Method for driving halftone display for a liquid crystal display

To correct the dependency of the transmissivity/applied voltage characteristics on color, a computing circuit is provided for generating corrected gray scale data by performing an addition or subtraction of the gray scale level related to at least one color. A delay circuit delays the gray scale data for uncorrected colors to maintain synchronization between the gray scale signals of all colors.

FIELD OF THE INVENTION 
The subject invention related to driving methods and control mechanisms in 
TFT liquid crystal displays (TFTLDCs). In particular, the subject 
invention relates to driving methods and control mechanisms for TFTLCD'S: 
in which the transition for each color in halftone display is effectively 
prevented. 
BACKGROUND ART 
The reduction in size of electronic equipment has been accompanied by an 
increase in the use of liquid crystal displays (LCDs). The LCD is not only 
used as a computer screen, but also is used as a television screen, a 
projection screen, etc. Utilizing liquid crystal has advantages such as 
low power consumption due to low driving voltage, and relatively fast 
response. It is expected that the field of application of LCDs will expand 
in the future. 
Most of the currently used LCDs are of the active matrix type. The active 
matrix type means the one in which a separate driving circuit element is 
provided for each pixel to improve display characteristics. Active matrix 
LCDs using thin-film three-terminal transistors (TFTs) as switching 
elements are called TFT liquid crystal displays (TFTLCDs). 
In using TFTLCDs to display pictures, it is necessary to provide gray scale 
data of the picture to the LCD to drive the LCD. FIG. 1 shows the 
construction of the control unit of the TFTLCD. The array/cell portion 1 
of the LCD is connected to an X-driver 3 and a Y-driver 5. The X-driver 3, 
when it is supplied with gray scale data, applies a voltage corresponding 
to the gray scale data to the cell. The Y-driver 5 is connected to the 
gate of a switching element, and conducts/does not conduct the voltage 
applied to the cell by the X-driver 3 at a predetermined time. 
Gray scale data is supplied to the X-driver by data control unit 10. The 
data control unit 10 consists of a data control circuit 12 for latching 
and storing the externally supplied R/G/B data in a buffer, and a timing 
control circuit 14 for outputting the gray scale data stored in the buffer 
to the X-driver 3 at a predetermined time. A clock signal is externally 
supplied to the data control circuit 12 and the timing control circuit 14 
to control the timing. A power supply 7 is connected to the X-driver, 
Y-driver 5, and data control unit 10. 
To display a picture on an LCD, a voltage corresponding to the gray scale 
is provided to each pixel of each color. That is, the driving of a pixel 
is not a simple on-off function, a voltage divided into several levels 
(gray scales) is provided to adjust the transmissivity of the pixel, so 
that intermediate color intensity can be displayed. To achieve such 
control in a color display, R/G/B signal levels are supplied to each 
pixel. For a display of a 64-level gray scale, 64-step voltage is used, 
and the voltage for each pixel is applied according to the respective gray 
scale data. Ideally, the same transmissivity can be achieved for all the 
colors when the voltage corresponding to a particular gray scale is used. 
The relationship for this is shown in FIG. 2. In FIG. 2, transmissivity is 
plotted on the ordinate, and applied voltage is plotted on the abscissa. 
Applied voltage is determined by the gray scale data. Accordingly, when a 
certain gray scale n is chosen, the applied voltage Vn is determined by 
that gray scale. Then, according to the relationship of FIG. 2, 
transmissivity Tn for the gray scale Vn is achieved. 
