Abstract:
An apparatus having a first filter means for adjusting an input signal based on past data output from the apparatus. In addition, a summing means is used to sum signals from the first filter means and from a second filter means to produce a sum signal. The apparatus includes a symbol detection means for generating an output signal from the sum signal. The second filter means provides adjustments in the output signal based on the peaks and plurality of past signals generated by the symbol detection means. A control means is included for controlling the filtering properties of both the first and second filter means, wherein the control means controls the filtering properties based on the past output signals from the symbol detection means.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to digital signal transmissions and in particular to equalizers. Still more particularly, the present invention relates to an improved decision feedback equalizer. 
     2. Description of the Related Art 
     The advent of the information age has increased the demand for the storage of digital data, along with the demands for processing and transmission of such data. The density of information stored in a single system has been increased to accommodate the growing demand. For example, the capacity of magnetic disks storage units has grown fueled by improvements in the design of heads and disks, improvements in magnetic media, decreases in head gap length and flying height, and improvements in servo accuracy for increased track density. 
     The growing demand for digital storage capacity has also prompted an interest in the use of digital signal processing methods as a means of continuing increases in density. The general similarity of read and write processes in disk and tape drive units to data detection in transmission and communication systems has focussed interest on the application of equalization and coding methods to channels for both tape and disk drive systems. In particular, read channels have received special attention because the information has to be processed faster. Equalizers are used in both tape and disk drive read channels. In particular, decision feedback equalizers have been used to recover digital signals in read channels. Two types of noise dominate magnetic recording and read channels. One is regular noise caused, by thermal noise in electronics and random variations in the magnetic media. The other type of noise is intersymbol interference (ISI). Regular noise is found everywhere and effects all channels. ISI becomes worse with increasing density. Pulses generated by transitions in the magnetic media tend to overlap as transitions become closer with higher recording densities. Many of these decision feedback equalizers incorporate finite impulse response (FIR) filters to reduce errors caused by interference between successive pulses of data. This interference is also known as intersymbol interference (ISI). Additionally, errors may occur when the peas of positive and negative pulses do not have the same magnitude. 
     Presently available equalizers used in read channels contain coefficients that are determined to reduce errors in data transmission (e.g., ISI). The presently available adaptive equalizers adapt or alter coefficients based on an error signal derived from errors in the output of the read channel. These equalizer coefficients, however, are not optimal for all possible types of data patterns. Therefore, it would be advantageous to have an improved apparatus for equalizing signals to reduce errors in transmission of data. 
     SUMMARY OF THE INVENTION 
     The apparatus of the present invention includes a first filter means for adjusting an input signal based on past data output from the apparatus. In addition, a summing means is used to sum signals from the first filter means and from a second filter means to produce a summed signal. The apparatus includes a symbol detection means for generating an output signal from the summed signal. The second filter means provides adjustments in the output signal based on the peaks and polarity of past signals generated by the symbol detection means. A control means is included for controlling the filtering properties of both the first and second filter means, wherein the control means controls the filtering properties based on the past output signals from the symbol detection means. 
     The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of a decision feedback equalizer (DFE) depicted in accordance with a preferred embodiment of the present invention; 
     FIGS. 2A-2D are more detailed block diagrams of DFEs depicted in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a flowchart of a process for calculating FIR filter coefficients based on data patterns and peak magnitudes of pulses depicted according to the present invention; 
     FIG. 4 is a block diagram of a read channel depicted according to the present invention; and 
     FIGS. 5A and 5B are disk drive systems depicted in which DFEs may be implemented into read channels, such as read channel  500  in FIG. 4 according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     With reference now to the figures, and in particular with reference to FIG. 1, a block diagram of a decision feedback equalizer (DFE)  100  is depicted in accordance with a preferred embodiment of the present invention. DFE  100  includes a forward equalizer  102 , a backward equalizer  104 , a summing circuit  106 , and a symbol detector  108 , such as, for example, a slicer. DFE  100  is used for suppressing inter-symbol interference (ISI) in high density magnetic recording, according to the present invention. Forward equalizer  102  receives an analog input waveform according to the present invention. Forward equalizer  102  filters the signal based on prior data patterns in accordance with a preferred embodiment of the present invention. The filtered signal is sent to summing circuit  106 , which sums the signal from forward equalizer  102  and from backward equalizer  104  to produce a summed signal. Backward equalizer  104  modifies the signal by subtracting the trailing edges of pulses in the waveform in accordance with a preferred embodiment of the present invention. Often times the trailing edges of a positive pulse and a negative pulse are different. Slicer  108  produces a best estimate of the data symbol, given an input from summing circuit  106 . 
