Patent Publication Number: US-6992964-B2

Title: Information reproducing device and information reproducing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-064354, filed Mar. 8, 2002, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an information reproducing device, and more particularly to an information reproducing device and an information reproducing method which perform decoding processing by using a PRML (Partial Response and Maximum Likelihood) method. 
   2. Description of the Related Art 
   Recently, there has been widely spread an information recording/reproducing device that performs a recording and reproducing process to a storage medium, e.g., an optical disk such as a DVD (Digital Versatile Disc), and the higher recording density with various methods has been demanded. With respect to this, for example, as a method of recording and reproducing process of an optical disk or the like, there is a PRML method. 
   That is, if the recording density becomes higher at the time of recording/reproducing information on a storage medium, a mark with a desired size can not be recorded due to an affect of the thermal interference when recording a shortest mark or a shortest space in a recording mode. Further, in reproduction, since the recorded shortest mark or shortest space is small, the waveform interference further affects, and an amplitude corresponding to the shortest mark/space becomes consequently small. That is, the shortest mark/space which is smaller than the desired size is formed in recording, and the amplitude corresponding to the shortest mark becomes small due to the affect of the waveform interference in the reproduction mode. 
   On the other hand, in a Viterbi algorithm which is the PRML method, the linearity is not necessarily demanded to the shortest mark/shortest space. Giving explanation taking an RLL ( 1 , k) modulation method as an example, the shortest mark/shortest space is  2 T in this modulation method.  1 T is not allowed in a state transition diagram of  FIG. 6  of the Viterbi algorithm determined by the modulation method. That is, a  2 T signal level does not necessarily have to match with a  2 T ideal signal level, and allows identification. 
   As a cited reference disclosing this PRML method, there is Jpn. Pat. Appln. KOKAI Publication No. 6-4810, in which the convolution integration of a decoding result and a PR (Partial Response) class is carried out in order to obtain an ideal waveform, an equalization error between the ideal waveform and an equalizer output is calculated and reproduction signal processing parameters (for example, a tap coefficient, a gain, an offset and the like of an equalizer) are changed by using an LMS (Least Mean Square) algorithm so as to minimize the equalization error. 
   When the recording density is improved, however, the shortest mark/space is generally hard to be formed, and in fact the shortest mark/space is formed with a smaller length than the desired length. According to the state transition diagram of a Viterbi decoder in  FIG. 6 , however, since a code of a length not more than the shortest mark/space is not admitted, a decoding error hardly occurs even if a value of a level corresponding to the shortest mark/space among levels of an actual reproduction signal is shifted in a direction to reduce the amplitude as compared with the ideal waveform. 
   Thus, as the recording density is increased, for example, when a tap coefficient of the equalizer is calculated by using the LMS algorithm so as to minimize the above-described equalization error, there is disadvantageously outputted an equalization coefficient amplified on a higher frequency side than the equalization coefficient of the equalizer which is required in the Viterbi decoder. As a result, the noise is amplified more than necessary, and the decoding error can not be consequently satisfactorily reduced. 
   That is, in this prior art, even if the recording density is increased and the amplitude of a response waveform corresponding to the shortest mark/shortest space becomes small, there is generated a tap coefficient of an FIR (Finite Impulse Response) filter having an equalizer characteristic which forcibly increases the gain on the high frequency side so as to increase the amplitude of the shortest mark/shortest space after equalization. 
   As a result, since equalization close to the  2 T signal level of the ideal waveform is performed beyond the  2 T signal level which is required in the Viterbi decoding processing, there is generated the equalization coefficient having the characteristic of the gain larger than that required in the Viterbi decoding processing in a high frequency band. There is, therefore, a problem that the noise is unnecessarily amplified and the decoding error can not be satisfactorily reduced. 
   BRIEF SUMMARY OF THE INVENTION 
   It is an embodiment of the present invention to provide an information reproducing device which can stably reproduce information recorded with the high density by optimizing reproduction processing parameters such as a filter coefficient of an equalizer in order to obtain an optimum equalization waveform demanded in Viterbi decoding processing. 
   To achieve this aim, according to one embodiment of the present invention, there is provided an information reproducing device which reproduces information recorded on a storage medium, comprising: detecting section for detecting information recorded on the storage medium and outputting a detection signal; converting section for converting the detection signal outputted by the detecting section into a digital signal; correcting section for correcting the digital signal converted by the converting section in accordance with a parameter; equalizing section for applying partial response equalization processing to the corrected digital signal corrected by the correcting section based on a predetermined coefficient and outputting an equalization signal; maximum likelihood decoding section for applying maximum likelihood decoding processing to the equalization signal outputted by the equalizing section based on a reference level and outputting a decoding signal; an ideal waveform generating section for generating and outputting an ideal waveform signal in accordance with the equalization signal outputted from the equalizing means; a target waveform generating section for generating and outputting a target waveform signal which can be a target of the equalizing section by changing at least one level in respective levels of the ideal waveform signal outputted by the ideal waveform generating means; and optimizing section for calculating an error between the target waveform signal outputted by the target waveform generating section and the equalization signal outputted by the equalizing section and optimizing at least one of the parameter of the correcting means, the predetermined coefficient of the equalizing section and the reference level of the maximum likelihood decoding section so as to minimize the error. 
