Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-11-04
2001-07-24
Jankus, Almis R. (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S093000, C345S096000, C345S100000
Reexamination Certificate
active
06266038
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an active matrix type liquid crystal display apparatus and, more particularly, it relates to an active matrix type liquid crystal display apparatus having a plurality of vertical signal lines and a plurality of switching transistors arranged for the liquid crystal device of each pixel.
2. Related Background Art
Known methods developed in recent years for driving liquid crystal display apparatus to display images include simple matrix drive methods typically to be conducted in a TN display mode, an STN display mode or a ferroelectric liquid crystal display mode, di-terminal type active matrix drive methods using MIMs or diodes and tri-terminal type active matrix drive methods using a-Si TFTs or poly-Si TFTS.
Meanwhile, known methods for driving liquid crystal panels include line-sequential scanning methods adapted to rewrite the voltage of all the pixels of a row in a single horizontal scanning period and dot-sequential scanning methods adapted to serially rewrite the voltage of each pixel. When a liquid rystal panel is driven by a DC voltage, electrochemical reactions are apt to occur in the liquid crystal material, the oriented film and/or the interface thereof to degrade the quality of the displayed image. A technique of polarity inversion of data signals or that of applying an AC to drive the liquid crystal panel is popularly used to avoid this problem. The AC drive technique utilizes both a line inversion system of inverting the polarity on a scanning line by scanning line basis and a field inversion system of inverting the polarity on a field by field basis in order to prevent inter-frame flickers and inter-line flickers from taking place.
FIG. 6
of the accompanying drawings schematically illustrates a circuit diagram of a pixel of a known active matrix circuit. Referring to
FIG. 6
, there are shown a vertical signal line
61
, a scanning line
62
and a switching pixel transistor
63
. Reference symbol Cadd denotes a holding capacitance and reference symbol LC denotes liquid crystal. Note that the switching pixel transistor
63
is an n-channel type transistor. A known active matrix circuit having the above described configuration is accompanied by the problems as pointed out below because the pixel transistor
63
is an n-channel type transistor.
The AC drive technique is normally used in liquid crystal display apparatus in order to prevent degradation (the sticking phenomenon) of the liquid crystal LC of the apparatus. Then, the image signal applied thereto can show either a positive polarity or a negative polarity relative to the middle potential as shown in FIG.
7
A and hence it is required to have a large amplitude. Then, as shown in
FIG. 7B
, the pulse of the scanning line
62
is required to have an even larger amplitude obtained by adding an amplitude corresponding to a threshold value of transistor
63
to that of the image signal. Furthermore, the apparent threshold value of the transistor
63
is raised as the source potential of the transistor
63
rises because of the back bias effect. Then, the amplitude of the pulses of the scanning line
62
becomes even larger if the biasing effect is taken into consideration so that consequently a high supply voltage is required to drive the circuit. The use of such a high voltage inevitably raise the power consumption rate.
FIG. 8
schematically illustrates a circuit diagram of a pixel of another known active matrix circuit. Referring to
FIG. 8
, the pixel comprises a signal line
61
, a scanning line
64
, a scanning line inverse relative to the scanning line
65
, an n-channel type pixel transistor
66
, a p-channel type pixel transistor
67
, a holding capacitance Cadd and liquid crystal LC. With such a circuit configuration, no additional amplitude corresponding to a threshold value is required and hence it suffices that the scanning line
64
has an amplitude substantially same as that of the image signal applied thereto because the ON-state resistance of the n-channel type transistor
67
is raised while that of the p-channel type transistor
66
is lowered in a range where the signal voltage is high, whereas the ON-state resistance of the n-channel type transistor
66
is lowered while that of the p-channel type transistor
67
is raised in a range where the signal voltage is low so that a constant ON-state resistance is realized over the entire range of change of the signal voltage.
In the above described active matrix circuit, both the n-channel type transistor
66
and the p-channel type transistor
67
are turned on simultaneously under any circumstances. However, it is sufficient to turn on only the p-channel type transistor
67
when an image signal (with a positive polarity) having a voltage higher than the middle potential is written onto a pixel and only the n-channel type transistor
66
when an image signal (with a negative polarity) having a voltage lower than the middle potential is written onto a pixel. It is not desirable to turn on the two transistors simultaneously from the viewpoint of reducing the power consumption rate.
FIG. 9A
shows a circuit diagram of a circuit adapted to transfer a signal to vertical signal lines
90
,
91
. Referring to
FIG. 9A
, image signal (
1
) is fed to polarity inversion circuit
81
, which forwards the signal to common communication signal line
87
to turn on/off CMOS transfer switches
83
,
84
according to control signals
88
,
89
from horizontal scanning circuit
82
and by way of inverters
85
,
86
so that the image signal is output to vertical signal lines
90
,
91
in an alternate fashion.
Now, as described above, a signal having its polarity inverted regularly and periodically has to be fed to the vertical signal lines
90
,
91
. Referring to
FIG. 9B
, the image signal (
1
) is transformed to show a waveform illustrated by (
3
) according to a polarity inversion signal INV (
2
). For the reason described above by referring to
FIG. 8
, CMOS transfer switches are preferably used for the transfer switches
83
,
84
so that the signal may be transferred without losing its amplitude. Thus, with any of the above described known techniques, a complicated signal processing circuit is required to invert an image signal according to a polarity inversion signal INV (
2
) and, additionally, CMOS transfer switches have to be used for the transfer switches
83
,
84
to consequently increase the circuit size.
SUMMARY OF THE INVENTION
In view of the above identified problems, it is therefore the object of the present invention to provide an active matrix type liquid crystal display apparatus that can be driven with a low voltage, a reduced power consumption rate and a reduced circuit size without sacrificing the quality of the image it displays.
According to a first aspect of the invention, the above object is achieved by providing an active matrix type liquid crystal display apparatus comprising a plurality of vertical signal lines (
14
,
15
), a plurality of scanning lines (
16
,
17
), a plurality of pixel electrode substrates carrying thereon respective pixel electrodes (
13
) arranged at the crossings of the vertical signal lines and the scanning lines, a counter electrode substrate and liquid crystal pinched between the pixel electrode substrates and the counter substrate, characterized in that
each of the pixel electrodes is connected to a pair of vertical signal lines selected from the vertical signal lines by way of a pair of switching devices (
11
,
12
), which switching devices are connected respectively to a pair of scanning lines (
16
,
17
), the pair of vertical signal lines (
14
,
15
) being adapted to individually supply a positive polarity image signal and a negative polarity image signal, the pair of scanning lines being adapted to alternately open and close the pair of switches so that,
while the positive polarity image signal is fed to the pixel electrode from one (
15
) of the pair of vertical signal lines by way of the corresponding one (
12
) of th
Koyama Osamu
Kurematsu Katsumi
Yoshida Daisuke
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Jankus Almis R.
Tran Henry N.
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