Abstract:
A data detection apparatus has an A/D (analog to digital) converter which samples and quantizes the output of a low-frequency-emphasizing-type equalization circuit and a digital filter which processes the output of the A/D converter. A Viterbi decoder performs Viterbi decoding of the digital output filter and determines a branchmetric on the basis of an equalization error.

Description:
This application is a continuation of application Ser. No. 08/076,779, filed Jun. 15, 1993. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to data detection, and more particularly, to data detection apparatus which reproduces and detects data subjected to digital recording, for example, on a magnetic recording medium. 
     2. Description of the Related Art 
     Data subjected to digital recording on a recording medium, such as a magnetic tape, a magnetic disk or the like, are reproduced (demodulated), for example, by (1) binary-value determination for each bit by integral detection, (2) three-value determination using differential equalization, partial-response (1, 0, -1) equalization, partial-response (1, 1) equalization, or the like, (3) combination of Viterbi decoding, which is a kind of maximum likelihood decoding, and three-value determination. 
     Since integral equalization is a kind of two-value determination, it is stable against variations in the level of the reproduced output and has a large margin in timing at a detection point. A low-frequency-emphasizing-type waveform equalizer used for integral equalization, partial-response (1, 1) equalization or the like has low high-frequency noise. Low-frequency compensation is unnecessary for differential equalization and partial-response (1, 0, -1) equalization. 
     Viterbi decoding is a decoding method which efficiently executes maximum likelihood decoding utilizing a repetitive structure of correlative codes, such as convolution codes, partial-response equalization or the like. Viterbi decoding is known to have a low error rate in a decoding operation, and has attracted notice as a means to realize high-density data recording. It has been known that by being combined with a three-value-determination equalization method, such as differential equalization or the like, Viterbi decoding can perform a decoding operation with a lower error rate than a decoding operation for each bit. (Refer, for example, to &#34;The Viterbi Algorithm&#34;, Proceeding of IEEE, Vol. 61, No. 3, March, 1973). 
     Low-frequency-emphasizing-type waveform equalization methods, such as integral equalization, partial-response (1,1) and the like, have the advantage of a relatively large margin in detection timing. However, such a method generally requires low-frequency cut-off, since low-frequency components are emphasized in the noise spectrum in the output of an equalizer. If low-frequency components are cut off at a high frequency, while the S/N ratio is improved, an equalization error caused by low-frequency cut-off distortion increases, and the I-pattern numerical aperture is reduced. 
     FIG. 1 illustrates a noise-spectrum distribution 100 for integral equalization. FIG. 2 illustrates a recorded waveform 110 and the corresponding integral-equalized waveform 120. In FIGS. 1 and 2, solid lines represent a case in which low-frequency components are not cut off, and broken lines represent a case in which low-frequency components are cut off. As can be understood from FIG. 2, an equalization error Δa is produced when low-frequency components are cut off, since low-frequency components are attenuated. In addition, as can be understood from FIG. 1, when low-frequency components are not cut off, the noise level increases as the frequency is reduced and finally diverges. On the other hand, when low-frequency components are cut off, the S/N ratio is improved since the low-frequency noise is cut off. 
     Since integral equalization is a kind of binary determination, Viterbi decoding cannot be applied to integral equalization in an unmodified state. 
     Equalization methods which do not require low-frequency compensation, such as differential equalization, partial response (1, 0, 1) equalization and the like, have a small margin in timing at a detection point, and have greater high-frequency noise than low-frequency-emphasizing-type equalization. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a data detection apparatus which has a large margin in detection timing and a small error rate. 
     According to one aspect, the present invention which achieves this objective relates to a data detection apparatus including a low-frequency-emphasizing-type equalizer, an A/D (analog-to-digital) converter for sampling and quantizing the output of the equalizer, a digital filter for performing calculation processing of the output of the A/D converter, and a Viterbi decoder for performing Viterbi decoding of the output of the digital filter. The Viterbi decoder determines branchmetric in consideration of an equalization error. 
     Since the low-frequency-emphasizing-type equalizer is used, a margin in detection timing is large, and the S/N ratio also has a large value. In addition, since Viterbi decoding is adopted, the error rate in a decoding operation has a small value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a noise-spectrum distribution of integral equalization; 
     FIG. 2 is a diagram showing a recorded waveform and the corresponding integral-equalized waveform; 
     FIG. 3 is a block diagram showing the schematic configuration of an apparatus according to an embodiment of the present invention; and 
     FIG. 4 is an enlarged view of an equalized waveform when the same state continues. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described with reference to the drawings. 
     FIG. 3 is a block diagram showing a schematic diagram of an apparatus according to the present embodiment. A reproduced signal (for example, an output from a reproducing amplifier) from a magnetic recording medium is input to input terminal 10. Integral equalizer 12 performs integral equalization. Low-frequency components are cut off at cutoff frequency fc in integral equalization. The output of the equalizer 12 is supplied to an A/D converter 14 to be subjected to sampling and quantization. The output from the A/D converter 14 is supplied to a digital filter 16. 
     The digital filter 16 includes delay circuit 18 for delaying the output of the A/D converter 14 by a one-bit period, and a subtracter 20 for subtracting the output of the delay circuit 18 from the output of the A/D converter 14. The digital filter 16 converts the supplied signal into a signal equivalent to a signal subjected to differential equalization instead of integral equalization. Accordingly, Viterbi decoding can be applied to the output of the filter 16. 
     The output of the filter 16 is supplied to a Viterbi decoder 22. The Viterbi decoder 22 includes a branchmetric determination circuit 24, an operation circuit 26 and a pass memory circuit 28. Data decoded by the Viterbi decoder 22 is output to the outside from an output terminal 30. 
     The configuration and the operation of a Viterbi decoder such as the Viterbi decoder 22 for a signal subjected to differential equalization are well known in the art as described in U.S. Pat. No. 4,606,027 issued Aug. 12, 1986 to S. Otani and in the article by &#34;H. Kobayashi, &#34;Application of Probabilitistic Decoding to Digital Magnetic Recording System&#34;, IBM J. Res. Develop. Vol. 15, pages 64-74, January 1971. 
     The term &#34;branchmetric&#34; indicates ambiguity levels of a change in a state for certain input information. For a general magnetic recording system, branchmetric is expressed by the logarithm of the probability of a change in a state when noise is assumed to have a white Gaussian distribution. 
     That is, 
     