Ideally, the relationship between gray scale, applied voltage, and 
transmissivity is the same for each of the R/G/B colors. However in 
actuality, the gray scale and the achieved transmissivity have a slight 
difference depending on color. This is because the degree of light 
modulation for the specific twist of the twisted noematic liquid crystal 
is slightly different depending on wavelength. That is, even though a 
light passes through a liquid crystal layer in a similarly twisted state, 
the degree of the modulation given to the passing light is wavelength 
dependent, and thus the scattering of brightness that occurs for a given 
gray scale is color dependent. This is shown in FIG. 3. The transmissivity 
of blue (B) is higher than that of both red (R) and green (G) for the same 
voltage over a wide range of applied voltage. That is, since the 
relationship between gray scale and applied voltage for each color is 
unique, the transmissivity of blue (B) is greater even if each color is 
selected with the same gray scale and the same voltage is applied in the 
displaying of intermediate colors. Thus, the correlation between 
transmissivity and applied voltage (hereinafter referred to as 
transmissivity/applied voltage characteristics) has a color (wavelength) 
dependency. If the displaying is performed without providing any 
correction, the graduation of color translates to blue more than called 
for by the halftone data, and the picture on the whole takes on a bluish 
hue. FIG. 4 shows this state represented by a chromaticity diagram. FIG. 4 
shows that L63 should be a white color state if an ideal state could be 
realized, but in actuality, L0, or a shift to blue, occurs because of the 
wavelength dependency of the transmissivity/applied voltage 
characteristics. 
Various methods have been proposed for correcting the above problem. These 
methods are roughly divided into (1) methods for making the correction by 
the modification of the structure of LCD, and (2) methods for making the 
correction by using electric control. 
A typical example of the first category (1) is the adoption of a multi-gap 
structure. A multi-gap structure is a structure in which the thickness of 
the color filter of the pixel of each color of R/G/B varies. That is, the 
thickness (gap) of the liquid crystal sealing portion is changed to 
achieve the matching of the transmissivity/applied voltage characteristics 
for each color. However, implementation of a multi-gap structure is 
accompanied by difficulties in the manufacturing process. Problems occur 
in the adjustment of the thickness of the color filter, and in the 
uniformization of the gap between the two glass substrates forming the 
liquid crystal cell. Yield is effected by these difficulties causing an 
increase in manufacturing cost. 
As an example of the second category (2), is a method in which the 
reference voltage (gray scale voltage) given to the data driver is 
tailored to the characteristics for each color. This method can compensate 
for the color dependency of the transmissivity/applied voltage 
characteristics. However, the circuits needed to independently control the 
reference voltages, raise the cost and cause difficulties in the 
implementation. Another method that falls within this second category, is 
to use the voltage for one of the colors of R/G/B as a reference voltage, 
and use offset voltages for each of other colors. This methods has the 
same problems as the method in which the reference voltages are separately 
applied, and in addition, cannot accomplish desired effect if the 
gradients of the curves showing the transmissivity/applied voltage 
characteristics of R/G/B vary with applied voltage. That is, in accordance 
with the offset voltage method, correction is carried out by applying a 
uniform offset voltage for all applied voltages, and thus the correction 
cannot be effectively performed unless the gradients of the curves showing 
the transmissivity/applied voltage characteristics are the same over the 
whole applied voltage range. 
Japanese Published Unexamined Patent Application No. 01-101586 discloses a 
technique in which different liquid crystal driving voltage levels are set 
for each of the colors, and that level is applied to each pixel. Japanese 
Published Unexamined Patent Application No. 03-6986 discloses a technique 
in which the driving voltage is made to vary a predetermined voltage from 
color to color to obtain uniformity in transmissivity. Japanese Published 
Unexamined Patent Application No. 03-290618 discloses a technique in which 
a similar object is accomplished by independently inputting a gray scale 
control signal for each color. 
Therefore, first object of the subject invention is to provide a driving 
method for a TFTLCD in which the dependency on color of the 
transmissivity/applied voltage characteristics is effectively corrected. 
A second object of the subject invention is to realize the effective 
correction using a very simple method which enables the above described 
correction to be made without increase in complexity of the control 
method, and the restrictions on the implementation by addition of 
circuits. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the above described problems are 
solved by gray scale data (a bit string for a color liquid crystal 
display) wherein the data control means includes a computing circuit for 
performing an addition or subtraction of the gray scale related to at 
least one color to generate a corrected gray scale, and also includes 
delay means for delaying the outputting of the uncorrected gray scales, 
during the time which the gray scale of the one color is being corrected.

