Abstract:
A data storage system identifies analog-to-digital conversion samples with amplitude below a certain threshold. Remaining samples are grouped according to phase into one or more quadrants. A multi-coordinate with overlapping quadrants is used to further differentiate sample points. The system then computes an average phase for zero phase start estimation.

Description:
PRIORITY 
     The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/939,530, filed Feb. 13, 2014, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Data storage systems utilize readback signals to calibrate a read head before reading. Zero phase start is used to provide an initial timing error estimate in frontend digital phase-locked-loop (DPLL) timing loop. When the readback signal includes defects, phase estimation is less reliable. Defects are currently handled by lengthening the sample window or shifting the sample window. Lengthening or shifting the sample window introduces undesirable latency. 
     Consequently, it would be advantageous if an apparatus existed that is suitable for producing a reliable zero phase start estimation from initial readback signal samples, regardless of defects. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and apparatus for producing a reliable zero phase start estimation from initial readback signal samples, regardless of defects. 
     In one embodiment of the present invention, a data storage system identifies analog-to-digital conversion samples with amplitude below a certain threshold. Remaining samples are grouped according to phase into one or more quadrants. The system then computes an average phase for zero phase start estimation. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of a data storage system according to at least one embodiment of the present invention; 
         FIG. 2  shows a representation of a readback signal with defects; 
         FIG. 3  shows a representation of a coordinate system for mapping components of readback signal samples; 
         FIG. 4  shows a representation of an alternative coordinate system for mapping components of readback signal samples; 
         FIG. 5  shows an exemplary representation of a combined coordinate system for mapping components of signal samples; 
         FIG. 6  shows a flowchart for a method of estimating input phase from a readback signal; 
         FIG. 7  shows a flowchart for a method of estimating input phase from a readback signal; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a block diagram of a data storage system according to at least one embodiment of the present invention is shown. In at least one embodiment, a computer apparatus, such as a data storage system, a processor  100  is connected to a memory  102  and a data storage element  104 . Computer executable program code configured to execute on the processor  100  receives a readback signal when beginning a data read operation from the data storage element  104 . 
     In at least one embodiment, the processor  100  computes a sin term and a cos term for each 4T in the readback signal. The sin term is computed according to the equation:
 
sin Term  n=x [4 n]−x[ 4 n+ 2]
 
the cos term is computed according to the equation:
 
cos Term  n=x [4 n+ 1]− x[ 4 n+ 4]
 
The processor  100  accumulates sin terms and cos terms for 16T or 32T. When the son terms and cos terms are accumulated, the processor  100  computes the input analog-to-digital conversion phase based on the accumulated terms.
 
