Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
2000-07-26
2003-02-11
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S063000
Reexamination Certificate
active
06518977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color image display apparatus which displays a color video image by controlling light emission of red (R), green (G) and blue (B) primary colors, and more particularly, to a color image display apparatus with an excellent dynamic resolution characteristic, which displays a high-quality moving image where color fringes at moving image edges are inconspicuous.
2. Description of the Prior Art
In recent years, in place of conventional Braun tube (CRT) display devices, flat-panel type display devices are becoming popular. These thin and light display panel devices, having a display panel where liquid crystal or plasma is sealed, display images with reduced image distortion, and receive reduced influence of earth magnetism. Among the flat-panel display devices, a plasma display device particularly draws public attention as a next-generation color image display device. The plasma display device is a spontaneous light emitting device, and therefore it has a wide view angle. Further, a large panel can be relatively easily constructed for this device. In this flat-panel display device, one pixel consists of red (R), green (G) and blue (B) light emitting cells. Color image display is realized by controlling the light emitting luminance levels of the respective light emitting cells.
Further, the plasma display device or the like having difficulty in displaying gray scale representation between “light emission (turned on)” and “non light emission (turned off)”, employs a so-called subfield method for displaying the gray scale representation by controlling the light emitting luminance levels of the respective R, G and B light emitting cells. In the subfield method, one field is divided into a plurality of subfields on a time base, then light emitting weights are uniquely allotted to the respective subfields, and light emission in the respective subfields are on/off controlled. This attains luminance gradation (or tonality) representation.
For example, in a case where one field is divided into six subfields SF
0
to SF
5
and light emitting weights in the ratios 1:2:4:8:16:32 are respectively allotted to the subfields, 64 level gradation can be represented. At level “0”, light emission is not performed in any of the subfields SF
0
to SF
5
. At level “63” (=1+2+4+8+16+32), light emission is performed in all the six subfields.
In this manner, in the color image display device which controls the light emitting luminance levels of respective R, G and B light emitting cells by the subfield method, the image quality of a displayed moving image is greatly influenced by time response characteristics related to light emission by the R, G and B cells (hereinafter may be simply referred to “light emitting response characteristics”) and the array of light emitting weights allotted to the respective subfields in each field.
The light emitting response characteristics of the R, G and B cells respectively indicate a light-emitting rise time characteristic from a point where a controller has instructed to start light emission to a point where light emitting luminance at the cell actually reaches a desired level, and a persistence time characteristic after the light emission instruction. Generally, if the persistence time is long, the light-emitting rise time is long. Accordingly, the persistence time is used as a representative characteristic of light emitting response characteristic. In the following description, the light emitting response characteristic is represented by the “persistence time” (a period from a point where the light emission is at the peak to a point where the light emission is at a level {fraction (1/10)} of the peak). The “persistence time” includes the “light-emitting rise time characteristic”.
The operation of this color image display device can be ideal operation as the light emitting response characteristics are short, however, the light emitting response characteristics cannot be reduced to zero. Further, as the light emitting response characteristics greatly depend on physical characteristics such as fluorescent materials used as the light emitting cells, it is very difficult to obtain uniform response characteristics in the R, G and B cells having different luminous wavelengths. For these reasons, when a moving image is displayed, the differences in time responses of the respective light emitting cells cause time lags in R, G and B light emission which overlap with each other, resulting in color shift (color fringing). The color shift appears at an edge portion where luminance greatly changes, e.g., from black to white or vice versa, as a phenomenon that a color different from the original image color is perceived. This seriously degrades image quality in moving image display.
Hereinbelow, the process of occurrence of color fringing interference at edge portions will be described with reference to FIG.
3
and
FIGS. 4A and 4B
. As shown in
FIG. 3
, a white rectangular pattern
32
on black background
31
is displayed on a display screen of a display device, and the white rectangular pattern
32
is moved rightward in FIG.
3
.
FIGS. 4A and 4B
show color fringes occurred on the boundaries between white and black colors.
FIG. 4A
shows the intensities (amplitudes) in the respective light emitting cells.
FIG. 4B
shows colors displayed on the display screen. As shown in
FIG. 4A
, as the G light emitting response is slower than the R and B light emitting responses, the G light emitting response represented with the broken line is delayed from the R and B light emitting responses represented with the solid lines. Thus, color fringing occurs in edge areas A and B. As shown in
FIG. 4B
, in the edge area A, a color of magenta (R+B) is perceived due to shortage of the amplitude of G with respect to R and B. In the edge area B, a color of green (G) is perceived due to excess amplitude of G. The edge area where color fringing occurs becomes wider as the speed of moving image increases.
In this manner, in the white and black video signal, colors not included in the original image (magenta and green) are perceived depending on the motion of the image. This seriously degrades the image quality. Especially, in the plasma display device and the like, material having persistence time of 12 ms or longer is often used as a G light emitting cell. As the response of the G cell using this material is slower than the responses of R and B cells, the consequent color fringing in edge areas is a main factor of degradation of image quality.
On the other hand, in the display devices which displays gray scale representation by the subfield method, the dynamic resolution is greatly influenced by the array of light emitting weights for the respective subfields in each field. To prevent degradation of dynamic resolution, it is preferable to perform light emission, based on a video signal that arrives for one field, as impulses for a very short period within each field period. In the CRT display devices, one field period is required for horizontal and vertical scan processing, however, impulse-like light emission is made for one pixel at a particular display screen position, in each field.
However, in the gradation representation by the subfield method, as the video signal that arrives for one field is divided into a plurality of subfields within the field for light emission and display, impulse light emission cannot be made for a short period. For this reason, it is difficult to realize a dynamic resolution characteristic equivalent to that of the CRT device.
Hereinbelow, the phenomenon where the dynamic resolution is degraded in correspondence with the array of light emitting weights for subfields will be described with reference to
FIG. 5
,
FIGS. 6A and 6B
and
FIGS. 7A and 7B
. In this case, the white rectangular pattern
32
shown in
FIG. 3
is displayed by a display device having a subfield arrangement for 64 (level “0” to level “63”) level representation with six subf
Kougami Akihiko
Naka Kazutaka
Ohsawa Michitaka
Ohtaka Hiroshi
Antonelli Terry Stout & Kraus LLP
Eisen Alexander
Hitachi , Ltd.
Hjerpe Richard
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