Abstract:
The present invention relates to a contrast enhancing apparatus for a video signal which is capable of enhancing a coefficient characteristic of a light and dark portion of a video signal in a video display apparatus such as TV, etc. In the conventional art, since the attenuating and amplifying coefficients of a low frequency component and a high frequency component are uniformly fixed for enhancing a contrast of a video signal, so that it is impossible to significantly enhancing a contrast of all kinds of videos. In addition, since a transition of a DC level occurs due to an attenuation of the low frequency component, the final output must be scaled again, and the minimum and maximum values of the signal component must be obtained. However, in the present invention, the coefficient which is varied based on the levels of the high frequency component and low frequency component for enhancing the low frequency component separated from the video signal, and the low frequency level is compensated with the thusly determined coefficient, and a coefficient which is in inverse proportion to the enhancement of the low frequency component. Therefore, in the present invention, it is possible to enhance the contrast based on the video signal which is inputted in real time by uniformly varying the enhancing coefficients of the high frequency component and the low frequency component based on a user control signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a contrast enhancing apparatus for a video signal, and in particular to a contrast enhancing apparatus which is capable of significantly enhancing a contrast of a video signal by enhancing(amplifying or attenuating) a video signal inputted. 
     2. Description of the Background Art 
     In a method for enhancing a contrast of a video signal, a boundary portion of a video signal is selected, and the boundary portion is enhanced for thereby enhancing a contrast of the same. In another method therefor, a light and darkness ratio between a light portion and a darkness portion is enhanced using a homomorphic filter for thereby enhancing a contrast. 
     As shown in FIG. 1, the contrast enhancing apparatus of a video signal formed of a conventional homomorphic filter includes an algebraical conversion unit  100  for converting a video signal Yin into an algebra form (log), a wave filtering unit  200  for separating an algebraically processed video signal Ylog 1  into a low frequency component Y 1 pf 1  and a high frequency component Yhpf 1 , a first multiplier  300  for attenuating the low frequency component Y 1 pf 1  using an attenuation coefficient “a”, a second multiplier  400  for amplifying the high frequency component Yhpf 1  using an amplifying coefficient “b”, a summing unit  500  for summing the attenuated low frequency component Y 1 pf 1  and the amplified high frequency component Yhpf 1 , and an exponential conversion unit  600  for converting the signal Ylog 2  summed by the summing unit  500  to a video signal. 
     The operation of the conventional contrast enhancing apparatus for a video signal will be explained with reference to the accompanying drawings. 
     First, the video signal Yin inputted from the algebraical conversion unit  100  is algebraically processed, and the thusly algebraically processed video signal Ylog 1  is divided into the low frequency component Y 1 pf 1  and the high frequency component Yhpf 1  by the wave filtering unit  200 . At this time, the low frequency component Y 1 pf is multiplied by the attenuating coefficient “a” and is attenuated by “a” times for enhancing the contrasts of the low frequency component Y 1 pf 1  and the high frequency component yhpf 1 , and the high frequency signal Yhpf 1  is amplified by the second multiplier  400  by the amplifying coefficient “b”. Here, since the attenuating coefficient “a” and the amplifying coefficient “b” have fixed values, the contrast of the video signal Yin is enhanced by uniformly attenuating or amplifying the inputted low frequency component Y 1 pf 1  and the high frequency component Yhpf 2 . 
     In addition, since the signal Ylog 1  that the low frequency component Y 1 pf 1  attenuated by the first multiplier  300  and the high frequency component Yhpf 1  amplified by the second multiplier  400  are summed by the summing unit  500  is an algebraically converted signal, the signal is inputted into the exponential conversion unit  600  and is exponentially converted and is recovered as a video signal Yout. 
     Therefore, in the homorphic filter, the contrast of the video signal is enhanced by attenuating the low frequency component Y 1 pf 1  of the input video signal Yin and amplifying the high frequency component Yhpf 1  using the fixed amplifying or attenuating coefficients “a” and “b”. However, since the above-described coefficients “a” and “b” have a fixed value, all video signals are amplified or attenuated by the fixed values, it is impossible to properly enhance the contrast of all video signals. 
     In addition, in the method in which the coefficients “a” and “b” are differently set in accordance with the video signal Yin, it is difficult to adapt in real time, and in the case that the low frequency component Y 1 pf 1  is attenuated, since a transition of the DC level occurs, the final output Yout must be scaled again, and the minimum and maximum values of the signal component must be obtained, so that a real time process is not implemented. 
