Patent Publication Number: US-7724636-B2

Title: Asymmetry compensator for partial response maximum likelihood (PRML) decoder

Description:
This application claims the benefit of Taiwan Patent Application No. 94106316, filed on Mar. 2, 2005, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to an asymmetry compensator, and in particular to an asymmetry compensator, which can be utilized in a Partial Response Maximum Likelihood (PRML) Decoder for adjusting the gains and the offsets of asymmetric components of the regenerating signal read from an optical recording medium. 
   2. Related Art 
   In general, the optical recording medium is utilized to record the data on the disk by optical means, and to read the data stored on the disk by optical means. As such, the device utilized to read the data stored on the optical recording medium is usually referred to as a signal regenerating device, it mainly comprises: a read/write head for reading/writing the data, a preamplifier, a waveform equalizer, a data detecting circuit, and a decoder. 
   Usually, in reading the data stored on the optical recording medium, the value of the data is determined by the size of the burned area. However, when the burned area is exceeding ‘large’ or exceeding ‘small’, the distinct asymmetry will appear in the optical disk regenerating signal, which affects the error occurrence rate of the regenerating signal of the optical disk, for example, PRSNR, SbER, which is utilized as an important indicator for indicating the optical disk error rates. Among them, PRSNR is disclosed by NEC, SbER is disclosed by Toshiba of Japan respectively. 
   U.S. Pat. No. 6,324,144 discloses a technology of eliminating the asymmetry, wherein the difference of numbers of the positive and negative signal is utilized to adjust the offset of the regenerating signal. However, to PRML, the adjustment of the offset is not able to entirely compensate the asymmetry in the signal. In addition, if this asymmetry were severe, the application of this technology would increase the difficulties in discriminating the signal. 
   U.S. Pat. No. 6,754,160 discloses another technical means, wherein the offset is obtained by comparing the signal of PRML with the regenerating signal of the optical disk. However, this means is less sensitive to the offset, and it needs additional hardware to record the regenerating signals, thus increasing the hardware complexity and expense. 
   U.S. Pat. No. 6,798,363 discloses another technical means, wherein in addition to adjusting the offset by the difference of numbers of the positive and negative signal, the offset is further adjusted by the symmetry of the short T signal. However, to the PRML, the adjustment of the offset alone is not capable of compensating the asymmetry entirely, for it needs the signal of the Viterbi decoder as the feedback signal, thus making the realization of the hardware even more difficult. 
   U.S. Pat. No. 6,693,863 discloses another technical means, in which the compensation of the asymmetry is achieved by adjusting the quantified level of A/D. However, the utilization of the analog circuit tends to have the problem of adjustment difficulties. 
   In addition, in the early open published No. 20030169665 discloses a technology, which is utilized to integrate the gain adjuster and the equalizer as a single unit, thus to reduce the hardware required, and the gain is adjusted by making use of the Viterbi feedback signal. However, the integration of gain adjuster and equalizer as a single unit increases the complexity of the circuit design, yet the offset has not been adjusted. 
   In other documents related to this subject, for example, as disclosed in “A study of Asymmetry Compensation for Partial-Response Maximum-Likelihood Detection in Optical Recording Media” (Sony paper, 1998), the offset of the signal is obtained by making use of the successive 6T, and then it is transmitted to the Viterbi decoder. However, the signals of succession are required, when it is not the case, then the correct offset can not be obtained. In another related document “Decision-directed adaptive nonlinear canceller for optical read channels” (Cirrus paper, 2001) is disclosed a technical means, the compensation signal output is obtained through the equations, yet in this process a set of predetermined input digital signals are required. In other related documents, for example, “Adaptive Signal Processing Method Using PRML For High Density Optical Disks” (Hitachi Ltd. 2002 IEEE), “Combined adaptive controlled PRML signal processing for high density optical disk” (Toshiba Corp. 2002 IEEE), “Adaptive Partial-Response Maximum-Likelihood Detection in Optical Recording Media” (Sony Corp. 2002 IRRR), the asymmetry of the RF (optical disk regenerating signal) is disclosed, and the solution of the asymmetry is by varying the level of the maximum-likelihood decoder. However, by doing so other problems are derived and have just yet to be solved, such as the convergence, the stability, and the interval between level and level of the optical disk regenerating signal. 