   The present invention generates and outputs a target waveform signal T which can be a target of the equalizing section by changing a level corresponding to a shortest mark/space in a plurality of levels of an ideal waveform signal, for example, as shown in  FIG. 5 . Furthermore, by optimizing reproduction processing parameters such as a tap coefficient of the FIR filter by using a target waveform signal T, it is possible to eliminate a problem of unnecessary amplification of the noise and occurrence of a decoding error by performing equalization close to a  2 T signal level of the ideal waveform beyond the  2 T signal level required in the Viterbi decoding processing, which has appeared in the prior art information reproducing device. 
   It is to be noted that a target of optimization in the present invention is the reproduction processing parameters and it is not restricted to the tap coefficient of the FIR filter. For example, parameters or the like such as a reference level of the Viterbi decoding processing, an amplitude or an offset value of a digital signal detected from a storage medium can be a target. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a block diagram showing an example of a primary part of an information reproducing device according to one embodiment of the present invention; 
       FIG. 2  is a block diagram showing an example of the information reproducing device according to one embodiment of the present invention; 
       FIG. 3  is a graph showing an example of a judged bit series, a response waveform of PR ( 1 ,  2 ,  2 ,  1 ) and an actually measured waveform of the information reproducing device according to one embodiment of the present invention; 
       FIG. 4  is a block diagram showing a structure of an example of a target waveform generator of the information reproducing device according to one embodiment of the present invention; 
       FIG. 5  is a flowchart showing an example of a target waveform generation method of the information reproducing device according to one embodiment of the present invention; 
       FIG. 6  is a view showing a state transition in a RLL modulation method and a PR ( 1 ,  2 ,  2 ,  1 ) ML decoding method of maximum likelihood decoding processing ( 1 ,  7 ) according to one embodiment of the present invention; 
       FIG. 7  is a block diagram for illustrating a method of determining tap coefficients of an FIR filter of the information reproducing device according to one embodiment of the present invention; 
       FIG. 8  is a block diagram for illustrating a method of determining Viterbi reference levels of the information reproducing device according to one embodiment of the present invention; 
       FIG. 9  is a block diagram for illustrating a method of controlling an ACG gain and an offset value of the information reproducing device according to one embodiment of the present invention; 
       FIG. 10  is a block diagram showing an example of a target waveform generator of an information reproducing device according to a second embodiment of the present invention; and 
       FIG. 11  is a flowchart showing an example of a target waveform generation method of the information reproducing device according to the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An information reproducing device and an information reproducing method according to one embodiment of the present invention will be described in detail hereinafter taking an optical disk recording/reproducing device as an example with reference to the accompanying drawings. Incidentally, although description is given as to an example of the optical disk recording/reproducing device in this embodiment, a target storage medium is not restricted to the optical disk, and the equivalent effects and advantages can be produced based on the equivalent principle as long as it is a storage medium, e.g., a magneto-optical disk. 
   &lt;First Embodiment&gt; 
   A first embodiment provides an information reproducing device and an information reproducing method with decoding errors reduced by generating a target waveform signal T and optimizing reproduction processing parameters in accordance with the generated signal. 
   &lt;Structure of Optical Disk Device According to Invention&gt; 
     FIG. 1  is a block diagram showing an example of a primary part of an information reproducing device according to one embodiment of the present invention, and  FIG. 2  is a block diagram showing an example of its entire structure. 
   (Basic Structure of Optical Disk Device) 
   In  FIG. 2 , an optical disk device A according to one embodiment of the present invention performs data recording or data reproduction with respect to an optical disk D. The optical disk device A has a tray  32  which carries the optical disk D accommodated in a disk cartridge, a motor  33  which drives this tray, a clamper  34  which holds the optical disk D, and a spindle motor  35  which rotates the optical disk D held by the clamper  34  at a predetermined number of revolutions. Moreover, a CPU  46  which executes the entire operation control as a control portion, a ROM  47  which stores therein a basic program or the like for this control operation and a RAM  48  which rewritably stores each control program or application data and the like are connected to each other through a control bus. In addition, a feed motor  36  which carries a pick up PU, a focus/tracking actuator driver/feed motor driver  40  which performs focusing or tracking control of the pick up, a spindle motor driver  41  which drives the spindle motor  35 , and a tray motor driver  42  which drives the tray motor are respectively provided and connected to a control portion such as the CPU  46 . 
   Additionally, there are provided a pre-amplifier  12  which is connected to a pick up head PUH and amplifies a detection signal and a servo seek control unit  39  which supplies a seek signal used to perform a seek operation to the driver. Further, there are provided a data processing unit  1  which is connected to the pick up head PUH, the pre-amplifier  12 , the servo seek control unit  39  and others, and a RAM  43  which stores therein data used for various kinds of processing. In order to transmit/receive a signal from the data processing unit  1  with respect to an external device, an interface control portion  45  is provided together with a RAM  44 . 
