Abstract:
An asymmetric error correction apparatus and method, and clock recovering apparatus and data recovering apparatus for a system for reading data from an optical recording medium such as a CD or DVD that has a multi-level input signal and irregular characteristic of zero-crossing transition. The signal inputted from the optical recording medium is digitized, and a zero-crossing detector extracts four sequential samples and detects a zero-crossing point from the two intermediate samples. An asymmetric error detector judges an asymmetric state and asymmetric polarity of the digital signal from a sum of the two side samples among the four samples if the zero-crossing point is detected. A correction section accumulates the judged asymmetric polarities, judges an asymmetric error of the digital signal if the accumulated value exceeds a predetermined threshold, and corrects the asymmetric error of the read signal caused by an inaccurate pit length.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an asymmetric error correction apparatus and method, and clock recovering apparatus and data recovering apparatus for an optical reading system employing the error correction apparatus. In particular, the present invention relates to an asymmetric error correction apparatus and method, and clock recovering apparatus and data recovering apparatus for an optical reading system employing the same that can correct distortion of a radio frequency (RF) signal caused by interference among data due to inaccurate pit and land lengths during data writing on an optical recording medium. The present application is based on Korean Application No. 2001-48227, filed Aug. 10, 2001, which is incorporated herein by reference. 
   2. Description of the Related Art 
   An optical recording system forms a pit corresponding to a signal to be recorded on an optical reading medium such as a compact disc (CD) or digital versatile disc (DVD). However, if data is recorded with an inaccurate pit length during a signal recording process, an asymmetric phenomenon of an RF signal is presented. In case that the pit violates an adjacent data bit region, a positive asymmetry of the signal is presented. In case that the pit is formed in a portion of the necessary data bit region, a negative asymmetry of the signal is presented. 
   In case that such an asymmetric phenomenon of the RF signal is presented, an inter-symbol interference (ISI) is produced while the optical reading system reads the signal recorded on the optical recording medium. The ISI makes it difficult to detect and correct a frequency error and phase error generated between the recorded data and read data. This causes the reproduced data to be distorted. 
   Conventionally, in order to solve this problem, a DC offset that is the asymmetric signal is removed using a digital sum value (DSV) asymmetric error correction apparatus applying a DSV algorithm as shown in  FIG. 1  or a zero-crossing  3  (ZC 3 ) asymmetric error correction apparatus applying three samples adjacent to a zero-crossing point as shown in  FIGS. 2 and 3 . 
   The conventional DSV asymmetric error correction apparatus  1  includes a binary quantization section  1 , a counting section  12 , a comparing section  13 , an error judgment section  14 , an integration section  15 , a correction section  16 , and a data detection section  17 . 
   The binary quantization section  11  determines a polarity value to be “1” if an average value of two samples is larger than “0” for each digital sample, and determines the polarity value to be “−1” if the average value of the two samples is smaller than “0”. The counting section  12  accumulates the detected polarity values to judge the degree of error. The comparing section  13  compares the accumulated polarity value with a predetermined threshold value. 
   The error judgment section  14  judges that the asymmetric error is generated if the accumulated polarity value exceeds the predetermined threshold value. The integration section  15  accumulates the detected asymmetric errors. The correction section  16  corrects an input signal Si and outputs a corrected signal Sc if the accumulated error value reaches a predetermined value. The data detection section  17  detects respective data bit values of the corrected signal Sc. 
   The ZC 3  asymmetric error correction apparatus  2 , which is another correction apparatus for correcting the asymmetric error, will now be explained with reference to  FIGS. 2 and 3 . 
   The ZC 3  asymmetric error correction apparatus  2  includes a zero-crossing detection section  21 , an absolute value comparing section  22 , an asymmetric polarity judgement section  23 , a counting section  24 , a comparing section  25 , an error judgment section  26 , an integration section  27 , a correction section  28 , and a data detection section  29 . 
   The zero-crossing detection section  21  detects a zero-crossing point by comparing sign bits of the two samples sequentially inputted. If the zero-crossing point is detected, the absolute value comparing section  22  compares absolute values of the two samples, judges that the asymmetry is generated in the sample having a smaller absolute value, and determines the sample having the smaller absolute value to be an intermediate sample. The asymmetric polarity judgement section  23  judges the polarity value of the signal from the sum of the intermediate sample and two neighboring samples, i.e., one sample at each side of the intermediate sample, thereby using the sum of three samples neighboring the zero-crossing point. 
