         [0053]    Due to shrinking read sensor dimensions on the HDD slider, the readback signal is weakened and may become too weak to support continued areal or track density increases. An alternative is to employ multiple readers on one head to increase the strength of the readback signal or use signal processing to process the signal from multiple heads to increase the signal-to-noise ratio (SNR). 
         [0054]    In a Two Dimensional Magnetic Recording (TDMR) head implementation, there may be multiple readers in a single head/slider. Multiple readers mean additional traces in the suspension that need to connect to the additional readers. Thus, manufacturing multiple traces/readers on a single head/slider/HGA is possible. It is also possible to add extra trace/readers for a single head/slider/HGA. The extra trace/reader can be designed and fabricated to have the same performance and specifications as the existing trace/reader on the head/slider/HGA. This extra trace will sustain the same write interference and write coupling as the trace of other read sensors and traces when writing occurs. In order to further improve the method, the extra read sensor connected to the extra trace can be designed to not sense any extrinsic signals, such as by shielding it or by positioning it at a large distance from the magnetic media. In this manner, the only signal it is reading is the write interference or write coupling signal. 
         [0055]      FIG. 12 , comprising  FIGS. 12A and 12B , illustrates the additional read sensor (reference reader) variant of the present embodiment.  FIG. 12A  is a bottom planar view  1200  of the slider  110  in accordance with the variant of the present embodiment, and  FIG. 12B  is a side planar view  1220  of the slider  110  in accordance with the variant of the present embodiment. Multiple read sensors  1210 ,  1212  including a reference read sensor  1214  are mounted on the slider along with the writer  116 . As can be seen from  FIG. 12B , the reference read sensor  1214  has a large clearance over the magnetic layer  124  on the disk media  120 . The reference reader  1214 , thus, will not pick up the readback signal from the magnetic disk media that the other read sensors  1210 ,  1212  pick up. However, the trace lines connected to the reference reader  1214  will read the same write interference as the other read trace lines. Therefore, the signal from the reference reader  1214  and its trace will consist only of the interference from the write operation and not include any readback signals for the magnetic disk media  124 . In this manner, the extra trace and its accompanying reader  1214  can generate a signal for suppression of the write interference and be subtracted from the signal from the active read sensor  1210 ,  1212  and its trace. Thus, by subtracting the write interference signal obtained from the reference reader  1214  and its trace, the readback signal obtained from the active read sensors  1210 ,  1212  can have the write coupled interference signal suppressed and, so, contain mostly useful signaling from reading the disk media. 
         [0056]      FIG. 13  illustrates a view  1300  of signals during writing when an unloaded HGA is used for generating the reference signal for suppressing write interference on the readback signal on a loaded HGA. A signal  1302  depicts the write signal. Signals  1306  and  1308  depict the write coupling response on the loaded HGA reader and the unloaded HGA reader, respectively. A signal  1310  depicts the write interference suppressed signal where the reference signal  1308  is subtracted from the readback signal  1306 . Reviewing the signal patterns  1302 ,  1306 ,  1308  during a time  1312 , it is clear to one skilled in the art that an unloaded HGA&#39;s response is a good predictor of write interference for another loaded HGA of the same type. This result can be applied to having an additional reference trace/reader on a single same head/HGA to be used as reference for write interference cancellation or suppression for other active traces/readers. 
         [0057]    The signals depicted in a view  1400  of  FIG. 14  show that write coupling/interference during a write operation can be cancelled and/or suppressed by subtracting the write coupling/interference signal from the raw readback signal. A write signal  1402  is shown above an existing signal  1404  read from the disk media  120 . A signal  1406  shows the write coupling interference and a signal  1408  shows a write coupling interfered signal  1408  read from the disk media  120 . Write interference can be removed from the signal  1408  by removing the signal  1406  to obtain a recovered write interference suppressed signal  1410 . As discussed above, the write coupling/interference signal for suppression can be obtained from the reference trace/reader/HGA. The resulting signal  1410  is a strong recovered signal that has write interference removed or suppressed. 
