Browse Prior Art Database

Autotransformer-Coupled Yoke for Resonant Flyback Deflection Circuits

IP.com Disclosure Number: IPCOM000116685D
Original Publication Date: 1995-Oct-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 6 page(s) / 314K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+2]

Abstract

The Video Electronics Standards Association (VESA) comprises of an association of several companies in the electronics industry which have an interest in video electronics. VESA now defines widely accepted standards for computer monitor video and sync pulse timings, amongst others.

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This is the abbreviated version, containing approximately 18% of the total text.

Autotransformer-Coupled Yoke for Resonant Flyback Deflection Circuits

      The Video Electronics Standards Association (VESA) comprises of
an association of several companies in the electronics industry which
have an interest in video electronics.  VESA now defines widely
accepted standards for computer monitor video and sync pulse timings,
amongst others.

      One such set of VESA timings is for an addressability
(information content) of 1024 x 768 pixels with a refresh rate of 75
Hz, which is sufficient to provide a flicker-free image on most
monitors.  IBM* had previously defined such a standard, but this was
not adopted by VESA for the following reason:

      IBM's standard is known as "XGA-75".  It is designed to run on
"IBM's XGA-2" video adapter card which has a maximum pixel clock
frequency of 90 MHz (90 Mpels/S).  XGA-75 has a pixel clock frequency
of 86 MHz.  There are, however, several other video adapter cards
available from other manufacturers.  Many of these are limited to a
maximum pixel clock frequency of 80 MHz and are unable to support
IBM's XGA-75 video standard.

      VESA, in an attempt to satisfy market demand for 1024 x 768
addressability with a flicker free image, and also to satisfy most
video adapter manufacturers including IBM, designed their 1024 x 768
@ 75 Hz mode to run with a pixel clock frequency of 78.750 MHz - well
within the capability of most video adapter cards on the market.

      VESA's timings, however, did not come without penalty.  In
order to reduce the pixel clock frequency from IBM's 86.000 MHz to
VESA's 78.750 MHz whilst maintaining the same addressability, both
horizontal and vertical blanking times had to be reduced.  These are
the periods within the horizontal and vertical deflection periods
during which the screen is blanked, and when the electron beams are
returned to their respective starting positions at the left and top
edges of the screen (flyback periods).  Relative to IBM's timings,
VESA reduced horizontal and vertical blanking times by 18.1% and
14.3%, respectively.  While this does not cause difficulties for the
vertical deflection stages of most CRT monitor designs, it does in
fact introduce a very significant problem for their horizontal
deflection circuits.  In designing this mode and its becoming a de
facto standard, VESA solved a big problem for the world's video
adapter manufacturers and at the same time created one for its
monitor manufacturers!

      Specifically, the problem to the horizontal deflection circuit
is as follows:

      In order to center the displayed image within the bezel in the
horizontal direction, it is necessary to shift the position of the
image within the raster.  This is achieved by introducing an
adjustable phase offset between the incoming horizontal sync pulse
and the deflection circuit's flyback pulse.

      However, in order to have an acceptable image quality and
adequate phase adjustment range,...