Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-04-20
2004-08-03
Awad, Amr (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S088000, C345S098000
Reexamination Certificate
active
06771238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to active matrix liquid crystal display devices incorporating therein driver circuits, and more particularly to a technology to enhance definition and image quality for the liquid crystal display devices.
2. Description of Related Art
In recent years, technological developments have being put forwarded in flat panels, such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescence (EL) displays, as cathode ray tubes (CRT) replacing displays. Among these flat displays, liquid crystal displays are the largest in marketplace and utilized for various display mediums including notebook personal computers, digital cameras with liquid crystal panels, car navigation systems, projectors and wide screen televisions.
The advantage of the liquid crystal display greater than the CRT lies in that the display area is obtained wide due to display section flatness and high definition given by the dot matrix display scheme.
The high definition is meant to increase in the number of pixels in the liquid crystal display. A drive frequency increases with increase in the number of pixels. For example, the number of pixels, although about four hundreds of thousands in NTSC rating, mounts to approximately two millions (1920×1080 pixels), in HDTV rating. Accordingly, in HDTV rating the input video signal has its maximum frequency reaching as high as 20 to 30 MHz, despite it was 6 MHz in the NTSC rating.
In order to display video signals with accuracy, a clock signal requires a frequency of several times (e.g., about 50 to 60 MHz) that of the video signal. It is expected that display with higher definition and image quality be furthermore required from now on and video signals with a dot clock extremely high in speed be dealt with.
FIG. 11A
shows a simplified routes for video signals to be inputted to the conventional liquid crystal display panel. The liquid crystal display panel
10
is arranged, as shown in
FIG. 11A
, with a pixel matrix area
11
, and a gate driver circuit
12
and a source driver circuit
13
. The gate driver circuit
12
is also called a scanning line driver circuit. The source driver circuit
13
is also called a signal line driver circuit or a data line driver circuit. The pixel matrix area
11
has pixels, each pixels having a liquid crystal cell
15
and a pixel TFT
16
. The liquid crystal cell
15
possesses a capacitor structure having dielectric sandwiched between a pixel electrode to be inputted by a video signal and an opposite electrode. The pixel TFT
16
includes a gate electrode, a source electrode and a drain electrode. The gate electrode is connected to a scanning line
17
, the source electrode (or the drain electrode) is connected to a signal line
18
and the drain electrode (or the source electrode) is connected to the pixel electrode of the liquid crystal cell
15
. The scanning line
17
is connected to the gate driver circuit
12
and the signal line
18
is connected to the source driver circuit
13
. The scanning line
17
is also called a gate line. The signal line
18
is also called a data line, a source line or a drain line.
The video signal to be applied to the pixel cell is processed suitably for display characteristics of the liquid crystal panel
10
by the video signal processing circuit
20
. The video signal processing circuit
20
mainly performs gamma correction, alternation and amplification to process on video signals inputted from the outside. The processed video signal is inputted from the source driver circuit
13
through the signal line
18
to the pixel matrix area
11
, thus applied to the pixel electrode of the liquid crystal cell
15
. The liquid crystal material in the liquid crystal cell
15
varies in light transmission rate depending upon a voltage applied to. The change of light transmission rate corresponds to tone whereby images are formed by the entire liquid crystal cells
15
.
In order to realize high quality display on the liquid crystal panel, the video signal processing circuit
20
requires an amplifier
21
(see
FIG. 11B
) to amplify signal waveforms with fidelity. This is because the amplifier
21
is at a final output end of the video signal processing circuit
20
where the video signal to be applied to the pixel electrode of the liquid crystal cell
15
is finally determined in amplitude and form. The video signal applied to the pixel electrode is a pulse-formed signal. Consequently, the amplifier
21
is required not to cause pulse signal amplitude deterioration and rounding of pulse waveforms.
It is known that the amplifier
21
generally has a frequency characteristic as shown in numeral
1101
of
FIG. 11C
wherein a voltage gain is nearly constant in a middle range but, in a range exceeding a certain frequency, decreases at a constant rate. The decrease rate is −20 dB/decade (−6 dB/octave) where the amplifier is in one stage. The cause of decreasing the gain in the higher range is due to output impedance increase in the single amplifier.
In the liquid crystal display, however, consideration has to be given not only to the output end voltage of the amplifier
21
but also to the voltage finally applied to the pixel electrode. Accordingly, there is a necessity for the frequency characteristic of the amplifier
21
in the video signal processing circuit to consider also the resistance R
LC
and capacitance C
LC
connected between the amplifier
21
and the liquid crystal cell
15
instead of the single amplifier
21
. Thereupon, as shown in numeral
1102
of the
FIG. 11C
the frequency range in which the gain of the pixel electrode of the liquid crystal cell
15
begins to lower is shifted to a lower side than the gain of the single amplifier
21
by impedance decrease due to the liquid crystal panel resistance R
LC
and capacitance C
LC
.
The increase of definition in the liquid crystal display is pixel and pixel density increase. The pixels, if increased, increases the number of connection lines, increasing liquid panel resistance R
LC
. The density increase actualizes; the problem of pixel matrix parasitic capacitance, giving rise to a tendency of increasing the capacitance C
LC
. Accordingly, the increase of definition results in a shift of the frequency range in which the gain of the amplifier
21
is flat toward the lower range side. In order to avoid the gain decrease, the resistance R
LC
may be decreased. In order to reduce the resistance R
LC
, the thickness of interconnection may be increased. However, the increase in interconnection thickness leads to increase in interconnect occupation area, running counter to a direction of a technological development called pixel shrinkage.
The increase in definition also requires high frequency drive. The video signal drive frequency in the HDTV rating requires as high as 20 to 30 MHz. If an HDTV rating display is realized by a liquid crystal panel, the video signal frequency f
vid
unavoidably comes to a frequency range that the gain on the pixel electrode is decreased due to the above-described increase in definition of the liquid crystal panel.
If a gain decrease on the pixel electrode occurs in the video signal frequency f
vid
, the video signal decreases in black or white level, resulting in image graying (muddy color in color display) and hence degradation in display quality.
High frequency drive has been unnecessary for such a VGA or SVGA rated liquid crystal panel as having the horizontal number of pixels of less than a thousand. Consequently, even if there has been a decrease on the high frequency side in the gain of the voltage applied to the pixel electrode, the amplifier
21
could be used at a frequency at which the gain is flat. The problem of the gain decrease concerning the frequency has not been recognized at all.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a liquid crystal display device which is capable of displaying with high quality, wherein the gain reduction in the high frequency
Awad Amr
Nelson Alecia D
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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