Abstract:
The present invention provides a display device which is capable of performing the gamma correction of a video signal voltage applied to respective pixels. The display device includes a drive circuit which supplies the video signal voltage to a plurality of video signal lines, wherein the drive circuit includes a storage circuit which stores display data inputted from the outside, a reference data generating circuit which generates reference data, a ramp voltage generating circuit which generates a ramp voltage, a plurality of comparing circuits which compares the display data stored in the storage circuit and the reference data generated by the reference data generating circuit, and a plurality of sampling circuits which sample the ramp voltage generated by the ramp voltage generating circuit based on a result of comparison of the comparing circuit and output the sampled ramp voltage as a video signal voltage to respective video signal lines, wherein the reference data generated by the reference data generating circuit is changed non-linearly with respect to time.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a display device, and, more particularly, to a technique which is applicable to gamma correction of a video signal voltage that is applied to respective pixels in a display device.  
           [0002]    A liquid crystal display module of a TFT (Thin Film Transistor) type has been used extensively as a display device in a notebook type personal computer or the like.  
           [0003]    As an example of a liquid crystal display module, a display module in which thin film transistors (TFT) are mounted on a polysilicon layer (hereinafter referred to as polysilicon type liquid crystal display module) has been known.  
           [0004]    Further, as such a liquid crystal display module, there is a display module that employs a method (hereinafter referred to as a PWM method) in which display data within one horizontal scanning line period is stored, and reference data which is sequentially increased or decreased within one horizontal scanning line period is generated; the stored display data and the reference data are compared to each other; and, when these data coincide with each other, a video signal voltage generated by a video signal voltage generating circuit is sampled and is applied to respective pixels (see Japanese Unexamined Patent Publication Hei 6 (1994)-178238 (hereinafter referred to as patent literature 1), Japanese Unexamined Patent Publication Hei 11 (1999)-272242 (hereinafter referred to as patent literature 2)).  
           [0005]    As the video signal voltage generated by the above-mentioned video signal voltage generating circuit, a voltage having an inclined voltage waveform (hereinafter referred to as a ramp voltage) is used.  
         SUMMARY OF THE INVENTION  
         [0006]    As described in the above-mentioned patent literature 1, with respect to the video signal voltage that is applied to each pixel, it is necessary to perform gamma correction by taking a transmissivity curve of the liquid crystal into consideration. In the liquid crystal display device described in the above-mentioned patent literature 1 and patent literature 2, gamma correction is performed by a video signal voltage generating circuit.  
           [0007]    [0007]FIG. 18 is a diagram showing one example of the conventional gamma correction technique disclosed in FIG. 7 of the above-mentioned patent literature 1 or in FIG. 14 of the above-mentioned patent literature 2. As can be understood from these drawings, in the gamma correction method described in the above-mentioned patent literature 1 and patent literature 2, an output of a ramp generating circuit is modulated in conformity with the required gamma characteristics.  
           [0008]    To be more specific, in accordance with this method, is a method in which the gamma characteristics are stored in a memory (MM) in advance, values of the memory (MM) are sequentially read out, and there values are converted into an analogue voltage using a digital/analogue converter (DAC). In FIG. 18, reference symbol AMP indicates an amplifier which amplifies the analogue voltage obtained by signal conversion in the digital/analogue converter (DAC), and a reference symbol RAMP indicates a ramp voltage outputted from the amplifier (AMP).  
           [0009]    However, in implementing the above-mentioned methods, it is necessary to provide a digital/analogue converter having a high resolution, such that the circuit scale of the digital/analogue converter having such high resolution becomes large-sized, and an extremely high accuracy is required, thus giving rise to the problem that it is difficult to form the digital/analogue converter on a substrate on which a display panel is formed.  
           [0010]    Further, although the output of the ramp generating circuit is delayed due to the wiring capacitance of the video signal lines (drain lines) in the display panel, the voltage error caused by this delay depends on the inclination of the ramp voltage with respect to time.  
           [0011]    In performing gamma correction, the inclination differs in respective regions, and the maximum inclination assumes a large value. Accordingly, there arises a drawback in that the error is increased, and, at the same time, the amount of error differs among the regions.  
           [0012]    The present invention has been made to solve the above-mentioned drawbacks of the related art, and it is an object of the present invention to provide a display device which is capable of performing gamma correction of a video signal voltage that is applied to respective pixels without modulating a ramp voltage.  
           [0013]    The above-mentioned and other objects and novel features of the present invention will become apparent from the following description in this specification and the attached drawings.  