   In such an optical disk device, the present invention supplies an optimum equalization signal demanded by Viterbi decoding processing by providing the data processing unit  1  including such a structure as shown in  FIG. 1 . That is, a target waveform signal T is generated from an ideal signal I, and parameters for reproduction processing such as a tap coefficient of an FIR filter are optimized based on the generated signal. As a result, it is possible to eliminate a problem of unnecessary amplification of the noise and occurrence of a decoding error by performing equalization close to a  2 T signal level of an ideal waveform beyond the  2 T signal level required in the Viterbi decoding processing which appears in the prior art device. 
   (Basic Operation of Optical Disk Device) 
   The optical disk device provided in the embodiment of the present invention having such a structure performs reproduction processing and recording processing of an optical disk as described below. That is, when the optical disk D is loaded in the optical disk device A, control information of the optical disk D recorded in a control data zone in an embossed data zone in a read-in area of the optical disk D is read and supplied to the CPU  46 . 
   In the optical disk A according to one embodiment of the present invention, energization is performed by a non-illustrated laser control unit and a laser beam is generated under control of the CPU  46  based on operation information by an operation of a user, the control information of the optical disk D recorded in the control data zone in the optical disk, a current status and others. 
   The generated laser beam is converged by an object lens  31  and a recording area of the disk is irradiated with this laser beam. As a result, data is recorded in the recording area of the optical disk D (generation of a mark row: data is recorded on the optical disk D based on a distance between marks with variable lengths and a length of each mark with a variable length), or the light having the intensity corresponding to the stored data is reflected and this reflection is detected, thereby reproducing this data. 
   In  FIG. 2 , although settings of the laser control unit included in the pick up head are set by the data processing unit  1 , the settings vary depending on a reproduction power to obtain a reproduction signal RF, a recording power to record data and an erasing power to erase data. The laser beam has powers of different levels, i.e., the reproduction power, the recording power and the erasing power, and the semiconductor laser unit is energized by the laser control unit so as to generate a laser beam having each power. 
   This laser control unit is constituted by a non-illustrated resistance and a transistor, and a power supply voltage is applied to the resistance, the transistor and a semiconductor laser as the semiconductor laser unit. As a result, an amplitude factor differs depending on a base current of the transistor, a different current flows through a semiconductor laser oscillator, and a laser beam with a different intensity is generated. Here, recording waveform compensation is carried out in accordance with a characteristic of each optical disk, a laser power is generated in accordance with a recording waveform pulse W outputted from a recording waveform generation circuit  11 , and recording processing is performed to the optical disk. 
   Furthermore, the optical disk D is carried into the device by the tray  32  directly or after being accommodated in a disk cartridge in such a manner that the optical disk D is arranged so as to be opposed to the object lens  31 . A tray motor  33  used to drive the tray  32  is provided in the device. Moreover, the loaded optical disk D is rotatably held on the spindle motor  35  by the clamper  34 , and rotated at a predetermined number of revolutions by the spindle motor  35 . 
   The pick up head PUH has therein a photo-detector (not shown) which detects a laser beam. This photo-detector detects a laser beam which has been reflected on the optical disk D and returned through the object lens  31 . A detection signal from the photo-detector (current signal) is converted into a voltage signal by a current/voltage converter (I/V), and this signal is supplied to the pre-amplifier  12  and the servo amplifier  34 . A signal for reproducing data in a header portion and a signal for reproducing data in the recording area are outputted from the pre-amplifier  12  to the data processing unit  1 . A servo signal (a track error signal, a focus error signal) from the servo amplifier  34  is outputted to the servo seek control unit  39 . 
   Here, as a method of optically detecting a quantity of focusing displacement, there are the following astigmatic method, a knife edge method and the like, for example. 
   That is, the astigmatic method is a method which arranges an optical element (not shown) which generates astigmatism in a detection light path of a laser beam reflected on a light reflection film layer or a light reflective recording film of the optical disk D and detects a change in shape of the laser beam with which the photo-detector is irradiated. A light detection area is diagonally divided into four. A difference in diagonal sum is taken with respect to the detection signal obtained from each detection area in the servo seek control unit  39 , and a focus error detection signal (focus signal) is obtained. 
   The knife edge method is a method which arranges a knife edge which partially asymmetrically prevents a laser beam reflected on the optical disk D. The light detection area is divided into two, and a difference between detection signals obtained from the respective detection areas is taken, thereby obtaining a focus error detection signal. 
   Usually, either the astigmatic method or the knife edge method is adopted. 
   The optical disk D has a spiral or concentric track, and information is recorded on the track. A condensing spot is traced along this track, and reproduction or recording/erasing of information is carried out. In order to stably trace the condensing spot along the track, a relative positional displacement of the track and the condensing spot must be optically detected. 
