GB2096815A - Liquid crystal picture display device - Google Patents

Abstract

A liquid crystal picture display device comprises a plurality of rows of picture elements each including an FET (1) formed on a semiconductor substrate. A first electrode (XI) is connected to each row of field effect transistors. A second transparent electrode (25) is deposited on a transparent substrate with a liquid crystal material between the substrates in contact with the first and second electrodes. A detecting circuit (20) detects the existence of a picture signal, (a) and a flip-flop (22) inverts the polarity of the video signal (d) and the voltage (c) applied to common electrode (25) if the existence of the picture signal is not detected for a given period of time (T0) such as a field period. Re-inversion takes place at the next interruption, giving the advantages of a.c. working in prolongation of cell life. <IMAGE>

Description

SPECIFICATION Liquid crystal picture display device This invention relates to liquid crystal picture display devices.

According to the present invention there is provided a liquid crystal picture display device comprising: a plurality of rows of picture elements each including a field effect transistor formed on a semiconductor substrate; a first electrode connected to each row of field effect transistors; a second transparent electrode deposited on a transparent substrate; a liquid crystal material disposed between the semiconductor substrate and the transparent substrate in contact with the first and second electrode; detecting means for detecting the existence of a picture signal; and inverting means for inverting the polarity of the voltage applied to the liquid crystal material if the existence of the picture signal is not detected for a given period of time.

In one embodiment the picture display device includes means for detecting the existence of a horizontal or vertical synchronous signal of a video signal in order to detect the existence of the picture signal.

In another embodiment the picture display device includes means for comparing an average voltage level of the picture signal with a reference voltage to detect the existence of the picture signal.

The invention is illustrated, merely by way of Example, in the accompanying drawings, in which: Figure 1 is a circuit diagram of a conventional liquid crystal picture display device; Figure 2 is a sectional view of a picture element of the conventional picture display device of Figure 1; Figure 3 is a block diagram of a liquid crystal picture display device according to the present invention; Figure 4 is a chart showing the waveform of various signals appearing in the picture display device of Figure 3; Figure 5 is a block diagram of a video signal detecting circuit of the picture display device of Figure 3; Figure 6 is a chart showing the waveform of various signals appearing in the detecting circuit of Figure 5; Figure 7 is a block diagram of another embodiment of the video signal detecting circuit of Figure 3; and Figure 8 is a chart showing the waveform of various signals appearing in the detecting circuit of Figure 7.

Figure 1 illustrates a conventional liquid crystal picture display device designed to display a picture from analog signals. This conventional picture display device comprises a plurality of rows of picture elements each consisting of a Pchannel MOSFET 1, a storage capacitor 2 and a liquid crystal cell 3. The fundamental operation of each picture element is as follows. When a negative voltage pulse is applied to a gate linex, as a gate signal, the MOSFET 1 becomes conductive and an analog video signal applied to a signal line y charges the storage capacitor 2 through the MOSFET 1. The MOSFET 1 is rendered non-conductive if the negative voltage pulse is removed.Since leak current through the MOSFET 1 and the liquid crystal cell 3 are quite small, a voltage proportional to the analog video signal and charged on the capacitor 2, is maintained on the liquid crystal cell 3 for a considerable period of time. Gate lines xj, xj+ are scanned in line sequence and the video signals corresponding to the gate lines are simultaneously applied to the signal lines Y Y+1 . so that an overall picture is produced.

Figure 2 is a sectional view of a picture element of the conventional picture display device of Figure 1. The MOSFET 1 has an n type silicon substrate 4, a source 5 and a drain 6 in a diffusion region, the source 5 being connected to one of the signal lines yi, y,,, Yj+ .... Figure 2 also shows a gate oxide layer, a gate electrode 8 connected to one of the gate lines xj, xj+ . . . and a picture electrode 9 of the liquid crystal cell 3. The electrode 9 together with a thin oxide film 11 and the substrate 4 form the capacitor 2. Liquid crystal material 12 is sandwiched between the electrode 9 and a transparent counter electrode 1 3 which serves as a common electrode for all the picture elements.The counter electrode 13 is formed on the surface of a glass substrate 14.

Reference numeral 10 denotes an insulating film.

The voltage polarity applied to the liquid crystal material is unidirectional so that the picture display device is driven by direct current.

Therefore, the liquid crystal material is of relatively short life and the picture display device has relatively poor reliability.

As an alternative method of driving a picture display device, it has been proposed to invert the video signal in synchronism with the signal applied to the counter electrode at a given frequency, thus driving the picture display device with alternating current. If the video signal is inverted at a frequency of 30 Hz, half the picture is displayed as a negative. However, if the video signal is inverted at a period of between 1 Hz and 10 Hz to avoid the negative display being produced, the picture is seen to flicker.

Figure 3 illustrates a liquid crystal picture display device according to the present invention having a gate line driving circuit 1 5 including a shift register (not shown), a clock signal input terminal 1 6 and a serial input terminal 17. A video signal inverting circuit 1 8 receives a video signal a from an input terminal 19. A video signal detecting circuit 20 also receives the video signal a from the terminal 1 9 and has a clock input terminal 21. A flip-flop circuit 22 receives a pulse signal b from the detecting circuit 20 and produces a control signal c which is applied to the inverting circuit 18. A signal dfrom the inverting circuit is fed to a drive circuit 23 which applies -video signals to the signal lines after sampling.

