JPH0389390A - Driving method for liquid crystal display element - Google Patents

Abstract

PURPOSE:To suppress the variation of brightness of a low frequency which becomes a wide area flicker by executing an interlaced scanning of an odd number ratio, and also, inverting the polarity of a voltage of a signal for driving a liquid crystal display element at every continuous field. CONSTITUTION:An interlaced scanning of an odd number ratio is executed, and also, the polarity of a voltage of a signal for driving a liquid crystal display element is inverted at every continuous field. That is, a ratio of the interlaced scanning is set to an odd number, and also, the polarity of a voltage of signal for driving a liquid crystal display element 10 is inverted at every continuous field, by which the variation of brightness of a frequency of 1/2 of a frame frequency is offset, and a wide area flicker of a frequency of 1/2 of the frame frequency is suppressed. In such a way, the frame frequency can be reduced without being accompanied with an extreme increase in flicker.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a method for driving an active namatrix type liquid crystal display element, and in particular to a method for driving an active namatrix type liquid crystal display element, in particular, by reducing the frame frequency without causing flicker. The present invention relates to a driving method that facilitates the realization of a display device with a large display capacity.

(Prior art) As active namatrix type liquid crystal display elements become larger and display capacity increases, the resistance and stray capacitance of electrodes within the liquid crystal display element increase, resulting in an increase in drive signals being transmitted through the liquid crystal display device. It has become difficult to operate liquid crystal display elements as expected.

In addition, as the number of display pixels increases and exceeds 100 pixels, the speed required of the signal source to drive the liquid crystal display element becomes non-negligible due to limitations in the operating frequency of the driving integrated circuit. Becomes faster.

The current frame frequency for driving a liquid crystal display element is generally selected to be 60 to 1200 Hz in consideration of the occurrence of flicker. If the frame frequency could be lowered, the frequency of the entire driving signal for the liquid crystal display element would be lowered, which would be convenient from the point of view of limiting the operating frequency of the driving integrated circuit.

Commonly used frame frequencies of 60 to 120
Hz is the frame frequency required for video display (approximately 25Hz
), so if there is no increase in flicker, the frame frequency can be reduced from the viewpoint of video display.

Heretofore, interlaced scanning has been well known as a technique for reducing frame frequency without increasing flicker, and 2:1 interlaced scanning is used in televisions (TVs). Note that interlaced scanning suppresses flicker that is felt when viewing a wide area.

However, in liquid crystal display elements, the liquid crystal is driven with alternating current, the response of the liquid crystal varies slightly depending on the polarity of the signal voltage, and the liquid crystal display element is an accumulation display element.
Flicker cannot be suppressed simply by 2:1 interlaced scanning.

In other words, in a liquid crystal display element, when the polarity of the signal voltage is reversed for each frame due to AC drive, a change in brightness occurs with a period twice the frame period, and even with 2:1 interlaced scanning, the change in brightness occurs. The frequency components are not canceled out, and this low frequency component is strongly perceived as flicker.

Figure 4 shows two active matrix liquid crystal display elements.
It shows the change in brightness when pair-one interlaced scanning is performed and the polarity of the signal voltage is reversed for each frame. Figure a schematically shows the temporal change in brightness of each of the four adjacent n I, ns n+1% n+2 scanning lines; This figure shows the change in average brightness over time. In addition, in the figure, the brightness depends on the polarity of the driving voltage, and the brightness changes immediately when a scanning line is selected, and remains at that brightness until the next scanning line is selected. It was assumed that As is clear from the figure, a change in brightness occurs with a period twice the frame period, and flicker is observed.

Therefore, various driving methods for reducing flicker have been proposed for liquid crystal display devices. For example, (1) a method of reversing the polarity of the drive voltage for each data line (Japanese Patent Laid-Open Publication No. 63-55590), (2) a method of reversing the polarity of the drive voltage every horizontal scan (Japanese Patent Laid-Open No. 62-17731) , ■ A method in which the polarity is reversed every horizontal scan and the common electrode is formed in a strip shape opposite to the gate electrode, thereby making it possible to reduce the amplitude of the drive signal of the signal electrode at the time of polarity reversal. 63-88523
(No. Publication) etc. have been proposed.

However, each of the above techniques has advantages and disadvantages, and the problems of each will be discussed in turn.

(2) Since the signal electrode driver integrated circuit element outputs a voltage in the range twice the signal amplitude applied to the liquid crystal layer, the signal electrode driver integrated circuit element is required to have a relatively high withstand voltage, which increases speed. It becomes a hindrance.

(2) Charges and discharges the stray capacitance of the signal electrode of the liquid crystal display device with a voltage twice the signal amplitude in a short period of time, that is, in a time that can be ignored for one horizontal scanning time. This places a heavy burden on the electrode driving integrated circuit element.

(2) Since the common electrode is patterned, the manufacturing cost of the liquid crystal display device increases.