   To solve the asymmetry problem of the optical disk regenerating signal, the adjustment of the offset or the adjustment of the gain is utilized in the prior art, yet the asymmetry correction of the signal must be achieved by offset adjustment and gain adjustment simultaneously, as such to obtain the correct and precise regenerating signal. Nevertheless, for this problem, the prior art does not have proper solution. 
   SUMMARY OF THE INVENTION 
   In view of the above-mentioned problems and shortcomings of the prior art, the invention is directed to an asymmetry compensator for a partial response maximum likelihood (PRML) decoder, thus solving the problem of asymmetry of the signal regeneration of the optical disk, increasing the signal decoding rate, as such solving the problems and the shortcomings of the prior art. 
   Therefore, the asymmetry compensator for the partial response maximum likelihood (PRML) decoder disclosed by the invention is utilized to eliminate the asymmetric portion of the regenerating signal read from an optical recording medium, including an offset controller and a gain controller. Said offset controller is used to determine the central level of the regenerating signal, thereby adjusting the offset of the regenerating signal, while said gain controller is used to adjust the gain of the regenerating signal. 
   According to one aspect of the invention, the asymmetric compensator disclosed by the invention can be used to effectively improve the asymmetry of the regenerating signal. 
   According to another aspect of the invention, the asymmetric compensator disclosed by the invention can be used to adjust the offset and gain simultaneously. 
   According to a further aspect of the invention, the asymmetric compensator disclosed by the invention can be used so that the regenerating signal does not have to go through the phase lock loop first, thereby improving the effectiveness of the phase lock loop. 
   According to yet another aspect of the invention, the asymmetric compensator disclosed by the invention can be used so that the offset controller is capable of solving the problem which is liable to be missed with the channel 2T by means of blue light. 
   According to a further aspect of the invention, the asymmetric compensator disclosed by the invention is easy to realize hardware-wise, easy to debug, thus reducing its hardware requirement. 
   According to yet a further aspect of the invention, the asymmetric compensator disclosed by the invention has excellent convergence, so the feedback signal from some other devices is not needed. 
   According to a further aspect of the invention, the asymmetric compensator disclosed by the invention can be fully digitalized and have high stability, so that the signal quality is improved and thus it will not deteriorate. 
   In the following description, for purposes of explanation numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structures and devices are shown in block diagram form in order to avoid obscuring the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, and wherein: 
       FIG. 1  is a system block diagram of the asymmetry compensator for partial response maximum likelihood (PRML) decoder according to the first embodiment of the invention; 
       FIG. 2  is a block diagram of the asymmetry compensator according to the first embodiment of the invention; 
       FIG. 3  is a block diagram of the gain controller of the asymmetry compensator according to the first embodiment of the invention; 
       FIG. 4  is a block diagram of the asymmetry compensator according to the second embodiment of the invention; 
       FIG. 5  is a block diagram of the asymmetry compensator according to the third embodiment of the invention; 
       FIG. 6  is curves showing the corresponding relations specified in the corresponding table of the asymmetry compensator according to the invention; 
       FIG. 7  is a system block diagram of the asymmetry compensator for a partial response maximum likelihood (PRML) decoder according to the fourth embodiment of the invention; 
       FIG. 8  is a system block diagram of the asymmetry compensator for a partial response maximum likelihood (PRML) decoder according to the fifth embodiment of the invention; 
       FIG. 9  is a block diagram of the asymmetry compensator according to the fifth embodiment of the invention; 
       FIG. 10  is an eye pattern showing the offset of the asymmetry signal before the application of the asymmetry compensator of the invention; 
       FIG. 11  is an eye pattern showing the offset of the compensated asymmetry signal after the application of the asymmetry compensator of the invention; and 
       FIGS. 12 &amp; 13  are the column charts corresponding to  FIGS. 10 &amp; 11  respectively showing the number of occurrences of the asymmetric signal and the compensated asymmetric signal before and after the application of the asymmetry compensator of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals are used throughout the drawings and the description to refer to the same or like parts. Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
   Refer to  FIG. 1  for the system block diagram of the Asymmetry Compensator for Partial Response Maximum Likelihood (PRML) Decoder according to the first embodiment of the invention. 
