JPH026069B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a so-called active matrix panel, which uses active elements to drive a liquid crystal.

BACKGROUND ART Liquid crystals have traditionally been used as displays mainly in watches, but in the future there will be an increasing demand for large-capacity liquid crystal displays for computer terminals, pocket TVs, and the like. However, in the conventional voltage averaging method, the drive duty is limited to 1/30 to 1/50, and it is difficult to increase the capacity to about 1/500. Therefore, active (MIM) devices such as thin film transistors (TFT) and metal-insulator-metal (MIM) devices are used.
is not strictly active, but is defined as such in the present invention), and writes to each pixel.
It has been proposed to improve drive duty by holding.

Figure 1 shows (n x m) cells arranged in a matrix, and is an active matrix panel in which each pixel is selected and write-hold operations are performed using n data lines and m gate lines. be.

Figure 2 shows the configuration and driving method of an active matrix using TFTs for a particular data line Di.
It shows about. Cells are vertically m from Pj ₁ to _{Pi n}
The transistors are turned on by the gate line to write the display contents to the pixels from the data line, and then the transistors are turned off to hold the written data until the next selection.
The transistor equivalently has the resistance R _L and capacitance of the liquid crystal.
_CL is connected. This data line is the image signal V.
S is sequentially selected from the shift register 1 and sampled and held in the capacitor C _Si via the transistor 2. This is so-called point sequential driving.

FIG. 3 shows an example of operation waveforms in this point sequential drive. In order to drive the liquid crystal with alternating current, one frame is divided into two fields, and a positive voltage is written in the odd fields and a negative voltage is written in the even fields. The data line voltage V _D is when the LCD is lit = V _DH , when it is not lit
V _DL is applied, and a voltage within this range is applied for the gradation. Further, the gate voltage V _G is selected sequentially from top to bottom as shown in the figure. For example, considering a television screen display using about 200 scanning lines, the selection time T _S of one scanning line is 60 μsec, and the field period is 16 msec. Therefore, each pixel is written in 60μsec, and 16m
Retains the written content for sec. Therefore, each Y line (corresponding to a scanning line) is sequentially written by gate signals V _G1 to V _G m, and the potential of each pixel is changed as shown in (c).
Hold V _C1 to V _Cn .

In addition, during one scanning line period, as shown in Fig. 4, the image signal V/S input is sampled and held sequentially from V _D1 to V _Do by the sample/hold clock S _H , and this data is stored at a predetermined level during the horizontal scanning period. written on the line.

FIG. 5A shows a video input signal used in this method, and the higher the voltage with respect to the pedestal level, the higher the brightness of the signal. This is simply because the video signal has a relationship between the signal voltage V and the luminance B according to the characteristics shown in FIG. 6 (BαV〓, γ is approximately 2.2) in accordance with the CRT. On the other hand, as shown in Figure 7, it is said that TN type positive display has the best display characteristics as the contrast approaches 100% as the voltage increases, that is, the black level increases. . At this time, while the original video signal is set so that the higher the voltage level, the whiter it becomes, a negative display on the guest/host liquid crystal is fine, but it is not suitable for the positive type display that is the purpose of the present invention. The higher the voltage, the higher the black level, and the polarity will be reversed from the original video signal. Therefore, the signal shown in FIG. 5A is inverted as it is to create the signal B, and the A signal and the B signal are switched for each field and used as the liquid crystal AC drive signal to generate the image signal V and S shown in FIG. 3 or the data line. signal
Although it is easy to create V _D from a circuit perspective,
This causes such inconvenience.

Therefore, an object of the present invention is to provide a method for forming an image signal that matches the characteristics of a positive type liquid crystal as shown in FIG.

FIG. 8 shows an image signal used in the present invention. First, a negative polarity video signal C was formed by inverting the white level and black level of the positive polarity video signal A shown in Fig. 5, and this negative polarity video signal was further inverted with respect to the common potential V _COM . Form a D signal, switch the C signal and D signal for each field,
Forms image signals for a positive type liquid crystal panel. Furthermore, the bias voltage V _B and amplitude Av were set to match the contrast curve of the liquid crystal shown in Fig. 7.
To form an image signal for optimal display.

FIG. 9 is a block diagram of an example of an image signal forming circuit used in the present invention. The output of the video IF or the external video signal V.IF is first converted into a negative polarity video signal by the inverting amplifier 20. Next, from the negative polarity video signal, the common potential V _COM
, the positive-phase and negative-phase output signals of the symmetrical amplifier 21, which form positive-phase and negative-phase negative polarity video signals whose bias voltages and amplitudes are symmetrical, are alternately switched for each field by the frame signal Sf. The selector 22 and its buffer 23 form an image signal V.S. Of course, there are other implementation examples, but in the eighth embodiment, the input video signal of the present invention is made negative in polarity.
A system that obtains an image signal as shown in the figure is included in the present invention.

