Abstract:
The invention is concerned with a liquid crystal display apparatus which displays data by driving a liquid crystal panel in synchronism with horizontal synchronizing signals and vertical synchronizing signals. The driver for driving the liquid crystal panel is provided with timing signals maintaining a predetermined period, and with a voltage in synchronism therewith, irrespective of the horizontal and vertical synchronizing signals. Therefore, even when the horizontal and vertical synchronizing signals have not been applied, the liquid crystal panel is provided with an a-c voltage of which the polarity is alternatingly reversed by the timing signals. Therefore, electrolysis does not take place in the liquid crystal panel, and the quality of display is not degraded.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display apparatus, and more specifically to an apparatus for driving a liquid crystal panel of a relatively large size, which displays characters and pictures. 
     2. Description of the Prior Art 
     In recent years, liquid crystals have been extensively used in display devices such as wristwatches, digital clocks, desk-top calculators, and the like. In particular, liquid crystals of the light-receiving type have been placed in extensive applications owing to such features that they can be seen in bright places, they consume small amounts of electric power, and they can be driven on low voltages. Depending upon the operation modes, the liquid crystals can be divided into those of the dynamic scattering effect type (DSM), and those of the field effect type (FEM). 
     A liquid crystal panel of the dynamic scattering effect type is constructed by, for example, forming an electrode of a transparent electrically conductive film obtained by the evaporation of tin oxide (SnO 2 ) or the like on a glass plate having a thickness of 0.5 mm to several millimeters, sticking two glass plates together via a spacer, and injecting the liquid crystals into space formed therebetween. The thus constructed liquid crystal panel is usually driven by a matrix driving method. Signal electrodes are arrayed on one side, and scanning electrodes are arrayed on the other side in the directions at right angles with each other. By applying a voltage to the selected signal electrodes and to the selected scanning electrodes, the voltage at the intersecting points exceeds a threshold value, whereby the parallel indicent light undergoes scattering and appears in a whitish turbid color. In this case, the liquid crystal panel is deteriorated within short periods of time if a d-c voltage is applied thereto. Therefore, an a-c voltage is usually applied to the liquid crystal panel. 
     In the case when the liquid crystal display apparatus which displays characters or pictures, and a control device for controlling the liquid crystal display apparatus, are accommodated in a single housing such as of a pocket calculator, drive signals are applied to the liquid crystal display apparatus simultaneously with the closure of the power source, and a direct current does not flow into the liquid crystals. However, when the liquid crystal display apparatus and the control device are separately provided such as in a monitor television, in which the liquid crystal display apparatus has been designed to be driven by input signals produced by the control device, a direct current flows into the liquid crystal panel when no input signals is applied thereto, presenting such a defect that the display quality is degraded. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a liquid crystal display apparatus which prevents the display quality of the liquid crystal panel from being degraded. 
     Another object of the present invention is to provide a liquid crystal display apparatus which is capable of applying an a-c voltage to the liquid crystal panel even when no horizontal synchronizing signal or no vertical synchronizing signal is input thereto. 
     The objects of the present invention are realized by a liquid crystal display apparatus which introduces horizontal synchronizing signals, vertical synchronizing signals and display data, and which drives a driver of liquid crystal panel in synchronism with the horizontal and vertical synchronizing signals to display the data on the liquid crystal panel. Namely, the liquid crystal display apparatus has an oscillator which generates timing signals maintaining a predetermined period irrespective of the horizontal and vertical synchronizing signals that are introduced. The driver of the liquid crystal panel is served with electric power from the power source and outputs of the oscillator. Even when the horizontal and vertical synchronizing signals are not applied to the driver, polarity of the driving voltage is reversed by the outputs of the oscillator, so that an a-c voltage of substantially a rectangular form is applied to the liquid crystal panel. Thus, since the driving voltage is applied in the form of an a-c voltage even when no horizontal or vertical synchronizing signal is input to the driving circuit of the liquid crystal panel, the display quality of the liquid crystal panel can be prevented from being degraded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a liquid crystal display apparatus according to an embodiment of the present invention; 
     FIGS. 2, 3 and 4 are circuit diagrams of major portions of a main circuit of FIG. 1; 
     FIG. 5 is diagram showing data poles and scanning poles of the liquid crystal panel; and 
