Abstract:
The present invention provides a read channel and a drive capable of suppressing deterioration in performance of a PLL and a Viterbi decoder by using a DC component eliminating means capable of higher-speed operation than hitherto. The location of an edge is determined by using differential of a read signal, and a DC component is detected from the midpoint level of the edge. Detection of a pseudo-edge due to a long mark or space signal is prevented by limiting the absolute value of a maximum or minimum of a differential coefficient when the location of the edge is determined from the differential coefficient of the read signal. Internal operation of a DC component detector is controlled according to the state of the PLL and the magnitude of the DC component.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese application JP 2007-127673 filed on May 14, 2007, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a read channel for an optical disc drive, a magnetic hard disc drive, or the like. 
   2. Description of the Related Art 
   One noteworthy feature of optical disc drive units and discs in conformity with DVD (digital versatile disc), Blu-ray Disc (hereinafter referred to simply as “BD”) and other standards is that recording media are interchangeable and thus the discs are interchangeable among different models for writing and reading. However, situations can, in actuality, possibly arise where reading is extremely difficult due to the discs becoming damaged or soiled on their information-bearing surfaces. Although the scope of application of the present invention is not limited to a BD, description will hereinafter be given assuming that the present invention is applied to the BD. Likewise, terms as employed herein are basically those used for the BD. 
     FIG. 2  shows an example of the configuration of a read channel in a very basic form. A read signal processing system that leads an analog read signal through decoding to a bit stream, including an analog equalizer and a PLL (phase locked loop), is herein called “read channel.” The read signal processing system is herein assumed as a Viterbi decoding system that performs AD (analog-to-digital) conversion on the analog read signal and then performs signal processing on the signal. Accordingly, the term “read signal” is herein employed almost exclusively for a digital signal after the AD conversion. However, both the analog and digital signals, if apparent in context, are called merely “read signal” for the sake of simplicity, because those skilled in the art are unlikely to confuse between the analog signal before the AD conversion and the digital signal. 
   An analog read signal is equalized by an analog equalizer  1  and is then converted into a digital signal by an AD converter  2 . At this point, the timing of sampling is determined by the channel clock. 
   After that, the signal is phase compared with the channel clock by a phase comparator  6 . A phase error signal is smoothed through a loop filter  9  and is converted through a DA converter  11  into an analog signal, which is then inputted as a control voltage signal to a VCO (voltage controlled oscillator)  10 . The VCO oscillates at the frequency indicated by the input voltage signal, and the oscillation frequency is used as the channel clock. In other words, the oscillation frequency is used as a drive clock for each element such as the AD converter, the phase comparator, the loop filter, the DA converter, and the Viterbi decoder  7 . Since the fact that this closed loop forms a PLL and acts to bring the channel clock into synchronization with the clock of the read signal, and the details of operation thereof are well known in the art, detailed description thereof is not given. Also as for the Viterbi decoder, detailed description thereof is not given herein because the details of operation thereof do not have a direct relation to the present invention. 
     FIG. 3  is an illustration of assistance in explaining the principle of phase comparison. The phase are compared using edges (points where read signal cross the zero level), that is, points corresponding to boundaries between marks and spaces. The channel clock is in synchronization with the edges. The timing of AD conversion is shifted T/2 from clock timing referred to the edge (where T denotes a channel clock cycle). The read signal subjected to sampling after a lapse of T/2 from channel clock time nT will hereinafter be expressed as x(n) for the sake of simplicity (where n denotes an integer). In  FIG. 3 , the edge and sample points are shown by a dashed line and open (or white) circles, respectively, in an instance where the phase of the channel clock is in perfect synchronization with that of the edge. The edge lies at the time nT. At this time, the values of two sample points with the edge in between are defined as x(n−1) and x(n), respectively. The read signal is assumed to be in linear form in the vicinity of the edge. At this time, the relation between the values is as follows: x(n)=−x(n−1). In  FIG. 3 , the edge and sample points are shown by a solid line and solidly shaded (or black) circles, respectively, in an instance where the phase of the same edge lags ΔT behind that of the channel clock. The edge is assumed to lie between the channel clock times (n−1)T and nT, and the values of the sample points at the times (n−1)T and nT are defined as x(n−1) and x(n), respectively. Clearly, the relation between the values is as follows: x(n)≠−x(n−1). Obviously, assuming that the edge has linearity leads to Expression (1).