   As a track displacement detection method, there are generally the following differential phase detection method, the push-pull method, the twin-spot method and others. 
   The differential phase detection method detects a change in intensity distribution of a laser beam on the photo-detector, which has been reflected on the light reflection film layer or the light reflective recording film of the optical disk D. The light detection area is diagonally divided into four. A phase difference between diagonal sums is taken in the servo seek control unit  39  with respect to the detection signal obtained from each detection area, and a track error detection signal (tracking signal) is obtained. 
   In the push-pull method, namely, in this method, a change in intensity distribution of a laser beam on the photo-detector, which has been reflected on the optical disk D, is detected. The light detection area is divided into two, a difference between detection signals obtained from the respective detection areas is taken, and a track error signal is obtained. 
   The twin-spot method performs wavefront division of the light into a plurality of beams by arranging a diffraction element or the like in a light sending system between the semiconductor laser element and the optical disk D, and detects a change in quantity of reflected light of ±1st diffraction ray with which the optical disk D is irradiated. A light detection area in which a quantity of reflected light of +1st diffraction ray and a quantity of reflected light of −1st diffraction ray are individually detected is arranged separately from the light detection area for detection a reproduction signal, and a difference between the respective detection signals is taken, thereby obtaining a track error signal. 
   By such focus control and track control, the focus signal, the tracking signal and the feed signal are supplied from the servo seek control unit  39  to the focus and tracking actuator driver and the feed motor driver  40 , and the object lens  31  is subjected to focus servo control and the tracking servo control by the driver  40 . In addition, an energization signal is supplied from the driver  40  to the feed motor  36  in accordance with an access signal, and the pick up head PUH is subjected to carriage control. 
   Additionally, the servo seek control unit  39  is controlled by the data processing unit  1 . For example, the access signal is supplied from the data processing unit  1  to the servo seek control unit  39 , and the feed signal is generated. 
   Further, the spindle motor driver  41  and the tray motor driver  42  are controlled based on a control signal from the data processing unit  1 , the spindle motor  35  and the tray motor  33  are energized, the spindle motor  35  is rotated at a predetermined number of revolutions, and the tray motor  33  can appropriately control the tray. 
   A reproduction signal RF corresponding to data of the header portion supplied to the data processing unit  1  is fed to the CPU  46 . A sector number as an address of the header portion is judged based on the reproduction signal RF and compared with a sector number as an address to be accessed (data is recorded or recorded data is reproduced). 
   As to the reproduction signal RF corresponding to data in the recording area supplied to the data processing unit  1 , necessary data is stored in the RAM  48 , the reproduction signal RF is processed in the data processing unit  1  and supplied to the interface control portion  45 , and a reproduction processing signal is supplied to, e.g., an external device such as a personal computer. 
   &lt;Viterbi Decoding Processing Involving Optimization of Reproduction Processing Parameters According to Present Invention&gt; 
   Optimization processing of reproduction processing parameters which is a characteristic of the present invention will now be described in detail hereinafter with reference to the accompanying drawings.  FIG. 1  is a block diagram of an optical disk device which is a first embodiment according to one embodiment of the present invention;  FIG. 3 , a graph showing a judged bit series D 1 , a response waveform I of a PR ( 1 ,  2 ,  2 ,  1 ) and an example D 2  of an actually measured waveform;  FIG. 4 , a block diagram showing a structure of an example of a target waveform generator;  FIG. 5 , a flowchart showing an example of a target waveform generation method of an information reproduction device according to one embodiment of the present invention;  FIG. 7 , a block diagram for illustrating a method of determining a tap coefficient of an FIR filter of the information reproducing device;  FIG. 8  is a block diagram for illustrating a method of determining Viterbi reference levels; and  FIG. 9 , a block diagram for illustrating a method of controlling an AGC gain and an offset value. 
   A primary part of the information reproducing device according to one embodiment of the present invention shown in  FIG. 1  illustrates a part of a structure of the data processing unit  1  and its periphery in  FIG. 2 . The description of the structure common to  FIG. 2  is omitted, and only a structure inherent to  FIG. 1  will be explained hereinafter. That is, the pick up head PUH has an object lens OL and drives it by an object lens actuator  11  upon receiving a drive signal from a servo block  13 . The data processing unit  1  has an A/D converter  14  which converts a reproduction signal RF which has been detected from the pick up head PUH and transmitted through a pre-amplifier  12  into a digital signal, and further includes an AGC (Automatic Gain Controller)  16  which controls a gain or the like of the converter, an FIR filter  17  as a transversal filter which is connected to the AGC  16  and performs equalization processing, and a Viterbi decoder  18  which receives an equalization signal P from the FIR filter  17 . Further, the data processing unit  1  has an ideal waveform generator  19  used to generate an ideal waveform signal I upon receiving a decoding signal d from the Viterbi decoder  18 , a target waveform generator  20  which receives the ideal waveform signal I from the ideal waveform generator  19  and generates a target waveform signal T based on the received signal, and a parameter control portion  15  used to adapt reproduction processing parameters characteristic of the present invention based on the target waveform signal T, and it is subjected to the operation control by the CPU  46  shown in  FIG. 1 . 
