JP2005202057A - Gamma correction circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress shifting of the luminance with respect to gradation levels after gamma correction with red, green and blue. <P>SOLUTION: At the time of converting gradation data expressed in 64 steps from a black level up to a white level relating to the respective colors; red, green and blue to gradation voltages, the resistance values of the portions corresponding to the black level in a ladder resistance section 21 for dividing down the reference voltage for generating the gradation voltages are changed over according to the respective colors. More specifically, resistors R<SB>R</SB>, R<SB>G</SB>, and R<SB>B</SB>corresponding to the respective colors; red, green and blue are parallel connected changeably between a VINO line and a VINI line and are changed over to the resistor R<SB>R</SB>when the color of the gradation data is red, to the resistor R<SB>G</SB>when the color of the gradation data is green, and to the resistor R<SB>B</SB>when the color of the gradation data is blue. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gamma correction circuit that corrects red, green, and blue luminance in a liquid crystal display device.

The slope of the reproduction characteristic curve when the reproduction characteristic is expressed with the logarithmic value of the brightness of the subject such as a landscape or a person as the horizontal axis and the logarithmic value of the luminance of the reproduced image displayed on the liquid crystal display device as the vertical axis. When θ is defined, tan θ is referred to as gamma. When the luminance of the subject is displayed faithfully, the reproduction characteristic curve is a straight line having an inclination angle θ of 45 ° and tan 45 ° = 1, so that gamma is 1. That is, in order to faithfully display the luminance of the subject, it is necessary to correct gamma to 1. As such a gamma correction technique, for example, the technique described in Patent Document 1 is known.
JP 2001-134242 A

  However, since the same gamma correction is conventionally performed for all three primary colors of red (R), green (G), and blue (B), each color is expressed with a certain number of gradations from the black level to the white level. The brightness of was shifted between red, blue, and blue. In particular, the luminance after blue gamma correction was shifted to the white level as a whole as compared with other colors.

  The present invention has been made in view of the above, and an object of the present invention is to provide a gamma correction circuit capable of suppressing the deviation of the luminance with respect to the gradation level after gamma correction between red, green, and blue. It is in.

  The gamma correction circuit according to the present invention includes a ladder resistor that divides a reference voltage used to convert gradation data representing red, green, and blue colors from a black level to a white level in a certain number of steps into a gradation voltage. And switching means for switching the resistance value of the portion corresponding to the black level in the ladder resistor in accordance with the color of the gradation data.

  In the present invention, the reference voltage for generating the gradation voltage is divided when the gradation data representing the red, green, and blue colors from the black level to the white level in a certain number of steps is converted into the gradation voltage. By switching the resistance value corresponding to the black level in the ladder resistance to be pressed according to each color, gamma correction is performed appropriately for each color, especially for blue, the luminance is shifted to the black level side as a whole. Let

  According to the gamma correction circuit of the present invention, it is possible to prevent the luminance with respect to the gradation level from being shifted between red, green, and blue. In particular, when the resistance value corresponding to the black level is optimally set, the luminances of red, green, and blue can be substantially matched.

  Hereinafter, a gamma correction circuit according to an embodiment will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of a liquid crystal display device to which the present gamma correction circuit 1 is applied. The liquid crystal display device shown in FIG. 1 includes a gamma correction circuit 1, a memory 2, a gradation control circuit 3, a liquid crystal display circuit 4, and a control circuit 5.

  The gamma correction circuit 1 includes a gradation amplifier 11 and a gradation adjustment register 12, and a gradation control circuit 3 generates a voltage obtained by dividing a reference voltage into 64 levels based on a value set in the gradation adjustment register 12. Output to. Details of the gamma correction circuit 1 will be described later.

In order to perform color display of 262,144 colors, the memory 2 stores 6-bit digital data for each gradation data of red, green, and blue, which are the three primary colors of light per display data. The total of the three colors is 18 bits. That is, as shown in the figure, 6 bits of R0 to R5 are stored for red gradation data, 6 bits of G0 to G5 are stored for green gradation data, and 6 bits of B0 to B5 are stored for blue gradation data. It is like that. Thus, each gradation data is 6 bits, and the gradation is divided into 64 (= 2 ⁶ ) stages.

  The gradation control circuit 3 has a one-to-one correspondence between the 64-level gradation data read from the memory 2 and the 64-level voltage output from the gamma correction circuit 1, and converts the gradation data to the gradation. The corresponding gradation voltage is converted and output to the liquid crystal display circuit 4. Since the gradation data is divided into 64 stages, the gradation voltage is also divided into 64 stages.

