Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
1998-10-01
2001-05-01
Ham, Seungsook (Department: 2878)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S368170
Reexamination Certificate
active
06225762
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for correcting the moiré effect on a multi-scan display monitor.
2. Background Information
Color cathode ray tubes (CRTs) are commonly used as visual display devices, employing up to three electrodes, typically one for each primary color: red, green, and blue. Most color CRTs use a shadow mask to selectively illuminate a matrix of each electrode's respective colored phosphors (i.e., red, green, and blue). The beam is typically larger than the shadow mask opening size, so the shadow mask blocks part of the beam and casts a portion of the original beam onto the desired phosphor. The dot pitch, or spacing, between adjacent shadow mask openings is typically between 0.2 and 0.3 mm to achieve a high resolution display.
A video pattern displayed on the CRT is comprised of a array of pixels, typically 640 to 1280 pixels wide and 480 to 1024 pixels high. One pixel will typically illuminate several screen dots. The number of screen dots illuminated is inversely proportional to the resolution of the video pattern. At higher video resolutions the number of screen dots illuminated can be less than three. At these higher resolutions an interference pattern, typically termed the moiré effect, can appear. From an operating standpoint, the moiré interference phenomenon poses a serious aesthetic problem, since the best electron beam focus and the highest image resolution results in generating an unacceptably noticeable moiré effect if the video signal being displayed includes alternating pixel patterns, which is a common occurrence.
One method of correcting the moiré effect is to rapidly shift the position of the image a small distance. The moiré patterns rapidly change in response to the shifting position and, because of persistence of vision, become unnoticeable. The required shift in position is small, typically about one-half of the screen dot pitch, and is imperceptible when the screen is viewed. One method of position shifting moiré cancellation is to shift all the lines of the display horizontally in every other display frame. Another method of position shifting moiré cancellation is to shift every other line of the display horizontally.
The disclosed methods add additional circuitry to the display monitor to produce the shifting of the image, increasing the cost and complexity of the display monitor. What is needed is a method and apparatus for correcting the moiré effect that reduces or eliminates the need for additional circuitry in the display monitor and which is suitable for use in display monitors which operate with a plurality of scanning resolutions and frequencies.
SUMMARY OF THE INVENTION
The present invention provides a method for correcting the moiré effect on a multi-scan display monitor with compensation for changes in the horizontal scanning frequency. A moiré adjustment signal is received which is substantially proportional to a desired horizontal spatial offset of alternate display lines on the CRT display. The horizontal spatial offset provides for the cancellation of the moiré pattern. A horizontal position signal is generated with a first frequency proportional to the horizontal scanning frequency and a position value substantially proportional to the moiré adjustment signal and the horizontal scanning frequency. A horizontal driver circuit generates a flyback pulse having a second frequency substantially equal to the horizontal scanning frequency and a phase that is shifted from a phase of the horizontal synchronization signal substantially in proportion to the horizontal position signal. The flyback pulse controls the position of lines on the display to correct the moiré effect. The increase in the moiré adjustment signal in proportion to the horizontal scanning frequency compensates for the lag of the horizontal driver circuit in responding to high frequency changes in the horizontal position signal.
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Fujimori Shin
Tadano Taro
Blakely , Sokoloff, Taylor & Zafman LLP
Ham Seungsook
Lee Shun
Sony Corporation
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