(Problems to be Solved by the Invention) Conventionally, several methods for reducing flicker as described above have been proposed, but all of them have drawbacks. Therefore, the present invention provides a driving method for an active matrix liquid crystal display element with a large display capacity, which can reduce flicker if the frame frequency is the same, or reduce the frame frequency if the flicker is at the same level. It provides: Another object of the present invention is to provide a driving method that does not increase the manufacturing cost of display elements and the cost of drive circuits for display elements.

[Structure of the Invention] (Means for Solving the Problem) The present invention performs odd ratio interlaced scanning and inverts the polarity of the voltage of the signal that drives the liquid crystal display element for each successive field. , which drives an active matrix type liquid crystal display element.

(Function) By setting the interlaced scanning ratio to an odd number and reversing the polarity of the voltage of the signal that drives the liquid crystal display element for each successive field, the brightness at a frequency that is half the frame frequency can be reduced. The changes are canceled out, and the wide-range flip force at a frequency that is half the frame frequency is suppressed. Therefore, it is possible to reduce the frame frequency without causing an extreme increase in flicker.

Note that if the interlaced scanning ratio is large, the observation area where the flicker suppression effect can be obtained will expand, so in practical terms,
Preferably, the interlaced scanning ratio is 3:1.

Hereinafter, the flicker suppression effect obtained by the present invention will be explained using 3:1 interlaced scanning as an example and referring to FIG. 1 in the same manner as in 2:1 interlaced scanning.

Fig. 1a shows adjacent n-1%n%n+1, n+2,
This figure shows the temporal change in brightness of the five n+3 scanning lines, and also shows the temporal average of the average brightness of three adjacent scanning lines. As in the case of FIG. 12, when a scanning line is selected, it is immediately reflected in the brightness, and the brightness is maintained until the next selection.

That is, as shown in FIG. 1a, when the liquid crystal display element is scanned in a 3:1 interlaced manner and the polarity of the voltage of the signal that drives the liquid crystal display element is reversed for each successive field, the liquid crystal display of each scanning line is The display element is driven with alternating current because the polarity of the signal voltage is reversed every frame. Therefore, the period of brightness change of each scanning line is twice the frame period, and the time difference of brightness change between adjacent scanning lines is approximately one-third of the field period, that is, the frame period. becomes. The phase difference in brightness changes between three adjacent scanning lines is about 120 degrees. If the brightness of these three scanning lines is equal, the average brightness of the three adjacent scanning lines will cancel each other out, as shown in Figure 1, and the brightness of the period twice the frame period will be The brightness changes are canceled out, the period of brightness changes becomes two-thirds of the frame period, and wide-area flicker at a frequency of one-half the frame frequency is suppressed. Therefore, according to the present invention, it is possible to reduce the frame frequency without causing an extreme increase in flicker.

The above is easy to understand if we consider the sum of vectors that express time as an angle and brightness as a magnitude. When one frame period is displayed as 360 degrees, when the interlaced scanning ratio is 3, the vector indicating the brightness on three adjacent scanning lines will be located somewhere between 360 degrees divided into three. It turns out. FIG. 2 shows that when the polarity of the driving voltage is reversed for each field in 3-to-1 interlaced scanning, the Nth, N+1, and
Two frames of +2, N+3, N+4, and N+5 fields are shown. Each vector is located 120 degrees apart. Note that the symbol in parentheses after the field number in the figure indicates the polarity of the driving voltage for the liquid crystal, and indicates that positive polarity is brighter than negative polarity.

Therefore, since the period of brightness change of one pixel is twice the frame period, when viewed in a time twice as long as the frame period,
If the vectors have the same magnitude with the same polarity, then
The sum of these vectors becomes zero, and brightness changes with a period twice the frame period, that is, flicker, are suppressed. Note that since the polarity of the driving voltage for the liquid crystal on the same scanning line is reversed every frame, the liquid crystal is AC driven at the frame period.

Note that the flicker suppression effect is based on the premise that in a graphic part with a large area, such as a background, the brightness between relatively close scanning lines is approximately equal.

Here, we assumed that the brightness of pixels on a scanning line changes immediately when a scanning line is selected and maintains that brightness until the next scanning line is selected. No matter what kind of delay or advance element the pixel response has, if it does not change depending on the scanning line, this flicker suppression effect will not change.

Therefore, according to the present invention, changes in brightness with a period twice the frame period are suppressed, and for example, in 3:1 interlaced scanning, the brightness period is two-thirds of the frame period. Therefore, if the period of brightness change is the same as the conventional one, according to the present invention, the frame period can be made longer, that is, the frame frequency can be made lower.

(Embodiment) The present invention is mainly applied to workstations, personal ◆computers, word processors, computer character terminals, facsimile soft copies, and the like. A block diagram of the circuit configuration of one embodiment of the present invention is shown in FIG.