   As shown in  FIG. 1 , it shows the system structure of the Partial Response Maximum Likelihood (PRML) Decoder, including: an analog-to-digital converter  100 , an asymmetric compensator  200 , a phase lock loop  300 , and a VETERBI decoder  400 . Firstly, the analog-to-digital converter  100  is utilized to convert the regenerating signal  900  read from the optical recording medium. The regenerating signal  900  read from the optical recording medium is of an analog format, which is converted to the regenerating signal  901  of a digital format through the analog-to-digital converter  100 . Then the regenerating signal  901  is removed from the asymmetric component through the asymmetric compensator  200 . Subsequently, the regenerating signal  906 , not having the asymmetric component, is phase locked by making use of the phase lock loop  300 . And finally, the regenerating signal  906  is decoded through the VITERBI decoder  400 . 
   In the above description, the asymmetric compensator  200  is composed of an offset controller  210  and a gain controller  250 , which can be used to eliminate the asymmetric component of the regenerating signal  901 . 
   Next, refer to  FIG. 2  for the system block diagram of the asymmetric compensator  200  according to the first embodiment of the invention, including an adder  211 , a comparator  212 , a first integrator  213 , a first controller  214 , a gain controller  250 , and a multiplier  270 . 
   For a more detailed description, the adder  211  is used to add an offset signal  905  to the regenerating signal  901  read from the optical recording medium, thus to adjust its central level and generate an added signal  902 . Thus the added signal  902  is the regenerating signal having its offset being adjusted. The comparator  212  is connected to the adder  211 , and is used to determine whether the central level of the added signal  902  is greater or less than zero. When it is greater than zero, then the output determination signal  903  is 1; otherwise, the output determination signal is −1. The determination signal  903  is received by the gain controller  250  and a first integrator  213 . The first integrator  213  is used to add the determination signal  903  successively and generate a corresponding first integration signal  904 . The first controller  214  is connected to the first integrator  213 , which is used to multiply the first integration signal  904  by a certain number, to generate the offset signal  905 . The first controller  214  for example can be a P controller. 
   Then, refer to  FIG. 3  for the structure of the gain controller  250  corresponding to that as shown in  FIG. 2 . As shown in  FIG. 3 , the gain controller  250  is composed of a second integrator  251 , a second controller  252 , and a calculator  253 , in which the second integrator  251  is used to add consecutively the regenerating signal  906  having no asymmetric component, thus generating a corresponding second integration signal  908 . The second controller  252  is connected to the second integrator  251 , which is utilized to multiply the second integration signal  908  by a specific number, to generate an operation signal  909 . In the above description, the second controller  252  for example can be a P controller. The asymmetric gain signal  907  is generated from the operation signal  909  as based on the determination signal  903 . When the determination signal is 1, the calculator  253  is used to subtract the operation signal  909  from 1 to obtain the asymmetric gain signal  907 , and when the determination signal  903  is −1, the calculator  253  is used to add 1 to the operation signal  909  to obtain the asymmetric gain signal  907 . 
   In the embodiment as shown in  FIGS. 2 and 3 , the regenerating signal  906  having no asymmetric component is used by the gain controller  250  as the feedback signal to calculate and generate the asymmetric gain signal  907  based on the determination signal  903 , then the multiplier  260  is used to multiply the added signal  902  and the asymmetric gain signal  907 , so as to generate and output the regenerating signal  906  not having the asymmetric component. 
   Subsequently, refer to  FIG. 4  for the system block diagram of the asymmetric compensator  200  according to the second embodiment of the invention, including: an adder  211 , a comparator  212 , an integrator  213 , a controller  214 , a gain controller  250 , and a multiplier  270 . The functions and the operations of the adder  211 , the comparator  212 , the integrator  213 , the controller  214 , and the multiplier  270  are the same as those of the first embodiment as shown in  FIG. 2 . The only difference is the gain controller  250 , which will be described in detail as follows. 
