Abstract:
A hard disk drive comprising a plurality of read/write heads oriented to serially read data from the magnetic media of the disk drive. The head output signals are delayed and combined to provide a time aligned composite signal for determining the value of the data bits read from the disk drive. An improved signal-to-noise ratio is provided according to the teachings of the present invention by combining the signal components from the plurality of heads, as the signal components are added algebraically while the noise components are combined as root mean square values. Thus the overall signal-to-noise ratio is improved, resulting in a greater probability of correctly determining the stored data.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. Ser. No. 10/619,057 filed Jul. 14, 2003, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to hard disk drives and other mass storage medias, and more particularly to a method and apparatus for reducing the signal-to-noise ratio of a signal representing a data bit read from such media.  
       BACKGROUND OF THE INVENTION  
       [0003]     A hard disk drive  10 , as illustrated in  FIG. 1 , comprises a platter  12  constructed of magnetic material for storing information, in the form of data bits, for processing by a computing or processing device. The information is stored on the platter  12  by magnetizing small magnetic domains that retain the magnetization and thus can be magnetized to store a zero data bit or a one data bit. A motor (not shown in  FIG. 1 ) spins the platter  12  (typically at speeds of 3,600 or 7,200 revolutions per minute) allowing a read/write head  14  to write data to or read data from the platter  12  as the read/write head  14  travels over the surface of the platter  12 . The read/write head  14  does not make physical contact with the platter  12 .  
         [0004]     The read/write head  14  is affixed to an arm  16  controlled by a positioning mechanism  18  for moving the arm across an upper surface of the platter  12 , between an edge  24  and a hub  26 . Data bits are stored on the platter  12  in sectors  30  on concentric tracks  32 . Typically, a sector contains a fixed number of bytes (for example, 256 or 512). A plurality of sectors are commonly grouped together into a cluster.  
         [0005]     As illustrated in  FIG. 2 , to increase storage capacity a hard disk drive typically comprises a plurality of parallel platters  12 A,  12 B and  12 C. Read/write heads  14 A through  14 F write data to and read data from a top and bottom surface of each of the platters  12 A,  12 B and  12 C. The depiction of three platters and six read/write heads illustrated in  FIG. 2  is merely exemplary.  
         [0006]     The positioning mechanism  18  conventionally employs a high-speed linear motor or a voice coil motor to move the arm  16 . In the voice coil embodiment, the voice coil is located adjacent to a magnet, which together operatively define the voice coil motor of the positioning mechanism  18 . The hard disk drive  10  further comprises a controller (not shown) for providing current to excite and control the voice coil motor of the positioning mechanism  18 . The excited voice coil motor rotates the arm  16 , moving the head  14  across the surface of the platter  12  along an arc.  
         [0007]     Data bits are written to and read from the hard disk drive  10 , utilizing a magneto-resistive transducer as a sensing and writing element within the read/write head  14 . The voice coil motor moves the arm  16  to a desired radial position on the surface of the platter  12 , after which the head  14  electromagnetically writes data to the platter  12  or senses magnetic field signal changes to read data from the platter  12 . The arm  16  is shaped and controlled such that it “flies” over the surface of the platter  12  as the latter rotates beneath it. Contact between the head  14  and the platter  12  is not desired.  
         [0008]     Conventional transducers comprising the read/write head  14  employ a magnetically permeable core coupled with a conductive coil to read and write data on the surface of the platter  12 . A write operation is typically performed by applying a current to the coil, thereby inducing a magnetic field in the adjacent magnetically permeable core. The magnetic field extends across the air gap between the head  14  and the platter  12  to magnetize a small region of magnetic domains to store the data bit. Information is read from the platter  12  when the magnetized region induces a voltage in the coil. Alternatively, reading can be performed using a magneto-resistive sensor, where the resistance varies as a function of the proximate magnetic field.  
         [0009]     To increase the amplitude (and thus the signal-to-noise ratio) and the detection accuracy of the data bits as they are read from the platter  12 , the head  14  is positioned as close to the platter  12  as possible. However, the low amplitude voltage signals produced in the head  14  during the read operation typically exhibit a low signal-to-noise ratio. Also, the high frequencies involved in the read operation tend to increase noise in the voltage signal. It is advantageous to improve the signal-to-noise ratio of the read signal to improve the accuracy (i.e., reduce the error rate) of data bit detection.  
