Patent Publication Number: US-7224407-B2

Title: Color demodulation device, color demodulation method and image display device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a color demodulation device and method for reproducing color difference signals included in a television signal, namely R-Y and B-Y signals. The present invention also relates to an image display device including the color demodulation device so as to produce color image. 
   2. Description of the Related Art 
   To produce color image from an analog video signal, for example, the composite video signal, an image display device includes a color demodulation device. The color demodulation device reproduces R-Y and B-Y signals from a chrominance signal included in the composite video signal. The chrominance signal is a sum of color subcarrier signals modulated by the respective R-Y and B-Y signals, and these color subcarrier signals are 90° out of phase each other. The color subcarrier frequency (fsc) is approximately 3.58 MHz. The chrominance signal includes a burst signal consisting of at least eight cycles of the color subcarrier signals, which is 90° out of phase with respect to the color subcarrier signal modulated by the R-Y signal and is 180° out of phase with respect to the color subcarrier signal modulated by the B-Y signal, as shown in  FIG. 1 . In a conventional color demodulation device, the chrominance signal is sampled at the timing of a burst-lock clock signal, that is a 4 fsc clock signal synchronized with the burst signal. As shown in  FIG. 1 , the chrominance signal is sampled at each peak of the color subcarrier signals modulated by the respective R-Y and B-Y signals, thereby alternatively producing digital samples of the R-Y and B-Y signals. 
   The burst signal is not necessarily in phase with the horizontal sync signal HD. Therefore, the digital samples of the respective R-Y and B-Y signals, which are produced at the timing of the burst-lock clock signal, need to be sampled again at the timing of a clock signal synchronized with the horizontal synch signal HD. In other words, the image display device with the conventional color demodulation device requires another clock signal synchronized with the horizontal synch signal HD, as shown in  FIG. 2 . The sync filter  1  extracts a vertical sync signal VD and the horizontal sync signal HD from the composite video signal. The line-lock clock generator  4  generates a line-lock clock signal synchronized with the horizontal sync signal HD. The line-lock clock signal is supplied to three sets of AD converters  8 , a matrix/image processor  11 , and a display  10 . The display  10  may be a liquid crystal panel (LCD), a cathode ray tube, or a plasma display panel (PDP). 
   The burst signal filter  3  extracts the burst signal from the composite color video signal. The burst-lock clock generator  22  generates the burst-lock clock signal based on the burst signal extracted by the burst signal filter  3 . The burst-lock clock signal is supplied to an AD converter  20 , a YC filter  21 , a color modulator  23 , and three sets of DA converters. 
   The AD converter  20  samples the composite video signal using the burst-lock clock signal as a sampling clock. The YC filter  21  separates digital samples of a luminance signal (Y) and a chrominance signal (C) included in the digital samples of the composite video signal. The color demodulator  23  produces the digital sample of the respective R-Y and B-Y signals from the digital samples of the chrominance signal. The digital samples of the respective R-Y, B-Y and Y signals are converted to analog signals by the DA converters  24 . The AD converters  8  sample the analog signals produced by the DA converters  24  at the timing of the line-lock clock signal, thereby producing the digital signals of the R-Y and B-Y and Y signals synchronized with the line-lock clock signal. The matrix/image processor  11  converts the digital samples of the R-Y, B-Y and B-Y signals into that of R, G, and B signals. Such a structure shown in  FIG. 2 , using two clock signals, i.e. the burst-lock clock and line-lock clock signals, appears in Japanese Patent Laid-Open No. 298913/1999. 
   As discussed above, in the conventional color demodulation device, the chrominance signal has to be sampled at the timing of the burst-lock clock signal, which is precisely synchronized with the burst signal. In other words, if a phase of the burst-lock clock signal deviates from that of the burst signal, the digital samples of the R-Y and B-Y signal will not be produced accurately. Furthermore, the image display device with the conventional color demodulation method requires two different clock signal generators, which not only increase circuit size but also cause noise resulting from interference of different clocks. 
   SUMMARY OF THE INVENTION 
   It is therefore, a primary object of the invention to provide a color demodulation device and method which is capable of producing digital samples of the respective R-Y and B-Y signals without the burst-lock clock signal. This object is achieved in accordance with one aspect of the present invention which is a color demodulation device having an AD converter, a phase signal generator, a burst data generator and a multiplier. The AD converter produces digital samples of chrominance signal at a frequency four times a color subcarrier frequency, and the phase signal generator generating a phase signal for identifying phases of a burst signal and color subcarrier signals modulated by respective B-Y and R-Y signals. The burst data generator produce burst data corresponding to the burst signals which have predetermined phase relationships with the respective color subcarrier signals according to the phase signal. The multiplier produces products of the burst data and the digital samples of the chrominance signal, from which digital samples of the respective R-Y and B-Y signals are produced. 
