Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
1998-07-21
2001-10-16
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S063000, C345S084000, C345S087000, C345S090000, C345S095000, C345S601000, C348S797000, C349S141000
Reexamination Certificate
active
06304254
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising two substrates, at least one of which is transparent, with
liquid crystal material between the substrates,
orientation means orienting the molecules of the liquid crystal material substantially parallel to the substrates,
electrodes on at least one of the substrates,
polarization means,
selection means for selecting at least one pixel, an electric field being applied substantially parallel to said substrates in the operating state during selection, and
drive means for supplying a signal voltage to an electrode coupled to the pixel.
Display devices of this type are used in flat-panel display devices such as television, monitors, alphanumerical displays etc.
A display device of the above-mentioned type is described in EP-A-0 667 555. This device is based on the “in plane switching effect” and switches between different electro-optical states with the aid of the electric field directed substantially parallel to the substrates and is therefore referred to as “horizontal electrical field type display”. However, this effect is much slower than the generally used (twisted nematic) TN effect so that it is not usable or hardly usable in applications where a high speed is required (video, monitors).
EP-A-0 667 555 describes how the switching speed can be increased, but the optimally realized switching times were between 25 msec and 30 msec, while a full transition within one field time, which is typically 16.7 msec at a signal frequency of 60 Hz, is desired for video applications.
OBJECTS AND SUMMARY OF THE INVENTION
It is, inter alia, an object of the present invention to provide a display device of the type described above, which switches considerably more rapidly. It is another object of the invention to provide such a display device which can be driven at video speed, possibly up to 100 Hz or more.
To this end, a display device according to the invention is characterized in that the drive means are provided with correction means which, during selection of a pixel, adapt the signal voltage to be supplied to the electrode coupled to the pixel, dependent on an external data voltage supplied during the selection and on the external data voltage supplied during the previous selection of the same pixel.
It appears that the switching speed of a pixel in a configuration as mentioned above (IPS or “in plane switching”) is enhanced by extending the range of the signal voltages to be supplied to the electrode coupled to the pixel to a considerable extent with respect to the range of external data voltages used for display purposes. In other electro-optical effects (for example, the guest-host effect or bistable effects such as SSFLC), this measure does not have any accelerating effect at all.
The correction means preferably adapt the signal voltage to be supplied to the electrode coupled to the pixel in such a way that the pixel reaches the transmission or reflection value of the pixel associated with the external data voltage before the next selection of the same pixel.
A correction in advance of externally supplied signals, based on signal values during the previous selection for a twisted nematic liquid crystal display device, is known per se from U.S. Pat. No. 5,495,265. However, this document deals with the correction of the charge across a pixel due to the capacitance change of this pixel upon a change of the voltage. Upon a transition between two extreme states, the dielectric constant changes from &egr;∥ to &egr;⊥, or conversely. At an increasing field, the dielectric constant having the highest value will gain more influence. In the configuration which is used in IPS, the dielectric constant of the substrate material (for example, glass) has a much greater influence than that of the liquid crystal material. Due to the specific structure, an at least equally large part of the electrical field lines extends through the substrates. The dielectric constant of the substrates does not change during switching. Due to the small rotation of the liquid crystal molecules, the dielectric constant of the liquid crystal material is also changed to a very small extent. The total capacitance between the electrodes of a pixel is thus substantially constant upon a change of the transmission state, so that the capacitance of the pixels in a display device based on the “in plane switching effect” does not change or hardly changes with the drive voltage (or the voltage across the pixel). The correction shown in U.S. Pat. No. 5,495,265 thus has hardly any influence in such a display device. In a display device which is based on the “in plane switching effect”, the duration of the selection and the hydrodynamical properties of the liquid crystal material define the extent of the correction to a large degree. This correction can be defined empirically (or approximated by means of a simple formula) and can be subsequently stored for one type of display device in, for example, a look-up table; alternatively, a microprocessor can be used for computing the correction.
If necessary, a change of the temperature of the liquid crystal or the ambient temperature can be included in the correction.
The invention is applicable to active matrices in which selection takes place by means of active switches such as three-pole switches (thin-film transistors) or two-pole switches, but also to Plasma Addressed Liquid Crystal displays (PALC), in which the selection takes place via plasma ducts.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
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Johnson Mark T.
Parker David W.
Kovalick Vincent E.
Shalwala Bipin
U.S. Philips Corporation
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