FIG. 10 shows a specific example of a circuit for forming an image signal according to the present invention, and FIG. 12 shows its operating waveform. The video signal V.IF is input to an inverting amplifier formed by a transistor 35. This input signal, that is, the positive polarity video signal I, is used to adjust the pedestal clamp level V _B ' of the video signal V.IF input by the transistor 32, variable resistor 30, and capacitor 31, and this V _B ' becomes the final image signal V. Determine the bias voltage V _B of S. The inverting amplifier includes a transistor 35, a collector resistor 34, an emitter resistor 36,
40. The capacitor 41 and variable resistor 42 are transistors, and by making the impedance on the emitter side of 35 variable, the amplitude Aυ' of the output signal of the inverting amplifier, that is, the negative video signal E, is made variable, and as a result, the amplitude Aυ' of the image signal VS is adjusted. can. The transistor 45 and the resistors 43 and 44 are symmetrical amplifiers, and the positive phase signal F and the negative phase signal G shown in FIG.
A symmetrical signal is formed as follows. Transmission gates, for example CMOS bilateral switches 46 and 47, alternately select signals F and G for each field in response to a frame signal Sf, and connect a video buffer consisting of transistors 48, 49, 52, 53 and resistors 50, 51. The AC image signal is output as VS via the .

The input video signal is, as mentioned above, only a signal for the CRT, and the signals for the liquid crystal display panel used in the present invention include one having the opposite polarity as described above, and one having a voltage-luminance characteristic. The nonlinear characteristics are significantly different. As shown in Fig. 6, the video signal is set so that the brightness is the γ power of the voltage, and therefore the input voltage is 100% higher than the desired brightness.
The characteristics are shown in Figure 2 A. On the other hand, as in the present invention, the polarity is set to negative, and an appropriate bias voltage is applied.
By setting V _B and amplitude Aυ, a curve like the one shown below can be obtained. Comparing this characteristic (B) with the characteristic of the liquid crystal used in the present invention shown in Fig. 7, it matches relatively well at the black level, that is, near 100% contrast.
They do not match on the white level and 0% contrast side.
As a result, it is no longer possible to reproduce images from white to gray. Therefore, if voltage compression (brightness expansion) on the white side is performed so as to reproduce the characteristics near the point Le in the contrast curve shown in FIG. 7, as shown in C, the gradation reproducibility will be improved. The variable resistor 37 and diode 38 in FIG. 10 are examples of implementation for this purpose. Due to the nonlinear effect of the diode, as shown in FIG. 13, the voltage is compressed when the amplitude is large, and this effect is obtained. The variable resistor 37 is for adjusting the degree of compression. This effect results in a signal in which the larger amplitude side of the signal in FIG. 8 is compressed both in + and -.

As described above, the present invention uses a video input signal with negative polarity and further compresses the white level to form an image signal.The method of the present invention provides the best display effect under optimal display conditions. Since it is possible to use a TN type positive liquid crystal with high brightness, its application to pocket televisions and the like is promoted.

As described above, the present invention includes an inverting amplification means for converting an input video signal into a negative polarity video signal, and a symmetric type amplifying means for forming a normal rotation and an inversion negative polarity video signal such that the bias voltage and amplitude of the negative polarity video signal are symmetrical. amplification means;
A selection means for alternately switching the normal rotation and inversion negative polarity video signals from the symmetrical amplification means for each field according to a frame signal, and an output signal from the selection means is supplied to a plurality of pixel electrodes arranged in a matrix. Since the bias voltage and amplitude of the negative polarity video signal can be set arbitrarily, ideal gradation reproduction can be achieved using a twisted nematic positive type liquid crystal display. can be realized. In particular, in order to obtain a gradation image display with high contrast and good color reproducibility, positive display is preferable to negative display. In order to obtain a preferable image with positive display, it is essential to reconstruct the input video signal by means such as the one described in the present application. Furthermore, since the bias voltage and amplitude are variably set, it is possible to obtain arbitrarily preferable image display conditions along the voltage contrast that differs depending on the liquid crystal material.

[Brief explanation of drawings]

FIG. 1 shows the arrangement of a matrix type liquid crystal panel display, and FIG. 2 shows the drive circuit and matrix cells of the active matrix panel.
Further, FIGS. 3 and 4 show driving waveforms of this circuit.
Further, FIG. 5 shows the video signal input waveform. FIG. 6 shows the brightness-voltage characteristics of the video signal, and FIG. 7 shows the brightness-voltage characteristics of the liquid crystal. FIG. 8 shows an image signal used in the present invention, FIG. 9 is a block diagram of an example of the waveform forming circuit of FIG. 8, FIG. 10 is a specific example thereof, and FIG. 11 shows its operating waveform. 12th
The figure shows the brightness-voltage characteristic obtained by the present invention, and FIG. 13 shows the voltage compression characteristic on the white side. 1... Data side shift register, 20... Inverting amplifier, 21... Symmetrical amplifier, 22... Selection circuit, 23... Buffer.

Claims (1)

[Claims] 1. Inverting amplification means for converting an input video signal into a negative polarity video signal; symmetrical amplification means for forming a positive and inverted negative polarity video signal such that the bias voltage and amplitude of the negative video signal are symmetrical; Selection means for alternately switching the normal rotation and inversion negative polarity video signals from the symmetrical amplification means for each field using a frame signal, and sampling for supplying the output signal from the selection means to a plurality of pixel electrodes arranged in a matrix. 1. A liquid crystal display device comprising means for controlling a bias voltage and an amplitude of the negative polarity video signal to be arbitrarily set variably.