   The counting section  24  accumulates the detected polarity value to judge the degree of error. The comparing section  25  compares the accumulated polarity value with the predetermined threshold value. The error judgment section  26  judges that the asymmetric error is generated if the accumulated polarity value exceeds the predetermined threshold value. The integration section  27  accumulates the detected asymmetric error. The correction section  28  corrects the input signal Si by adjusting the gain of the accumulated error value, and outputs the corrected signal Sc. The data detection section  29  detects the respective data bit values of the corrected signal Sc. 
   The detailed construction and operation of the zero-crossing section  21 , absolute value comparing section  22 , and asymmetric polarity judgement section  23  are illustrated in  FIG. 3   
   The zero-crossing detection section  21  includes an exclusive OR gate  21   a  that judges the zero-crossing point from the sign bits of the two sequential samples D 2   k  and D 3   k . If the zero-crossing point is detected between the two sequential samples D 2   k  and D 3   k , the zero-crossing detection section  21  sets the detected zero-crossing point value ZC to 1. 
   The absolute value comparing section  22  obtains absolute values of the two sequential samples D 2   k  and D 3   k  between which the zero-crossing point is detected, and compares the absolute values. The absolute value comparing section  22  judges that the asymmetry is generated in the sample D 2   k  or D 3   k  having a smaller absolute value. For example, if |D 2   k | is larger than |D 3   k |, the absolute value comparing section  22  selects D 3   k  as the intermediate sample, and an adder  23   a  obtains the sum sum — 1 of the intermediate sample D 3   k , previous sample D 2   k , and later sample D 4   k . Meanwhile, if |D 2   k | is smaller than or equal to |D 3   k |, the absolute value comparing section  22  selects D 2   k  as the intermediate sample, and an adder  23   b  obtains the sum sum — 2 of the intermediate sample D 2   k , previous sample D 1   k , and later sample D 3   k . 
   A polarity judgment section  23   c  judges the polarity value to be negative if the detected zero-crossing value ZC is 1, and the sum — n is larger than 0. The polarity judgment section  23   c  judges the polarity value to be positive if the detected zero-crossing value ZC is 1, and the sum — n is smaller than or equal to 0. Here, n is 1 or 2. The polarity value obtained as above is inputted to the counting section. 
   The conventional DSV asymmetric error correction apparatus has an advantage that it can achieve a relatively stable operation without being greatly affected by the amount of the asymmetric polarity and the timing error value. However, since the average value of two samples should be obtained for each sample, the conventional apparatus has the disadvantage of slow correction speed as shown in  FIG. 12 . 
   Also, the conventional ZC 3  asymmetric error correction apparatus, which judges the polarity using the two sample values at the zero-crossing point, can easily judge the polarity in case that the asymmetric polarity is small and the timing error is small as shown in  FIG. 13 . However, since it uses three samples neighboring the zero-crossing point, it has the disadvantage that it is difficult to accurately judge the polarity of the asymmetric signal in case that the environment of the optical channels is inferior. Accordingly, the amount of jitter in a normal state becomes larger during the error tracking in the correction process as shown in  FIGS. 14 and 15 , and thus it has the disadvantage that it is difficult to achieve a stable operation of the following PLL loop. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a primary object of the present invention to provide an asymmetric error correction apparatus and method, and clock recovering apparatus and data recovering apparatus for an optical reading system employing the same that can improve the correction speed of the asymmetric error. 
   Another object of the present invention is to provide an asymmetric error correction apparatus and method, and clock recovering apparatus and data recovering apparatus for an optical reading system employing the same that can achieve a stable operation even when a signal is distorted through circumstances such as increased frequency error, phase error, and amount of asymmetry. 
