Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
2000-06-22
2003-12-02
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S087000, C345S088000, C345S589000, C345S690000, C348S674000, C348S675000, C348S677000, C348S694000, C358S518000, C358S519000
Reexamination Certificate
active
06657619
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a clamping circuit for a liquid crystal display device, and more particularly to a clamping circuit for a large-screen and high-definition color liquid crystal display device of an active-matrix type.
2. Description of the Prior Art
Since a liquid crystal display device (LCD) of an active-matrix type using thin film transistors has advantages that it can drive two-dimensionally arranged pixel electrodes independently of one another, can implement a large-screen, high-definition and fine-gradation image display device, can be made compact thanks to its flat panel, can be driven by a low voltage and can be low in power consumption, the demand on it has been more and more expanded.
Since a liquid crystal display device of this kind sets a picture signal at a voltage suitable for driving liquid crystal, a black level in a picture signal is clamped to a predetermined clamping voltage. Furthermore, since the characteristic of transmissivity of liquid crystal to voltage applied to the liquid crystal is steep in the vicinity of the black level, the degree of gradation is lowered in the vicinity of the black level. To this end, so called gamma correction is performed, in which a picture signal is amplified by changing the degree of amplification in response to a picture signal level, which is called a gamma correction, is performed.
A clamping circuit for a liquid crystal display device clamps in general the black levels of analog RGB (Red, Green and Blue) picture signals to the same voltage.
In recent years, however, incorrect operation of a gamma-correction circuit and a coloring problem caused by variation in black level after a gamma correction are getting a great deal of attention as a problem to be solved.
On the other hand, in order to prevent the coloring problem caused by the difference in the characteristic of applied voltage to brightness among R, G and B signals of a LCD panel, a clamping circuit for a conventional liquid crystal display device described in Japanese Patent Laid-Open Publication No.Sho 64-78,592 has proposed to set pedestal clamping levels so that at least one of them is different from the other according to R, G and B color signals.
Referring to
FIG. 1
, a conventional clamping circuit for a liquid crystal display device is provided with three clamping circuits connected between three coupling capacitors and three gamma-correction circuits, respectively. The coupling capacitors
1
,
2
and
3
are provided for receiving input color signals R, G and B of a picture signal VI to remove a direct current (DC) bias. Output signals from the coupling capacitors
1
,
2
and
3
are inputted into the clamp units
400
,
500
and
600
, respectively, to output clamped color signals RC, GC and BC to the gamma-correction circuits
7
,
8
and
9
, respectively. The clamped color signals RC, GC and BC are obtained by newly adding a predetermined clamping voltage, which is the pedestal level of the picture signal, to the AC component of each of the color signals R, G and B whose DC bias has been cut. The gamma-correction circuits
7
,
8
and
9
perform a predetermined gamma correction and amplification on each of the clamped color signals RC, GC and BC to output the respective output color signals RG, GG and BG.
The clamp unit
400
is provided with a clamping portion
410
for performing a clamping operation of adding a clamping voltage to the AC component of an inputted color signal R and outputting the clamped color signal RC, and a clamping voltage generating circuit
420
for generating and supplying a predetermined clamping voltage to the clamp unit
410
.
The clamp unit
500
is provided with a clamping portion
510
for performing a clamping operation of adding a clamping voltage to the AC component of the inputted color signal G and outputting the clamped color signal GC.
The clamp unit
600
is provided with a clamping portion
610
for performing a clamping operation of adding a clamping voltage to the AC component of an inputted color signal B and outputting the clamped color signal BC, and a clamping voltage generating circuit
620
for generating and supplying a predetermined clamping voltage to the clamp units
510
and
610
.
The clamping voltage generating circuits
420
and
620
are of the same composition, and referring to
FIG. 2
showing with a block diagram the composition of the clamping voltage generating circuit
420
for the color signal R as a representative, the clamping voltage generating circuit
420
is provided with a variable resistor
423
for generating a divided voltage FR by dividing variably the voltage VDD of a power source and a buffer circuit
422
consisting of a voltage follower circuit for buffer-amplifying the divided voltage FR and outputting a clamping voltage CR.
In the same way, the clamping voltage generating circuit
620
is provided with a variable resistor and a buffer circuit.
Now, operation of a clamping circuit of a conventional liquid crystal display circuit is described with reference to FIG.
1
and FIG.
2
. The color signals R, G and B of the input picture signal VI are supplied to the respective systems. The coupling capacitors
1
,
2
and
3
are provided to eliminate only the DC components of the color signals R, G and B since the respective DC components are not constant. The AC component of each of the color signals R, G and B, in which the DC components are eliminated by the capacitors
1
to
3
, varies in black level according to the kind of a picture signal in a state of leaving as it is (for example, an inverse character display and a normal character display are different in black level), and therefore it is difficult to perform a later-stage process such as a gamma correction and the like. Thereupon, in order to prevent the black level of a picture signal from varying, pedestal level clamping circuits are provided for reproducing the DC components to the AC components of the color signals R, G and B. The foregoing circuits are called the clamp units
400
,
500
and
600
.
The processing system of the color signal R is explained as a representative in the following. The clamping voltage generating circuit
420
in the clamp unit
400
generating a divided voltage FR corresponding to a desired clamping voltage CR by dividing a power voltage VDD by means of the variable resistor
423
. The buffer circuit
422
buffer-amplifies the divided voltage, generates and supplies a clamping voltage CR to a clamping portion
410
. The clamping portion
410
operates so as to superpose the supplied clamping voltage CR on the pedestal level of the AC component of a color signal R, namely, the black level of a color signal R and outputs a clamped color signal RC. This is a clamping operation.
In the same way, clamped color signals GC and BC are outputted by performing clamping operations in the systems of color signals G and B.
In such a way, in a conventional clamping circuit the clamped color signals RC, and GC and BC can set their black levels independently. In this example the clamped color signals GC and BC are interlocked, but they can be composed so that they can be set independently of each other. However, once these clamped voltages, namely, the black levels are set, their settings are fixed as they are.
On the other hand, the black levels to be set of the gamma-correction circuits
7
,
8
and
9
at a later stage of the clamp units
4
,
5
and
6
are ideally set aiming at a predetermined voltage. As a matter of fact, however, the black levels vary among signals R, G and B or even among the gamma-correction circuits of the same color due to variation in accuracy of the gamma-correction circuits.
As shown in
FIG. 3
showing an example of the input/output characteristic of a gamma-correction circuit, however, when the black level of an output of a clamping circuit is fixed, the black level of the output of the clamping circuit may become different from the black level of the gamma-correction circuit. In such a s
Hjerpe Richard
NEC LCD Technologies Ltd.
Scully Scott Murphy & Presser
Tran Henry N.
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