Abstract:
When restoring signals from an optical recording medium, one usually obtains nonlinearly distorted signals. The equalizer disclosed in the prior art can only process linear signals. A new structure that can process nonlinearly distorted signals is disclosed, which including an A/D converter for sampling restoring signals. The restored signals are processed by an adaptive linear equalizer to extract the errors. The errors are then fed into a nonlinear equalizer for correcting the nonlinear distortion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The invention relates to a wave equalizer in a signal restoration device for optical storage media and, in particular, to a wave equalizer that uses an adaptive equalizer to correct nonlinear distorted signals when restoring signals from an optical recording medium.  
         [0003]     1. Related Art  
         [0004]     Generally speaking, optical recording systems have data stored on or read out from a disk in an optical way. The device that is used to read data on the optical recording medium is called the signal restoring device. It generally includes a pickup head for reading data, a pre-amp, a wave equalizer, a data detection circuit, and a decoder.  
         [0005]     After the pickup head obtains data signals, the pre-amp amplifies the signals and the wave equalizer equalizes each of the data signals. The data detection circuit detects the binary data from the equalized data signals and outputs the detected signals to the decoder for decoding.  
         [0006]     The wave equalizer in the restoring system is mainly used to eliminate the distortion in the data signals. In other words, it converts the recording/restoring channel into a linear model and removes the linear distortion during the data restoring process.  
         [0007]     In order to ensure good quality when restoring signals from an optical recording medium, one usually uses a nonlinear equalizer for eliminating nonlinear distortion in addition to a normal linear equalizer as the latter alone cannot achieve a satisfactory effect. There are many reasons for nonlinear distortions. For example, the nonlinear distortion in asymmetric signals is related to the product of the input signal and its one channel bit duration.  
         [0008]     Moreover, as the optical recording media have higher storage capacities nowadays, it is almost impossible to build a linear model for the binary data on an optical disk. The primary problem is the system response. Therefore, the establishment of a linear model has to take into account the nonlinear distortion in data signals. It is thus an important subject to solve the problem of nonlinear distortion in the development of a signal restoring system.  
         [0009]     In recent years, the development of high-density disks generally uses the signal-reading detection technique in the partial response maximum likelihood (PRML) technology in restoring optical recording medium signals. The purpose of partial response (PR) is to equalize sampled radio frequency (RF) signals into target PR for the detection of maximum likelihood.  
         [0010]     The PRML chip has a signal model database. It compares the digital signal obtained from the recording medium with the signal model in the database, finding a closest signal model for output. The hard drive disks (HDD&#39;s) on the market have adopted the PRML reading channel. The optical disk drives, or called the signal restoring systems, generally also use the PRML technology. In comparison with the conventional analog trough detection, the PRML makes use of the sampling and frequency responses of digital signals to achieve higher signal stability and storage capacities.  
         [0011]     The U.S. Pat. No. 6,052,349 addressed the question of how to eliminate nonlinear distortion and proposed a technical solution. It uses a neural network to build a nonlinear equalizer. However, it requires a huge amount of memory in practice. In one of its embodiments, the device contains over ten multipliers, complicated functional operations, and complex decision mechanisms. Since the linear equalizer and the nonlinear equalize are connected in series, delays happen frequently when extracting feedback signals from the data decoder, making the whole circuit very unstable.  
         [0012]     As the nonlinear equalizer using the neural network technology requires a lot of computations, the prior art thus results in many delays during the extraction of feedback signals.  
       SUMMARY OF THE INVENTION  
       [0013]     In view of the foregoing, it is an objective of the invention to provide a signal restoring device for optical recording media to solve/improve existing technical problem in the prior art. It uses a simpler structure to reduce the amount of necessary multipliers.  
         [0014]     When restoring signals from an optical recording medium, nonlinear distortions often occur to the signals when reading or writing the signals. The usual equalizer can only process linear signals. The invention discloses a structure to process nonlinearly distorted signals to achieve the goal of correcting nonlinear distortions.  
         [0015]     To achieve the above objectives, the disclosed signal restoring device includes an analog-to-digital (A/D) converter and a wave equalizer. The wave equalizer further contains a first adaptive equalizer and a nonlinear distortion cancellation equalizer. The adaptive linear equalizer is used to perform a linear equalization on the digital signals and to output a target wave and an error signal. The nonlinear distortion cancellation equalizer takes the error signal as its target level. It performs a nonlinear distortion cancellation according to an estimated nonlinear form signal. Moreover, the invention also includes a first adder, which sums up the outputs of the adaptive linear equalizer and the nonlinear distortion cancellation equalizer.  
         [0016]     The nonlinear distortion cancellation equalizer further contains a second adaptive linear equalizer and a second adder. The output from the second adaptive linear equalizer is fed back into the second adder so that the second adder can output a second error signal according to the error signal and the feedback signal. The second error is the nonlinear input signal of the second adaptive linear equalizer.  
