Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-10-06
2002-02-05
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S088000
Reexamination Certificate
active
06344839
ABSTRACT:
TECHNICAL FIELD
This invention relates to a drive method and drive circuit intended to compensate for degraded picture quality of moving image in a display device so designed as to display multitonal image signal making up one frame with plural subframes of different relative ratios of brightness.
BACKGROUND TECHNOLOGY
The PDP (Plasma Display Panel) has recently attracted public attention as a thin, light-weight display device. Completely different from the conventional CRT driving method, the drive method of this PDP is a direct drive by digitalized image input signal. The brightness and tone emitted from the panel face depend therefore on the number of bits dealt with
The PDP may be roughly divided into AC type and DC type methods whose basic characteristics are different from each other. As for the tonal display, however, 64-tone display was the maximum reported from the trial manufacture level. The Address/Display Separation type drive method (ADS subframe method) has been proposed as an approach to solve this problem.
FIGS.
1
(
a
) and
1
(
b
) show the drive sequence and drive waveform of the PDP used in this ADS subframe method.
In FIG.
1
(
a
), which gives an example of 256 tones, one frame is composed of eight subframes whose relative ratios of brightness are 1, 2, 4, 8, 16, 32, 64 and 128, respectively. Combination of this brightness of eight screens enables a display in 256 tones.
In FIG.
2
(
b
), the respective subframes SF
1
to SF
8
are composed of the address duration AD
1
, . . . that write one screen of refreshed data and the sustaining duration ST
1
, . . . that defines the brightness level of these subframes. In the address duration, a wall charge is formed initially at each pixel simultaneously over all the screens, and then the sustaining pulses are given to all the screens for display. The brightness of the subframes is proportional to the number of sustaining pulses to be set to the predetermined brightness. Two hundred and fifty-six tone display is thus performed.
In such an AC drive method, the greater the number of tones, the number of bits of the address duration as preparation time for the panel to emit light and brightness within one frame duration becomes. This relatively shortens the sustaining duration as emission time, lowering thus the maximal brightness.
Hence, the brightness and tone emitted from the panel face depend on the number of bits to be dealt with. With the increased number of bits of the signal processed, the picture quality improves, but the emission brightness reduces. If, on the contrary, the number of bits of the signal processed is diminished, the emission brightness augments, but the tonal display reduces, deteriorating thus the picture quality.
The error variance processing intended to minimize the grayness error between input signal and emission brightness reducing rather the bit number of output drive signal than that of input signal is a processing to represent a pseudo-intermediate (half) tone, which is used when representing the grayness with fewer tones.
In the conventional general error variance processing circuit, the image signal of n-bit (n being
8
for instance) original pixels Ai, j enters an image signal input terminal, and passes through vertical adder and horizontal adders. Further, in the bit conversion circuit, the image signal reduces its bit number to m (4, for instance, and m<n). After passing through the PDP drive circuit, it emits light from the PDP.
The error variance signal from said horizontal adder is compared with data stored beforehand by an error detect circuit, and the difference between this signal and the data is weighted by predetermined coefficient in an error load circuit. The error detect output is added to said vertical adder through the intermediary of the h line delay circuit that outputs the reproduction error Ej-
1
produced at the pixel going back by h lines from the original pixel Aj, i, for example, by one line in the past, and at the same time, added to said horizontal adder through the intermediary of a d-dot delay circuit that outputs the reproduction error Ei-
1
produced at the pixel going back by d lines from the original pixel Ai, j, for example, by one dot in the past. In general, the coefficients at said error load circuit are to be set so that their total sum may be 1 (one).
As a result, a stepwise emission brightness level represented by 4 bits is output momentarily at the output terminal of the bit conversion circuit. Nevertheless, the emission brightness levels above and below the step-like level are actually output alternately in predetermined proportion, which will be recognized as an averaged state. This allows for a correction brightness line with approximate y=x
However, the subframe lighting method was problematical in that the picture quality worsens in a part of screen when the input level of original signal somewhat changes.
In a case where 4-bit image signal scanning from SF
4
to SF
1
in the sequential order of brightness as shown in FIG.
2
(
a
), the level 7 is quantized by 0111 and 8 is quantized by 1000 when the input of the first and second frames of the original signal change at levels 7 and 8, respectively. At the point of change from 7 to 8, therefore, the level becomes 01111000 as shown in FIG.
2
(
b
) with indiscriminate emission at the levels 7 and 8. The brightness at that time reaching about 2 times the level 7 or level 8, it looks like a white line.
Conversely at the point of change from 8 to 7, the level becoming 10000111, the non-emission duration looks like a continuous black line.
The sampling signal a before conversion as shown in FIG.
3
(
c
) and the signal b converted into the waveform of ADS subfield method as shown in FIG.
3
(
b
) were filtered by the LPF (Low Pass Filter) with the half of frame frequency as the cutoff frequency and compared. The comparison of these signals revealed a large difference between the point of change of the image signal level from 7 to 8 and the point of change from 8 to 7 as shown in FIG.
3
(
e
),where A represents the LPF output waveform of a, and B, that of b.
In such a display and reproduction system where the image signal is time-shared into plural subframes, there exists at a point of level change a level that does not always coincide with the change of original signal when a moving image changing in the time axial direction is displayed. This was problematical since it degrades the picture quality.
It was problematical particularly because pseudo-half tone, for example, by an error variance in one tone level was accompanied by flickering in the time axial direction.
The first purpose of this invention is to provide a method to compensate for the degradation of picture quality of a moving image arising from the half-tone display of the subframe method.
DISCLOSURE OF THE INVENTION
The drive method of a display device by this invention consists in that in a display unit so designed as to display a multi-tone image signal composing one frame from plural subframes of different relative ratios of brightness, two subframes of minimal brightness are arranged adjacently to each other so that the subframe selection and lighting may be possible in response to the change of image brightness in the time axial direction.
When, for example, the level of original signal changes from 7 to 8 or from 8 to 7, the brightness of 5-bit 5-screens is used, SF
3
, SF
2
, SF
1
and SF
1
of 4, 2, 1, and 1 are selected as the subframes for level 8, and SF
3
, SF
2
and SF
1
of 4, 2 and 1 are selected as subframes for level 7.
More materially, when one frame changes from level 7 to 8, or from 8 to 7, the level 7 is quantized at [01110] by SF
3
, SF
2
and SF
1
out of SF
4
, SF
3
, SF
2
, SF
1
and SF
1
, while the level 8 is quantized at [01111] by SF
3
, SF
2
, SF
1
and SF
1
out of SF
4
, SF
3
, SF
2
, SF
1
and SF
1
. At the point of change from level 7 to 8, the level becomes [01110] [01111], and the lighting is discontinuous at the levels 7 and 8. At the point
Denda Hayato
Kobayashi Masayuki
Kosakai Asao
Matsunaga Seiji
Nakajima Masamichi
Flynn ,Thiel, Boutell & Tanis, P.C.
Frenel Vanel
Fujitsu General Limited
Shalwala Bipin
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