Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-03-15
2002-12-03
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S097000, C345S100000
Reexamination Certificate
active
06489941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and a method for driving the apparatus. More particularly, the invention relates to a driving circuit adapted advantageously to a liquid crystal display apparatus utilizing anti-ferroelectric liquid crystal as its liquid crystal material, and a method for driving the LCD apparatus.
2. Description of the Related Art
Anti-ferroelectric liquid crystal (AFLC) is one of a variety of liquid crystal materials used by liquid crystal displays (LCD). An LCD apparatus that utilizes AFLC causes liquid crystal molecules to block or transmit light therethrough by driving the molecules between two phases: an anti-ferroelectric phase with no electric field turned on, and a ferroelectric phase with an electric field activated.
Threshold-less anti-ferroelectric liquid crystal (TL-AFLC) is particularly noted for its wide angles of visibility and its high response speed. A voltage-transmittance curve (V-T curve) of TL-AFLC has a symmetrical V-shaped characteristic around an origin, as shown in FIG.
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. In terms of response times, twisted nematic (TN) liquid crystal takes dozens of milliseconds to respond when driven, while TL-AFLC merely takes tens of microseconds to act. That means TL-AFLC is quicker to respond than TN liquid crystal by as many as three orders of magnitude.
Alternate driving is one of the commonly employed methods for driving LCD. The alternate driving method involves driving LCD by alternating, illustratively in increments of frames, the polarity of video signals (voltages) applied to the liquid crystal through the use of an AC voltage. Generally, one frame lasts about 16 milliseconds. Within that time frame, each scanning line is fed with a gate pulse whose width is about 16 microseconds needed to drive all scanning lines of the frame illustratively on an XGA display. The pulse width varies with the number of scanning lines used.
One disadvantage of the conventional alternate driving method above is that when applied to an LCD apparatus using TL-AFLC as its liquid crystal material, the method causes the LCD to lose its response speed resulting in a moving picture afterimage phenomenon. The reason is as follows: if the width of a gate pulse applied to each scanning line is illustratively 16 microseconds, that means the write time per scanning line is 16 microseconds as opposed to the response time of tens of microseconds of TL-AFLC. When the response time of TL-AFLC is longer than its write time, the writing of data within a single frame time fails to let TL-AFLC fully respond. Hence the inability to obtain an adequate transmittance.
To obtain the necessary transmittance requires writing data not in a single frame but across multiple frames. This translates into an effectively prolonged response time for the LCD as a whole. For example, the writing of data in five frames involves a response time of 16 milliseconds×5=80 milliseconds. That is, the response time of TL-AFLC ends up being equivalent to that of TN-LCD. In order to suppress moving picture afterimages, it is necessary ideally to complete the writing of data within a single frame time. With the conventional driving method, however, afterimages are unavoidable because the method requires carrying out data writes over a plurality of frames. Although TL-AFLC used in the LCD is supposed to offer a high response speed, the actual response speed of the liquid crystal material when driven is reduced to levels of other slow-to-respond liquid crystals. There has been a need for methods that would make full use of the inherently quick response of TL-AFLC used in LCD apparatuses.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances and provides a liquid crystal display apparatus having a driving circuit to make full use of the inherently high response speed of TL-AFLC, and a method for driving the apparatus.
In carrying out the invention and according to a first aspect thereof, there is provided a liquid crystal display apparatus having a driving circuit comprising: a signal line driving means having anti-ferroelectric liquid crystal sandwiched between an active matrix substrate and an opposed substrate, the active matrix substrate having a plurality of signal lines and a plurality of scanning lines arrayed in a matrix fashion to constitute a plurality of pixels, the signal line driving means driving the plurality of signal lines; a scanning line driving means for driving the plurality of scanning lines; and a reset voltage applying means for use when video signals are written to all pixels on any one of the plurality of scanning lines in one horizontal period, the reset voltage applying means applying a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines which are contiguous to the one scanning line and to which the video signals are written following the one horizontal period, the applying of the reset voltage being performed prior to the one horizontal period in which to write the video signals to all pixels on the one scanning line and over a plurality of horizontal periods temporally continuous to the one horizontal period.
According to a second aspect of the invention, there is provided a liquid crystal display apparatus having a driving circuit comprising: a signal line driving means having anti-ferroelectric liquid crystal sandwiched between an active matrix substrate and an opposed substrate, the active matrix substrate having a plurality of signal lines and a plurality of scanning lines arrayed in a matrix fashion to constitute a plurality of pixels, the signal line driving means driving the plurality of signal lines; a scanning line driving means for driving the plurality of scanning lines; and a reset voltage applying means for use when video signals are written to all pixels on any one of the plurality of scanning lines in one horizontal period, the reset voltage applying means applying a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines which are separated from the one scanning line and to which the video signals are written following the one horizontal period, the applying of the reset voltage being performed prior to the one horizontal period in which to write the video signals to all pixels on the one scanning line and over a plurality of horizontal periods temporally separated from the one horizontal period.
There are liquid crystal display apparatuses that adopt what is known as a line sequential driving method whereby scanning lines are sequentially scanned from top to bottom while being fed with a signal one line at a time. For such LCD apparatuses, one frame time divided by the number of scanning lines gives a driving time per scanning line (for one horizontal (1H) period). That is, there is no spare time left in each frame time. On the other hand, writing a signal to the signal line driving means (source driver) leaves enough time to spare. A clock signal for the signal line driving means generally contains per horizontal period the pluses numbering greater than the number of signal lines. Upon completion of the writing of data corresponding to the actual number of signal lines, a short time (e.g., equivalent to about 10% of the total number of pulses in each horizontal period) is left out as a blanking period.
Taking account of such a spare time existing per horizontal period upon writing of a signal to the signal line driving means, the inventors of this invention came up with an idea: that any voltage remaining in the liquid crystal should be initially reset by taking advantage of the spare time, followed by a write operation (i.e. application of a voltage) in order to let the liquid crystal fully respond to the subsequently applied voltage. The term “reset” refers herein to a state where no voltage is being applied to the liquid crystal. In other words, the reset voltage is a zero voltage.
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Inage Fumiaki
Ito Naoki
Ping Chen Guo
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Dinh Duc Q
Hjerpe Richard
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