         [0017]     The disclosed signal restoring device modifies the adaptive linear equalizer structure into a nonlinear distortion cancellation equalizer. The number of required multipliers is only related to the complexity of the design. Using the invention, an extremely good signal quality can be achieved with only a few multipliers and without complicated functional operations and decision mechanisms. In comparison with the prior art, there is no such problem as time delays. It has a better feedback control. Its design in the nonlinear distortion cancellation equalizer is also superior to the prior art.  
         [0018]     In addition, the disclosed device uses the disclosed nonlinear distortion cancellation equalizer in partial response to achieve effects that are more satisfactory. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:  
         [0020]      FIG. 1  is a system block diagram of the disclosed signal restoring device;  
         [0021]      FIG. 2  is a schematic view of the disclosed nonlinear distortion cancellation equalizer;  
         [0022]      FIG. 3  is a schematic view of an estimated nonlinear signal;  
         [0023]      FIG. 4  is a schematic block diagram of the second adaptive linear equalizer;  
         [0024]      FIG. 5  is another schematic block diagram of the second adaptive linear equalizer;  
         [0025]      FIG. 6  shows the PR output from the linear equalizer only;  
         [0026]      FIG. 7  is a histogram for  FIG. 6 ;  
         [0027]      FIG. 8  shows the PR output from the linear equalizer and the nonlinear distortion cancellation equalizer; and  
         [0028]      FIG. 9  is a histogram for  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     In general, one uses nonlinear equalizer to process nonlinear distortion signals. The nonlinear distortion cancellation equalizer is not a truly nonlinear equalizer. The basic concept is as follows. After a signal is processed by an adaptive linear equalizer and equalized by a linear equalizer, the residual error term is extracted to be the nonlinear distortion signal in the signal. The error term is output to the nonlinear distortion cancellation equalizer, whose inputs have to be nonlinear signals. This is because it is a linear equalizer in effect. Therefore, it cannot have any equalization effect if the inputs are linear signals.  
         [0030]     Please refer to  FIG. 1 . The disclosed signal restoring device contains at least an analog-to-digital (A/D) signal converter  100 , an adaptive linear equalizer  200 , a nonlinear distortion cancellation equalizer  300 , a data detector,  400 , and a decoder  500 . The input terminal of the adaptive linear equalizer  200  is connected to the output terminal of the A/D converter  100 . The error signal output terminal of the adaptive linear equalizer  200  is connected to the output terminal of the nonlinear distortion cancellation equalizer  300 . The output terminal of the linear signal is connected to a first adder  700 . The first adder  700  simultaneously receives the output from the nonlinear distortion cancellation equalizer  300 . After operations, the adder  700  outputs signals to the data detector  400 .  
         [0031]     The adaptive linear equalizer  200  extracts some signals and feeds them into the nonlinear distortion cancellation equalizer  300 . The outputs of the adaptive linear equalizer  200  and the nonlinear distortion cancellation equalizer  300  are added by the first adder  700  to be the input of the data detector  400 , recovering the binary data original stored on the medium. Finally, the decoder  500  outputs the resulting data to an external system via an interface  600 .  
         [0032]     The biggest difference between the disclosed structure and the prior art is that: the invention uses the first adder  700  to process the outputs from the adaptive linear equalizer  200  and the nonlinear distortion cancellation equalizer  300 . This greatly reduces signal delays.  
         [0033]     The interface  600  is used to communicate with an external system, such as a computer or a television, for outputting restored signals. Of course, the external system also uses the interface  600  along with other circuits to record data signals on optical recording media.  
         [0034]     The signal extracted by a pickup head is first amplified by a pre-amp. The resulting radio frequency (RF) signal becomes the input of the A/D converter  100 . After processing, the A/D converter outputs sampling signals as the input of the adaptive linear equalizer  200 .  
         [0035]     The nonlinear distortion cancellation equalizer  300  further contains a second adaptive linear equalizer  800  and a second adder  900 , as illustrated in  FIG. 2 . The output from the second adaptive linear equalizer  800  is fed back into the second adder  900  so that the second adder  900  can output a second error signal ê according to the error signal e and the feedback signal Cn. The second error is the nonlinear input signal of the second adaptive linear equalizer.  
         [0036]     The nonlinear distortion cancellation equalizer  300  is used to eliminate the nonlinear distortions in the signals. The prior art uses a lot of multipliers in the linear and nonlinear equalizers, resulting in signal delays and complicated circuit designs. The disclosed nonlinear distortion cancellation equalizer  300  has a simpler operation model and thus fewer multipliers.  
         [0037]     In the partial response channel technology, the data detector  400  is a signal processing circuit using maximum likelihood estimation to detect the partial response in the equalized signals. A preferred embodiment of such a data detector is a Viterbi decoder.  
         [0038]     In the following, we give a detailed description for the nonlinear distortion cancellation equalizer  300 . The adaptive linear equalizer extracts a target PR and an error value e.  
         [0039]     The error value e represents the nonlinear distortion in the signals and is treated as the target level of the nonlinear distortion cancellation equalizer. The input is the estimated nonlinear signal u i . Suppose the output of the nonlinear distortion cancellation equalizer  300  is C n , then  
           c   n     =         ∑   i     ⁢         W   i     ·     u   i       ⁢           ⁢   or   ⁢           ⁢     c   n         =       ∑   j     ⁢       ∑   i     ⁢       W   i   j     ·     u   i   j               ;       
 