     Since the video sinal is not processed in a real time, it is impossible to uniformly enhance the contrast of the video signal. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a contrast enhancing apparatus for a video signal which is capable of determining an enhancing coefficient of the low frequency component using a high frequency component level and a low frequency component level in a method for enhancing the low frequency component of the video signal, compensating the thusly determined coefficient using a DC level, obtaining a low frequency component, and the determining the enhancing coefficient of the high frequency component based on the enhancing coefficient of the low frequency component. In addition, the enhancing coefficient of the high frequency component and the enhancing coefficient of the low frequency component are given an off-set value in accordance with a control signal of a user, so that it is possible to enhance a light darkness ratio by variably amplifying or attenuating the contrast of the video signal in accordance with the levels of the high frequency component and low frequency component. 
     To achieve the above objects, there is provided a contrast enhancing apparatus for a video signal which includes an algebraical conversion unit for algebraically converting an inputted video signal, a wave filtering unit for separating the algebraically converted video signal into a low frequency signal and a high frequency signal, a low frequency coefficient mapping unit for generating a level of a high frequency signal which is an output of the wave filtering unit and a level of the low frequency signal and a low frequency enhancing coefficient based on a user&#39;s adjustment, a high frequency coefficient mapping unit for generating a high frequency enhancing coefficient which is in inverse proportion to the low frequency enhancing coefficient, a first multiplier for multiplying the low frequency signal and the low frequency enhancing coefficient, a low frequency signal processing compensation unit for receiving an output of the low frequency mapping unit and an output of the first multiplier and compensating a low frequency level, a second multiplier for multiplying the high frequency signal and the high frequency enhancing coefficient, a summing unit for summing an output of the low frequency amplifying compensation unit and an output of the second multiplier, and an exponential conversion unit for exponentially converting an output of the summing unit. 
     Additional advantages, objects and features of the invention will become more apparent from the description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a view illustrating the construction of a conventional contrast enhancing apparatus for a video signal; 
     FIG. 2 is a view illustrating a contrast enhancing apparatus for a video signal according to a first embodiment of the present invention; 
     FIG. 3 is a view illustrating a contrast enhancing apparatus for a video signal according to a second embodiment of the present invention; 
     FIG. 4 is a view illustrating a contrast enhancing apparatus for a video signal according to a third embodiment of the present invention; 
     FIG. 5 is a block diagram illustrating a contrast enhancing apparatus for a video signal according to the present invention; and 
     FIG. 6 is a graph of a variation of a low frequency coefficient based on an integrated video signal and a variation of a user control signal at a video signal coefficient generator according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The contrast enhancing apparatus for a video signal according to the present invention includes an algebra conversion unit for algebraically converting an inputted video signal, a wave filtering unit for separating the algebraically converted video signal into a low frequency signal and a high frequency signal, a low frequency coefficient mapping unit for generating a high level frequency signal and a low level frequency signal which are the outputs of the wave filtering unit and a low frequency enhancing coefficient based on a user&#39;s adjustment, a high frequency coefficient mapping unit for generating a high frequency enhancing coefficient which is in inverse proportion to the low frequency enhancing coefficient, a first multiplier for multiplying the low frequency signal and the low frequency enhancing coefficient, a low frequency signal processing compensation unit for receiving an output of the low frequency coefficient mapping unit and an output of the first multiplier, a second multiplier for multiplying the high frequency signal and the high frequency enhancing coefficient, a summing unit for summing an output of the low frequency amplifying compensation unit and an output of the second multiplier, and an exponential conversion unit for exponentially converting an output of the summing unit. The above-described construction is defined as a first feature of the present invention. 
     In the present invention, as a second feature of the present invention, the low frequency counting mapping unit generates a coefficient signal proportional to the size of the high frequency component of the input signal, and the size of the coefficient signal is varied by a low frequency level of the input video signal and is in inverse proportion to the average value of the input signal, and the coefficient is varied by an adjusting signal of a user. 
     As a third feature of the present invention, the low frequency amplifying mapping unit is determined by a minimum value and a maximum value of the input video signal range and a low frequency coefficient mapping unit. 
     As a fourth feature of the present invention, the high frequency coefficient mapping unit generates a signal which is inverse proportion to the low frequency coefficient mapping unit, and the size of the same is varied by an adjusting signal of a user. 