   To achieve the above objects, the present invention provides an asymmetric error correction apparatus comprising: a zero-crossing detecting means for extracting four sequential samples from a digital signal inputted from an analog-to-digital (A/D) converter and detecting a zero-crossing point from the two intermediate samples; an asymmetric error detecting means for judging an asymmetric state and an asymmetric polarity of the digital signal from a sum of the two side samples among the four samples if the zero-crossing point is detected; and a correcting means for correcting an asymmetric error of the digital signal according to the detected asymmetric polarity. 
   The zero-crossing detecting means detects the zero-crossing point by detecting an inversion of signs of the two intermediate samples among the four samples. 
   The asymmetric error detecting means includes an asymmetric state judging means for judging the asymmetric state of the digital signal by obtaining the sum of the two side samples among the four samples if the zero-crossing point is detected; and a polarity judging means for judging the asymmetric polarity of the digital signal according to the sign of the sum if the zero-crossing point is detected. 
   The polarity judging means judges the asymmetric polarity of the digital signal to be negative if the sum is larger than 0, while it judges the asymmetric polarity of the digital signal to be positive if the sum is smaller than or equal to 0. 
   The correcting means includes a polarity counter for counting the judged asymmetric polarities; a comparing means for comparing a counted value of the asymmetric polarities with a predetermined threshold value to output a result of comparison, and resetting the polarity counter if the counted value of the asymmetric polarities exceeds the threshold value; an asymmetric error generating means for generating the asymmetric error according to the result of comparison; an integrating means for integrating the generated asymmetric error; and an error correcting means for correcting the asymmetric error of the digital signal inputted from the A/D converter according to a result of integration. 
   In another aspect of the present invention, there is provided an asymmetric error correction method comprising the steps of extracting four sequential samples from an input digital signal and detecting a zero-crossing point from the two intermediate samples; judging an asymmetric state and an asymmetric polarity of the digital signal from a sum of the two side samples among the four samples if the zero-crossing point is detected; and correcting an asymmetric error of the digital signal according to the detected asymmetric polarity. 
   According to the asymmetric error correction apparatus and method as above, the asymmetric error is detected using only two samples, not for each sample or three samples, and thus they have the advantage of a high correction speed. Also, the asymmetric polarity is judged according to a sample before one timing and a sample after one timing (called an external sample) rather than two samples neighboring the zero-crossing point, without using the two intermediate samples neighboring the zero-crossing point. Accordingly, a stable operation can be achieved even if signal distortion such as an increase of the frequency error, phase error, and amount of asymmetry exists. 
   In still another aspect of the present invention, there is provided a clock recovering apparatus for an optical reading system for reading an analog signal from an optical recording medium, the apparatus comprising a filter for calculating a timing of a digital signal sequentially inputted from an analog-to-digital (A/D) converter; an asymmetric error corrector for extracting four sequential samples from the digital signal inputted from the filter, detecting an asymmetric error of the digital signal from a sum of the two side samples among the four samples if a zero-crossing point is detected, and correcting the detected asymmetric error; and a phase locked loop for correcting a timing error and a phase error of the asymmetric-error-corrected digital signal. 
   In still another aspect of the present invention, there is provided a data recovering apparatus for an optical reading system for reading an analog signal from an optical recording medium, the apparatus comprising an analog-to-digital (A/D) converter for converting the analog signal into a digital signal; a clock recovering means for correcting an asymmetric error by sequentially extracting four samples from the digital signal inputted from the A/D converter, and recovering a clock by correcting a timing error and a phase error of the asymmetric-error-corrected digital signal; and a data detector for detecting data by recovering the digital signal inputted from the A/D converter according to the recovered clock. 