  The liquid crystal display circuit 4 is wired so that a plurality of signal lines and a plurality of scanning lines intersect, and a signal line driving circuit that drives each signal line, a scanning line driving circuit that drives each scanning line, and a signal line and scanning. A pixel is provided at each intersection of the lines. The gradation voltage is input to the signal line driver circuit and is sent to each pixel through the signal line. Each pixel includes a switching element and a pixel electrode that are controlled to be turned on and off by a scanning signal sent through the scanning line, and the gradation voltage sent through the signal line when the switching element is turned on is applied to the pixel electrode. It is to be written. The control circuit 5 outputs various control signals for controlling operations of the gamma correction circuit 1, the memory 2, and the like.

  As shown in the circuit diagram of FIG. 2, the gradation amplifier 11 of the gamma correction circuit 1 includes a ladder resistor unit 21, selectors 25 a to 25 f, and an amplifier unit 26, and the gradation adjustment register 12 has an inclination adjustment. The configuration includes a register 22, a fine adjustment register 23, and an amplitude adjustment register 24.

A reference voltage is supplied to the ladder resistor unit 21 by an upper limit voltage VDH and a lower limit voltage VGS. The ladder resistor unit 21 includes a plurality of resistors for dividing the reference voltage into a plurality of voltages and performing gamma correction. Specifically, the variable resistor VR0, the resistor PKH, the variable resistor VRH, the resistor PKM, the variable resistor VRL, the resistor PKL, the resistor R1, and the variable resistor VR1 are connected in series in this order, and further between the variable resistor VR0 and the resistor PKH. Resistors R _R , R _G , and R _B are connected in parallel to be switchable by a switch SW1.

The variable resistors VR0 and VR1 are for adjusting the gradation voltage amplitude. Switching control of the resistors R _R , R _G and R _B is performed by the control circuit 5. Resistor R _R is used when a red gamma correction, resistor R _G is used when the green gamma correction, R _B is used when the blue gamma correction. Resistor _{R R,} R _G, for the resistance value of R _B, is assumed to be preset to a value suitable for the gamma correction of each color.

  The resistors PKH, PKM, and PKL are for finely adjusting the magnitude of the gradation voltage with respect to the gradation data. The variable resistors VRH and VRL are for adjusting the slope of the characteristic curve indicating the characteristic of the gradation voltage with respect to the gradation data.

  The inclination adjustment register 22 stores 3 bits of values for determining the resistance values of the variable resistors VRH and VRL. In addition, there are provided registers for the case where the gradation data is for positive polarity and negative polarity, respectively, and independent setting according to the polarity is possible. As shown in the table of FIG. 3, the signal name that determines the resistance value of the variable resistor VRH is PRP0 for positive polarity, and PRN0 for negative polarity, and the signal name that determines the resistance value of the variable resistor VRL is positive polarity. PRP1 and PRN1 for negative polarity are used. By setting the value of the slope adjustment register 22, as shown in FIG. 4, it is possible to adjust the slope of the characteristic curve indicating the gradation voltage characteristics with respect to the gradation data.

  The amplitude adjustment register 24 stores 3 bits of values for determining the resistance values of the variable resistors VR0 and VR1. As shown in the list of FIG. 3, the signal name that determines the resistance value of the variable resistor VR0 is VRP0 for the positive polarity and VRN0 for the negative polarity, and the signal name that defines the resistance value of the variable resistor VR1 is that for the positive polarity. VRP1 and VRN1 are used for negative polarity. By setting the value of the amplitude adjustment register 24, it is possible to adjust the amplitude of the gradation voltage as shown in FIG.

  The fine adjustment register 23 stores the values for controlling the 8-input 1-output type selectors 25a to 25f for 3 bits. The selector 25 a has eight input terminals connected to the resistor PKH, and selects one of the eight divided voltages in the resistor PKH based on the set value of the fine adjustment register 23. The selectors 25b to 25e have their input terminals sequentially connected to the resistor PKM, and each selects one of eight divided voltages in the resistor PKM based on the set value of the fine adjustment register 23. . The eight input terminals of the selector 25f are connected to the resistor PKL, and selects one of eight divided voltages in the resistor PKL based on the set value of the fine adjustment register 23. As shown in the list of FIG. 3, the signal names for setting the selection by the selector 25a are PKP0 for positive polarity and PKN0 for negative polarity. The signal names for setting selection by the selector 25b are PKP1 for positive polarity and PKN1 for negative polarity, and the signal names for setting selection by the selector 25c are PKP2 for positive polarity and PKN2 for negative polarity. The signal name for setting the selection by the selector 25d is PKP3 for the positive polarity and PKN3 for the negative polarity. The signal name for setting the selection by the selector 25e is PKP4 for the positive polarity and PKN4 for the negative polarity. The selector 25f The signal name for setting the selection according to is PKP5 for positive polarity and PKN5 for negative polarity. By setting the value of the fine adjustment register 23, it is possible to finely adjust the magnitude of the gradation voltage with respect to the gradation data as shown in FIG.

