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Browse Prior Art Database

Self Adjusting Multi-Frequency Dynamic Focus Circuit for Cathode Ray Tube Displays

IP.com Disclosure Number: IPCOM000115128D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 6 page(s) / 356K

Publishing Venue

IBM

Related People

Jackson, FS: AUTHOR [+2]

Abstract

Large sized CRT monitors require Dynamic Astigmatic Focus (DAF) correction circuits which ensure that the electron beam remains optimally focused at all points across the screen surface area. The DAF circuit is required to generate a complex waveform which typically consists of a combination of horizontal and vertical parabolic components. This waveform is superimposed upon an adjustable DC voltage of approximately 7KV for a color CRT before it is passed to one of the CRT's focus electrodes.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 16% of the total text.

Self Adjusting Multi-Frequency Dynamic Focus Circuit for Cathode
Ray Tube Displays

      Large sized CRT monitors require Dynamic Astigmatic Focus (DAF)
correction circuits which ensure that the electron beam remains
optimally focused at all points across the screen surface area.  The
DAF circuit is required to generate a complex waveform which
typically consists of a combination of horizontal and vertical
parabolic components.  This waveform is superimposed upon an
adjustable DC voltage of approximately 7KV for a color CRT before it
is passed to one of the CRT's focus electrodes.

      The parabolic form and amplitude of the DAF signal result
directly from the curvature of the CRT's screen, the radius of
deflection of the electron beam, several electron gun design
parameters and the voltages applied to the electrodes within the gun.
These parabolic voltage components of the composite DAF signal must
therefore be synchronized to both horizontal and vertical deflection
currents of the raster scan circuits.  Their parabolic waveshapes and
amplitudes must remain constant, since they depend primarily on
screen geometry, which is fixed for a given CRT design.  In
particular, they must not vary with either horizontal or vertical
deflection frequencies.  In multifrequency monitors (multisync or
autosync), this becomes a problem.

      Provision of a constant-amplitude parabolic waveform at
vertical frequencies, synchronized to the vertical deflection
current, does not normally present any difficulties in a colour
monitor design since such a waveform is usually required for
East-West Pincushion correction purposes.  It is therefore readily
available for use by the DAF circuit.  To generate a typical 200V
peak-to-peak vertical parabola, the DAF circuit simply needs to
amplify the East-West Pincushion correction signal with a simple
amplifier circuit.

      By contrast, generation of a constant-amplitude horizontal
parabola is not straightforward in a multifrequency design.  The
prior
art provides several techniques, all of which are relatively complex
and costly, either in parts cost, in manufacturing set-up, or both.

      Described is a particularly low cost and relatively simple
approach to the generation of a self-adjusting, constant-amplitude
parabolic waveform which is synchronized to the horizontal deflection
system of a CRT display over a large range of horizontal deflection
frequencies.  The remaining parts of the circuit utilize conventional
techniques to generate the composite DAF waveform and are not the
subject of this invention disclosure.  They are, however, described
briefly in order to aid the reader's understanding of the complete
DAF circuit.  The constant-amplitude, multifrequency parabolic
waveform is then amplified to typically 400V p-p, prior to being
summed with a 200V p-p vertical parabola and 7KV (adjustable) DC
focus voltage.  The composite waveform; horizontal parabola, vertical
parabola...