           [0014]    A summary of representative aspects of the invention disclosed in this specification is as follows.  
           [0015]    The present invention is directed to a display device which includes a display part having a plurality of pixels, a plurality of video signal lines which apply a video signal voltage to the plurality of pixels, and a drive circuit which supplies the video signal voltage to the plurality of video signal lines. The drive circuit includes a storage circuit which stores display data inputted from the outside, a reference data generating circuit which generates reference data, a ramp voltage generating circuit which generates a ramp voltage, a plurality of comparing circuits which compare the display data stored in the storage circuit and the reference data generated by the reference data generating circuit, and a plurality of sampling circuits which sample the ramp voltage generated by the ramp voltage generating circuit based on a result of comparison of the comparing circuit. The sampled ramp voltage is output as a video signal voltage to respective video signal lines, wherein the reference data generated by the reference data generating circuit is changed non-linearly with respect to time.  
           [0016]    Here, the above-mentioned reference data generating circuit, includes a selection circuit to which a plurality of clocks which have different frequencies from each other are inputted, and which selects one clock out of the plurality of clocks in response to a selection control signal; a counter which counts clocks selected by the selection circuit and outputs the number of counts as reference data; and a control part which transmits the selection control signal, which indicates the clocks to be selected by the selection circuit in response to a preset number of counts and the number of counts of the counter, to the selection circuit.  
           [0017]    Further, the control part includes a plurality of registers which store a preset number, a plurality of comparators which compare the number stored in respective registers and the number of counts of the counter, and a control circuit which generates the selection control signals in response to the result of a comparison at the plurality of comparators.  
           [0018]    Further, in accordance with the present invention, the ramp voltage generating circuit generates a ramp voltage of positive polarity and a ramp voltage of negative polarity, and the sampling circuit samples the ramp voltage of positive polarity or the ramp voltage of negative polarity generated by the ramp voltage generating circuit in response to an alternating signal inputted from the outside and the result of comparison of the comparing circuits and outputs the sampled ramp voltage to respective video signal lines as a video signal voltage.  
           [0019]    Further, according to the present invention, the ramp voltage generating circuit generates the ramp voltage of positive polarity and the ramp voltage of negative polarity, and the sampling circuit includes a first sampling circuit, which samples the ramp voltage of positive polarity generated by the ramp voltage generating circuit in response to an inputted result of comparison of one comparing circuit out of two comparing circuits; a second sampling circuit, which samples the ramp voltage of negative polarity generated by the ramp voltage generating circuit in response to an inputted result of comparison of another comparing circuit out of two comparing circuits; a first switching circuit, which inputs the inputted result of comparison of one comparing circuit out of two comparing circuits to either the first sampling circuit or the second sampling circuit and the inputted result of comparison of another comparing circuit out of two comparing circuits into either the second sampling circuit or the first sampling circuit in response to an alternating signal inputted from the outside; and a second switching circuit, which outputs the ramp voltage of positive polarity sampled by the first sampling circuit to one video signal line or another video signal line out of neighboring video signal lines as a video signal voltage, or outputs the ramp voltage of negative polarity which is sampled by the second sampling circuit to another video signal line or one video signal line out of the neighboring video signal lines as a video signal voltage in synchronism with the changeover at the first switching circuit and in response to the alternating signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic diagram showing the constitution of a liquid crystal display device according to an embodiment 1 of the present invention;  
         [0021]    [0021]FIG. 2 is a block diagram showing an example of the reference data generating circuit shown in FIG. 1;  
         [0022]    [0022]FIG. 3 is a schematic circuit diagram showing an example of the ramp voltage generating circuit shown in FIG. 1;  
         [0023]    [0023]FIG. 4 is a table showing the relationship between a count value (Nc) of the counter shown in FIG. 2 and the frequency of an input signal (fin) inputted to the counter;  
         [0024]    [0024]FIG. 5 is a graph showing a time response of a count value of the reference data generating circuit shown in FIG. 1.  
