Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Patent
1997-05-30
1999-02-16
Blum, Theodore M.
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
348806, 348745, H01J 2958
Patent
active
058724320
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to raster-scanned cathode ray tube (CRT) displays with means for reducing Moire interference.
High performance raster-scanned cathode ray tube (CRT) displays are becoming increasingly susceptible to visual performance degradation by Moire interference patterns. Factors contributing to the susceptibility of these displays includes, but are not limited to, exceptionally small electron beam spot size, finer shadow masks or aperture grilles, user controls allowing variable picture width and height, dithered pixels patterns generated by graphics user interface software for improved colour richness, a large number of possible display modes such as 640.times.480 and 1024.times.768 pixel modes, and synchronisation to wide frequency range of line and frame synchronisation (sync) signals.
Moire interference is an interference fringe pattern produced in the picture displayed on a CRT when the spatial frequency of the shadow mask or aperture grille of the CRT and the spacing between adjacent pixels of the picture are approximately equal. The "critical pixel frequency" is obtained when the pixel spacing exactly equals the spacing of adjacent phosphors dots on the CRT screen. Moire interference is particularly prevalent when uniform patterns are displayed. Such patterns are typically displayed as backgrounds to a graphical user interface. These backgrounds typically have a dithered or speckled picture content.
Previously, Moire interference has been reduced in high performance CRT displays by changing the pitch of the shadow mask. This was a practical solution because the scan dimensions were generally fixed and there were few possible applications for the display to address. Moire interference could therefore be reduced to the point where it was not noticeable. Furthermore, the electron beam spot size of the CRTs used was relatively poor compared with more modern CRTs. This aided Moire suppression.
More recent advances in CRT performance and graphics software have caused Moire interference to once again become noticeable. A further complication stems from the introduction of CRTs having a non-linear dot pitch. Moire interference affects different regions of these CRTs at different critical pixel frequencies for each individual graphics application.
The display industry in general has recognised the re-emergence of Moire interference as a problem in high performance displays and some systems have been developed to reduce the effect by increasing spot size.
EPA 557 970 describes a technique for reducing Moire interference by shifting the displayed image vertically on every alternate frame, thereby smoothing out Moire effects. The vertical shifting however increase the susceptibility of the displayed image to vertical jitter.
In general, conventional solutions cannot detect if the Moire interference conditions are present in the display device. Instead, they attempt to reduce Moire interference, whether or not it is noticeable. The operation of these systems therefore tends to degrade the overall performance of the display. In particular, picture resolution is reduced.
In accordance with the present invention, there is now provided cathode ray tube display apparatus comprising: a cathode ray display tube in which an electron beam spot is scanned in a raster pattern to produce an output image; drive means for generating drive voltages on the focus electrode, first and second grid electrodes, and a cathode of the cathode ray tube to generate the electron beam spot in the cathode ray tube; spot size control means for simultaneously varying, in response to a spot control signal, the voltages generated on the focus electrode, first and second gird electrodes and the cathode by the drive means to vary the size of the electron beam spot in the cathode ray tube; and, Moire interference detection means comprising: a band-pass filter for generating the spot control signal in response to a signal indicative of the pixel frequency of a displayed image in a direction of raster scan falling within the pass
REFERENCES:
patent: 3806636 (1974-04-01), True et al.
Blum Theodore M.
International Business Machines - Corporation
McKinley Martin J.
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