     According to the present invention, DFE  100  sets the coefficients in forward equalizer  102  based on the data pattern generated by symbol detector  108  rather than on an error measure as is the case for presently available adaptive equalizers. Similarly, the coefficients in backward equalizer  104  are selected on the basis of the polarity of past pulses rather than on a measurement of error in the output from DFE  100  according to the present invention. These coefficients are selected to reduce errors caused by various data patterns and peaks having different magnitudes. 
     For example, forward equalizer  102  in DFE  100  can be designed to compensate for various effects in the magnetic read channel. Forward equalizer  102  is employed to sharpen the leading edge of pulses received by DFE  100 . Under linear channel assumptions, without noise, a single set of forward equalizer coefficients is optimal for all data patterns. According to a preferred embodiment of the present invention, forward a equalizer  102  has coefficients adapted for filtering pulses based on the polarity of the waveform as determined by symbol detector  108  according to the present invention. Typically with MR heads, a bigger pulse occurs in one direction than the other. Forward equalizer  102  is adapted to correct for the difference between the pulse sizes. While forward equalizer  102  does not know what bit is coming next, the expected polarity of the next pulse can be determined from the output of symbol detector  108 . With magnetic media, the polarity of the next pulse is opposite of the last pulse. For example, if a “0” comes next, nothing will be present to equalize but if a “1” is next, forward equalizer  102  can be set correctly for the polarity for the pulse using different sets of coefficients according to the present invention. Once noise, however, is added, some data patterns are more susceptible to noise than other data patterns. Thus, designing or setting forward equalizer  102  with less noise boost for the most noise sensitive patterns usually reduces this type of susceptibility and improves performance within the read channel. 
     Alternatively forward equalizer  102  also can be set based on past data patterns in the form of a series of zeros and ones. For example, if the most error prone pattern were an isolated one, such as 0 0 0 1 0 0 0, the three zeros could be detected and the coefficients switched from the normal forward equalizer coefficients, optimized over all patterns, to a second set of coefficients that is optimized for patterns with three leading zeros. Forward equalizer  102  adjusts for past data patterns. Backward equalizer  104  is used to adjust for offsets between peaks in a signal. 
     Backward equalizer  104  subtracts off the trailing edges of pulses. Similarly, the coefficients in backward equalizer  104  could be altered depending on the polarity of the pulses and the magnitude of the peaks of the pulses according to the present invention. According to the present invention, forward equalizer  102  and backward equalizer  104  may be implemented either by using an adaptive or time varying filter and placing different sets of coefficients within the equalizers depending on the past data. Alternatively, forward equalizer  102  and backward equalizer  104  may be implemented using a number of different filters, with each filter having a fixed set of coefficients according to the present invention. In particular, the coefficients for the filters are identified using a time domain adaptive filter method called least means square (LMS), which uses a training sequence and adjusts the filter coefficients to obtain the right response. More information of DFEs may be found in Proakis,  Digital Communications,  McGraw-Hill, Inc., 3d ed., 1995, ISBN No. 0-07-051726-6. More information on filters, filter design, and selection of coefficients for filters may be found in Ifeachor and Jervis,  Digital Signal Processing: A Practical Approach,  Addison-Wesley Publishers Company, Inc., 1995, ISBN No. 0-201-54413-X. More information of the use of DFEs with respect to magnetic recording can be found in Cioffi, et al.,  Adaptive Equalization in Magnetic - Disk Storage Channels,  IEEE Communications Magazine, February 1990, pages 14-29 and Density Improvements in Digital Magnetic Recording  by Decision Feedback Equalization,  Bergmans, IEEE transactions on Magnetics, vol. MAG-22, number 3, May 1986, pages 157-162. According to a preferred embodiment of the present invention, LMS is preferred for determining filter coefficient. 
