1. Field of the Invention
The present invention relates to a method for driving a liquid crystal display being used as a monitor for a personal computer, a TV (television) set or a like, a liquid crystal display device and a monitor provided with the same, and more particularly to the method for driving a liquid crystal display to display a gray shade by placing light and shade to a character, image, or a mike, in a step-by-step manner and the liquid crystal display device employing the above method for driving a liquid crystal display and the monitor being provided with such the liquid crystal display device described above.
The present application claims priority of Japanese Patent Application No.2001-200095 filed on Jun. 29, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
FIG. 13 is a block diagram showing an example of configurations of a conventional liquid crystal display device disclosed in Japanese Patent Application Laid-open No. 2001-134242. As shown in FIG. 13, the conventional liquid crystal display device 1 includes a color liquid crystal display 1, a control circuit 2, a gray-scale power circuit 3, a data electrode driving circuit 4, and a scanning electrode driving circuit 5.
The color liquid crystal display 1 uses, for example, an active-matrix driving type color liquid crystal display employing a thin-film transistor (TFT) as a switching element. In the color liquid crystal display 1, a region surrounded by a plurality of scanning electrodes (gate lines) arranged at specified intervals in a row direction and by a plurality of data electrodes (source lines) arranged at specified intervals in a column direction is used as a pixel. In the color liquid crystal display 1, a liquid crystal cell being equivalently a capacitive load, a common electrode, a TFT used to drive a corresponding liquid crystal cell, and a capacitor used to accumulate a data electrode during one vertical sync period are arranged for each pixel. To drive the color liquid crystal display 1, while a common potential Vcom is being applied to a common electrode, a data red signal, a data green signal, and a data blue signal produced based on red data DR, green data DG, and blue data DB each being digital video data are fed to data electrodes, while a scanning signal produced based on a horizontal sync signal SH, vertical sync signal SV, or a like is fed to scanning electrodes. This causes color characters or images to be displayed on a display screen of the color liquid crystal display 1. The color liquid crystal display 1 is of an SXGA (Super Extended Graphics Array)—type liquid crystal display with 1280-by-1024 pixel resolution.
The control circuit 2 is made up of, for example, an ASIC (Application Specific Integrated Circuit) and, as shown in FIG. 14, has a control section 6 and gamma correcting sections 71 to 73. The control section 6 generates a horizontal scanning pulse PH, a vertical scanning pulse PV, and a polarity reversing pulse POL used to drive the Color liquid crystal display 1 with alternating current and feeds them to the data electrode driving circuit 4 and the scanning electrode driving circuit 5. Moreover, the control section 6 feeds control signals SCR, SCG, and SCB used to control the gamma correcting sections 71 to 73 to gamma correcting sections 71 to 73. The gamma correcting sections 71 to 73 provide a gray shade by making a gamma correction individually to each of the red data DR, green data DG, and blue data DB each being of 8 bits and being fed from an external by arithmetic operations based on control signals SCR, SCG, and SCB to be fed from the control section 6. The gamma correcting sections 71 to 73 feed results from the gamma correction to the data electrode driving circuit 4 as corrected red data DRG, corrected green data DGG, and corrected blue data DBG.
Next, a gamma correction is described. A reproduction characteristic is expressed by plotting a logarithmic value of luminance that a subject of, for example, a landscape, person or a like appearing in a photograph taken by a video camera originally has, as abscissa, and by plotting a logarithmic value of luminance of a reproduced image displayed by a video signal fed from a video camera as ordinate and, when an angle of inclination of a curve of the above reproduction characteristic is given as “θ”, a value of tan θ is defined as a gamma (γ). When luminance of a subject is faithfully reproduced on a display, that is, when the logarithmic value (input value) plotted as abscissa increases or decreases by 1 (one), the logarithmic value (output value) plotted as ordinate increases or decreases by 1 (one), a curve for the reproduction characteristic proves to be a linear line having an angle of inclination θ of 45° and, since tan 45°=1, the gamma becomes 1 (one). Therefore, to faithfully reproduce luminance of a subject, it is necessary that a gamma of a whole system including an imaging device making up a video camera used to take a picture of a subject to a CRT (Cathode Ray Tube) display used to reproduce an image is 1 (one). However, each of imaging devices such as a CCD (Charged Coupled Device) or a like making up a video camera and each of CRT displays have their own specific gamma. For example, a gamma of the CCD is 1 and a gamma of the CRT display is about 2.2. Thus, in order to have a gamma of a whole system become 1 (one) and to obtain a reproduction image providing a good gray shade, a correction of an image signal is required, and this correction is referred to as a “gamma correction”. In ordinary cases, a gamma correction is made to a video signal so that the image signal can be matched to a characteristic (gamma characteristic) of a CRT display.
The gamma correction to be made by the gamma correcting sections 71 to 73 includes a first gamma correction and a second gamma fine correction used to correct a difference among a red color, a green color, and a blue color that can not be fully corrected by another second gamma coarse correction to be made by the data electrode driving circuit 4 which makes a gamma correction commonly to the red color, green color, and blue color (to be described later). Here, the first gamma correction represents a gamma correction to be made to arbitrarily provide a luminance characteristic of a reproduced image to luminance of an input image, for example, to have an input image signal be matched to a gamma characteristic of a CRT display (its gamma being about 2.2). Moreover, the second gamma correction represents a gamma correction to be made to have an input image signal be matched to a transmittance characteristic of each of applied voltages for a red color, a green color, and a blue color in a color liquid crystal display 1.