 ΔT∝x(n)−x(n−1)  (1) 
   In other words, detection of phase error can be accomplished by sampling the read signal by the channel clock and distinguish the edges, then the phase error can be determined from the difference in read signal levels between the two points with the edge in between. 
   When the phase error is determined from the signal level in the manner as mentioned above, the phase error cannot be accurately determined if an unwanted DC (direct current) component is superimposed on the read signal. Such a situation will be described with reference to  FIG. 4 . In  FIG. 4 , the edge and the sample points are shown by a dashed line and open (or white) circles, respectively, in an instance where the unwanted DC component is absent and the phase of the read signal is in perfect synchronization with that of the channel clock. The edge and the sample points are shown by a solid line and solidly shaded (or black) circles, respectively, in an instance where the phase of the read signal is in synchronization with that of the channel clock and the DC component is superimposed by Δx on the read signal. Even if the read signal is in synchronization with the channel clock, the superimposed DC component causes the phase comparison based on definition by Expression (1) to output an erroneous phase error value. For this reason, the DC component of the read signal is removed by use of a high-pass filter before entering the phase comparator. Even under this condition, however, a pattern-dependent DC component variation or the like remains in the read signal. The DC component variation depending on the pattern is removed by use of a DFB (duty feedback) slicer, which utilizes the fact that the bit stream recorded on an optical disc is modulated by using a modulation code that, if integrated at given or more intervals, exhibits the appearance possibility of “0” becoming equal to the appearance possibility of “1.” Since the DFB slicer is a technology well known in the art, detailed description thereof is not given. 
   Description will now be given with regard to a JFB (jitter feedback) DC compensator. When the DC component is zero and the PLL is completely locked, the phase error is zero, that is, the midpoint of the edge coincides with the zero level. When the read signal rather undergoes the DC variation with the PLL locked, the midpoint of the edge lies outside the zero level. Accordingly, the integration of the midpoint level of the edge can lead to a DC level. This method is employed provided that the PLL is locked, because the edge is used to detect the DC component. 
   PRML (partial response most-likely) decoding method involves decoding a read signal into a most-likely bit stream, while comparing the read signal at plural consecutive times with a target signal. Viterbi decoding method that is one of ML (most-likely) decoding methods is widely in practical use because of enabling a substantial reduction in the scale of the circuit. The PRML method has come into use also as a reading device for the optical disc in order to achieve higher speeds and larger capacities. Since the target signal is used provided that an unwanted DC component is completely absent, decoding performance undergoes deterioration if the DC component is superimposed on the read signal at the time of comparison of the read signal with the target signal. 
   When the device in a read mode, the device is made to minimize the occurrence of read error even if the disc is in bad condition such as situations where the disc is defective or soiled. For example, when the surface of the disc is so soiled that the read signal is almost entirely shielded, a defect detection technique such as described in Japanese Patent Application Publication Laid-open Application No. 2003-30850 can be used to minimize the influence of defects. It is well known in the art that the same or similar approach is used for the optical discs in general. In general outline, the approach involves a circuit that monitors the top envelope of the read signal and outputs a defect signal if the amplitude of the signal is equal to or less than a threshold for a given or longer time, as shown in  FIG. 5 . The approach involves, for the duration of output of the defect signal, holding the tracking, focusing and other controls and also holding the PLL of the read signal processing system and doing the like, thereby preventing undesirable operation resulting from the defects and hence minimizing the influence of the defects. 