         [0025]    [0025]FIG. 6 is a signal diagram showing a time response of the ramp voltage generating circuit shown in FIG. 1;  
         [0026]    [0026]FIG. 7 is a schematic circuit diagram showing one example of the comparator used in the reference data generating circuit shown in FIG. 1;  
         [0027]    [0027]FIG. 8 is a truth table of the comparator circuit shown in FIG. 7;  
         [0028]    [0028]FIG. 9 is a timing chart showing the operation when b=011 is set in the comparator circuit shown in FIG. 7;  
         [0029]    [0029]FIG. 10 is a schematic circuit diagram showing one example of the counter shown in FIG. 2;  
         [0030]    [0030]FIG. 11 is a schematic circuit diagram showing one example of the control circuit and selector shown in FIG. 2;  
         [0031]    [0031]FIG. 12 is a schematic circuit diagram showing an example of the comparator shown in FIG. 2 constituted as a dynamic circuit;  
         [0032]    [0032]FIG. 13 is a schematic circuit diagram showing an example of the dynamic circuit shown in FIG. 12 constituted of thin film transistors;  
         [0033]    [0033]FIG. 14 is a schematic circuit diagram showing an example of the dynamic circuit shown in FIG. 12 constituted of thin film transistors;  
         [0034]    [0034]FIG. 15 is a schematic circuit diagram showing an example of the operational amplifier, which constitutes the ramp voltage generating circuit shown in FIG. 3, constituted of thin film transistors;  
         [0035]    [0035]FIG. 16 is a schematic circuit diagram showing an example of the operational amplifier, which constitutes the ramp voltage generating circuit shown in FIG. 3, constituted of thin film transistors;  
         [0036]    [0036]FIG. 17 is a schematic diagram showing an example of a liquid crystal display device according to the embodiment 2 of the present invention; and  
         [0037]    [0037]FIG. 18 is a block diagram showing one example of a circuit for performing gamma correction. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    Hereinafter, embodiments of the present invention will be explained in detail in conjunction with the drawings. In all of drawings, parts having the same functions are indicated by the same symbols, and their repeated explanation is omitted.  
         [0039]    [Embodiment 1] 
         [0040]    [0040]FIG. 1 is a diagram showing a liquid crystal display device according to an embodiment of the present invention. The liquid crystal display device of this embodiment is a polysilicon type liquid crystal display module in which thin film transistors (TFT) are formed on a polysilicon layer.  
         [0041]    The liquid crystal display device of this embodiment includes a drain driver  100 , a timing control circuit  200 , a reference data generating circuit  300 , a ramp voltage generating circuit  400 , a gate driver  500  and a display part  800 .  
         [0042]    In the display part  800 , there are a plurality of pixels, which are arranged in a matrix array, drain signal lines D, which supply a video signal voltage to respective pixels, and gate signal lines G, which supply a scanning signal voltage to the respective pixels.  
         [0043]    Each pixel includes a pixel transistor (GTFT), which is constituted of a thin film transistor, and the pixel transistor (GTFT) is connected between a drain signal line D and a pixel electrode (ITO 1 ), and a gate thereof is connected to a gate signal line G. Between the pixel electrode (ITO 1 ) and a counter electrode (also referred to as a “common electrode”, although not shown in the drawing), liquid crystal is sealed, and, hence, a pixel capacity (CLC) is equivalently connected between the pixel electrode (ITO 1 ) and the counter electrode. In FIG. 1, for the sake of brevity of illustration, only one thin film transistor (GTFT) is shown.  
         [0044]    The drain driver  100  is constituted of a shift register  110 , a latch circuit  120 , a latch circuit  130 , a comparator  140  and a sample holding circuit  150 .  
         [0045]    The timing control circuit  200  receives a clock (CLK), a horizontal synchronous signal (Hs), a vertical synchronous signal (Vs), a display timing signal (DTMG) and display data (Di) as input signals, and it generates signals which control the drain driver  100 , the reference data generating circuit  300 , the ramp voltage generating circuit  400  and the gate driver  500 .  
         [0046]    Hereinafter, the driving method employed in the liquid crystal display device of this embodiment will be explained.  
         [0047]    In general, for the purpose of preventing degradation of the liquid crystal, a liquid crystal display device typically adopts an alternating driving method. In the liquid crystal display device of this embodiment, as an alternating driving method, a dot inversion method is adopted. According to the dot inversion method, video signals which are applied to pixels which are arranged close to each other assume polarities opposite to each other in the row direction as well as in the column direction.  
         [0048]    The shift register  110  is operated in response to a start signal (HST) and a clock signal (HCK) transmitted from the timing control circuit  200 , and it outputs a multi-phase pulse which controls the latch circuits  120 .  
         [0049]    The latch circuits  120 , in response to this multi-phase pulse, sequentially hold the display data (DATA) transmitted from the timing control circuit  200 , one after another, for one horizontal scanning line.  
         [0050]    Upon receipt of a timing signal (LT), which is indicative of the completion of transfer of display data for one horizontal scanning line, from the timing control circuit  200 , the latch circuit  130  receives and holds the display data in the latch circuit  120  at the same timing and at the same time.  
