Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Application Serial No. 60/152,382, filed Sep. 3, 1999 and from U.S. Provisional Application Serial No. 60/129,654, filed Apr. 16,1999, and is a continuation-in-part of U.S. patent application Ser. No. 09/480,314, filed Jan. 10, 2000, titled “An Acquisition Signal Error Estimator,” all of which are hereby incorporated by reference in their entireties as if fully set forth herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to magnetic recording and, particularly, to an improved error estimator for a sampled amplitude read channel.  
           [0004]    2. Description of the Related Art  
           [0005]    Sampled amplitude detectors used in magnetic recording require timing recovery in order to correctly extract the digital sequence. As shown in FIG. 1, data sectors  100  on magnetic disks are formatted to include an acquisition preamble  102 , a sync or synchronization mark  104 , and user data  106 . Timing recovery uses the acquisition preamble  102  to acquire the correct sampling frequency and phase before reading the user data  106 . The synchronization mark  104  demarcates the beginning of the user data. The preamble  102  is written using the periodic non-return-to-zero (NRZ) sequence 001100110011. . . which causes the pattern of magnetization SSNNSSNNSSNN . . . to be written on the magnetic medium. The pattern is periodic, having period 4T, where T is the bit period. The pattern is sometimes called a 2T pattern because the interval between successive magnetic field direction transitions is 2T. During the read operation, the sequence of samples [x i , x i+1 , . . . ], produced by the preamble is also of period 4T. In the case of PR4 (partial response) equalization, the sinusoid is ideally sampled at π/4, 3π/4, 5π/4, 7π/4 and so on, resulting in an equalized sequence of [1, 1, −1, −1, 1, 1, −1, −1, 1, 1, . . . ]. In the case of EPR4 (extended partial response) equalization, the sinusoid is ideally sampled at phases 0, π/2, π, 3π/2 and so on, which results in the equalized sequence [2, 0, −2, 0, 2, 0, −2, 0, 2, 0, . . . ]. In the general case of E 2n PR4, where n is a non-negative integer, the sinusoid is ideally sampled at phases π/4, 3π/4, 5π4, 7π/4 and so on, resulting in an equalized sequence of [2 n , 2 n , −2 n , −2 n , 2 n , 2 n , −2 n , −2 n , . . . ]. For E 2n +1 PR4 equalization, the sinusoid is ideally sampled at phases 0, π/2, π, 3π/2 and so on, which results in the equalized sequence [ 2   n+1 , 0, −2 n+1 , 0, 2 n+1 , 0, −2 n+1 , . . . ].  
           [0006]    Conventionally, the error between the received sample and its ideal value is estimated as x i −{overscore (x)} i  where x i  is the received sample value and {overscore (x)} i is the nearest ideal sample value to the received value x i . The nearest ideal sample value {overscore (x)} i  is computed simply by comparing the received value x i  to each of the ideal signal levels and declaring {overscore (x)} i  to be the closest ideal level (i.e., the ideal level that minimizes the absolute value |x i −{overscore (x)} i |of the error). This is referred to as a slicer or threshold detector estimate.  
           [0007]    However, the slicer estimate is disadvantageous in that it is sensitive to distortions in gain, DC offset, and magneto-resistive signal asymmetry. As such, there is a need for an improved error estimator.  
         SUMMARY OF THE INVENTION  
         [0008]    These and other drawbacks in the prior art are overcome in large part by a system and method according to the present invention. An improved system and method for acquisition signal error estimation is provided which uses one or more past values of the sequence to determine the nearest ideal sample value. According to one embodiment, three consecutive samples are used. According to another embodiment, two consecutive samples are used. Finally, according to another embodiment of the invention, consecutive samples are used, but no slicer estimate is required.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    A better understanding of the invention is obtained when the following detailed description is considered in conjunction with the following drawings in which:  
         [0010]    [0010]FIG. 1 is a diagram of an exemplary data format of user data;  
         [0011]    [0011]FIG. 2 is a block diagram of an exemplary read/write channel according to an embodiment of the invention; and  
         [0012]    [0012]FIG. 3A, FIG. 3B, and FIG. 3C are diagrams of exemplary acquisition signal error estimators according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    FIGS.  2 - 3  illustrate an improved acquisition signal error estimator according to an implementation of the present invention. The acquisition signal error estimator uses a plurality of received samples to estimate the signal error. Turning now to the drawings and, with particular attention to FIG. 2, a block diagram of a sampled amplitude read channel according to an embodiment of the invention is shown and identified by the reference numeral  200 . During a write operation, data are written onto the media. The data are encoded in an encoder  202 , such as an RLL or other encoder. A precoder  204  precodes the sequence to compensate for the transfer function of the magnetic recording channel  208  and equalizing filters. The write circuitry  206  modulates the current in the recording head coil to record a binary sequence onto the medium. A reference frequency f ref  provides a write clock to the write circuitry  206 .  
