Abstract:
A display driver adjustable for a gamma specification of a liquid display panel, including: a system interface receiving display data from an external; a memory storing the display data; a grayscale voltage generator generating a plurality of grayscale voltages; a gamma adjusting circuit adjusting the gamma specification; and an output circuit outputting the grayscale voltage in response to the display data from the memory to the liquid display panel, wherein the gamma adjusting circuit includes: a gradient adjustment register controlling variable resistors of a ladder resistor; an amplitude/reference adjustment register; and a micro adjustment register, wherein the gradient adjustment register, the amplitude/reference adjustment register and micro adjustment register, are independently set in accordance with red, green and blue, respectively.

Description:
CROSS REFERNCE TO RELATED APPLICATION 
   This is a continuation of U.S. application Ser. No. 10/161,635, filed Jun. 5, 2002 now U.S. Pat. No. 7,023,458, the subject matter of which is incorporate by reference herein. This application also relates to U.S. application Ser. No. 11/126,160, filed on May 11, 2005, the subject matter of which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a display apparatus having a display panel in which display pixels are arranged in a matrix and a driving device for supplying to the display panel a gray scale voltage corresponding to display data. More specifically, the invention relates to a display apparatus that uses a liquid crystal material, organic EL, and plasma and its driving device for displaying. 
   JP-A-2001-13478 discloses a liquid crystal display apparatus source driver that constitutes a reference voltage generating circuit for generating a gamma correction reference voltage by resistive voltage division, and a resistance setting circuit for selecting a resistance to be used for the resistive voltage division from among a plurality of resistances. The reference further discloses that a gamma correction setting register receives data for setting the value of resistance, appeared on a display data line, in response to a clock signal CK when an enable signal E goes to “H”, and then switching on or off respective switches for resistances and other switches that comprise the reference voltage generating circuit according to the bit value of the received data for setting the value of resistance, thereby determining the reference voltage. 
   JP-A-6-348235 discloses a liquid crystal display apparatus that constitutes a liquid crystal display panel having a X signal line and a Y signal line, a horizontal driver for selecting a gray scale signal from among a plurality of gray scale signals supplied from a gray scale voltage generating circuit, on the basis of a data signal of an image to be displayed, for supply onto the X signal line of the liquid crystal display panel, and a vertical driver for supplying a liquid panel scanning signal onto the Y signal line of the liquid crystal display panel. The reference further discloses that the gray scale voltage generating circuit constitutes a plurality of fixed resistances interposed in series between the sides of the reference voltage of a high potential and the reference voltage of a low potential, and voltage varying unit for varying a voltage at a connection point between the fixed resistances to a voltage between the high potential reference voltage and the low potential reference voltage, thereby supplying the voltage at the connection point between the fixed resistances as a gray scale signal. The reference furthermore discloses that by adjusting the resistance value of a variable resistance in the above-mentioned manner, the voltage level of the gray scale signal or a gray scale voltage can be arbitrarily adjusted, so that gray scale characteristics can be freely modified. 
   JP-A-11-24037 discloses a gray scale voltage generating circuit that constitutes amplification unit for generating a variable intermediate-level gray scale voltage from an intermediate-level reference voltage and amplification unit for supplying gray scale voltages of negative polarity. The former amplification unit divides a reference supply voltage with the resistance divided for amplification, thereby generating a higher gray scale voltage of positive polarity and a lower gray scale voltage of positive polarity. Then, the amplification unit further divides these voltages with the resistance divided, thereby generating the intermediate-level reference voltage. Finally, the amplification unit generates the variable intermediate level-gray scale voltage from the intermediate-level reference voltage, using a variable resistance as a feedback resistance. The latter amplification unit inverse-amplifies all the gray scale voltages of positive polarity, obtained by dividing the resistive voltage and then amplifying the reference supply voltage, at the same amplification factor with respect to a liquid crystal GND potential, for supply as the gray scale voltages of negative polarity. The reference further discloses that the gray scale characteristics can be adjusted just by adjusting a single variable resistance. 
   In the above-mentioned art, however, among 64 gray scale levels of voltages, the voltages at the two ends are fixed as a GND voltage or the reference voltage externally supplied. Accordingly, adjustment to the gray scale voltage fixed as the GND voltage is impossible. Further, for adjustment to the gray scale voltage fixed as the reference voltage, an additional adjustment circuit becomes necessary outside the gray scale voltage generating circuit, thus leading to an increase in the number of components. Though there are some cases where adjustment to the voltages of the gray scale levels at the two ends becomes necessary due to the characteristic differences of liquid crystal display panels, the above-mentioned techniques did not take such cases into consideration. 
   JP-A-11-175027 discloses a liquid crystal driving circuit that constitutes a latch address control circuit, a first holding circuit, a second holding circuit, setting registers, a gray scale voltage generating circuit, a gray scale voltage selector circuit, and an amplifier circuit. The latch address control circuit sequentially generates latch signals that receive display data. The first holding circuit holds the number of display data equivalent to the number of output data lines in response to a latch signal, and the second holding circuit receives and then holds the number of display data held in the first holding circuit, equivalent to the number of the output data lines in response to a horizontal synchronization signal. The setting registers control the value of a gray scale voltage. The gray scale voltage generating circuit receives a plurality of different reference voltages to generate a gray scale voltage specified by one of the setting registers. The gray scale voltage selector circuit selects a gray scale voltage according to the display data held in the second holding circuit, and the amplifier circuit shifts the gray scale voltage selected by the selector circuit so as to be more closer to an offset voltage, and amplifies the gray scale voltage by an amplitude factor specified by one of the setting registers, for supply. The reference further discloses that the setting registers for setting the amplification factor of respective operational amplifiers in the amplifier circuit are provided for respective R, G, and B display colors, and that a voltage setting can be changed according to each of the colors. The reference further discloses that an offset voltage setting can be changed, because the offset voltage of the amplifier circuit is generated by dividing an offset reference voltage with the resistance divided and a common voltage, using a plurality of variable resistances, the resistance value of which can be set. In the above-mentioned art, however, an offset adjustment circuit becomes necessary in the amplified circuit. Thus the size of the driving circuit becomes large, so that the cost of the circuit increases. Further, in this art, a gamma correction control register sets the resistance values of all the variable resistances in a resistance ladder for adjustment so as to obtain a desired gamma characteristic. Accordingly, if the resistance value of a single variable resistance is adjusted, the overall resistive voltage division ratio would be changed. This leads to a change in all the gray scale voltages. Thus, in order to adjust gray scale voltages according to the respective characteristics completely, it would take much time. Further, The reference does not disclose adjustment to the gray scale voltage amplitude. 
   JP-A-2001-22325 discloses a liquid crystal display apparatus that constitutes a pair of amplifiers, a voltage dividing circuit for generating a plurality of a pair of symmetrical reference voltages of positive and negative polarities from standard voltages of positive and negative polarities, and a variable voltage generating circuit for supplying a pair of symmetrical reference voltages of positive and negative polarities for gray scale adjustment to a pair of voltage dividing points in the voltage dividing circuit, associated with specific intermediate gray scale levels. The reference further discloses that by increasing a positive reference voltage V x−2  from a positive reference voltage V x−1  by a desired value and decreasing a negative V x+1  from V x  by the desired value simultaneously in the variable voltage generating circuit in a normally white mode, the voltage values of reference voltages V 0  to V x−2 , V x+1  to V 2x−1  can be changed smoothly. The reference discloses that, with this arrangement, adjustment to and modification of a gray scale level-brightness characteristic can be easily performed by a single variable voltage generating circuit. 
