Abstract:
A perpendicular recording system includes a summing module that has a first input that receives a read signal. A DC correction module selectively generates a DC correction signal to reduce DC offset in the read signal. The DC correction signal is output to a second input of the summing module. A detecting module compares an output of the summing module to a predetermined threshold and selectively detects Thermal Asperity (TA).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. patent application Ser. No. 10/737,648, filed on Dec. 15, 2003, and claims the benefit of U.S. Provisional Application No. 60/514,519, filed on Oct. 24, 2003, both of which are hereby incorporated by reference in their entirety. 

   FIELD OF THE INVENTION 
   The magnetic storage industry has been increasing the storage capacity of hard drives using perpendicular recording systems and magneto-resistive (MR) heads. The MR head includes an MR element that is made of a material that changes electrical resistance in response to magnetic fields. The MR head normally glides over the spinning magnetic disk. When the MR head hits a protruding object on the disk surface, the MR element heats up rapidly. Afterwards, the heat decays relatively slowly. The effect of this transient phenomenon is a change in the baseline of the read-back signal that is output by the MR head. The baseline change contains a substantial low-frequency component, which may cause loss of read-back data. The severity of the loss depends upon the robustness of the data detection system and the rate at which TA events occur. 
   TA events usually occur more often in perpendicular recording systems due to the relatively close proximity of the recording head and the magnetic media. Heating of the recording head from the TA event is higher than that experienced in longitudinal recording systems due to the smaller distance between the recording head and the media. The increased heating causes a transient of increased duration and amplitude in the output signal of the recording head. In other words, the recording head signal of perpendicular recording systems typically includes both an increased rate of TA events and TA events that have a longer duration. With the increased likelihood of errors, it is possible that the error correcting code (ECC) that is used in the perpendicular recording system may not be able to regenerate the user data. 
   In longitudinal recording systems, TA can be detected by low pass filtering the analog read-back signal at, for example, the output of a variable gain amplifier (VGA). The output of a low pass filter (LPF) is compared to a threshold. If the output exceeds the threshold, a TA event is declared. This technique can apply to the absolute value of the output of the LPF as well. Since the perpendicular recording read-back signal contains a strong low frequency component, it is more likely that false detection of TA events will occur in perpendicular recording systems. 
   Referring now to  FIG. 1 , a block diagram of an exemplary read channel for a perpendicular magnetic recording system is shown and is generally designated  20 . A preamp module  24  receives a read signal from a MR head  26 , which is positioned by a read/write arm  28  relative to a magnetic storage medium  30 . A spindle motor  32  rotates the magnetic storage medium  30 . An output of the preamp module  24  is coupled to an input network  40 , which has an output that is connected to a variable gain amplifier (VGA) module  42 . The input network  40  typically includes one or more resistors and/or capacitors. 
   A TA detector (TAD) module  46  and a continuous time filter (CTF) module  50  receive an output of the VGA module  42 . An output signal of the TAD module  46  indicates whether a TA event is detected. The CTF module  50  low pass filters and shapes the output of the VGA module  42 . An analog to digital converter (ADC) module  60  samples the output of the CTF module  50  and generates a digital output. A finite impulse response (FIR) equalizer  64  equalizes an output of the ADC module  60 . A data detector module  66  detects the read data in the filtered read data signal. A reconstruction filter module (RF)  70  receives an output of the data detector module  66  and reconstructs the input signal without the TA and DC offset errors. The reconstruction filter module  70  is typically an FIR filter whose taps are the equalization target, although other reconstruction filters may be used. For example, if the equalization target is (5, 6, 0, −1), then the FIR taps should be f 0 =5, f 1 =6, and f 3 =−1. 
   A timing control module  74  receives outputs of the ADC module  60  and the reconstruction filter module  70  and generates timing signals for the ADC module  60 . An automatic gain control (AGC) module  78  adjusts the gain of the VGA module  42  based on the output of the ADC module  60 , the FIR equalizer module  64  and the reconstruction filter module  70 . Additional details of the reconstruction filter module  70  can be found in U.S. patent Ser. No. 10/754,325, which was filed on Jan. 9, 2004 and which is hereby incorporated by reference in its entirety. 
   Referring now to  FIGS. 2 ,  3 A and  3 B, an example of one TAD module  46 - 1  is shown in  FIG. 2  to include a LPF  90 , a rectifier  92  and a threshold comparator  94 . The rectifier  92  rectifies negative signals when the MR head inputs are reversed. The input signal x is filtered by the LPF  90 , which has an output signal y that is input to the rectifier  92 . An output signal w of the rectifier  92  is input to a threshold comparator  94 . If the input signal w to the threshold comparator  94  is larger than a predetermined threshold, the output signal z=1, otherwise z=0. In  FIG. 3A , an example of a TAD module  46 - 2  without the rectifier  92  is shown. In  FIG. 3B , a first threshold comparator  95 - 1  compares the output of the LPF  90  to a first threshold Th 1 . A second comparator  95 - 2  compares the output of the LPF to a second threshold Th 2 . The first and second thresholds may have the same magnitude and opposite polarity. 
