JPH10206826A - Liquid crystal driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to distinctly discriminates gradation differences of a displayed multi-gradational image. SOLUTION: The liquid crystal driving circuit is equipped with a Y driver 201 which selects display pixels of a liquid crystal display and an X driver 101 which controls the light transmissivity of the display pixels selected by the Y driver 201 corresponding to gradations specified with a video signal, and the X driver 101 includes a reference voltage generation part which sets the relation between the gradations and light transmissivity so that unit changes of the gradations become luminance changes that a human can easily visually observe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a liquid crystal drive circuit for driving an active matrix type liquid crystal display,
In particular, the present invention relates to a liquid crystal drive circuit that drives this liquid crystal display so as to perform multi-tone display.

[0002]

2. Description of the Related Art In recent years, multi-tone images have been developed in personal computers. This is the same in a portable computer using a flat display represented by a liquid crystal display for displaying an image.

A general liquid crystal display has an array substrate on which a matrix array of a plurality of pixel electrodes is formed, a counter substrate on which a counter electrode is formed to face the matrix array of these pixel electrodes, and an array substrate and a counter substrate. And a liquid crystal composition layer held therebetween. Each pixel electrode forms one pixel in cooperation with a counter electrode portion and a liquid crystal composition portion corresponding to the pixel electrode region. The liquid crystal display displays an image corresponding to a light transmittance distribution controlled by a signal voltage applied to each pixel electrode with respect to a counter electrode.

[0004] In particular, an active matrix type liquid crystal display has attracted attention as a flat display capable of displaying images of higher quality than before. This active matrix type liquid crystal display includes a plurality of switching elements formed of, for example, thin film transistors (TFTs) for selectively supplying a signal voltage to each pixel electrode. These thin film transistors are formed on the array substrate adjacent to intersections of a plurality of scanning lines formed along the rows of the pixel electrodes and a plurality of signal lines formed along the columns of the pixel electrodes on the array substrate. Is done. The gate electrode of each thin film transistor is connected to the corresponding scanning line, and the source / drain path is connected between the corresponding signal line and the corresponding pixel electrode. In this case, the thin film transistors in each row conduct only for one horizontal scanning period in response to a scanning signal supplied through a corresponding one of the plurality of scanning lines, and are supplied through the plurality of signal lines during this period. A signal voltage is supplied to each pixel electrode in the corresponding row. The potential of each pixel electrode is held until the corresponding thin film transistor conducts again after one frame period. Incidentally, in the multi-tone display, the signal voltage is controlled so that the light transmittance of the liquid crystal display, that is, the luminance changes at a predetermined ratio with respect to a unit change of the gradation.

[0005]

By the way, in a portable computer, setting a maximum luminance of a liquid crystal display to be high is restricted for power saving, and a good contrast ratio cannot be obtained in many cases. In this case, it was difficult to clearly identify the tone difference of the multi-tone image. SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal driving circuit that can clearly identify a gradation difference of a displayed multi-gradation image.

[0006]

According to a first aspect of the present invention, a selection means for selecting a display pixel of a liquid crystal display, and a light transmittance of the display pixel selected by the selection means is specified by a video signal. Control means for controlling in accordance with the gradation, wherein the control means sets a relationship between the gradation and the light transmittance such that a unit change of the gradation is a luminance change which is easily observed by human eyes. Is provided.

According to a second aspect of the present invention, in addition to the configuration of the liquid crystal driving circuit of the first aspect, the setting means has a value obtained by converting the luminance of the halftone into a common logarithm with respect to the grayscale of the halftone. The gradation and the light transmittance have a constant gradient, gradually approach a constant gradient from the intermediate gradation to a high luminance gradation, and become larger than the constant inclination at a gradation lower in luminance than the intermediate gradation. The liquid crystal driving circuit according to claim 1, further comprising means for setting a relationship.

According to a third aspect of the present invention, in addition to the configuration of the liquid crystal driving circuit according to the first aspect, the setting means includes a transmittance of approximately one-third of the gradation from the highest gradation in all gradations. Means for setting the relationship between the gray level and the light transmittance such that the gray level becomes half of the maximum luminance transmittance, and the gray level lower than this gray level is reduced by about 90% of the gray level. A liquid crystal driving circuit is provided.