   However, the above-mentioned art does not display insertion of a variable resistance into the reference voltage generating circuit, and does not disclose adjustment to the amplitude of a gray scale voltage. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a display apparatus and a display driving device in which, by adjusting both of the gradient and the amplitude of a gray scale number-gray scale voltage characteristic, adjusting accuracy is improved, and image quality is thereby improved. 
   Therefore, a display apparatus and a display driving device according to the present invention comprise a gray scale voltage generating circuit for generating a plurality of levels of a gray scale voltage from a reference voltage, an amplitude adjustment register capable of setting the amplitude of a characteristic curve of a plurality of levels of the gray scale voltage with respect to gray scale numbers, and a gradient adjustment register capable of setting the gradient of the characteristic curve. 
   Then, preferably, the display apparatus and the display driving device according to the present invention further comprise resistive voltage dividing circuits for dividing the reference voltage with resistance divided, an amplitude adjustment variable resister connected in series with the side of the reference voltage closer to the side of the reference voltage than the resistive voltage dividing circuits, the resistance setting of which is adjustable according to a setting in the amplitude adjustment register, and a gradient adjustment variable resister connected in series with the resistive voltage display circuits, the resistance setting of which is adjustable according to a setting in the gradient adjustment register. 
   Alternatively, preferably, the display apparatus and the display driving device according to the present invention further comprise resistive voltage dividing circuits for dividing the reference voltage with the resistance divided, an amplitude adjustment variable resister connected in series with ground, closer to the ground than the resistive voltage dividing circuits, the resistance setting of which is adjustable according to a setting in the amplitude adjustment register, and a gradient adjustment variable resister connected in series with the resistive voltage dividing circuits, the resistance setting of which is adjustable according to a setting in the gradient adjustment register. 
   According to the present invention, both of the gradient and the amplitude of the gray scale number-gray scale voltage characteristic can be adjusted. Thus, adjusting accuracy is improved, and image quality is thereby improved. 
   Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B, and  1 C are characteristic curves showing a gamma characteristic of a typical liquid crystal display panel; 
       FIGS. 2A ,  2 B,  2 C and  2 D are characteristic curves showing adjustments to the gamma characteristic according to the present invention; 
       FIG. 3  is a block diagram showing a configuration of a gray scale voltage generating circuit according to a first embodiment of the present invention; 
       FIGS. 4A and 4B  are a block diagram showing configurations of a variable resister according to the first embodiment of the present invention; 
       FIG. 4C  is a table showing a relationship between a register setting and the resistance value of the variable resister according to the first embodiment of the present invention, respectively; 
       FIGS. 5A ,  5 B, and  5 C are characteristic curves showing adjustment operations of the gamma characteristic using settings of an amplitude adjustment register according to the present invention; 
       FIGS. 6A ,  6 B, and  6 C are characteristic curves showing adjustment operations of the gamma characteristic using settings of a gradient adjustment register according to the present invention; 
       FIGS. 7A and 7B  are a block diagram showing a configuration of a selector circuit, showing a relationship between a register setting value and a resistance divided voltage according to the first embodiment of the present invention, respectively; 
       FIG. 8  is a characteristic curve showing an adjustment operation of the gamma characteristic using settings of a micro adjustment register according to the present invention; 
       FIG. 9  is a block diagram showing a configuration of a liquid crystal display apparatus system according to a first embodiment of the present invention; 
       FIGS. 10A and 10B  are timing diagrams showing a flow for a register setting according to the present invention; 
       FIG. 11  are characteristic curves showing asymmetrical gamma characteristics of a liquid crystal display panel; 
       FIG. 12  is a block diagram showing a configuration of a gray scale voltage generating circuit according to a second embodiment of the present invention; 
       FIG. 13  is a block diagram showing a configuration of a gray scale voltage generating circuit according to a third embodiment of the present invention; 
       FIG. 14  is a block diagram showing a configuration of a liquid crystal display apparatus system according to a second embodiment of the present invention; 
       FIG. 15  is a block diagram showing a configuration of a liquid crystal display apparatus system according to a third embodiment of the present invention; and 
       FIG. 16  is a block diagram showing a configuration of a liquid crystal display apparatus system according to a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   A typical gamma characteristic will be described with reference to  FIGS. 1A ,  1 B, and  1 C.  FIG. 1A  shows an applied voltage-brightness characteristic when a liquid crystal display panel is in a normally black mode. The smaller the applied voltage is, the lower the brightness becomes, and the larger the applied voltage is, the higher the brightness becomes. It can be seen from this characteristic curve that a change in the brightness with respect to the applied voltage is slow or becomes saturated in a low applied voltage region and a high applied voltage region. 
   In addition to liquid crystal display panels in the normally black mode, there are also liquid crystal display panels in a normally white mode. However, a description herein will be directed to the case where the liquid crystal display panel is in the normally black mode. Incidentally, the present invention can be practiced irrespective of the mode of the liquid crystal display panel. 
   Next,  FIG. 1B  shows gray scale number-brightness characteristics. This characteristic is commonly referred to as the gamma characteristic. A solid line indicated by reference numeral  101  shows the characteristic that the brightness linearly increases as the gray scale number increases, and this characteristic is defined as the characteristic when γ=1.0. The value of γ is obtained from the following expression (1):
 
(gray scale number) γ =brightness[ cd/m   2 ]  (1)
 
   From the above expression (1), it can be seen that curves indicated by reference numerals  102  and  103  show the characteristics when γ=2.2 and γ=3.0, respectively. Traditionally, when display data is displayed on the liquid crystal display panel, the gamma characteristic a person perceives has the highest image quality is generally the characteristic indicated by the curve  102  when γ=2.2. 
   Thus, in a liquid crystal display apparatus, by adjusting an applied voltage for each gray scale number, adjustment to the gamma characteristic is made. 
     FIG. 1C  is a characteristic curve showing the relationship between gray scale number and applied voltage when the number of gray scale levels is set to 64. The applied voltage-brightness characteristic shown in  FIGS. 1A ,  1 B, and  1 C varies from one liquid crystal display panel to another liquid crystal display panel. When an applied voltage is adjusted such that γ becomes equal to 2.2, for example, an adjusted value of the applied voltage becomes different according to each of the liquid crystal display panels. A curve indicated by reference numeral  104  in  FIG. 1C  shows the relationship between gray scale number and applied voltage when γ=2.2. Curves indicated by reference numerals  105  and  106  show relationships between gray scale number and applied voltage when γ=2.2 in liquid crystal display panels different from the one for the curve  104 . As described above, in a liquid crystal display apparatus, a gray scale voltage generating circuit becomes necessary that can adjust an applied voltage, which will be referred to as a gray scale voltage, according to the characteristic of each liquid crystal display panel so as to obtain a desired gamma characteristic. 
   In order to allow adjustment to voltages of the gray scale levels at the two ends, the present invention is configured to have a resistance ladder. In this configuration, variable resistances are disposed at both ends of the resistance ladder. A reference voltage is externally supplied to one of the ends and the other end is coupled to ground. Voltages of the gray scale levels at the two ends such as the ones indicated by reference numerals  107  and  108  in  FIG. 1C  are generated by resistive voltage division using the variable resisters. Further, it is arranged such that a register, which will be referred to as an amplitude adjustment register, can set the resistance values of the variable resisters, and that offset adjustment which was conventionally made by an amplifier circuit was also made possible by the resistance ladder. 