 and the second error signal of the adaptive linear equalizer ê=e−C n . 
 
         [0041]     The parameter of the adaptive linear equalizer is W i   n+1 =W i   n +r·{overscore (e)}·u i , where r is the step size parameter. The estimated nonlinear signal u i  can also be generated from the synthesized signal of the adaptive linear equalizer.  
         [0042]     The operation structure for  
         c   n     =       ∑   i     ⁢       W   i     ·     u   i             
 
 is shown in  FIG. 4 . Each u i  is multiplied by the corresponding weight. For example, the multiplier  810  multiplies u 1  with W 1 , the multiplier  820  multiplies u 2  with W 2 , the multiplier  830  multiplies u 3  with W 3 , and the multiplier  840  multiplies u 4  with W 4 . The multiplication results are added by the adder  850  to output Cn. 
 
         [0044]     The operation structure of  
         c   n     =       ∑   j     ⁢       ∑   i     ⁢       W   i   j     ·     u   i   j               
 
 is shown in  FIG. 5 . Each u i  here is the delay of the previous u i . For example, u 2  is obtained by delaying u 1  by one channel bit duration using a delayer  860 , u 3  is obtained by delaying u 2  by one channel bit duration using a delayer  870 , and u 4  is obtained by delaying u 3  by one channel bit duration using a delayer  880 . After obtaining the delayed values, the multipliers  810 ,  820 ,  830 ,  840  multiply them by the corresponding weights. Finally, the adder  850  sums them up to output Cn. 
 
         [0046]     In the following, we give an example of generating u 1 . If y i  are the RF signal sampling points separated by one channel bit duration. As shown in  FIG. 3 , since the nonlinear distortion signals usually occur at zero crossing points, u i  are selected from: 
 
 u   i    ε{y   i−k1   ·y   i−k2+1   , y   i−k1   ·y   i−k1−1   , y   i−k2   ·y   i−k2+1   , y   i−k2   ·y   1−k2−1 }. 
 
 That is, each u i  is represented by four values: the two coordinates of the zero crossing point and the products of the previous and the next points. Therefore, the generation of each u i  requires four multipliers. 
 
         [0048]     Suppose the index j in u i   j  and W i   j  indicates the sampling point separated by one channel bit duration, then  
         c   n     =       ∑   j     ⁢       ∑   i     ⁢       W   i   j     ·       u   i   j     .               
 
         [0049]     Consequently, if we select u i  to be three symmetric TAP adaptive linear equalizers, then each w i   ·  u i  requires four multipliers. All four w i    ·  u i  require 16 multipliers. In addition, each u i  requires one multiplier. Therefore, the system requires 20 multipliers in total for the four u i  in order to achieve extremely good signals.  
         [0050]      FIG. 6  shows the RF output signal of the adaptive linear equalizer.  FIG. 7  is a histogram for  FIG. 6 .  FIG. 8  shows the output signal of the adaptive linear equalizer and the nonlinear distortion cancellation equalizer.  FIG. 9  is a histogram for  FIG. 8 . The nonlinear distortion cancellation equalizer is implemented according to the disclosed embodiment, where 20 multipliers are used. One can easily see that the PR after the insertion of the nonlinear distortion cancellation equalizer accurately falls on the +3, +2, +1, 0, −1, −2, and −3 levels.  
         [0051]     Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.