Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-12-16
2004-03-02
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S103000, C349S041000, C349S151000
Reexamination Certificate
active
06700562
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an active matrix liquid crystal display device comprising a row and column array of liquid crystal display elements, each display element having an associated switching device, sets of row and column address conductors connected to the display elements via which selection signals and data signals, respectively, are applied to the display elements, a row drive circuit for applying selection signals to the set of row address conductors and a column drive circuit for applying data signals to the set of column address conductors, which column drive circuit is operable to transfer the data signals for the display elements of a row to groups of column address conductors in sequence, in respective group address periods, each group comprising a plurality of column address conductors.
Active matrix liquid crystal display devices, are well known, typical examples of such, and the general manner in which they operate, are described in U.S. Pat. No. 5,130,829. In these devices, display element electrodes are provided on a first substrate together with the switching devices, in the form of TFTs (thin film transistors), and sets of row and column address conductors. A second substrate carrying a transparent common electrode is arranged spaced from the first substrate and LC (liquid crystal) material is disposed between the two substrates. Each He display element electrode is connected to the drain electrode of its associated TFT. The gates of all of the TFTs in a row of display elements are connected to a respective row address conductor and the source electrodes of all the TFTs in a column of display elements are connected to a respective column address conductor. A row drive circuit connected to the set of row address conductors scans the row conductors by applying a selection (gating) signal to each row conductor in sequence to turn on the TFTs of a row of display elements, and a column drive circuit connected to the set of column conductors applies data signals to the column conductors in synchronism with the scanning of the row conductors by the row drive circuit, whereby the display elements of a selected row are charged via their respective TFTs to a level dependent on the value of the data signal on their associated column conductors to produce a required display output. The rows are driven individually in turn during respective row address periods in this manner so as to build up a display picture over one field period, and the array of display elements is repeatedly addressed in a similar manner in successive field periods.
For convenience of manufacture and compactness, the row and/or column drive circuits in some display devices, and especially those using polysilicon TFTs, have been integrated on the substrate carrying the TFTs peripherally of the display element array using the same large area electronics technology as that employed for the active matrix circuitry of the array with the circuitry of the drive circuits being fabricated simultaneously with this circuitry and similarly comprising TFTs, conductor lines, etc. Due to certain limitations in operational performance of the TFTs and the kinds of circuit possible when using TFTs, the column drive circuit is customarily provided in the form of a simple multiplexing circuit, examples of which are described in U.S. Pat. No. 4,890,101, the paper entitled “Fully Integrated Poly-Si TFT CMOS Drivers for Self-Scanned Light Valve” by Y. Nishihara et al in SID 92 Digest, pages 609-612, and in the paper entitled “A 1.8-in Poly-Si TFT—LCD for HDTV Projectors with a 5-V Fully Integrated Driver” by S. Higashi et al in SID 95 Digest, pages 81 to 84. This type of column drive circuit operates in the manner described in the opening paragraph, the operation being based on a multiplexing technique in which analogue video information (data) is sequentially transferred via multiplexing switches from a plurality of video input lines, to which video information is applied simultaneously, to corresponding groups or blocks of column address conductors in the display with each column conductor in a group being connected via a multiplexer switch to a different video input line. Each column address conductor is connected to a respective output of the circuit and typically in these circuits, the operation is such that an output associated with one column conductor becomes high impedance prior to, or while, the data signal for an adjacent column conductor is applied. During a row address (video line) period the multiplexer circuit operates to charge each group of column conductors in turn until all the column conductors in the display device have been charged to a level corresponding to the level of the video information on the input lines. Once a group of column conductors has been charged the associated multiplexing switches open and the column conductors become high impedance nodes with the voltage applied being maintained on the column conductor capacitance, and then the next group is charged. The circuit operates in this manner so as to charge all the groups in sequence and to drive each row of display elements in turn in this way during respective row address periods.
Whilst the provision of an integrated, multiplexing type, column drive circuit has benefits with regard to the simplication of fabrication of the display device, it has been found that problems can occur in the display output from the display element array during operation of the device. Particularly, certain columns in the array may show errors by virtue of the display elements in these columns having incorrect drive levels which results for example in a lack of display uniformity when displaying uniform grey fields that manifests itself as highly visible vertical lines in the displayed image.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an active matrix display device of the kind using a column drive circuit which operates in the manner of a multiplexing circuit in which the problem of the aforementioned undesirable display output artefacts is overcome or reduced at least to some extent.
According to the present invention there is provided an active matrix liquid crystal display device of the kind described in the opening paragraph which is characterised in that the column drive circuit is arranged to apply during an address period for one group a pre-charging signal to the adjacent column address conductor of the next-addressed group according to the value of a data signal for a column address conductor in that one group. Such pre-charging of a column conductor in a group at the time the preceding group is being charged is intended to reduce the size of the voltage transition on that column conductor occurring when that column conductor is charged in the subsequent group address period. As a result of this pre-charging, it has been found that the extent of the unwanted display artefacts is considerably reduced.
The invention stems from an appreciation of the reason for these display artefacts when using a multiplexing type of column drive circuit. Capacitive couplings can occur between adjacent column address conductors, either directly or indirectly. Where each column conductor extends between facing edges or sides of an adjacent pair of display element electrodes in a row, capacitance coupling between an adjacent pair of column address conductors indirectly via the electrode can be significant. Direct capacitive coupling between two column conductors can occur in the case of an alternative lay-out in which pairs of column conductors are provided adjacent one another and columns of display element electrodes are provided to either side of the pair, one column of electrodes being addressed via one of column conductors and the other addressed via the second column conductor. The presence of such indirect or direct capacitance means that as the voltage on the first column conductor of one group is changed in operation of the column drive circuit, the change in voltage can be coupled onto
Edwards Martin J.
Knapp Alan G.
Hjerpe Richard
Koninklijke Philips Electronics N.V
Tran Henry N.
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