According to the liquid crystal driving circuit of the present invention, it is possible to easily identify a gradation difference irrespective of a bright portion and a dark portion of a display image. That is, even when a good contrast ratio cannot be obtained, a high-quality multi-tone image can be displayed.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a liquid crystal display device according to an embodiment of the present invention will be described with reference to the drawings. FIG.
1 schematically shows the configuration of this liquid crystal display device. This liquid crystal display device includes an active matrix type liquid crystal panel 11 for performing color display at a resolution of (640 × 3) × 480, and an X driver 10 connected to the liquid crystal panel 11.
1, a Y driver 201 connected to the liquid crystal panel 11, and a liquid crystal controller 251 for controlling the X driver 101 and the Y driver 201. The LCD controller 251 controls a vertical synchronization signal, a horizontal synchronization signal,
And the gray scale display data, and controls the X driver 101 and the Y driver 201 based on them. The X driver 101 generates a signal voltage under the control of the liquid crystal controller 251. Y driver 201 is LCD controller 251
A scanning signal is generated by the control of.

The liquid crystal panel 11 has a conventionally known structure, and is arranged in a matrix (640 × 3) × 480.
A plurality of pixel electrodes 21 and a thin film transistor (TFT) 31 for selectively supplying a signal voltage to these pixel electrodes are provided. These thin film transistors 31 are formed by a plurality of scanning lines formed along rows of the pixel electrodes 21 on the array substrate.
13 (Y1, Y2, ..., Y480) and a plurality of signal lines 15 (X1, X2, ..., X (630x3)) formed along the column of these pixel electrodes adjacent to the intersection. It is formed on an array substrate. The gate electrode of each thin film transistor 31 is connected to the corresponding scanning line 13, and the source / drain path is connected between the corresponding signal line 15 and the corresponding pixel electrode 21. In this case, the thin film transistors 31 in each row conduct for one horizontal scanning period in response to a scanning signal supplied from the Y driver 201 via the corresponding one of the plurality of scanning lines 13, and during this time, the X driver 101 The signal voltage supplied via the plurality of signal lines 15 is applied to the pixel electrode 21 of the corresponding row.
Respectively. This signal voltage is supplied to the liquid crystal capacitor CLC composed of the pixel electrode 21 and the counter electrode 41 and the pixel electrode 21.
And the storage capacitor CS formed by the storage capacitor line 51 holds the data for one frame (F).

The X driver 101 includes a shift register 111 and a D / A converter DAC as shown in FIG. The shift register 111 is composed of 640 × 3 flip-flops connected in series, and sequentially shifts the start pulse ST in response to the shift clock CK. Start pulse
ST is generated from the liquid crystal controller 11 in synchronization with the horizontal synchronization signal, and the shift clock CK is generated from the liquid crystal controller 11 in synchronization with the gradation display data. D / A converter DAC
Responds to the start pulse ST stored in the corresponding flip-flop of the shift register 111 to sample the gray scale display data, and generate a reference voltage V0, V1,..., V9 from the reference voltage generator 114. The reference voltage is divided by an internal resistor to output a 64 gradation analog gradation voltage. The analog gradation voltage is latched by the latch circuit 117 based on the latch enable signal EN, and is supplied to the signal line 15 in each horizontal scanning period.

The reference voltage generating circuit 114 is composed of resistors R1-R11 connected in series as a square voltage dividing circuit having an amplitude of the power supply voltages VDD to VSS, for example, as shown in FIG. By dividing the voltage, the reference voltage V0, V
Generates 1, ..., V9. These reference voltages V0, V1,..., V9, specifically, the resistance values of the resistors R1 to R11 are determined by the relationship between the light transmittance of the pixel with respect to the gradation specified by the gradation display data, for example, as shown in FIG. Is set to be

Here, the relationship between the gradation and the light transmittance shown in FIG. 4 will be further described. The change in screen brightness is perceived as a change in brightness by human vision, and between the two
The ber-Fechner rule holds. According to this law, the brightness L perceived by a human when a liquid crystal panel having a transmittance T is viewed.
Is represented by the following definition formula.

L = k * logT (where k is a constant) (1) The brightness perceived by a human when the liquid crystal panel has 100% transmittance T100 is L100, and the transmittance T at the time of n-gradation display is
If the brightness perceived by a human when n is Ln, from equation (1), Ln / L100 = (k * logTn) / (k * logT100)） Tn = T100 ^{(Ln / L100)} --- (2 However, in the equation (2), the contrast can be set only to 100 at the maximum. Therefore, in order to increase the contrast, a coefficient a in equation (2) is added. The coefficient a is set so as to be in a proportional relationship with the gradation when the transmittance is displayed in log, as in the equation (2). For example, when the target contrast is CT, the coefficient a in the case of the 64-gradation module is as follows: a = (CT / 100) ^{{-(63-n) / 63}} --- (3) Equations (2) and (3) Accordingly, the target transmittance Tn of the n-th gradation is expressed by Expression (4).

