Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-11-13
2002-07-02
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C348S673000, C348S678000
Reexamination Certificate
active
06414664
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of display devices such as liquid crystal display (“LCD”) devices and the like. More specifically, the present invention relates to a method of and apparatus for controlling contrast for such LCDs, especially active-matrix LCDs, while receiving large dynamic range video data.
An “image” is a pattern of physical light. An “image output device” is a device that can provide an output defining an image. A “display” is an image output device that provides information to an observer in a visible form. A “liquid crystal display” (“LCD”) is a display device that includes a liquid crystal cell with a light transmission characteristic that can be controlled in parts of the cell by an array of light control units to cause presentation of an image. A “liquid crystal cell” is an enclosure containing a liquid crystal material. An “active-matrix liquid-crystal display” (“AMLCD”) is an LCD in which each light control unit has a nonlinear switching element that causes presentation of an image segment by controlling a light transmission characteristic of an adjacent part of the liquid crystal cell. An LCD can have a plurality of electrically-separated display regions, each display region also being known as a display cell, or when the regions designate a small portion of the display, each display region is known as a “pixel.” Each pixel in a high density display matrix, such as for LCDs, requires its own active (switching element) driver (e.g., a thin film transistor). The light control units can, for example, be binary control units.
In recent years, due to the great needs of avionics displays, LCD devices are more popularly used in avionics displays than other solid image display elements because of the low power consumption of the LCD elements. Also, personal computers, portable game machines, and other devices requiring a visual interface often use LCDs to display data. Such LCDs can be matrix addressed, such as an active-matrix LCD, but the use of a thin film transistor with every pixel in an active-matrix LCD is required for high resolution. Recently, color LCDs have come into common usage also. The increased usage of the color LCDs is partially because of their availability and a color pixel density of 100 to the inch can be easily achieved.
LCDs are generally classified into two categories: passive-matrix LCDs and active-matrix LCDs. Active-matrix LCDs are more popular than passive-matrix LCDs because of their excellent image quality, high speed, high contrast ratio (i.e., ratio of maximum to minimum luminance values in the LCD), and superior color quality. Although the passive-matrix LCDs are advantageously used for high-density integration because of their simple structures and lower manufacturing costs, the passive-matrix LCD elements have crosstalk to a non-selected cell, and an increase in resolution, which is an object of the high-density integration, cannot be achieved. In contrast to this, in the active-matrix LCDs, crosstalk to a non-selected cell can be suppressed without posing any problem, and an image having a high resolution can be obtained, thereby considerably improving image quality. In this manner, a large number of active-matrix LCDs have been used in recent years. Also, passive-matrix and active-matrix LCDs operate with a back light, which is typically a fluorescent lamp.
Both the active-matrix and passive-matrix LCDs are a matrix of row and column electrodes with a pixel at the intersection of each row and column. The active-matrix LCD provides a transistor at the intersection of each row and column electrode to greatly improve the voltage control of each pixel. The LCD is driven by providing the video voltage to the pixels one row at a time. The LCD is refreshed at a frequency that minimizes the flicker of the LCD, typically greater than 30-Hz. In a typical LCD architecture, the row electrodes are used to select the row which is to be driven and the column electrode provides the drive voltage that is used to determine the gray shade or level of the pixel at the intersection of the selected row and column. In a passive-matrix LCD, the root-mean-square voltage across the pixel, as determined by the select line voltage and the gray level voltage, determine the gray level of the pixel. In an active-matrix LCD, the gray level voltage delivered by the transistor at the pixel determines the gray level.
Both categories of LCDs require light rays from a back light to generate the colors. The back light generates an image plane of light beneath the LCD, which in turn generates the color display. In both passive-matrix and active-matrix systems, the color is generated by an array of color filters.
However, in these LCDs, the following problems are posed. The image quality of active-matrix LCDs is substandard at some contrast settings and viewing angles. Also, image quality changes as the contrast is changed. From a usability standpoint, there exists a considerable amount of dissatisfaction with the contrast and image quality of active-matrix LCDs. Contrast control works on a CRT and users desire that type of interface because they comfortable with it, and the display image is appealing. In a CRT, when the contrast is adjusted up and down it looks good and it adjusts the contrast as one would expect. The contrast control in a CRT is very smooth and very continuous. The situation of the LCD contrast being difficult to adjust in comparison to the CRT is directly related to the fact that an LCD has a limited number of shades of gray, e.g., 64 shades of gray, whereas a CRT essentially has infinite shades of analog. Thus, a need exists to obtain better image quality and better control of the LCD's contrast of the video input to make it more closely resemble or match the quality that is obtained with a CRT when its contrast is adjusted. There is a desire to achieve that parody with an LCD when its video contrast is adjusted. A discussion of manual contrast control of CRTs can be found in most text books, for example, Bernard Grob,
Basic Television Principles and Servicing,
pp. 267-268 (4
th
Ed. 1975).
LCDs having the above drawbacks are not satisfactorily used in image display devices which are popularly used in avionics and industrial applications, especially in military aircraft; image display devices free from the above drawbacks are desired. To date, some of the attempted solutions to the problem have included classical contrast gain function, digital contrast to input video, and contrast changes. The classical contrast gain function requires brightness as a video adjustment. On LCDs, the brightness of the video is controlled by adjusting the back light. The contrast change solution controls the contrast by selecting from the existing shades of gray as determined by the LCD driver system.
The viewability of an image on an LCD is generally determined by the brightness and contrast of the LCD and video signal corresponding to the image. The luminance of each LCD pixel corresponds to the amplitude of the video signal for the pixel. High amplitudes typically correspond to very bright pixels, while low amplitudes generally correspond to dark pixels. The range between the minimum and maximum amplitudes and the corresponding degrees of luminance may be subdivided into an almost infinite number of luminance levels, reflecting subtleties of shading and color represented by the video signal. The brightness and contrast adjustments of the LCD, on the other hand, are essentially static. Conventionally, brightness corresponds to a direct current signal added to the video signal so that the overall signal level increases. As a result, the overall display becomes brighter. For CRT displays, the DC component is added to the video signal. For LCDs, the backlight system responds to the brightness control.
Contrast, on the other hand, relates to the amplification of the video signal. Thus, as contrast increases, bright pixels become very bright, while re
Conover Kurt M.
Wood Teddy J.
Anderson William C.
Awad Amr
Honeywell Inc.
Saras Steven
Yeadon Loria B.
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