CORRECTION CIRCUIT TO COMPENSATE FOR PARAMETER CHANGES IN AN ACTIVE MATRIX DISPLAY

Abstract: Correction circuit in which the influence of drive signals, particularly column signals on feedback lines (39) signalling changes in elements (notably ageing of switches (35)) is eliminated. To this end, a fraction of the incoming (video) signal (42) is mixed with the signal from the feedback line (39).

Description: CORRECTION CIRCUIT TO COMPENSATE FOR PARAMETER CHANGES IN AN ACTIVE MATRIX DISPLAY. The invention relates to a display device comprising a matrix of pixels arranged in rows and columns, a group of row electrodes and a group of column electrodes for driving the pixels, each pixel comprising an electro-optical display element which is connected to a row electrode or a column electrode via a switching element, drive means for presenting row selection signals to the row electrodes and drive means for presenting data signals to the column electrodes, the display device comprising compensation means for compensating row selection signals. Such display devices are used, for example in monitors but also, for example in devices for video applications. A display device of the kind mentioned above is described in EP-A-O 523 797 (PHB 33.732). In the devices shown in these documents, variations of the behaviour of MIMs, for example due to ageing are detected by means of a sensor circuit. The sensor circuit comprises one or more capacitors which are constituted, for example by the capacitance of additionally provided pixels which consist of liquid crystal pixels in the relevant example. Dependent on the difference between a voltage across the capacitor (which may be an average voltage over a given period of time, for example approximately a picture selection time) and a reference voltage, the row selection signals are compensated for, for example ageing or temperature influences. The reference voltage is then preferably set at a value associated with mid-grey. To inhibit degradation of the liquid crystal material, the sign of the drive voltages across a liquid crystal display element is periodically changed, for example each line selection period. In the device mentioned above the problem presents itself that for the relevant type of liquid crystalline material for the different fields the average value of the (data) voltages associated with different luminances on a column electrode are not equal to each other and also differ from the value associated with mid-grey. Since the voltages on the column electrodes intended for picture display influence the voltage across the capacitor of the sensor circuit via capacitive crosstalk, this reference voltage is also too high and too low and the compensation of the selection voltages is set too high or too low. This does not only lead to image flicker but also to a reduced contrast and image retention. It is, inter alia an object of the invention to obviate the above-mentioned problem. To this end, a display device according to the invention is characterized in that the compensation means are provided with correction means which correct the extent of compensation, dependent on presented data signals. In a field in which signals associated with black (white) are presented and the reference voltage could become too low (or too high) due to capacitive crosstalk, said correction means provide such an adaptation of the selection voltages that possible deviations due to capacitive crosstalk on the capacitor of the sensor circuit are eliminated. Said correction means may be implemented by means of a low-frequency filter which averages, for example the incoming signals over an image selection period (and inverts them in the example mentioned) and an attenuator for the filtered signal. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows diagrammatically a display device according to the invention, Fig. 2 shows some typical values of column voltages as are used in such device, while Fig. 3 shows diagrammatically the way of correction as is used in a device as shown in EP-A-0 523 797, and Fig. 4 shows diagrammatically the way of correction as is used in a device according to the invention. Fig. 1 shows a display device for displaying information, for example video images, comprising an active matrix or liquid crystal display panel 10 consisting of m rows (1 to m) and n pixels (1 to n) in each row. In this example, each pixel 12 consists of a twisted nematic liquid crystal display element 14 which is electrically connected in series with a non-linear two-pole switching element 15 (in this example a metal-isolator-metal element or MIM, or metal-semiconductor-metal element or MSM) which functions as a switching element between a row or selection conductor 16 and a column or data conductor 17. The pixels 12 are driven via the row and column conductors which are arranged on opposite, facing sides of two substrates (not shown) of, for example glass on which also the facing electrodes of the pixels are provided. The switching elements 15 are arranged on the same substrate as the group of row electrodes or row conductors 16. The row conductors 16 are successively selected by means of a row-drive circuit 20, while simultaneously data signals are presented to the column conductors or column electrodes via a column-drive circuit 22. In their turn, these drive circuits are controlled by a control circuit 25 in which also the timing is controlled. Timing signals and signals determining the voltages of the selection signals are applied from the control circuit 25 to the row drive circuit 20 via lines 26 and 27, respectively. Signals determining the voltages of the data signals (video signals) are applied from the control circuit 25 to the column drive circuit 22 via one or more lines 28, while timing signals are applied in synchronization with those for the row selection via one or more lines 29. In this embodiment the two-pole switching elements are constituted by MIMs. However, other non-linear two-pole switching elements may alternatively be used such as, for example diode rings, back-to-back diodes or other diode structures. The device of Fig. 1 also includes a reference circuit 34 which comprises a MIM 35 arranged in series with a capacitor 36. By selecting a pixel for the capacitor 36 and by manufacturing the MIM 35 simultaneously with the other MIMs 15, the reference circuit 34 behaves in substantially the same way as the pixels with associated switching elements. In this way the reference circuit 34 is suitable for detecting variations of, for example the voltage across the MIM due to temperature changes or ageing. The MIM 35 is connected to an extra (m+ l)th row conductor 16' which is selected in the same way as the rows 1-m by means of similar signals after selection of the mth row. As described in EP-0 523 797, an electric voltage is presented to the terminal of the capacitor not connected to the MIM during selection of the capacitor 36 or a corresponding pixel. The voltage difference across the capacitor is compared with a reference voltage and, dependent on possible voltage differences, selection voltages are corrected or not corrected. In the present example, a given voltage is presented by means of the column drive circuit 22 via, for example column electrode 1 during selection of the extra (m+l)th row conductor 16'. The presented voltage is, for example the voltage associated with mid- grey. The voltage at the capacitor terminal connected to the MIM is then presented via the line 39 to a control circuit which forms part of the control circuit 25. This voltage (every time at the same voltage at the column electrode (mid-grey)) is directly related to the voltage difference across the capacitor. Similarly as the voltage difference across the capacitor is compared with a reference voltage so as to record changes in the behaviour of the MIM, the voltage on the line 39 may be compared, for example with a voltage V β which is associated with the presented value on the column electrode. This is diagrammatically shown in Fig. 3 in which a signal of the line 39 is buffered first, if necessary, in a buffer circuit 40 so as to be subsequently compared with the desired voltage V a in the operational amplifier circuit 41. Dependent on the extent to which this signal deviates from V B , a correction signal is supplied which corrects the row selection signals V i+ , V„. As is shown by way of the broken lines in Fig. 1, a plurality of capacitors (extra pixels) can be selected during selection of the (m+ l)th row. Approximately the average is then detected on the line 39. Notably, but not exclusively, if the capacitor terminal not connected to the MIM (or the display cell) is connected to a column electrode, the signal generated across the capacitor may deviate from said value due to crosstalk of other column electrodes. This will be elucidated with reference to Fig. 2 in which the column voltages are shown for a typical liquid crystal pixel. In most drive schemes, the sign of the voltage across the pixels is regularly changed, for example each frame period. A positive and a negative signal will hereinafter be referred to. A complication is that the voltage swing to be used is not the same for both signs; for example, Fig. 2a shows that the column voltages vary between 3 V (black) and 7 V (white) for the relevant pixel in the positive field, whereas they vary between 3.6 V (white) and 6.4 V (black) in the negative field, while the column voltage for mid-grey is 5 V for both fields. If, for example one or more of the pixels of column 1 is driven to be completely black (Fig. 2b), a column voltage of 3 V is presented in the positive field and a column voltage of 6.4 V is presented in the negative field. The average voltage on the column electrode is thus 4.7 V, i.e. 0.3 V too low. However, if on the other hand one or more of the pixels of column 1 is driven to be completely white (Fig. 2c) a column voltage of 7 V is alternately presented in the positive field and a column voltage of 3.6 V is presented in the negative field. The average voltage on the column electrode is thus 4.7 V, i.e. 0.3 V too high. An analog reasoning applies for intermediate grey tints. These too high or too low values of the average column voltage cause capacitive crosstalk on the capacitor terminal (or display cell) not connected to the MIM of the reference circuit and influence a satisfactory feedback via the circuit shown in Fig. 3. Once set, a column electrode value corresponding to mid-grey for obtaining minimum flicker for this setting is now disturbed by crosstalk in the reference circuit. According to the invention, to avoid this, the selection voltage is corrected in advance, dependent on the crosstalk of the signal from the reference circuit 34 on the line 39. After having been first inverted in this example, the incoming (video) signal 42 is mixed via a low-pass filter 43 and an attenuator 44 with the signal 39 from the buffer circuit 40. In this example, the signals are added in a summing circuit 45. Since the voltage in the positive field is too high, a -fraction of the inverted positive signal is added thereto so that the voltage on line 46 is decreased. Since the voltage in the negative field is too low, a fraction of the inverted negative signal is added thereto so that the voltage on line 46 is increased. By a suitable choice of the attenuation factor of the attenuator 44 and of the filter 43, the signal 39 is corrected in such a way that the influence of crosstalk via the columns is negligible. Although the (video) signal is inverted, materials and/or drive methods are feasible in which the mid-grey level for the two fields is exactly at the other side of the average voltage on the column; in that case the inversion is not necessary. In summary, the invention provides a correction circuit in which the influence of drive signals, particularly column signals on feedback lines signalling changes in elements (notably ageing of switches) is eliminated. To this end, a fraction of the incoming (video) signal is mixed with the signal from the feedback line.