         I(y.sub.k |S.sub.i ;S.sub.j)=lnp(y.sub.k |S.sub.i ;S.sub.j)                                                 (1), 
    
     where S i  and S j  represent states, and y k  represents input information including noise. 
     Since branchmetric includes a common term for respective changes in states, each branchmetric can be simplified by erasing the common term. According to the above-described literature by H. Kobayashi, if simplified branchmetric is expressed by normalized branchmetric 
     
         I&#39;(y.sub.k |S.sub.i ;S.sub.j)=lnp(y.sub.k |S.sub.i ;S.sub.j), 
    
     the normalized branchmetric for differential equalization can be determined by the following expressions. That is, ##EQU1## where S 0  and S 1  represent the states in which the recorded waveform is L (low) and H (high) in FIG. 2, respectively. 
     A branchmetric determination circuit 24 in the present embodiment adds correction term a k  in consideration of an equalization error and defines branchmetric by the following expressions for a case in which the same state continues. 
     That is, ##EQU2## 
     The normalized branchmetric is obtained from the following expressions. That is, ##EQU3## where a 1  =Δa/A, and a 2  =Δa 2  /2a. 
     Correction terms a 1  and a 2  are determined from the amount of equalization error Δa shown in FIG. 2. 
     As shown in FIG. 4, the amount of equalization error Δa changes with time when the same state continues. Accordingly, an equalization-error-amount setting circuit 32 may determine the expected value of the amount of equalization error Δa by linear approximation, or statistical determination in accordance with the frequency of the occurrence of continuation of the same state. Alternatively, the equalization-error-amount setting circuit 32 may statistically process the past history of decoded signals and select the amount of equalization error Δa from several values based on the result of the statistical processing. 
     Expressions (3) are more complicated than expressions (1), causing a burden on the branchmetric determination circuit 24. Actually, however, since the operation circuit 26 has the greatest temporal restriction, the adoption of expressions (3) influences the entire operational speed and the circuit scale less. 
     The Viterbi decoder 22 determines branchmetric using expressions (3) and (4), and performs Viterbi decoding. 
     Although the foregoing description has been made illustrating detection of reproduced data from a magnetic recording medium, the present invention may, of couse, be applied to data detection at a reception side in digital data transmission. 
     As can be easily understood from the foregoing description, according to the present embodiment, equalization having a large margin in timing and an excellent S/N ratio can be performed by cutting off low frequency components at a higher frequency by equalization. Furthermore, by determining branchmetric in consideration of an equalization error caused by low-frequency cut-off, digital information can be detected with a low error rate. 
     The individual components shown in outline or designated by blocks in the drawings are all well-known in the data detection apparatus arts and their specific construction and operation are not critical to the operation or best mode for carrying out the invention. 
     While the present invention has been described with respect to what is presently considered to be the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.