   In this embodiment, the target waveform signal T including the non-linearity corresponding to the reproduction signal RF is defined from the ideal waveform signal T, and the reproduction processing parameters such as tap coefficients C 0  to C N  of the FIR filter  17  are obtained based on the LMS algorithm by using a difference between the target waveform signal T and the reproduction equalization signal. 
   (Method of Generating Target Waveform Signal T) 
   A method of generating the target waveform signal T which is characteristic of the present invention will now be described hereinafter. 
   First of all,  FIG. 4  shows structures of the ideal waveform generator  19  and the target waveform generator  20 . The ideal waveform generator  19  has a plurality of delay circuits  51  which receive a decoding signal d from the Viterbi decoder  18 , a plurality of amplifier circuits  52  provided to be connected to these delay circuits  51 , and an adder  53  which receives outputs from these amplifier circuits  52 . 
   Further, the target waveform generator  20  has a level selector  54  which receives an ideal waveform signal I from the ideal waveform generator  19 , and a level shifter  55  which receives level signals of a level  2  and a level  4  corresponding to an amplitude of a shortest mark/space of the level sector  54 , for example. 
   With such a structure, the ideal waveform generator  19  generates the ideal waveform signal I by the convolution integral of the decoding signal d from the Viterbi decoder  18  and a PR ( 1 ,  2 ,  2 ,  1 ) characteristic, and supplies it to the target waveform generator  20 . 
   In the target waveform generator  20 , the level sector  54  judges the level  2  and the level  4  corresponding to the amplitude of the shortest mark/space of the ideal waveform signal I. If it is determined that the ideal waveform signal I is on the level  2  and level  4 , the level is changed in accordance with the later-described arithmetic expression in order to generate the target waveform signal T. 
   Here, a method of generating the target waveform signal T including the non-linearity corresponding to the reproduction signal RF will now be described in detail hereinafter in conjunction with a flowchart of  FIG. 5 . Description will be given on an example where the ( 1 ,  7 ) modulation method is used as a modulation method for data to be recorded and the PR ( 1 ,  2 ,  2 ,  1 ) ML decoding method is used as a decoding method. It is to be noted that the information recording/reproducing device using a combination of recorded data based on another modulation method and another decoding method also demonstrates the same effects and advantages within the scope of the present invention. 
   The decoding signal d is first supplied from the Viterbi decoder  18 , and the convolution integral of this decoding signal d and the PR ( 1 ,  2 ,  2 ,  1 ) class is carried out, thereby generating the ideal waveform signal I (S 11 ). The level judgment of the ideal waveform signal I is executed by the level selector  54  of the target waveform generator  20  (S 12 ). Here, the ideal waveform signal I is divided into seven levels from the level  0  to the level  6  (S 13 ). Among these levels, the level  2  and the level  4  correspond to the shortest mark/space  2 T.  1 T does not exist in the state transition drawing of the Viterbi decoder even if the amplitude corresponding to the shortest mark/space is small. Therefore, a bit error is not generated even if the amplitude is one close to the level  3 . 
   As shown in (a) in a graph of  FIG. 3 , the target waveform signal T is generated from the ideal waveform signal I (signal D 1  is a corresponding original data bit). That is, in (a) of  FIG. 3 , with respect to the levels corresponding to the level  2  and the level  4  in the ideal waveform signal I (a solid line and a broken line), the level of the target waveform signal T is changed to the level  3  side, and the target waveform signal T (solid line) is generated (S 14 , S 15 ). That is, the waveform of the level corresponding to the shortest mark/space is changed so as to be close to the center of the ideal waveform, thereby generating the target waveform signal T. 
   Representing this processing by numerical expressions, the following expressions can be obtained:
 
 Lv   12 ′= Lv   12 +α( Lv   13 − Lv   12 )
 
 Lv   14 ′= Lv   14 −α( Lv   14 − Lv   13 )
 
Lv 12 ′ and Lv 14 ′ denote levels corresponding to the shortest mark/space of the target waveform signal T, and Lv 12  and Lv 14  designate levels corresponding to the shortest mark/space of the ideal waveform signal I. Here, α is a coefficient satisfying 0≦α≦1. In this manner, the target waveform signal T is generated and outputted (S 16 ). Furthermore, (b) in  FIG. 3  shows an actually measured waveform D 2 .
 
   (Method of Controlling Reproduction Processing Parameters) 
   Description will now be given as to a method of controlling reproduction processing parameters by using the thus obtained target waveform signal T obtained as described above. 
   1. Method of Controlling Tap Coefficients of FIR Filter 
   A method of controlling the FIR filter  17  using the target waveform signal T will now be described. Here, description will be given as to a case where the FIR filter is a filter having seven taps and respective tap coefficients are C 0  to C 6  for the simplicity. 