   Besides local factors such as defects on the disc, a phenomenon occurs in which reading performance undergoes deterioration over an extremely wide range of the disc due to a disc&#39;s structure or the like, such as inter-layer interference of a dual layered disc.  FIG. 6  shows an example of a read signal disturbed under the influence of the inter-layer interference. This is an instance where data is read from a layer-L 1  of a rewritable dual layered Blu-ray Disc, that is, a layer close to the surface of the disc. As can be seen from  FIG. 6 , both the top and the bottom envelopes that should be, by nature, substantially flat undergo great disturbance under the influence of the inter-layer interference. During the reading of the data from the layer-L 1 , read light is focused on that layer. Part of the read light passes through the layer-L 1  and reflected at layer-L 0 , and partially reaches a photodetector of an optical head. Since the light beams from the layer-L 0  and the layer-L 1  reach the photodetector simultaneously, the interference of the light beams occurs. Generally, a distance between the layer-L 0  and the layer-L 1  slightly varies according to their positions on the disc. When data is read from the disc under this condition, an interference pattern formed on the photodetector by the light beams from the layer-L 0  and the layer-L 1 , changes with time. As a result, the read signal undergoes disturbance as shown in  FIG. 6 . When the disturbance is encountered in the signal as shown in  FIG. 6 , the signal recorded in the disturbed place cannot be accurately decoded and thus results in a burst error, as in the case of the defects. In the example shown in  FIG. 6 , the burst error is a few hundreds of bytes long. This length does not get in the way of reading in terms of the capability of an error correcting code of the Blu-ray Disc system. However, when data is read from a region where the distance between the layers varies greatly in the tangential direction of the disc, the state of the interference also changes more rapidly on the photodetector. Accordingly, the frequency of occurrence of disturbance of the signal as shown in  FIG. 6  becomes higher, and thus the disturbance occurs plural times in one recording unit block (RUB). Under this condition, the probability of occurrence of the read error cannot be ignored. Incidentally, factors that cause the same or similar disturbance of the read signal include fingerprints and track deviation. 
   SUMMARY OF THE INVENTION 
   The read signal disturbed by the inter-layer interference of the dual layered disc or the like is characterized by involving local DC component variation. In this case, the read signal is characterized in that an interval of DC component variation is as short as a few tens of microseconds in time for BD1X. When the DC component is superimposed on the read signal as previously mentioned, the DC component causes deterioration in the performance of the PLL and the Viterbi decoder. Devices for removing the unwanted DC component from the read signal include the DFB slicer as previously mentioned. However, the DFB slicer requires a sufficiently long integrating time in order to eliminate statistical fluctuation, because of utilizing the appearance possibility of “0” equaling the appearance possibility of “1.” In other words, the DFB slicer cannot handle the DC component variation caused by the inter-layer interference or the like. 
   The JFB DC compensator obtains the DC level by integrating the midpoint level of the edges, utilizing the fact that when the DC variation occurs in the read signal with the PLL locked, the midpoint of the edge lies outside the zero level, as described in Japanese Patent Application Laid-open Publication No. 2006-4465. This method requires the PLL to be locked because the edge is used for DC component detection. With the use of this method alone, DC component variation compensation is therefore difficult. Moreover, when the amplitude of the DC component superimposed on the read signal is very great, a problem arises as described below: two points adjacent to each other with the zero level in between are shifted from the position of a true edge, thus leading to an erroneous result. The DC component variation due to the inter-layer interference or the like of the dual layered disc which the present invention is intended particularly for is a phenomenon that occurs fast as compared to the operating speed of the DFB slicer as mentioned above, and moreover, its amplitude is as much as a few tens of percent of the amplitude of the read signal. Consequently, the method is likely to fall into a situation where a pseudo-edge is detected. 
   The Viterbi decoder can also handle the DC component variation in the read signal by causing a target signal level (or a target levels) of the Viterbi decoder to adaptively follow the read signal according to the level of the read signal. This technology is described in Japanese Patent Application Laid-open Publication No. Hei 11-296987. The technology must reduce the influence of short defects or the like that cannot be detected by a defect detecting mechanism, because the target levels for use in Viterbi decoding is allowed to follow the read signal. Accordingly, the length of integrating time of an integrator that determines the following speed of the targets must be such that the mechanism is not excessively sensitive to minute defects or the like. For this reason, an adaptive Viterbi decoder is also unsuitable for the handling of the local DC component variation such as the inter-layer interference. 
   An object of the present invention is to provide a read channel capable of reducing a local DC component variation such as inter-layer interference superimposed on a read signal, and thereby suppressing deterioration in performance of a PLL and a Viterbi decoder. 
   A read channel according to the present invention includes a means for detecting a local DC component. Moreover, the read channel determines the location of an edge of a read signal from a differential coefficient of the read signal, counts the number of edges occurring within a given interval, and calculates average DC amplitude using the number of edges. The read channel includes a means for preventing detection of a pseudo-edge at a long mark or space when determining the location of the edge from the differential coefficient of the read signal. Furthermore, the read channel includes a means for observing the state of a PLL, and controls the operation of a DC component detector according to the state of the PLL. Still furthermore, the read channel controls the operation of each element in the read channel according to the magnitude of DC component amplitude. 