   Another aspect of the present invention is a color demodulation method for producing digital samples of the respective R-Y and B-Y signals from products obtained by multiplying digital samples of the chrominance signal produced at a frequency four times a color subcarrier frequency by burst data. The burst data are digital samples corresponding to burst signals having predetermined phase relationships with the respective color subcarrier signals modulated by the respective R-Y and B-Y signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram for explaining a conventional color demodulation process. 
       FIG. 2  is a block diagram illustrating a structure of a display device including a conventional color demodulation device. 
       FIG. 3  is a block diagram illustrating a structure of a display device including a color demodulation device according to the first embodiment. 
       FIGS. 4 ,  6 , and  14  are diagrams showing signal waveforms or data represented by signals for explaining the operation of the color demodulation device according to the first embodiment. 
       FIGS. 5 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12  and  13  are block diagrams showing various portions of the image display device shown in  FIG. 1 . 
       FIGS. 15 and 16  are block diagrams each showing an alternative structure of the first embodiment. 
       FIG. 17  is a block diagram illustrating a structure of each of low pass filters according to the second embodiment. 
       FIG. 18  is a block diagram illustrating a structure of amplitude adjusters according to the third embodiment. 
       FIGS. 19 ,  20  and  21  are diagrams showing characteristics of the burst amplitude detector according to the fourth embodiment. 
       FIGS. 22 and 23  are diagrams showing signal waveforms for explaining the operation of the color demodulation devices according to the fifth and sixth embodiment, respectively. 
       FIG. 24  is a vector diagram useful for understanding the operation of the color demodulation device according to the eighth embodiment. 
       FIG. 25  is a block diagram illustrating a structure of an image display device including a color demodulation device according to the eighth embodiment. 
       FIG. 26  is a block diagram illustrating an alternative structure of the image display device according to the eighth embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
     FIG. 3  is a block diagram illustrating a structure of an image display device having a color demodulation device according to the first embodiment of the present invention. The color demodulation device and method described below can be implemented by a circuit including discrete elements or an IC, as well as software. 
   A sync separator  51  extracts a vertical sync signal VD and a horizontal sync signal HD from a composite color video signal, which is shown in  FIG. 6(   a ). A line-lock clock generator  52  generates a line-lock clock signal. The line-lock clock signal is a 4 fsc (frequency four times a color subcarrier frequency, approximately 3.58 MHz) clock signal synchronized with the horizontal sync signal HD. The line-lock clock generator  52  also generates a reset signal, which occurs at the timing of the vertical sync signal VD. 
   An AD converter  54  samples the composite color video signal at 4 fsc by using the line-lock clock signal as a sampling clock. A YC separator  55  separates digital samples Y representing a luminance signal and digital samples C representing a chrominance signal both of which art included in the digital samples of the composite video signal. The digital samples C are sent to a color demodulator  56 , and the digital samples Y are sent to a matrix/image processor  11 . 
   The color demodulator  56  produces digital samples R-Y and B-Y, which represent R-Y and B-Y signals, respectively. The matrix/image processor  11  receives the digital samples R-Y, B-Y and Y and develops R, G, and B signals using conventional method. The matrix/image processor  11  also performs image processing such as OSD (on-screen display) processing and contrast adjustment. The display  10  presents color image based on the R, G, and B signals. 
   As shown in  FIG. 6(   a ) or ( b ), the chrominance signal includes a burst signal, which can be expressed as E sin ωt, where E is an amplitude of the burst signal, herein after referred to as burst amplitude. As shown in  FIG. 4 , the color subcarrier signals modulated by the B-Y and R-Y signals lag the burst signal by 90° and 180°, respectively. Therefore, the color subcarrier signals modulated by the respective B-Y and R-Y signals can be expressed as −B sin ωt and −R cos ωt, respectively, where B and R are amplitudes of the B-Y and R-Y signal. And furthermore, the chrominance signal consisting of these color subcarrier signals, −B sin ωt and −R cos ωt, can be expressed as −(B sin ωt+R cos ωt). 
   The amplitudes B and R vary with time, they can however be regarded as constants in several cycles of the color subcarrier signal, since frequency bands of the B-Y and R-Y signals are sufficiently lower than color subcarrier frequency. 
   Since the line-lock clock signal has frequency four times the color subcarrier frequency, the interval between each sampling point is equal to 90° phase angle of the burst signal. Therefore, phase angles of four consecutive sampling points can be expressed as θ, θ+90°, θ+180°, and θ+270°, where θ is a phase difference between the line-lock clock signal and the burst signal. 
   Accordingly, the digital samples C at θ, θ+90°, θ+180°, and θ+270° are expressed as −(B sin θ+R cos θ), (B cos θ−R sin θ), −(B sin θ+R cos θ), and −(B sin θ−R cos θ), respectively. At the same time, sample values of the burst signal at θ, θ+90°, θ+180°, and θ+270°, herein after referred to as burst data A, B, C and D, are expressed as follows:
 