         [0051]    The comparator  140  compares a quantity of display data held by the latch circuit  130  and a quantity of reference data (NCNT) transmitted from the reference data generating circuit  300 . To be more specific, the comparator  140  is initialized in response to an initializing signal (RS) transmitted from the timing control circuit  200 , and, thereafter, it outputs a High level (hereinafter referred to as “H level”) when the reference data (NCNT) is smaller than the display data or equal to the display data. The reference data generating circuit  300  is an up counter which receives the clock (CK) and the initializing signal (RS) transmitted from the timing control circuit  200  as inputs. The sample holding circuit  150  receives an output of the comparator  140 , the alternating signals (M, MB), and outputs (RAMP 1 , RAMP 2 ) of the ramp voltage generating circuit  400  as inputs, and it outputs a video signal voltage to the drain signal lines D on the display part  800 . Here, the alternating signal (M) and the alternating signal (MB) are logic signals which control the polarity of the video signal voltage that is applied to the pixel electrode of the display part  800  and have the relationship of inversion, and, hence, their logics are inverted for every line or for every frame.  
         [0052]    The output (RAMP 1 ) of the ramp voltage generating circuit  400  is a ramp voltage of positive polarity, and the output (RAMP 2 ) of the ramp voltage generating circuit  400  is a ramp voltage of negative polarity. With respect to respective ramp voltages (RAMP 1 ) and (RAMP 2 ), their absolute values of inclination are set to be equal to each other.  
         [0053]    The sample holding circuit  150  samples a ramp voltage (RAMP 1 ) using the switching element (SWA) or a ramp voltage (RAMP 2 ) using the switching element (SWB) in response to an output signal of the comparator  140  under the control of alternating signals (M, MB) and outputs the sampled voltage to the drain signal lines (D) as video signal voltages.  
         [0054]    In the case shown in FIG. 1, when the alternating signal (M) assumes the H level and the alternating signal (MB) assumes the L level, the sampled ramp voltage (RAMP 1 ) of positive polarity is outputted to the drain lines (D 1 ) and the sampled ramp voltage (RAMP 2 ) of negative polarity is outputted to the drain lines (D 2 ). Further, when the alternating signal (M) assumes the L level and the alternating signal (MB) assumes the H level, the sampled ramp voltage (RAMP 2 ) of negative polarity is outputted to the drain lines (D 1 ) and the sampled ramp voltage (RAMP 1 ) of positive polarity is outputted to the drain lines (D 2 ). Due to such a constitution, the polarity of the video signal outputted to the drain lines D can be inverted for every one horizontal line between neighboring drain signal lines. In FIG. 1, reference symbol LS indicates a level shift circuit.  
         [0055]    The gate driver  500  is operated in response to a start signal (VST) and a clock (VCK) transmitted from the timing control circuit  200 , and it outputs a scanning signal which sequentially turns on pixel transistors (GTFT) for one horizontal scanning line period to the gate signal lines G of the display part  800 .  
         [0056]    In this way, images are displayed on the display part  800 . In this embodiment, since the alternating operation is performed by the sample holding circuit  150 , the ramp voltages (RAMP 1 , RAMP 2 ) which are outputted from the ramp voltage generating circuit  400  can be held at the positive polarity and the negative polarity without changing the polarity, whereby the voltage amplitude can be decreased and the power consumption can be reduced. Further, the output impedance of the ramp generating circuit  400  can be reduced, and, hence, the delay time can be shortened, whereby the display images of high quality can be obtained.  
         [0057]    In this embodiment, gamma correction is performed using the reference data generating circuit  300 . FIG. 2 is a block diagram showing the reference data generating circuit  300  shown in FIG. 1.  
         [0058]    The reference data generating circuit  300  is constituted of a frequency dividing circuit  310 , a selector  320 , a counter  330 , a register  340 , a comparator  350  and a control circuit  360 .  
         [0059]    The frequency dividing circuit  310  divides the frequency of the input clock CK and outputs four divided frequency signals (f 1 , f 2 , f 3 , f 4 ). Here, in FIG. 2, RS indicates an initializing signal. The frequencies of the respective outputs of the frequency dividing circuit  310 , when f 0  is set as the reference frequency, become f 1 /f 0 =1, f 2 /f 0 =½, f 3 /f 0 =¼, f 4 /f 0 =⅛.  
         [0060]    The selector  320 , in response to an output signal of the control circuit  360 , selects one signal (input signal (fin)) from the four divided frequency signals (f 1 , f 2 , f 3 , f 4 ) outputted from the frequency circuit  310  and outputs the input signal (fin) to the counter  330 . The counter  330  is an up counter which counts the input signal (fin).  