     As a fifth feature of the present invention, a minimum value is detected from an image signal, and the detected value is subtracted by a subtractor with respect to the image signal and is inputted into an algebra conversion unit, and an output of the exponential conversion unit is summed with the detected minimum value by the summing unit. 
     As a sixth feature of the present invention, the coefficient of the coefficient mapping unit is varied in accordance with an average value of the video signal inputted. 
     FIG. 2 illustrates a contrast enhancing apparatus for a video signal according to the present invention which includes an algebra conversion unit  110  for algebraically converting a video signal Yin, a wave filtering unit  120  for filtering the algebraically converted video signal Ylog 1  and separating into a low frequency component Y 1 pf 1  and a high frequency component Yhpf 1 , a low frequency coefficient mapping unit  130  for generating a low frequency enhancing coefficient “a” having a value varied based on the level of the low frequency component Y 1 pf 1  and the level of the high frequency component Yhpf 1 , a high frequency coefficient mapping unit  140  for generating a high frequency enhancing coefficient “b” having a value which is in inverse proportion to the low frequency enhancing coefficient “a” generated by the low frequency coefficient mapping unit  130 , a first multiplier  150  for multiplying to enhance(amplify or attenuate) the low frequency component Y 1 pf 1  filtered by the wave filtering unit  120  based on a low frequency enhancing coefficient “a” generated by the low frequency coefficient mapping unit  130 , a second multiplier  160  for multiplying to enhance the high frequency component Yhpf 1  filtered by the wave filtering unit  120  based on the high frequency enhancing coefficient “b” generated by the high frequency coefficient mapping unit  140 , a low frequency amplifying unit  170  for detecting a DC level compensation coefficient “c” from a relative enhancing value  1 −a of the low frequency enhancing coefficient “a” generated by the low frequency mapping unit  130  and a central value log(Ymed) of the video signal and compensating a DC level of the enhanced low frequency component Y 1 pf 2 , a summing unit  180  for summing the low frequency component Y 1 pf 1  having a DC level compensated by the low frequency amplifying unit  170  and a high frequency component Yhpf 2  enhanced by the second multiplier  160 , and an exponential conversion unit  190  for exponentially converting the signal Ylog 2  summed by the summing unit  180  and outputting a recovered video signal Yout. 
     The low frequency mapping unit  130  outputs a low frequency enhancing coefficient “a” which is varied based on the levels of the low frequency component Y 1 pf 1  and the high frequency component Yhpf 1 , and the outputted low frequency enhancing coefficient “a” is varied by a user control signal Ui 1  for thereby uniformly varying the amplitude and the level of the low frequency component Y 1 pf 1 . The high frequency mapping unit  140  outputs a high frequency enhancing coefficient “b” which is in inverse proportion to the low frequency enhancing coefficient ‘a”, and the high frequency enhancing coefficient ‘b” is varied by a user control signal Ui 2  for thereby uniformly varying the amplitude and level of the high frequency component Yhpf 1 . 
     The low frequency amplifying unit  170  includes a low frequency amplifying mapping unit  170 A for detecting a low frequency compensation coefficient “c” using a central value log(Ymed) of a video signal range with respect to the relative enhancing value  1 −a of the low frequency enhancing coefficient “a” generated by the low frequency mapping unit  130 , and a second summing unit  170 B for compensating a low frequency level of the low frequency component Y 1 pf 2  enhanced by the first multiplier  150  by the low frequency compensation coefficient “c” detected by the low frequency amplifying mapping unit  170 A. 
     FIG. 3 illustrates a contrast enhancing apparatus according to another embodiment of the present invention. In this embodiment of the present invention, there are further provided a subtractor  101  for subtracting the inputted video signal Yin by a certain level, a minimum value detector  102  for detecting a minimum value of the inputted video signal Yin, and a third summing unit  191  for summing the values by the value subtracted by the subtractor  101 , compared to the construction of FIG.  2 . 
     FIG. 4 illustrates a contrast enhancing apparatus according to another embodiment of the present invention. In this embodiment of the present invention, there is further provided a meaning unit  102  for meaning the inputted video signal Yin, compared to the construction of FIG. 3, 
     FIG. 5 is a detailed block diagram of FIG.  2 . The algebraical conversion unit  110  stores an algebraically converted value using a ROM and outputs an algebraically converted signal Ylog 1  which is a data value corresponding to an address ADDR of the input video signal Yin, and the wave filter  120  includes a low pass filter(LPF)  120 A for filtering an algebraically converted video signal Ylog 1  and outputting a low frequency component Y 1 pf 1 , and a first subtractor  120 B for subtracting the low frequency component Y 1 pf 1  from the algebraically converted video signal Y 1 log 1  and outputting a high frequency component. 
