Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
1999-12-02
2002-05-28
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
06396508
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to video displays and more particularly, to a method and system for improving the image quality of a display in which a pixel is illuminated by pulses generated in subfields of a frame of the image in accordance with a pulse distribution function. A maximum pixel value to be imaged during the frame is determined, and the pulse distribution is modified based on the maximum pixel value. The invention is particularly suited for use with plasma display panels.
BACKGROUND OF THE INVENTION
Digital displays such as alternating current (AC) Plasma Display Panels (PDPs) are evolving as an attractive choice to view television programming, especially with regard to the emerging digital television and high definition television (DTV/HDTV) formats. Conventional cathode ray tubes (CRTs) have an established high picture quality, and PDPs are striving to achieve a similar quality in order to attract widespread consumer acceptance.
PDPs, i.e., gas discharge panels, are well known in the art and, in general, comprise a structure including a pair of substrates respectively supporting column and row electrodes, each coated with a dielectric layer and disposed in parallel spaced relation to define a gap therebetween in which an ionizable gas is sealed. The substrates are arranged such that the electrodes are disposed in orthogonal relation to each other, thereby defining points of intersection which, in turn, define discharge pixel sites at which selective discharges may be established to provide a desired storage or display function.
It is known to operate such panels with AC voltages and particularly to provide a write voltage which exceeds a firing voltage at a given discharge site, as defined by selected column and row electrodes, thereby to produce a discharge at a selected cell. The discharge can be continuously “sustained” by applying an alternating sustain voltage, which, by itself, is insufficient to initiate a discharge. The technique relies upon wall charges generated on the dielectric layers of the substrates which, in conjunction with the sustain voltage, operate to maintain continuing discharges.
Referring to
FIG. 1
, the structure of a full color AC plasma panel is schematically illustrated. Plasma panel
410
includes a back substrate
412
upon which plural column address electrodes
414
are supported. Column address electrodes
414
are separated by barrier ribs
416
and are covered by red, green and blue phosphors
418
,
420
and
422
, respectively. A front transparent substrate
424
includes a-pair of sustain electrodes
426
and
428
for each row of pixel sites. A dielectric layer
430
is emplaced on front substrate
424
and a magnesium oxide overcoat layer
432
covers the entire lower surface thereof, including all of sustain electrodes
426
and
428
.
The structure of
FIG. 1
is sometimes called a single substrate AC plasma display since both sustain electrodes
426
and
428
, for each row, are on a single substrate of the panel. An inert gas mixture is positioned between substrates
412
and
424
and is excited to a discharge state by sustain voltages applied by sustain electrodes
426
and
428
. The discharging inert gas produces ultra-violet light that excites the red, green and blue phosphor layers
418
,
420
and
422
, respectively to emit visible light. If the driving voltages applied to column address electrodes
414
and sustain electrodes
426
,
428
are appropriately controlled, a full color image is visible through front substrate
424
.
In order to cause the AC plasma panel of
FIG. 1
to exhibit a full color image for applications such as television or computer display terminals, a means of achieving a gray scale is needed. Since it is desirable to operate AC plasma panels in a memory mode to achieve high luminance and low flicker, an addressing technique is utilized to achieve image gray levels in pixels that only exist in the ON or OFF states. Such addressing technique is described by Yoshikawa et al. in “A Full Color AC Plasma Display With 256 Gray Scale”, Japan Display, 1992, pp. 605-608. Because a PDP is a digital device, it can provide only affixed number of gray scale gradations. In the case of an 8-bit red-green-blue (RGB) signal, 256 gradations are possible.
FIG. 2
illustrates the driving sequence used by Yoshikawa et al. to achieve a 256 gray scale. The drive sequence is sometimes called the sub-field addressing method. The plasma. display panel is addressed in a conventional video manner that divides images into frames. A typical video image may be presented at 60 frames per second, which corresponds to a frame time of 16.6 milliseconds. The sub-field addressing method shown in FIG.
2
. divides each frame into 8 sub-fields, SF1-SF8.
As shown in FIG.
3
,.each of the 8 sub-fields is further divided into an address period and a sustain period. During the sustain period, a sustain voltage is applied to sustain electrodes
426
and
428
, shown in FIG.
1
. Thus, if a given pixel site is in the ON state, it is caused to emit light by one or more sustain pulses. By contrast, the sustain voltage is insufficient to cause a discharge at any pixel site that is in the OFF state.
Note in
FIG. 2
that the length of the sustain period of each of the 8 sub-fields is different. The first sub-field has a sustain period with only 1 complete sustain cycle period. The second sub-field has 2 sustain cycles, the third sub-field has a sustain period with 4 sustain cycles and, so forth, until the 8th sub-field which has a sustain period with 128 sustain cycles.
By controlling the sustaining of a given pixel site that has been addressed, the perceived intensity of the pixel site can be varied to any one of the 256 gray scale levels. Suppose it is desired for a selected pixel site to emit at half-intensity or at level 128 out of 256. In such a case, referring to
FIG. 1
, a selective write address pulse is applied to the pixel site during sub-field 8 by applying an appropriate voltage to a column address electrode
414
, and utilizing one of sustain lines
426
,
428
as the opposing address conductor. No address pulses are applied during the other sub-fields to the addressed pixel site. This means that during the first 7 sub-fields, there is no writing action and therefore no light is emitted during the sustain periods. However, for sub-field
8
, the selective write action turns ON the selected pixel site and causes an emission of light therefrom during the sub-field
8
sustain period, in this case for 128 sustain cycles. The 128 sustain cycle per frame energization corresponds to a half-intensity for a frame time.
If, alternatively, it is desired for the selected pixel site to emit at one-quarter intensity or at level
64
out of 256, then a selective write address pulse is applied to the pixel site during sub-field
7
and no address pulses are applied during the other sub-fields. Thus, during sub-fields
1
,
2
,
3
,
4
,
5
,
6
and
8
, there is no writing and therefore no light is emitted during the respective sustain periods. However, for sub-field
7
, the selective write turns ON the selected pixel site and causes an emission of light during the sub-field sustain period (in this case, for 64 sustain cycles corresponding to a 1-quarter intensity). For a full-intensity case, the selective write address pulse is applied during all 8 sub-fields so that the pixel site emits light for all sustain periods for each of the 8 sub-fields, corresponding to a full-intensity for the frame.
The Yoshikawa et al. procedure enables any of 256 different intensities to be achieved-through the action of a display processor supplying an 8-bit data word for each sub-pixel site, the data word corresponding to the desired gray intensity level. By routing each of the bits of the data word to control the selective write pulse of each of the 8 address periods of the 8 sub-fields in a given frame, the 8-bit data word controls the number-of sustain cycles during which the selected pixel site will emit light for that f
Eisen Alexander
Hjerpe Richard
Matsushita Electronics Corp.
Ohlandt Greeley Ruggiero & Perle L.L.P.
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