         [0061]    In the register  340 , data for gamma correction (N 1  to N 6 ) is preliminarily stored. In this embodiment, the data is stored at six points. The comparator  350  compares an output value of the counter  330  and a value of the gamma correction data stored in the register  340 . The control circuit  360  receives an output of the comparator  350  as an input and controls the selector  320 .  
         [0062]    [0062]FIG. 4 shows the relationship between the count value (Nc) of the counter  330  shown in FIG. 2 and the frequency of the input signal (fin) which is inputted to the counter  330 . In response to the value (N 1  to N 6 ) stored in the register  340  and the count value (Nc) of the counter  330 , the frequency of the input signal (fin) of the counter  330  is controlled as shown in FIG. 4.  
         [0063]    [0063]FIG. 5 is a view showing time-sequential response of the count value of the reference data generating circuit  300 . In FIG. 5, reference symbol T indicates time and reference symbol Nc indicates the count value. The counter  330  is reset in response to the initializing signal RS and, thereafter, the frequency of the input signal (fin) is changed, as shown in FIG. 4, in the sequence f 4 →f 3 →f 2 →f 1 →f 2 →f 3 →f 4 .  
         [0064]    In this case, with respect to the count value (Nc) of the reference data generating circuit  300 , the inclination is gentle when the frequency of the input signal (fin) is low and is steep when the frequency of the input signal (fin) is high. As a result, the time sequential response of the count value of the reference data generating circuit  300  exhibits the characteristic shown in FIG. 5.  
         [0065]    [0065]FIG. 3 is a circuit diagram showing the ramp voltage generating circuit  400  shown in FIG. 1.  
         [0066]    The ramp voltage generating circuit  400  is, as shown in FIG. 3, constituted of two ramp generating circuits which generate a ramp voltage (RAMP 1 ) of positive polarity and a ramp voltage (RAMP 2 ) of negative polarity. The ramp generating circuit which generates the ramp voltage (RAMP 1 ) is constituted of an operational amplifier  411 , an inverter  412 , switching elements ( 413 ,  415 ), a resistor  414  and a capacitor  416 ; while, the ramp generating circuit which generates the ramp voltage (RAMP 2 ) is constituted of an operational amplifier  421 , an inverter  422 , switching elements ( 423 ,  425 ), a resistor  424  and a capacitor  426 .  
         [0067]    With respect to respective ramp generating circuits, when the initializing signal (RS) assumes the H level, the switching elements ( 413 ,  423 ) are turned off and the switching elements ( 415 ,  425 ) are turned on. In this state, the respective ramp generating circuits constitute voltage follower circuits, and, hence, the respective outputs assume a ground potential (GND). Next, when the initializing signal (RS) assumes the L level, the switching elements ( 413 ,  423 ) are turned on and the switching elements ( 415 ,  425 ) are turned off. Accordingly, the capacitors ( 416 ,  426 ) are charged, and, hence, the ramp voltage (RAMP 1 ) rises along with a lapse of time and the ramp voltage (RAMP 2 ) is decreased along with a lapse of time.  
         [0068]    [0068]FIG. 6 is a view showing the time-sequential response of the ramp voltage generating circuit  400 . In FIG. 6, reference symbol T indicates time and reference symbol V indicates voltage.  
         [0069]    In view of the time-sequential response of the count value (Nc) of the reference data generating circuit  300  shown in FIG. 5 and the time-sequential response of the ramp voltage generating circuit  400  shown in FIG. 6, the relationship between the count value (Nc) of the reference data generating circuit  300  and the output voltage (V) of the ramp voltage generating circuit  400  assumes an inverse function of the time-sequential response of the count value (Nc) of the reference data generating circuit  300 . That is, the relationship of voltage and transmissivity (gamma characteristics) of the driven liquid crystal can be corrected by setting the time-sequential response of the count value of the reference data generating circuit  300  to a relationship similar to the gamma characteristics.  
         [0070]    In this manner, according to this embodiment, by changing over the frequency of the input signal of the counter  330 , which forms part of the reference data generating circuit  300 , in response to the count value (Nc) of the reference data generating circuit  300 , the gamma characteristics of the driven liquid crystal can be corrected.  
         [0071]    According to this method, the ramp voltages (RAMP 1 , RAMP 2 ) outputted from the ramp voltage generating circuit  400  may be always set to a fixed inclination, and, hence, even when a delay is present in the drain signal line D, since an absolute value of the error is fixed, the influence to on display quality can be reduced.  