     In addition, the low frequency mapping unit  130  includes a first integrator  130 A for integrating to detect the level of the low frequency component Y 1 pf 1 , a coefficient generation unit  130 B for generating a low frequency coefficient “d” which is varied in accordance with the level of the integrated low frequency component Y 1 pf 1  and an off-set value based on a user control signal Ui 1 , a high frequency size detector  130 C for obtaining an absolute value of the high frequency component Yhpf 1  and detecting a size Yhabs 1  of the same, a second subtractor  130 D for subtracting to detect a relative size Yhabs 2  of the high frequency component Yhabs 1  detected by the high frequency size detector  130 C, a second integrator  130 E for integrating to detect the level of the high frequency component subtracted by the second subtractor  130 D, and a third multiplier  130 F for multiplying a high frequency component level Yhabs 3  integrated by the second integrator  130 E and a low frequency coefficient “d” generated by the coefficient generation unit  130 B and generating a low frequency enhancing coefficient “a”. 
     The high frequency coefficient mapping unit  140  includes a fourth multiplier  140 B for multiplying a value subtracted by the third subtractor  140 A which outputs a value subtracted from the maximum variable range of the low frequency enhancing coefficient “a” outputted from the third multiplier  130 F and a user control signal Ui 2  and generating a high frequency enhancing coefficient “b”. 
     In addition, the exponential conversion unit  190  stores an exponentially converted data using the ROM, search an address ADDR of the signal Ylog 2  summed by the summing unit  180 , and outputs an exponentially converted video signal which is a corresponding data. 
     The operation of the present invention will be explained with reference to FIG.  2 . 
     The input video signal Yin is inputted into the algebraical conversion unit  110  and is converted into an algebra(log function), and this algebraically converted video signal Ylog 1  is inputted into the wave filtering unit  120 , and the wave filtering unit  120  separates the inputted video signal into a log frequency component Y 1 pf 1  and a high frequency component Yhpf 1 . 
     In addition, the low frequency component Y 1 pf 1  separated by the wave filtering unit  120  is inputted into the low frequency coefficient mapping unit  130  and the first multiplier  150 , respectively, and the high frequency component Yhpf 1  is inputted into the coefficient mapping unit  130  and the second multiplier  160 , respectively. 
     At this time, the coefficient mapping unit  130  generates a low frequency enhancing coefficient “a” which is varied based on the levels of the low frequency component Y 1 pf 1  and the high frequency component Yhpf 1  and enhances the low frequency component Y 1 pf 1  inputted into the first multiplier  150  for thereby outputting the enhanced low frequency component Y 1 pf 2 . 
     The low frequency coefficient mapping unit  130  uniformly varies the low frequency enhancing coefficient “a” by the off-set value based on a user control signal. 
     If the low frequency enhancing coefficient is in a range of a&gt;0, 0.5≦a≦1.5, when a=1, the low frequency component Y 1 pf 1  is not enhanced, and when a=1.5, the low frequency component Y 1 pf 1  is amplified 1.5 times, and when a=0.5, the low frequency component Y 1 pf 1  is attenuated 0.5 times. 
     The low frequency enhancing coefficient “a” generated by the low frequency coefficient mapping unit  130  is inputted into the high frequency coefficient mapping unit  140  and the low frequency amplifying compensation unit  170 , respectively. 
     The high frequency mapping unit  140  generates a high frequency enhancing coefficient “b” which is in inverse proportion to the inputted low frequency enhancing coefficient “a”, and the thusly generated high frequency enhancing coefficient “b” is uniformly varied by the off-set value in accordance with a user control signal. 
     The high frequency enhancing coefficient “b” generated by the high frequency mapping unit  140  enhances the high frequency component Yhaf 1  inputted into the second multiplier  160  and varies the level and amplitude of the high frequency component Yhpf 2  in accordance with a user control signal Ui 2 . 
     Here, if the high frequency enhancing coefficient is in a range of b&gt;0, 0.5≦b≦1.5, when b=1, the low frequency component Y 1 pf 1  is not enhanced, and when b=1.5, the low frequency component Y 1 pf 1  is amplified 1.5 times, and when b=0.5, the low frequency component Y 1 pf 1  is attenuated 0.5 times. 