   According to the clock recovering apparatus provided with the asymmetric error correction apparatus, it has the advantage that the time required for recovering the asymmetric error is shortened, and an accurate correction is possible, enabling easy clock recovery. Also, according to the data recovering apparatus provided with the asymmetric error correction apparatus, it has the advantage that quick data recovery can be achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
       FIG. 1  is a block diagram illustrating the construction of a conventional asymmetric error correction apparatus using the DSV algorithm; 
       FIG. 2  is a block diagram illustrating the construction of a conventional asymmetric error correction apparatus using three-sample signal values at a zero-crossing point; 
       FIG. 3  is a block diagram illustrating the detailed construction of a zero-crossing detection section and an asymmetric error detection section of  FIG. 2 ; 
       FIG. 4  is a block diagram illustrating the construction of a data recovering apparatus for an optical reading system according to a preferred embodiment of the present invention; 
       FIG. 5  is a block diagram illustrating the detailed construction of a clock recovering section of  FIG. 4 ; 
       FIG. 6  is a block diagram illustrating the detailed construction of an asymmetric error correction section of  FIG. 5 ; 
       FIG. 7  is a block diagram illustrating the detailed construction of a zero-crossing detection section and an asymmetric error detection section of  FIG. 6 ; 
       FIG. 8  is a graph illustrating positions of respective samples when the zero-crossing point is generated; 
       FIG. 9  is a flowchart illustrating an asymmetric error correction method according to a preferred embodiment of the present invention; 
       FIG. 10  is a flowchart illustrating the detailed process of an asymmetric error detection step of  FIG. 9 ; 
       FIG. 11  is a flowchart illustrating the detailed process of an error correction step of  FIG. 9 ; 
       FIG. 12  is a graph illustrating the correction speed of the asymmetric error correction apparatuses according to the prior art and the present invention in case that the asymmetric rate (ASM) is 15.6% and the error value is estimated at optimum; 
       FIG. 13  is a graph illustrating the jitter amount in a normal state during the error tracking of the asymmetric error correction apparatuses according to the prior art and the present invention in case that the tracking speed is identical and the timing error does not exist; 
       FIG. 14  is a graph illustrating the jitter amount in a normal state according to the asymmetric rate (ASM) during the error tracking of the asymmetric error correction apparatuses according to the prior art and the present invention in case that the tracking speed is identical and the timing error exists; 
       FIG. 15  is a graph illustrating the jitter amount in a normal state according to the asymmetric rate (ASM) during the error tracking of the asymmetric error correction apparatuses according to the prior art and the present invention in case that the timing error does not exist and an optimum error value tracking is possible; 
       FIG. 16  is a graph illustrating the bit error rate (BER) of the asymmetric error correction apparatuses according to the prior art and the present invention when the asymmetric rate (ASM) is 9.8% in case that the timing error does not exist and an optimum error value tracking is possible; and 
       FIG. 17  is a graph illustrating the bit error rate (BER) of the asymmetric error correction apparatuses according to the prior art and the present invention when the asymmetric rate (ASM) is 15.6% in case that the timing error does not exist and an optimum error value tracking is possible. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A data recovering apparatus A for an optical reading system for reading an analog signal from an optical recording medium of the present invention will now be described in detail with reference to  FIG. 4 . 
   Referring to  FIG. 4 , the data recovering apparatus A for an optical reading system includes an A/D converter A 1 , a clock recovering section A 2 , and a data detection section A 3 . 
   The A/D converter A 1  converts the analog signal sequentially inputted into a digital signal Si. The clock recovering section S 2  sequentially extracts four samples from samples of the digital signal Si inputted from the A/D converter A 1 .  FIG. 8  illustrates the four samples D 1   k , D 2   k , D 3   k , D 4   k , or D 1   k-1 , D 2   k-1 , D 3   k-1 , D 4   k-1  including a zero-crossing point t k  or t k-1  extracted by the clock recovering section A 2 . 
   The clock recovering section A 2  corrects the asymmetric error of the digital signal Si on the basis of the sum of the two side samples D 1   k  and D 4   k , or D 1   k-1  and D 4   k-1  among the four samples D 1   k , D 2   k , D 3   k , D 4   k , or D 1   k-1 , D 2   k-1 , D 3   k-1 , D 4   k-1 . Also, the clock recovering section A 2  corrects the timing error and the phase error of the asymmetric-error-corrected digital signal. The data detection section A 3  detects a recovered digital signal Sc from the digital signal in which the asymmetric error, timing error, and phase error are corrected. 
   Referring to  FIG. 5 , the detailed construction and operation of the clock recovering section A 2  will be explained. In the embodiment of the present invention, it is assumed that the zero-crossing point is detected at t k . 
   The clock recovering section A 2  includes a filter  200 , an asymmetric error correction section  300 , and a PLL section  400 . The filter  200  calculates the timing of the digital signal inputted from the A/D converter A 1 . The filter can be implemented using an interpolating filter. 