         [0014]    The bit sequence is then provided to a variable gain amplifier  210  to adjust the amplitude of the signal. DC offset control  212  and loop filter/gain error correction  214  according to the present invention may be provided to control the adjustment of the VGA  210 . Further, an asymmetry control unit  215  including an asymmetry adjustment unit  216  and asymmetry control  218  may be provided to compensate for magneto-resistive asymmetry effects. It is noted that, while described in the context of gain correction, the teachings of the present invention are equally applicable for use in the DC offset and asymmetry control loops. As will be described in greater detail below, the acquisition signal error estimator uses a plurality of received samples to estimate the signal error.  
         [0015]    Turning back to FIG. 2, the signal is then provided to a continuous time filter  220 , which may be a Butterworth filter, for example, to attenuate high frequency noise and minimize aliasing into baseband after sampling. The signal is then provided to an analog-to-digital converter  222  to sample the output of the continuous time filter  220 .  
         [0016]    A finite impulse response filter  224  provides additional equalization of the signal to the desired response. The output of the FIR  224  is provided to an interpolated timing recovery unit  228 , including an acquisition signal error estimator  229  according to the present invention, which is used to recover the discrete time sequence. The output of the interpolated timing recovery unit is used to provide a feedback control to the DC offset control  212 , the gain error  214 , the asymmetry control  218  and the FIR  224  control  226 . The output of the interpolated timing recovery  228  is provided to a Viterbi detector  232  to provide maximum likelihood detection. Further, the ITR output is provided to a sync detector  234  according to the present invention. The sync detector  234  detects the sync mark using phase information gleaned from having read the immediately preceding preamble. This information is then provided to the Viterbi detector  232  for use in sequence detection. The Viterbi detector output is then provided to the decoder  236  which decodes the encoding provided by the encoder  202 . After acquiring the preamble, the sync mark detector searches for the sync mark which demarcates the beginning of the data field. When the sync mark is detected, the sync mark detector enables the Viterbi detector  232  and decoder  236 .  
         [0017]    The gain control signal provided by the loop filter/gain control unit  214  minimizes the error given by e i =gx i −{overscore (x)} i  where g is the system gain. It can be shown that the system gain is updated according to g i+1 =g i −Be i x i =g i −Bd i , where B is a constant.  
         [0018]    According to one embodiment of the invention, the gain error term d i  is given by  
           d   i =( x   i   −{overscore (x)}   i ) {overscore (x)}   i +( x   i−1   −{overscore (x)}   i−1 ) {overscore (x)}   i−1    
         [0019]    Thus, the term gain d i  is dependent on the signal error term. As discussed above, the signal error term depends of the selection of {overscore (x)} i . According to the present invention, rather than employing a threshold detector, the selection of {overscore (x)} i  depends upon past values of x i .  
         [0020]    In particular, in the case where the preamble signal is ideally sampled at phases 0, π/2, π, 3π/2 and so on, (i.e., as for E 2n+1 PR4 equalization), the ideal sample sequence takes the form [a, 0, −a, 0, a, 0, −a . . . ], where a is the amplitude of the sinusoid. In this case, the error x i −{overscore (x)} i  is estimated, where x i  is the received sample value, and {overscore (x)} i  is computed as follows:  
           x   _     i     =     {             a                 if                          x   i     -     x     i   -   2                ≥                x     i   -   1       -     x     i   -   3                            and                   x   i       -     x     i   -   2         ≥   0                   -   a                   if                          x   i     -     x     i   -   2                ≥                x     i   -   1       -     x     i   -   3                            and                   x   i       -     x     i   -   2         &lt;   0                 0                 if                          x   i     -     x     i   -   2                &lt;            x     i   -   1       -     x     i   -   3                                              
 
         [0021]    One implementation of the error signal estimator  229   a  described above is shown in FIG. 3A. As shown, an input signal x i  is input along line  302  to a pair of delay operators  320 ,  322 . The resulting output of the delay operators is provided to an arithmetic operator circuit  324 . The signal x i  is also provided along line  323  to the arithmetic operator  324 . The arithmetic operator  324  performs the operation x i −x i−2 . The output of the arithmetic operator  324  is provided to circuit  318  which determines the sign (i.e., whether the output is greater than or less than zero). The output of the circuit  318  controls a multiplexer  314 , as will be explained in greater detail below.  