         [0072]    [0072]FIG. 7 is a circuit diagram showing one example of the comparator  350  used in the reference data generating circuit  300 . The circuit shown in FIG. 7 is of a comparator having a 3 bit input and is constituted of inverters ( 31 ,  32 ,  33 ), OR circuits ( 34 ,  45 ,  36 ), an AND circuit  37  and an SR flip-flop  38 . In FIG. 7, reference symbols a 0 , a 1 , a 2  indicate signals from the counter  330  and reference symbols b 0 , b 1 , b 2  indicate signals from the register  340 .  
         [0073]    A truth table of the comparator circuit shown in FIG. 7 is shown in FIG. 8. FIG. 8 indicates an output c of the AND circuit  37 . When the count value of the counter  330  is increased from 0, the output c changes 0 to 1 at a point of time that the value of b becomes equal to the count value of the counter  330 . By inputting this output c to the SR flip-flop  38 , the output d assumes the H level when the relationship a≧b is satisfied.  
         [0074]    [0074]FIG. 9 is a timing chart which shows when the signal b is set as b=011 in the comparator circuit shown in FIG. 7. The output c assumes the H level when the signal is a=011 and a=111, while the output d of the SR flip-flop  38  assumes the H level when the signals a, b assume the relationship a≧b.  
         [0075]    [0075]FIG. 10 is a circuit diagram showing one example of the counter  330  shown in FIG. 2. The circuit shown in FIG. 10 is a 4 bit counter and is constituted of a latch circuit  380  and an incrementer  370 .  
         [0076]    The latch circuit  380  is constituted of D-type flip-flops ( 381  to  384 ), and it is operated in response to the clock (CK), the initializing signal (RS) and inputs (ei 0  to ei 3 ), latches the inputs (ei 0  to ei 3 ) at the timing of the clock (CK), and generates the outputs (eo 0  to eo 3 ). The incrementer  370  is constituted of AND circuits ( 375  to  377 ) and EOR circuits (exclusive “or” circuit) ( 371  to  374 ) and an output of the incrementer  370  is inputted to the latch circuit  380  and “1” is added to the latch circuit  380 .  
         [0077]    Due to such a constitution, it is possible to realize a synchronous-type counter  330  which can add “1” to the output of the latch circuit  380  at the timing of the clock (CK). The counter  330  shown in FIG. is also applicable to the frequency dividing circuit  310 .  
         [0078]    [0078]FIG. 11 is a circuit diagram showing one example of the control circuit  360  and the selector  320  shown in FIG. 2.  
         [0079]    The control circuit  360  shown in FIG. 11 is constituted of inverters ( 361  to  366 ), AND circuits ( 391  to  395 ) and OR circuits ( 396  to  398 ), and it receives the output of the comparator  350  as an input thereof and outputs selector signals (s 1  to s 4 ). The selector  320  is constituted of AND circuits ( 321  to  324 ) and OR circuits ( 325  to  327 ), and it selects one of the output signals (f 1  to f 4 ) of the frequency dividing circuit in response to the selector signals (s 1  to s 4 ) and outputs the input signal (fin).  
         [0080]    As mentioned previously, the output of the comparator  350  assumes the H level in the order of C 1 →C 2 →C 3 →C 4 →C 5 →C 6 . Assuming that the outputs (C 1  to C 6 ) of the comparator  350  are set to the L level, the selector signal (s 1 ) assumes the H level, and, hence, due to the AND circuit  321 , the frequency dividing signal having the frequency of f 4  is selected as the input signal (fin).  
         [0081]    Next, when the output (c 1 ) of the comparator  350  assumes the H level, due to the AND circuit  391 , the selector signal (s 2 ) assumes the H level, and, hence, due to the AND circuit  322 , the frequency dividing signal having the frequency of f 3  is selected as the input signal (fin).  
         [0082]    Hereinafter, in the same manner, the frequency dividing signal selected by the selector  320  is changed in the order of f 4 →f 3 →f 2 →f 1 →f 2 →f 3 →f 4 .  
         [0083]    [0083]FIG. 12 is a circuit diagram showing the circuit constitution when the comparator  350  shown in FIG. 2 is constituted as a dynamic circuit. The circuit shown in FIG. 12 is constituted of switching elements ( 41  to  48 ), inverters ( 52  to  55 ) and a capacitor  51 .  