burst data A=E sin θ  (1)
 
burst data  B=E  sin(θ+90°)= E  cos θ  (2)
 
burst data  C=E  sin(θ+180°)=− E  sin θ  (3)
 
burst data  D=E  sin(θ+270°)=− E  cos θ  (4)
 
   By using the burst data, the digital samples B-Y are produced by the process represented by following mathematical operation. 
   First, the digital samples C are multiplied by the digital samples of the burst signal which is delayed by 180° phase angle with respect to the B-Y signal included in the chrominance signal. This process is performed by multiplying the digital samples of the chromnance signal at θ, θ+90°, θ+180°, and θ+270° by the burst data A, B, C and D, respectively. Products a 1 , a 2 , a 3  and a 4  produced by this process are expressed as follows:
 
 a 1=− E  sin θ( B  sin θ+ R  cos θ)=− EB  sin 2    θ−ER  sin θ cos θ  (5)
 
 a 2=− E  cos θ( B  cos θ− R  sin θ)=− EB  cos 2    θ+ER  sin θ cos θ  (6)
 
 a 3=− E  sin θ( B  sin θ+ R  cos θ)=− EB  sin 2    θ−ER  sin θ cos θ  (7)
 
 a 4=− E  cos θ( B  cos θ− R  sin θ)=− EB  cos 2    θ+ER  sin θ cos θ  (8)
 
   Then, the products a 1  and a 2 , a 2  and a 3 , a 3  and a 4 , and a 4  and a 1  are added together, thereby producing products EB, each of which corresponds to a product of the B-Y signal (B) and the burst amplitude (E) at each of the sampling points. This process is expressed as follows: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           a1 
                           + 
                           a2 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 
                                   - 
                                   E 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 B 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   sin 
                                   2 
                                 
                                 ⁢ 
                                 θ 
                               
                               - 
                               
                                 E 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θcosθ 
                               
                             
                             ) 
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           ( 
                           
                             
                               
                                 - 
                                 E 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                             + 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               B 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     
                                       sin 
                                       2 
                                     
                                     ⁢ 
                                     θ 
                                   
                                   + 
                                   
                                     
                                       cos 
                                       2 
                                     
                                     ⁢ 
                                     θ 
                                   
                                 
                                 ) 
                               
                             
                           
                           = 
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               B 
                               . 
                             
                           
                         
                       
                     
                   
                 
                   
               
             
             
               
                 ( 
                 9 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         a2 
                         + 
                         a3 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 - 
                                 E 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                             + 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
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                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                           
                           ) 
                         
                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         ( 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                         = 
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         a3 
                         + 
                         a4 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 - 
                                 E 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
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                                 sin 
                                 2 
                               
                               ⁢ 
                               θ 
                             
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                               ⁢ 
                               
                                   
                               
                               ⁢ 
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                               ⁢ 
                               
                                   
                               
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                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         ( 
                         
                           
                             
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                             ⁢ 
                             
                                 
                             
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           + 
                           
                             E 
                             ⁢ 
                             
                                 
                             
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                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
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                                 + 
                                 
                                   
                                     cos 
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                         = 
                         
                           
                             - 
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                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 11 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         a4 
                         + 
                         a1 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 - 
                                 E 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                             + 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                           
                           ) 
                         
                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         ( 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                         = 
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 12 
                 ) 
               
             
           
         
       
     
   
   The digital samples B-Y, i.e. the amplitude of the B-Y signal (B), are obtained by dividing each sum of the products, a 1 +a 2 , a 2 +a 3 , a 3 +a 4  and a 1 +a 4 , by the burst amplitude (E). 
   The digital samples B-Y can also be obtained by multiplying the digital samples C by the digital samples of the burst signal which is in phase with the B-Y signal in the process described above. In this case, the digital samples of the chrominance signal at θ, θ+90°, θ+180°, and θ+270° are multiplied by the burst data C, D, A and B, respectively, thereby producing products corresponding to a 1 , a 2 , a 3  and a 4 . With this alternative method, the products EB is obtained without minus sign. 
   On the other hand, the digital samples R-Y is produced by the process represented by following mathematical operation using the burst data A, B, C and D. 
   First, the digital samples Care multiplied by the digital samples of the burst signal which is delayed by 180° phase angle with respect to the R-Y signal included in the chrominance signal. This process is performed by multiplying the digital samples C at θ, θ+90°, θ+180°, and θ+270° by the burst data D, A, B and C, respectively. Products b 1 , b 2 , b 3  and b 4  produced by this process are expressed as follows:
 
 b 1=− E  cos θ( B  sin θ+ R  cos θ)=− EB  sin θ cos θ− ER  cos 2  θ  (13)
 
 b 2= E  sin θ( B  sin θ− R  cos θ)= EB  sin θ cos θ− ER  sin 2  θ  (14)
 
 b 3=− E  cos θ( B  cos θ+ R  sin θ)=− EB  sin θ cos θ− ER  cos 2  θ  (15)
 
 b 4= E  sin θ( B  cos θ− R  sin θ)= EB  sin θ cos θ− ER  sin 2  θ  (16)
 
   Then, the products b 1  and b 2 , b 2  and b 3 , b 3  and b 4 , and, b 4  and b 1  are added together, thereby producing products EB each of which corresponds to a product of the R-Y signal (R) and the burst amplitude at each of the sampling points. This process is expressed as follows: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           b1 
                           + 
                           b2 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 
                                   - 
                                   E 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 B 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θcosθ 
                               
                               - 
                               
                                 E 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   cos 
                                   2 
                                 
                                 ⁢ 
                                 θ 
                               
                             
                             ) 
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           ( 
                           
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                             - 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               R 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     
                                       sin 
                                       2 
                                     
                                     ⁢ 
                                     θ 
                                   
                                   + 
                                   
                                     
                                       cos 
                                       2 
                                     
                                     ⁢ 
                                     θ 
                                   
                                 
                                 ) 
                               
                             
                           
                           = 
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               R 
                               . 
                             
