Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-12-17
2002-04-30
Wu, Xiao (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S211000, C345S210000, C345S095000
Reexamination Certificate
active
06380917
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a technical field of a driving circuit and a driving method for driving an electro-optical device such as a liquid crystal device, the electro-optical device, and electronic equipment employing the electro-optical device and, more particularly, to a driving circuit and a driving method of an electro-optical device that receives a digital image signal and has a DA (Digital to Analog) converting function and a &ggr; correcting function for an electro-optical device, the electro-optical device, and electronic equipment using the electro-optical device.
2. Description of Related Art
Hitherto, as a driving circuit for driving a liquid crystal device, which is an example of one type of electro-optical device, there is available, for example, a so-called digital driving circuit configured to receive digital image data indicating an arbitrary step of gray scale among a plurality of steps of gray scale, generate analog image data having a driving voltage corresponding to the step of gray scale, and supply the generated analog image data to a signal line of the liquid crystal device. Such a driving circuit is usually provided with a digital-to-analog converter (hereinafter referred to as “DA converter” or “DAC” as necessary) for converting digital image data to analog image data; it is configured to latch the digital image data, which has been input via a digital interface, by a latching circuit, then subject it to analog conversion through a switched capacitor type DA converter (hereinafter referred to as “SC-DAC” (Switched Capacitor—DAC: switch control capacity type DAC) as necessary), a DAC composed of a resistance ladder circuit or the like.
In a liquid crystal device or the like, the changes in optical characteristics (transmittance, optical density, luminance or the like) with respect to the changes in the driving voltage (or a voltage applied to the liquid crystal) are generally nonlinear according to the saturation characteristic or threshold value characteristic that the liquid crystal or the like has and they exhibit a so-called “&ggr; characteristic.” Hence, this type of driving circuit is normally provided with &ggr; correcting means for making a correction on digital image data in a stage preceding the latching circuit.
The &ggr; correcting means, for example, carries out &ggr; correction on 6-bit digital image data D
A
by referring to a table stored in RAM or ROM so as to convert it into 8-bit digital image data D
B
(D&ggr;
1
, D&ggr;
2
, . . . , D&ggr;
8
). The processing by the &ggr; correcting means is implemented, considering the input/output characteristics of the DAC and the characteristic of the transmittance of liquid crystal pixels with respect to the voltage applied to a signal line (characteristics of transmittance vs. the voltage applied to liquid crystal). The transmittance characteristic of the liquid crystal pixels refers to the characteristic of changes in the transmittance of light obtained by transmitting through a liquid crystal layer with respect to the voltage applied to the liquid crystal layer held between a pair of substrates (transmitting through polarizer if they are disposed outside the substrates as necessary).
On the other hand, the aforesaid SC-DAC is constituted by a plurality of capacitive elements disposed in parallel. The respective capacitive elements have binary ratios of, for example, 2
0
C, 2C, 2
2
C, 2
4
C and so on. Using these capacitive elements, a pair of reference voltages are subjected to voltage division or the like (charge share) thereby to output analog image data having a driving voltage that changes according to the changes in the gray scale of image data D
B
. The DAC such as the SC-DAC configured as described above is connected to a signal line of a liquid crystal device or the like; a buffer circuit or the like is provided between the output terminal of the DAC and the signal line so as to protect the output voltage from the influences of the parasitic capacitance of the signal line.
As set forth above, the driving circuit causes a voltage corresponding to the digital image data D
B
to be applied to the respective signal lines of a liquid crystal device or the like.
Graph (A) on the left in
FIG. 21
shows the relationship between the decimal values of image data D
A
and output voltage Vc of the DAC; graph (B) on the right in
FIG. 21
shows the relationship between transmittance S
LP
of liquid crystal pixels and voltage V
LP
applied to the signal line (the axis of the transmittance is based on the logarithm). At the center in
FIG. 21
, the binary values of 8-bit digital image data D
B
are given between the two graphs (A) and (B).
In graph (B) on the right in
FIG. 21
, 2
6
pieces of 8-bit data capable of distinguishably representing the transmittance characteristic of the liquid crystal pixels are selected among 2
8
pieces of 8-bit data obtained from the 8-bit input data to make the &ggr; correction and the selected pieces of data are tabulated. And when 6-bit image data D
A
is input, the &ggr; correcting means converts it into 8-bit data D
B
according to the table and outputs it to the DAC. More specifically, image data D
A
is represented in 64-step gray scale; therefore, the foregoing conversion is carried out so that the data D
A
for 64 steps of gray scale may be specified among the 256 steps of gray scale that can be represented by image data D
B
in order to provide even changing ratio of the transmittance in the liquid crystal when image data D
A
expressed in the 64-step gray scale is changed.
Thus,
FIG. 21
illustrates the correspondence relationship between the 6-bit image data D
A
and the 8-bit image data D
B
and output voltage Vc (equivalent to V
LP
) of the DAC.
SUMMARY OF THE INVENTION
The foregoing conventional driving circuit, however, requires &ggr; correcting means and RAM or ROM or the like for storing the conversion table for the &ggr; correction which are provided in the stage preceding the latching circuit in order to make &ggr; correction. These components, therefore, provide obstacles in an attempt to reduce the size of the driving circuit. It would be possible to make up the DAC by using many amplifiers so as to provide it with the &ggr; correcting function without using the aforesaid SC-DAC. This, however, would pose such a problem as a more complicated circuit. In addition, forming operational amplifiers on a glass substrate tends to cause more variations in operating characteristics to occur.
Accordingly, it is an object of the present invention to provide a driving circuit of an electro-optical device that is compatible with digital image signals and has a relatively simple and small-scale circuit configuration to provide a DA converting function and a &ggr; correcting function (or an auxiliary function for making a &ggr; correction), the electro-optical device, and electronic equipment employing the electro-optical device.
To this end, according to one aspect of the present invention, there is provided a driving circuit of an electro-optical device that supplies an analog image signal, which has a driving voltage corresponding to an arbitrary step of gray scale among 2
N
(where N is a natural number) steps of gray scale, to a signal line of an electro-optical device in which the changes in the optical characteristics with respect to the changes in the driving voltage are nonlinear; the driving circuit of the electro-optical device being provided with: an input interface to which an N-bit digital image signal indicative of the arbitrary step of gray scale is applied; and a digital-to-analog converter that generates a voltage within a range of a pair of first reference voltages according to the bit value of the foregoing digital image signal to produce the driving voltage within a first driving voltage range corresponding to the step of gray scale of the digital image signal so that the changes in the driving voltage with respect to the changes in the step of gray scale of the digital image si
Matsueda Yojiro
Ozawa Tokuroh
Oliff & Berridg,e PLC
Seiko Epson Corporation
Wu Xiao
LandOfFree
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