Abstract:
A method and apparatus for signal equalization in a light storage system is disclosed. The method includes observing run length of an RF signal read from the optical disc; classifying run lengths into sets; calculating the mean of certain run lengths; comparing the values with expected values of said run lengths to generate an error value; and adjusting equalizing parameters of boost and frequency if the error value lies outside an expected range.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to signal equalization, and more particularly, to a method and apparatus of signal equalization in an optical disc drive system.  
         [0003]     2. Description of the Prior Art  
         [0004]     In a read channel of a light storage system, a pick-up head of the light storage system reads information from an optical disc to generate a radio-frequency (RF) signal. The RF signal is then processed by a specific low-pass filter and an equalizer. A phase-locked device synchronizes the processed RF signal and the pit/land can be scanned according to the clock of the synchronization signal.  
         [0005]     Performance of the read channel relates to jitters between edge timing of the RF data and synchronization signals. The frequency response of the signals read from an optical disc varies from disc to disc. When reading from different disc, the EQ parameters must be adjusted accordingly to obtain the optimum equalizing performance. For variances of the optical discs and the read channel, as well as different requirements for rotational speed, the low-pass filter and the equalizer of the read channel must be adjusted to meet the jitter performance. The conventional adjusting method for the low-pass filter and equalizer is to look up a table for choosing parameters of filters and equalizers according to the types of discs, and utilize a method of trial and error to adjust the parameters to meet the jitter performance.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore one of the objectives of the present invention to provide an apparatus and method for signal equalization in an optical disc system, to solve the above-mentioned problem.  
         [0007]     The claimed method involves monitoring the run length of an outputted signal to determine which direction to vary the equalizing parameters.  
         [0008]     Briefly described, a method for signal equalization in a light storage system is disclosed. The method comprises: monitoring the run lengths of the signal read from the optical disc; classifying the run lengths into sets; calculating a first average value of a first run length set; calculating a second average value of a second run length set; comparing the first average value of the first run length set with an expected value of that run length set; adjusting a first equalizing parameter in order to make the first average value approximately equal to the expected value of that run length set; comparing the second average value of the second run length set with an expected value of that run length set; adjusting a second equalizing parameter in order to make the second average value approximately equal to the expected value of that run length set; and alternately adjusting first and second equalizing parameters.  
         [0009]     An apparatus of signal equalization in a light storage system is also disclosed. The apparatus comprises a run length meter and a processor. The run length meter comprises: a measuring module for measuring a plurality of run lengths of the signal; a classifying module, coupled to the measuring module, for classifying measured run lengths into a plurality of run length sets; a calculating module, coupled to the classifying module, for calculating a first average value of a first run length set corresponding to a first run length and for calculating a second average value of a second run length set corresponding to a second run length; and a comparing module, coupled to the calculating module, for comparing the first average value with a first expected value of the first run length set to determine a first difference value and for comparing the second average value with a second expected value of the second run length set to determine a second difference value. The processor, coupled to the equalizer and the run length meter, is used for adjusting a first equalizing parameter to reduce the first difference value and adjusting a second equalizing parameter to reduce the second difference value until the first difference value falls within a first range and the second difference value falls within a second range.  
         [0010]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a diagram of an equalizing circuit according to an embodiment of the present invention.  
         [0012]      FIG. 2  is a diagram illustrating the characteristic of an equalizer shown in  FIG. 1 .  
         [0013]      FIG. 3  is a flowchart illustrating the steps of equalizing frequency response of an optical disc signal. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The present invention provides a method and apparatus for equalizing the frequency response of an optical disc signal by utilizing the run length of the optical disc signal. Please refer to  FIG. 1 , which is a diagram of an equalizing circuit  100  according to an embodiment of the present invention. In this embodiment, the equalizing circuit  100  comprises an equalizer  102 ; a run length meter  104 ; and a processor (e.g. a DSP)  106 . The run length meter  104  comprises a measuring module  108 , a classifying module  110 , a calculating module  112 , and a comparing module  114 .  