   The present invention is not limited to this arrangement, and is configured to have the resistance ladder by which other voltages of gray scale levels than the ones of gray scale levels at the two ends can also be adjusted by register settings. The contents of the adjustments will be explained with reference to  FIGS. 2A ,  2 B, and  2 C. 
     FIG. 2A  shows gray scale number-vs.-gray scale voltage characteristics in the cases where the resistance values of the variable resistances at both ends of the resistance ladder have been set by the amplitude adjustment register. Dotted lines indicated by reference numeral  201  show the characteristics where an amplitude voltage adjustment to gray scale voltages is made such that the gray scale voltage of the highest scale level is changed without changing the gray scale voltage of the lowest gray scale level. Solid lines indicated by reference numeral  202  show the characteristics where the amplitude voltage adjustment to the gray scale voltages is made such that the gray scale voltage of the lowest scale level is changed without changing the gray scale voltage of the highest gray scale level. Both of the characteristic lines  201  and  202  show the cases where one of the variable resisters at both ends of the resistance ladder or the variable resisters on both of the reference voltage side and the ground side of the resistance ladder has been set by the amplitude adjustment register. Solid lines indicated by reference numeral  203  on  FIG. 2B  show characteristics where the variable voltages at both ends of the resistance ladder have been simultaneously set by the amplitude adjustment register. In this case, the same effect as in the case of offset adjustment that was made by the amplifier circuit can be obtained. 
   Next, solid lines indicated by reference numeral  204  in  FIG. 2C  show gray scale number-gray scale voltage characteristics where the gradient characteristic of voltages of intermediate gray scale levels is adjusted. This adjustment can be made by the gradient adjustment register. This register allows setting of the resistance values of the variable resisters that generate gray scale voltages  205  and  206  that determine the gradient characteristic in the resistance ladder. 
   As described above, gray scale voltages indicated by the curves  104  to  106  in  FIG. 1D  in accordance with the characteristics of respective liquid crystal display panels can be roughly set by the amplitude adjustment register and the gradient adjustment register. Adjustment to obtain a desired gamma characteristic according to the characteristics of respective liquid crystal display panels can be thereby facilitated, so that an adjustment time can be shortened. 
   Next, solid lines indicated by reference numeral  207  in  FIG. 2D  show gray scale number-gray scale voltage characteristics where respective gray scale voltages are micro adjusted. This micro adjustment becomes possible by providing resistive voltage dividing circuits for further dividing the respective voltages of gray scale levels resistive-voltage-divided by one or a plurality of the variable resisters and then allowing a desired gray scale voltage to be selected from among the voltages generated by the resistive voltage division according to a setting in a micro adjustment register. With this arrangement, even if a single variable resistance value is changed, which is the case where the problem described above would occur, respective gray scale voltages resistive-voltage-divided by this variable resister are further resistive-voltage-divided to select a desired voltage. Only the desired gray scale voltage can be thereby adjusted with no other gray scale voltages changed so much. 
   Further, by allowing the micro adjustment of respective gray scale voltages, adjustment to the gamma characteristic can be made with higher accuracy, so that higher image quality can be effected. 
   As described above, the present invention is configured to have a resistance ladder. With this configuration, when adjustment to the gamma characteristic is made, rough gray scale adjustment such as amplitude voltage adjustment to the gray scale voltages and the gradient characteristic adjustment to the voltages of intermediate gray scale levels according to the characteristics of respective liquid crystal display panels can be made by using settings of the amplitude register and the gradient register. Adjustment to the gamma characteristic can be thereby facilitated, so that an adjustment time can be shortened. Further, by providing the micro adjustment register, micro adjustment to the gray scale voltages which have been adjusted by the amplitude adjustment register and the gradient adjustment register can be further made. Adjusting accuracy can be thereby improved, so that high image quality can be effected. Still further, a degree of freedom in an adjustment range is increased. Thus, versatility of adjustment is obtained. 
   A configuration of a liquid crystal display apparatus according to a first embodiment of the present invention will be described with reference to  FIGS. 3 to 10 . 
     FIG. 3  is a block diagram showing a configuration of a gray scale voltage generating circuit according to the present invention. Reference numeral  301  denotes a control register for holing settings for adjusting the gamma characteristic, reference numeral  302  denotes the gray scale voltage generating circuit, and reference numeral  303  denotes a decoder circuit for decoding a gray scale voltage corresponding to display data. The control register  301  constitutes an amplitude adjustment register  304 , a gradient adjustment register  305 , and a micro adjustment register  306 , described above. Incidentally, the values in the control register  301  may also be stored in a non-volatile memory in a CPU to which the liquid crystal display apparatus is connected. 
   The gray scale voltage generating circuit  302  constitutes a resistance ladder  307  disposed between the sides of a reference voltage  316  externally supplied and GND, for generating voltages of gray scale levels, variable resisters  321  to  324  and resistive voltage division circuits  326  to  331  for further dividing voltages with resistance divided by the variable resisters, all of which constitutes the resistance ladder  307 , selector circuits  308  to  313  for selecting a gray scale voltage generated by the resistive voltage dividing circuits  326  to  331  according to a setting in the micro adjustment register  306 , an amplifier circuit  314  for buffering the output voltage of the respective selector circuits, and an output unit resistance ladder  315  for dividing the output voltage with resistance divided of the amplifier circuit  314  into a desired number of gray scale levels (herein 64) of voltages. 
   The lower variable resistance  321  disposed at the bottom of the resistance ladder  307  is configured to allow setting of its resistance value according to a lower variable resistance setting  317  set in the amplitude adjustment register  304 . The upper variable resister  322  disposed on the top of the resistance ladder  307  is configured to allow setting of its resistance value according to an upper variable resistance setting  318  set in the amplitude adjustment register  304 . Then, it is arranged such that the voltages divided by the variable resisters  321  and  322  are set to the voltages of the gray scale levels at the two ends, and amplitude adjustment of a gray scale voltage can be set by the amplitude adjustment register  304 . The lower variable resister  321  is connected to the GND side in series, being closer to the GND side than the resistive voltage dividing circuit  331  and the lowest level of the gray scale voltage. The upper variable resister  322  is connected to the side of the reference voltage  316  in series, being closer to the side of the reference voltage  316  than the resistive voltage dividing circuit  326  and the highest level of the gray scale voltage. That is, the lower variable resister  321  and the upper variable resister  322  are disposed outside the resistive voltage dividing circuits. When the gray scale voltage amplitude is reduced by the variable resisters  321  and  322 , power dissipation can be reduced. For this purpose, either one of the variable resisters  321  and  322  may be employed. 
   The lower-middle variable resister  323  disposed in the lower position from the middle of the resistance ladder  307  is configured to allow setting of its resistance value according to a lower-middle variable resistance setting set in the gradient adjustment register  305 . The upper-middle variable resister  324  disposed in the upper position from the middle of the resistance ladder  307  is configured to allow setting of its resistance value according to an upper-middle variable resistance setting set in the gradient adjustment register  305 . The voltages divided by both of the variable resisters  323  and  324  with the resistance divided are set to voltages of gray scale levels that determine the gradient characteristic of the voltages of intermediate gray scale levels, and it is arranged such that the gray scale voltage gradient characteristic can be set by the gradient adjustment register  305 . The variable resisters  319  and  320  are connected with the resistive voltage dividing circuits in series. Even if the variable resistance settings  319  of the variable resister  323  and the variable resistance setting  320  of the variable resister  324  change, the gray scale voltage amplitude is not affected so much. By adjusting both of the variable resisters  323  and  324 , the contrast of an image can be improved. For this purpose, either one of the variable resisters  323  and  324  may be employed. 