   The asymmetric error correction section  300  sequentially extracts the four samples D 1   k , D 2   k , D 3   k , and D 4   k  from the digital signal inputted from the filter  200 . The asymmetric error correction section  300  detects the zero-crossing point in case that the sign bits of the two samples D 2   k  and D 3   k  among the four samples D 1   k , D 2   k , D 3   k , and D 4   k , are inverted. The asymmetric error correction section  300  detects the asymmetric error of the digital signal from the sum of the two side samples D 1   k  and D 4   k  among the four samples D 1   k , D 2   k , D 3   k , and D 4   k . The asymmetric error correction section  300  corrects the asymmetric error with respect to the digital signal. 
   The PLL section  400  corrects the timing error and the phase error of the asymmetric-error-corrected digital signal, and outputs the corrected digital signal to the filter  200 . 
   The detailed construction and operation of the asymmetric error correction section  300  will be explained with reference to  FIGS. 6 to 8 . 
   The asymmetric error correction section  300  includes a zero-crossing detection section  320 , an asymmetric error detection section  340 , and a correction section  360 . The zero-crossing detection section  320  detects the zero-crossing point from the sign bits of the digital signal Si sequentially inputted from the A/D converter A 1 . The zero-crossing detection section  320  may comprise an exclusive OR gate  320   a  as shown in  FIG. 7 . 
   The zero-crossing detection section  320  sequentially extracts the four samples D 1   k , D 2   k , D 3   k , and D 4   k  from the digital signal inputted from the A/D converter A 1 . The zero-crossing detection section  320  detects the zero-crossing point by exclusive-OR-gating the sign bits of the two intermediate samples D 2   k  and D 3   k  among the four samples D 1   k , D 2   k , D 3   k , and D 4   k . The four sequential samples D 1   k , D 2   k , D 3   k , and D 4   k  are signals that have passed delay elements  340   a ,  340   b , and  340   c , respectively. Referring to  FIG. 8 , it can be recognized that the zero-crossing point is generated between the two intermediate samples D 2   k  and D 3   k  among the four sequential samples D 1   k , D 2   k , D 3   k , and D 4   k  at the time point of t k . 
   Referring to  FIGS. 7 and 8 , if the zero-crossing point is generated at the time point of t k , the zero-crossing detection variable ZC is set to 1. 
   If the zero-crossing point is detected, the asymmetric error detection section  340  judges the asymmetric state and the asymmetric polarity of the digital signal from the sum of the two side samples D 1   k  and D 4   k  among the four samples D 1   k , D 2   k , D 3   k , and D 4   k . 
   The asymmetric error detection section  340  includes an asymmetric state judgment section  340   d  and a polarity judgment section  340   e . The asymmetric state judgment section  340   d  may be implemented using an adder. If the zero-crossing point is detected, the asymmetric state judgment section  340  calculates the sum of the two side samples D 1   k  and D 4   k  among the four samples D 1   k , D 2   k , D 3   k , and D 4   k . The asymmetric state judgment section  340   d  judges the digital signal to be asymmetric if the sum is not 0, while it judges the digital signal to be symmetric if the sum is 0. 
   If the zero-crossing point is detected, the polarity judgment section  340   e  judges the asymmetric polarity of the digital signal according to the sign of the sum in case that the zero-crossing detection variable ZC is set to 1. If the sum is larger than 0, the polarity judgment section  340   e  judges the polarity of the digital signal to be negative, and sets +1 as the polarity value. Meanwhile, if the sum is smaller than or equal to 0, the polarity judgment section  340   e  judges the polarity of the digital signal to be positive, and sets −1 as the polarity value. In principle, the sum being 0 means that the asymmetry is not generated. However, practically, it is very rare for the sum to be 0, and does not affect the counting of the asymmetric polarity values. Accordingly, the operation of the asymmetric error correction section is not affected by the case that the sum is 0 even if the polarity is judged to be either negative or positive. 
