Liquid crystal matrix display device

The present invention comprises a selective supply of image signals and a desired direct current potential to picture elements during each period of field scanning, whereby flickering and shading unevenness are prevented.

BACKGROUND OF THE INVENTION 
The present invention relates to a liquid crystal matrix display device 
such as used for a television display. 
Liquid crystal televisions have recently been commercialized and the demand 
for them has rapidly increased. In general, NTSC-type television 
broadcasting is received by liquid crystal televisions, but in this type 
of broadcasting, 60 fields are transmitted per second and when the 
polarity of an image signal is reversed for each field in order to drive 
the liquid crystal with alternating current, the liquid crystal is 
subjected to a 30 Hz drive. 
Generally, if a liquid crystal is not driven above 40 Hz, flickering is 
produced quite strikingly. 
For this reason, an art for eliminating flickering has been proposed, which 
is disclosed in Japanese Patent Laid-Open No. 15338/1984. In this art, a 
single polarity image signal and a given potential of direct current are 
supplied to a picture element alternatively while switching them for each 
field. Consequently, an image signal to be supplied has a single polarity, 
and thus it is possible to restrain the production of flickering. 
However, the above-mentioned art has a disadvantage in that shading 
irregularity occurs between the upper and lower portions of a picture. In 
other words, as regards to the picture elements in the upper portion of a 
picture, a signal is written on a source line immediately after switching 
to an image signal or direct current and the source line is then held in 
the signal switched state, and thus the amount of leakage of charges 
stored in the picture elements to the source line is relatively small and 
does not lead to any significant problem. As regards those in the lower 
portions of a picture, however, either an image signal or direct current 
is written on the source line at the end of a field scanning and the 
source line is thus switched to an image signal or direct current, and the 
potential of the charges stored in the picture element is therefore 
greatly different from that of the source line, and leakage of the charges 
results. Furthermore, since this leakage of the charges continues for a 
period of time which is substantially equivalent to one field, it is 
impossible to reproduce a true image in the lower portion of a picture, 
whereby shading unevenness is produced as between the upper and the lower 
portions of a picture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, reference numeral 1 denotes a shift register for selecting a 
source line; or column electrode reference number 2 denotes a sample hold 
circuit for holding image signals; and S.sub.11, S.sub.12 . . . S.sub.21, 
S.sub.22 . . . denotes switching elements for selectively supplying an 
image signal and a desired direct current potential V.sub.H to a source 
line S. Reference number 4 denotes a control circuit for controlling 
timings. L.sub.11, L.sub.12 . . . denote picture elements arranged in a 
matrix of rows and columns and M.sub.11, M.sub.12 denote switching 
elements series-connected to corresponding picture elements. 
Operations will be described hereinafter with reference to the time charts 
shown in FIG. 2. Vertical synchronizing signals (v. SYNC) and horizontal 
synchronizing signals (H. SYNC) shown in FIG. 4 are supplied to the 
control or synchronizing circuit 4 and timing signals output from this 
control circuit control the operations of the shift register 1 and the 
gate driver 3. Firstly, image signals are subjected to sample-holding in 
the sample-hold circuit 2 each horizontal scanning by the output from the 
shift register 1. These image signals are supplied to the buffer 
amplifiers S.sub.11, S.sub.12 . . . . 
On the other hand, a predetermined signal in the form of a desired direct 
current potential V.sub.H is supplied to the switching elements S.sub.21, 
S.sub.22 . . . . This direct current potential and the image signals are 
supplied to the source line or column electrode S while they are being 
switched once during each period of horizontal scanning by means of the 
pulse B shown in FIG. 2. Namely, when a logic level of a terminal B is 
"1", the direct current potential V.sub.H is supplied to the source line 
S, and when it is "0", image signals are supplied thereto. 
The signal output from the source line is written in each picture element 
to drive the same by means of scanning signals from the gate driver 3, as 
described below. The gate driver 3 applies scanning signals shown by 
G.sub.1, G.sub.2 . . . in FIG. 2 to gate lines or row electrodes G.sub.1, 
G.sub.2 . . . , and the image signals and the direct current potential 
V.sub.H are respectively written in each picture element once during each 
field scanning period by means of these scanning signals. 
A pulse p.sub.1 of the scanning signal G.sub.1 shown in FIG. 2 is generated 
in synchronization with an image signal writing start pulse C shown in 
FIG. 2 which is output from the control circuit 4. Since this pulse 
p.sub.1 is generated at a timing at which image signals are supplied to 
the source line S to select the first gate line G.sub.1, the image signals 
are written in the first row of picture elements L.sub.11, L.sub.12, 
L.sub.13 . . . in parallel. 
In a similar manner, pulses of the scanning signals G.sub.2, G.sub.3 . . . 
shown in FIG. 2 are sequentially applied to the gate lines G.sub.2, 
G.sub.3, . . . G.sub.130 in synchronization with the generation of image 
signals and the image signals are sequentially in the rows of picture 
elements corresponding to the gate lines G.sub.2, G.sub.3 . . . G.sub.130. 