         [0084]    When the initializing signal (RS) assumes the H level, the switching element  41  is turned off and the switching element  48  is turned on, so that the output d assumes the L level. Next, when the initializing signal (RS) assumes the L level, the switching element  41  is turned on and the switching element  48  is turned off, and, hence, an output d is controlled based on a switching element logic provided by the switching elements ( 42  to  47 ).  
         [0085]    In the switching element logic, the parallel connection constitutes an OR operation and the serial connection constitutes an AND operation, and, hence, the constitution of the switching elements ( 42  to  47 ) are equivalent to the constitution of the circuit shown in FIG. 7.  
         [0086]    The circuit constitution of the dynamic circuit shown in FIG. 12, when the dynamic circuit is constituted of thin film transistors, is shown in FIG. 13 and FIG. 14. The circuit shown in FIG. 13 constitutes the switching element logic using P-type MOS transistors (hereinafter referred to as PMOS) and the circuit shown in FIG. 14 constitutes the switching element logic using N-type MOS transistors (hereinafter referred to as NMOS).  
         [0087]    [0087]FIG. 15 and FIG. 16 are circuit diagrams showing the circuit constitution when the operational amplifiers ( 411 ,  421 ) which constitute the ramp voltage generating circuit  400  shown in FIG. 3 are constituted of thin film transistors. The circuit shown in FIG. 15 is the circuit of the operational amplifier used in the ramp generating circuit which generates the ramp voltage (RAMP 1 ) of positive polarity, while the circuit shown in FIG. 16 is the circuit of the operational amplifier used in the ramp generating circuit which generates the ramp voltage (RAMP 2 ) of negative polarity.  
         [0088]    In the circuit shown in FIG. 15, the output transistor  435  is constituted of a PMOS transistor having the source connected to ground. Due to such a constitution, at the time of generating the ramp voltage (RAMP 1 ) of positive polarity, it is possible to ensure a current (source current) in the outputting direction from a required output terminal, and, at the same time, it is possible to raise the output voltage to a voltage in the vicinity of the power source voltage.  
         [0089]    In the circuit shown in FIG. 16, the output transistor  445  is constituted of a NMOS transistor having its source connected to the ground. Due to such a constitution, at the time of generating the ramp voltage (RAMP 2 ) of negative polarity, it is possible to ensure a current (sink current) in the inputting direction at the required output terminal, and, at the same time, it is possible to lower the output voltage to a voltage in the vicinity of a negative power source voltage.  
         [0090]    [Embodiment  2 ] 
         [0091]    [0091]FIG. 17 is a schematic diagram showing the constitution of a liquid crystal display device according to embodiment 2 of the present invention.  
         [0092]    The feature which makes this embodiment 2 different from the previously-mentioned embodiment 1 lies in the constitution of the sample holding circuit  150 .  
         [0093]    In this embodiment, in the sample holding circuit  150 , a buffer amplifier (BAA) which amplifies the ramp voltage (RAMP 1 ) of positive polarity and a buffer amplifier (BAB) which amplifies the ramp voltage (RAMP 2 ) of negative polarity are provided so as to drive the drain signal lines D using the buffer amplifiers. As a result, a fluctuation of the load of the ramp voltage generating circuit  400  attributed to display images can be suppressed, and, hence, images of high quality can be displayed.  
         [0094]    Here, the buffer amplifier (BAA) and the buffer amplifier (BAB) are provided for every two neighboring drain signal lines (for example, the drain signal line (D 1 ) and the drain signal line (D 2 ) shown in FIG. 17), wherein two drain signal lines use the buffer amplifier (BAA) and the buffer amplifier (BAB) in common. Accordingly, in this embodiment, the outputs of two comparators  140 , which correspond to two neighboring drain signal lines, are inputted to the sample holding circuit  150 .  
         [0095]    Then, due to the operation of switching elements (SW 1 ), which are controlled in response to the alternating signals (M, MB), an output of one comparator  140  is outputted to a switching element (SWA) which samples the ramp voltage (RAMP 1 ) of positive polarity to or a switching element (SWB) which samples the ramp voltage (RAMP 2 ) of negative polarity.  
         [0096]    Simultaneously, an output of another comparator  140  is outputted to the switching element (SWB) or the switching element (SWA).  
         [0097]    Further, due to the operation of switching elements (SW 2 ), which are controlled in response to the alternating signals (M, MB), an output of the buffer amplifier (BAA) which amplifies the ramp voltage (RAMP 1 ) of positive polarity, is inputted to one drain signal line; or another drain signal line and, at the same time, an output of the buffer amplifier (BAB), which amplifies the ramp voltage (RAMP 2 ) of negative polarity, is inputted to the other drain signal line or the one drain signal line.  