   Baseline wander may increase the probability of false TA detection, or, if the detection threshold is permanently adjusted for baseline wander, the probability of detecting an actual TA event decreases. In  FIGS. 4-7 , examples of different waveforms are shown at the output of the VGA module  42  and the output of the LPF  90  in the TAD module  46 . In these examples, the TAD module  46 - 2  shown in  FIG. 3A  is used. 
   Referring now to  FIG. 4A , an output signal  98  of the VGA module  42  for a perpendicular waveform corresponding to a random data sequence is shown. In  FIG. 4B , an output  100  of the LPF  90  of the TAD module  46 - 2  is shown with a TA threshold  102 . Since the output  100  of the LPF  90  is below the TA threshold  102 , a TA event is not detected. 
   Referring now to  FIGS. 5A and 5B , an output signal  98 ′ of the VGA module  42  that is similar to the waveform in  FIG. 4A  is shown. A TA event is added to the random data sequence as shown at  110 . An output  112  of the LPF  90  exceeds the TA threshold  102  and a TA event is detected. 
   Referring now to  FIGS. 6A and 6B , an input waveform includes a relatively long negative signal that is followed by a relatively long positive signal. In  FIG. 6A , a solid waveform  120  represents a read-back signal without the high pass filtering effect of the preamplifier  24  and input network  40 . A dashed waveform  124  in  FIG. 6A  represents a read-back signal that is distorted by the high pass filtering effect of preamplifier  24  and the input network  40 . An output  126  of the LPF  90  of the TA detector  46 - 2  is shown in  FIG. 6B  along with the threshold  102 . Since the output signal  126  exceeds the TA threshold  102 , a false TA event is detected. The reason for this false detection is that baseline wander, which is the difference between the non-distorted waveform  120  and the distorted waveform  124  in  FIG. 6A  adversely impacts the output signal  126  of the low pass filter  90  as well. In this case, the output signal  126  of the LPF  90  exceeds the TA threshold  102  around the transition. Raising the threshold would prevent this problem. 
   Referring now to  FIGS. 7A and 7B , an output signal  130  corresponding to a random waveform with TA is shown. In this case, undetected TA  132  occurs in an output signal  134  of the LPF  90  when the waveform without the TA is negative. In this example, a TA event would not be detected unless the TA threshold  102  is lowered. 
   As can be appreciated from the forgoing, in some cases the TA event is detected when it should not have been detected. In other cases, the TA event is not detected when it should have been detected. There is a trade-off between false detection and missed detection in conventional approaches. If the threshold is lowered, the likelihood of detecting false TA events increases. If the threshold is increased, the likelihood of correctly detecting TA events decreases. 
   SUMMARY OF THE INVENTION 
   A perpendicular recording system according to the present invention includes a summing module that has a first input that receives a read signal. A DC correction module selectively generates a DC correction signal to reduce DC offset in the read signal. The DC correction signal is output to a second input of the summing module. A detecting module compares an output of the summing module to a predetermined threshold and selectively detects Thermal Asperity (TA) events based on the comparison. 
   In other features, an input filter receives the read signal from a preamplifier and filters the read signal. A variable gain amplifier (VGA) module amplifies the read signal and outputs the read signal to the first input of the summing module. A magnetic storage medium stores magnetic fields. A magneto-resistive head reads the magnetic fields stored on the magnetic medium and generates the read signal. 
   In other features, a variable gain amplifier (VGA) module amplifies the read signal with the DC offset. A filter module receives an output of at least one of the VGA module and the summing module. An analog to digital converting (ADC) module samples the read signal with the DC offset that is output by the filter module and generates a digital read signal with DC offset. An equalizing module communicates with the ADC module and equalizes the digital read signal with the DC offset. 
   In other features, a data detecting module communicates with the equalizing module and detects data in the digital read signal with the DC offset. A reconstruction filter module communicates with the data detecting module and reconstructs the read signal without TA and DC offset errors. The DC correction module generates the DC correction signal based on outputs of the ADC module, the equalizing module and the reconstruction filter module. An automatic gain control (AGC) module generates a gain correction signal that is based on outputs of the ADC module, the equalizing module and the reconstruction filter module. 
   In other features, the filter module includes a finite impulse response filter. The data detecting module includes a Viterbi detector. The detecting module includes a low pass filter and a threshold comparator that compares an output of the LPF to the threshold. 
   In still other features, the detecting module includes a low pass filter (LPF) and a rectifier module that receives an output of the LPF. A threshold comparator compares an output of the rectifier to the threshold. The detecting module includes a high bass filter that has an output that communicates with the (LPF). 
