Television – Video display – Cathode-ray tube
Reexamination Certificate
1997-11-24
2001-06-12
Lee, Michael H. (Department: 2614)
Television
Video display
Cathode-ray tube
C348S607000, C315S382100, C315S382000, C315S370000
Reexamination Certificate
active
06246447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general, to video moiré reduction, and in particular, to a method and device for adaptively defocusing the electron beam size in dependence on the incoming video format.
2. Description of the Prior Art
Moiré is a common word for patterns which look like waves on water. An example in the real world is the interference between two fences on a bridge. Moiré appears when two sampling processes don't match each other and there is no sufficient filtering (interpolation/decimation).
The description of moiré in monitors is complex. The origin is the uncorrelated sampling process between the video format at the electrical side, and the two-dimensional sampling by the shadow mask at the mechanical-optical side. In addition, the scan line frequency and its higher harmonics as they interfere with the shadow mask are responsible for so-called scan line moiré. Scan line moiré is most noticeable when the picture content has constant luminance and no picture details.
Although high resolution video formats and high monitor bandwidth are desirable for sharp and detailed images, the visibility of moiré usually increases with both of these. Reducing scan line moiré is possible by influencing the shape and width of the electron beam itself, but unfortunately, the reduction of moiré by this method and preservation of resolution is a contradiction.
For standard monitors where there is a fixed horizontal and vertical scan frequency, moiré suppression is achieved by choosing a suitable mask pitch (mask sampling frequency ~1/mask pitch) in the horizontal and vertical directions. This is not possible for multi-sync high resolution monitors having large screens which are able to display various video graphics formats. These monitors are optimized for the highest resolution video formats for an electron beam having a very small spot size. In these types of monitors, even if the pitch size is chosen to optimize the reduction of moiré for the high resolution video format, moiré will still appear at lower resolution video formats. Also, taking into account that the customer is allowed to change the window size by about 15%, which changes the distance between pixels, an optimal pitch cannot be found for several scan modes.
European Patent Application EPA 707,300, corresponding to U.S. Pat. No. 5,747,933, describes a moiré detector for controlling the electron beam size in these multi-sync monitors. One of it's main components is an adaptive band-pass filter. The center frequency of the band-pass filter is adjusted to a moiré frequency. In the case where the incoming video signal is at the moiré frequency, or where there are fine details in the video image which match the moiré-frequency, the electron beam is defocused, thus losing resolution in the video image.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to avoid the conflict between increased resolution and increased moiré by using a video format adaptive electron beam spot size.
It is another object of the invention to reduce moiré by increasing the electron beam spot size for incoming low resolution video formats.
It is a further object of the invention to increase the spot size for incoming high resolution video formats when the image regions have low resolution picture content, yet maintaining the small spot size for the image regions which have a high resolution picture content.
It is yet another object of the invention to reduce moiré with minimum loss of resolution.
It is yet a further object of the invention to reduce moiré with minimum loss of sharpness.
It is even another object of the invention to reduce moiré without requiring feedback from the monitor, or the use of adaptive band-pass filters.
It is even a further object of the invention to provide a method and device for reducing moiré which can be used on various active display sizes and screen pitch sizes.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.
REFERENCES:
patent: 5483128 (1996-01-01), Chen
patent: 5747933 (1998-05-01), Beeteson et al.
patent: 5959414 (1999-09-01), Yoshida et al.
patent: 0707300A2 (1996-04-01), None
patent: 0707418A1 (1996-04-01), None
patent: 575885A (1993-03-01), None
patent: 575886A (1993-03-01), None
Goodman Edward W.
Lee Michael H.
Philips Electronics North America Corporation
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