   The present invention enables reducing deterioration in performance due to the local DC component variation in the read signal resulting from the inter-layer interference of the dual layered disc, fingerprints on the surface of the disc, or the like, thereby making it possible to provide an optical disc drive capable of reading with higher reliability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing an example of the configuration of a read channel according to the present invention. 
       FIG. 2  is a diagram showing the configuration of the read channel. 
       FIG. 3  is an illustration of assistance in explaining the principle of phase comparison. 
       FIG. 4  is an illustration of assistance in explaining the influence of a DC component superimposed on a read signal upon the phase comparison. 
       FIG. 5  is a graph of assistance in explaining the operation of a defect detector. 
       FIG. 6  is an illustration of a read signal affected by inter-layer interference. 
       FIG. 7  is a plot of assistance in explaining the principle of detection of the amplitude of an instantaneous DC component. 
       FIG. 8  is a diagram showing an example of the configuration of a DC component detector. 
       FIG. 9  is a diagram showing an example of the configuration of a read channel in which edge discrimination rate is improved by reducing inter symbol interference. 
       FIG. 10  is a graph showing the frequency characteristic of amplitude response of an equalizer for reducing the inter symbol interference. 
       FIGS. 11A and 11B  are illustrations of assistance in explaining the effect of the present invention. 
       FIG. 12  is a diagram showing a configuration in which a feedback method is used for DC component compensation. 
       FIG. 13  is a representation showing an example of wave distortion at the centers of a long mark and space. 
       FIG. 14  is a diagram showing an example of the configuration of a DC component detector designed to avoid erroneous edge determination in the long mark or space. 
       FIG. 15  is a diagram showing an example of a configuration with the addition of a hold function under given conditions. 
       FIG. 16  is a diagram of the configuration of an optical disc drive using the read channel according to the present invention. 
       FIG. 17  is a diagram of the configuration of a read channel digital unit. 
       FIG. 18  is a diagram of the configuration of an optical disc drive using the read channel according to the present invention. 
       FIG. 19  is a diagram showing an example of the configuration of a DC component detector in which an extrapolator is used to eliminate delay in DC component detection. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the present invention will be described below. The embodiment is intended for an optical disc drive. For the sake of simplicity, however, only parts which the present invention is intended directly for are shown and described, and other parts are omitted. 
     FIG. 1  shows an embodiment of the present invention.  FIG. 1  is a schematic block diagram of a read signal processing system of the optical disc drive which decodes an analog read signal into a bit stream.  FIG. 1  is given provided that the present invention is applied to a BD. 
   An analog read signal outputted by a pickup is fed to a high-pass filter  12 , which in turn removes almost all of DC components contained in the output from the pickup. After that, the analog read signal is subjected to waveform equalization by an analog equalizer  1  and is then converted into a digital signal by an AD converter  2 . Here, sampling is done in synchronization with a channel clock. Then, a DFB slicer  3  removes DC component which varies depending on the influence of asymmetry and data pattern of the read signal. The output from the DFB slicer is inputted to a DC component detector  8 . The DC component detector detects the amplitude of DC component remaining in the read signal. The configuration and operation of the DC component detector will be described later. The detected DC component is subtracted from the read signal by use of a subtracter  5 . Thereby, a varying DC component is removed from the read signal before phase comparison and decoding, if the DC component detector operates at sufficiently high speed. After the DC component has been removed from the read signal, the read signal is inputted to a phase comparator  6 , which in turn performs a phase comparison with a channel clock signal. An output from the phase comparator is inputted through a loop filter  9  to a DA converter  11 . The DA converter converts the digital signal into an analog voltage signal. This signal is used as a frequency control signal for a VCO  10 . The VCO oscillates at a frequency specified by the frequency control signal. After the DC component has been removed from the read signal, the read signal is inputted also to a Viterbi decoder  7 , which in turn decodes the read signal into a bit stream. 
