Abstract:
Lines or sections of lines on a scanned CRT display are checked to determine whether a moire pattern exists. The existence is determined by comparing the number of pixels occurring on a line or a section of a line to a reference count corresponding to a moire pattern. Substantial equality of the pixel count and the reference count enables a moire correction circuit for the section of the display containing the moire pattern. In sections of the display where the moire pattern does not occur, i.e., the pixel count and the reference count are not substantially equal, the moire correction circuit is automatically disabled.

Description:
DOCUMENTS INCORPORATED BY REFERENCE 
     U.S. Pat. No. 5,440,353 is incorporated by reference to show a horizontal moire cancellation circuit which is useful in the invention described below. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to cathode-ray tube displays and particularly to raster scanned displays, especially color displays. More particularly, it relates to automatically correcting such displays to eliminate or at least substantially reduce horizontal moire interference. 
     2. Background 
     Moire effects create ripple effects in video displays which smear detail and detract from the picture quality. Corrections may be applied to the video signals to suppress moire effects. To correct for horizontal moire, the lines of the display are slightly delayed. In the prior art, correction of horizontal moire are manually activated and the correction is applied to the entire screen. When only a section of the display requires correction, applying the correction to the entire screen degrades resolution of the displayed image and defocuses all or part of the display. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention, the existence of a moire pattern on a display is determined automatically. A count of the number of pixels occurring in horizontal lines of a scanned CRT display is made and compared to a reference count. If the counts are substantially equal, a moire correction circuit is enabled for that section of the display. Sections of each horizontal line can be separately checked for the presence of a moire pattern. The moire correction circuit is automatically disabled for sections of the display for which it is not required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is described in detail by referring to the various figures of the drawing which illustrate specific embodiments of the invention, and wherein like numerals refer to like elements. 
     FIG. 1 is a flowchart depicting a sequence of operations according to the invention. 
     FIG. 2 is schematic diagram of an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principle of operation according to the invention is shown in FIG.  1 . In the following description, references are made to a flowchart depicting the sequence of operations performed by a program. The symbols used are standard flowchart symbols accepted by the American National Standards Institute and the International Standards Organization. In the explanation, an operation may be described as being performed by a particular block in the flowchart. This is to be interpreted as meaning that the operations referred to are performed by programming and executing a sequence of instructions that produces the result said to be performed by the described block. Alternatively, the operation may be performed by apparatus designed for the purpose. The actual program instructions used depend on the particular processor used to implement the invention. Different processors have different instruction sets but persons of ordinary skill in the art are familiar with the instruction sets with which they work and can implement the operations set forth in the blocks of the flowchart. 
     A color display is produced by modulating the three electron beams, one for each of the basic colors: red, blue, and green. The electron beams strike a phosphor which produces the corresponding color. The electron beam for each color is directed to the correct phosphor and is blocked, using a mask, for example, from striking the wrong color phosphor on the screen. Some color displays use a timing circuit and alternating stripes of phosphors to cause each electron beam to strike the correct phosphor and to avoid striking the other phosphors. 
     The electrons striking the phosphor create a dot of the corresponding color. Such dots are referred to as picture elements or pixels for short. The close proximity of varying pixel colors creates a blending of the basic colors to produce combinations of colors. 
     Because of mechanical tolerances in the mask or in the timing, the electron stream may be misaligned for certain color pixels so that the electrons do not always strike the correct phosphor dot or stripe exactly in the latter&#39;s center. As a result, the pixel is not completely illuminated. This creates a moire pattern. The problem is most noticeable when the display driver is attempting to turn on and then off adjacent pixels of the same color. For purposes of this explanation, the situation where an electronic beam is turned on and off at a rate that creates a moire interference is referred to as overmodulation. 
     Correction of moire patterns is more complicated when a monitor is capable of multifrequency operation. That is, for example, a monitor designed to work in cooperation with more than one type of computer display driver. Not all computer display drivers operate at the same frequency so the monitor is designed to perform at more than one frequency. 
     The presence of a moire pattern in the display can be detected by the occurrence of a predetermined number of pixels during a given time period. 
     In FIG. 1, a modulation (or pixel) detector recognizes the occurrence of pixels in a display stream. When a pixel is detected ( 101 ), a pixel count value (PCNT) is incremented ( 103 ). If the TIME OUT signal does not occur ( 104 ) but the horizontal synchronization signal (HSYNCH) does ( 109 ), then PCNT is reset ( 110 ). 
     If the TIME OUT signal does occur ( 104 ), then PCNT is compared to a predetermined reference count value (RCNT). If PCNT is equal or substantially equal to RCNT ( 105 ), a moire correction is enabled ( 107 ). The circuit than waits for an HSYNCH signal ( 106 ) to reset PCNT ( 110 ). If, after the TIME OUT signal occurs ( 104 ), PCNT does not substantially equal the reference count RCNT ( 105 ), then the moire correction circuit is disabled ( 111 ) and PCNT is reset ( 110 ). 