                           
                         
                       
                     
                   
                 
                   
               
             
             
               
                 ( 
                 17 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         b2 
                         + 
                         b3 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                             - 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                           
                           ) 
                         
                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         ( 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             θ 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                         = 
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 18 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         b3 
                         + 
                         b4 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 - 
                                 E 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                             - 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                           
                           ) 
                         
                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         ( 
                         
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                         = 
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 19 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         b4 
                         + 
                         b1 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 - 
                                 E 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               B 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θcosθ 
                             
                             - 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               θ 
                             
                           
                           ) 
                         
                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         ( 
                         
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                         = 
                         
                           
                             - 
                             E 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 20 
                 ) 
               
             
           
         
       
     
   
   The digital samples R-Y, i.e. the amplitude of the R-Y signal (R), are obtained by dividing each sum of two products b 1 +b 2 , b 2 +b 3 , b 3 +b 4 , and b 4 +b 1  by the burst amplitude (E). 
   The digital samples R-Y can also be obtained by multiplying digital samples C by the digital samples of the burst signal which is in phase with the B-Y signal in the process described above. In this case, the digital samples C at θ, θ+90°, θ+180°, and θ+270° are multiplied by the burst data D, A, B and C, respectively, thereby producing products corresponding to b 1 , b 2 , b 3  and b 4 . With this alternative method, the products EB is obtained without minus sign. 
   A color demodulation process discussed above is performed by the color demodulator  56 .  FIG. 5  is a block diagram illustrating an internal structure of the color demodulator  56 . As shown in  FIG. 6(   e ), a phase signal generator  58  generates a phase signal indicating four phases ‘ 1 ’, ‘ 2 ’, ‘ 3 ’ and ‘ 4 ’ which correspond to the phase angles θ, θ+90°, θ+180° and θ+270°, respectively. The phase signal indicates each of the four phases at the timing of the line-lock clock. The phase signal is provided to a burst data generator  57  and a selector  61 . A BGP generator  59  generates a burst gate pulse (BGP) including a burst interval, as shown in  FIG. 6(   d ). 
   The burst data generator  57  extracts digital samples of the burst signal included in the digital samples C, and identifies their respective phase angles, according to the phase signal. The digital samples of the burst signal at the respective four phases ‘ 1 ’, ‘ 2 ’, ‘ 3 ’ and ‘ 4 ’ correspond to the burst data A, B, C and D. Therefore, the burst data generator  57  produces the burst data A, B, C and D, for example, by averaging those samples at the respective four phases. 
   The selector  61  selectively outputs the burst data D, A, B, and C according to the phase signal, as shown in  FIG. 6(   f ), so as to provide digital samples of the burst data which is 180° out of phase with respect to the R-Y signal to a multiplier. The multiplier  63  derives the products b 1 , b 2 , b 3  and b 4  by multiplying the burst data provided by the selector  61  and the digital samples C. At the same time, a D-type flip flop (D-FF)  62  selectively outputs the burst data A, B, C and D according to the phase signal, as shown in  FIG. 6(   g ), so as to provide digital samples of the burst signal 180° out of phase with the B-Y signal to a multiplier  64 . The multiplier  63  derives the products a 1 , a 2 , a 3  and a 4  by multiplying the burst data provided by the D-FF  62  and the digital samples C. 
   The products a 1 , a 2 , a 3  and a 4  derived by the multiplier  64  are sent to a LPF  66 , and the products b 1 , b 2 , b 3  and b 4  derived by the multiplier  63  are sent to a LPF  65 .  FIG. 7  is a block diagram illustrating a structure of each of the LPFs  65  and  66 . A D-FF  86  delays each input by one line-lock clock cycle. Accordingly, the products a 2 , a 3 , a 4  and a 1 , or b 2 , b 3 , b 4  and b 1  are introduced to one of two inputs of an adder  87 , when the products a 1 , a 2 , a 3  and a 4 , or b 1 , b 2 , b 3  and b 4 , are introduced to another input. The adder  87  produce the product ER or EB by adding incoming inputs a 1  and a 2 , a 2  and a 3 , a 3  and a 4 , and a 4  and a 1 , or b 1  and b 2 , b 2  and b 3 , b 3  and b 4  and b 4  and b 1 . 
   The products ER and EB derived by the LPFs  65  and  66  are sent to amplitude adjusters  67  and  68 , respectively. The amplitude adjusters  67  and  68  produce the digital samples R-Y and B-Y by dividing the products EB and EB by a burst amplitude E detected by a burst amplitude detector  60 . The burst amplitude detector  60  detects the burst amplitude E based on the burst data A, B, C and D. 