   Description will now be given with regard to the DC component detector. DC component detection takes place utilizing the following: when the DC component is zero and the PLL is locked, phase error is zero or equivalently the midpoints of the edges coincides with the zero level, whereas when a local DC variation occurs in the read signal with the PLL locked, the midpoint of the edge lies outside the zero level. The constant capability of detecting the edges is required because the edge is used for the DC component detection. Generally, the edges are distinguished by detecting that the values of two consecutive points of the read signal are in opposite sign. When the amplitude of the DC component superimposed on the read signal is large, however, a situation arises where the values of two points adjacent to each other with a true edge in between are in the same sign and the values of two points adjacent to each other with a false edge in between are of opposite sign. This is the very thing that has a great adverse influence on the phase comparison and Viterbi decoding. 
   In this embodiment, a differential edge discrimination method is therefore used in order to locate the true edge even in the above situation. This method utilizes the fact that a time differential coefficient of the read signal is a maximum or minimum at the edge.  FIG. 7  is a plot of assistance in explaining the method. As previously mentioned, the read signal is expressed as x(n). Discrete time differential of x(n) is expressed as y(n) given by Equation (2). Hereinafter, a differential signal of the read signal refers simply to y(n) unless otherwise specified.
 
 y ( n )= x ( n )− x ( n− 1)  (2)
 
   As shown for example in  FIG. 7 , y(i) is the maximum or minimum, assuming that the edge lies between x(i−1) and x(i). Even when the varying DC component is superimposed on the read signal, it is obviously possible to locate the true edge by time differential if the rate of change of the DC component relative to time is sufficiently slow as compared to the channel clock. An instantaneous value d(n) of the DC component at the edge is the mean value of the values of the two points adjacent to each other with the edge in between, as represented by Equation (3).
 
 d ( i )={ x ( i− 1)+ x ( i )}/2  (3)
 
   It is necessary to compare the values of y(n−1), y(n) and y(n+1) at three consecutive times with one another in order to determine whether y(n) is the maximum or minimum. 
   Even if the edges are discriminated by using the differential edge discrimination and an instantaneous DC component value is obtained, it may possibly lead to a problem if it is simply subtracted from the read signal. In other words, the subtraction of the instantaneous value from the read signal is not proper, because the instantaneous value of the DC component takes a finite value even when a phase difference arises between the read signal and the channel clock. In the present embodiment, a moving average of instantaneous DC components is worked out and subtracted from the read signal in order to avoid the problem. 
     FIG. 8  shows the configuration of the DC component detector based on the above principle. Since a signal time in  FIG. 8  indicates the instant of differential edge decision and instantaneous DC component detection at time i, an input to the DC component detector is expressed as x(i+1) allowing for required delay. The output from the DFB slicer is an input signal to the DC component detector. This signal is herein called merely “read signal.” First, the read signal branches into two lines. The first line computes the amplitude of the DC component. In the first line, two 1 T delay circuits  4  are used to obtain x(i) and x(i−1), and an adder  14  and a multiplier  15  are used to find the mean value of x(i) and x(i−1) or equivalently obtain a candidate c(i) for the amplitude of the DC component. The term “candidate” is employed because the resulting value is not treated as the amplitude value unless judged as the true edge. 
   The second line dose the differential edge discrimination. First, the subtracter  5  is used to obtain the differential signal y(i+1) of the read signal from x(i+1) and x(i), which was used to compute the amplitude of the DC component. 1 T delay circuits  4  are used to obtain y(i) and y(i−1), based on y(i+1). y(i+1), y(i) and y(i−1) are inputted to a maximum/minimum detector  19 , which in turn determines whether or not y(i) is the maximum or minimum. The result p(i) of determination is true if y(i) is the maximum or minimum. The result p(i) of determination, together with the candidate c(i) for the amplitude of the DC component, is inputted to a decider  16 . If p(i) is true, the decider  16  outputs an instantaneous DC component d of the edge to a moving averager  17 . A general moving average is not proper for the moving averager  17  to perform a moving average computation. The reason is that the edge appearance frequency is a fraction of the frequency of occurrence of the channel clock and the edges occur at irregular intervals. For this reason, a counter  18  counts the number of edges occurring within an average interval length specified by L, and the moving averager  17  divides the sum of the amplitudes of the instantaneous DC components at the edges within the interval by the number N of edges, thereby determining the average amplitude of the DC component. 