   By configuring the gray scale voltage generating circuit to have the resistance ladder as described above and setting variable resistance values in the resistance ladder by means of the amplitude adjustment register  304  and the gradient adjustment register  305 , a resistive voltage division ratio can be changed, so that the amplitude voltage adjustment to the gray scale voltages and the gradient characteristic adjustment to the voltages of the intermediate gray scale levels can be adjusted. Details of these operations will be described later. 
   Gray scale voltages generated according to the variable resistance values set in the amplitude adjustment register  304  and the gradient adjustment register  305  are further divided by the resistive voltage dividing circuits  326  to  331  with the resistance divided to generate micro-adjustment gray scale voltages to which micro adjustment is made. Next, the micro-adjustment gray scale voltages are supplied to the selector circuits  308  to  313  to select a desired gray scale voltage according to a setting  325  set in the micro adjustment register  306 . With this arrangement, micro adjustment to the respective gray scale voltages can be made, and the accuracy of adjustment to the gamma characteristic can be improved, so that the degree of freedom of adjustment is also improved. Details of this operation will be described later. 
   The respective gray scale voltages generated as described above are buffered at the amplifier circuit  314  in a subsequent stage. Then, in order to generate desired voltages of 64 gray scale levels, the gray scale voltages are divided by the output unit resistance ladder  315  with the resistance divided so as have a linear relationship to one another, and thereby the 64 gray scale voltages are generated. With this arrangement, among the 64 gray scale voltages generated by the gray scale voltage generating circuit  302 , a gray scale voltage corresponding to display data is decoded to become an applied voltage to the liquid crystal display panel. 
   The circuit as described above constitutes a resistance ladder that can make rough gray scale voltage adjustments such as the amplitude voltage adjustment to the gray scale voltages and the gradient characteristic adjustment to the voltages of intermediate gray scale levels by using settings in the amplitude adjustment register  304  and the gradient adjustment register  305 , when the gamma characteristic is adjusted. Then, it is arranged such that micro adjustment to the respective gray scale voltages generated by the resistance ladder can be further made according to a setting in the micro adjustment register  306 . Adjustment to the gamma characteristic can be thereby facilitated, so that an adjustment time can be shortened. Then, the adjusting accuracy and the degree of freedom of adjustment are improved, so that a small-sized gray scale voltage generating circuit that can effect high image quality and versatility is thereby realized at a low cost. 
   Next, the settings in the registers and the operations of the variable resisters  321  to  324  in  FIG. 3  according to this embodiment will be described with reference to  FIGS. 4A ,  4 B, and  4 C. Reference numeral  401  shows the internal configuration of the variable resister  321 ,  322 ,  323 , or  324 . The variable resisters  321  to  324  herein are configured such that for each decrease of bit in settings in the registers which are the amplitude adjustment resister  304  and the gradient adjustment register  305 , the resistance is incremented by 4R, where R indicates a unit of resistance. If a setting in the register is “111”[BIN] as indicated by reference numeral  402 , switches  403  to  405  connected to the terminals of the resisters in the variable resister  401  are switched ON, thereby bringing the variable resister  401  into a short-circuited state. Accordingly, the total resistance of the variable resister  401  becomes 0R. Incidentally, the switches  403  to  405  are controlled on a bit-to-bit basis of a setting in the register; the switch  403  is controlled to be switched ON or OFF according to the second bit of a setting in the register, the switch  404  is controlled to be switched ON or OFF according to the first bit of the setting in the register, and the switch  405  is controlled to be switched ON or OFF according to the zeroth bit of the setting of the register. Next, if a setting in the register is “000”[BIN] as indicated by reference numeral  406 , the switches  403  to  405  connected to the terminals of the resistances in the variable resister  401  are switched OFF. The total resistance of the variable resister  401  becomes the sum of the resistances inside the variable resister, or 28R. The relationship between setting of the register and variable resister value in the above-described circuit configuration becomes the one shown in the table indicated by reference numeral  407 . 
   The relationship between setting in the register and variable resistance value is just an example for setting. If the respective bits of a setting in the register are inverted, the relationship between setting of the register and variable resistance value becomes inverted; if a setting in the register increases, the resistance value of the variable resister also increases. The relationship between setting in the register and variable resister may also be inverted, as described above. The change ratio of a variable resistance value with respect to a setting in the register is herein set to 4R for each setting. The change ratio may also be smaller or larger than 4R. If the change ratio of a variable resistance value for each setting in the register is decreased, the accuracy of adjustment is improved. However, the range of adjustment becomes smaller. Conversely, if the change ratio of a variable resistance value for each setting in the register is increased, the adjustment range becomes more extended. However, the accuracy of adjustment deteriorates. Preferably, the resistance unit R constitutes several tens of kiloohms, because current dissipation can be reduced. Though the number of bits of a setting in the register described above is set to three bits, the number of the bits of the setting may be increased. In this case, though the adjustment range increases, the size of the gray scale voltage generating circuit increases. 
   With the arrangement described above, the resistance values of the variable resisters can be changed according to a setting in the register. 
   Next, adjustment operations of the gamma characteristic by the amplitude adjustment register  304  and the variable resisters  321  and  322  in the resistance ladder  307  in  FIG. 3  will be described with reference to  FIGS. 5A ,  5 B, and  5 C. 
     FIG. 5A  shows an adjustment operation when the resistance value of the lower variable resister  321  in the resistance ladder  307  in  FIG. 3  is set by the amplitude adjustment register  304 . A solid line indicated by reference numeral  501  shows a gray scale number-gray scale voltage characteristic when the amplitude adjustment register  304  is set to a default setting. If the gray scale voltage of the lowest gray scale level is to be changed without changing the gray scale voltage of the highest gray scale level to make amplitude adjustment to the gray scale voltages to a small degree, as shown by a dotted line indicated by reference numeral  502 , a setting in the amplitude adjustment register  304  should be set such that the resistance value of the lower variable resister  321  becomes large. If the gray scale voltage of the lowest gray scale level is to be changed without changing the gray scale voltage of the highest gray scale level to make amplitude adjustment to the gray scale voltages to a great degree, as shown by a dotted line indicated by reference numeral  503 , a setting in the amplitude adjustment register  304  should be set such that the resistance value of the lower variable resister  321  becomes small. 
   By changing the resistance value of the lower variable resister  321  according to a setting in the amplitude adjustment register  304  in this manner, the gray scale voltage of the lowest gray scale level can be changed without changing the gray scale voltage of the highest gray scale level, thereby allowing amplitude adjustment to the gray scale voltages. 
   Next,  FIG. 5B  shows an adjustment operation when the resistance value of the upper variable resister  322  in the resistance ladder  307  in  FIG. 3  is set by the amplitude adjustment register  304 . As described above, the solid line  501  in  FIG. 5B  shows the gray scale number-gray scale voltage characteristic when the amplitude adjustment register  304  is set to the default setting. If the gray scale voltage of the highest scale level is to be changed without changing the gray scale voltage of the lowest gray scale level as shown in a dotted line indicated by reference numeral  504  to make amplitude adjustment to the gray voltages to a small degree, a setting in the amplitude adjustment register  304  should be set such that the resistance value of the upper variable resister  322  becomes large. If the gray scale voltage of the highest gray scale level is to be changed without changing the gray scale voltage of the lowest gray scale level as shown by a dotted line indicated by reference numeral  505  to make amplitude adjustment to the gray scale voltages to a great degree, a setting in the amplitude adjustment register  304  should be set such that the resistance value of the upper variable resister  322  becomes small. 
