Computer graphics processing and selective visual display system – Display driving control circuitry – Waveform generator coupled to display elements
Reexamination Certificate
1998-06-30
2001-04-03
Chow, Dennis-Doon (Department: 2775)
Computer graphics processing and selective visual display system
Display driving control circuitry
Waveform generator coupled to display elements
C345S210000, C345S094000, C345S095000, C345S096000
Reexamination Certificate
active
06211866
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a liquid crystal display device and, more particularly, to a grayscale voltage generating circuit used in circuitry for driving a liquid crystal display device which presents a multi-level grayscale (graduation) display.
BACKGROUND OF THE INVENTION
A grayscale voltage generating circuit for a liquid crystal display device of this type has been disclosed in the specification of Japanese Patent Kokai Publication JP-A-6-348235, by way of example. Specifically, the specification proposes a grayscale voltage generating circuit comprising a plurality of fixed resistors serially connected between a high-potential reference voltage and a low-potential reference voltage, and voltage varying means capable of varying the voltages at the nodes between the fixed resistors between the high-potential reference voltage and the low-potential reference voltage, with the voltages at the nodes between the fixed resistors being used as grayscale signals.
FIG. 5
illustrates part of this prior-art grayscale voltage generating circuit. Though there are eight outputs of positive polarity and eight outputs of negative polarity according to the description given in the above-mentioned Japanese Patent Kokai Publication JP-A-6-348235 (see FIG. 1 of the specification), here the grayscale voltage generating circuit will be described as a circuit having three outputs of each of the positive and negative polarities.
As shown in
FIG. 5
, the prior-art grayscale voltage generating circuit includes fixed resistors R
1
to R
4
and a variable resistor VR
3
serially connected between a reference voltage VH and a reference voltage VL, variable resistors VR
1
, VR
2
serially connected between the reference voltage VH and reference voltage VL, an amplifier circuit (a voltage-follower operational amplifier) A
1
connected between the node of the fixed resistors R
1
, R
2
and the variable resistor VR
1
, and an amplifier circuit A
2
connected between the node of the fixed resistors R
3
, R
4
and the variable resistor VR
2
.
The reference voltages VH and VL in this grayscale voltage generating circuit are output as is as a high-potential grayscale voltage V
0
H on the side of positive polarity and a high-potential grayscale voltage V
0
L on the side of negative polarity, respectively. A halftone grayscale voltage V
1
H and a low-potential grayscale voltage V
2
H on the side of positive polarity as well as a low-potential grayscale voltage V
2
L and a halftone grayscale voltage V
1
L on the side of negative polarity can be adjusted by adjusting the resistance values of the variable resistors VR
1
, VR
2
connected to the input side of the amplifier circuits A
1
, A
2
, respectively, and the resistance value of the variable resistor VR
3
located between the grayscale voltage V
2
H and grayscale voltage V
2
L.
The above-described grayscale voltage generating circuit makes it possible to improve upon a change in grayscale characteristic brought about by a change in viewing angle, namely the angle from which a liquid crystal display monitor is viewed. This change in grayscale characteristic due to a change in viewing angle is one characteristic of a liquid crystal display monitor primarily of the twisted nematic type.
Owing to the characteristics of the liquid crystal in a liquid crystal monitor, it is required that identical positive and negative voltages be applied in AC drive. Furthermore, it is necessary to correct for a shift (referred to as a “feed-through characteristic”) in ground potential of the liquid crystal caused by a difference in the voltages applied to the liquid crystal (e.g., as when 1 V is applied and when 5 V is applied). This correction is referred to as a “feed-through correction.”
In the above-described grayscale voltage generating circuit, feed-through correction values are decided by the potential dividing ratio of the serially connected fixed resistors R
2
, R
3
and variable resistor VR
3
and the adjustment of the resistance values of the variable resistors VR
1
, VR
2
. Consequently, even if the variable resistors VR
1
, VR
2
are varied to obtain the optimum grayscale characteristic and optimize the feed-through correction values of the grayscale voltages V
1
H, V
1
L in order to improve upon the grayscale characteristic based upon the viewing angle, the feed-through correction values of the low-potential grayscale voltages V
2
H, V
2
L are decided solely by the potential dividing ratio determined by the fixed resistors R
2
, R
3
and variable resistor VR
3
between the halftone-level grayscale voltages V
1
H, V
1
L. In other words, adjusting the variable resistor VR
3
merely changes the potential difference between the low-potential grayscale voltages V
2
H and V
2
L. This means that these feed-through correction values cannot be adjusted individually for each of the positive and negative polarities.
As a consequence of the foregoing, the feed-through correction values of the grayscale voltages V
2
H, V
2
L differ from the appropriate values.
SUMMARY OF THE DISCLOSURE
In the course of the investigations toward the present invention the following problems have been encountered in the prior art.
First, the variable resistances which decide the grayscale voltages (grayscale characteristic) on the side of positive and negative polarity are required to be adjusted by identical ratios at adjusting points provided on both the positive- and negative-polarity sides, and this necessitates the use of measuring equipment such as a voltmeter. Consequently, it is difficult for the user of the liquid crystal monitor to adjust the grayscale characteristic without using measuring equipment. The reason for this is that in order to operate a liquid crystal panel, it is necessary to drive the liquid crystal by an AC voltage and apply the grayscale voltages on the sides of positive and negative polarity at identical ratios with respect to the ground potential of the applied voltage.
The second problem is that when a grayscale voltage is varied, the feed-through correction must be performed again. For example, if grayscale voltage is varied in the grayscale voltage generating device shown in
FIG. 5
, the ideal feed-through correction values to be applied to the liquid crystal panel will no longer be obtained. The reason for this is as follows: In the grayscale voltage generating circuit shown in
FIG. 5
, the high-potential grayscale voltages V
0
H, V
0
L serve as the references and the high-potential feed-through correction values need not be varied to accomplish the feed-through correction. The feed-through correction of the halftone levels can be performed by adjusting the variable resistor VR
1
in such a manner that the grayscale voltage V
1
H will agree with a grayscale voltage to which a feed-through correction value on the positive polarity side has been added and adjusting the variable resistor VR
2
in such a manner that the grayscale voltage V
1
L will agree with a grayscale voltage to which a feed-through correction value on the negative polarity side has been added. However, since the low-potential feed-through correction is decided by the potential dividing ratio of the potential dividing resistors R
2
, R
3
, VR
3
located between the grayscale voltages V
1
H and V
1
L, the correction is influenced by a change in the halftone-level grayscale voltages V
1
H, V
1
L and low-potential feed-through correction values that should be constant levels are no longer obtained.
Accordingly, the present invention seeks to solve the aforementioned problems and, it is an object thereof to provide a grayscale voltage generating circuit for a liquid crystal display device in which, by adjusting a variable resistor at only one location without use of measuring equipment, halftone-level grayscale voltages on the positive and negative polarity sides can be varied simultaneously over identical voltage ranges without affecting high- and low-potential grayscale voltages.
It is another object to provide a grayscale generating circuit in which ideal halftone-level fe
Awad Amr
Chow Dennis-Doon
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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