   When the phase difference arises between the read signal and the channel clock, the influence of the phase difference on the DC component detection on adjacent edges are opposite sign. Thus, taking the average as described above makes it possible to reduce the influence of the phase difference on the DC component detection, even when the phase difference arises between the read signal and the channel clock. 
   Longer average interval length L yields less susceptibility to noise or the like. However, if the average interval length L is too long, it produces the side effect of causing computing error due excess delay between the DC component and the output from the DFB slicer at the time of subtraction of the DC component from the output from the DFB slicer. An average interval length L of 100 to 1000 T is proper for handling the inter-layer interference in BD. 
   In  FIG. 1 , the result of subtraction of the DC component from the read signal is inputted as it is to the Viterbi decoder. Obviously, however, an adaptive equalizer may be inserted immediately before the Viterbi decoder to improve decoding performance. 
   In the BD, errors in differential edge discrimination is more likely to occur due to the influence of inter symbol interference in a place where a 2 T symbol that is a shortest symbol is adjacent to a 4 T or longer symbol. To reduce the likelihood of the differential edge decision error, it should be equalized prior to the differential edge decision so as to reduce the inter symbol interference.  FIG. 9  shows a configuration in which this equalization is implemented. An FIR (finite impulse response) equalizer  20  is inserted immediately before the DFB slicer to perform equalization so as to reduce the inter symbol interference.  FIG. 10  shows the equalization characteristics of the FIR equalizer. In this instance, moreover, an adaptive equalizer  21  is inserted immediately before the Viterbi decoder  7  to enhance the decoding performance. 
     FIGS. 11A and 11B  are illustrations of assistance in explaining the effect of the configuration shown in  FIG. 9 . A BD dual layered disc having a surface recording capacity of 25 gigabytes was used.  FIGS. 11A and 11B  each show a read signal wave processed by a signal processing simulator, the read signal wave being read from the disc in a place where the influence of inter-layer interference manifests itself. The read signal wave shown in  FIG. 11A  is the result obtained by a conventional channel, and the read signal is disturbed between the vicinity of time A and the vicinity of time D. Specifically, in an interval between the times A and B, the amplitude decreases sharply, and subsequently a large positive DC component is superimposed on the signal, and thus, in some period, a top envelope lies beyond a processing range. In an interval between the time B and the vicinity of time C, a negative DC component is superimposed on the signal, and its amplitude is as much as about 20% of the amplitude of the read signal. In an interval between the vicinity of the time C and the vicinity of the time D, another positive DC component, although small, is superimposed on the signal. The signal wave shown in  FIG. 11B  is that processed by a system shown in  FIG. 9 . Incidentally, both the signal waves shown in  FIGS. 11A and 11B  are those immediately before entering the adaptive equalizer. The signal processing simulator has the function of decoding an error correcting code and displaying an error location. Since a continuous line is used to display the error location, a burst error location is displayed as solidly shaded, as shown in the interval between the times A and B in  FIG. 11B . An error is not displayed in  FIG. 11A , because the inter-layer interference had a significant influence, thus caused a read error, and thus made it impossible to acquire error location information that should have been obtained through error correction. 
   As is apparent from comparison of the intervals between the times B and C shown in  FIGS. 11A and 11B , the negative DC component is substantially eliminated from the signal wave of  FIG. 11B  although it is observed in the signal wave of  FIG. 11A , and moreover, no error occurs in the signal wave of  FIG. 11B . The same goes for the interval between the times C and D. As for the interval between the times A and B, the effect of improvement is not achieved because of the decrease in the amplitude and the superimposed DC component was beyond the processing range. 
   It is also possible to achieve the same effect with a configuration in which a feedback method is used to subtract the detected DC component from the read signal. In an example shown in  FIG. 12 , a feedback loop is configured in parallel with a loop of the DFB slicer in order to subtract outputs from the DC component detector  8  and a DFB circuit  22  from an output from the AD converter  2 . As employed herein, the DFB circuit refers to every circuit element that constitutes the DFB slicer, except for the adder (or the subtracter). 