     Where the readback signal includes defects, the processor  100  determines a phase estimation by eliminating samples with sin terms and cos terms below a certain energy threshold. In at least one embodiment of the present invention, the processor  100  filters samples where the sin term squared plus the cos term squared is below a desired threshold. Furthermore, the processor  100  weighs samples based on accumulated sample phase location in a coordinate system; defective samples tend toward a random phase distribution. 
     Referring to  FIG. 2 , a representation of a readback signal with defects is shown. The readback signal may include one or more valid, substantially noise free portions  200 ,  204  and one or more defective portions  202 . Defective portions  202  introduce noise to zero phase start calculations. Noise caused by a defective portion  202  can be mitigated by extending the sampling window to include more valid portions  200 ,  204 , or by sliding the sampling window until only valid portions  200 ,  204  are included. Both solutions add latency to the processes. If defective portions  202  are not excluded, variance of the phase estimation increases and becomes unreliable, increasing the probability of a loss-of-lock during data read. 
     Referring to  FIG. 3 , a representation of a coordinate system for mapping components of readback signal samples is shown. Each sample falls into a defined coordinate system region  304 ,  306 ,  308 ,  310  based on the sample&#39;s sin term  300  and cos term  302 . A first region  304  includes samples having a positive sin term  300  and a negative cos term  302 . A second region  306  includes samples having a positive sin term  300  and a positive cos term  302 . A third region  308  includes samples having a negative sin term  300  and a negative cos term  302 . And a fourth region  310  includes samples having a negative sin term  300  and a positive cos term  302 . As sin terms  300  and cos terms  302  are computed, the samples are assigned an appropriate region  304 ,  306 ,  308 ,  310  (or quadrant in the present example). Valid samples tend to congregate in the same region  304 ,  306 ,  308 ,  310 ; defective samples tend to distribute randomly. 
     Referring to  FIG. 4 , a representation of an alternative coordinate system for mapping components of readback signal samples is shown. Where input phases are near a boundary in the coordinate system shown in  FIG. 3 , samples may not be properly associated. To account for such possibility, the alternative coordinate system defines a first region  404  including samples having a positive sin term  400  and a negative cos term  402 , offset by a predetermined amount such as 45°, a second region  406  including samples having a positive sin term  400  and a positive cos term  402 , a third region  408  including samples having a negative sin term  400  and a negative cos term  402  and a fourth region  410  including samples having a negative sin term  400  and a positive cos term  402 . By including a second coordinate system offset from the coordinate system shown in  FIG. 3 , samples that would fall into an ambiguous boundary area in one coordinate system fall squarely into a well-defined region of the other coordinate system. 
     Referring to  FIG. 5 , an exemplary representation of a combined coordinate system for mapping components of signal samples is shown. In at least one embodiment, the combined coordinate system includes a first sin term axis  502  and a second sin term axis  506  offset by a certain number of degrees such as 45°. The exemplary combined coordinate system also includes a first cos term axis  500  and a second cos term axis  504  offset by a certain number of degrees such as 45°. While the exemplary embodiments shown herein specify 45°, a person skilled in the art will appreciate that other offsets are applicable in other embodiments. A person skilled in the art may appreciate that the number of axes is dependent on the number of samples. In one exemplary embodiment using 4T samples in the readback signal, each coordinate system is divided into four regions (quadrants); other embodiments may use 6T or other samples, dictating corresponding numbers of regions. 
     Where the first sin terms axis  502  and first cos terms axis  500  overlap with the second sin term axis  506  and second cos term axis  504 , the combined coordinate system defines eight regions  508 ,  510 ,  512 ,  514 ,  516 ,  518 ,  520 ,  522 . Each region  508 ,  510 ,  512 ,  514 ,  516 ,  518 ,  520 ,  522  defines an overlap portion of the two underlying coordinate systems. While the regions  508 ,  510 ,  512 ,  514 ,  516 ,  518 ,  520 ,  522  in  FIG. 5  are illustrated individually, in actual implementation, the regions defined by the individual coordinate systems are more useful. For example, the first coordinate system defined by the first sin term axis  502  and first cos term axis  500  includes a first quadrant containing a first region  508  and second region  510 ; a second quadrant containing a third region  512  and fourth region  514 ; a third quadrant containing a fifth region  516  and sixth region  518  and a fourth quadrant containing a seventh region  520  and eighth region  522 . Likewise, the second coordinate system defined by the second sin term axis  506  and second cos term axis  504  includes a first quadrant containing the first region  508  and eighth region  522 ; a second quadrant containing the second region  510  and third region  512 ; a third quadrant containing a fourth region  514  and fifth region  516  and a fourth quadrant containing the sixth region  518  and seventh region  520 . 
     In at least one embodiment of the present invention, each calculated sin term and cos term of each readback signal point falls into one of the regions (in this case quadrants) defined by each coordinate system. Generally, valid readback signal points  526 ,  528 ,  530 ,  532  will fall into one quadrant of either the first coordinate system or the second coordinate system. Defective readback signal points  534  will be distributed randomly. In the present example, all of the valid readback signal points  526 ,  528 ,  530 ,  532  appear in the first quadrant of the first coordinate system. Furthermore, defective readback signal points  534  tend to have lower amplitude than valid readback signal points  526 ,  528 ,  530 ,  532 . Therefore, in at least one embodiment of the present invention, a threshold amplitude  524  defines a cut-off below which readback signal points are considered defective. Once one of the quadrants includes a predetermined number of valid readback signal points  526 ,  528 ,  530 ,  532 , the final input phase is estimated. 
     Referring to  FIG. 6 , a flowchart for a method of estimating input phase from a readback signal is shown. Where a computer system is analyzing a readback signal, the system initializes  600  region or quadrant counters. Each region or quadrant counter is associated with a phase defined region in a coordinate system. In at least one embodiment, the computer system defines more than one coordinate system, each coordinate system being offset from the others. The system then computes  602  sin and cos terms for each input sample received from the readback signal. In at least one embodiment, defective signals are assumed to have a smaller energy than valid, non-defective signals; therefore, signals with energy below a certain threshold are excluded  603  from further analysis. The sin and cos terms are placed  604  in regions of the one or more coordinate systems and the corresponding region or quadrant counters are increased. If the computer system determines  608  that it has sufficient sample points in any one quadrant, the system determines  610  an estimated input phase based on the points in that quadrant. Otherwise, the computer system computes  602  additional sin and cos terms of additional samples. 
     Referring to  FIG. 7 , a flowchart for a method of estimating input phase from a readback signal is shown. Where a computer system is analyzing a readback signal, the system initializes  700  region or quadrant counters. Each region or quadrant counter is associated with a phase defined region in a coordinate system. In at least one embodiment, the computer system defines more than one coordinate system, each coordinate system being offset from the others. The system then computes  702  sin and cos terms for each input sample received from the readback signal. In at least one embodiment, defective signals are assumed to have a smaller energy than valid, non-defective signals; therefore, signals with energy below a certain threshold are excluded  703  from further analysis. The sin and cos terms are placed  704  in regions of the one or more coordinate systems and the corresponding region or quadrant counters are increased. If the computer system determines  708  that the zero phase start sample length has been reached, the system determines  710  which region or quadrant has the maximum number of points based on the region or quadrant counters. The system then determines  712  an estimated input phase based on the points in that quadrant. Otherwise, the computer system computes  702  additional sin and cos terms of additional samples. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description of embodiments of the present invention, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.