Computer graphics processing and selective visual display system – Data responsive crt display control
Reexamination Certificate
1998-02-17
2001-05-29
Liang, Regina (Department: 2774)
Computer graphics processing and selective visual display system
Data responsive crt display control
C315S370000
Reexamination Certificate
active
06239772
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of and a device for moiré reduction, and to an image display apparatus comprising a moiré reduction device.
A display monitor operating under the raster scanning system repeats the line scanning of an electron beam by moving the resulting scanning lines in a direction substantially perpendicular to the line direction. In a usual raster scan, the lines are scanned in the horizontal direction while the lines succeed each other in the vertical direction. In the so called transposed scan, the lines are scanned in the vertical direction and succeed each other in the horizontal direction. In a cathode ray tube, the dot pitch of the fluorescent material on the screen of the CRT that can emit a fluorescence is determined by the aperture pitch of the shadow mask. Thus, in case of displaying a video pattern in which consecutive pixels in a line repeat in an ON and OFF sequence an interference with the aperture pitch of the shadow mask may occur, thereby causing a moiré.
WO 97/09820 discloses a method of reducing the moiré phenomenon upon display of a video signal on a display device by low-pass filtering the video signal. The low-pass filter has a cut-off frequency substantially corresponding to a maximum frequency of a baseband portion of the video signal to obtain a low-pass filtered video signal to be applied to the display device. It is a drawback of the prior art that a complex and expensive low pass filter is required to suppress all the harmonics of the baseband signal to a high degree to reach an optimum moiré reduction, without attenuating the baseband signal to obtain an optimum sharpness of the displayed video.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved video moiré reduction. To this end, a first aspect of the invention provides a method of reducing, a moiré phenomenon. A second aspect of the invention provides a device for reducing moiré. A third aspect of the invention provides an image display apparatus. Advantageous embodiments are defined in the dependent claims.
The prior art low pass filter has to be designed to pass the base band video without a substantial suppression to avoid to deteriorate the sharpness of the displayed picture. On the other hand, the prior art low pass filter has to suppress all higher harmonics of the baseband video signal to a high degree to avoid visible moiré disturbances.
The invention is based on the recognition that at a specific scan frequency belonging to a specific video mode (for example: VGA, SVGA, XGA), a specific spatial higher harmonic of a baseband video signal causes the or the majority of the video moiré. Therefore, according to the invention, it is sufficient to only suppress this specific higher harmonic to reduced the video moiré effectively. Thus, a notch filter with a first passband to pass the baseband video, and a notch or stopband at or near the specific higher harmonic reduces the video moiré to a very high degree. The behavior of this notch filter outside the first passband and the stopband is not relevant. It is thus not required to suppress all higher harmonics of the baseband video signal with a low pass filter.
The specific higher harmonic of the baseband video signal is that higher harmonic that is near to a spatial frequency of the mask-phosphor pattern of the display screen. According to an insight of the invention, in practice, the most relevant higher harmonic for moiré is caused by a baseband video signal comprising a sequence of pixels which are alternatively on and off. Such a sequence determines the highest video frequency to be displayed, preferably with full amplitude.
In an embodiment of the invention, the notch filter may have a further passband outside the above mentioned passband and stopband, the video harmonics in this passband will not contribute to moiré. Further, the notch is a zero (or at least a substantially zero) in the amplitude characteristic. Such a filter may be a simple comb filter.
In an embodiment of the invention, the notch frequency of the notch filter is controlled in response to the detected video mode displayed on the screen. A specific number of video pixels in a video line period is displayed on a line on the screen within a predetermined width, thereby determining the spatial frequencies of the higher harmonics of the video pixels. Thus, according to an insight of the invention, it is known in each video mode which spatial frequencies of the higher harmonics occur. Further, the spatial frequencies of the dots of the mask-phosphor pattern which are determined by the display device used are known. Consequently, it can be easily established which higher harmonic is nearest to one of the spatial frequencies of the dots. Only this higher harmonic, or the relevant dot frequency needs to be substantially suppressed to avoid moiré. In this way, the moiré is suppressed to a very high degree independent on the video mode displayed. In the prior art the amount of suppression of the moiré depends on which higher harmonic is responsible for the moiré. The nearer this higher harmonic is to the baseband, the less the suppression will be.
An embodiment of the invention, has the advantage that a very simple combfilter is able to substantially completely suppress the relevant higher harmonic, without substantially attenuating the baseband video.
An embodiment of the invention, a non-linear function succeeds the notch filter. The non-linear function compensates for the gamma of the picture display. In this way, the moiré is even suppressed better because no higher harmonics of the output video are regenerated by the non-linear characteristic of the picture display.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 0719042 (1996-06-01), None
patent: 59-71024A (1984-04-01), None
patent: WO9709820 (1997-03-01), None
Hentschel Christaan
Vriens Leendert
Biren Steven R.
Liang Regina
U.S. Philips Corporation
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