   In this embodiment, the gain controller  250  is utilized to perform calculation by making use of the offset signal  905  and the amplitude of the regenerating signal  901  read from the optical recording medium, so as to output the asymmetric gain signal  907 . When the determination signal  903  is 1, the calculator in the gain controller  250  is used to obtain the asymmetric gain signal  907  through dividing the amplitude of the regenerating signal  901 , read from the optical recording medium by the result of the amplitude of the regenerating signal  901 , read from the optical recording medium plus the offset signal  905 . And when the determination signal  903  is −1, the calculator in the gain controller  250  is used to obtain the asymmetric gain signal  907  through dividing the amplitude of the regenerating signal  901 , read from the optical recording medium by the result of the amplitude of the regenerating signal  901 , read from the optical recording medium minus the offset signal  905 . 
   The above-mentioned calculation may be expressed as the following mathematical equations. 
   When the determination signal  903  is 1, then the asymmetric gain signal  907  is: Asym_gain=Amp/(Amp+offset), wherein Amp indicates the amplitude of the regenerating signal  901 , offset indicates the offset signal  905 . 
   When the determination signal  903  is −1, then the asymmetric gain signal  907  is: Asym_gain=Amp/(Amp-offset), wherein Amp indicates the amplitude of the regenerating signal  901 , offset indicates the offset  905 . 
   Then, refer to  FIG. 5  for the asymmetric compensator according to the third embodiment of the invention. In this embodiment, a corresponding table is established for the asymmetric compensator. From the table, the corresponding regenerating signal, not having the asymmetric component, can be obtained directly by its corresponding relations with the regenerating signal having the asymmetric component as read from the optical recording medium, in which the corresponding table is calculated from the amplitude and offset of the regenerating signal of the optical disk. 
   In  FIG. 5 , the asymmetric compensator as disclosed by the invention is composed of an adder  211 , a comparator  212 , an integrator  213 , a controller  214 , and a gain controller  250 . Among them, the gain controller  250  is used to store a corresponding table indicating the corresponding relations between the regenerated signal having the asymmetric component, and the regenerating signal not having the asymmetric component. The functions and operations of the adder  211 , the comparator  212 , the integrator  213 , and the controller  214  are the same as those as shown in  FIG. 2  and the related first embodiment. The establishment of the corresponding table contained in the gain controller  250  will be described in detail as follows. 
   Next, refer to  FIG. 6  for the linear and curvature relations respectively between the regenerating signal having the asymmetric component and the regenerating signal not having the asymmetric component. As shown in  FIG. 6 , the relation can be a linear relation or a non-linear curvature relation, with its vertical axis (y axis) representing the asymmetric compensation signal, and its horizontal axis (x axis) representing the asymmetric signal. 
   The linear relation corresponding table is calculated and obtained by the equation y=ax+b; wherein, when x≦offset, then a=(amplitude/(amplitude+offset)), b=−a×offset; and when x≧offset, then a=(amplitude/(amplitude+−offset), b=−a×offset. 
   The non-linear relation corresponding table is calculated and obtained by the following quadratic equation. Assuming that 
   
     
       
         
           
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   wherein x1,y1 represent amplitude, as represents the offset, x3,y3 represent negative amplitude, and y2 is 0. 
   Furthermore, refer to  FIG. 7  for the system block diagram of the Asymmetry Compensator for the Partial Response Maximum Likelihood (PRML) Decoder according to the fourth embodiment of the invention. 
   As shown in  FIG. 7 , it shows the system structure of the Partial Response Maximum Likelihood (PRML) Decoder, including: an analog-to-digital converter  110 , an asymmetric compensator  200 , a phase lock loop  310 , and a VITERBI decoder  410 . Firstly, the analog-to-digital converter  100  converter is utilized to convert the regenerating signal  900  read from the optical recording medium. The regenerating signal  900  read from the optical recording medium is of an analog format, which is converted to the regenerating signal  901  of the digital format through the analog-to-digital converter  100 . Next, the regenerating signal  901  is phase looked by making use of the phase lock loop  310 . Then, the phase-locked regenerating signal is removed from the asymmetric component through the asymmetric compensator  200 . Finally, the regenerating signal is decoded through the VITERBI decoder  410 . 
   Moreover, refer to  FIG. 8  for the system block diagram of the Asymmetry Compensator for Partial Response Maximum Likelihood (PRML) Decoder according to the fifth embodiment of the invention. 