     The TIME OUT signal is generated by a timing circuit and can be adjusted so that several sections of a horizontal line can be checked for the occurrence of a moire pattern. 
     Details of an embodiment of the invention are shown in FIG.  2 . Three operational amplifiers  201 A,  201 B, and  201 C are coupled to receive pixel information from an associated video driver circuit for displaying color signals on a scanned cathode-ray tube display system. When any one of the red, green, or blue video signals exceeds a reference value, an enabling signal is supplied to an OR gate  203 . The output signal from the OR gate  203  increments the count value (PCNT) in a pixel counter  205 . As a result, the pixel counter  205  contains the number of pixels which have been displayed since a reset signal had been applied. 
     The output count from the pixel counter  205  is applied to one set of input terminals of a comparator  207 . Input signals to the other input terminals of the comparator  207  are signals representing a reference count (RCNT) supplied by a controller  221  which can include a microprocessor. The reference count corresponds to a number of pixels over a predetermined period of time that would indicate that a horizontal moire problem existed. Such a value depends on the horizontal frequency of the particular display system in which the invention is installed. 
     The reference count depends on several factors. As an example, the reference count can be determined as            horizontal                 display                 size       2        (     dot                 pitch     )          (     number                 of                 sections                 per                 horizontal                 line     )         .                          
     That is, the reference count,        RCNT   =     pps   2                            
     where        pps   =       np   s     .                            
     The value of np is the total number of pixels in one horizontal line across the screen and is given by        d   p                          
     where d is the horizontal display size, e.g., in millimeters, and p is the CRT&#39;s dot pitch, spacing between adjacent dots of the same color phosphor, e.g., in millimeters. The value s is the number of sections to be checked along a horizontal line. For example, a typical value might be 10. The divisor 2 in the RCNT equation arises from the fact that an on/off sequence is being checked for. 
     The TIME OUT signal on line  223  in FIG. 2 is generated by a check count. A check counter  213  is incremented by the output signal from an AND gate  217  which is primed by the reset output signal from a MATCH flip-flop  209  and enabled by an output signal from an oscillator  215 . The check count is supplied to the controller  221  and when it reaches a predetermined value, the TIME OUT signal  223  is generated. Alternatively, the check count can be maintained inside the controller  221  by an internal clock and enabled by the reset signal from the MATCH flip-flop  209  via an input port. Other implementations will be recognized by those of ordinary skill in the art. 
     The check count, CC, depends on the number of sections to be checked on each horizontal line, designated by s. The horizontal frequency, h, of the display is determined by the controller. Therefore, the time required for a beam to travel across the entire screen is        t   =       1   h     -   r                            
     where r is the horizontal retrace time, i.e., the time for the beam to go from the end of one line to the beginning of the next line. Dividing t by s gives the time for the beam to move across a section of a line to be evaluated; that is, τ=t/s and the check count is CC=τ×c where c is the frequency of the oscillator  215 . 
     As an example, if 640 pixels in a given time during the scanning of a horizontal line would produce a moire effect, then the reference count would be set to        640   2                          
     or 320. This would be represented by a binary signal of 101000000, requiring nine bits. In one embodiment, only the most significant seven bits of both counts would be applied to the comparator  207 . By eliminating the least significant two bits of the pixel count and the reference count, the comparison of the pixel count and the reference count will indicate equality within a value of ±4. 
     When a TIME OUT signal occurs on line  223 , the MATCH flip-flop  209  will be set if PCNT=RCNT±4, for example. When the TIME OUT signal goes low, the setting of the MATCH flip-flop  209  is transferred to a flip-flop  211  by a clock signal derived from an inverter  216 . On a line  231 , the set condition of the flip-flop  211  will enable a horizontal moire circuit during the time it is set. A suitable horizontal moire canceling circuit is described in detail in U.S. Pat. No. 5,440,353 incorporated herein by reference. 
     The occurrence of a horizontal synchronization signal resets the MATCH flip-flop  209  and the check counter  213  via an OR gate  219 . The check counter  213  is also reset by the TIME OUT signal via the OR gate  219 . 
     Any or all parts of the invention described can be incorporated in software to operate according to the description set forth above. A hardware implementation has several advantages including faster operation independently from a microprocessor. Performing the reference count in the controller  221  has the advantage that the system of the invention can be incorporated to accommodate display systems with differing horizontal frequencies. 
     The arrangement of the system according to the invention permits the moire correction circuit to be enabled by other means such as by the user via an input to the controller in accordance with the prior art. 
     While the Invention Has Been Particularly Shown and Described with Reference to a Preferred Embodiment Thereof, it Will Be Understood by Those Skilled in the Art That Various Changes and Modifications in Form and Details May Be Made Therein Without Departing from the Spirit and Scope of the Invention According to the Following Claims.