   By changing the resistance value of the upper variable resister  322  according to a setting in the amplitude adjustment register  304  in this manner, the gray scale voltage of the highest gray scale level can be changed without changing the gray scale voltage of the lowest gray scale level, so that amplitude voltage adjustment to the gray scale voltages can be made. 
   Next,  FIG. 5C  shows an adjustment operation when the resister values of the lower variable resister  321  and the upper variable resister  322  are simultaneously set by the amplitude adjustment register  304 . As described above, the solid line  501  in  FIG. 5C  shows the gray scale number-gray scale voltage characteristic when the amplitude adjustment register  304  is set to the default setting. If the gray scale voltages of the highest and lowest gray scale levels are to be increased with the gray scale number-gray scale voltage characteristic and the amplitude voltage kept to be the same as those in the case of the solid line  501 , as shown in a dotted line indicated by reference numeral  506 , a setting in the amplitude adjustment register  304  should be set such that the resistance value of the lower variable resister  321  becomes large and the resistance value of the upper variable resister  322  becomes small. Further, if the gray scale voltages of the highest and lowest gray scale levels are to be decreased with the gray scale number-gray scale voltage characteristic and the amplitude voltage kept to be the same as the ones indicated by the solid line  501 , as shown in a dotted line indicated by reference numeral  507 , a setting in the amplitude adjustment register  304  should be set such that the resistance value of the lower variable resister  321  becomes small and the resistance value of the upper variable resister  322  becomes large. 
   If the resistance values of the lower and upper variable resisters  321  and  322  are simultaneously set according to a setting in the amplitude adjustment register  304  in this manner, the characteristic becomes the one obtained by making offset adjustment to the gray scale number-gray scale voltage characteristic when the amplitude adjustment register  304  is set to the default setting. 
   As described above, the amplitude adjustment register  304  in  FIG. 3  can make amplitude voltage adjustment to the gray scale voltages according to the characteristics of respective liquid crystal display panels. 
   Next, adjustment operations of the gamma characteristic using the gradient adjustment register  305  and the variable resisters  323  and  324  in the resistance ladder  307  in  FIG. 3  will be described with reference to  FIGS. 6A ,  6 B, and  6 C. 
     FIG. 6A  shows an adjustment operation when the resistance value of the lower-middle variable resister  323  in the resistance ladder  307  in  FIG. 3  is set by the gradient adjustment register  305 . A solid line indicated by reference numeral  601  shows a gray scale number-gray scale voltage characteristic when the gradient adjustment register  305  is set to a default setting. As shown in a dotted line indicated by reference numeral  602 , if the gray scale voltages of low gray scale levels are to be changed without changing the gradient characteristic of the gray scale voltages of high gray scale levels to make adjustment such that the gradient of the gray scale voltages of intermediate gray scale levels is reduced, a setting in the gradient adjustment register  305  should be set such that the resistance value of the lower-middle variable resister  323  becomes large. 
   As shown in a dotted line indicated by reference numeral  603 , if the gray scale voltages of low gray scale levels are to be changed without changing the gradient characteristic of the gray scale voltages of high gray scale levels to make adjustment such that the gradient of the gray scale voltages of intermediate gray scale levels is increased, a setting in the gradient adjustment register  305  should be set such that the resistance value of the lower-middle variable resister  323  becomes small. 
   By changing the resistance value of the lower-middle variable resister  323  according to a setting in the gradient adjustment register  305  in this manner, the gray scale voltages of low gray scale levels can be changed without changing the gradient characteristic of the gray scale voltages of high gray scale levels, so that the gradient of the gray scale voltages of intermediate gray scale levels can be adjusted. 
   Next,  FIG. 6B  shows an adjustment operation when the resistance value of the upper-middle variable resister  324  in the resistance ladder  307  in  FIG. 3  is set by the gradient adjustment register  305 . As described above, the line  601  shows the gray scale number-gray scale voltage characteristic when the gradient adjustment register  305  is set to the default setting. As shown in a dotted line indicated by reference numeral  604 , if the gray scale voltages of high gray scale levels are to be changed without changing the gradient characteristic of the gray scale voltages of low gray scale levels to make adjustment such that the gradient of the gray scale voltages of intermediate gray scale levels is reduced, a setting in the gradient adjustment register  305  should be set such that the resistance value of the upper-middle variable resister  324  becomes large. Further, as shown in a dotted line indicated by reference numeral  605 , if the gray scale voltages of high gray scale levels are to be changed without changing the gradient characteristic of the gray scale voltages of low gray scale levels to make adjustment such that the gradient of the gray scale voltages of intermediate gray scale levels becomes large, a setting in the gradient adjustment register  305  should be set such that the resistance value of the upper-middle variable resister  324  becomes small. 
   By changing the resistance value of the upper-middle variable resister  324  according to a setting in the gradient adjustment register  305 , the gray scale voltages of high gray scale levels can be changed, so that the gradient of the gray scale voltages of intermediate gray scale levels can be adjusted. 
     FIG. 6C  shows an adjustment operation when the resistance values of the lower-middle variable resister  323  and the upper-middle variable resister  324  are simultaneously set by the gradient adjustment register  305 . As described above, the line  601  shows the gray scale number-gray scale voltage characteristic when the gradient adjustment register  305  is set to the default setting. As shown in a dotted line indicated by reference numeral  606 , if the gradient characteristic is to be the same as that of the line  601  and gray scale voltages  608  that determine the gradient characteristic are to be increased, a setting in the gradient adjustment register  305  should be set such that the resistance value of the lower-middle variable resister  323  is large and the resistance value of the upper-middle variable resister  324  is small. Further, as shown in a dotted line indicated by reference numeral  607 , if the gradient characteristic is to be the same as that of the line  601  and the gray scale voltages  608  that determine the gradient characteristic are to be reduced, a setting in the gradient adjustment register  305  should be set such that the resistance value of the lower-middle variable resister  323  is small and the resistance value of the upper-middle variable resister  324  is large. 
   If the resistances of the lower-middle resister  323  and the upper-middle variable resister  324  are simultaneously set according to a setting in the gradient adjustment register  305 , the gradient characteristic of the gray scale number-gray scale voltage remains the same as the characteristic when the gradient adjustment register  305  is set to the default setting. However, the voltage values of the gray scale voltages  608  that determine the gradient characteristic are adjusted. 
   As described above, the gradient adjustment register  305  in  FIG. 3  can adjust only the gradient characteristic of the gray scale voltages of intermediate gray scale levels according to the characteristics of respective liquid crystal display panels, with no amplitude voltage change in the gray scale voltages. 
   Next, the relationship between setting in the micro adjustment register  306  and the selector circuits  308  to  313  in  FIG. 3  according to this embodiment will be described with reference to  FIGS. 7A ,  7 B, and  7 C. 
   Referring to  FIG. 7A , reference numeral  701  denotes one of the selector circuits  308  to  313 , the internal configuration of which is shown. Reference numeral  702  denotes one of the resistive voltage dividing circuits  326  to  331  in the resistance ladder  307  in  FIG. 3 , the internal configuration of which is shown.  FIG. 7A  shows a configuration in which resistive voltage division with a resistance value of 1R is performed to generate eight micro adjustment gray scale voltages A to H. The selector circuit  701  selects one of the micro adjustment gray scale voltages A to H generated by the resistive voltage dividing circuit  702  according to a setting  703  in the micro adjustment register  306 . 