         [0098]    For example, with respect to the case shown in FIG. 17, when the alternating signal (M) assumes the H level and the alternating signal (MB) assumes the L level, the output of the comparator  140  corresponding to the drain signal line (D 1 ) is inputted to the switching element (SWA) and the output of the comparator  140  corresponding to the drain signal line (D 2 ) is inputted to the switching element (SWB). Further, the output voltage of the buffer amplifier (BAA) is inputted to the drain signal line (D 1 ) and the output voltage of the buffer amplifier (BAB) is inputted to the drain signal line (D 2 ).  
         [0099]    Further, when the alternating signal (M) assumes the L level and the alternating signal (MB) assumes the H level, the output of the comparator  140  corresponding to the drain signal line (D 1 ) is inputted to the switching element (SWB) and the output of the comparator  140  corresponding to the drain signal line (D 2 ) is inputted to the switching element (SWA). Further, the output voltage of the buffer amplifier (BAB) is inputted to the drain signal line (D 1 ) and the output voltage of the buffer amplifier (BAA) is inputted to the drain signal line (D 2 ).  
         [0100]    Accordingly, the polarity of the video signal supplied to the drain signal lines D can be inverted for every horizontal scanning line between the neighboring drain signal lines.  
         [0101]    As has been explained heretofore, gamma correction of the video signal voltage applied to the liquid crystal is performed using the reference data generating circuit  300 , and, hence, the ramp voltage outputted from the ramp voltage generating circuit  400  can have a fixed inclination. Thus, even when a delay is present in the voltage waveform of the ramp voltage on the drain signal line D, it is possible to set the error to a fixed value, whereby the present invention is applicable to a drain driver of high accuracy.  
         [0102]    Further, the reference data generating circuit  300  can be realized using a logic circuit, and, hence, the reference data generating circuit  300  can be easily formed on the same substrate as the display part  800 . Further, since the data used for gamma correction is stored in a register, it is possible to set the data particularly for every product or for every panel individually.  
         [0103]    Further, since the ramp voltages (RAMP 1 , RAMP 2 ) which are outputted from the ramp voltage generating circuit  400  can be held at the positive polarity and the negative polarity, respectively, without changing the polarity, it is possible to simplify the circuit, and, at the same time, the ramp voltage generating circuit  400  can be formed on the substrate on which the display part  800  is formed.  
         [0104]    In this manner, according to the liquid crystal display device of this embodiment, by performing gamma correction individually, or performing temperature compensation which changes the correction value in response to temperature, it is possible to realize a display of high quality.  
         [0105]    Further, by forming the drain driver and the peripheral circuit on the substrate on which the display part  800  is formed, the number of parts and the number of connection terminals can be reduced, and, hence, a display having high reliability can be realized.  
         [0106]    Further, since the alternating operation is performed by the sample holding circuit  150 , the ramp voltages (RAMP 1 , RAMP 2 ) which are outputted from the ramp voltage generating circuit  400  can be held at the positive polarity and the negative polarity respectively, without changing the polarity, whereby the voltage amplitude can be decreased and the power consumption can be reduced.  
         [0107]    Further, the output impedance of the ramp generating circuit  400  can be reduced, and, hence, the delay time can be shortened whereby display images of high quality can be obtained.  
         [0108]    Here, although the present invention has been explained in conjunction with various embodiments, which are applied to a liquid crystal display module, it is needless to say that the present invention is not limited to these embodiments, but is applicable to other display devices, such as an EL display device.  
         [0109]    Although the present invention has been specifically described based on certain embodiments, it is needless to say that the present invention is not limited to these embodiments and that various modifications are conceivable without departing from the gist of the present invention.  
         [0110]    A brief explanation of the advantageous effects obtained by typical aspects of the invention disclosed in this specification is provided, as follows.  
         [0111]    (1) According to the present invention, the data for gamma correction can be stored in a register, and, hence, it is possible to set the data for every product or for every panel individually; whereby, by performing gamma correction individually or by performing temperature compensation which changes the correction value in response to temperature at the time of shipping, it is possible to realize a display of high quality.  
         [0112]    (2) According to the present invention, since the driving circuit can be formed on the substrate on which the display part is formed, it is possible to realize a highly reliable display, while reducing the number of parts and the number of connection terminals.  
         [0113]    (3) According to the present invention, the voltage amplitude of the ramp voltage generating circuit can be decreased, and, hence, the power consumption can be reduced; and, at the same time, the output impedance of the ramp generating circuit can be reduced and the delay time can be shortened, whereby display images of high quality can be obtained.