   A long space or mark in the read signal can possibly be concave or convex at its center, as shown in  FIG. 13 . This concavity or convexity can possibly appear according to the shape of the mark on the disc or the conditions of equalization. The presence of the concavity or convexity can possibly lead to erroneous edge discriminations, because the differential signal of the read signal exhibit a maximum or minimum in the vicinity of the center of the mark or space. However, the absolute value of the differential coefficient is small because the concavity or the like is gentle in form. The maximum and minimum having a small absolute value can be therefore ignored to avoid the erroneous edge discriminations.  FIG. 14  shows the configuration of the DC component detector in which this function is implemented. 
   Its configuration is that the above function is added to the DC component detector shown in  FIG. 8 . Specifically, the maximum and minimum values to be off target for the edge determination are predetermined, and an absolute value h thereof is placed in an absolute value comparator  24 . The differential y(i) of the read signal at a point targeted for the edge determination is fed to the other input of the absolute value comparator  24 . An output from the absolute value comparator  24  is a logical signal, and the absolute value comparator  24  makes a comparison between the absolute values and outputs “true” if y(i) is more than h. This output is inputted to one terminal of an AND circuit  25 . An output from the maximum/minimum detector  19  is inputted to the other terminal of the AND circuit  25 . An output from the AND circuit  25  is the result of the edge determination. Other operations are the same as shown for example in  FIG. 8 . 
   The PLL is required to be in a locked state for detecting the DC component. It is therefore necessary to automatically control the operation of the DC component detector according to the state of the PLL. A situation such that the amplitude of the varying DC component superimposed on the read signal is significantly large, thus the DC component cannot be fully compensated, as shown for example in  FIGS. 11A and 11B , can also possibly arise In such a situation, the operation of each element of a read channel can be temporarily held to reduce deterioration in performance. 
     FIG. 15  shows a configuration with the addition of the function of automatically controlling the operation of the DC component detector according to the state of the PLL, and also holding the operation of some elements of the read channel during detection of the DC component above a specified level. Although the configuration shown in  FIG. 15  is based on the configuration shown in  FIG. 9 , the subject matter of this embodiment may be likewise applied to the configurations shown in  FIGS. 1 and 12 . 
   The state of the PLL is monitored by a PLL lock monitor  23 . Since circuit for observing the state of the PLL is well known in the art, detailed description is not given herein with regard to the configuration and operation thereof. The PLL lock monitor determines whether or not the PLL is locked, and the DC component detector  8  operates under control of an output from the PLL lock monitor. Specifically, if the PLL is in its locked state, the operation for DC component detection is enabled to compensate the DC component of the read signal, or if the PLL is not locked, the operation for DC component detection is held so that an output from the DC component detector  8  becomes zero. 
   If the DC component detector  8  detects the DC component above the predetermined specified level, the DC component detector  8  outputs a hold signal while the value of the DC component exceeds the specified level. The hold signal is inputted to the following elements: the phase comparator  6 , the adaptive equalizer  21  and the Viterbi decoder  7 . While receiving the hold signal, the phase comparator  6  produces outputs, all of which are zeros. Thereby, the oscillation frequency of the VCO  10  is fixed during that time. This enables avoiding an erroneous phase comparison resulting from inadequate compensation due to extreme DC components. 
   While receiving the hold signal, the adaptive equalizer  21  holds its adaptive operation and maintains the tap coefficients immediately before receiving the hold signal. This enables preventing abnormal coefficient learning. In this instance, the Viterbi decoder  7  uses an adaptive Viterbi method, and thus, while receiving the hold signal, the Viterbi decoder  7  also holds its adaptive operation in order to prevent abnormal target learning, as in the case of the adaptive equalizer. 
     FIG. 16  shows, in schematic form, the configuration of an optical disc drive implemented according to the present invention. Only parts requiring description in connection with the present invention are shown in  FIG. 16 , and other parts are omitted therefrom. Also as for details of parts, description of parts that can be readily understood by those skilled in the art is omitted. 
   Firstly, the operation of the optical disc drive will be described in outline. Information recorded on an optical disc  101  is optically read out and converted into an electric signal (or a read signal) by a pickup  102 . The read signal is amplified by an AFE-IC (analog front end integrated circuit)  103  to reach given amplitude and is equalized by a built-in analog equalizer within the AFE-IC, and then the read signal is inputted to a read channel digital unit  105  in a DSP (digital signal processor)  104 . As described with reference to the above embodiment, the read signal is decoded into a bit stream by the read channel digital unit. In the present embodiment, the optical disc drive is configured using two types of LSI (large-scale integrated) circuits: the AFE-IC including integration of almost all analog circuits of the optical disc drive and the DSP including integration of almost all digital circuits thereof. The configuration using a combination of the LSI circuits in this manner is generally used for the optical disc drive, and in this case, the analog equalizer is typically integrated in the AFE-IC. As employed herein, the read channel digital unit refers to a section of the read channel shown in  FIG. 1 , following after the AD converter and formed mainly of digital circuits, as shown in  FIG. 17 . 