     The thusly generated low frequency enhancing coefficient “a” enhances the low frequency component Y 1 pf 1  inputted into the first multiplier  150  by “a”-times based on the low frequency component Y 1 pf 1  and the high frequency component Yhpf 1 , and the since the high frequency enhancing coefficient “b” is in inverse proportion to the low frequency enhancing coefficient “a”, the high frequency component Yhpf 1  inputted; into the second multiplier  160  is enhanced by b-times. 
     In addition, in order to compensate the DC level of the low frequency component Y 1 pf 2 , the low frequency amplifying mapping unit  170 A of the low frequency amplifying compensation unit  170  detects a relative enhancing value  1 −a with respect to the inputted low frequency enhancing coefficient “a” and multiplies the thusly detected value and the central value log(Ymed) of the video signal range for thereby generating a low frequency amplifying compensation coefficient “c”. Namely, c=( 1 −a)(log(Ymed)). 
     The low frequency amplifying compensation coefficient “c” compensates the DC level transition based on the enhanced low frequency component Y 1 pf 2 . 
     Namely, when the low frequency enhancing coefficient “a” is 1, since the low frequency component Y 1 pf 1  is not enhanced, the low frequency amplifying compensation coefficient “c” is 0, and if the low frequency enhancing coefficient “a” is higher than 1, the relative enhancing value  1 −a has a negative value, the DC level of the low frequency component Y 1 pf 2  amplified by the DC level compensation coefficient “c” is decreased. If the low frequency enhancing coefficient “a” is smaller than 1, the relative enhancing value  1 −a has a positive value, the DC level of the low frequency component Y 1 pf 2  amplified by the DC level compensation coefficient “c” is increased. 
     Therefore, the DC level compensation coefficient “c” detected by the low frequency amplifying compensation unit  170  finally compensates the DC level of the low frequency component Y 1 pf 2  enhanced by the first multiplier  150 . The thusly compensated low frequency component Y 1 pf 3  is inputted into the first summing unit  180 . 
     The summing unit  180  sums a low frequency component Y 1 pf 3  outputted from the low frequency amplifying compensation unit  170  and a high frequency component Yhpf 2  enhanced by the second multiplier  160 , and the thusly summed signal Ylog 1  is exponentially converted by the exponential conversion unit  190 . 
     FIG. 3 illustrates another embodiment of the contrast enhancing apparatus according to the present invention. In this embodiment of the present invention, the minimum value is detected by the inputted video signal and is subtracted from the video signal by the subtractor, and a resultant value is inputted into the algebraical conversion unit. The output of the exponential conversion unit is summed with the minimum value detected by the summing unit. The description of the same elements as the construction of FIG. 2 will be omitted. 
     The minimum value detector  102  detects a minimum value of the input video signal Yin, and the thusly detected minimum value is transferred to the subtractor  101 . The subtractor  101  the minimum value detected by the minimum value detector  102  from the input video signal Yin, and an output signal Yclp is transmitted to the algebraical conversion unit  110 . In addition, the value is summed by the summing unit as much as the value that the original video signal Yexp which is an output signal of the exponential conversion unit  190  is subtracted from the input video signal Yin by transmitting the value to the third summing unit  191  for thereby outputting a recovering video signal Yout. 
     In addition, assuming that the maximum value of the video signal range is Ymax, an equation of c=( 1 −a)log(Ymax) is obtained. At this time, the minimum value of the video signal is “0” by the subtractor  101  connected with the input video signal Yin, the value “c” is not affected. 
     FIG. 4 illustrates another embodiment of the contrast enhancing apparatus according to the present invention. As shown therein, in addition to the construction of FIG. 3, a low frequency processing level adjusting unit  230  is further provided for analyzing an input video signal and adjusting an output level of the low frequency processing unit. 
     The low frequency processing level adjusting unit  230  outputs an adjusting coefficient “a”, which is obtained by adjusting the level of an adjusting coefficient Ya from the low frequency coefficient mapping unit  130  in accordance with a mean signal, by summing and subtracting a certain level corresponding to the mean level by computing a mean value of the input video signal. Namely, when the mean value is high, the value “a” is decreased, and when the mean value is low, the value “a” is increased, so that it is possible to obtain the value “a” which is adaptiye to the mean value. 