   In other features, the detecting module includes a bandpass filter. A threshold comparator compares an output of the bandpass filter to the threshold. A timing control module generates a timing correction signal that is based on outputs of the ADC module and the reconstruction filter module. 
   A perpendicular recording system according to the present invention comprises a DC correction module that generates a DC correction signal. A detecting module has a threshold, receives a read signal with DC offset and the DC correction signal, adjusts one of the read signal with the DC offset and the threshold based on the DC correction signal. The detecting module compares the adjusted one of the read signal with the DC offset and the threshold to the other of the read signal with the DC offset and the threshold, and selectively detects Thermal Asperity (TA) events based on the comparison. 
   In still other features, an input network receives a read signal from the preamplifier and filters the read signal. A variable gain amplifier (VGA) module amplifies the read signal with the DC offset. A magnetic storage medium stores magnetic fields. A magneto-resistive head reads the magnetic fields stored on the magnetic medium and generates the read signal. 
   In other features, a variable gain amplifier (VGA) module amplifies the read signal. A filter module receives an output of the VGA module. An analog to digital converting (ADC) module samples the read signal that is output by the filter module and generates a digital read signal. An equalizing module communicates with the ADC module and equalizes the digital read signal. A data detecting module communicates with the equalizing module and detects data in the digital read signal. A reconstruction filter module communicates with the data detecting module and reconstructs the read signal without TA and DC offset errors. 
   In other features, the DC correction module generates the DC correction signal based on outputs of the ADC module, the equalizing module and the reconstruction filter module. An automatic gain control (AGC) module generates a gain correction signal that is based on outputs of the ADC module, the equalizing module and the reconstruction filter module. The filter module includes a finite impulse response filter. The data detecting module includes a Viterbi detector. A timing control module generates a timing correction signal that is based on outputs of the ADC module and the reconstruction filter module. 
   In other features, the detecting module comprises a low pass filter (LPF) and a summing module that has a first input that communicates with an output of the LPF and a second input that receives the DC correction signal. A threshold comparator compares an output of summing module to the threshold. 
   In other features, the detecting module comprises a low pass filter (LPF). A threshold comparator adjusts the threshold based on the DC correction signal to create an adjusted threshold and compares an output of LPF to the adjusted threshold. A high pass filter has an output that communicates with the LPF. A rectifier communicates with an output of the summing module and the threshold comparator. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a functional block diagram of a read channel for a perpendicular recording system according to the prior art; 
       FIG. 2  is a functional block diagram of one Thermal Asperity (TA) detector according to the prior art; 
       FIGS. 3A and 3B  are functional block diagrams of other TA detectors according to the prior art; 
       FIGS. 4A and 4B  illustrate the output of a variable gain amplifier (VGA) module and a low pass filter (LPF) module in the TA detector of  FIG. 3 , respectively, for a random input waveform without TA; 
       FIGS. 5A and 5B  illustrate the output of the VGA module and the LPF module in the TA detector, respectively, for a random input waveform with detected TA; 
       FIGS. 6A and 6B  illustrate the output of the VGA module and the LPF module in the TA detector, respectively, for a random input waveform with false detected TA; 
       FIGS. 7A and 7B  illustrate the output of the VGA module and the LPF module in the TA detector, respectively, for a random input waveform with a higher TA threshold and without false detected TA; 
       FIGS. 8A and 8B  illustrate the output of the VGA module with and without high pass filtering and the output of the LPF module in the TA detector with and without DC correction, respectively, for a random input waveform with false detected TA; 
       FIG. 9  is a functional block diagram of a read channel for a perpendicular recording system according to the present inventions; 
       FIG. 10  illustrates part of the read channel of  FIG. 9  corresponding to the VGA module, DC correction circuit and TA detector; 
       FIG. 11  illustrates a first alternate embodiment of the VGA module, DC correction circuit and TA detector; 
       FIG. 12  illustrates a second alternate embodiment of the VGA module, DC correction circuit and TA detector; 
       FIG. 13  illustrates one implementation of the TA detector of  FIG. 12  in further detail; 
       FIG. 14  illustrates a second implementation of the TA detector of  FIG. 12  in further detail; 
       FIG. 15  illustrates a third implementation of the TA detector of  FIG. 12  in further detail; 
       FIG. 16  illustrates a fourth implementation of the TA detector of  FIG. 12  in further detail; 
       FIG. 17  illustrates a TA detector according to the present invention with a high pass filter; and 
       FIG. 18  illustrates a TA detector according to the present invention with a band pass filter. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify the same elements. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
   In the example in  FIGS. 6A and 6B , false detection of the TA event occurred due to baseline wander. The baseline wander occurs due to high pass filtering effects of the preamplifier  24  and the input network  40 . TA detection according to the present invention improves the performance of perpendicular recording systems by compensating for baseline wander as will be described further below. 