     FIG. 8  is a block diagram illustrating a structure of the amplitude adjusters  67  and  68 . Dividers  100  and  101  divide the products EB and EB by the burst amplitude E, thereby producing the digital samples R-Y and B-Y. Outputs of the divider  100  and  101 , i.e. the digital samples R-Y and B-Y, are sent to muting circuits  102  and  103 , respectively. The muting circuits  102  and  103  shut off the outputs of the divider  102  and  103  when the burst amplitude E is smaller than a predetermined value, thereby performing a color killer process. 
   Detailed structures of the burst data generator  57 , the line-lock clock generator  52 , and the burst amplitude detector  60  will be described below. 
     FIG. 9  is a block diagram illustrating a structure of the burst data generator  57 . During the burst interval, a gate control circuit  69  outputs enable signals to gates  70 ,  71 ,  72  and  73  according to the phase signal. More specifically, the gate control circuit  69  outputs the enable signals to the respective gates  70 ,  71 ,  72  and  73 , when the phase signal indicates ‘ 1 ’, ‘ 2 ’, ‘ 3 ’ and ‘ 4 ’. Accordingly, digital samples sampled at the respective four phases, are sent to averagers  75 ,  76 ,  77  and  78 , respectively. The averagers  75 ,  76 ,  77  and  78  produce the burst data A, B, C and D by averaging the digital samples outputted by the respective gate  70 ,  71 ,  72  and  73 . A reset-enable pulse generator  74  generates a reset-enable pulse, which is supplied to the averager  75 ,  76 ,  77  and  78 , and the memories  79 ,  80 ,  81  and  82 , at the end of the burst interval. Memories  79 ,  80 ,  81  and  82  hold the respective burst data A, B, C and D at the timing of the reset-enable signal, and at the same time, the averagers  75 ,  76 ,  77  and  78  reset data processing. 
     FIG. 10  is an alternative structure of the burst data generator  57 , which produce one of the burst data A, B, C or D. An enable pulse generator  69  generates an enable pulse, which occurs when the phase signal indicates one of the four phases ‘ 1 ’, ‘ 2 ’, ‘ 3 ’ or ‘ 4 ’ during the burst interval. The D-FF  84  provides its output to an adder  83  in response to this enable pulse. Accordingly, the adder  83  produces a sum of digital samples of the burst signal at one of the four phases. The output of the adder  83  is also provided to a D-FF  85  via D-FF  84 . A reset-enable pulse generator  74  generates a reset-enable pulse, which occurs at the end of the burst interval, and supplies it to the. D-FFs  84  and  85 . The D-FF  85  holds the output of the D-FF  84  at the timing of the reset-enable pulse, thereby obtaining the sum produced by the adder  83 . At the same time, the D-FF  84  resets data processing. The D-FF  85  produce one of the burst data A, B, C or D, by dividing the sum produced by the adder  83  by a number of the digital samples added to this sum. All of the four burst data A, B, C and D can be obtained by arranging four sets of the structure shown in  FIG. 8 . 
     FIG. 11  is a block diagram illustrating a structure of the line-lock clock generator  52 . A phase comparator  88  produces a dc error voltage, which is proportional to a phase difference between the horizontal sync signal HD and a pulse signal produced by a counter  90 . A VCO  89  generates a clock signal whose frequency depends on this dc error voltage. The clock signal produced by the VCO  89  is supplied to the counter  90  and a phase shifter  91 . The counter  90  generates a pulse signal by dividing a frequency of the clock signal produced by the VCO  89  so as to produce a 4 fsc clock signal, which is supplied to the phase comparator  88 . Accordingly, the VCO  89  supplies 4 fsc clock signal which is synchronized to the horizontal sync signal HD to the phase shifter  91 . 
   The phase shifter  91  provides phase shifts equal to a multiple of 1/32 period of the 4 fsc clock signal to the input, clock signal, thereby producing thirty-two patterns of 4 fsc clock signal. A counter  92  counts sync pulses included in the vertical sync signal VD and outputs a number of the counted synch pulses to a selector  93 . The counter  92  resets counting and outputs the reset signal when thirty-two sync pulses are counted. This reset signal is sent to the burst amplitude detector  60  every thirty-two vertical periods. 
   The selector  93  selects one of a 4 fsc clock signal according to the count value of the counter  92 , and outputs the selected clock signal as a line-lock clock signal. This means a phase of the line-lock clock shifts every vertical period by a multiple of 1/32 period of the line-lock clock signal. The reason that the line-lock clock signal is shifted in this manner is to enable the burst amplitude detector  60  to produce the burst amplitude E. The operation of the burst amplitude detector  60  will be described later. 
   Although the line-lock clock generator shown in  FIG. 11  shifts the phase of the line-lock clock signal as discribed above, the invention is not limited to such a case. The phase shifter  91  may provide phase shift equal to a multiple of, for example, 1/16 period of the line-lock clock. 
     FIG. 12  is a block diagram illustrating a structure of the burst amplitude detector  60 . The burst data A, B, C and D are sent to a maximum selector  94 . The maximum selector  94  selects one of the four burst data A, B, C and D having maximum value as a maximum burst data. The maximum burst data is sent to a data processor  96 . An enable pulse generator  95  generates an enable pulse, which occurs at the end of the burst interval, and supplies it to the data processor  96  sixty-four times per vertical period. 
   The data processor  96  calculates the burst amplitude based on the maximum burst data. The memory  97  holds the burst amplitude calculated by the data processor  96  at the timing of the reset signal, which is released by the counter  92  every thirty-two vertical periods. 
     FIG. 13  is a block diagram illustrating a structure of the data processor  96 . An adder  98  adds the maximum burst data selected by the maximum selector  94  and an output of a D-FF  99 . The D-FF  99  feeds back its output to the adder  98 , in response to the enable pulse generated sixty-four times per vertical period by the enable pulse generator  95 . The D-FF  99  holds an output of the adder  98 , in response to the reset signal released every thirty-two vertical periods by the counter  92 . As a result, a sum of the sixty-four maximum burst data of thirty-two consecutive frames is obtained when the D-FF  99  receives the reset signal. The D-FF  99  discards the lowest 11 bits of the sum, and outputs the upper bits, which corresponds to a sum of thirty-two digital samples of the burst signals in a range of π/4&lt;θ&lt;3π/4. This sum is equal to an area indicated by a hatched portion in  FIG. 14 , which calculated as follows: 
   