     FIG. 6 is a diagram of signals for illustrating the operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram showing a liquid crystal display apparatus according to an embodiment of the present invention. In FIG. 1, the liquid crystal display device is connected to a control device (not shown) via, for example, a cable. A video signal 1 is sent from the control device via the cable. The video signal 1 consists of a horizontal synchronizing signal H.SYNC, a vertical synchronizing signal V.SYNC, and a data signal, like a signal applied to the existing television sets. The video signal 1 is applied to a data separator circuit 5 and to a synchronizing signal separator circuit 6. FIG. 2 illustrates details of the date separator circuit 5 and the synchronizing signal separator circuit 6. 
     That is, the video signal 1 is sent to one terminal of a comparator 51 in the data separator circuit 5, while another terminal of the comparator 51 is served with a predetermined voltage (+E 1 ). Therefore, the comparator 51 produces a data signal that is compared with the voltage of +E 1  volts; the data signal is then sent to a sampling circuit 8. 
     The sampling circuit 8 consists of a flip-flop 81, and the data signal is input to a data terminal D of the flip-flop 81. Further, timing signals CLK of a predetermined period produced by a counter 4 that will be mentioned later, are input to a trigger terminal T of the flip-flop 81. Therefore, the data signals are sampled and produced by the flip-flop 81 responsive to the timing signals CLK. 
     The video signal is also input to one terminal of a comparator 62 in the synchronizing signal separator 6. The other terminal of the comparator 62 is served with a predetermined voltage -E 2 . The comparator 62 produces a synchronizing signal that is compared with a voltage of -E 2  volts. The synchronizing signal is input to an inverter 52 which produces a horizontal synchronizing signal H.SYNC that will be sent to a latch circuit 10 and to a shift register 12. Output of the comparator 62 is also input to an integration circuit 63 which integrates synchronizing signals to produce a vertical synchronizing signal V.SYNC that will be sent to the shift register 12. 
     A clock generating circuit 7 produces clock signals maintaining a predetermined period at all times. As illustrated in detail in FIG. 3, the clock generating circuit 7 consists of a parallel circuit of quartz oscillator 71 and inverter 72, that is connected to a resistor 73 of which the one end is grounded. Clock signals produced by the clock generating circuit 7 are input to the counter 4. 
     FIG. 3 illustrates in detail the counter 4 which consists of flip-flop circuits 41 of a plurality of stages. The clock signal is input to a trigger terminal T of the flip-flop circuit 41. Reset terminals R of the individual flip-flop circuits 41 are served, as reset signals, with outputs of the comparator 62 of the synchronizing signal separator circuit 6. Outputs on the side &#34;0&#34; of the individual flip-flop circuits 41 are input to their own data terminals D, and to the trigger terminals T of the flip-flop circuits 41 of the subsequent stages. 
     Outputs on the side &#34;1&#34; of the individual flip-flop circuits 41 serve as binary signals for the counter 4, and are counted up upon each receipt of clock signal from the clock generating circuit 7. Output on the side &#34;1&#34; of the flip-flop 41 of the second stage is sent as a clock signal CLK to the sampling circuit 8 and to a shift register 9 that will be mentioned later. Further, output of the side &#34;1&#34; of the flip-flop 41 of the final stage is sent to drivers 11 and 13 of the liquid crystal panel 2 as a signal M for switching the drive level of the liquid crystal panel 2. 
     The shift register 9 stores the data in an amount of one horizontal period, and shifts the data produced by the sampling circuit 8 according to a predetermined shift clock CLK. 
     The latch circuit 10 works as a register for temporarily storing the data in the shift register 9. When the data of an amount of one horizontal period is stored in the shift register 9, the data in the shift register 9 is set to the latch circuit 10 in parallel responsive to a horizontal synchronizing signal H.SYNC. 
     The shift register 12 is reset by a vertical synchronizing signal V.SYNC produced by the synchronizing signal separator 6, and is shifted upon each receipt of horizontal synchronizing signal H.SYNC, to drive scanning poles or electrodes 14 of the liquid crystal panel 2. 
     The drivers 11 and 13 are served with a predetermined voltage from a liquid crystal driving power source 3. 
     As is well known, the liquid crystal panel 2 has scanning poles or electrodes 14 and data poles or electrodes 15 that are arrayed in the form of a matrix. The scanning poles 14 and the data poles 15 are driven by the drivers 13 and 11. The driver 13 for the scanning poles 14 and the driver 11 for the data poles 15 so work as to apply alternating-current voltage waveforms to the liquid crystal panel 2 even when horizontal and vertical synchronizing signals have not been applied thereto. That is, as shown, for example, in FIG. 4, the driver 13 is made up of widely known transmission gates 132 to drive the scanning poles. Here, the scanning voltage consisting of four different voltage levels V 1  to V 4  is applied to the transmission gates 132 of the driver. The driver 11 for the data poles 15 is also constructed in the same manner as shown in FIG. 4, and in which the voltage consisting of four different voltage levels V 5  to V 8  is applied to the transmission gates of the driver 11. These voltage levels are switched by the voltage-level switching signals M produced by the counter 4, directly or via an inverter 131, such that a-c voltage is applied to the liquid crystal panel 2 even when the poles of the liquid crystal panel 2 have not been selected. 
     To simplify the description, here, let it be presumed that the character &#34;H&#34; is displayed on the liquid crystal panel 2 which consists of scanning poles X 1  to X 5  and data poles Y 1  to Y 4 . 
     Further, the four voltage levels V 1  to V 4  applied to the driver 13 for the scanning poles are given as follows: 
     
