Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
1997-05-19
2001-07-31
Ham, Seungsook (Department: 2878)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S368280
Reexamination Certificate
active
06268705
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technological Field
The present invention relates to a method and apparatus for reducing vertical raster modulation in a raster scanned cathode ray tube (CRT) display.
2. Prior Art
Electron gun technology in high resolution CRT displays give very small electron beam spots on the CRT screen to provide good resolution in high addressability video formats. An increase in desired addressability stems from the increasing popularity of graphical user interfaces such as the Windows operating system from Microsoft and the OS/2 operating system from IBM. However, displays are also frequently used in more conventional text modes such as the Disk Operating System (DOS) text mode. These older text modes have lower addressabilities, leading to vertical break up of characters when a display with a small electron beam spot size is used. The character break up is generally known as vertical raster modulation. Displays having aperture grille CRTs are particularly susceptible to vertical raster modulation.
Conventionally, vertical raster modulation has been avoided by “doubling” the pixels in both the horizontal and vertical directions of the raster, thereby doubling the line density of the displayed image. However, many computer graphics adaptors do not have pixel doubling capability. Consequently, when they drive a high resolution display, vertical raster modulation can be come noticeable.
Vertical raster modulation in CRT displays having a single electron beam, such as for example, monochrome CRT displays or colour displays having a “Trinitron” CRT or similar CRT (Trinitron is a trade mark of Sony Corporation), the above mentioned problem of vertical raster modulation has been solved by stretching the electron beam spot in a vertical direction. This practice is generally referred in the art as applying an astigmatism to the spot. Conventionally, the astigmatism is applied by adding a four pole radially spaced electro-magnet structure to the electron gun assembly of the CRT. The four poles are equally spaced around the circumference of the neck of the CRT to generate a magnetic field which compresses the electron beam in a first direction and stretches it in a second direction orthogonal to the first direction, depending on the direction of current flow through the electro-magnet. Because the magnetic field generated by the four pole magnet structure acts in two orthogonal directions, this arrangement is not suitable for application to conventional in-line electron gun colour CRTs. Specifically, if used with such CRTs, this arrangement would cause convergence errors in the horizontal direction.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is now provided raster demodulation apparatus for a raster-scanned cathode ray tube display, the apparatus comprising control means for varying the shape of at least one scanned electron beam spot in a direction perpendicular to scan lines of the raster; characterised in that the control means comprises an electro-magnet for mounting on the neck of the cathode ray tube, the magnet having eight pole pieces spaced around the neck of the cathode ray tube and a winding passing around each of the pole pieces in succession in opposite directions so that, when an electrical current is passed through the winding, adjacent pole pieces around the neck of the cathode ray tube have different magnetic polarity to generate, within the neck of the cathode ray tube, a magnetic field in which magnetic lines of force pass through the or each electron beam only in a direction parallel to the scan lines of the raster.
The present invention advantageously enables a vertical astigmatism to be selectively introduced in both single electron beam CRTs and in-line multiple electron beam CRTs without introducing unwanted electron beam distortions, such as convergence errors.
Preferably, the pole pieces are divided into first group of four pole pieces comprising two pairs of pole pieces facing each other in a direction parallel to the lines of the raster and a second group of four pole pieces comprising two pairs of pole pieces facing each other in a direction perpendicular to the lines of the raster, the first group each having a greater number of turns of the winding than the second group. This advantageously draws any lines of magnetic force perpendicular to the raster lines away from the region of the CRT neck occupied by the or each electron beam. In a preferred embodiment of the present invention, the poles in the first group each carry three times the number of turns of the winding carried by each of the poles of the second group. The pole pieces are preferably directed radially inwardly.
In a particularly preferred embodiment of the present invention, the control means is arranged to vary the current passing through the winding in dependence on the raster line density of an image displayed on the display.
This advantageously enables raster modulation to be reduced to below a noticeable level without impairing overall image quality.
The control means preferably comprises a detector for determining the raster line density in dependence on raster line and frame synchronisation signals corresponding to the image displayed on the display. The detector enables the demodulator to vary the shape of the electron beam spot in real time.
The detector may comprise a switched capacitor low pass filter having a clock input triggered by the line synchronisation signal, a signal input responsive to the frame synchronisation signal, and an output for varying the shape of the scanned electron beam. This is especially attractive because it is relatively inexpensive to implement, involving only a few relatively low cost electrical components. For optimum performance, the switched capacitor filter is preferably configured as a second order low pass filter.
Preferably, the detector comprises: first duty cycle means for converting the line synchronisation signal to a line square wave signals; and second duty cycle means for converting the frame synchronisation signal to a frame square wave signal. This effectively reduces harmonic content of the line and frame sync signals, optimising the operation of the switched capacitor filter.
In preferred embodiments of the present invention, the detector comprises an anti-aliasing filter having an input connected to the output of the second duty cycle means and an output connected to the signal input of the switched capacitor filter. This prevents aliasing in the switched capacitor filter.
The detector may comprise a divide by N counter having an input connected to the first duty cycle means and an output connected to the clock input of the switched capacitor filter. The divisor of the counter may be varied under the control of a microprocessor to adjust the response of the filter.
Preferably, the detector comprises a rectifier for generating a DC control level in response to the output of the filter.
It will be appreciated that the present invention extends to a cathode ray tube display comprising a cathode ray tube and raster demodulation apparatus as described above.
Viewing the present invention from another aspect, there is now provided a method for reducing raster modulation in a raster-scanned cathode ray tube display, the method comprising varying the shape of an scanned electron beam spot scanned by the display in dependence on the raster line density of an image displayed on the display.
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N. H. Dekkers, A universal deflection unit generating a field of any order and azimuth, Journal Phys. D: Appl. Phys. 7:805-814 (1974).
Beeteson John
Knox Andrew
Ham Seungsook
International Business Machines - Corporation
Lee Shun
Morris, Esq. Daniel P.
Scully Scott Murphy & Presser
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