  In FIG. 2, the output voltage of the variable resistor VR0 is VIN0, the output voltage of the selector 25a is VIN1, the output voltage of the selector 25b is VIN2, the output voltage of the selector 25c is VIN3, the output voltage of the selector 25d is VIN4, and the selector 25e The output voltage is VIN5, the output voltage of the selector 25f is VIN6, and the input stage voltage of the variable resistor VR1 is VIN7. That is, each of the selectors 25a to 25f selects a voltage at these VIN1 to VIN6.

  The amplifier unit 26 amplifies and outputs each voltage of VIN0 to VIN7. VIN0 corresponds to V0 of 64 output voltages V0 to V63 of the gamma correction circuit 1, VIN1 corresponds to V1, and VIN2 corresponds to V8. A resistor is connected between the V1 line and the V8 line, and voltages divided in six stages by the resistor are output as output voltages V2 to V7 of the gamma correction circuit 1. Similarly, VIN3 corresponds to V20, and voltages divided in 11 steps by a resistor connected between the V8 line and the V20 line are output as the output voltages V9 to V19 of the gamma correction circuit 1. VIN4 corresponds to V43, and voltages divided in 22 steps by a resistor connected between the V20 line and the V43 line are output as output voltages V21 to V42 of the gamma correction circuit 1. VIN5 corresponds to V55, and voltages divided in 11 steps by a resistor connected between the V43 line and the V55 line are output as output voltages V44 to V54 of the gamma correction circuit 1. VIN6 corresponds to V62, and voltages divided in six stages by resistors connected between the V55 line and the V62 line are output as the output voltages V56 to V61 of the gamma correction circuit 1. VIN7 corresponds to V63. In this way, the gamma correction circuit 1 outputs voltages V0 to V63.

The voltage V0 corresponds to the darkest black level, the voltage V63 corresponds to the brightest white level, and the resistors R _R , R _G , and R _B that switch according to red, green, and blue colors correspond to the black level. It is configured to be connected between the VIN0 line and the VIN1 line.

Next, a gamma correction circuit of a comparative example will be described. As shown in FIG. 7, the gamma correction circuit of the comparative example is configured by replacing the resistors R _R , R _G , and R _B that can be switched by the switch SW1 shown in FIG. It is the structure connected between. In addition, the same components as those in FIG. 2 are denoted by the same reference numerals, and redundant description is omitted here.

  With such a configuration, the gamma correction circuit of the comparative example performs the same gamma correction for each color without switching the resistor R0 depending on the color of the gradation data.

  Next, a difference in gamma correction between the gamma correction circuit 1 of the present embodiment and the gamma correction circuit of the comparative example will be described. FIG. 8 is a graph showing the relationship between gradation level and luminance before gamma correction. The luminance characteristics of red (R), green (G), and blue (B) are significantly different from the luminance characteristics of white (W).

The present gamma correction circuit 1, the resistor _{R R,} R G, previously set the _{R B} in the appropriate resistance value, by switching the red, green, resistor _R R according to the colors _blue, R G, and _{R B} When gamma correction is performed, as shown in FIG. 9, a graph is obtained in which the luminance characteristics of the colors red, green, and blue match the luminance characteristics of white. Note that the vertical axis of the graph of FIG. 9 is the normalized luminance that is normalized so that the luminance is 100 when the gradation level is 63. In the graph of FIG. 9, when the gradation level is 0, the black level has the lowest luminance, and when the gradation level is 63, the white level has the highest luminance.

  On the other hand, when the same gamma correction is performed for each of the red, green, and blue colors without switching the resistor R0 by the gamma correction circuit of the comparative example, the luminances of red, green, and blue are as shown in FIG. Although the characteristic approaches the luminance characteristic of white, it does not reach a perfect match. In particular, for blue, the deviation at the black level is large.

This gamma correction circuit 1, the resistance in a portion corresponding to the black level R _R, R _G, and R _B are connected in parallel, when the red gamma correction switched to resistance RR, is switched to the resistor RG when the green gamma correction, In the case of blue gamma correction, the gamma correction at the black level is appropriately performed by switching to the resistor RB.

  Therefore, according to the present embodiment, when converting the gradation data representing 64 levels from the black level to the white level for each color of red, green, and blue into the gradation voltage, the reference voltage for generating the gradation voltage is set. By switching the resistance value of the portion corresponding to the black level in the ladder resistance unit 21 to be divided according to each color, the gamma correction is appropriately performed for each color, and particularly for blue, the luminance is shifted to the black level side. In addition, it is possible to suppress the deviation of luminance with respect to the gradation level in red, green, and blue. In particular, when the resistance value corresponding to the black level is optimally set, the luminances of the red, green, and blue colors can be made to substantially match.