On the other hand, pulses are applied to gate lines G.sub.131 . . . 
G.sub.240 immediately before the pulses are applied to the gate lines 
G.sub.1 . . . G.sub.130 (at the timings at which the direct current 
potential V.sub.H is supplied to the source line S), as shown in FIG. 2, 
thereby the direct current potential V.sub.H is sequentially in the rows 
of the picture electrodes corresponding to the gate lines G.sub.131 to 
G.sub.240. 
In such a manner, image signals are written in the rows of picture elements 
corresponding to the gate lines G.sub.1 to G.sub.130 and the direct 
current potential V.sub.H is written in the rows of picture elements 
corresponding to the gate lines G.sub.131 to G.sub.240, in the first half 
of each field scanning period. 
When the above-described writing is completed, a pulse p.sub.2 of the first 
scanning signal G.sub.1 shown in FIG. 2 is applied to the gate line 
G.sub.1 in synchronization with the writing start pulse D for the direct 
current potential shown in FIG. 2 at the timing at which the direct 
current potential V.sub.H is supplied to the source lines S. The direct 
current potential V.sub.H is written in the first row of picture elements 
corresponding to the gate line G.sub.1 by means of this pulse p.sub.2 at a 
different timing than the timing at which the image signals are applied to 
the first row of the picture elements. In a similar manner, the direct 
current potential V.sub.H is sequentially written in the rows of picture 
elements corresponding to the gate lines G.sub.2, G.sub.3, . . . G.sub.130 
in the second half of each field scanning period. 
On the other hand, image signals are sequentially written in the rows of 
picture elements corresponding to the gate lines G.sub.131 to G.sub.240. 
Therefore, when one picture element is selected twice during one field 
scanning period, the image signal and the direct current potential V.sub.H 
are written in the picture element each time while they are being switched 
during each field scanning period. 
Thus, it is substantially possible to effect a 60 Hz drive and to eliminate 
all flickering. 
In FIG. 2, E denotes a signal to be written in the picture elements 
connected to the gate line G.sub.1 and F denotes a signal to be written in 
the picture elements connected to the gate line G.sub.131. 
Since the source lines S are switched to the image signals or the direct 
current potential during the period of one horizontal scanning in response 
to the signal B as shown in FIG. 2, the charge stored in each picture 
element does not substantially leak at all and the picture elements in the 
upper and the lower portions of the picture element matrix are under the 
same condition so that no shading unevenness is produced. 
The detailed configuration of the gate driver 3 is described below with 
reference to FIG. 3. In the drawing, reference numbers 5 and 6 denote 
respective shift registers of 480 bits each; outputs only in odd number 
steps are derived from the shift register 5 and those only in even number 
steps are derived from the shift register 6. In addition, the writing 
start pulses C, D (which are the same as C, D in FIG. 2) for starting the 
application of the image signals and the direct current potential are 
supplied to the shift registers 5, 6, respectively. 
480 clock pulses are supplied to the clock input CK of each shift register 
5, 6 during each period of field scanning. Thus, pulses b.sub.1 to 
b.sub.240 shown in FIG. 4 are generated from the terminals b.sub.1 to 
b.sub.240 of the shift register 6 and pulses a.sub.1 to a.sub.240 shown in 
FIG. 4 are generated from the terminals a.sub.1 to a.sub.240. As the gate 
circuits g.sub.1 to g.sub.240 and the inverters t.sub.1 to t.sub.240 
receive these pulse signals, they generate the pulses G.sub.1 to G.sub.240 
shown in FIG. 4 which are respectively supplied to the gate lines G.sub.1 
to G.sub.240, the pulses shown in FIG. 2 thereby being obtained. 
In the above description, the ratio of the image signal time to the direct 
current potential time is 1:1, but the ratio is not limited to this value 
and it may, for example, be set to about 2:1. 
Furthermore, it is possible to control brightness by making the direct 
current potential V.sub.H variable. 
In the above embodiment, the gate lines comprise 240 lines, but in the case 
of 480 gate lines, it is possible to employ the present invention by 
doubling the speed of image signal, without any other change. 
In addition, although the above embodiment relates to a black and white 
television, the present invention can also be applied to color televisions 
in a similar manner. 
The present invention in which the image signal and the direct current 
potential are selectively supplied to the picture elements during the 
period of each field scanning is capable of providing a 60 Hz drive, and 
the drive frequency is doubled twice as that of the conventional drive in 
the case of the NTSC type. Any possible flickering is thus eliminated. 
This effect is particularly significant in the cases of the and SECAM 
types in which the transmission speed is small. Since no flickering is 
produced, the composition of liquid crystal can be freely selected and it 
is possible to use a liquid crystal having high speed, high resistance, 
and high reliability. 
In addition, all picture elements in the matrix are driven under the same 
condition and it is thus possible to display a picture of good quality 
without any shading being experienced. Therefore, the requirements 
relating to the leak current of switching elements is moderated. 
Furthermore, image signals need not be inverted and a frame memory for 
double-speed scanning is made unnecessary, resulting in a simple circuit 
configuration.