         [0015]     As shown in  FIG. 1 , the equalizing circuit  100  is coupled to a photo detector of a pick-up head for receiving an RF signal generated from the photo detector. The optical pick-up head scans the surface of an optical disc, and then the photo detector therein converts the received reflected laser beams into an RF signal and sends the RF signal to the equalizer  102 , which sets initial parameters of boost (i.e. gain) and central frequency for equalizing the RF signal. The run length meter  104  detects all run lengths present in the RF signal from 3T to 11T in multiples of T, by utilizing the measuring module  108  to measure all run lengths, and utilizing the classifying module  110  to classify run lengths into run length sets from 3T to 11T. In this embodiment, the measuring module  108  is implemented by utilizing an interpolator to over-sample the RF signal, an edge detector to find zero-crossing points of the over-sampled RF signal, a run length counter to count the number of points between every two zero-crossings, and a run length selector to obtain the run length of a particular pulse through the number of points counted between zero-crossings. However, the scope of the invention is not limited to this embodiment. Any known circuit architecture capable of measuring the run lengths can be adopted to act as the desired measuring module  108 .  
         [0016]     Dimensions of the equalizer  102  represent sets of parameters. In this embodiment, the parameters of the equalizer  102  are adjusted to change the dimensions of the equalizer  102 . The calculating module  112  calculates a first average value of a first run length set and calculates a second average value of a second run length set, where in this embodiment the first run length set and the second run length set are the 3T run length and 4T run length respectively. However, the selection of the first run length set and second run length set is adjustable. For example, in another embodiment of the present invention, the 4T run length set and the 5T run length set are utilized to tune the parameters set to the equalizer  102 .  
         [0017]     Utilizing the comparing module  114 , a first error value is generated by subtracting a first expected value from the first average value of the first run length set, where in this embodiment the first expected value is equal to 3T. If the first error value lies outside a first range delimited by two threshold values T 1  and T 2 , the processor  106  will implement an algorithm to the equalizer  102  in order to adjust a first equalizing parameter until the first error value lies within the desired first range. Please note that the first expected value is not limited to be 3T. If the above-mentioned desired first range is properly adjusted, the first expected value can be different from 3T, such as 3T+Δ. Then the threshold values for this expected value 3T+Δ are adjusted to be T 1 +Δ and T 2 +Δ, accordingly. The objective of tuning the equalizing parameters is still achieved.  
         [0018]     Once the average value of run lengths in the 3T run length set is approximately equal to the first expected value, a second error value is generated by subtracting a second expected value from the second average value of the second run length set, where in this embodiment the second expected value is equal to 4T. Please note that the second expected value is not limited to be 4T. If the second error value lies outside a desired second range delimited by two threshold values T 3  and T 4 , the processor  106  will implement another algorithm to the equalizer  102  in order to adjust a second equalizing parameter until the second error value lies within the second range. Please note that the second expected value is not limited to be 4T exactly. The second expected value is allowed to be different from the ideal value, 4T. For example, if the above-mentioned desired second range is properly adjusted, the second expected value is allowed to be different from 4T. The same goal of tuning the equalizing parameters is achieved. The first equalizing parameter and the second equalizing parameter are boost G m  and central frequency f c  respectively. However, these equalizing parameters are for illustrative purposes only, and not meant to be limitations.  
         [0019]     The method of equalizing the RF signal response will now be described in detail. The RF signal contains a plurality of run lengths in multiples of T. Please note that, as mentioned above, any two run length sets can be utilized, but 3T and 4T are ideally chosen as they are affected the most by boost and central frequency. The choice of these run lengths is not intended to be a limitation of the present invention, however. Through the utilization of the measuring and classifying modules  108  and  110 , the 3T and 4T run lengths in the read signal are identified from a plurality of run length sets and the average values of the 3T and 4T run lengths in the read signal are calculated.  
         [0020]     Please refer to  FIG. 2 .  FIG. 2  is a diagram illustrating the characteristic of the equalizer  102  shown in  FIG. 1 . The dotted line L 1  represents that the first expected value is equal to 3T, and the other dotted line L 2  represents that the second expected value is equal to 4T. Therefore, these two dotted lines L 1  and L 2  define four regions A, B, C, and D on the plane. As can be seen from the diagram, region A means that the average value of the measured 3T run lengths is greater than the ideal 3T run length, and average value of the measured 4T run lengths is less than the ideal 4T run length; region B means that the average value of the measured 3T run lengths is less than the ideal 3T run length, and the average value of the measured 4T run lengths is less than the ideal 4T run length; region C means that the average value of the measured 3T run lengths is less than the ideal 3T run length, and the average value of the measured 4T run lengths is greater than the ideal 4T run length; and region D means that the average value of the measured 3T run lengths is greater than the ideal 3T run length, and the average value of the measured 4T run lengths is greater than the ideal 4T run length.  
         [0021]     As such, the equalizing parameters are to be adjusted if the first error value between the average value of measured run lengths in a selected run length set and the expected value of the selected run length does not lie in a predetermined range. The first error value is obtained by subtracting the expected value of the selected run length from the average value of measured run lengths in a selected run length set. An example of tuning the equalizing parameters is illustrated. As shown in  FIG. 2 , initially the setting for equalizing parameters, central frequency f c  and boost G m , is (F 1 , G 1 ). The comparing module  114  compares the average value of the measured 3T run lengths with the expected value of 3T run length, detects the first error value, and determines that the average value of the measured 3T run lengths is greater than the expected value of the 3T run length. Therefore, the comparing module  114  then notifies the processor  106  of the comparing result. The processor  106  will implement an algorithm to the equalizer  102  in order to adjust the equalizing parameter, boost G m , in a direction to reduce the first error value.  
         [0022]     Once the difference, the first error value, between the average value of the measured 3T run lengths and the expected value of 3T run length lies within the desired first range, i.e. between threshold values T 1  and T 2 , the average value of the measured 4T run lengths is compared with the expected value of the 4T run length to obtain the second error value by subtracting the expected value of the 4T run length from the average value of the measured 4T run lengths. Please note that the current setting for the equalizing parameters, central frequency f c  and boost G m , is (F 1 , G 2 ). Then, the difference between the average value of measured 4T run lengths and the expected value of 4T run length is compared. The comparing module  114  detects the second error value, and determines that the average value of the measured 4T run lengths is less than the expected value of the 4T run length. Therefore, the comparing module  114  notifies the processor  106  of the comparing result. The processor  106  will implement another algorithm to the equalizer  102 , in order to adjust the equalizing parameter, central frequency, in a direction that reduces the error. In this embodiment, the equalizing parameter, central frequency f c , is tuned in steps of size d by the processor  106 . Therefore, the setting for the equalizing parameters, central frequency f c  and boost G m , is (F 2 , G 2 ). Please note that F 2  is equal to F 1  minus d.  
         [0023]     As indicated by  FIG. 2 , the dotted lines L 1  and L 2  will cross at a certain point, which stands for an optimum setting (F 0 , G 0 ) for the equalizing parameters, i.e. central frequency f c  and boost G m . In other words, if the equalizer  102  adopts the above optimum setting (F 0 , G 0 ), the average value of measured 3T run lengths is equal to 3T and the average value of measured 4T run lengths is equal to 4T. However, since the current setting (F 2 , G 2 ) for the equalizing parameters is unable to make the first error value between the average value of measured 3T run lengths and expected value of 3T run length fall in the desired first range and the second error value between the average value of measured 4T run lengths and expected value of 4T run length fall in the desired second range, the above-mentioned parameter tuning procedure is repeated. Therefore, as shown in  FIG. 2 , the setting (F 2 , G 2 ) is updated by a new setting (F 2 , G 3 ); the setting (F 2 , G 3 ) is updated by a new setting (F 3 , G 3 ), where F 3  is equal to F 2  minus d; and the setting (F 3 , G 3 ) is updated by a new setting (F 3 , G 4 ). After the equalizer  102  utilizes the setting (F 3 , G 4 ) to set the equalizing parameters, central frequency f c  and boost G m , the comparing module  114  finds out that the second error value between the average value of measured 4T run lengths and the expected value of the 4T run length falls in the desired second range, i.e. between two threshold values T 3  and T 4 . Therefore, the processor  106  will hold the current setting (F 3 , G 4 ). That is, because the first error value between the average value of measured 3T run lengths and the expected value of 3T run length lies in the desired first range and the second error value between the average value of measured 4T run lengths and the expected value of 4T run length lies in the desired second range, the parameter tuning procedure is deemed successful even though the final setting (F 3 , G 4 ) is different from the optimum setting (F 0 , G 0 ). In addition, the above parameter tuning procedure will keep monitoring the measured run lengths to optimize the equalizing parameters set to the equalizer  102 .  