   A parameter control portion  15  performs the control of reproduction processing parameters which is characteristic of the present invention and, as shown in  FIG. 7 , it has a difference circuit  62  which receives the target waveform signal T from the target waveform generation portion  20  and receives an output from the delay circuit  61  which accepts an equalization signal P from the FIR filter  17 , and average value calculation portions  64  corresponding to the number of lines which receive synthetic signals of the reproduction signals RF delayed by a plurality of delay circuits  63  which gradually delay outputs from the difference circuit  62  and calculate average values of the received signals. Moreover, it has amplifier circuits  65  corresponding to the number of lines which respectively receive control signals from the CPU  46  and amplify them, and memories  66  corresponding to the number of lines which receive outputs from the amplifier circuits  65  as tap coefficients C 0  to C N  corresponding to the number of lines and store them. 
   In addition, as shown in  FIG. 7 , the FIR filter  17  receives the tap coefficients C 0  to C N  from the delay circuits  57  corresponding to the number of lines which receive the reproduction signals RF and sequentially apply predetermined delay to them and the parameter control portion  15 , adequately amplifies the delayed reproduction signals FF in accordance with these coefficients, adds them in the adder, and outputs an equalization signal P. 
   In these structures, the difference circuit  62  outputs a difference between the target waveform signal T supplied from the target waveform generator  20  and the equalization signal P obtained by taking a delay time generated in the Viterbi decoder  18  and the target waveform generator  20  into consideration, namely, an equalization error signal E. Subsequently, the correlation between the equalization error signal E and the input signal with an arithmetic delay being taken into consideration is taken, and an average value of the correlation is calculated by each average value calculation portion  64 . The control sensitivity is applied to the calculation result by the amplifier circuit  65  in accordance with the control signal from the CPU  46 , and new tap coefficients C 0  to C N  are calculated and stored in the memories  66 , respectively. 
   The tap coefficients C 0  to C N  adapted by the target waveform signal T are respectively supplied to the amplifier circuits  58  of the FIR filter  17 . In the FIR filter  17 , the Viterbi decoding processing with decoding errors greatly reduced as compared with the prior art device is enabled by applying the equalization processing to the reproduction signals RF based on the optimized tap coefficients C 0  to C N . 
   2. Method of Controlling Viterbi Reference Level 
   Description will now be given as to a method of controlling Viterbi reference levels R 0  to R 6  using the target waveform signal T. 
   The parameter control portion  15  controls the reproduction processing parameters characteristic of the present invention and, as shown in  FIG. 8 , it has a difference circuit  72  which receives a target waveform signal T from the target waveform generation portion  20  and receives an output P′ from a delay circuit  71  which accepts the equalization signal P from the FIR filter, and a selector  73  which receives an output E from the difference circuit  72  and the ideal waveform signal I from the ideal waveform generation portion  19 . In addition, it has a plurality of accumulators  74  and counters  76  which are connected to outputs of the selector  73  corresponding to the level number, and correspond to the level number. Additionally, dividers  75  to which outputs of the accumulators  74  are connected are provided, and outputs from the counters  76  are supplied to these dividers  75 . Further, outputs from the dividers  75  are supplied to amplifier circuits  77  and connected to memories  78  corresponding to the level number. 
   Furthermore, the Viterbi decoder  18  has a branch metric circuits (Branch Metric)  71  corresponding to the level numbers, their outputs are supplied to an ACS circuit (Add Compare Selector)  73 , and an output from the ACS circuit  73  is a pathmetric memory (Pathmetric Memory)  72 . This output is supplied to a pass selector (Pass Selector)  75  and, on the other hand, an output from the ACS circuit  73  is supplied to the pass selector  75  through a path memory (Path Memory)  74 , thereby obtaining a decoding signal d. 
   In such a configuration, an equalization error E is first obtained as with the method of controlling the tap coefficients of the FIR filter  17 . Then, the selector  73  is operated in accordance with a value of the ideal waveform signal I corresponding to the reference level judged by the Viterbi decoder  18 , and the accumulator  74  and the counter circuit  75  provided in accordance with each reference level are used to calculate an average value of the equalization error on each judged reference level. Then, the control sensitivity is applied by the amplifier circuit  77 , and the optimized reference levels R 0  to R 6  are calculated. 
   In the Viterbi decoder  18 , the branch metric circuit  71  obtains a difference of the supplied equalization signal P based on the reference levels R 0  to R 6  optimized in accordance with the target waveform signal T. The obtained difference signal is supplied to the ACS circuit  73 , and the decoding signal d of the supplied equalization signal P is obtained by the pass selector  75  through the pathmetric memory  72  and the path memory  74 . 
   As described above, the present invention enables reproduction of information with decoding errors greatly reduced as compared with the prior art device by performing Viterbi decoding processing based on the reference levels R 0  to R 6  optimized by using the target waveform signal T. 
   3. Method of Controlling AGC Gain and Offset Value 
   Description will now be given as to a method of controlling an AGC gain and an offset value using the target waveform signal T. 