Tn = a * {T100 ^{(Ln / L100)} } (4) In this embodiment, as shown by a solid line in FIG. The gradation is set so as to gradually approach a in the gradation region III where the luminance is higher than the region II, and is set so as to be larger than a in the gradation region I where the luminance is lower than the halftone region II.

When the halftone luminance is set high, as shown by the broken line in FIG. %
(With a maximum luminance of 100% transmittance), and lowering the gray level lower than that level by about 90% of the previous gray level, the brightness of the halftone level when viewed from the front is reduced. Higher and the difference can be equally divided.

According to this embodiment, the transmittance is 2% at 100%.
In a liquid crystal panel of about 00 nit, the effective contrast ratio can be improved to 300 by setting the conventional value to 100.

In the present embodiment, the reference voltages V0-V
9 is generated, and from the reference voltages V0-V9, the D / A converter DAC is further configured to generate 64 analog gray scale voltages. However, the reference voltage V10 is further generated, and according to the product type. The reference voltage V10 may be used.

Further, in the present embodiment, the X driver 101 is shown as a resistor DAC system, but this may be changed to a capacitor DAC system operating based on a reference voltage or a voltage selection system. In the case of the voltage selection method, 64 reference voltages are required for 64 gradation display.

[0021]

As described above, according to the present invention, it is easy to observe a gradation change in a bright part in which human senses are present. The value converted to the common logarithm has a constant slope, and for gradations with higher luminance than the halftone, the gradation gradually decreases from the halftone to the certain gradient determined by the halftone (nonlinearity). The key is set to be larger than a certain gradient determined in the halftone. As a result, a sufficient contrast ratio can be obtained from the viewpoint of human vision, and the luminance difference of all gradations can be equally divided at all viewing angles.

When the luminance is higher than that of the halftone, the transmittance of the tone which is almost 1/3 from the highest tone of all the tone is about 50% (the transmittance of the maximum brightness is 100%). ), And the gray level lower than the gray level is set to be lower by about 90% of the previous gray level. In this case, when viewed from the front, the brightness of the halftone gradation can be increased, and the difference can be equally divided.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a liquid crystal drive circuit according to one embodiment of the present invention.

FIG. 2 is a diagram showing the configuration of an X driver shown in FIG. 1 in further detail.

FIG. 3 is a diagram illustrating a configuration example of a reference voltage generation circuit illustrated in FIG. 2;

4 is a graph showing a relationship between gradation and light transmittance obtained by a resistance ratio of a series resistor shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Liquid crystal panel 13 ... Signal line 15 ... Scanning line 21 ... Pixel electrode 31 ... Thin film transistor 101 ... X driver 201 ... Y driver 251 ... Liquid crystal controller 111 ... Shift register 114 ... Reference voltage generation part 117 ... Latch circuit R1-R11 ... Resistance

Claims (3)

[Claims] 1. A display device comprising: a selection unit for selecting a display pixel of a liquid crystal display; and a control unit for controlling a light transmittance of a display pixel selected by the selection unit in accordance with a gradation specified by a video signal. A liquid crystal drive circuit, wherein the control means includes a setting means for setting a relationship between the gradation and the light transmittance such that a unit change of the gradation becomes a luminance change that is easily observed by human eyes. 2. The image processing apparatus according to claim 1, wherein the setting unit has a constant gradient of a value obtained by converting halftone luminance into a common logarithm with respect to the halftone grayscale, and the constant gradient from the intermediate grayscale to a high-luminance grayscale. 2. The liquid crystal driving circuit according to claim 1, further comprising: means for setting the relationship such that the relationship gradually becomes larger than the predetermined gradient at a gradation lower in luminance than the intermediate gradation. 3. The setting means reduces the transmittance of a tone approximately one-third from the highest of all the brightness levels to half of the transmittance of the maximum brightness, and sets a tone having a brightness lower than this tone. 2. The liquid crystal driving circuit according to claim 1, further comprising: means for setting the relationship such that the relationship becomes lower at a value of about 90% of the gradation.