   The selector circuit  701  comprises two-input one-output selector circuits, and selects the output of a selector circuit in a first-stage selector circuit group  704  according to the zeroth bit of the register setting  703 , selects the output of a selector circuit in a second stage selector circuit group  705  according to the first bit of the register setting  703 , and selects an output in a third-stage selector circuit  706  according to the second bit of the register setting  703 . 
   If the register setting  703  is set to “000” [BIN], the selector circuit  701  supplies the micro adjustment gray scale voltage A divided by the resistive voltage dividing circuit  702  with the resistance divided. If the register setting  703  is set to “111” [BIN], the selector circuit  701  supplies the micro adjustment gray scale voltage H divided by the resistive voltage division circuit  702  with the resistance divided. In this way, for each increase of bit in the register setting  703  in the micro adjustment register  306 , the selector circuit  701  sequentially selects one of the micro adjustment gray scale voltages A to H, each divided by the resistive voltage dividing circuit  702  with the resistance divided. The relationship between the register setting  703  and the micro adjustment gray scale voltages A to H selected by the selector circuit  701  is shown in a table indicated by reference numeral  707 . 
   The relationship between a register setting and the selector circuit is just an example. If the respective bits of a register setting are inverted, the relationship between the register setting and the selector circuit is inverted. If the register setting increases, the selector circuit sequentially selects one of the micro adjustment gray scale voltages H to A in this stated order. As described above, the relationship between register setting and variable resistance may also be inverted. 
   The number of bits of a setting in the register for the selector circuit described above is three bits, and the selector circuit selects one of the eight micro adjustment gray scale voltages. The number of the bits of a setting may be increased to increase the number of selectable gray scale levels. In this case, a gray scale voltage micro adjustment range becomes more extended. However, the size of the gray scale voltage generating circuit increases. Further, although the resistance value used for resistive voltage division in the resistive voltage dividing circuit is set to 1R, this value may be set to be smaller or larger. If the resistance value is reduced, the micro adjustment range becomes narrower. However, the adjusting accuracy is improved. If the resistance value is increased, the micro adjustment range becomes more extended, but the adjusting accuracy deteriorates. Further, like the variable resisters in  FIG. 4A , preferably, the unit resistance R constitutes several tens of kiloohms, because power dissipation can be thereby reduced. 
   Next, adjustment to the gamma characteristic by the micro adjustment register  306  and the selector circuits  308  to  313  in  FIG. 3  will be described with reference to  FIG. 8 . 
   Referring to  FIG. 8 , a solid line indicated by reference numeral  801  shows a gray scale number-gray scale voltage characteristic when the micro adjustment register  306  is set to a default setting. A dotted line indicated by reference numeral  802  shows a characteristic when a setting in the micro adjustment register  306  is set such that the voltage value selected by the selector circuits  308  to  313  is maximized. A dotted line indicated by reference numeral  803  shows a characteristic when a setting in the micro adjustment register  306  is set such that the voltage value selected by the selector circuits  308  to  313  is minimized. Accordingly, the voltages in a region from the dotted line  802  to the dotted line  803  constitute the range of gray scale voltages that can be set for micro adjustment by the micro adjustment register  306 . Reference numerals  804  to  809  denote the outputs of the selector circuits  308  to  313  or the gray scale voltages that can be micro adjusted, and they can be micro adjusted within the range of the gray scale voltages from the dotted line  802  to the dotted line  803 . 
   As described above, according to a setting in the micro adjustment register  306  in  FIG. 3 , one gray scale voltage is selected from among the gray scale voltages generated by the voltage dividing circuits  326  to  331  in the resistance ladder  307 , respectively so as to allow micro adjustment. With this arrangement, micro adjustment to gray scale voltages according to the characteristics of respective liquid crystal display panels becomes possible. The adjusting accuracy is thereby improved, so that high image quality can be effected. 
   A configuration of a liquid crystal display apparatus system where the gray scale voltage generating circuit that can adjust the gamma characteristic using three types of the adjustment registers is included in a signal line driving circuit will be illustrated in  FIG. 9 . The three types of the adjustment registers are the amplitude adjustment register, gradient adjustment register, and micro adjustment register described above. Reference numeral  900  denotes the liquid crystal display apparatus according to the present invention. Reference numeral  901  denotes a liquid crystal display panel, reference numeral  902  denotes the signal line driving circuit that includes the gray scale voltage generating circuit  302  in  FIG. 3  for supplying a gray scale voltage corresponding to display data to the signal line of the liquid crystal display panel  901 . Reference numeral  903  denotes a scanning line driving circuit for scanning scan lines on the liquid crystal display panel  901 , reference numeral  904  denotes a system power generation circuit for supplying power for operating the signal line driving circuit  902  and the scanning line driving circuit  903 . A supply voltage  905  supplied from the system power generation circuit  904  to the signal line driving circuit  902  includes the reference voltage  316  in  FIG. 3 . Next, reference numeral  906  is an MPU (micro processor unit) for performing various control and processing for displaying an image on the liquid crystal display panel  901 . The signal line driving circuit  902  constitutes a system interface  907  for exchanging display data with the MPU  906  and exchanging data with the control register, a display memory  909  for temporarily storing display data  908  supplied from the system interface  907 , and the control register  301 , gray scale voltage generating circuit  302 , and decoder circuit  303 , illustrated in  FIG. 3 . The control register  301  includes the amplitude adjustment register  304 , gradient adjustment register  305 , and micro adjustment register  306  illustrated in  FIG. 3 . The signal line driving circuit  902  and the scanning line driving circuit  903  may also be included in the liquid crystal display  901 . 
   The MPU  906  conforms to the bus interface of the 16-bit bus 68xxx general-purpose MPU family, for example. From the MPU  906 , a CS (Chip Select) signal for indicating chip selection, an RS (Register Select) signal for selecting whether an address or data in the control register  301  is specified, an E (Enable) signal for commanding the start of processing, an R/N (Read/Write) signal for selecting data writing or reading, and a Data signal indicating a 16-bit data that represents an actual address or data setting in the control register  301 . By means of these control signals, settings in the amplitude adjustment register  304 , gradient adjustment register  305 , and micro adjustment register  306  are assigned to respective addresses in the control register  301 , and data writing and reading operations are performed onto each address in the control register  301  to which setting data is assigned. 
   Next, the operations of the control signals supplied from the MPU  906  to the system interface  907  in the signal line driving circuit  902  will be described with reference to  FIGS. 10A and 10B . First, the CS signal is set to “Low”, and the control register  301  is brought into an accessible state. During the period in which the RS signal is “Low”, address specification is performed. During the period in which the RS signal is “High”, data specification is performed. If data writing is performed into the control register  301 , the R/W signal is held “Low”. A predetermined address value is set for the Data signal during the period of address specification. During the period of data specification, data to be written into the register at this address, such as a setting in the amplitude adjustment register  304 , gradient adjustment register  305 , or micro adjustment register  306 , all described above, is set. Thereafter, the E signal is driven “high” for a given period, and data is thereby written into the control register  301 . 
   When reading out data that has been set in the control register  301 , the CS signal and the RS signal are set in the same manner as that described above. Then, the R/W signal is held “High”. A predetermined address is set during the period of address specification. After this setting, by holding the E signal “High” for the given period, the data written in the register during the period of data specification is read out. 
   By writing settings in the amplitude adjustment register  304 , gradient adjustment register  305 , micro adjustment register  306  at the respective assigned addresses in the control register  301 , when adjustment to the gamma characteristic is made, amplitude voltage adjustment to the gray scale voltages, gradient characteristic adjustment to the gray scale voltages of intermediate gray scale levels, and micro adjustment become possible. Adjustment to the gamma characteristic is thereby facilitated, and gray scale voltages in accordance with the characteristics of the respective liquid crystal display panels can be thereby set. 