     The input video signal Yin generates a signal Ylog 1  which is algebraically converted by the algebraical conversion unit  110 , and the thusly generated signal is outputted to the wave filtering unit  120 . The algebraical conversion unit  110  stores the algebraically converted value into a ROM, and the signal Ylog 1  which is algebraically converted as a signal Yclip outputted from the subtractor  101  is applied to an address of the ROM is outputted based on the data of the ROM. The wave filtering unit  120  outputs a high frequency signal Yhpf 1  that the low frequency signal Y 1 pf 1  is subtracted from the low frequency signal Y 1 pf 1  and the algebraically converted signal Ylog 1 . 
     The low frequency signal Y 1 pf 1  and the high frequency signal Yhpf 1  which are outputted from the wave filtering unit are applied to the low frequency coefficient mapping unit  130 . As shown in FIG. 6, in the construction of the low frequency coefficient mapping unit, in order to detect the amount of the high frequency component among the signal components, the value 1−Yhabs 1  is computed by the ABS unit  130 C and the relative detector  130 D. When the value Yhabs 2  is outputted, an integrated value is obtained by the integrator  130 E for thereby outputting a value Yhabs 3 . 
     In addition, in order to detect the amount of the low frequency component, a signal Y 1 pf 1  that the low frequency Y 1 pf 1  is integrated by the integrator  130 A is outputted. The user control signal Ui 1  and the thusly integrated signal Y 1 pf 1  are inputted into the coefficient generator  130 B for thereby outputting a signal “d”. An interrelationship between the coefficient “d” outputted from the coefficient generator  130 B and the user control signal Ui 1  and the integrated signal Y 1 pf 1  is shown in FIG.  6 . 
     The output signal Yhabs 3  of the integrator  130 E and the coefficient “d” outputted from the coefficient generator  130 B becomes a signal Ya by the multiplier. In the low frequency processing level adjusting unit  230 , the mean unit  103  computes a mean value of the input video signal, and the algebraical conversion unit  130 G algebraically converts the output signal Yavg of the mean unit  103 . The output Ylogavg of the algebraical conversion unit is applied to the level shifter  130 H for thereby outputting a level-shifted signal Ylavgs. 
     The signal Ya subtracts the level-shifted signal Ylavgs for thereby obtaining a low frequency amplifying coefficient “a”. 
     FIG. 5 illustrates a detailed construction of the circuit of FIG.  2 . 
     The algebraical conversion unit  110  stores the algebraically converted value using the ROM for implementing a real time process. When the video signal Yin is applied to the address ADDR of the ROM, a corresponding data is searched and is outputted as an algebraically converted video signal Ylog 1 . 
     In addition, the wave filtering unit  120  filters the algebraically converted video signal Ylog 1  using the low pass filter(LPF)  120 A and outputs a low frequency component Y 1 pf 1 , and the filtered low frequency component Y 1 pf 1  is subtracted from the algebraically converted video signal Y 1  by the first subtractor  120 B for thereby outputting a high frequency component Yhpf 1 . 
     The thusly separated low frequency component Y 1 pf 1  is inputted into the coefficient mapping unit  130  and the first multiplier  150 , and the high frequency component Yhpf 1  is inputted into the coefficient mapping unit  130  and the second multiplier  160 . 
     The coefficient mapping unit  130  detects the levels of the low frequency component Y 1 pg 1  and the high frequency component Yhpf 1  and outputs a low frequency enhancing coefficient “a”. The first integrator  130 A of the coefficient mapping unit  130  integrates the low frequency component Y 1 pf 1  and detects the level of the same and outputs to the coefficient generator  130 B. The coefficient generator.  130 B outputs a low frequency coefficient “d” in accordance with the level of the integrated low frequency component Y 1 pf 1 . 
     At this time, the coefficient generation unit  130 B varies the low frequency coefficient “d” by the off-set value of FIG. 6 in accordance with a user control signal Ui 1 . 
     Namely, as shown in FIG. 6, the low frequency coefficient “d” outputted from the coefficient generation unit  130 B has a minimum value at a threshold value of the integrated low frequency component Y 1 pf 1  and has a large value in the case that the same is relatively larger or smaller than the threshold value Th. The low frequency coefficient “d” is uniformly varied by the off-set value in accordance with a user control signal Ui 2 . 
     At this time, the coefficient generation unit  130 B varies the low frequency coefficient “d” by the off-set value as shown in FIG. 6 by a user control signal Ui 1 . 