   The correction section  360  corrects the asymmetric polarity of the digital signal according to the asymmetric polarity. The correction section  360  includes a polarity counter  361 , a comparing section  363 , an asymmetric error generation section  365 , an integration section  367 , and an error correction section  369 . The polarity counter  361  obtains and stores the counted value of asymmetric polarities by accumulating the polarity values outputted from the polarity judgment section  340   e.    
   The comparing section  363  compares the counted value of the asymmetric polarities with the predetermined threshold value, and outputs the result of comparison. If the counted value of the polarities exceeds the threshold value, the comparing section  363  resets the polarity counter  361 . 
   By resetting the polarity counter  361  after the correction is performed, the accuracy of the judgment is secured, and the jitter in the normal state is prevented during the tracking of the asymmetric signal. 
   The asymmetric error generation section  365  generates the asymmetric error according to the result of comparison. That is, if the counted value of the asymmetric polarities exceeds the threshold value, the asymmetric error generation section  365  generates the asymmetric error having the opposite sign value. The threshold value may be set as an optimum value considering the correction speed and the normal state value during the tracking. 
   The integration section  367  integrates the generated asymmetric error. The error correction section  369  corrects the asymmetric error of the digital signal Si inputted from the A/D converter A 1  according to the result of integration, and outputs the corrected digital signal Sc. 
   The clock recovering section A 2  and the asymmetric error correction section  300  can be implemented as separate devices. 
   Hereinafter, the asymmetric error correction method according to the preferred embodiment of the present invention will be explained with reference to  FIGS. 9 to 11 . Since the asymmetric error correction method is similar to the operation of the asymmetric error correction section  300 , the detailed explanation thereof will be omitted. 
   The asymmetric error correction method comprises a zero-crossing detection step S 100 , an asymmetric polarity detection step S 200 , and an error correction step S 300 . The zero-crossing detection step S 100  sequentially extracts the four samples from the input digital signal, and detects the zero-crossing point by the sign bits of the two intermediate samples. The asymmetric polarity detection step S 200  judges the asymmetric state and the asymmetric polarity of the digital signal from the sum of the two side samples among the four samples if the zero-crossing point is detected. 
   The asymmetric polarity detection step S 200  includes an asymmetric error state judgment step S 220  and an asymmetric polarity judgment step S 240 . The asymmetric error state judgment step S 220  calculates the sum of the two side samples among the four samples if the zero-crossing point is detected. The asymmetric error state judgment step S 220  judges the asymmetric state of the digital signal from the sum of the two samples. 
   If the zero-crossing point is detected, the asymmetric polarity judgment step S 240  judges the polarity of the digital signal according to the sign of the sum. The asymmetric polarity judgment step S 240  judges the polarity of the digital signal to be negative if the sum is larger than 0, while it judges the polarity of the digital signal to be positive if the sum is smaller than or equal to 0. 
   The error correction step S 300  corrects the asymmetric error of the digital signal according to the result of the polarity judgment. The error correction step S 300  includes a step S 310  of counting the asymmetric polarities determined at the asymmetric polarity judgment step S 240 , a comparing step S 320 , an asymmetric error generation step S 330 , an asymmetric error integration step S 340 , and an asymmetric error correction step S 350 . 
   The comparing step S 320  compares the counted value of the asymmetric polarities with the predetermined threshold value, and outputs the result of comparison. The comparing step S 320  resets the polarity counted value if the polarity counted value exceeds the threshold value. The asymmetric error generation step S 330  generates the asymmetric error corresponding to the result of comparison. 
   The asymmetric error integration step S 340  integrates the generated asymmetric error. The asymmetric error correction step S 350  corrects the asymmetric error of the digital signal according to the result of integration. 
   Referring to  FIGS. 12 to 17 , the comparison result of the performance of the conventional DSC and ZC 3  asymmetric error correction method with that of the asymmetric error correction method according to the present invention will be explained. 
     FIG. 12  is a graph illustrating the tracking time when the remaining asymmetric amount converges into 0 through the respective methods in the environment that the SNR is 17 dB and the asymmetric error is 15.6%. As shown in  FIG. 12 , in case of using the parameter having the optimum tracking performance for the respective methods, the correction speed achieved by the method according to the present invention is faster than that of the conventional DSV method by 15 μs. Accordingly, the method according to the present invention is suitable for being applied to a system for reading a signal from a high-density optical recording medium at a high speed, and can secure a stable operation. 