PREFERRED EMBODIMENT 
The subject invention can be realized by improving the data control unit 10 
of FIG. 1 as is shown in FIG. 5. In the background art, the data control 
unit consists only of a latch and a buffer. However, in the subject 
invention, the gray scale data related to a color, that is to be 
corrected, is temporarily inputted to a computing circuit. An addition or 
subtraction operation is applied to that gray scale data to shift it by 
one or more gray scale levels, to thereby achieve transmissivity 
equivalent to the other colors which are not to be corrected. 
In FIG. 5, the color to be corrected is blue (B), and the colors which are 
not to be corrected are red (R) and green (G). The gray scale data related 
to R or G are shown by R0 to R5 or G0 to G5 in FIG. 5. 
A portion 20 to which gray scale data related to R and G are inputted 
includes a data latch circuit 22 and a buffer circuit 26, like that in the 
data control unit in the background art. However, in addition to the data 
control unit in the background art, it includes a delay circuit 24. This 
is to compensate for the time during which the gray scale data B0 to B5 
related to B is operated on by a computing circuit 32 in accordance with a 
condition determination table 36, as described later. The delay circuit 25 
thereby assumes the outputting of the R and G gray scale data to the 
driver with the same timing as the corrected B gray scale data. 
The gray data B0 to B5 for blue is a bit string for representing a 64-level 
gray scale. It is comprised of a bit string (B0, B1, B2, B3, B4, B5). For 
instance, if the gray scale is "4", (B0, B1, B2, B3, B4, B5)=(001000), and 
if the gray scale is "28", (B0, B1, B2, B3, B4, B5)=(001110). The same 
applied for R0 to R5 or G0 to G5 which are the gray scale data for reg or 
green, respectively. 
Circuit 30 is for adjusting the Blue gray scale data B0 to B5. To 
accomplish this, the gray scale data related to Blue is first supplied to 
a computing circuit 32. In the computing circuit 32, the gray scale data 
for blue is reduced, for instance, by zero to four levels in comparison 
with the grey scale data for red and green. By correcting gray scale data 
in this way, results in matching the transmissivity of blue to that of Red 
and Green. 
Further, the gray scale data for Blue is also supplied to a condition 
determination table 33. The condition determination table 33 determines 
the amount of the adjustment of the gray scale data. A diagrammatic 
representation of the condition determination table 33 is shown in FIG. 6. 
As shown, conditions A to C, corresponding to various gray scale levels, 
are set in the condition determination table 33. The condition 
corresponding to a gray scale is outputted from the condition 
determination table 33 to an addition/subtraction table 34. The 
addition/subtraction table 34 has the function of setting the actual 
amount of the addition or subtraction. A diagrammatic representation of 
the addition/subtraction table 34 is shown in FIG. 7. That is, the 
addition/subtraction tables set the amount to be added or subtracted 
according to the condition provided from the condition determination table 
33. The amount of the addition or subtraction to correct the gray scale is 
supplied to the computing circuit 32. 
The condition determination table 33 and the addition/subtraction table 34 
can be implemented by software. The condition determination table can also 
be implemented by hardware by using the logic circuit shown in FIG. 8. To 
implement the specific conditions represented in FIG. 6, the gray scale 
data B0 to B5 are inputted to the logic circuit as shown. The gray scale 
data of B2 to B5 are inverted and inputted to an AND circuit 101 to create 
a condition corresponding to condition A in FIG. 6 for gray scale levels 0 
to 3. Similarly, the gray scale data B0, B2 to B5 for gray scale levels 61 
to 63 corresponding to condition A is inputted into AND circuit 102. The 
outputs of the AND circuit 101 and the AND circuit 102 are inputted to an 
OR circuit 106, and the condition A is outputted by circuit 110. AND 
circuit 103 and AND circuit 104 are circuits for generating condition B. 