The gray-scale power circuit 3, as shown in FIG. 14, includes resistors 81 to 819 being cascaded between a terminal for a reference voltage Vaa and a ground and voltage followers 91 to 917 an input terminal of each of which is connected to a connection point among the resistors 81 to 819 adjacent to one another. The gray-scale power circuit 3 amplifies and buffers each of gray-scale voltages V0 to V17 occurring at each of the connection points of the resistors 81 to 819 adjacent to one another and being set to make the second gamma coarse correction and feeds them to the data electrode driving circuit 4. The data electrode driving circuit 4, as shown in FIG. 14, chiefly includes a multiplexer (MPX) 10, a 8-bit DAC (Digital-to-Analog Converter), and voltage followers 121 to 12381. The MPX 10 switches a set of the gray-scale voltages V0 to V8 or a set of the gray-scale voltages V9 to V17 out of the gray-scale voltages V0 to V17 fed from the gray-scale power circuit 3 based on a polarity reversing pulse POL fed from the control circuit 2 and feeds the switched voltages to the DAC 11. The DAC 11 makes the second coarse correction described above to corrected red data DRC, corrected green data DGG, and corrected blue DBG each being of 8 bits based on the set of gray-scale voltages V0 to V8 or the set of gray-scale voltages V9 to V17 fed from the MPX 10. Then, the DAC 11 converts the corrected red data DBC, corrected green data DGG, and corrected blue DBG all having undergone the second gamma coarse correction to an analog data red signal, analog data green signal, and analog data blue signal and then feeds them to each of corresponding voltage followers 121 to 12384. Each of the voltage followers 121 to 12384 amplifies and buffers the data red signal, data green signal, and data blue signal fed from the DAC 11 and feeds the amplified and buffered signal to each of corresponding data electrodes in the color liquid crystal display 11. The scanning electrode driving circuit 5, with timing when a vertical scanning pulse PV is fed from the control circuit 2, sequentially generates a scanning signal and sequentially applies the generated signal to each of corresponding scanning electrodes in the color liquid crystal display 1.
As described above, in the conventional liquid crystal display device, the control circuit 2 makes the first gamma correction and the second gamma coarse correction individually and separately to each of the red data DR, green data DG, and blue data DB each being of 8 bits fed from an external. Now let it be assumed that a curve “a” in FIG. 15 shows a gamma characteristic (gray scale—normalized luminance characteristic) of the red data DR, green data DG, and blue data DB each being of 8 bits fed from an external and that the first gamma correction is to be made to have the input data be matched to a gamma characteristic (gray scale—normalized luminance characteristic, gamma being about 2.2) of a CRT display shown by a curve “b” in FIG. 15. In FIG. 15, the normalized luminance denotes relative luminance obtained when luminance occurring when a maximum gray level (8 bits, that is, 255 gray levels) is displayed is 1 (one).
Therefore, the gamma characteristic of the corrected red data DRG, corrected green data DGG, and corrected blue data DBG to be output from the control circuit 2, as shown by a curve “c” in FIG. 15, is almost matched to the gamma characteristic (gamma being about 2.2) of the CRT display shown by the curve “b” in FIG. 15. However, as shown in FIG. 16, when portions on the curves “b” and “c” existing, for example, between 150 gray levels and 160 gray levels are magnified, there is no complete matching between values on the curves “b” and “c”. Moreover, in FIG. 16, though a relation of the gray shade to the normalized luminance is cyclically reversed, this has occurred due to an error in the measurement and values in the reversed portions are theoretically same. This is because, since red data DR, green data DG, and blue data DB each being of 8 bits are converted to corrected red data DRG, corrected green data DGG, and corrected blue data DBG each being 8 bits by arithmetic operations, no gray levels to be originally converted exist and there is no way but to be converted to a nearest gray level. This causes impairment of linearity of a gamma characteristic obtained after the gamma correction has been made.
As a result, for example, as shown in FIG. 17, when an image in which its display luminance increases linearly (the image being called a “gray scale image”) from a left to right direction in FIG. 17 is displayed in the color liquid crystal display 1, though a gray level originally should rise gradually from the left to right direction in FIG. 17, however, the gray level on a right side becomes equal to the gray level on a left side, thus causing vertical stripes to be displayed. Therefore, the conventional liquid crystal display cannot be used as a display device for a medical electronic apparatus requiring a display of an image with high definition in particular. To solve this problem, a method in which a number of bits for the red data DR, green data DG, and blue data DB is increased seems to be available, however, this method causes a circuit size of a whole liquid crystal display device to be made large and expensive.
Moreover, in the conventional liquid crystal display device described above, red data DR, green data DG, blue data DB each being of 8 bits are converted merely to corrected red data DRG, corrected green data DGG, and corrected blue data DBG each being of 8 bits. Thus, the conventional liquid crystal display device has shortcomings in that it cannot solve problems related to an environmental chance in ambient temperature, ambient illumination, frequency characteristic of timing signal fed from an external, change in a gamma characteristic of the color liquid crystal display 1 corresponding to luminance of a backlight used to provide light from a rear of the color liquid crystal display 1, and dispersion in a gamma characteristic occurring during a manufacturing process of the color liquid crystal display 1. These disadvantages described above also occur in a driving circuit of a monochrome liquid crystal display in a same manner.