The drive circuit has a clock signal input terminal 24. The control signal c is also applied to a common electrode terminal 25. The construction of each picture element is the same as that described in relation to Figure 1 and so will not be discussed further. Figure 4 shows the waveforms of the signals a to d.

Initially the video signal a is of positive polarity and is applied to the terminal 1 9. When the control signal c is level 0, the video signal is applied to the drive circuit 23 as the signal d.

When the video signal is not applied to the drive circuit for some reason at a given instant, the pulse signal b is produced by the detecting circuit 20 to invert the flip-flop 22. Then the control signal c is inverted and the signal d becomes the video signal but with negative polarity.

Simultaneously the voltage polarity at the common electrode terminal 25 is inverted. Thus the voltage polarity applied across the liquid crystal material is inverted. Subsequently the polarity is inverted whenever the video signal a is interrupted and the life of the liquid crystal material is substantially prolonged. Moreover, since the inverting operation is carried out when the video signal is interrupted, flickering of the picture caused by the inversion is negligible.

Because the polarity can be inverted by an instantaneous interruption of the video signal, the inverting operation may be carried out even if the picture is scarcely disturbed. Therefore, it is desirable that the inverting operation is carried out when the video signal is interrupted for a given time period To e.g. more than one field (about 16 msec).

Figure 5 shows the video signal detecting circuit 20 in greater detail. When the video signal a is fed to an input terminal 27 of a synchronous separator 26, a synchronous signal e is produced.

The signal e is shown in Figure 6, the period T denoting the time for one scanning line. The signal e is fed to a counter circuit 28 as a reset signal. The counter circuit 28 counts the number of pulses of a pulse signal ffrom a clock input terminal 29, and if the reset signal is not produced in a period of 1 6 msec, the pulse signal b is produced from the counter circuit 28. While Figure 6 shows that the existence of the video signal is detected from a horizontal synchronous signal with the video signal, it will be appreciated that the existence of the video signal can also be detected from a vertical synchronous signal.

Figure 7 shows another embodiment of the video signal detecting circuit 20. When the video signal a is fed to an input terminal 31 of an integration circuit 30, an output signal g is produced representing the average voltage level of the video signal, and a voltage Vs is produced when the video signal is present and a voltage VN is produced when the video signal is interrupted.

The output from the integration circuit 30 is applied to a comparator 32 which receives a reference voltage V0 which is between the voltage Vs and the voltage VN via an input terminal 33. An output signal ifrom the comparator 32 is at level 1 when the video signal is present and at level 0 when it is interrupted.

The output signal ifunctions as a reset signal of a counter 34. By applying the clock signal fto the input terminal 35 of the counter 34 the pulse signal b is produced when the video signal is interrupted for the period T0.

While the inverting operation is carried out when the video signal is interrupted for a period of one field in this embodiment, it is to be noted that the same effect may be obtained by carrying out the inverting operation when the video signal is interrupted for a period of one frame (about 32 msec), one second etc.

The present invention is applicable to TN-type liquid crystal picture display devices, DSM liquid crystal picture display devices etc. The inverting period is comparatively long since it is carried out when the video signal is interrupted, for example when channels are switched or by a disorder of the received electric wave. Therefore, the present invention is particularly effective when applied to picture display devices with guest-host type liquid crystal material whose life can be prolonged by inversion of a comparatively long period. It has been found that the liquid crystal material whose life is 5,000 hours when driven by direct current has a life increased to more than 30,000 hours, i.e. more than six times as long, when driven by the inversion of the present invention.

The picture display device according to the present invention and described above inverts the video signal when it is interrupted for a given period of time by channel switching or a disorder of the received electric wave. Accordingly, disorder or flickering of the picture with the inverting operation is not noticeable and flickering of the picture caused by the inverting operation is substantially eliminated. Moreover, it has been confirmed from experiment that the life of guesthost liquid crystal material is prolonged by change, though irregular, of the polarity of the signal applied to the liquid crystal material and that high reliability is achieved.

Claims (5)

Claims

1. A liquid crystal picture display device comprising: a plurality of rows of picture elements each including a field effect transistor formed on a semiconductor substrate; a first electrode connected to each row of field effect transistors; a second transparent electrode deposited on a transparent substrate; a liquid crystal material disposed between the semiconductor substrate and the transparent substrate in contact with the first and second electrodes; detecting means for detecting the existence of a picture signal; and inverting means for inverting the polarity of the voltage applied to the liquid crystal material if the existence of the picture signal is not detected for a given period of time.

2. A picture display device as claimed in claim 1 including means for detecting the existence of a horizontal or vertical synchronous signal of a video signal in order to detect the existence of the picture signal.

3. A picture display device as claimed in claim 1 including means for comparing an average voltage level of the picture signal with a reference voltage to detect the existence of the picture signal.

4. A liquid crystal picture display device substantially as herein described with reference to and as shown in Figures 3 to 7 of the accompanying drawings.

5. In a picture display device comprising: electric field effect transistors deposited on a semiconductor layer formed on an insulating substrate such as glass or on a semiconductor substrate in line and row configuration; a first electrode which is one electrode of said electric field effect transistor; a second electrode which is a transparent electrode deposited on a transparent substrate opposite to said first electrode; and a liquid crystal held between the first electrode and the second electrode, the improvement comprising: a means for detecting the existence of picture signals and a means for inverting voltage polarity applied to the liquid crystal held between the first electrode and the second electrode if the picture signals are not applied for more than a given period of time.