     
       
         
           
             
               
                 
                   
                     
                       ∫ 
                       
                         
                           1 
                           4 
                         
                         ⁢ 
                         π 
                       
                       
                         
                           3 
                           4 
                         
                         ⁢ 
                         π 
                       
                     
                     ⁢ 
                     
                       E 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                       ⁢ 
                       
                         ⅆ 
                         θ 
                       
                     
                   
                   
                     
                       ( 
                       
                         
                           3 
                           4 
                         
                         - 
                         
                           1 
                           4 
                         
                       
                       ) 
                     
                     ⁢ 
                     π 
                   
                 
                 = 
                 
                   
                     
                       
                         2 
                         ⁢ 
                         
                           2 
                         
                       
                       π 
                     
                     ⁢ 
                     E 
                   
                   ≅ 
                   
                     0.9 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     E 
                   
                 
               
             
             
               
                 ( 
                 21 
                 ) 
               
             
           
         
       
     
   
   Therefore, the burst amplitude (E) is calculated by dividing the output data of the D-FF  99  by 0.9 or multiplying by 1.11 which is the reciprocal of 0.9. The value of the amplitude of the burst signal may be varied to control a color gain. 
   According to the color demodulation device and the color demodulation method discussed above, the digital samples B-Y and R-Y can be obtained using the line-lock clock signal, which is not necessarily in phase with the burst signal. 
   Although a frequency of the line-lock clock is assumed to be four times the color subcarrier frequency (4 fsc) in the above description, the line-lock clock signal of two times the color subcarrier signal with 50% duty cycle may also be used. In this case, the AD converter  54  samples the composite video signal at both rising and trailing edges of the line-lock clock signal, reducing clock generating power. Furthermore, the line-lock clock frequency may be 4n multiple the color subcarrier frequency, where n is a natural number. By increasing the frequency of the line-lock clock signal, the amplitude of the burst signal is detected more accurately. 
     FIGS. 15 and 16  are block diagrams each illustrating alternative structure of an image display device according to the first embodiment. The image display device shown in  FIG. 15  receives the luminance signal (Y signal) and the chrominance signal (C signal) separately. A sync Filter  51  extracts the horizontal sync signal HD and the vertical sync signal VD from the luminance signal. An AD converter  119  produces the digital samples Y. At the same time, an AD converter  120  produces the digital samples C. The AD converters  119  and  120  use the line-lock clock signal as a sampling clock to produce the digital samples Y and C. The image display device shown in  FIG. 16  uses CRT  122  for displaying an image. A DA converter  121  provides R, G and B analog signals to the CRT  122 . 
   Second Embodiment 
   The product EB can be obtained based on three of the products a 1 , a 2 , a 3  and a 4 , which are associated with adjacent three phases. For example, two multiple of the product EB associated with the phase angle θ+180° is obtained by adding the three products a 1 , a 2  and a 3  as follows; 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           a1 
                           + 
                           
                             2 
                             × 
                             a2 
                           
                           + 
                           a3 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             2 
                             ⁢ 
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           + 
                           
                             2 
                             ⁢ 
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             - 
                             2 
                           
                           ⁢ 
                           E 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             B 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             - 
                             2 
                           
                           ⁢ 
                           E 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           B 
                         