   The resultant bit stream is subjected to error correction by an ECC (error correction code) decoder  106  to yield user data. The resultant user data goes out of the drive through an interface circuit  107 . The use of the read channel capable of handling a local DC component variation occurring in the read signal brings about an improvement in reading performance of the dual layered disc or the disc having fingerprints thereon. 
     FIG. 18  is a schematic illustration of the configuration of an optical disc drive using the read channel of a type shown in  FIG. 15 . The operation of the optical disc drive will be described in outline. Information recorded on the optical disc  101  is optically read out and converted into an electric signal (or a read signal) by the pickup  102 . The read signal is amplified by the AFE-IC  103  to reach given amplitude and is equalized by the analog equalizer contained within the AFE-IC, and then the read signal is inputted to a monitor-equipped read channel digital unit  109  in the DSP  104 . As described with reference to the above embodiment, the read signal is decoded into a bit stream by the monitor-equipped read channel digital unit. As employed herein, the monitor-equipped read channel digital unit refers to a section of the read channel shown in  FIG. 1 , following after the AD converter and formed mainly of digital circuits, as shown in  FIG. 15 . 
   The resultant bit stream is subjected to error correction by the ECC decoder  106  to yield user data. The resultant user data goes out of the drive through the interface circuit  107 . 
   The read channel, as used in this example, is characterized by being provided with two types of signals: the signal indicative of the state of the PLL; and the hold signal which the DC component detector outputs while detecting the DC component of given or greater amplitude, as described for  FIG. 15 . Under normal conditions, these signals are used to control the operation of each element in the read channel, as previously mentioned. In the example, the signals can be referred to by firmware  108 . Thereby, the firmware  108  can be informed of the occurrence of trouble during reading. In the example, if a read error occurs while a block of data is read, the PLL state signal is monitored to see whether or not the PLL has fallen into its unlocked state while the block of data is read. If the PLL is in its unlocked state, the firmware  108  tries avoiding the unlocked state by placing PLL parameters in the DSP so as to temporarily increase or decrease a time constant of the PLL. 
   The instantaneous value of the DC component which the DC component detector obtains using the differential edge discrimination, as shown in  FIG. 8 , has an error due to the influence of noise present in the read signal. The instantaneous value of the DC component can also possibly be the erroneous value due to the erroneous edge determination under the influence of the inter symbol interference. To reduce these influences, the DC component detector determines an average DC component, using the moving average or integral of the instantaneous value of the DC component determined by the differential edge discrimination. The average DC component determined in this manner, of course, lags behind the read signal. To enhance the effect of averaging, it is necessary to lengthen an interval targeted for averaging. Clearly, a large amount of delay, however, affects the subtraction of the determined DC component from the read signal, which is therefore unlikely to achieve its full effect. 
     FIG. 19  shows, in schematic form, the configuration of a DC component detector capable of avoiding the above problem. The configuration leading to an average DC component value is the same as shown in  FIG. 8 . The configuration shown in  FIG. 19  is different from that shown in  FIG. 8  in that an output from the moving averager is further inputted to an extrapolator  201  so that a final output from the DC component detector is an output from the extrapolator. The extrapolator estimates a value at a time after the latest input time by extrapolation based on past values. Since the extrapolator is widely used for a signal processing circuit, detailed description is not given herein with regard to the configuration thereof and so on. In the present case, the delay between the average DC component and the read signal as mentioned above can be substantially eliminated by determining a value at a time after averaging time L, because the main cause of delay is averaging. 
   The present invention relates to a read channel mainly for use in an optical disc drive. The present invention is effective particularly for a drive for BD and HDDVD media having two or more recording layers. The present invention may be applied to a perpendicular magnetic recording hard disc drive or the like including a channel for reading out a signal having the same or similar problems, besides the optical disc drive.