   A parameter control portion  15  controls reproduction processing parameters characteristic of the present invention and, as shown in  FIG. 9 , it has a difference circuit  82  which receives the target waveform signal T from the target waveform generation portion  20  and receives an output from a delay circuit  81  which accepts the equalization signal P from the FIR filter  17 , and a selector  83  which receives an output E from the difference circuit  82  and the ideal waveform signal I from the ideal waveform generation portion  19 . Furthermore, it has a plurality of accumulators  84  and counters  86  which are connected to outputs of a level  0 , a level  3  and a level  6  of the selector  83 . Moreover, there are provided dividers  85  to which outputs of the accumulators  84  are connected, and outputs from the counters  86  are supplied to the dividers  85 . In addition, outputs from the dividers  85  are supplied to amplifier circuits  87 , and there is a difference circuit  88  which receives an output from the amplifier circuit  87  on the level  0  and an output from the amplifier  87  on the level  6 . Additionally, there are a memory  89  which receives an output from the difference circuit  88  and a memory  89  which receives an output from the amplifier circuit  87  on the level  3 . 
   Further, an AGC circuit  16  automatically supplies an adequate gain, an offset value and the like and at least has a gain controller  91  which receives the reproduction signal RF and an offset controller  92  which receives an output from the gain controller  91 . Furthermore, a gain G stored in the memory  89  connected to the difference circuit  88  corresponding to the level  0  and the level  6  is supplied to the gain controller  91 , and an offset value F stored in the memory  89  corresponding to the level  3  is supplied to the offset controller  92 . 
   In such a structure, like the method of controlling the Viterbi reference level, the equalization error E is first obtained. Then, the selector  83  is operated in accordance with a value of the ideal waveform signal I corresponding to the reference level judged by the Viterbi decoder  18 , and an average value of the equalization error E according to each reference level is obtained by the accumulators  84 , the dividers  85  and the counters  86 . In the respective reference levels, as to the gain G, a difference in average value of the two equalization errors is calculated by the difference circuit  88  by using a maximum value and a minimum value of the reference levels, namely, the equalization errors of the level  0  and the level  6 , a new gain G is calculated and optimized by applying the control sensitivity by the amplifier circuit  87 , and this is stored in the memory  89 . 
   Moreover, the offset value F is optimized as a new offset value F by applying the control sensitivity to a central value of the reference levels, namely, the average value of the equalization errors on the level  3  with the similar procedures, and this is stored in the memory  89 . In each control, the control sensitivity is controlled by the CPU  46 , and such a value that a control value can be sufficiently converged is selected. 
   The optimized gain G and offset value F are respectively supplied to the gain controller  91  and the offset controller  92 , and the gain control and the offset control corresponding to the target waveform signal T are executed. 
   As described above, according to one embodiment of the present invention, since the reproduction signal RF is adapted based on the gain G and the offset value F optimized by using the target waveform signal T, information with decoding errors greatly reduced as compared with the prior art device can be reproduced in the following Viterbi decoding processing. 
   As described above in detail, in the information reproducing device according to one embodiment of the present invention, even if the linearity is lost due to formation of a small shortest mark/space, the target waveform signal T is generated, and at least the reproduction processing parameters such as {circle around (1)} the tap coefficients of the FIR filter, {circle around (2)} the reference levels of Viterbi decoding, {circle around (3)} the gain and the offset value of the AGC circuit and others are optimized. As a result, in the Viterbi decoding processing, there are provided the information reproducing device and the information reproducing method which can reproduce information with decoding errors greatly reduced as compared with the prior art device. 
   &lt;Second Embodiment&gt; 
   A second embodiment provides an information reproducing device and an information reproducing method when changing not only levels corresponding to the shortest mark/space of the ideal waveform but also all the levels in case of obtaining the target waveform signal T. 
     FIG. 10  is a block diagram showing an example of the target waveform generator of the information reproducing device according to the second embodiment of the present invention, and  FIG. 11  is a flowchart showing an example of the target waveform generation method of the information reproducing device according to the second embodiment of the present invention. 
   In  FIG. 10 , the ideal waveform generator  19  is the same as that in  FIG. 4 , thereby omitting the explanation. A target waveform generator  20  which receives the ideal waveform signal I from an ideal waveform generator  19  has a level detector  93  to which Viterbi reference levels R 0  to R 6  and the ideal waveform signal I are supplied, and a level shifter  94  to which the Viterbi reference levels R 0  to R 6 , the ideal waveform signal I and a detection signal from the level detector  93  are supplied. 
   In such a structure, the procedures of generating the target waveform signal T according to the second embodiment will now be described hereinafter with reference to a flowchart of  FIG. 11 . The explanation will be given as to a case where the ( 1 ,  7 ) RLL modulation method is used as the modulation method for data to be recorded and the PR ( 1 ,  2 ,  2 ,  1 ) decoding method is used as the decoding method for the simplicity. This is also equivalent in the information recording/reproducing device using a combination of recorded data based on another modulation method or another decoding method within the scope of the present invention. 