     FIG. 13  is a graph illustrating the RMS jitter values in a normal state according to the asymmetric rate (ASM) in the environment that the conventional and present methods have the same tracking speed, the SNR is 17 dB, and the timing error does not exist.  FIG. 14  is a graph illustrating the RMS jitter values in a normal state according to the asymmetric rate (ASM) in the environment that the conventional and the present methods have the same tracking speed, and the timing error exists.  FIG. 15  is a graph illustrating the RMS jitter values in a normal state according to the asymmetric rate (ASM) in the environment that the conventional and present methods have the respective optimum tracking conditions, the SNR is 17 dB, and the timing error does not exist. 
   Referring to  FIGS. 13 to 15 , the asymmetric error correction method according to the present invention enables stable operation and accurate polarity judgment in an optical reading system in which the degree of asymmetry is great and phase and frequency errors exist. The reason why the asymmetric error correction method according to the present invention is not affected by the frequency error and the phase error is that it uses the samples that are not greatly affected by the timing error and the AWGN. That is, the asymmetric error correction method according to the present invention uses the zero-crossing point in the same manner as the conventional ZC 3  method. 
   However, the conventional ZC 3  method uses the sample values neighboring the zero-crossing point, which are greatly affected by the timing error and AWGN, for judgment of the asymmetric polarity. On the contrary, the asymmetric error correction method according to the present invention does not use the two samples neighboring the zero-crossing point, but judges the asymmetric polarity according to the sum of the sample before one timing and the sample (called the external sample) after one timing rather than the two samples neighboring the zero-crossing point. Accordingly, it can achieve stable operation even if signal distortion such as increased frequency error, phase error, and amount of asymmetry exists. 
     FIG. 16  is a graph illustrating the bit error rate (BER) performance of the asymmetric error correction apparatuses according to the prior art and the present invention according to the variation of the SNR value [dB] when the asymmetric rate (ASM) is 9.8% in case that the timing error does not exist and the optimum error value tracking condition is satisfied.  FIG. 17  is a graph illustrating the bit error rate (BER) performance of the asymmetric error correction apparatuses according to the prior art and the present invention according to the variation of the SNR value [dB] when the asymmetric rate (ASM) is 15.6% in case that the timing error does not exist and the optimum error value tracking condition is satisfied. 
   When the ASM is 9.8%, the BER performance of the asymmetric error correction apparatus and method according to the present invention is far better in comparison to the conventional DSV method. Also, when the ASM is 15.6%, the jitter performance and the BER performance of the asymmetric error correction apparatus and method according to the present invention are most superior. 
   As described above, the present invention detects the asymmetric error only in case that the zero-crossing point is detected from the two samples among the four sequential samples. Also, the present invention does not use the two intermediate samples neighboring the zero-crossing point, but judges the asymmetric polarity according to the sum of the sample before one timing and the sample (called the external sample) after one timing. Accordingly, it can achieve stable operation even if signal distortion such as increased frequency error, phase error, and amount of asymmetry exists. 
   Also, the asymmetric error correction apparatus and method according to the present invention enable stable operation and accurate polarity judgment in an optical reading system in which the degree of asymmetry is great and the phase and frequency errors exist. That is, when the ASM is 15.6%, the SNR margin of the asymmetric error correction apparatus and method according to the present invention is higher than the conventional ZC 3  method by about 0.5 dB, and higher than the conventional DSV method by about 2.2 dB. 
   Also, in case of using the parameter having the optimum tracking performance, the correction speed achieved by the asymmetric error correction apparatus and method according to the present invention is faster than that of the conventional DSV method by about 15 μs. Accordingly, the present invention is suitable to being applied to a system for reading a signal from a high-density optical recording medium at a high speed, and can secure a stable operation by more rapidly correcting the distorted input data. Further, since the external sample value is varied according to the variation of the signal level, the present invention can reduce the error in judging the polarity in comparison to the conventional ZC 3  method using the three samples neighboring the zero-crossing point. 
   Although the preferred embodiment of the present invention has been described, it is understood that the present invention should not be limited to this preferred embodiment but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed.