Patent Publication Number: US-2019199891-A1

Title: Circuit applied to display apparatus and associated signal processing method

Description:
This application claims the benefit of Taiwan application Serial No. 106145007, filed Dec. 21, 2017, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to signal processing in a display apparatus, and more particularly to an impulsive interference detection circuit applied to a display apparatus and an associated signal processing method. 
     Description of the Related Art 
     In the Digital Video Broadcasting-Second Generation Terrestrial (DVB-T2) standard, impulsive interference is considered as an issue severely affecting image display. Impulsive interference has large sudden and periodical amplitudes, and is usually generated by factors in the ambient environment, e.g., an operating washing machine or dishwasher, and a fast automobile passing by. Due to the influence of the impulsive interference, distortion may be caused by offsets in noise variances during a signal-to-noise (SNR) calculation process performed by an SNR calculation circuit, leading to subsequent signal processing errors. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a method for calculating a noise variance, wherein the method is capable of outputting more accurate noise variances even in the presence of impulsive interference so as to resolve issues of the prior art. 
     A circuit applied to a display apparatus is disclosed according to an embodiment of the present invention. The circuit includes a first noise variance estimation circuit, an impulsive interference determination circuit, a second noise variance estimation circuit and a selection circuit. The first noise variance estimation circuit calculates a first noise variance of an input signal. The impulsive interference determination circuit determines whether the input signal has impulsive interference according to the first noise variance to generate a detection result. The second noise variance estimation circuit calculates a second noise variance based on the input signal. The selection circuit selectively outputs one of the first noise variance and the second noise variance according to the selection result. 
     A signal processing method applied to a display apparatus is disclosed according to another embodiment of the present invention. The signal processing method includes: calculating a first noise variance of an input signal; determining whether the input signal has impulsive interference according to the first noise variance to generate a detection result; calculating a second noise variance of the input signal; and selectively outputting one of the first noise variance and the second noise variance according to the detection result. 
     The above and other aspects of the invention will become better understood with regard to the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a circuit applied to a display apparatus according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of a circuit applied to a display apparatus according to another embodiment of the present invention; 
         FIG. 3  is a block diagram of a first noise variance estimation circuit according to an embodiment of the present invention; 
         FIG. 4  is a detailed block diagram of the first noise variance estimation circuit according to an embodiment of the present invention; 
         FIG. 5  is a schematic diagram of a frequency-domain signal; 
         FIG. 6  is a schematic diagram of a receiver according to an embodiment of the present invention; and 
         FIG. 7  is a flowchart of a signal processing method applied to a display apparatus according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a block diagram of a circuit  100  applied to a display apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , the circuit  100  in  FIG. 1  includes a first noise variance estimation circuit  110 , an impulsive interference determination circuit  112 , a second noise variance estimation circuit  120  and a selection circuit  124 . In this embodiment, the circuit  100  is provided in a Digital Video Broadcasting-Second Generation Terrestrial (DVB-T2)-conforming receiver of a television or a set-top box (STB), and a signal received by the receiver adopts an orthogonal frequency-division multiplexing (OFDM) modulation scheme. 
     In the operation of the circuit  100 , the first noise variance estimation circuit  110  calculates a first noise variance σ n   2  based on an input signal Vin, and the impulsive interference determination circuit  112  determines whether the input signal Vin has impulsive interference according to the first noise variance σ n   2 . More specifically, in this embodiment, the input signal Vin is a frequency-domain signal, which includes multiple symbols, and the first noise variance estimation circuit  110  calculates the first noise variance σ n   2  based on multiple pilot cells in each symbol, where the subscript “n” represents the symbol number. Associated implementation details are to be given shortly in the following disclosure. After the first noise variance σ n   2  has been calculated, the impulsive interference determination circuit  112  determines whether the first noise variance σ n   2  is greater than a threshold to determine whether the input signal Vin has impulsive interference to generate a detection result Vc. For example, if the first noise variance is greater than the threshold, it is determined that the input signal Vin has impulsive interference, otherwise it is determined that the input signal Vin does not have impulsive interference. 
     The second noise variance estimation circuit  120  real-time calculates a second noise variance σ n,k   2  according to multiple observation values y n,k  of the input signal Vin, an estimated channel response h n,k , and multiple ideal values x n,k  of the input signal Vin, wherein the subscript “n” represents the symbol number and the subscript “k” represents the carrier number. In one embodiment, the calculation method of the second noise variance σ n,k   2  is: σ n,k   2 =|N n,k | 2 |y n,k −h n,k ·x n,k | 2 , where N n,k  is a noise variance statistical value of the k th  carrier of the n th  symbol. 
     It should be noted that, because the first noise variance σ n   2  is calculated in regard to determining whether the input signal Vin has impulsive interference, the first noise variance σ n   2  is capable of fully reflecting the influence of impulsive interference. In contrast, the second noise variance σ n,k   2  is calculated in regard to the difference between the observation value y n,k  and the product of the estimated channel response h n,k  and the ideal value x n,k  of the k th  carrier of the n th  symbol, the second noise variance σ n,k   2  does not actually reflect the influence of impulsive interference. Therefore, the selection circuit  124  in this embodiment may select the first noise variance σ n   2  or the second noise variance σ n,k   2  according to the detection result Vc, and provides the selected noise variance for subsequent use. More specifically, when the detection result Vc indicates the presence of impulsive interference in the input signal Vin, the selection circuit  124  outputs the first noise variance σ n   2 ; when the detection result Vc indicates that the input signal Vin does not have impulsive interference, the selection circuit  124  outputs the second noise variance σ n,k   2 . 
     As described above, since the selection circuit  124  may select the more appropriate noise variance according to whether the input signal Vin has impulsive interference, the issue of the prior art, in which distortion caused by offsets in noise variances during the SNR calculation process results in subsequent signal processing errors, is resolved. 
       FIG. 2  shows a block diagram of a circuit  200  applied to a display apparatus according to another embodiment of the present invention. The difference of the circuit  200  from the circuit  100  in  FIG. 1  is that, a second noise variance estimation circuit  220  includes a calculation circuit  222  and a filter  226 . The operation of the calculation circuit  222  is identical to that of the second noise variance estimation circuit  120  in  FIG. 1 , and the operations of the other components are identical to those having the same names. Thus, description in regard to only the filter  226  is given below. 
     In the circuit  200 , the filter  226  may perform a filtering operation (i.e., smoothing processing) on the noise variance calculated by the calculation circuit  222 . For example, the second noise variance σ n,k   2  may be calculated according to a calculation method: σ n,k   2 =σ n-1,k   2 +α·(σ n,k   2 −σ n-1,k   2 ), where α may be any value between 0 and 1, σ n,k   2  is the noise variance outputted by the filter  226  for the k th  carrier of the n th  symbol, σ n-1,k   2  is the noise variance outputted by the filter  226  for the k th  carrier of the (n−1) th  symbol, and σ n,k   2 ′ is the noise variance outputted by the calculation circuit  222  for the k th  carrier of the n th  symbol. In another embodiment, the impulsive interference determination circuit  212  may also send the detection result Vc to the filter  226 , and the filter  226  is turned off when the indication result Vc indicates that the input signal Vc has impulsive interference. 
       FIG. 3  shows a block diagram of the first noise variance estimation circuit  110 / 210  according to an embodiment of the present invention. As shown in  FIG. 3 , the first noise variance estimation circuit  110 / 210  includes a noise capture circuit  310  and a variance calculation circuit  320 .  FIG. 4  shows a detailed block diagram of the first noise variance estimation circuit  110 / 210  according to an embodiment. In this embodiment, the noise capture circuit  310  is implemented by a filter. In  FIG. 4  and the following description, the noise capture circuit  310  is a second-order filter as an example for illustrations. Thus, the noise capture circuit  310  of this embodiment includes two delay circuits  412  and  414 , two multipliers  415  and  416  (having a multiplier of 0.5), and two adders  417  and  418 ; however, the present invention is not limited to the above. In other embodiments, the noise capture circuit  310  may also be implemented as a filter of an order higher than the second order. The variance calculation circuit  320  includes an intensity calculation circuit  422  and a summation circuit  424 .  FIG. 5  shows a schematic diagram of the input signal Vin (a frequency-domain signal) in this embodiment, where the vertical axis represents OFDM symbols at different time points, and each row represents one OFDM symbol, and each OFDM symbol includes an edge pilot cell, multiple data cells and multiple scatter pilot cells; the horizontal axis represents the frequency, and the columns respectively correspond to different carriers. In this embodiment, the first noise variance estimation circuit  110 / 210  sequentially generates the variance statistical information of noise of pilot cells of each symbol (i.e., the OFDM symbol at each row in  FIG. 5 ), and the impulsive interference determination circuit  112 / 212  accordingly generates a detection result. Operation details of each of the circuit components are given below with reference to equations. 
     The pilot cells of the input signal Vin (a frequency-domain signal) are captured by a pilot cell capture circuit, and the channel frequency response thereof may be represented as Ĥ n,k =H n,k +N n,k , where the subscript “n” represents the sequence number of the symbol (i.e., which row in  FIG. 5 ), and the subscript “k” represents the number of carrier (i.e., which column in  FIG. 5 ), H n,k  represents the channel frequency response of the pilot cells, N n,k  represents of noise of the pilot cells, and the noise includes additive white Gaussian noise (AWGN), inter-carrier interference (ICI), adjacent-channel interference (ACI), co-channel interference (CCI) and impulsive interference. Further, the channel impulse response of the pilot cells may be represented as: 
     
       
         
           
             
               
                 h 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
               = 
               
                 
                   ∑ 
                   
                     m 
                     = 
                     0 
                   
                   
                     M 
                     - 
                     1 
                   
                 
                  
                 
                   
                     δ 
                      
                     
                       ( 
                       
                         t 
                         - 
                         
                           τ 
                           m 
                         
                       
                       ) 
                     
                   
                   · 
                   
                     e 
                     
                       j 
                        
                       
                           
                       
                        
                       
                         θ 
                         m 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     where δ(t) is a delta function, τ m  and θ m  are delay and phase of the corresponding path, and M is the quantity of paths. The noise capture circuit  310  may be represented as: H k   diff =δ[k]−0.5·(δ[k+1]+δ[k−1]), and is 
     
       
         
           
             
               
                 h 
                 diff 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               1 
               - 
               
                 cos 
                  
                 
                   ( 
                   
                     2 
                      
                     π 
                      
                     
                       t 
                       
                         T 
                         sp 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
     on a corresponding time domain. Thus, the output from the noise capture circuit  310  in  FIG. 4  may be represented as: 
     
       
         
           
             
               
                 
                   H 
                   ^ 
                 
                 
                   n 
                   , 
                   k 
                 
               
               - 
               
                 0.5 
                  
                 
                   ( 
                   
                     
                       
                         H 
                         ^ 
                       
                       
                         n 
                         , 
                         
                           k 
                           - 
                           1 
                         
                       
                     
                     + 
                     
                       
                         H 
                         ^ 
                       
                       
                         n 
                         , 
                         
                           k 
                           + 
                           1 
                         
                       
                     
                   
                   ) 
                 
               
             
             = 
             
               
                 
                   ( 
                   
                     
                       δ 
                        
                       
                         [ 
                         k 
                         ] 
                       
                     
                     - 
                     
                       0.5 
                       · 
                       
                         ( 
                         
                           
                             δ 
                              
                             
                               [ 
                               
                                 k 
                                 + 
                                 1 
                               
                               ] 
                             
                           
                           + 
                           
                             δ 
                              
                             
                               [ 
                               
                                 k 
                                 - 
                                 1 
                               
                               ] 
                             
                           
                         
                         ) 
                       
                     
                   
                   ) 
                 
                 ⊗ 
                 
                   
                     H 
                     ^ 
                   
                   
                     n 
                     , 
                     k 
                   
                 
               
               = 
               
                 
                   
                     H 
                     k 
                     diff 
                   
                   ⊗ 
                   
                     
                       H 
                       ^ 
                     
                     
                       n 
                       , 
                       k 
                     
                   
                 
                 = 
                 
                   
                     
                       H 
                       k 
                       diff 
                     
                     ⊗ 
                     
                       ( 
                       
                         
                           H 
                           
                             n 
                             , 
                             k 
                           
                         
                         + 
                         
                           n 
                           
                             n 
                             , 
                             k 
                           
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             H 
                             k 
                             diff 
                           
                           ⊗ 
                           
                             H 
                             
                               n 
                               , 
                               k 
                             
                           
                         
                         + 
                         
                           
                             H 
                             k 
                             dif 
                           
                           ⊗ 
                           
                             N 
                             
                               n 
                               , 
                               k 
                             
                           
                         
                       
                       ≈ 
                       
                         
                           H 
                           k 
                           dif 
                         
                         ⊗ 
                         
                           N 
                           
                             n 
                             , 
                             k 
                           
                         
                       
                     
                     = 
                     
                       
                         N 
                         
                           n 
                           , 
                           k 
                         
                       
                       - 
                       
                         0.5 
                          
                         
                           ( 
                           
                             
                               N 
                               
                                 n 
                                 , 
                                 
                                   k 
                                   + 
                                   1 
                                 
                               
                             
                             + 
                             
                               N 
                               
                                 n 
                                 , 
                                 
                                   k 
                                   - 
                                   1 
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     In brief, because adjacent pilot cells theoretically have substantially the same signal intensity, the data outputted by the noise capture circuit  310  each time is a difference between noise components of one pilot cell and an average of noise components of two adjacent pilot cells on the left and right of the pilot cell. 
     The variance calculation circuit  320  calculates the variance statistical information of noise of pilot cells of each symbol. More specifically, the intensity calculation circuit  422  calculates a discrepancy level between differences of noise captured by the noise capture circuit  310 ; for example, the intensity calculation circuit  422  squares the output from the noise capture circuit  310  as its output, and the summation circuit  424  sums up the output from the intensity calculation circuit  422  to generate the first noise variance. More specifically, calculation equations of the filter  310 , the intensity calculation circuit  422  and the summation circuit  424  may be represented as follows: 
     
       
         
           
             
               
                 σ 
                 n 
                 2 
               
               ≈ 
               
                 
                   2 
                   3 
                 
                  
                 
                   1 
                   
                     K 
                     - 
                     2 
                   
                 
                  
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       1 
                     
                     
                       K 
                       - 
                       2 
                     
                   
                    
                   
                     
                        
                       
                         
                           N 
                           
                             n 
                             , 
                             k 
                           
                         
                         - 
                         
                           
                             1 
                             2 
                           
                            
                           
                             ( 
                             
                               
                                 N 
                                 
                                   n 
                                   , 
                                   
                                     k 
                                     - 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 N 
                                 
                                   n 
                                   , 
                                   
                                     k 
                                     + 
                                     1 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                        
                     
                     2 
                   
                 
               
             
             = 
             
               
                 2 
                 3 
               
                
               
                 1 
                 
                   K 
                   - 
                   2 
                 
               
                
               
                 
                   ∑ 
                   
                     k 
                     = 
                     1 
                   
                   
                     K 
                     - 
                     2 
                   
                 
                  
                 
                   { 
                   
                     
                       
                          
                         
                           N 
                           
                             n 
                             , 
                             k 
                           
                         
                          
                       
                       2 
                     
                     + 
                     
                       
                         1 
                         4 
                       
                        
                       
                         ( 
                         
                           
                             
                                
                               
                                 N 
                                 
                                   n 
                                   , 
                                   
                                     k 
                                     - 
                                     1 
                                   
                                 
                               
                                
                             
                             2 
                           
                           + 
                           
                             
                                
                               
                                 N 
                                 
                                   n 
                                   , 
                                   
                                     k 
                                     + 
                                     1 
                                   
                                 
                               
                                
                             
                             2 
                           
                         
                         ) 
                       
                     
                     - 
                     
                       Re 
                        
                       
                         { 
                         
                           
                             
                               N 
                               
                                 n 
                                 , 
                                 k 
                               
                             
                             ( 
                             
                               
                                 N 
                                 
                                   n 
                                   , 
                                   
                                     k 
                                     - 
                                     1 
                                   
                                 
                                 * 
                               
                               + 
                               
                                 N 
                                 
                                   n 
                                   , 
                                   
                                     k 
                                     + 
                                     1 
                                   
                                 
                                 * 
                               
                             
                             } 
                           
                           + 
                           
                             
                               1 
                               2 
                             
                              
                             
                               N 
                               
                                 n 
                                 , 
                                 
                                   k 
                                   - 
                                   1 
                                 
                               
                             
                              
                             
                               N 
                               
                                 n 
                                 , 
                                 
                                   k 
                                   + 
                                   1 
                                 
                               
                               * 
                             
                           
                         
                         } 
                       
                     
                   
                   } 
                 
               
             
           
         
       
     
     In the above equation, “K−2” represents the quantity of pilot cells calculated, and 
     
       
         
           
             “ 
             
               
                 2 
                 3 
               
                
               
                 1 
                 
                   K 
                   - 
                   2 
                 
               
             
             ” 
           
         
       
     
     is an adjustment ratio. If the noise variance of each pilot cell is defined as σ n,k   2  ≡E{|n n,k | 2 }, the above calculation equation may be represented as follows: 
     
       
         
           
             
               E 
                
               
                 { 
                 
                   
                     σ 
                     ^ 
                   
                   n 
                   2 
                 
                 } 
               
             
             = 
             
               
                 
                   2 
                   3 
                 
                  
                 
                   1 
                   
                     K 
                     - 
                     2 
                   
                 
                  
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       1 
                     
                     
                       K 
                       - 
                       2 
                     
                   
                    
                   
                     E 
                      
                     
                       { 
                       
                         
                           
                              
                             
                               n 
                               
                                 n 
                                 , 
                                 k 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                             1 
                             4 
                           
                            
                           
                             ( 
                             
                               
                                 
                                    
                                   
                                     n 
                                     
                                       n 
                                       , 
                                       
                                         k 
                                         - 
                                         1 
                                       
                                     
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     n 
                                     
                                       n 
                                       , 
                                       
                                         k 
                                         + 
                                         1 
                                       
                                     
                                   
                                    
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                         - 
                         
                           Re 
                            
                           
                             { 
                             
                               
                                 
                                   n 
                                   
                                     n 
                                     , 
                                     k 
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       n 
                                       
                                         n 
                                         , 
                                         
                                           k 
                                           - 
                                           1 
                                         
                                       
                                       * 
                                     
                                     + 
                                     
                                       n 
                                       
                                         n 
                                         , 
                                         
                                           k 
                                           + 
                                           1 
                                         
                                       
                                       * 
                                     
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   n 
                                   
                                     n 
                                     , 
                                     
                                       k 
                                       - 
                                       1 
                                     
                                   
                                 
                                  
                                 
                                   n 
                                   
                                     n 
                                     , 
                                     
                                       k 
                                       + 
                                       1 
                                     
                                   
                                   * 
                                 
                               
                             
                             } 
                           
                         
                       
                       } 
                     
                   
                 
               
               = 
               
                 
                   
                     2 
                     3 
                   
                    
                   
                     1 
                     
                       K 
                       - 
                       2 
                     
                   
                    
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         1 
                       
                       
                         K 
                         - 
                         2 
                       
                     
                      
                     
                       ( 
                       
                         
                           σ 
                           
                             n 
                             , 
                             k 
                           
                           2 
                         
                         + 
                         
                           
                             1 
                             4 
                           
                            
                           
                             σ 
                             
                               n 
                               , 
                               
                                 k 
                                 - 
                                 1 
                               
                             
                             2 
                           
                         
                         + 
                         
                           
                             1 
                             4 
                           
                            
                           
                             σ 
                             
                               n 
                               , 
                               
                                 k 
                                 + 
                                 1 
                               
                             
                             2 
                           
                         
                       
                       ) 
                     
                   
                 
                 = 
                 
                   
                     
                       1 
                       
                         K 
                         - 
                         2 
                       
                     
                      
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           0 
                         
                         
                           K 
                           - 
                           1 
                         
                       
                        
                       
                         σ 
                         
                           n 
                           , 
                           k 
                         
                         2 
                       
                     
                   
                   - 
                   
                     
                       1 
                       
                         6 
                          
                         
                           ( 
                           
                             K 
                             - 
                             2 
                           
                           ) 
                         
                       
                     
                      
                     
                       ( 
                       
                         
                           5 
                            
                           
                             σ 
                             
                               n 
                               , 
                               0 
                             
                             2 
                           
                         
                         + 
                         
                           5 
                            
                           
                             σ 
                             
                               n 
                               , 
                               
                                 K 
                                 - 
                                 1 
                               
                             
                             2 
                           
                         
                         + 
                         
                           σ 
                           
                             n 
                             , 
                             1 
                           
                           2 
                         
                         + 
                         
                           σ 
                           
                             n 
                             , 
                             
                               K 
                               - 
                               2 
                             
                           
                           2 
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     The noise variance of the symbol is again defined as the average of the variance of each pilot cell, and the noise variance of the symbol may be represented as: 
     
       
         
           
             
               
                 σ 
                 _ 
               
               n 
               2 
             
             ≡ 
             
               
                 1 
                 K 
               
                
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     K 
                     - 
                     1 
                   
                 
                  
                 
                   
                     σ 
                     
                       n 
                       , 
                       k 
                     
                     2 
                   
                   . 
                 
               
             
           
         
       
     
     If the value of K is large enough, the output from the first noise variance estimation circuit  110 / 210  may be represented as: 
     
       
         
           
             
               E 
                
               
                 { 
                 
                   
                     σ 
                     ^ 
                   
                   n 
                   2 
                 
                 } 
               
             
             = 
             
               
                 
                   
                     
                       K 
                       
                         K 
                         - 
                         2 
                       
                     
                      
                     
                       ( 
                       
                         
                           1 
                           K 
                         
                          
                         
                           
                             ∑ 
                             
                               k 
                               = 
                               0 
                             
                             
                               K 
                               - 
                               1 
                             
                           
                            
                           
                             σ 
                             
                               n 
                               , 
                               k 
                             
                             2 
                           
                         
                       
                       ) 
                     
                   
                   - 
                   
                     
                       1 
                       
                         6 
                          
                         
                           ( 
                           
                             K 
                             - 
                             2 
                           
                           ) 
                         
                       
                     
                      
                     
                       ( 
                       
                         
                           5 
                            
                           
                             σ 
                             
                               n 
                               , 
                               0 
                             
                             2 
                           
                         
                         + 
                         
                           5 
                            
                           
                             σ 
                             
                               n 
                               , 
                               
                                 K 
                                 - 
                                 1 
                               
                             
                             2 
                           
                         
                         + 
                         
                           σ 
                           
                             n 
                             , 
                             1 
                           
                           2 
                         
                         + 
                         
                           σ 
                           
                             n 
                             , 
                             
                               K 
                               - 
                               2 
                             
                           
                           2 
                         
                       
                       ) 
                     
                   
                 
                 → 
                 
                   
                     1 
                     K 
                   
                    
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         0 
                       
                       
                         K 
                         - 
                         1 
                       
                     
                      
                     
                       σ 
                       
                         n 
                         , 
                         k 
                       
                       2 
                     
                   
                 
               
               = 
               
                 
                   σ 
                   _ 
                 
                 n 
                 2 
               
             
           
         
       
     
     As described above, the first noise variance estimation circuit  110 / 210  is capable of outputting the noise variance average of the carrier frequencies in each symbol as the first noise variance. 
     Noise of each pilot cell includes normally occurring noise and noise caused by impulsive interference, wherein the normally occurring noise may be AWGN, ICI, ACI and CCI, and so the noise variance of each symbol outputted by the first noise variance estimation circuit  110 / 210  also includes normally occurring noise and impulsive interference. However, in the above calculation process, particularly noticeable values are generated based on sporadic characteristics of impulsive interference. Thus, the method of the embodiment is capable of accurately calculating the noise variance (i.e., the first noise variance σ n   2 ), and is specifically capable of determining whether each symbol has impulsive interference by determining whether the first noise variance is greater than a threshold. 
     The circuits  100  and  200  shown in  FIG. 1  and  FIG. 2  may be applied in a receiver.  FIG. 6  shows a schematic diagram of a receiver  600  according to an embodiment of the present invention. As shown in  FIG. 6 , the circuit  600  includes a front-end circuit  610 , a time-domain/frequency-domain conversion circuit  630 , a pilot signal capture circuit  640 , a data capture circuit  642 , a first noise variance estimation circuit  110 / 120 , an impulsive interference detection circuit  112 / 212 , a channel estimation circuit  670 , an equalizer  680 , a second noise variance estimation circuit  120 / 220 , a selection circuit  124 / 224 , an SNR estimation circuit  690  and a back-end circuit  692 . In this embodiment, the receiver  600  processes an analog input signal from an antenna, and generates an output signal to a back-end processing circuit in a television or in an STB so as to play the output signal on a screen. 
     In the circuit  600 , the front-end circuit  610  performs analog-to-digital conversion on the received signal, and filters out adjacent channel interference (ACI) from the digital input signal to generate a digital input signal. The time-domain/frequency-domain conversion circuit  630  converts the digital input signal from a time domain to a frequency domain to generate a frequency-domain signal. The pilot signal capture circuit  640  captures multiple pilot cells (may be edge pilot cells and/or scattered pilot cells) in one symbol. Operation details of the first noise variance estimation circuit  110 / 210  and the impulsive interference determination circuit  112 / 212  are similar to those in  FIGS. 3 and 4 , and are omitted herein. The channel estimation circuit  670  calculates the channel frequency response CE and signal intensity corresponding to the symbol in the frequency-domain signal according to the pilot cells. On the other hand, the data capture circuit  642  captures multiple data cells in the symbol from the frequency-domain signal. The equalizer  680  performs equalization on the multiple data cells according to the channel frequency response calculated by the channel estimation circuit  670  to generate an equalized signal EQ. Operation details of the second noise variance estimation circuit  120 / 220  are similar to those shown in  FIGS. 1 and 2 , and the selection circuit  124 / 224  selectively outputs the first noise variance σ n   2  calculated by the first noise variance estimation circuit  110 / 210  or the second noise variance σ n,k   2  calculated by the second noise variance estimation circuit  120 / 220 . The SNR estimation circuit  690  performs SNR estimation according to the first noise variance σ n   2  or the second noise variance σ n,k   2  to generate an estimated SNR result. The back-end circuit  692  performs operations such as de-interleaving, demapping and decoding on the equalized signal EQ according to the estimated SNR result. 
     In one embodiment, the SNR estimation circuit  690  generates the estimated SNR result by using the calculation method below: 
     
       
         
           
             
               
                 SNR 
                 
                   n 
                   , 
                   k 
                 
               
               = 
               
                 
                   S 
                   
                     n 
                     , 
                     k 
                   
                 
                 
                   σ 
                   
                     n 
                     , 
                     k 
                   
                   2 
                 
               
             
             , 
           
         
       
     
     where SNR n,k  is the SNR of the k th  carrier of the n th  symbol, and S n,k  is the signal intensity of the k th  carrier of the n th  symbol. 
       FIG. 7  shows a flowchart of a signal processing method applied to a display apparatus according to an embodiment of the present invention. Referring to  FIGS. 1 to 7  and the above disclosure, the process of  FIG. 7  includes following steps. 
     In step  700 , the process begins. 
     In step  702 , a first noise variance of an input signal is calculated, and it is determined according to the first noise variance whether the input signal has impulsive interference to generate a detection result. 
     In step  704 , a second noise variance is calculated according to multiple observation values of the input signal, an estimated channel response and multiple ideal values of the input signal. 
     In step  706 , one of the first noise variance and the second noise variance is selectively outputted according to the detection result, wherein the outputted first noise variance or second noise variance is used for performing SNR estimation. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.