1. Field of the Invention
The present invention relates to a liquid crystal display device of active matrix type and a method of controlling the liquid crystal display device. Especially, the present invention relates to a technique for preferable image display.
2. Description of the Related Art
Liquid crystal display devices of active matrix type using TFTs (Thin Film Transistors) as driving elements have been in wide use as display devices for personal computers or the like. Generally, liquid crystal devices of this kind often adopt a display method called a TN (Twisted Nematic) type. A liquid crystal display device of TN type are formed with twisted nematic cells in which arrangement of liquid crystal molecules are consecutively twisted by 90 degrees, with the liquid crystal cells sandwiched between two transparent electrode-plated substrates. The liquid crystal display device lets light penetrate through when a voltage is not supplied between the electrode-plated substrates.
FIG. 1 shows an outline of a TFT (Thin Film Transistor) driving liquid crystal display device described above.
This device comprises TFTs and pixel electrodes 1 laid out in the form of a matrix. Gate electrodes of the TFTs which are switching elements are connected to scanning lines G1, G2, . . . , Gn each transmitting a gate signal output from a Y driver 2. Drain electrodes of the TFTs are connected to signal lines D1, D2, . . . , Dm each transmitting a data signal output from an X driver 3. Source electrodes of the TFTs are connected the pixel electrodes 1. Counter electrodes facing the pixel electrodes 1 are also laid out (not shown). Liquid crystals (not shown) are sandwiched by the pixel electrodes 1 and the counter electrodes, forming liquid crystal cells C.
Data are written in the liquid crystal cells C by sequentially causing TFTs to be on by pulse-like gate signals sequentially supplied to the scanning lines and by transmitting the data signals simultaneously supplied to the signal lines to the pixel electrodes 1 (line-sequential driving). Information of the data signals written in the liquid crystal cells C is retained until the pixel electrodes 1 are driven in a subsequent frame. This control of retaining the information in the liquid crystal cells C until next data signal writing is generally called hold driving.
FIG. 2 shows a waveform of a driving voltage and a response waveform of the liquid crystal cells C when the TFT driving liquid crystal display device described above is driven in the hold driving method. The waveform of the pixel response corresponds to the amount of light penetrated through the liquid crystal cells C. A state of writing data in one of the liquid crystal cells C is shown here.
The Y driver shown in FIG. 1 drives each of the scanning lines in every 16 ms, and generates an high level pulse of the gate signal. The X driver 3 generates the data signal in synchronization with the gate signal. Polarity of the data signal is inverted at every frame scan and so-called frame inversion driving is carried out. Within the 16 ms period shown in FIG. 2, all the scanning lines are scanned although the waveforms thereof are not shown.
For example, in a period of first three frames, an absolute value of a voltage supplied between the pixel electrode 1 and the counter electrode (not shown) is 5 V in all the frames. Therefore, the liquid crystal cell C in FIG. 2 lets the light penetrate through the cell C and white is displayed on a screen. For the remaining three-frame period, the voltage between the pixel electrode 1 and the counter electrode (not shown) is 0 V. Therefore, the liquid crystal cell C shuts the light and black is displayed on the screen.
Generally, a response time of the liquid crystal cells C in the TN type liquid crystal display device is longer than the scanning period of one frame. Especially, the response time of the liquid crystal cells C in a half tone continues for several frames, as shown by a dashed line in FIG. 2. Recently, a liquid crystal cell called a π cell having a short response time has been developed.
As has been described above, the TN type liquid crystal display device displays an image by being driven in the hold driving method. In hold driving, information in the liquid crystal cells C is retained until a subsequent data signal is written. As a result, blurring (image tailing) occurs in a moving image due to partial overlap of display data in a previous frame. Such blurring does not occur on a CRT (Cathode Ray Tube) display.
FIG. 3 shows waveforms of a voltage driving a CRT according to a so-called impulse driving method. Light is emitted from a pixel only in the case where the voltage is supplied to the driving signal and an electron beam is emitted on the pixel. Data scanned in an immediately preceding frame disappears with a shift of the driving signal to low level so that no blurring occurs.
In order to alleviate the blurring on the liquid crystal display device, impulse driving has been tried on the liquid crystal display device. Details of this trial have been described in Digest of SID98 pp. 143-146. Liquid crystal display devices of this type uses the π cells or the like having a short response time.
FIG. 4 shows waveforms of a driving voltage and a response waveform of a liquid crystal cell observed in the case of impulse driving of a liquid crystal display device. As in the case shown in FIG. 2, white is displayed for first three frames and black is displayed in the remaining three frames.
The liquid crystal display device scans each of the scanning lines twice in every 16 ms (one frame). A first scan is used for receiving a data signal and a second scan is used for resetting the liquid crystal cells. In other words, impulse driving is realized by writing black data after a predetermined time has elapsed since the data signal were written in the liquid crystal cells C. “W” shown with arrows in FIG. 4 refers to an operation of writing white, while “B” means an operation of writing black. “R” refers to a resetting operation. In this manner, display data in the liquid crystal cells C are retained only for a predetermined period T1 in one frame and blurring in a moving image is alleviated.
FIG. 5 shows an example of a display screen in the case where the impulse driving described above is carried out. In FIG. 5, liquid crystal cells in white display white and hatched cells display black.
As waveforms in FIG. 5 shows, display data (white) are written at the first scan in a display period (16 ms) of one frame. At the second scan in one frame period, reset data (black) are written in the liquid crystal cells. In other words, as shown in top of FIG. 5, the display data and the reset data having band-like shapes move from the top to the bottom in the scanning in one frame.
However, line-sequential writing of the display data (white) and the reset data (black) in alternation causes flicker. Especially, when a display speed of the liquid crystal cells C is low, or when a scanning period (refresh rate) is long, flicker becomes large.
Japanese Patent Application Laid-open Publication No. HEI 10-62811 describes a liquid crystal display device comprising a plurality of X drivers and Y drivers and individually driving neighboring liquid crystal cells. This liquid crystal display device secures time to write and reset for each of the liquid crystal cells by carrying out partially overlapping write and reset operations on the cells. In this manner, contrast of display data is improved. However, liquid crystal display devices of this kind have the plurality of X drivers and Y drivers, which leads an increase in circuit size. Furthermore, since the number of signal lines becomes double, a problem of aperture ratio reduction also occurs.
In order to improve brightness of a display image, a backlight is generally arranged, facing a liquid crystal panel comprising pixel electrodes, TFTs, and a control circuit thereof. However, when the impulse driving described above is carried out, pixel electrodes having reset data written therein and thus displaying black absorb light from the backlight. As a result, a problem of wasteful power consumption occurs. Moreover, since an image displayed by impulse driving has lower brightness than an image displayed by hold driving, it is necessary to increase the brightness of the backlight. As a result, power consumption increases.
In the case where a plurality of fluorescent tubes laid out in parallel are used as the backlight, a problem of uneven image display caused by a difference in a degradation speed of each fluorescent tube also occurs.