Inputted to ANDs 103 and 104 is an output 122 separately created in a 
group of logic circuits 120, to thereby output the condition B for desired 
gray scale data levels 4 to 10 and 54 to 60. If there is no output from OR 
circuits 106 and 107, condition C is set. In this case, an output is 
provided by an AND circuit 108 to the circuit 110 to achieve the 
generation of condition C. Conditions A, B, and C are outputted from Q1 to 
Q3 of the circuit 110. 
Operation of the circuit 30 to which gray scale data for blue is inputted, 
and of the circuit 20 to which gray scale data related to Red and Green 
are inputted is as follows. When a gray scale level "2" is received, or 
(B0, B1, B2, B3, B4, B5)=(010000) is inputted, the input to the display is 
determined by the condition determination table 33. As shown in FIG. 6, in 
the condition determination table 33, the condition A is outputted to the 
addition/subtraction table 34, and thereafter, in the addition/subtraction 
table 34, "0" is outputted to the computing circuit as the addition or 
subtraction amount as shown in FIG. 7. Accordingly, the gray scale "2" is 
provided unconnected to the X-driver via a buffer circuit 36. The above 
described processing causes a predetermined delay. Thus, the gray scale 
data for Red and Green corresponding to the gray scale data related to 
Blue are delayed for time taken for the processing by a delay circuit 24. 
As a result, the gray scale data related to B is outputted from the buffer 
circuit 36 to the X-driver is synchronized with the gray scale data for 
Red and Green for simultaneous output from the buffer circuit 26 to the 
X-driver. 
Where the gray scale data level is "20," or the grey scale level signal 
(B0, B1, B2, B3, B4, B5)=(001010), the condition determination table 33 
provides condition C signal to the addition/subtraction table 34 as shown 
in FIG. 6. In response, the addition/subtraction table 34 provides a 
signal to the computing circuit to subtract four grey scale levels (the 
amount as shown as -4 in FIG. 7). Accordingly, the gray scale level "20" 
is corrected by the computing circuit 32 to a gray scale level 
"16"(20-4=16) which level is provided to the X-driver via the buffer 
circuit 36. In this way, corrections are made to the 
transmissivity/applied voltage characteristics where, as shown in FIG. 3, 
they are not uniform for each color. 
FIG. 9 shows the affect the correction of the present invention has on the 
transmissivity/applied voltage characteristics. In this figure, the 
ordinate indicates transmissivity and the abscissa indicates gray scale 
level all of R/G/B, the same transmissivity is achieved for the same gray 
scale level. Thus, it is seen that the problem of the subject invention of 
effectively correcting the difference in the dependency of the 
transmissivity/applied voltage for each color has been solved. 
In accordance with the subject invention, the difference in the dependency 
of the transmissivity/applied voltage characteristics for each color can 
be effectively compensated for. Further, the amount of the adjustment can 
be varied with the grey scale level for accurate compensation. 
With the method of the subject invention, only an additional circuit such 
as a computing circuit, is needed to effectively correct the differences 
in the transmissivity/applied voltage characteristics for colors. The 
above correction is made while avoiding the problems in complexity of 
control methods in the background art. That is, to implement the subject 
invention, only a condition determination circuit is needed in the data 
control circuit. It is not necessary to change the structure of the 
X-driver or the structure of the cell. 
Although, in this embodiment, the gray scale data related to B has been 
made to match the gray scale data related to R and G by performing a 
subtraction thereof, it should be self evident to those skilled in the art 
that an addition of the gray scale data related to Red and Green can be 
used to match the gray scale data for those colors with the gray scale 
data related to Blue using the teaching of the present invention. 
Therefore, it should be understood that many changes can be made in the 
described embodiment without departing from the spirit and scope of the 
present invention.