     Namely, as shown in the graph of FIG. 6, the low frequency coefficient “d” outputted from the coefficient generation unit  130 B has a minimum value at a threshold value of the integrated low frequency component Y 1 pf 1 , and in the case that the low frequency coefficient “d” is relatively larger than or smaller than the threshold value Th, the low frequency coefficient “d” is uniformly varied by the off-set value by the user control signal Ui 2 . 
     A high frequency size detector  130 C which receives a high frequency component Yhpf 1  has an absolute value |Yhpf 1 | of the high frequency component yhpf 1  and detects the size Yhabs 1 , and the thusly detected size is subtracted(1−Yhabs 1 ) from the second subtractor  130 D for thereby detecting a relative size Yhabs 2 . The thusly subtracted value is integrated by the second integrator  130 E, and a relative level Yhabs 3  of the high frequency component Yhpf 1  is detected. 
     The relative levels of the low frequency coefficient “d” and the high frequency component Yhpf 1  are multiplied by the multiplier  130 E and are outputted to the first multiplier as a low frequency enhancing coefficient “a”, so that the low frequency component Y 1 pf 1  is enhanced(amplified or attenuated) by a coefficient “a” which is varied based on the levels of the low frequency component Y 1 pf 1  and the high frequency component Yhpf 1 . 
     The third subtractor of the high frequency mapping unit  140  which receives a low frequency enhancing coefficient “a” subtracts the low frequency enhancing coefficient “a” from the maximum varying range (2:0.5≦a≦1.5) of the low frequency enhancing coefficient “a”, and the multiplier  140 B multiplies by the off-set value in accordance with a user control signal and outputs a high frequency enhancing coefficient “b”. 
     Namely, the subtraction is performed using “2−a” based on the low frequency enhancing coefficient “a”, and a high frequency enhancing coefficient “b” which is in inverse proportion to the low frequency enhancing coefficient “a”, and the high frequency enhancing coefficient “b” is varied by the off-set value based on a user control signal Ui 2 , and the level of the amplitude of the enhanced high frequency component Uhpf 2  are varied. 
     The high frequency enhancing coefficient “b” enhances the high frequency component inputted into the second multiplier  150 , and the thusly enhanced high frequency component Yhpf 2  is inputted into the first summing unit  180 . 
     In addition, the low frequency component Y 1 pf 2  which is enhanced by the first multiplier  150  by the low frequency enhancing coefficient “a” of the coefficient mapping unit  130  is inputted into the first summing unit  180  as a low frequency component Y 1 pf 3  in which the low frequency level is compensated by the relative enhancing value by the low frequency amplitude compensation unit  170 . Namely, a difference value between the low frequency amplitude maximum value obtained when the low frequency component Y 1 pf 1  passes through the first multiplier  150  and the maximum value of the video signal is compensated by the second summing unit  170 B. 
     The first summing unit  180  outputs a signal Ylog 2  that the low frequency component Y 1 pf 3  which is low-frequency-compensated by the low frequency amplitude compensation unit  170  and the high frequency component Yhpf 2  enhanced by the second multiplier  160  to the exponential conversion unit  190 . 
     The exponential conversion unit  190  stores the value, which will be exponentially converted, using the ROM for a real time process of the algebraically converted video signal and searches a corresponding data when the algebraically converted video signal Ylog 2  is applied to the address ADDR of the ROM and outputs the searched data as an exponentially converted video signal Yout. 
     Therefore, in order to enhance the filtered low frequency component, the low frequency enhancing coefficient “a” which is varied in accordance with the high frequency component Yhpf 1  and the low frequency component Y 1 pf 1 , is determined, and the inversely proportional high frequency enhancing coefficient “b” is determined based on the thusly determined coefficient “a”. 
     In order to prevent a transition of the enhanced low frequency component Y 1 pf 1 , the low frequency level of the finally filtered low frequency component Y 1 pf 2  is compensated using the low frequency level compensation coefficient “c” with respect to the relative enhancing value  1 −a of the low frequency enhancing coefficient “a” and the central value log(Ymed) of the video signal. 
     In addition, in order to prevent a saturation of the low frequency component Y 1 pf 1  and the high frequency component Yhpf 1 , the level and amplitude of the same are varied using the user control signals Ui 1  and Ui 2 . 
     Although the preferred embodiment of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as recited in the accompanying claims.