   Referring now to  FIGS. 8A and 8B , the waveforms from  FIGS. 6A and 6B  are shown with an additional waveform  150  added to  FIG. 8B . The additional waveform  150  corresponds to the output the LPF when DC correction (for baseline wander) is added according to the present invention before the TA detector. If the DC corrected (or baseline wander corrected) signal is used by the TA detector, false detection occurs less often. 
   Referring now to  FIGS. 9 and 10 , a DC correction module  154  generates a DC correction signal that is input to a summing module  160 . The DC correction signal is related to a difference between the waveforms  120  and  124  in  FIG. 8A . In other words, the DC correction signal compensates the read back signal for the high pass filtering effects of the preamplifier  24  and the input network  40 . The DC correction signal reduces the DC offset of the read-back signal to which is it applied. Alternately, the DC correction signal adjusts a threshold of a comparator based on the calculated DC offset, as will be described further below. 
   In one embodiment, the DC correction module  154  receives outputs of the ADC module  60 , the FIR equalizer module  64  and the reconstruction filter module  70 . Additional details of DC correction modules  154  can be found in U.S. patent application Ser. No. 10/737,648, to Oberg, filed on Dec. 15, 2003, which is hereby incorporated by reference in its entirety. The output of the VGA module  42  is summed by a summing module  160  with the DC correction signal and input to the TAD module  164 . The TAD module  164 , in turn, determines whether a TA event occurs based, in part, upon a comparison of the output of the summer  160  to a threshold. 
   Referring now to  FIG. 11 , the DC correction signal can also be applied to a main data path. In other words, the DC correction module  154  outputs the DC correction signal to a summing module  184 , which also receives an output of the VGA module  42 . The output of the summing module  184  is fed to a TAD module  186  and to the CTF module  50  as shown. 
   Referring now to  FIG. 12 , the DC correction module  154  outputs the DC correction signal to a TAD module  194 , which also receives the output of the VGA module  42 . The TAD module  194  determines whether a TA event occurred based on the output of the VGA module  42  and the DC correction signal, as will be described below in conjunction with  FIGS. 13-15 . 
   Referring now to  FIG. 13 , one implementation  194 - 1  of the TAD module  194  of  FIG. 12  is shown. The TAD module  194 - 1  includes the LPF  90 , a summing module  210  and a threshold comparator  214 . The DC correction signal adjusts the output of the LPF  90 . The output of the LPF  90  is input to the threshold comparator  214 . The threshold comparator  214  compares the output of the summing module to a threshold. 
   Referring now to  FIG. 14 , another implementation  194 - 2  of the TAD module  194  of  FIG. 12  is shown. The TAD module  194 - 2  includes the LPF  90  and a threshold comparator  218 . The DC correction signal is input to the threshold comparator  214 , which determines whether a TA event occurred based on the DC correction signal, the output of the LPF  90  and an adjustable threshold Th. The TAD module  194 - 2  adjusts the threshold Th based on the DC correction signal. 
   Referring now to  FIG. 15 , another implementation  194 - 3  of the TAD module  194  of  FIG. 12  is shown. The TAD module  194 - 3  includes the LPF  90 , a summing module  220 , a rectifier  222  and a threshold comparator  224 . The DC correction signal and an output of the LPF  90  are input to the summing module  220 . An output of the summing module  220  is input to the rectifier  222 . The output of the rectifier  222  is input to a threshold comparator  214 , which determines whether a TA event occurred based on the threshold Th. 
   Referring now to  FIGS. 17 and 18 , high pass filters can be added to the TAD modules according to the present invention. In  FIG. 17 , another implementation  194 - 4  of the TAD module  194  of  FIG. 12  is shown. A high pass filter (HPF)  240  reduces DC levels of the data signal so that a transition following a long DC run (as shown in  FIG. 6 ) will not trigger a false detection. A cut-off frequency of the high-pass filter  240  should be set low enough to allow actual TA to be detected as a TA event. The high pass filter  240  can be added to other TAD modules described herein. For example in  FIG. 17 , a TAD module  250  includes the high pass filter  240 , the LPF  90  and the threshold comparator  224 . 
   Referring now to  FIG. 18 , when both high pass and low pass filtering is desired, a band pass filter  260  can be used in a TAD module  270 . As can be appreciated, the high pass filtering and/or band pass filtering can be used in any of the embodiments described herein. 
   As can be appreciated by the foregoing, the read channel according to the present invention adjusts either the signal input to the TAD module or the threshold Th used by the TAD module based on the DC offset. As a result, the probability of failing to detect TA events when they occur is reduced. Likewise, the probability of faulty detection of TA events when they do not occur is also reduced. Therefore, the probability of correctly identifying TA events is increased. 
   Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.