   As shown in  FIG. 8 , it shows the system structure of the Partial Response Maximum Likelihood (PRML) Decoder, including: an analog-to-digital converter  120 , an asymmetric compensator  200 , a phase lock loop  320 , and a VITERBI decoder  420 . Firstly, the phase lock loop  320  is utilized to phase-lock the regenerated signal  900  read from the optical recording medium, then the signal is converted by the analog-to-digital converter  120  from the analog format to the digital format. Subsequently, the converted signal is removed of its asymmetric component by making use of the asymmetric compensator  200 . And finally, the regenerating signal is decoded through the VITERBI decoder  420 . 
   In the above description, an offset controller  210  in the asymmetric compensator  200  is utilized to find the central level of the regenerating signal  901  through the control loop by making use of the principle of the digital sum of the regenerating signals, equaling to zero. As shown in  FIG. 9 , an equalizer  215  is added in the control loop of the offset controller  210 , so as to increase the ratio of the high frequency portion in the added signal  902  deriving from the regenerating signal  901 , thus solving the problem that certain portions of the regenerating signal don&#39;t pass the central level. In addition, the adjustment of the central level is more dependent on the high frequency portion of the regenerating signal of the optical disk, thus the addition of the equalizer can result in the increase of data decoding rate. Furthermore, the gain controller  250  is utilized to control the symmetry of the regenerating signal of the optical disk through the control loop by making use of the characteristics of the amplitude symmetry of the regenerating signal of the ideal optical disk. 
   As mentioned earlier, the purpose of adding an equalizer in front of the comparator  212  is to increase the proportion of the high frequency portion in the regenerated signal, so that the problem that certain portions of the regenerating signal don&#39;t pass the central level can be solved. The reason is that the adjustment of the central level is more dependant on the high frequency portion of the regenerating signal of the optical disk. Therefore, the data decoding rate can be increased by making use of the equalizer to raise the proportion of the high frequency portion of the regenerated signal, so as to adjust the high frequency gain of the regenerating signal, and raise the zero point detection capability of the offset controller. 
   Finally, refer to  FIGS. 10 and 11  for the eye patterns of the asymmetric signals and the compensated asymmetric signals. In addition, refer to  FIGS. 12 and 13  for the column shape charts corresponding to  FIGS. 10 and 11  respectively. From the above drawings, the effectiveness of the invention is evident, wherein, the asymmetric signal represents the regenerating signal of the optical disk entering the asymmetry compensator, of which about 25% is the asymmetric component, while the compensated asymmetric signal represents the regenerating signal of the optical disk output by the asymmetry compensator of the invention. From the drawings it is evident that the asymmetric component of the compensated asymmetric regenerated signal of the optical disk is significantly reduced. 
   Presently, the signal processing method utilized for the blue light optical disk regenerating signal (RF) is realized by means of the “Partial Response Maximum Likelihood (PRML) Decoder”. Through the application of this method, the optical disk regenerating signal, having higher capacity and noise resistance, can be achieved. In the PRML structure, the RF optical disk regenerating signal can be modulated to the partial response model of the maximum likelihood (ML) through the utilization of the adaptive filter. However, in the present structure, only the linear feature of the RF optical disk regenerating signal is considered, yet the non-linear feature of the RF optical disk regenerating signal is treated as noise. The Asymmetry Compensator of the invention is mainly used to compensate for the non-linear feature of the asymmetry of the RF optical disk regenerating signal, which can be used to effectively restrain and reduce the asymmetry of the RF optical disk regenerating signal. Then, the signal thus processed is sent to the PRML decoder for decoding, thus achieving the reduction of the error occurrence rate of the signal. 
   Summing up the above, the signal asymmetry compensation steps of the invention may be classified mainly into the following two signal asymmetry compensation procedures. In the first signal asymmetry compensation procedure, the central level of the regenerating signal is located through the control loop by means of the principle of the digital sum value (DSV) of the signals equaling to zero, and proportion of the signal high frequency portion of the regenerating signal is increased through an additionally added equalizer, which is used to solve the problem of certain high frequency RF signals not passing the central level; moreover, the data decoding rate is increased due to the fact that the adjustment of the central level is more dependent on the high frequency portion of the RF optical disk regenerating signal. In the second signal asymmetry compensation procedure, the amplitude symmetry feature of the ideal optical disk regenerating signal is utilized to control and reduce the asymmetry of the optical disk regenerating signal through the control loop, thereby effectively solving the asymmetry problem of the optical disk regenerating signal. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.