Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Patent
1996-03-13
1998-05-05
Saras, Steven
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
345 94, 345 95, 349100, 349133, G09G 336
Patent
active
057481660
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the addressing of ferroelectric liquid crystal displays.
Liquid crystal display devices are well known. They typically comprise a liquid crystal cell formed by a thin layer of a liquid crystal material held between two glass walls. These walls carry transparent electrodes which apply an electric field across the liquid crystal layer to cause a reorientation of the molecules of liquid crystal material. The liquid crystal molecules in many displays adopt one of two states of molecular arrangement. Information is displayed by areas of liquid crystal material in one state contrasting with areas in the other state. One known display is formed as a matrix of pixels or display elements produced at the intersections between column electrodes on one wall and row electrodes on the other wall. The display is often addressed in a multiplex manner by applying voltages to successive row and column electrodes.
Liquid crystal materials are of three basic types, nematic, cholesteric, and smectic each having a distinctive molecular arrangement.
The present invention concerns ferroelectric smectic liquid crystal materials. Devices using this material form the surface stabilised ferroelectric liquid crystal (SSFLC) device. These devices can show bistability, ie the liquid crystal molecules, more correctly the molecular director, adopt one of two alignment states on switching by positive and negative voltage pulses and remain in the switched state after removal of the voltage. This behaviour depends upon the surface alignment properties.
Some types of surface alignment will produce a device in which the switched states remain after removal of the voltage, other types of surface alignment will produce a device in which the states may randomly decay on removal of the voltage. The switched states may be stabilised by the presence of an ac bias. The actual states achieved may be dependent upon the amplitude of any ac bias present. The ac bias may be provided by the data (column) voltages in a multiplexed device.
There are a number of known systems for multiplex addressing ferroelectric displays; see for example article by Harada et al 1985 S.I.D. Paper 8.4 pp 131-134, and Lagerwall et al 1985 I.D.R.C. pp 213-221. See also GB 2,173,336-A and GB 2,173,629-A. Multiplex addressing schemes for SSFLCs employ a strobe waveform that is applied in sequence to rows but not necessarily to successive rows simultaneously with data waveforms applied to e.g. column electrodes. The time taken to scan down N lines is termed a field time and equals N times the time taken to address each line--the line address time. For some multiplex modes two field times are required to switch all the pixels to the required state; the total time to completely address a matrix is the frame time. A characteristic of SSFLCs is that they switch on receipt of a pulse of suitable voltage amplitude and length of time of application, ie pulse width, termed a voltage time product V.t. Thus both amplitude and pulse width need to be considered in designing multiplex addressing schemes.
The bistability property, together with the fast switching speed, makes SSFLC devices suitable for large displays with a large number of pixels or display elements. Such ferroelectric displays are described for example in: N A Clark and S T Lagerwall, Applied Physics Letters Vol 36, No 11 pp 889-901, June, 1980; GB-2,166,256-A; U.S. Pat. No. 4,367,924; U.S. Pat. J Phys Lett 36, L69, 1975.
For displays having a large number N the time taken by two field times can be significant. One way of reducing this is to blank all pixels to one state with a single blanking pulse, then scan each line with a strobe pulse during one field time to switch selected pixels to the other state. In this case the total time to address is one field time. A disadvantage of whole frame blanking is display appearance, and loss of information whilst the blanked display is being written. Alternatively the blanking pulse may also scan the lines preceding the strobe pulse by, e.g. five lines. In thi
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Hughes Jonathan Rennie
Towler Michael John
Lewis David L.
Saras Steven
The Secretary of State for Defense
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