   Next, a configuration of a liquid crystal display apparatus according to a second embodiment of the present invention will be described. 
   First, generally, when a gray scale voltage is applied to a liquid crystal display panel, the polarity of the gray scale voltage must be reversed by an alternating current having a given period, which is hereinafter referred to as an M signal, so as to alternating-current drive the liquid crystal display panel. 
   The gray scale number-gray scale voltage characteristic of the liquid crystal display panel also differs according to the polarity of the M signal, and it sometimes happens that adjustment must be made for each polarity of the M signal so as to obtain a desired gamma characteristic.  FIG. 11  shows changes in the gray scale number-gray scale voltage characteristics when a liquid crystal display panel is alternating-current driven. A curve indicated by reference numeral  1101  shows a gray scale number-gray scale voltage characteristic when the polarity of the M signal is positive or equals to zero. This curve shows that, when the liquid crystal display panel is in the normally black mode, as the gray scale number increases, the gray scale voltage increases. A curve indicated by reference numeral  1102  shows a gray scale number-gray scale voltage characteristic when the polarity of the M signal is negative or one. This curve shows that, as the gray scale number increases, the gray scale voltage decreases. The curve  1101  and the curve  1102  are symmetrical with respect to a center line  1103 . Suppose that the positive and negative gray scale number-gray scale voltage characteristics are symmetrical. Then, if the output order of the 64 gray scale voltages is reversed, or the relationship between gray scale voltage and gray scale number is reversed in such a way that the 64th gray scale voltage is output as the first gray scale voltage and the first gray scale voltage is output as the 64th gray scale voltage, and other gray scale voltages are output in descending order of gray scale numbers in the gray scale voltage generating circuit in  FIG. 3 , it is not necessary to make adjustment to the gamma characteristic of according to the polarity of the M signal. However, depending on a liquid crystal display panel, there is a case where positive and negative gray scale number-gray scale voltage characteristics are not symmetrical, as shown in a curve indicated by reference numeral  1104 . In this case, in the gray scale voltage generating circuit in  FIG. 3  according to the first embodiment, setting in the registers must be performed whenever necessary in accordance with the positive or negative gray scale number-gray scale voltage characteristic in order to make adjustment to obtain a desired gamma characteristic. In order to solve the problem described above, in the second embodiment of the present invention, resistance ladders for positive and negative gray scale voltages, which have the same effect as that in the first embodiment are provided separately to allow adjustment to both of the positive and negative gamma characteristics. 
   A configuration of a liquid crystal display apparatus according to the second embodiment of the present invention will be described with reference to  FIG. 12 . 
     FIG. 12  shows the gray scale voltage generating circuit  302  in  FIG. 3  according to the first embodiment, of which only the internal configuration is modified. The configurations and operations of the control register  301  and the decoder circuit  303  are the same as those according to the first embodiment. The gray scale voltage generating circuit  302  in  FIG. 12  includes a resistance ladder  1202  for positive gray scale voltages and a resistance ladder  1203  for negative gray scale voltages obtained by dividing the resistance ladder  307  in  FIG. 3  according to the first embodiment. 
   The resistance ladders  1202  and  1203  for positive and negative gray scale voltages are configured such that they can achieve the same effect as the first embodiment according to settings in the amplitude adjustment register  304  and the gradient adjustment register  305 . 
   The resistance ladders  1202  and  1203  for positive and negative gray scale voltages are configured to commonly use settings in the amplitude adjustment register  304  and the gradient adjustment register  305  to allow the same amplitude voltage adjustment to gray scale voltages and the same adjustment to the gradient characteristic as those in the first embodiment by using the settings, according to the polarity of a gray scale voltage. It is arranged such that setting of resistance values in the resistance ladder  1202  for positive gray scale voltages is different from setting of resistance values in the resistance ladder  1203  for negative voltages to allow different gray scale voltage adjustments depending on the polarity of a gray scale voltage according to the settings in the amplitude adjustment register  304  and the gradient adjustment register  305 . 
   Further, as described above, since two resistance ladders  1202  and  1203  for positive and negative gray scale voltages are provided, two types of selector circuits, which are a selector circuit  1204  for positive gray scale voltages and a selector circuit  1205  for negative gray scale voltages become necessary, in place of the selector circuits  308  to  313  in  FIG. 3 . The selector circuit  1204  for positive gray scale voltages and the selector circuit  1205  for negative gray scale voltages have the same configuration as the selector circuits  308  to  313  in  FIG. 3  according to the first embodiment, thus allowing micro adjustment which is the same as that in the first embodiment by using settings in the micro adjustment register  306 . 
   In the gray scale voltage generating circuit  302  having the configuration as described above, polarity selector circuits  1201  and  1206  for performing selection in response to the M signal makes selection between the outputs of the resistance ladders  1202  and  1203  for positive and negative gray scale voltages and the outputs of the selector circuits  1204  and  1205  for positive and negative gray scale voltages according to the polarity of the M signal. When the polarity of the M signal equals to zero, the polarity selectors  1201  and  1206  select the outputs of the resistance ladder  1202  for positive gray scale voltages and the selector circuit  1204  for positive gray scale voltages. When the polarity of the M signal equals to one, the polarity selectors  1201  and  1206  selects the outputs of the resistance ladder  1203  for negative gray scale voltages and the selector circuit  1205  for negative gray scale voltages. 
   By configuring the gray scale voltage generating circuit as described above, and including this circuit in the liquid crystal display apparatus system that is the same as the liquid crystal display apparatus system in  FIG. 9  according to the first embodiment, a liquid crystal display apparatus that can separately adjust gamma characteristics for positive and negative gray scale voltages is realized. Settings in the respective adjustment registers  304  to  306  are assigned to respective addresses in the control register  301  to perform writing of the settings into the respective registers in response to the control signals in  FIG. 10  as in the first embodiment. 
   Next, a configuration of a gray scale voltage generating circuit according to a third embodiment will be shown in  FIG. 13 . In this embodiment, a single resistance ladder is provided in place of two resistance ladders according to the second embodiment. The adjustment registers according to the first embodiment such as the amplitude adjustment register, gradient adjustment register, and micro adjustment register are provided separately according to the polarities of gray scale voltage, thereby allowing separate adjustments to the gamma characteristics for both positive and negative gray scale voltages.  FIG. 13  shows the gray scale voltage generating circuit in  FIG. 3  according to the first embodiment, of which only the internal configuration of the control register  301  is modified. Thus, the configurations and the operations of the gray scale voltage generating circuit  302  and the decoder circuit  303  are the same as those in  FIG. 1 . Referring to the internal configuration of the control register  301  in  FIG. 13 , reference numeral  1301  denotes an amplitude adjustment register for positive gray scale voltages, reference numeral  1302  denotes an amplitude adjustment register for negative gray scale voltages, reference numeral  1303  denotes a gradient adjustment register for positive gray scale voltages, reference numeral  1304  denotes a gradient adjustment register for negative gray scale voltages, reference numeral  1305  denotes a micro adjustment register for positive gray scale voltages, and reference numeral  1306  denotes a micro adjustment register for negative gray scale voltages, in each of which setting can be performed separately according to the polarity of a gray scale voltage. The adjustment registers  1301  to  1306  select settings in the registers  1301  to  1306  according to the polarity of a gray scale voltage by using selector circuits  1307  to  1309  for performing selection in response to the M signal. When the polarity of the M signal is zero, the selector circuits  1307  to  1309  select settings in the registers  1301 ,  1303 , and  1305  for positive gray scale voltages, respectively. When the polarity of the M signal is one, the selector circuits  1307  to  1309  select settings in the registers  1302 ,  1304 , and  1306  for negative gray scale voltages, respectively. The amplitude adjustment registers  1301  and  1302  for positive and negative gray scale voltages achieve the same effects shown in  FIGS. 5A ,  5 B, and  5 C as the amplitude adjustment register according to the first embodiment. The gradient adjustment registers  1303  and  1304  for positive and negative gray scale voltages achieve the same effects shown in  FIGS. 6A ,  6 B, and  6 C as the gradient adjustment register according to the first embodiment. The micro adjustment registers  1305  and  1306  for positive and negative gray scale voltages achieve the same effects shown in  FIG. 8  as the micro adjustment register according to the first embodiment. 
