Liquid-crystal control circuit display device with selection...

Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix

Utility Patent

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C345S097000, C345S208000, C349S133000, C349S172000

Utility Patent

active

06169531

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a display device comprising a number of display elements, which are arranged in rows and columns in accordance with a matrix, and comprising, between a first substrate and a second substrate, a liquid-crystal material which belongs to the group of smectic liquid-crystal materials, which include ferroelectric liquid-crystal material with deformable helix, ferroelectric liquid-crystal material with twisted smectic structure, monostable ferroelectric liquid-crystal material, electroclinic smectic A liquid-crystal material and anti-ferroelectric liquid-crystal material, and comprising a group of row electrodes and a group of column electrodes, each pixel including, on at least one substrate, a picture electrode which is connected to a column electrode or row electrode via an active switching element, and said display device comprising means for applying selection voltages to the row electrodes and data voltages to the column electrodes.
Such display devices can be used as video displays, but also, for example, in datagraphic monitors or as view finders.
In general, a ferroelectric liquid-crystal material with a deformed helix is to be understood to mean a ferroelectric liquid-crystal material with a natural helix whose pitch is smaller than the wavelength of visible light (up to approximately 400 nm). An electric field which extends perpendicularly to the axis of the helix causes deformation of said helix, which results in rotation of the optical axis. As a result, between crossed polarizers, one of which extends parallel to the axis of the helix, the transmission increases for both positive and negative fields.
Just like the other materials mentioned hereinabove, the ferroelectric liquid-crystal material with a deformed helix exhibits a high degree of polarization in the fully driven state. If they are provided between a polarizer and an analyzer, the above-mentioned materials have another characteristic in common, namely that they can switch between substantially transparent and substantially opaque if the polarizer and the analyzer are at a specific angle with respect to each other, while intermediate levels of transparency (grey levels) can also be realized.
A display device of the type mentioned hereinabove is described in “A Full-Color DHF-AMLCD with Wide Viewing Angle” in SID 94 DIGEST pp. 430-433. According to said document, the use of devices with DHFLC material (Deformed Helix Ferroelectric Liquid Crystal) is advantageous as compared to so-called SSFLC devices (Surface Stabilized Ferroelectric Liquid Crystal) due to the absence of multidomains, while a more continuous change of the transmission voltage characteristic enables a better realization of grey levels to be achieved. Despite the high switching rate of the mixture used in the display device, the field frequency remains too low for video applications (NTSC or PAL), which is in complete opposition to the expectations based on the high switching rate of the DHFLC material. This finds its explanation in the high value of the spontaneous polarization of these materials. The customary pulse duration of drive pulses (which, in practice, is comparable to a pulse duration of the order of, for example, half the row-selection time of the drive system, which amounts, for example, in TV systems to 64 &mgr;sec.) is too short to supply the polarization current. Transient phenomena occur which may extend over a plurality of frame or field times.
In the device described in the above-mentioned document, also “image sticking” (image retention) occurs. The other above-mentioned liquid-crystal effects with a high degree of polarization exhibit the same problems, while the degree of polarization increases as the electric field increases. In general, the voltage/transmission curve for positive and negative voltages is symmetrical. Examples of such effects are described in
J. S. Patel: “Ferroelectric Liquid-crystal Modulator using Twisted Smectic Structure”, Appl. Phys. Lett. Vol. 60(3) pp. 280-282 (1992)
H. Okado et al: “New Display Mode of Ferroelectric Liquid Crystals with Large Tilt Angle”, Ferroelectrics Vol. 149, 171-181 (1993),
D. M. Walba et al: “High Performance Electroclinic Materials”, Ferroelectrics Vol. 148, 435-442 (1993).
Yet another example is the anti-ferroelectric liquid-crystal effect, as described, for example, in Asia Display '95 pp. 61-64.
It is one of the objects of the invention to provide, inter alia, a display device of the type mentioned in the opening paragraph, which can operate at field frequencies in excess of 20 Hz (for example 50 Hz (PAL)).
Another object of the invention is to provide a device in which little, if any, “after image” occurs.
To this end, a display device in accordance with the invention is characterized in that the display device comprises a control circuit which, during a first part of a line-selection time, provides a first number of successive row electrodes with a selection signal for the purpose of reset and which, during a corresponding part of a subsequent line-selection time, provides a second number of successive row electrodes, yet shifted by at least one line position, with a selection signal for the purpose of reset and which, during another part of a line-selection time, provides one or two row electrodes with a selection signal for the purpose of writing data and which provides the column electrodes with data signals.
In this context, “successive row electrodes” can also be understood to mean a number of row electrodes at one extreme edge (for example the last row electrodes) together with a number of row electrodes at the other extreme edge (for example the first row electrodes). In specific applications (double row drive), the even fields and the odd fields, for example, of a television picture are partly interlaced. The type of application determines whether selection offset by one or two line positions takes place and whether, during other parts of a line-selection time, one or two row electrodes are provided with a selection signal for writing data.
The invention is based on the recognition that, unlike known (ferroelectric) liquid-crystal display devices, upon application of a voltage across a pixel, the spontaneous polarization of DHFLC materials (and the other materials mentioned hereinabove) plays such an important part that either it requires such a long time that the display device as a whole becomes too slow or the pixel does not always receive the desired charge and the associated transmission value. In the above-mentioned article, it is proposed to bring a row of display elements to an extreme optical transmission state by means of an auxiliary voltage (reset), prior to selection, but also in this case the pixel does not always receive the desired charge owing to the great importance of the polarization, so that incomplete reset occurs. As the charge (and hence the transmission value) across the pixel is undefined again after this reset, the data signal provided in a subsequent selection process will not lead to the intended final value of the charge (and hence of the transmission value) across the pixel, etc. Even in the case of an identical grey level of the pixel to be written over a period covering a plurality of field times, it may take a number of field times to eliminate this “memory effect”.
In a display device in accordance with the invention, the “memory effect” is eliminated at least substantially by providing the pixels in a first number of successive rows with a reset signal during a first part of the line-selection time, repeating this in subsequent line-selection times for a second number of successive rows, yet shifted by at least one line position, and providing the column electrodes with data signals during every second part of the line-selection time. In this case, the joint effect of the reset signals presented during the first parts of the number of successive line-selection times is sufficient, if the numbers are suitably chosen, to achieve a complete reset.
Preferably, resetting c

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