     With reference now to FIGS. 2A-2D, more detailed block diagrams of DFEs are depicted in accordance with a preferred embodiment of the present invention. According to the present invention the forward equalizers (e.g., a filter bank containing FIR filters or a single FIR filter connected to a memory containing different sets of coefficients) are employed to sharpen the leading edges of pulses based on a data pattern, such as a series of logic zeros followed by a logic one or on the expected polarity of the next pulse. The backward equalizer cancels tails based on the polarity of the pulses according to the present invention. The backward equalizer detects the polarity of the pulse and knows that the trailing edge will interfere with the next pulse and the proper set of coefficients is selected to cancel the trailing edge to eliminate interference according to the present invention. DFE  300  in FIG. 2A includes a filter bank  302  containing FIR filters  304 ,  306 , and  308  according to the present invention. The output of FIR filters  304 ,  306 , and  308  in filter bank  302  are connected to summing circuit  304  with the output of summing circuit  304  being connected to slicer  306 . DFE  300  also includes a backward equalizer  308  with the output bits from slicer  306  used as an input into backward equalizer  308  as in a conventional DFE. Additionally, the data bits from slicer  306  are directed into selection logic  310 , which selects one of FIR filters  304 ,  306 , or  308  in filter bank  302  based on the data output from slicer  306  according to the present invention. In this manner, the best FIR filter in filter bank  302  may be selected for a given prior data pattern for different symbol decisions generated by slicer  308  to minimize data errors caused by a particular data pattern according to the present invention. 
     Although filter bank  302  contains FIR filters, other filters may be implemented in accordance with a preferred embodiment of the present invention. For example, infinite impulse response (IIR) filters or continuous time filters may be implemented in place of FIR filters  304 ,  306 , and  308 . Although the depicted example shows only three filters, other numbers of filters may be implemented according to the present invention. 
     In FIG. 2B, DFE  312  employs a single FIR filter  314  with time varying coefficients instead of a filter bank  302  in FIG.  3 A. Different sets of coefficients are stored within coefficient memory  316 . A set of coefficients from the different sets of coefficients is selected from coefficient memory  316  by selection logic  310  in response to data patterns output by slicer  306  according to the present invention. The set of coefficients selected for use in FIR filter  314  is selected to minimize data errors caused by a particular waveform being sent through DFE  312  according to the present invention. 
     Slicer  306  should be able to provide immediate decisions on each symbol so that selection logic  310  can function properly. Although the depicted examples show as slicer, such as slicer  306 , more complex detectors can be employed according to the present invention. For example, slicer  306  maybe employed to estimate pulse polarity and a more complex detector may be employed to generate feedback bits according to the present invention. 
     Feedback cancellation terms for cancelling ISI caused by positive and negative pulses having different peak magnitudes may be implemented within an FIR filter  318  in DFE  320  in FIG. 2C as coefficients according to the present invention. Different sets of coefficients are used to provide feedback cancellation terms, depending on the polarity and magnitude of the pulse whose ISI is being cancelled. These coefficients are stored within coefficient memory  322  and are designed to cancel tails caused by a positive and a negative pulse having different peak magnitudes according to the present invention. The particular sets of coefficients used by FIR filter  318  are selected by selection logic  310  based on the output from slicer  306 . 
     Alternatively, the FIR filter  318  may be implemented in DFE  324  in FIG. 2D using a filter bank  326  according to the present invention. Filter bank  326  includes FIR filter  328  and FIR filter  330 , which have their outputs connected to summing circuit  304 . The output of slicer  306  is routed to FIR filter  328  or FIR filter  330 , based on pulse polarity, using separator  332 . If the output of slicer  306  is being directed to FIR filter  328 , a zero is being directed to FIR filter  330 , and vice versa. FIR filter  328  and FIR filter  330  each contain coefficients that result in positive and negative pulses canceling to eliminate the occurrence of tails. In this manner, the presently claimed invention reduces data errors. 