   Accordingly, the adjustment registers  1301  to  1306  for positive and negative gray scale voltages, described above can provide the same effect as the first embodiment. Adjustment to gray scale voltages and the gamma characteristics according to the characteristics of respective liquid crystal display panels can be thereby made separately for both of positive and negative gray scale voltages. 
   By including the control register  301  having the configuration as described above in a liquid crystal display apparatus system in  FIG. 14 , a liquid crystal display apparatus with a circuit size smaller than that according to the second embodiment is realized, which can adjust the gamma characteristics for both positive and negative gray scale voltages. Settings in the adjustment registers  1301  to  1306  for positive and negative gray scale voltages are written into the control register  301  at the respective addresses assigned to the positive and negative adjustment registers  1301  to  1306  in response to the control signals like those in  FIG. 10 . 
   Next, a configuration of a liquid crystal display apparatus according to a third embodiment of the present invention will be described. 
   In liquid crystal display panels, depending on an application, an image is sometimes displayed by backlighting. In this case, the gray scale number-gray scale voltage characteristic of a liquid crystal display panel sometimes changes according to turning ON or OFF of backlight, so that adjustment to the gamma characteristic should be made. In this embodiment, a method of adjusting the gamma characteristic during the period where the backlight is turned ON or OFF as described above will be described with reference to  FIG. 15 . 
     FIG. 15  is the liquid crystal display apparatus system in  FIG. 9  according to the first embodiment, in which the internal configurations of the MPU  906  and the control register  301  in the signal line driving circuit  902  are modified. Although the configurations and the operations of other blocks are the same as those in the first embodiment, the liquid crystal display panel  901  includes a circuit for backlighting described above. Backlight ON/OFF determination unit  1501  for determining whether the backlight is turned ON or OFF is provided inside the MPU  906 , and a backlight ON time register  1502  and a backlight OFF time register  1503  are provided separately inside the control register  301 . The backlight ON time register  1502  includes the amplitude adjustment register  304 , gradient adjustment register  305 , and micro adjustment register  306  that achieve the same effects as those according to the first embodiment. The backlight OFF time register  1503  also includes the amplitude adjustment register  304 , gradient adjustment register  305 , and micro adjustment register  306  that achieve the same effects as those according to the first embodiment. In response to a determination signal  1504  indicating the state where the backlight is turned ON or OFF, supplied from the backlight ON/OFF determination unit  1501 , the selector circuit  1505  makes selection between a setting in the backlight ON time register  1502  and a setting in the backlight OFF time register  1503  to use the register setting selected by the selector circuit  1505  in the gray scale voltage generating circuit  302  which has the same configuration as that according to the first embodiment. 
   As described above, by providing for the control register  301  two types of amplitude adjustment registers, gradient adjustment registers, and micro adjustment registers all of which achieve the same effects as those according to the first embodiment during the periods where the backlight is turned ON and OFF, separate adjustments to the gamma characteristic of the respective liquid crystal display panels can be made, depending on whether the backlight is turned ON or OFF. A liquid crystal display apparatus where high image quality can be effected is thereby realized. Settings in the backlight ON time register  1402  and the backlight OFF time register  1403  are assigned to respective addresses in the control register  301  and written into the control register  301  at the respective addresses in response to control signals in  FIG. 10 , as in the first embodiment. 
   Next, a configuration of a liquid crystal display apparatus according to a fifth embodiment of the present invention will be described. 
   This embodiment allows separate gamma characteristic adjustments for respective liquid crystal display panel colors of red, green, and blue (to be referred to as R, G, and B, respectively). The configuration of the apparatus will be described with reference to  FIG. 16 . 
     FIG. 16  is the liquid crystal display apparatus system in  FIG. 9  according to the first embodiment, in which only the internal configuration of the control register  301  is modified, as in  FIG. 15  according to the fourth embodiment. The configurations and the operations of other blocks are the same as those in the first embodiment. In order to make separate gamma characteristic adjustments for respective R, G, and B, an R adjustment register  1601 , a G adjustment register  1602 , and a B adjustment register  1603  are provided separately in the control register  1603 . All of the adjustment registers  1601  to  1603  include the amplitude adjustment register  304 , gradient adjustment register  305 , and micro adjustment register  306 , respectively, which achieve the same effects as those according to the first embodiment. 
   As described above, registers for respective display colors are separately provided in the control register  301  in the liquid crystal display. These registers include the R adjustment register  1601 , G adjustment register  1602 , and B adjustment register  1603  each of which comprise the amplitude adjustment register, gradient adjustment register, and micro adjustment register that achieve the same effects as those according to the first embodiment. With this arrangement, separate gamma characteristic adjustments for the respective display colors of R, G, and B in the liquid crystal display panel become possible, so that the liquid crystal display apparatus is realized in which high image quality can be effected. Settings in the R adjustment register  1601 , G adjustment register  1602 , and B adjustment register  1603  are assigned to respective addresses in the control register  301  and written into the control register  301  at the respective addresses in response to the control signals in  FIG. 10 , as in the first embodiment. 
   The present invention is not limited to the embodiments described above, and various modifications are possible. To take an example, the above description was given, assuming that the liquid crystal display panel is in the normally black mode. The present invention, however, can be practiced irrespective of the modes of the liquid crystal display panel. Further, a description was given, assuming that the number of gray scale levels is 64. The present invention, however, can be practiced irrespective of the number of gray scale levels. 
   According to the first to fourth embodiments, in order to make adjustment to the gamma characteristic, the amplitude adjustment register and the gradient adjustment register are provided. Then, a resistance ladder is provided which can make rough adjustments to gray scale voltages such as amplitude voltage adjustments to the gray scale voltages and the gradient characteristic of the gray scale voltages of intermediate gray scale levels. These adjustments are made according to the characteristics of the respective liquid crystal display panels, by using settings in the registers. With this arrangement, adjustment to the gamma characteristic can be facilitated, so that an adjustment time can be shortened. Further, by using the resistance ladder to allow the adjustments to be made, the size of the gray scale voltage generating circuit can be reduced at a low cost. 
   Further, in addition to the amplitude adjustment register and the gradient adjustment register, the micro adjustment register is provided. With this arrangement, micro adjustment to the gray scale voltages which have been adjusted by the amplitude and gradient adjustment registers becomes possible. Adjusting accuracy can be thereby increased, and high image quality can be effected. 
   Still further, according to the first to fourth embodiments, gamma characteristic adjustments according to the characteristics of respective liquid crystal display panels become possible. Thus, a versatile circuit configuration can be constructed. 
   According to the present invention, the accuracy of gamma characteristic adjustment is improved in a liquid crystal display apparatus. Image quality is thereby improved. 
   It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.