         [0010]     Known techniques for increasing the signal-to-noise ratio have focused on increasing the signal level and reducing noise in the head output signal, thus reducing noise effects that must otherwise be accommodated during the subsequent signal processing. Error detecting/correcting codes can be appended to the data words to account for noise effects. However, this technique increases the total number of bits (i.e., data bits plus error detecting/correcting bits) required to store information on the hard disk drive  10  and thus reduces the effective hard disk drive capacity. So called “giant magneto resistance detectors” generally produce a higher output voltage and thus have a higher signal-to-noise ratio than the inductive coil described above. Also, as the materials comprising the magneto-resistive device are improved to generate less noise during the reading process, the signal-to-noise ratio improves. Certain regions of the noise spectrum can also be filtered from the read signal using spectral filters. However, noise voltage remains in the spectral region processed by the signal processing circuitry to detect the data bits.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     A hard disk drive comprises a magnetic storage disk having magnetic regions that are magnetized to store data bits. A plurality of read heads in proximate relation to the storage disk determine the magnetization of the magnetic regions as the storage disk moves relative to the read heads. The heads are oriented to successively read the same magnetic region, each producing a signal representative of the magnetization of a given region. A detector responsive to the signals determines the data bit value represented by the magnetized region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention can be more easily understood and the advantages and uses thereof more readily apparent, when considered in view of the following detailed description of the preferred embodiment when read in conjunction with the following figures wherein:  
         [0013]      FIGS. 1 and 2  illustrate elements of a prior art hard disk drive;  
         [0014]      FIG. 3  illustrates a hard disk drive head constructed according to the teachings of the present invention.  
         [0015]      FIGS. 5A and 5B  illustrates the output signal from a prior art read head;  
         [0016]      FIGS. 6A and 6B  illustrate the output signals from a read head according to the teachings of the present invention. 
     
    
       [0017]     In accordance with common practice, the various features of the present invention are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Reference characters denote like elements throughout the figures and text.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Before describing in detail the particular hard disk drive in accordance with the present invention, it should be observed that the present invention resides primarily in a novel combination of elements. Accordingly, the elements have been represented by conventional elements in the drawings, showing only those specific details that are pertinent to the present invention so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein.  
         [0019]      FIG. 3  illustrates three heads  40 A,  40 B, and  40 C disposed at an end  41  of an arm  42 . The heads  40 A through  40 C are positioned on the arm  41  such that each passes over the same segment of the track  32  and thus each head  40 A through  40 C reads the same data from the platter  12 , with a time delay between each of the three read operations. According to the teachings of the present invention, the earlier read signals are delayed such that the three read signals are concurrent in time for processing to determine the read data bit value. In this embodiment, the platter  12  is assumed to rotate in the direction indicated by an arrowhead  44 . Thus the head  40 A first encounters the data bit to be read from the track  32 , followed by the read heads  40 B and  40 C.  
         [0020]     As illustrated in  FIG. 4 , a delay element  50  is disposed between the head  40 B and a detector  52 . A delay element  54  is disposed between the head  40 A and the detector  52 . It is assumed that the head  40 C is the last to read the data bit from the platter  12 ; thus in this embodiment it is unnecessary to delay the output signal therefrom. According to the present invention, three time-aligned signals representing the read data bit value are presented as inputs to the detector  52 . The output signal from the detector  52  represents the data bit. Although the embodiment of  FIG. 4  illustrates the delay elements  50  and  54  as separate components, this is merely done for explanatory purposes, while in another embodiment, the delay functions can be incorporated into the detector  52 . Signal processing elements capable of functioning as the delay elements  50  and  54 , are known in the art. For example, the signal processing elements can comprise sample and hold circuits to implement the required delay. The delay can also be implemented using resistor-capacitor and resistor inductor combinations, and delay lines.  
         [0021]      FIG. 5A  is a cross-sectional view of a section  60  of the platter  12 , as disclosed by the prior art. An arrowhead  62  indicates direction of movement of the section  60  relative to the arm  16  and the head  14 .  