     With reference now to FIG. 3, a flowchart of a process or calculating FIR filter coefficients based on patterns or polarities of pulses is depicted according to the present invention. The LMS algorithm is used in each case, and two or more sets of FIR filter coefficients are calculated at once. A long sequence of pulses, called a training sequence, is used in the LMS algorithm. A known data sequence, which represents the correct binary pattern for the training sequence is also used in the LMS algorithm. 
     In FIG. 3, coefficients of all filters are initialized in step  400 . Next, training data is fed into each filter, (step  402 ) and all filter outputs are calculated (step  404 ). The filter outputs are compared to the known data sequence, generating an error signal for each FIR filter (step  406 ). A determination is made as to whether the error signal is less than a preselected limit or value (step  408 ). If the error is small enough, the process is terminated. If not, the FIR filter coefficients are adapted to reduce the error (step  410 ). Note that coefficients for each FIR filter are only adapted at those points in the training sequence where that filter will be used. In this way, the LMS algorithm is used to train several filters at once. Following step  410 , the process is repeated starting at step  402  until the error is reduced to an acceptable level. Many variations of this process are possible, based on known methods in adaptive filtering. The coefficients determined in this process are then stored in coefficient memories  316  and  322  for use in the respective FIRs or they are used to generate FIRs for use in filter banks according to the present invention. More information on calculating FIR filter coefficients using LMS may be found in Ifeachor and Jervis,  Digital Signal Processing: A Practical Approach,  Addison-Wesley Publishers Company, Inc., 1995, ISBN No. 0-201-54413-X. 
     With reference now to FIG. 4, a block diagram of a read channel is depicted according to the present invention. Read channel  500  includes automatic gain control  502 , which receives an input signal from a read head (not shown). The adjusted signal is sent to antialiasing filter  504  for initial modification and then to analog to digital (A/D) converter  506 . Thereafter, the signal is modified by DFE  508  in the manner described above. 
     With reference now to FIGS. 5A and 5B, magnetic data storage systems, a disk drive system  600  and a tape drive system  602 , are depicted in which DFEs may be implemented into read channels, such as read channel  500  in FIG. 4 according to the present invention. 
     Nonlinear effects are common in magnetic recording, and these can be partially compensated by forward equalizer  102  according to the present invention. Magneto resistive (MR) heads have several nonlinear characteristics that give rise to asymmetric dipulses. By keeping track of the pulse polarity, the selection logic can easily switch between two backward equalizers or two sets of coefficients one for positive pulses and one for negative pulses. 
     Manufacturing variation is one cause of MR head nonlinearities. Thus, a disk drive with several MR heads may contain several different types of nonlinearity. In such a situation, several sets of FIR filter coefficients can be used and chosen based on prior data and on the particular MR head being used. The coefficients may be determined through a training sequence performed during manufacturing or on power up to generate the filter coefficients for use with each individual MR head. Additionally, different coefficients may be used for each head in a multi-head drive system. 
     Thus, the present invention provides an improved method and apparatus for filtering data by employing a DFE having a forward equalizer whose filtering properties can be varied in response to past data output by the apparatus (e.g., a series of zeros and a one or pulse polarity). Additionally, the present invention provides additional processing of data signals using a backward equalizer that has filtering properties that can be varied in response to peaks of positive and negative pulses already processed by the apparatus. 
     The forward equalizer contains coefficients varying with time, being chosen from a set of possible coefficients. The selection logic examines past data and loads the best set of filter coefficients at each symbol time. Thus, the forward equalizer is tuned to specific data patterns or pulse polarity for optimal performance. Similarly, the backward equalizer has its coefficients selected based on the polarity of the detected data. The use of specific data patterns and polarity of detected data provides for optimal performance when the apparatus of the present invention is used in read channels for drive systems, such as disk drive system  600  and tape drive system  602 . Thus, with the present inventions nonlinear characteristics caused by MR heads and by various read channels may be minimized using a DFE according to the present invention. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, although the present invention is described with the respect to the magnetic recording channels, such as recording channels in a tape drive system or a disk drive system, the equalizers of the present invention maybe employed in other types of data channels according to the present invention.