                       
                     
                   
                 
                   
               
             
             
               
                 ( 
                 22 
                 ) 
               
             
           
         
       
     
   
   Similarly, the product ER can be obtained based on three of the products b 1 , b 2 , b 3  and b 4  associated with adjacent three phases. For example, two multiple of the product ER associated with the phase angle θ+180° is obtained by adding the three products b 1 , b 2  and b 3  as follows; 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           b1 
                           + 
                           
                             2 
                             × 
                             b2 
                           
                           + 
                           b3 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             2 
                             ⁢ 
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                           
                             2 
                             ⁢ 
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               - 
                               E 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             B 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θcosθ 
                           
                           - 
                           
                             E 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             θ 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             - 
                             2 
                           
                           ⁢ 
                           E 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     sin 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                                 + 
                                 
                                   
                                     cos 
                                     2 
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             - 
                             2 
                           
                           ⁢ 
                           E 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           R 
                         
                       
                     
                   
                 
                 ⁢ 
                 
                     
                     
                 
               
             
             
               
                 ( 
                 23 
                 ) 
               
             
           
         
       
     
   
   By obtaining the product ER and EB based on the three of the products, the digital samples R-Y and B-Y are produced without “color deviation”, where the digital samples R-Y and B-Y do not match with the digital sample Y, which in turn results in less clear image. 
   The operation discussed above can be performed by each of the LPFs  65  and  66  according to the second embodiment shown in  FIG. 17 . Each of the products a 1 , a 2 , a 3  and a 4 , or b 1 , b 2 , b 3  and b 4  is provided to a D-FF  104  and an adder  107  from the multiplier  63 , or  64 . The D-FF  104  delays an input thereto by one line-lock clock period. The output of the D-FF  104  is provided to a D-FF  105  and a twofold multiplier  105 . The D-FF  105  delays the output of the D-FF  104  by one line-lock clock period. A twofold multiplier  106  doubles an output of the D-FF  104 . The adder  107  produces the two multiple of the product AR or AB by adding three input thereto. 
   The structure of each of the LPFs  65  and  66  is not limited to the one shown in  FIG. 13 . Any kind of filters, which provides zero gain to the 2 fsc and DC components, is employable as the LPFs  65  and  66 . 
   Third Embodiment 
     FIG. 18  is a block diagram illustrating the amplitude adjusters  67  and  68  (see  FIG. 5 ) according to the third embodiment. The amplitude adjuster shown in  FIG. 18  further includes a squaring circuit  108 , which squares the burst amplitude (E) provided by the burst amplitude detector  60 . By squaring the burst amplitude, the digital samples R-Y and B-Y are normalized by the amplitude of the burst signal, which means the amplitudes of the digital samples R-Y and B-Y are kept in an appropriate range. 
   Fourth Embodiment 
   As shown in  FIGS. 8 and 16 , the amplitude adjusters  67  and  68  adjust amplitude of the digital samples R-Y and B-Y by dividing the products AR and AB by the burst amplitude. Therefore, The burst amplitude with extremely small value causes noise components in the R, G and B signals. Conversely, the burst amplitude with extremely large value results in the R, G and B signals with tint color. The burst amplitude detector  60  according to the fourth embodiment solves these problems by correcting the burst amplitude as follows. 
     FIGS. 19 ,  20  and  21  are diagrams showing examples of output characteristics of the burst amplitude detector  60  according to the fourth embodiment. In  FIGS. 19 ,  20  and  21 , each of horizontal axes represents the detected value of the burst amplitude, and each of vertical axes represents an output value of the burst amplitude, which is provided to the amplitude adjusters  67  and  68 . As shown in  FIG. 19 , by limiting the burst amplitude to a predetermined range, the amplitudes of the digital samples R-Y and B-Y are kept in appropriate range, thereby preventing noise components from being contained in the R, G and B signals. This noise can be prevented more effectively by increasing the burst amplitude when it becomes less than predetermined value as shown in  FIG. 20 . The output characteristic shown in  FIG. 21 , which is a combination of that of  FIGS. 15 and 16  is also employable. 
   By correcting a value of the burst amplitude as discussed above, the amplitude of the digital samples R-Y and B-Y can be adjusted accurately, thereby improving a quality of colors represented by the R, G and B signals. 
   Fifth Embodiment 
     FIG. 22  is a diagram for explaining an alternative method for detecting the burst amplitude in the burst data generator  57  (see  FIGS. 9 and 10 ) according to the fifth embodiment. In  FIG. 22 , samples An, Bn, Cn and Dn (n=1, 2, 3 . . . ) indicated by black points correspond to the digital samples of the burst signal sampled at the respective four phases, i.e. θ, θ+90, θ+180, θ+270, at which the phase signal indicates ‘ 1 ’, ‘ 2 ’, ‘ 3 ’ and ‘ 4 ’. 
   The burst data generator  57  according to the fifth embodiment determines. the burst data A, B, C and D based on plurality of the respective samples An, Bn, Cn and Dn. For example, the burst data generator  57  selects a sample A 3  as the burst data A, when A 1 &lt;A 3 &lt;A 2 . The burst data is obtained more accurately by averaging selected samples. For example, when A 3 &lt;A 1 &lt;A 4 &lt;A 2 , the burst data A is calculated as (A 1 +A 4 )/2. 
   Sixth Embodiment 
   As mentioned in the first embodiment, the digital samples R-Y and B-Y can be produced more accurately by increasing the line-lock clock signal frequency by 4×n multiples of the color subcarrier frequency. A color modulator according to the sixth embodiment has the line-lock clock signal with frequency eight times the color subcarrier (8 fsc).  FIG. 23  is a diagram showing the chrominance signal being sampled at frequency eight times the color subcarrier frequency (8 fsc). As shown in  FIG. 23 , the digital samples C are sampled at eight phase angles, θ, θ+45°, θ+90°, θ+135°, θ+180°, θ+225°, θ+270° and θ+315°, where the phase signal indicates ‘ 1 ’, ‘ 1 . 5 ’, ‘ 2 ’, ‘ 2 . 5 ’, ‘ 3 ’, ‘ 3 . 5 ’, ‘ 4 ’ and ‘ 4 . 5 ’. 
   The color demodulator according to the sixth embodiment produces the digital samples R-Y and B-Y by adding two products associated with phase angles θ+315° and θ+45°, θ+45° and θ+135°, θ+135° and θ+225°, θ+225° and θ+315°, and θ+315° and θ+45°, thereby preventing the “color deviation” discussed in the second embodiment. Therefore, in this case, the burst data A, B, C and D are produced based on digital samples of the burst signal sampled at the phase angles θ+45°, θ+135°, θ+225° and θ+315°, respectively. 
   Seventh Embodiment 
   Each of the burst data generator  57  shown in  FIGS. 9 and 10  produces the burst data by averaging digital samples of the burst signal included in the digital samples C. The seventh embodiment presents alternative way to produce the burst data. 
   Value of the respective burst data A, B, C and D of are given by following equations;
 