         [0022]      FIG. 5B  illustrates the output voltage from the head  14 , with respect to time, as induced in the head  14  by a magnetic domain region of the platter  12  during a read operation. The voltage signal is processed (within a detector such as the detector  52  of  FIG. 4 ) to determine whether the magnetic domain region stores a one bit or a zero bit.  
         [0023]     As illustrated in the cross-sectional view of  FIG. 6A , according to the teachings of the present invention, the arm  42  carries the three heads  40 A through  40 C. The output signals from the heads  40 A through  40 C are represented in  FIG. 6B  by three voltage signals  70 A,  70 B and  70 C, respectively. The signal  70 A indicates that during a read operation the output voltage from the head  40 A dropped at time t 0 . The signal  70 B indicates that the output voltage from the head  40 B dropped at time t 1  during the same read operation, but later in time by the difference between t 0  and t 1 . The signal  70 C indicates that the output voltage from the head  40 C dropped at time t 2 , again during the same read operation. As described with reference to  FIG. 4 , the voltage waveforms  70 B and  70 C are delayed to achieve time alignment with the voltage waveform  70 A. As can be appreciated by those skilled in the art, the depiction of three heads is merely exemplary, while in another embodiment more or fewer, but at least two, heads can be used during the read operation.  
         [0024]     Within the detector  52 , the head output voltage signals are analyzed to determine whether the voltage represents a one bit or a zero bit. Those skilled in the art are familiar with such techniques for the detection of digital data from a voltage signal such as obtained by reading from magnetic domain regions. Detection accuracy (e.g., as measured by the bit error rate) is important for successful operation of the computing or data processing device operative with the hard disk drive  10 . According to the embodiment comprising three heads  40 A,  40 B and  40 C, within the detector  52  of  FIG. 4 , the three time aligned head signals are averaged, thereby increasing the signal component and reducing the noise component to effect an improvement in the signal-to-noise ratio of the combined voltage waveform.  
         [0025]     The signal averaging function can be accomplished using various known techniques, including transmission delay lines, all pass filters, and resistance-capacitance filters. With an improvement in the signal-to-noise ratio as taught by the present invention, the likelihood of a correct detection is improved and the likelihood of an incorrect detection is reduced. Thus, use of a hard disk drive incorporating the teachings of the present invention may render unnecessary the prior art error detection and correction techniques, such as the use of error correcting techniques. With a reduction in the error correction/detection bits, the amount of hard disk space allocated to information bits is commensurately increased, as is the data storage capacity of the hard disk drive.  
         [0026]     Let the output signals from the heads  40 A,  40 B and  40 C be designated as signals S 1 , S 2  and S 3  respectively, and let the noise components of the output signals from each head  40 A,  40 B and  40 C be designated as n 1 , n 2  and n 3 . Within the detector  52 , the signals are summed such that S total =S 1 +S 2 +S 3 . For the more general case where there are N heads, the total signal magnitude is S total =N*S 1  (assuming the equivalent output signals from each of the heads).  
         [0027]     However, the noise components are random and add as root means square (RMS) values, such that n total =SQRT (n 1 ˆ2+n 2 ˆ2+n 3 ˆ2). For the more general case of N heads, the total noise voltage magnitude is approximately n total =SQRT (N)*n 1  (assuming an equivalent noise voltage at each head).  
         [0028]     According to the prior art hard disk drives, the signal-to-noise ratio of a single read head is S 1 /n 1 . The signal-to-noise ratio of the combined head output signals, according to the teachings of the present invention is S total /n total =N*S 1 /(SQRT(N)*n 1 )=SQRT(N)*(S 1 /n 1 ). As can be seen there is a SQRT(N) improvement in the signal-to-noise ratio according to the teachings of the present invention.  
         [0029]     The teachings of the present invention are further applicable to other types of data storage media, such as magnetic storage devices, including floppy disks, magnetic tapes, and magnetic card strips. Optical storage devices, e.g., digital video disks (DVD&#39;s) and compact disk read only memories (CD ROM&#39;s) can also benefit from the teachings of the present invention. The reading apparatus of such devices is modified, according to the teachings of the present invention, to produce at least two output signals representative of the stored data bit. The output signals are processed according to the teachings of the present invention to produce a composite signal having a reduced signal-to-noise ratio, thus improving detection accuracy of the read operation.  
         [0030]     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. Further, the scope of the present invention may include any combination of the elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.