In FIG. 3, reference numeral 10 denotes an active matrix type liquid crystal display element having a well-known structure in which a switching element such as a thin film transistor is provided for each pixel, and a drive circuit is connected to this element 10. A signal line drive circuit 12 is connected to the signal line 10a, and a scan line drive circuit 13 is connected to the scan line 10b.
Further, a common electrode drive circuit 14 is connected to each common electrode. Signals from main processor 16 cause drawing processor 18 to write display data to frame memory 22 via arbiter 20. During display, the display data reading section 24 sequentially reads display data for each field from the frame memory 22 via the arbiter 20 and supplies it to the drive circuit 11. For example, frame
Display data for one horizontal line is read out as a serial signal from the memory 22 and input to the signal line drive circuit 12. Note that the scanning line drive circuit 13 performs 3:1 interlaced scanning on the scanning lines 10b.

Next, the display operation will be explained. In this embodiment, 3:1 interlaced scanning is performed, so one screen is composed of three fields.

First, in displaying the first field, display data for one horizontal line is read out from the frame memory 22 as a serial signal and input to the signal line drive circuit 12. The read data for one horizontal line is converted by the signal line driving circuit 12 into a parallel signal that drives the signal line 10a. on the other hand,
In synchronization with this, the scanning line drive circuit 13 drives the scanning electrode 10b.
For example, the first scanning line is turned on, and display data is written to the pixels on the first scanning line.

Next, display data for the next horizontal line is read from the frame memory 22 and input to the signal line drive circuit 12.

The scanning line drive circuit 13 turns on the fourth scanning line,
Display data is written to pixels on the fourth scanning line. Thereafter, in the same manner, signal voltages are sequentially written to the pixels on the 3n+1st, . . . scanning lines to display the first field.

Next, the second field is displayed in the same manner. The display data is read from the frame memory 22, and the 2nd, 5th, . . . , 3n+2nd, . . .
A display signal is written to the pixels on the scanning line.

At this time, the polarity of the write signal is reversed to the polarity of the first field.

When the display of the second field is finished, the display data is read out from the frame e memory 22 in the same way, and the display data is read out in the same way as the 3rd, 6th, . . . , 3n, 3rd, . .
A display signal is written to the pixels on the scanning line . . . to display the third field. At this time, the polarity of the write signal is
Reverse the polarity of the second field.

As described above, the display of one frame is completed by displaying the first, second, and third fields.

Subsequently, the fourth, fifth, and sixth fields are displayed to display the next frame. At this time, in the display of the fourth field, the write signal to the pixel has the polarity reversed from the write signal voltage of the first field. That is,
The polarity of the signal voltage is reversed for each successive field,
In addition, since odd ratio interlaced scanning is performed, the polarity of the write signal voltage is reversed every frame for pixels on the same scanning line, and the liquid crystal is driven with alternating current whose polarity is reversed every frame.

Note that when the liquid crystal is driven with alternating current, the brightness at each scanning line is as shown in Figure 1 in this example because the optical response of the liquid crystal varies depending on the polarity of the signal voltage, as mentioned earlier. become. Also, as explained in the (effect) section,
The period of the average brightness change of three adjacent scanning lines is two-thirds of the frame period, compared to the brightness change of twice the frame period in the case of conventional 2:1 interlaced scanning. significantly shorter.

In the above embodiment, the case of interlaced scanning of 3:1 was explained as an example, but the present invention is not limited to this, and may be applied to interlaced scanning of other odd ratios.

【Effect of the invention〕

When an active matrix type liquid crystal display element is driven with alternating current, wide-area flicker with a frequency that is half the frame frequency generally occurs.However, according to the present invention, which drives the liquid crystal display device using odd ratio interlaced scanning, the brightness can be improved. The frequency of the change in brightness can be made higher than before, and low frequency brightness changes that cause wide-area flicker can be suppressed.

In addition, since the polarity of the drive voltage only needs to be reversed for each field, compared to the case where the polarity is reversed for each horizontal scan, the output current of the driving integrated circuit element of the data line of the liquid crystal display device The current flowing through can be reduced.

Furthermore, the frame frequency can be lowered without worrying about wide-area flicker, and as a result, the distortion of the drive signal waveform due to the electrode resistance and stray capacitance inside the liquid crystal display device is reduced, and the electrode resistance can be increased somewhat. , it becomes easier to manufacture liquid crystal display elements.

Furthermore, the power consumption of the drive signal generation circuit for the liquid crystal display element can be reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram showing changes in brightness in adjacent scanning lines in the case of 3:1 interlaced scanning, which explains the present invention in detail, and FIG.
The figure is a vector diagram showing brightness in adjacent scanning lines, the third
The figure is a block diagram of an embodiment of a liquid crystal display device embodying the present invention, and FIG. 4 is a diagram showing changes in brightness in adjacent scanning lines in the case of conventional 2:1 interlaced scanning.

Claims (1)

[Claims] A method for driving an active matrix type liquid crystal display element, characterized in that odd ratio interlaced scanning is performed and the polarity of the voltage of a signal for driving the liquid crystal display element is inverted for each successive field.