burst data A=sin θ  (23)
 
burst data  B =sin(θ+90°)  (24)
 
burst data  C =sin(θ+180°)  (25)
 
burst data  D =sin(θ+270°)  (26),
 
where θ is the phase difference between the line-lock clock signal and the burst signal (0≦θ&lt;2π).
 
   The burst data generator  57  produces the burst data A, B, C and D according to the equation (23)-(26), by adjusting θ so that the digital samples R-Y and B-Y are modulated correctly. Users may control these operations, which is performed by the burst data generator  57 , by observing reference color in a reproduced image. 
   The burst data generator  57  according to the seventh embodiment can be formed with a microcomputer, which calculates value of the respective burst data by using the equation (23)-(26). It is possible to use the digital samples C including at least one cycle of the burst signal as the burst data, since the color subcarrier frequency is much higher than that of the chrominance signal. 
   Eighth Embodimient 
     FIG. 25  is a block diagram illustrating a structure of an image display device according to the eighth embodiment, which includes the color demodulation device according to each of the embodiments described above. As shown in  FIG. 25 , the. image display device according to the eighth embodiment further includes a line memory  110 , adders  110  and  111 , and two sets of the burst data generator  112  and  111 . The image display device shown in  FIG. 21  is directed to the PAL system. 
     FIG. 24  is a vector diagram showing a phase relationship among the burst signal, the R-Y and B-Y signals and the chrominance signal in the PAL system. As shown in  FIG. 24 , the burst signals of two adjacent lines n and n-1 are 90° out of phase. 
   Therefore, the burst signals 180° out of phase with respect to the B-Y signal is obtained by adding two adjacent lines n and n−1, and the burst signal 180° out of phase with respect to the R-Y signal is obtained by subtracting the line n from the line n−1. 
   The adder  110  subtracts the digital sample C of the line n−1 from that of the line n provided by the line memory  109 , thereby producing digital samples corresponding to the burst signal 180° out of phase with respect to the R-Y signal. At the same time, the adder  111  adds the digital sample C of the line n+1 and that of the line n, thereby producing digital samples corresponding to the burst signal 180° out of phase with respect to the B-Y signal. The burst data generators  112  and  113  produce the burst data A, B, C and D based on the digital samples produced by the adder  110  and  111 . 
   The structure of other parts and operations thereof are the same as each of the embodiments described above. 
     FIG. 26  is a block diagram illustrating another structure of an image display device according to the eighth embodiment. A memory  116  holds digital samples of the burst signal and output them to an adder  117  after one horizontal period. The adder  117  adds the digital samples of the burst signal and that of after one horizontal period, thereby producing digital samples corresponding to the burst signal 180° out of phase with the R-Y signal. The burst data generator  57  produces the burst data A, B, C and D according to the digital samples produced by the adder  117 . As shown in  FIG. 24 , in the PAL system, the chrominance signal varies its polarity for each line. Therefore, an amplitude adjuster  118  changes polarity of an output of the LPF  65 .