Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-08-17
2001-11-20
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000, C345S088000
Reexamination Certificate
active
06320565
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to color display systems which employ one or more electro-optic display devices such as liquid crystal display devices, and more particularly relates to DAC driver circuitry employed in such a system to convert incoming digital display signals to analog signals, and to address the display device with such analog signals.
Color display systems are known in which light bars of different colors are sequentially scrolled across a single electro-optic light modulator panel to produce a color display. See, for example, commonly assigned U.S. Pat. No. 5,532,763, incorporated herein by reference.
These display systems are particularly suitable for displaying color information in the form of continuously updated image information signals arranged in successive frames, such as color video information, in which each frame is composed of component color sub-frames, eg., red, green and blue sub-frames.
These systems employ an electro-optic light modulator comprised of a row-and-column matrix array of pixels, for modulating light in accordance with the image information signals during successive frame periods. The signal information is applied to the pixel rows of the array a line at a time during each frame period. Such a matrix array is preferably “active”, ie., each pixel is connected to the array via an active switching element, in order to prevent cross-talk between pixels.
Because each sub-frame of information in a three color system must be addressed on a single electro-optic light modulator panel during one frame period, the addressing rate is three times faster than the rate for a three-panel system. At present, a reflective active-matrix liquid crystal display (AMLCD) on silicon employing a liquid crystal exhibiting the twisted nematic (TN) effect and transistors as the active switching elements, is the preferred electro-optic light modulator. Such a reflective device can support a high density of pixels, due to the fact that the wiring and switches can be integrated on the silicon substrate beneath the reflective electrodes. Moreover, such a device can be addressed at a much higher rate than prior transmissive devices.
Nevertheless, the fastest response time of the TN LC material, the time needed to drive the LC material to the dark state, is on the order of 100 microseconds, while the time for addressing a row is only 5 microseconds. Moreover, the pixel capacitance varies with both the applied voltage and time.
As a result, the accuracy of the display data transfer depends on the residual state of the previously stored information. This means that the brightness of a particular color depends on the brightness of the previous color. Two-dimensional look-up tables are presently used to provide correction values to the new signals, in order to avoid such color artifacts.
In published PCT application WO 96/00479, the problems of slow response time and varying pixel capacitance with voltage of a TN LC material in a single panel color sequential projector is approached by substituting an electro-optic material with a faster response time, and with capacitance subsantially independent of voltage, such as a ferroelectric LC material.
However, ferro-electric LC materials and similar electro-optic materials have a memory effect. Thus, WO 96/00479 teaches the use of a “blanking pulse”, ie, an auxiliary signal, to reset the pixels prior to each new addressing with display data. Such a blanking pulse is applied during some part of the line selection period via either the row electrode or the column electrodes in combination with a reference electrode in the case where two-terminal devices such as diodes are used as the active matrix switching elements, or via a common counterelectrode in the case where three terminal devices such as TFTs are used as the active matrix switching elements.
Such a scheme requires separate means for generating the reset signal, either directly or as a combination of an auxiliary voltage and a reference voltage via a separate set of reference electrodes. Such circuit means are not readily integratable into drive circuitry which is designed to convert incoming digital information signals (eg., digital tv signals or computer display signals) to analog signals suitable for driving the display.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide driver circuitry for driving an electro-optic display device, in which the color artifacts caused by the residual state of the electro-optic material are reduced or eliminated without resort to correction of data signals based on the residual state of the pixels from previously addressed data signals, and without resort to separate means for generating a reset signal.
It is another object of the invention to provide an electro-optic display device incorporating such drive circuitry.
It is yet another object of the invention to provide a color display system incorporating such an electro-optic display device.
In accordance with the invention, color artifacts caused by the residual state of the pixels in an electro-optic display device from previously addressed data signals are substantially reduced or eliminated by providing means in a DAC driver circuit for resetting the pixels to a pre-determined state prior to addressing with the next set of data signals.
Preferably, the reset signal is derived from a global ramp generator and applied to selected pixels in advance of addressing with the next set of data signals.
In accordance with one aspect of the invention, a DAC driver circuit for an electro-optic display device comprises:
a control circuit for sequentially addressing the pixels of the device with analog display signals, the control circuit comprising selection means for selecting a row of pixels during a row selection period, and writing means for applying the analog signals to the selected pixels over one or more columns during the row selection period;
a DAC for converting a stream of incoming digital display signals into the analog display signals, the DAC including a ramp generator for generating an analog ramp down and sample-and-hold means for selecting an analog signal from the ramp corresponding to the sampled digital signal, and holding the analog signal for application to the selected pixels;
characterized in that the control circuit includes means for selecting pixels to be reset to a pre-determined state prior to writing, and means for applying a resetting pulse from the analog ramp generator to the selected pixels prior to selection of the analog information signals from the DAC, whereby the color artifacts caused by the residual state of the electro-optic material are reduced or eliminated.
Preferably, the resetting pulse from the analog ramp generator is applied to the selected pixels prior to generation of the ramp, and the pixels selected for resetting are advanced in the addressing sequence from the pixels selected for addressing.
For example, in a display device having N rows of pixels, in which the control circuit is set to select row n for addressing and row(s) m for reset, where m is from one to ten rows in advance of row n, eg., m=n+8.
In accordance with a preferred embodiment of the invention, the control circuit is set to select rows for addressing in the following sequence: n, n+1/3N, n+2/3N, n+1, n+1/3N+1, n+2/3N+1, n+2, n+1/3N+2, etc., and the rows selected for reset are in the sequence m, m+1/3N, m+2/3N, m+1, m+1/3N+1, m+2/3N+1, m+2, m+1/3N+2, etc.
An electro-optic display device incorporating the drive circuitry of the invention, as well as a display system incorporating such an electro-optic display device are included within the scope of the invention. Preferably, such a display system is a single-panel, scrolling color display system.
Use of the reset pulse obviates the need for guard bands between the scrolling color bands employed in prior versions of such a display system to guard ag
Albu Lucian Remus
Janssen Peter J.
Gathman Laurie E.
Philips Electronics North America Corporation
Wu Xiao
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