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Adaptive Pulse Stretch Circuit for Moire Reduction

IP.com Disclosure Number: IPCOM000115084D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 55K

Publishing Venue

IBM

Related People

Beeteson, JS: AUTHOR [+2]

Abstract

The formation of moire interference patterns on CRT computer displays is becoming a problem again due to the sharper spot performance of modern CRTs, to the large number of modes in use, and to the use of certain background shading patterns on GUI Operating Systems.

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Adaptive Pulse Stretch Circuit for Moire Reduction

      The formation of moire interference patterns on CRT computer
displays is becoming a problem again due to the sharper spot
performance
of modern CRTs, to the large number of modes in use, and to the use
of
certain background shading patterns on GUI Operating Systems.

      Disclosed is a low-cost technique to widen the spot profile so
as to reduce the visibility of video moire.  The circuit to be
described can operate on a pel-by-pel basis so that moire control can
rapidly be switched in and out.

Since widening the spot can affect brightness, a compensation method
is also described.

Pulse stretching - The circuit shows a typical video amplifier in a
modern high frequency computer display monitor where a cascode input
amplifier is followed by a push-pull driver.

      Additions to the standard circuit are the Q1 connection and the
the Q2 connection.  Q1 allows the switching in and out of a capacitor
on one of the output bases.  When switched in this capacitor will
have the effect of slowing turn on and so widening the output pulse.
Switching the capacitor out (by transistor Q2) will remove the pulse
stretching effect as illustrated in the timing diagrams.

Gain control - Because pulse stretching can affect brightness, an
optional gain control may be needed.  In practice the amount of pulse
stretch needed to eliminate moire is quite small - about 10% - and
the brightness change may not be visually signif...