   At first, the decoding signal d is supplied from the Viterbi decoder  18 , and the convolution integral of the decoding signal d and the PR ( 1 ,  2 ,  2 ,  1 ) class is performed by functions of a plurality of delay circuits  51 , a plurality of amplifier circuits  52  and the adder  53 , thereby generating the ideal waveform signal I (S 21 ). Then, the level judgment of the ideal waveform signal I is performed by the level detector  93  of the target waveform generator  20  (S 22 ). Here, the ideal waveform signal I is divided into seven levels from level  0  to level  6  (S 23 ). The level shifter  94  converts the signal levels of the ideal waveform signal I into reference levels R 0  to R 6  Of the Viterbi decoder in accordance with the judged level, respectively. 
   In the second embodiment, as described above, the target waveform signal T is converted into the reference level R 0  to R 6  of the Viterbi decoder in all the levels. By obtaining such a target waveform signal T, the optimum control value can be supplied to the Viterbi decoder  18  even if the linearity is lost due to formation of a small shortest mark/space. Moreover, the similar advantages can be demonstrated in reproduction of data which is asymmetry because optimization of the conditions in recording is insufficient. 
   &lt;Third Embodiment&gt; 
   A third embodiment provides the case of giving predetermined conditions to the reproduction processing parameters in order to stabilize the operations of the AGC, the FIR filter, the Viterbi decoder and the adaptation control. 
   That is, the third embodiment at least {circle around (1)} determines a total value of the tap coefficients C 0  to C 6  of the FIR filter as 1 and {circle around (2)} optimizes the reference levels R 0  to R N  of the Viterbi decoder  18  while fixing a value of the minimum reference level R 0  and a value of the maximum reference level R N  in order to stabilize the operation of each adaptation control when adapting the reproduction processing parameters in accordance with the target waveform signal T as described above. 
   (Problems) 
   In the first embodiment, when optimization of the gain using the AGC circuit  16  according to the target waveform signal T is performed concurrently with optimization of the tap coefficients C 0  to C 6  of the FIR filter  17  or optimization of the reference levels R 0  to R N  of the Viterbi decoder, the gain which should have been optimized unnecessarily varies due to a change in the tap coefficients C 0  to C 6  or the reference levels R 0  to R N  of the Viterbi decoder. As a result, each adaptation control unstably operates, and the control system may diverge in the worst case. 
   (Tap Coefficients C 0  to C 6  of FIR Filter  17 ) 
   In optimization of the tap coefficients C 0  to C 6 , unintentional fluctuations in the gain can be avoided by taking values of the tap coefficients C 0  to C 6  according to the target waveform signal T in such a manner that a total value of the tap coefficients C 0  to C 6  becomes 1. 
   Therefore, it is possible to provide the information recording device and the information recording method which can stably reproduce information recorded with the high density while maintaining the adequate gain by obtaining in the parameter control portion  15  such tap coefficients C 0  to C 6  corresponding to the target waveform signal T acquired in the above-described method as that their total value becomes 1 and performing the equalization processing by using the FIR filter  17  based on the tap coefficients C 0  to C 6 . 
   (Maximum/minimum Reference Levels of Viterbi Decoder) 
   In case of optimizing the reference levels R 0  to R N  Of the Viterbi decoder, unintentional fluctuations in the gain can be avoided by setting the reference levels R 0  and R N  which are maximum and minimum reference levels as fixed values and then determining and using the reference levels R 0  to R N  according to the target waveform signal T. 
   It is, therefore, possible to provide the information recording device and the information recording method which can stably reproduce information recorded with the high density while maintaining the adequate gain by adapting in the parameter control portion  15  the reference levels R 0  to R N  corresponding to the target waveform signal T acquired by the above-described method other than the maximum/minimum reference levels without changing values of the maximum/minimum reference levels and performing the Viterbi decoding processing based on the reference levels R 0  to R N . 
   According to the third embodiment, unintentional fluctuations in the gain can be avoided, stable reproduction of recorded information is enabled and the identification accuracy of recorded information can be greatly improved by controlling both or either the equalizer (FIR filter) and the Viterbi decoder which are required in the PRML signal processing under the conditions mentioned above. 
   Although a person skilled in the art can realize the present invention based on the above-described various embodiments, he/she can readily conceive various modifications of these embodiments and can apply to various embodiments without the inventive capability. The present invention, therefore, covers a wide range which is not inconsistent with the disclosed principle and the novel characteristics, and is not restricted to the foregoing embodiments. 
   As described above, according to one embodiment of the present invention, it is possible to provide the information reproducing device and the information reproducing method which can give the reproduction processing parameters optimum for the Viterbi decoder and stably reproduce information recorded with the high density even if the linearity is lost due to formation of a small shortest mark/space. 
   Further, the similar advantages can be demonstrated in reproduction of data which is asymmetric because optimization of the conditions in recording is insufficient.