Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-03-15
2004-04-20
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S094000
Reexamination Certificate
active
06724358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix type display apparatus that is suitable for active matrix type display of images, in particular moving images, using for example liquid crystals, and to a driving method for the same.
2. Description of the Related Art
Conventionally, cathode ray tubes (CRTs) and liquid crystal displays (LCDs) are used for television receivers and computer displays. In liquid crystal display devices for image display, a display pattern is formed on the image screen by selective driving of pixel electrodes arranged in a matrix. When a voltage is applied between a selected pixel electrode and the counter electrode in opposition thereto, then the liquid crystal disposed between the electrodes is optically modulated, which can be seen as a display pattern. As a method for driving the pixel electrodes, the active matrix type driving method is known, in which the individual pixel electrodes are arranged in a matrix, and the pixel electrodes are connected to corresponding switching elements and driven. Generally well known as switching elements for selectively driving the pixel electrodes are thin film transistors (TFTs), and switching elements with so-called MIM (metal/insulator/metal) structure.
Liquid crystal display apparatuses are not only used for the display of still images, but also for the display of moving images. However, the display of moving images poses the problem that pronounced after-images can be observed, and that moving features appear to be followed by a tail. A major reason for the problem of after-images is the slow response of the liquid crystals that are ordinarily used, which is several dozen milliseconds. To solve this problem, not only the development of liquid crystals with faster response has been advancing, but as shown in Japanese Unexamined Patent Publication JP-A 4-288589 (1992), efforts are made to compensate the problem of the slow response of the liquid crystal by anticipatorily emphasizing changes of the voltage applied to the pixel electrodes. Also Japanese Unexamined Patent Publication JP-A 9-258169 (1997) discloses the idea of improving the after-images by anticipatorily emphasizing changes of the voltage applied to the liquid crystal for the display of moving images.
However, in recent years, it has been shown that the problem of after-images is not only caused by the slow responsiveness of the liquid crystals, but also by an after-image effect in human eyesight. That is to say, ordinary liquid crystal display apparatuses use hold mode display elements, which hold the voltage information written into the pixel electrodes for one vertical scanning period that lasts until the next writing process in the pixel capacitor between the pixel electrode and the counter electrode in opposition to the pixel electrode, often leading to after-images in human eyesight. When new information is written into the pixels, the information of the old frame, which was written in the previous vertical scanning period, is perceived as an after-image by the human eye. In image display with CRTs, on the other hand, the information is displayed only in the moment when the electron beam hits the screen, and during the remaining period, black display is performed in which nothing is displayed, so that the human eye does not perceive an after-image. Consequently, to realize a high-speed moving image with a liquid crystal display apparatus, it is necessary to display the information only during a portion of each vertical scanning period and to perform black display in which nothing is displayed during the rest of the vertical scanning period, so as to approximate an impulse mode, as is done in the case of CRTs.
FIG. 17
illustrates one idea for improving the after-images of liquid crystals with a pseudo-impulse mode. When the liquid crystal display is performed by transmission-type liquid crystal display, then it is necessary to turn on a backlight. If the backlight is turned off during a portion of each cycle of the vertical scanning signal, a substantially black display is possible. Japanese Unexamined Patent Publication JP-A 64-82019 (1989) discloses the idea of dividing one frame period for driving the liquid crystal to display one image frame into one vertical period in which a scanning signal is applied successively to the plurality of scanning lines Y
1
, Y
2
, etc., a liquid crystal response period lasting until display is performed with the driven liquid crystal, and a backlight ON period, so that the backlight is only on for a portion of one frame period. Also Japanese Unexamined Patent Publications JP-A 11-202285 (1999) and JP-A 11-202286 (1999) disclose the idea of partially turning the backlight off.
FIG. 18
shows another idea for displaying a pseudo-impulse mode on a liquid crystal display apparatus. For example Japanese Unexamined Patent Publications JP-A 9-127917 (1997) and JP-A 11-109921 (1999) disclose dividing one frame period into a vertical period and a black writing period, writing the original image display video signal during the vertical period, and writing a black signal to the pixels during the black writing period.
Improving the responsiveness of the liquid crystal by compensation, anticipatorily emphasizing changes of the voltage applied to the pixel capacitors as disclosed in JP-A 4-288589 and JP-A 9-258169, does not improve the after-image effect of human eyesight. And when turning off the backlight to perform display in pseudo-impulse mode as shown in
FIG. 17
, in the conventional technology disclosed in JP-A 64-82019, the backlight is turned off simultaneously on the entire display screen. Therefore, it is necessary to turn on the backlight after the vertical period, in which signals are written into the pixels in all display regions, and after the liquid crystal response period that lasts until the liquid crystal of the pixels that are scanned and into which the signal is written last has responded sufficiently. This means, the scanning time allotted per scanning line has to be made shorter than in the ordinary case when the backlight is not turned off. For example, when the backlight is turned on for ⅓ of each frame period, and ⅓ of each frame period is taken for the response of the liquid crystal, then the scanning time allotted as one vertical period is only ⅓ of the scanning time in the ordinary case. This corresponds to a display with a driving frequency that is three times as high, which puts a considerable load on the wiring resistances, switching performance of the TFTs, driver performance and the structure of the backlight, leading to lower display quality and higher costs. Moreover, it has also been suggested to shorten the time for the response of the liquid crystal and increase the scanning time serving as the vertical period by sequentially turning a plurality of back lights on and off, as shown in JP-A 11-202285 and JP-A 11-202286. However, also in this conventional technology, the fact that the vertical period for scanning is shorter than before remains unchanged, and there is also the problem of increased costs for the backlight structure.
Also when a black signal is written into the pixels and display is performed in pseudo-impulse mode as shown in
FIG. 18
, it is necessary to allot a black signal writing time of about one half of each frame period, so that the actual driving frequency is increased, and the same problems occur as in the prior art, in which the backlight is turned off. As a countermeasure, it has been suggested to provide scanning lines and signal lines for the application of the black signal as shown in JP-A 9-127917, but this leads to problems regarding lower yield due to an increased number of lines, an increased number of drivers, and increased costs for the source driver. It has also been suggested to partition the display portion and perform black display and video display in alternation as shown in JP-A 11-109921, but this leads to increased costs because of a more complicated circuit system
Ban Atsushi
Nakamura Wataru
Okada Yoshihiro
Conlin David G.
Dharia Prabodh M.
Edwards & Angell LLP
Jensen Steven M.
Mengistu Amare
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