         [0022]    Th output of the arithmetic operator  324  is also provided to circuit  326  which performs the absolute value operation. The resulting output is then provided to a delay operator  328  and also to an arithmetic operator  330 . The output of the delay operator  328  is also provided to the arithmetic operator  330 , which performs the operation |x i −x i−2 |−|x i−1 −x i−3 |. Finally, the output of the arithmetic operator  330  is compared with zero by circuit  332  and used to control the multiplexer  316 , as will be described in greater detail below.  
         [0023]    The input signal x i  is input along line  304  to the multiplexer  316  and, along lines  306  and  310  to arithmetic operators  308 ,  312 , respectively. The arithmetic operator  308  performs the operation x i −a, and the arithmetic operator  312  performs the operation x i +a. The outputs of the arithmetic operators  308 ,  312  are provided as inputs to the multiplexer  314 . The multiplexer  314  outputs one or the other based on the sign of x i −x i−2  provided by circuit  318 . The output of the multiplexer  314  is provided as the other input to the multiplexer  316 . Finally, the output of the multiplexer  316  is then selected based on the sign of |x i −x i−2 |−|x i−1 −x i−3 |provided by circuit  332 .  
         [0024]    In the case where the preamble signal is ideally sampled at the phases π/4, 3π/4, 5π/4, 7π/4 and so on (i.e., as for E 2n PR4 equalization), the ideal sample sequence takes the form [b, b, −b, −b, b, b, −b, −b, . . . ] where {square root}{square root over (2)} b is the amplitude of the sinusoid. In this case, the error x i −{overscore (x)} i  is estimated, where x i  is the received sample value, and {overscore (x)} i  is computed as follows:  
           x   _     i     =     {                            b                 if                   x   i       -     x     i   -   2         ≥   0                       -   b                   if                   x   i       -     x     i   -   2         &lt;   0                                   
 
         [0025]    One implementation of the error signal estimator  229   b  described above is shown in FIG. 3B. As shown, an input signal x i  is input along line  350  to a pair of delay operators  358 ,  360 . The resulting output of the delay operators is provided to an arithmetic operator circuit  362 . The signal x i  is also provided along line  352  to the arithmetic operator  362 . The arithmetic operator  362  performs the operation x i −x i−2 . The output of the arithmetic operator  362  is provided to circuit  364  which determines the sign (i.e., whether the output is greater than or less than zero). The output of the circuit  364  controls a multiplexer  370 , as will be explained in greater detail below.  
         [0026]    The signal x i  is input along lines  354  and  356  to arithmetic operators  366 ,  368 , respectively. The arithmetic operator  368  performs the operation x i −b, and the arithmetic operator  366  performs the operation x i +b. The outputs of the arithmetic operators  366 ,  368  are provided as inputs to the multiplexer  370 . The multiplexer  370  outputs one or the other based on the sign of x i −x i−2  provided by circuit  364 .  
         [0027]    An alternate method for obtaining the gain error term d i  is to avoid using the slicer estimate altogether. More particularly, one method of doing so is to use the error term  
           d   i   =x   i   2   +x   i−1   2 −4  
         [0028]    An implementation of this method for estimating the gain error term is shown in FIG. 3C. As shown therein, sample x i  is input along line  401  to a squaring operator  408  and along line  403  to a delay operator  402 . The output of the delay operator  403 , x i−1 , is provided to a squaring operator  404 . The outputs of the squaring operators  404 ,  408  are summed by adder  406 . Finally, the output of the adder  406  is input to the adder  410 , which subtracts a constant (e.g., 4). The resulting output d i  is used to calculate system gain g i , as described above.  
         [0029]    It is noted that, while described above as discrete components, the gain control systems may typically implemented as software or firmware. The invention described in the above detailed description is not intended to be limited to the specific form set forth herein, but is intended to cover such alternatives, modifications and equivalents as can reasonably be included within the spirit and scope of the appended claims.

Technology Category: 3