         [0058]    The readback signal  1404  is a single frequency pattern written on the disk. Referring to  FIG. 15 , an illustration  1500  depicts similar signaling as  FIG. 14  except that a readback signal from the disk is from a track written with a pseudorandom pattern. Thus, a write signal  1502  is shown above a signal  1504  showing the write coupling interference and a write coupling interfered signal  1506  read from the disk media  120 . Write interference can be removed from the signal  1506  by removing the signal  1504  to obtain a recovered write interference suppressed signal  1508 . Comparing a portion  1520  of the recovered write interference suppressed signal  1508  to a portion  1510  of the write coupling interfered signal  1506 , the portion  1520  shows reduced or suppressed interference. Similarly, comparing recovered another portion.  1540  of the recovered write interference suppressed signal  1508  with another portion  1530  of the write coupling interfered signal  1506 , the portion  1540  shows reduced or suppressed interference. 
         [0059]    In current HDDs, there is a need to design for reduction of and/or shielding of electromagnetic interference (EMI). EMI or electromagnetic noise from surroundings can affect HDD operation. Thus, HDDs have design considerations such as (but not limited to) overlapping or overhanging edges on the HDD enclosure, metallic shielding tape, or metallic particle bond or fillers to reduce and shield EMI. 
         [0060]    It is possible to use the reference trace/reader approach discussed above as an in-situ sensor for EMI. The EMI signal detected by the reference trace/reader will be similar to that experienced by other reader/traces and can be used for cancellation of the EMI noise in read line signals during read or write operation. In this manner, cost savings can result as EMI design considerations for HDD can be relaxed when using an EMI suppression scheme based on the reference trace/reader approach. 
         [0061]    Various embodiments for implementing reference trace and reader are possible. The reference trace and reader can make use of an existing trace and reader on a multiple reader TDMR head and/or slider or be an additional trace and reader added to an existing HGA. 
         [0062]      FIG. 16  and  FIG. 18  depict two possible embodiments for reference reader location (the reference trace leading to the reference reader is not shown). Referring to  FIG. 16 , including  FIGS. 16A  (a bottom planar view  1600 ) and  16 B (a side planar cross-sectional view  1620 ), a first embodiment depicts a first shield  1602  and a second shield  1604  protecting a reader  1606  from EMI. A reference reader  1608  (a read sensor) is located far from the magnetic media and also shielded from the media by the second shield  1604 . First and second leads  1622 ,  1624  also shield the reference reader  1608  and provide connection to the flex connector. In this manner, the reference reader  1608  will not pick up any readback signals from the disk media. Thus, signals from the reference reader  1608  and its traces  1622 ,  1624  can be used as a reference signal for suppression of write interference from other readers  1606  and/or for suppression of EMI noise. 
         [0063]    For the reference trace to be more effective, symmetry in design should also be considered. Referring to  FIG. 17A , an illustration  1700  of a preamplifier  1702  shows that EMI noise is typically (A+B)sin ωt. Referring to  FIG. 17B , an illustration  1720  illustrates a possible implementation for the embodiment of  FIG. 16  where trace lines from a preamplifier  1722  to the head/slider reader and reference reader completely cancel the EMI noise. 
         [0064]    Referring to  FIG. 18 , a second embodiment is depicted in  FIG. 18A  (a bottom planar view  1800 ) and  FIG. 18B  (a side planar cross-sectional view  1820 ). The second embodiment differs from the first embodiment ( FIG. 16 ) in that the second shield  1604  is merged with a second lead  1822  for the reference reader  1608  and they share the same trace lead to the pre-amplifier.  FIG. 19  is an illustration  1900  for the pre-amplifier  1902  where the trace line Rdx comes from the shield  1822 , the trace line Rdy comes from the shield  1602  and the trace Rdz comes from the lead  1622 . In this manner, EMI Interference (A−B)sin ω t between Rdx and Rdy traces can be suppressed using EMI Interference (A−C)sin ω t between Rdz and Rdy. For complete cancellation, Rdx−Rdy=a(Rdz−Rdy) where a=(B−A)/(C−A) applies. 
         [0065]    Thus, it can be seen that suppressing the write interference in accordance with the present embodiments enables improved servo information recovery during the writing process, thereby overcoming drawbacks in the current state of the art which lacks such write interference suppression schemes. With recovery of servo information during the writing process in a dedicated servo system, an HDD design can have better tracking capability, leading to increased achievable track density. As those skilled in the art can realize, increased track density and servo capabilities have direct impact on areal density of the disk media  120 . 
         [0066]    In addition, suppression of write interference allows read-while-write which can be used for other technologies such as for write synchronization as needed in two dimensional magnetic recording and bit patterned media. Further, reference read sensors and traces can be used for EMI suppression and cancellation and may relax EMI design requirements for HDDs. While exemplary embodiments have been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. 
         [0067]    It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, operation, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements and method of operation described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.