Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-08-20
2001-11-06
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S100000
Reexamination Certificate
active
06313817
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising 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, drive means for driving the column electrodes in conformity with an image to be displayed, and drive means for driving the row electrodes. Such display devices are used in, for example portable apparatuses such as laptop computers, notebook computers and telephones.
Passive matrix displays of this type are generally known and, to be able to realize driving of a large number of rows, they are more and more based on the (S)TN ((Super)-Twisted Nematic)) effect.
In (S)TN liquid crystal display devices, the pixels react to the effective value (rms value) of the supplied voltage. The drive of liquids (pixels) reacting in this manner is described in Alt & Pleshko's article “Scanning Limitations of Liquid Crystal Displays”, IEEE Trans. on El. Dev., Vol. ED 21, No.2, February 1974, pp. 146-155.
In these devices, one row is consecutively driven each time. When rapidly switching (S)TN liquid crystal material is used, there is relaxation of the directors within one frame period. This leads to loss of contrast and is sometimes also referred to as “frame response”.
Notably in applications in display devices built into portable apparatuses (mobile telephone, laptop computers) the aim is to drive these apparatuses with a minimal energy. It is notably attempted to minimize the drive voltages as much as possible in these cases.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a display device of the type described above in which a drive voltage which is chosen to be as favorable as possible is sufficient.
Moreover, the invention aims at a maximal “frame response” reduction.
To this end, a display device according to the invention is characterized in that the multiplexibility m of the liquid crystal material is larger than or equal to the number of row electrodes N, and that the drive means for driving the row electrodes in the operating state sequentially provide groups of p electrodes with p mutually orthogonal signals, the value of p of the number of rows driven simultaneously being an integer which is chosen to be as proximate as possible to the optimum value p
opt
={square root over (m
eff
+L )}±{square root over ((m
eff
+L −N))}, in which N<m
eff
<m.
In this application, the multiplexibility of the liquid crystal material m is understood to mean the maximum number of rows which can be driven with a maximum contrast by means of the relevant liquid crystal material, which is determined by the so-called Alt&Pleshko maximum, as described in the above-mentioned article.
The invention is based on the recognition that, when driving p rows simultaneously, the drive voltage of the rows and the maximal drive voltages of the columns can be chosen to be substantially equal to each other. Notably in drive-ICs, which supply row voltages as well as column voltages, this leads to lower power supply voltages.
Preferably, p
opt
={square root over (m)}−{square root over ((m−N))}. This yields equal row voltages and (maximally possible) column voltages and leads to the lowest supply voltage for a drive IC where the supply voltage is determined by the highest of the two voltages.
A power of two is preferably chosen for p, which is as proximate as possible to p
opt
because a set of orthogonal signals consists of a number of functions which is a power of two, and each function of this set further consists of a number of elementary pulses which is the same power of two. If fewer functions for driving are chosen than are present in the set of orthogonal functions, the elementary period of time of the pulses decreases proportionally, which is unfavorable for RC time effects across the columns and rows. Since P
opt
is not always a power of two, the voltages for the orthogonal signals are not always equal to each other. The mutual deviation remains limited to about 38%.
It is to be noted that 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 “frame response” is avoided by making use of “Active Addressing”, in which all rows are driven during the entire field 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 field period) instead of once per field period.
In an article by T. N. Ruckmongathan et al. “A New Addressing Technique for Fast Responding STN LCDs”, Japan Display 92, pp. 65-68, a group of L rows is driven with mutually orthogonal signals. Since a set of orthogonal signals, such as Walsh functions, consists of a number of functions which is a power of 2, hence 2
s
, L is preferably chosen to be as equal as possible thereto, hence generally L=2
s
, or L=2
s
−1. The orthogonal row signals F
i
(t) are preferably square-shaped and consist of the voltages +F and −F, while the row voltage is equal to zero outside the selection period. The elementary voltage pulses of which the orthogonal signals are composed, are regularly distributed in the field period. Thus, the pixels are then excited 2
s
or (2
s
−1) times per field period with regular intervals instead of once per field period. Even for low values of L, such as L=3 or L=7, it appears that the “frame response” is suppressed just as well as the driving of all rows simultaneously, as in “Active Addressing”, but much less electronic hardware is required for this purpose. However, neither of the two articles states how drive voltages can be optimized.
REFERENCES:
patent: 5420604 (1995-05-01), Scheffer et al.
patent: 5485173 (1996-01-01), Scheffer et al.
patent: 5546102 (1996-08-01), Scheffer et al.
patent: 54-22856 (1979-08-01), None
patent: 08179731-A (1996-07-01), None
T.N. Ruckmongathan and N.V. Mahusudana, New Addressing Techniques for Multiplexed Liquid Crystal Displays, Proceedings of the SID, vol. 24/3, 1983.*
Jurgen Nehring and Allan R. Kmetz, Ultimate Limits for Matrix Addressing of RMS-Responding Liquid Crystal Displays, IEEE Transactions on Electronic Devices, vol. ED-26, No. 5, May 1979.*
“Scanning Limitations of Liquid Crystal Displays”, in IEEE Trans. on El. Dev., vol. ED 21, No. 2, Feb., 1974, pp. 146-155.
“Active Addressing Method for High-Contrast Video-Rate STN Displays” by T.J. Scheffer and B. Clifton, in SID Digest 92, pp. 228-231.
“A New Addressing Technique for Fast Responding STN LCDs” by T.N. Ruckmongathan et al., in Japan Display '92, pp. 65-68.
Eisen Alexander
Hjerpe Richard
U.S. Philips Corporation
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