         V.sub.1 =V.sub.0 
    
     
         V.sub.2 =0 
    
     
         V.sub.3 =1/aV.sub.0 
    
     
         V.sub.4 =(1-1/a)V.sub.0 
    
     where 
     V 0  : reference voltage 
     a: bias ratio (usually, a=√N+1 where N is the number of scanning lines) 
     The four voltage levels V 5  to V 8  applied to the data or driver 11 for the signal poles are also given as follows: 
     
         V.sub.5 =0 
    
     
         V.sub.6 =V.sub.0 
    
     
         V.sub.7 =2/aV.sub.0 
    
     
         V.sub.8 =(1-2/a)V.sub.0 
    
     In the following description, furthermore, voltages V 9  to V 14  are given as follows: 
     
         V.sub.9 =1/aV.sub.0 
    
     
         V.sub.10 =-1/aV.sub.0 
    
     
         V.sub.11 =V.sub.0 
    
     
         V.sub.12 =-V.sub.0 
    
     
         V.sub.13 =(1-2/a)V.sub.0 
    
     
         V.sub.14 =-(1-2/a)V.sub.0 
    
     Referring to the diagram (4) of FIG. 6, the scanning poles X 1  to X 5  are successively scanned upon receipt of horizontal synchronizing signals. In the diagram (5), on the other hand, signals are input to the data poles Y 1  to Y 4  responsive to the character &#34;H&#34; set to the latch circuit 10. 
     As shown in the diagram (6), therefore, the driving waveform of the scanning poles are successively selected starting from X 1 . Namely, a voltage V 1  is applied to the selected poles, and a voltage V 4  -V 3  is applied to the poles which are not selected. 
     In the diagram (7), on the other hand, a voltage V 6  is applied to the selected poles 15, and a voltage V 7  -V 8  is applied to the poles 15 which are not selected. 
     As shown in the diagram (8), therefore, a voltage V 11  -V 12  is applied to a picture element A (coordinate X 2  -Y 1 ) consisting of liquid crystal when it is selected, and a voltage V 9  -V 10  is applied thereto when it is not selected. On the other hand, an a-c voltage V 13  -V 14  is applied to a picture element B (coordinate X 2  -Y 2 ) which is not selected. Thus, a-c rectangular pulses of a peak voltage are applied to the selected picture elements in the liquid crystal panel, and a-c rectangular pulses of a voltage smaller than the peak voltage are applied to the picture elements which are not selected. 
     Here, it should be noted that the present invention is in no way limited to the above-mentioned embodiment only, but can be modified in a variety of other ways. For instance, a one-shot multivibrator may be employed in place of the counter 4, to utilize its output as a voltage-switching signal M. 
     Further, in the above-mentioned embodiment, the shift register 12 on the scanning side does not perform the scanning when horizontal and vertical synchronizing signals are not applied. However, it is also possible to operate the shift register 12 on output signals M produced by the counter 4, so that the scanning is performed by the output signals of the counter 12 when the horizontal and vertical synchronizing signals are not input to the register.