According to the present embodiment, three resistors R _R , R _G , and R _B corresponding to the respective colors of red, green, and blue are connected in parallel to the portion corresponding to the black level of the ladder resistor 21 so that the floor is By switching these resistors R _R , R _G , and R _B according to the color of the tone data, the resistance value can be switched according to the color with a simple configuration. In addition to providing the three resistors R _R , R _G , and R _B in a switchable manner, the resistance values may be switched according to the color by using a variable resistor.

  According to the present embodiment, the variable resistors VRH and VRL are provided at both ends of the central resistor PKM of the ladder resistor unit 21, and the inclination adjustment register 22 for setting the resistance values of these variable resistors VRH and VRL is provided. By adjusting the resistance values of the variable resistors VRH and VRL according to the values set in the slope adjustment register 22, the slope of the characteristic curve indicating the gradation voltage characteristics with respect to the gradation data can be adjusted. Can do.

  According to the present embodiment, the variable resistors VR0 and VR1 are provided at both end portions of the ladder resistor unit 21, and the amplitude adjustment register 24 for setting the resistance values of these variable resistors VR0 and VR1 is provided. By adjusting the resistance values of the variable resistors VR0 and VR1 according to the value set in the adjustment register 24, the amplitude of the gradation voltage can be adjusted.

  According to the present embodiment, the selectors 25a to 25f are connected to the resistors PKH, PKM, and PKL in the central portion of the ladder resistor unit 21, and the fine adjustment register 23 for setting the selection by the selectors 25a to 25f is provided for fine adjustment. Since the selectors 25a to 25f select the divided voltage output from the ladder resistor unit 21 in accordance with the value set in the register 23, the magnitude of the gradation voltage with respect to the gradation data can be adjusted. it can.

In this embodiment, inside three resistors R _R of the ladder resistor portion _21, R _G, was to switch to R _B, defined red, blue, the resistance ratio in accordance with the respective colors of green 3 Two ladder resistor units may be provided, and these ladder resistor units may be switched according to each color.

It is a block diagram which shows the structure of the liquid crystal display device provided with the gamma correction circuit in one embodiment. FIG. 2 is a circuit diagram showing a configuration of a gamma correction circuit shown in FIG. 1. It is a graph which shows a mode that the inclination is adjusted in the characteristic of the gradation voltage with respect to gradation data. It is a graph which shows a mode that the amplitude of a gradation voltage is adjusted in the characteristic of the gradation voltage with respect to gradation data. It is a graph which shows a mode that the magnitude | size of a gradation voltage is finely adjusted in the characteristic of the gradation voltage with respect to gradation data. 3 is a table showing a list of signal names and setting contents for each register shown in FIG. 2. It is a circuit diagram which shows the structure of the gamma correction circuit of a comparative example. It is a graph which shows the relationship between the gradation level before a gamma correction, and a brightness | luminance. It is a graph which shows the relationship between the gradation level after performing gamma correction by the gamma correction circuit of an Example, and a brightness | luminance. It is a graph which shows the relationship between the gradation level after performing a gamma correction with the gamma correction circuit of a comparative example, and a brightness | luminance.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Gamma correction circuit 2 ... Memory 3 ... Gradation control circuit 4 ... Liquid crystal display circuit 5 ... Control circuit 11 ... Gradation amplifier 12 ... Gradation adjustment register 21 ... Ladder resistance part 22 ... Inclination adjustment register 23 ... Fine adjustment register 24 ... Amplitude adjustment registers 25a to 25f ... Selector 26 ... Amplifier section

Claims (5)

A ladder resistor that divides a reference voltage used to convert gradation data expressed in a certain number of steps from black level to white level for each color of red, green, and blue into gradation voltage;
Switching means for switching the resistance value of the portion corresponding to the black level in the ladder resistor according to the color of the gradation data;
A gamma correction circuit comprising: The ladder resistor includes three resistors corresponding to each color of red, green, and blue in a portion corresponding to a black level,
2. The gamma correction circuit according to claim 1, wherein the switching means switches the three resistors according to the color of the gradation data. A variable resistor provided in the ladder resistor to adjust the slope of the characteristic curve indicating the characteristic of the gradation voltage with respect to the gradation data;
An inclination adjustment register in which the value of the variable resistor is set;
The gamma correction circuit according to claim 1, wherein: A variable resistor provided in the ladder resistor to adjust the amplitude of the gradation voltage;
An amplitude adjustment register in which the value of the variable resistor is set;
The gamma correction circuit according to claim 1, comprising: A selector for selecting a divided voltage output from the ladder resistor in order to adjust the magnitude of the gradation voltage;
A fine adjustment register in which selection by the selector is set;
5. The gamma correction circuit according to claim 1, comprising:

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