         [0024]     Please refer to  FIG. 3 .  FIG. 3  is a flowchart illustrating the steps of equalizing frequency response, as detailed in the previous paragraphs.  
         [0025]     Step  300 : Start;  
         [0026]     Step  302 : Read an RF signal from an optical disc;  
         [0027]     Step  304 : The equalizer  102  enables initial setting of equalizing parameters, central frequency f c  and boost G m ;  
         [0028]     Step  306 : The measuring module  108  measures the run lengths of the RF signal;  
         [0029]     Step  308 : The classifying module  110  classifies the run lengths measured by the measuring module  108  into a plurality of run length sets, so the run length sets are in multiples of T from 3T to 11T;  
         [0030]     Step  310 : The calculating module  112  calculates a first average value of a selected first run length set and a second average value of a selected second run length set;  
         [0031]     Step  312 : The comparing module  114  compares the first average value with an expected value of the first run length set.  
         [0032]     Step  314 : Does an error value E 1  between the first average value and the expected value fall in a first range delimited by two threshold values T 1  and T 2 ? If yes, go to step  322 ; otherwise, go to step  316 ;  
         [0033]     Step  316 : Is the error value E 1  greater than the threshold value T 1 ? If yes, go to step  318 ; otherwise, go to step  320 ;  
         [0034]     Step  318 : The processor  106  decreases the equalizing parameter, boost G m , set to the equalizer  102 . Go to step  306 ;  
         [0035]     Step  320 : The processor  106  increases the equalizing parameter, boost G m , set to the equalizer  102 . Go to step  306 ;  
         [0036]     Step  322 : The comparing module  114  compares the second average value with an expected value of the second run length set.  
         [0037]     Step  324 : Does an error value E 2  between the second average value and the expected value fall in a second range delimited by two threshold values T 3  and T 4 ? If yes, go to step  306 ; otherwise, go to step  326 ;  
         [0038]     Step  326 : The processor  106  utilizes a step size to adjust the other equalizing parameter, central frequency f c , in a direction to reduce the error value E 2  of the second run length set. Then return to Step  306 .  
         [0039]     In the above-mentioned embodiment, the average value of measured 3T run lengths decreases as the equalizing parameter, boost G m , decreases. If the architecture of the equalizer  102  is modified, the characteristic of the equalizer  102  might be changed. For example, the average value of measured 3T run lengths increases as the equalizing parameter, boost G m , decreases. In this case, step  316  is modified to determine if the error value E 1  is lower than the threshold value T 2 , and step  324  is modified to determine if the error value E 2  between the second average value and the expected value falls in the second range. Similarly, adjustment to the other equalizing parameter, central frequency f c , varies with the architecture of the equalizer  102 . In the above-mentioned embodiment, the equalizing parameter, central frequency f c , decreases in a predetermined step size if the error value between the average value of measured 4T run lengths and the expected value of the 4T run length is above the desired second range. However, in another embodiment having an equalizer with a specific architecture, the equalizing parameter, central frequency f c , increases in a predetermined step size if the error value between the average value of measured 4T run lengths and the expected value of the 4T run length is above the desired second range. In this case, step  326  should be updated accordingly to the design requirements. The same objective of finding out the optimum setting of equalizing parameters is achieved.  
         [0040]     Unlike the prior art, the present invention can adjust the parameters in a correct direction without a method of trial and error. Equalization by observing run length rather than jitter is therefore more efficient and achieves efficiency and speed.  
         [0041]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.