1. Field of the Invention
The invention relates to a display device employing a liquid crystal material between a first substrate provided with row or selection electrodes and a second substrate provided with column or data electrodes, in which overlapping parts of the row and column electrodes define pixels. The display device further employs drivers for driving the column electrodes in conformity with an image to be displayed, and for driving the row electrodes which, in the operating condition, sequentially supply groups of p row electrodes with p mutually orthogonal signals. Such display devices are used in, for example, portable apparatuses such as laptop computers, notebook computers and telephones.
2. Description of the Related Art
Passive-matrix displays of this type are generally known and, for realizing a high number of lines, they are increasingly based on the STN (Super-Twisted Nematic) effect. An article by T.J. Scheffer and B. Clifton “Active Addressing Method for High-Contrast Video Rate STN Displays”, SID Digest 92, pp. 228-231 describes how the phenomenon of “frame response” which occurs with rapidly switching liquid crystal materials is avoided by making use of “Active Addressing”. In this method, all rows are driven throughout the frame period with mutually orthogonal signals, for example, Walsh functions. The result is that each pixel is continuously excited by pulses (in an STN LCD of 240 rows: 256 times per frame period) instead of once per frame period. In “multiple row addressing”, a (sub-)group of p rows is driven with mutually orthogonal signals. Since a set of orthogonal signals, such as Walsh functions, consists of a plurality of functions which is a power of 2, i.e. 2S, p is preferably chosen to be equal thereto as much as possible, i.e. generally p=2s (or also p=2s−l). The orthogonal row signals Fi(t) are preferably square-wave shaped and consist of voltages +F and −F, while the row voltage is equal to zero outside the selection period. The elementary voltage pulses from which the orthogonal signals are built up are regularly distributed across the frame period. In this way, the pixels are then excited 2s (or (2s−1)) times per frame period with regular intermissions instead of once per frame period. Even for low values of p such as p=3 (or 4) or p=7 (or 8) the frame response appears to be suppressed just as satisfactorily as when driving all rows simultaneously, such as in “Active Addressing”, but it requires much less electronic hardware.
However, it appears that, notably for Walsh functions, the frequency content of the functions from a complete set of functions is greatly different. Since the dielectric constant of liquid crystalline material is frequency-dependent, this may cause the liquid crystalline material to react differently at different positions in, for example, a matrix display, dependent on the image contents. This leads to artefacts in the image such as different forms of crosstalk.
It is an object of the invention to provide a display device in which a minimal number of artefacts occurs in the image.
To this end, a display device according to the invention is characterized in that the mutually orthogonal signals are obtained from at least two types of orthogonal functions having four elementary units of time, within which four elementary units of time one pulse each time has a polarity which is different from that of the other pulses.
It is found that orthogonal signals can thereby be generated which differ little in frequency content and thus do not give rise or hardly give rise to artefacts in the image. Such orthogonal signals are obtained, for example, from orthogonal functions having four elementary units of time, within which four elementary units of time the pulse having a polarity which differs from that of the other pulses each time shifts by one elementary unit of time. The use of four elementary units of time has the additional advantage that the number of column voltage levels remains limited to five, while this number is six when using, for example, three elementary units of time, within which three elementary units of time one pulse having a polarity which differs from that of the other pulses shifts by only one unit of time. A larger number of column voltage levels to be used of course leads to more expensive drive electronics.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
FIG. 8. shows mixed orthogonal signals according to the invention, derived from
The device further comprises a row function generator 7 in the form of, for example, a ROM for generating orthogonal signals Fi(t) for driving the rows 2. Similarly as described in said article by Scheffer and Clifton, row vectors driving a group ofp rows via drive circuits 8 are defined during each elementary time interval. The row vectors are written into a row function register 9.
Information 10 to be displayed is stored in a pxM buffer memory 11 and read as information vectors per elementary unit of time. Signals for the column electrodes 3 are obtained by multiplying the then valid values of the row vector and the information vector during each elementary unit of time and by subsequently adding the p obtained products. The multiplication of the values which are valid during an elementary unit of time of the row and column vectors is realized by comparing them in an array 12 of M exclusive ORs. The addition of the products is effected by applying the outputs of the array of exclusive ORs to the summing logic 13. The signals 16 from the summing logic 13 drive a column drive circuit 14 which provides the columns 3 with voltages Gj(i) having p+1 possible voltage levels. Every time, p rows are driven simultaneously, in which p<N (“multiple row addressing”). The row vectors therefore only have p elements, as well as the information vectors, which results in a saving of the required hardware such as the number of exclusive ORs and the size of the summing circuit, as compared with the method in which all rows are driven simultaneously with mutually orthogonal signals (“Active Addressing”).
It is possible to use less drive electronics by choosing p to be low, for example, in the range between 3 and 8.
−2An.Ap+(p−2).Ap2=0; which yields An=Ap.(p−2) /2 (1)
In addition, the effective value of the function must be 1 (normalized for the function F). This leads to
It follows from (1) and (2) for Ap and An that Ap=2/√{square root over (p)} and
respectively.
For p=4 it holds that Ap=An=1 and the number of possible column voltages is 5. This is higher for other values; for p=3, the number of possible column voltages is 6, namely (−5/2)Ap, (−3/2)Ap, (−1/2)Ap, (1/2)Ap, (3/2)Ap en (5/2)Ap.
However, when using Walsh functions, the number of required column voltage levels would be 4 for p=3 (a subject chosen from a set of 4 Walsh functions).
The invention is based on the recognition that orthogonal functions are selected as starting points based on mutually orthogonal signals obtained from at least two types of orthogonal functions with four elementary units of time, as is shown in FIG. 4. Starting from the functions of
The pulse patterns derived from (1), (2), (3) and (4) comprise a DC component. To reduce its influence, preferably 2 of these pulse patterns in a set to be chosen are inverted (the DC content is now opposed). For a completely DC-free drive, all signals from the used set are inverted after each frame period.
This set is denoted as K8(5,r) (Kuijk function) because in the fifth (5,*) pattern, the negative pulse starts in the second half period with a negative pulse (at the fifth position) which shifts to the right (5,r) in the subsequent patterns.
The set of K(uijk) functions can be further extended by mixing, as it were, the two types of orthogonal functions shown in
The invention is of course not limited to the embodiments shown. Similarly as described above, more than 2 functions of
The protective scope of the invention is not limited to the embodiments described. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope. The use of the verb “to comprise” and its conjugations does not exclude the presence of elements other than those stated in the claims. The use of the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.
Number | Date | Country | Kind |
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00200508 | Feb 2000 | EP | regional |
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Number | Date | Country | |
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20010020926 A1 | Sep 2001 | US |