Active matrix liquid crystal display device with interdigitated

Details Active matrix liquid crystal display device with interdigitated Active matrix liquid crystal display device with interdigitated

1994-03-02

1995-09-05

Gross, Anita Pellman

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

359 87, 359 89, G02F 11343

Patent

active

054483859

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a so-called "active matrix" liquid crystal display screen device comprising a thin layer of liquid crystals disposed between a plane backing electrode and control electrodes each co-operating with the backing electrode to define a capacitor and a picture element such that each pixel corresponds to a row and to a column, each control electrode being connected to a control element such as a thin-film transistor enabling it either to be raised to the potential of a conductor which is common to all of the pixels in the column to which it belongs, or else to isolate it therefrom and cause it to take up a floating potential.
Flat display screens of the type described above are already known. In general, the backing electrode constitutes a common potential plane covering the entire screen. Means are often provided for varying the potential of the backing electrode so as to reduce the dynamic range of the voltage required on the column conductors that receive data.
To avoid residual charge accumulating and which would give rise to ghost images, it is necessary for the mean value over time of the voltage applied to the capacitor of each pixel to be zero. This makes it necessary to reverse the polarity of the voltage applied to the capacitor at regular intervals. However, since the voltages at 50 Hz or at 60 Hz in general use are not perfectly symmetrical, it is impossible to avoid flicker which becomes invisible only when polarities are alternated at a high spatial frequency. In general, that means that the spatial frequency used is the spatial frequency at which the pixels are distributed in rows or in columns.
The solution which comes immediately to mind consists in inverting the voltage applied to the backing electrode (of absolute value Vce) both from one frame to the next and also from one row to the next, and inverting the polarity applied to the column conductors correspondingly. This amounts to saying that the voltages applied to the backing electrode and to the column conductors while displaying rows of order p and p+1 for images of order i and of order i+1 are as follows:


______________________________________ image "i" (or frame "i") row "p" backing electrode: Vce+ Vce+ Vce+ Vce+ column: V- V- V- V- row backing electrode: Vce- Vce- Vce- Vce- "p + 1" column: V+ V+ V+ V+ image (or frame "i+ ") "i + 1" row "p" backing electrode: Vce- Vce- Vce- Vce- column V+ V+ V+ V+ row backing electrode Vce+ Vce+ Vce+ Vce+ "p + 1" column V- V- V- V- ______________________________________
Under such circumstances, the backing electrode conserves an unchanging polarity Vce+ or Vce- throughout the duration of one row. This makes it easier to control. On the other hand, it is difficult to obtain sufficiently fast convergence of the data presented on each of the column conductors since the polarity of a column conductor is inverted on each row, i.e. at a frequency of a few tens of kHz for a 625-line television type image.
It might be thought that the problem could be avoided by inverting column polarity once per frame, thereby improving the accuracy with which each pixel is controlled. The voltage excursion on the column conductor is then smaller from one row to the next. However, it would then be necessary to invert the polarity of the backing electrode for each "column". Since all of the pixels along a row are written simultaneously, that amounts to saying that it would be necessary for Vce+=Vce-. The advantage of small dynamic range on the columns would then be lost.
The invention seeks to provide a display screen of the type defined above but satisfying practical requirements better than those known in the past. To this end, the invention proposes a device having a screen in which the backing electrode is made up of two fractions provided with means enabling them to be taken to different potentials that are inverted on each frame or multiple of the frame frequency, and in which successive columns (or successive groups of a few columns each) of control-ele

REFERENCES:
patent: 4973135 (1990-11-01), Okada et al.
Lechner et al., Liquid Crystal Matrix Displays, Nov. 1971, pp. 1566-1579, oceedings of the IEEE, vol. 59, No. 11.

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