Claims: Claims: 1. A display device comprising a matrix of pixels arranged in rows and columns, a group of row electrodes and a group of column electrodes for driving the pixels, each pixel comprising an electro-optical display element which is connected to a row electrode or a column electrode via a switching element, drive means for presenting row selection signals to the row electrodes and drive means for presenting data signals to the column electrodes, the display device comprising compensation means for compensating row selection signals, characterized in that the compensation means are provided with correction means which correct the extent of compensation, dependent on presented data signals. 2. A display device as claimed in Claim 1, characterized in that the switching elements comprise non-linear two-pole switching elements. 3. A display device as claimed in Claim 1 or 2, characterized in that the compensation means for compensating drive signals comprise scanning means for detecting property changes of the switching elements. 4. A display device as claimed in Claim 1, 2 or 3, characterized in that the correction means process the signal from the compensation means and incoming data signals to corrected drive signals. 5. A display device as claimed in Claim 4, characterized in that the correction means comprise a low-pass filter and an attenuator for the incoming data signal.

Inventors: VAN GORP HENRICUS THEODORUS FR, VAN MOURIK JOHANNES GERARDUS R, VAN GORP, HENRICUS, THEODORUS, FRANSISCUS, VAN MOURIK, JOHANNES, GERARDUS, RIJK
Applicants: FLAT PANEL DISPLAY CO FPD BV, FLAT PANEL DISPLAY CO. (FPD) B.V.
Application year: 1995
Main classification: G09G3
IPC codes: G02F1, G09G3