Method for Automatic Cut-Off Set Up of Displays

Method for Automatic Cut-Off Set Up of Displays

      Disclosed is a method which allows a display to automatically
set up its own cutoffs in a simple, accurate and very cost effective
manner.  The concept could be readily extended to include setting up
the video gains and also setting G2 given suitable additional
circuitry.

      Current display repair strategies tend to involve 'on site
repair'.  During this repair, a new CRT/card assembly combination
needs to be set up.  With microprocessor-based monitors it has been
found that it is sufficient to set up the spare card assembly at the
factory with the geometry and video gain settings set to 'process
average' values.  However, the CRT cut offs are extremely variable
and using average cut off values does not always give ideal results.
The effect of this variability is that the background raster is not
always the same colour and poor colour tracking results.

      An existing solution is to allow the repairer to manually
adjust the cutoffs by using a coded 'second menu' (available via the
user keypad).  This requires a certain level of skill and will still
not ensure perfect colour tracking.  The new solution is simple and
does not require any skills on the part of the repairer.  In
addition, it provides a perfect background raster colour print.

      The key element of the solution is for the monitor's
microprocessor to monitor the beam current and by varying the
individual cut off settings to determine the point at which each beam
is just cut off.  In this way the cut offs can be perfectly set up
automatically by the microprocessor.

      First, consider the process involved in setting up the cutoffs
in this way.  Possible hardware implementations will then be
detailed.  The process is:
  1.  Microprocessor sets brightness to midpoint.
  2.  Microprocessor sets cut offs to minimum (blackest).
  3.  Screen image blanked (either by the microprocessor or by
       displaying a black screen).
  4.  G2 set so that the raster is just extinguished.  This can be
done
       either by the repairer or else by the microprocessor,
monitoring
       beam current.
  5.  Each colour cut off increased in turn whilst monitoring the
beam
       current.  To improve noise immunity it is suggested that this
is
       done until a beam current of (say) 1uA is detected.  The
cutoff
       setting is then backed off by a fixed number of steps to take
the
       beam current back to zero (at the point where beam current is
       about to start to flow).

      The simplest way of measuring beam current is by monitoring the
voltage on the bottom of the beam current limit resistor (R1 in Fig.
1,
a typical beam current limit circuit).  The current through R1
consists
of the flyback transformer (FBT) bleed current (Typically 50uA) plus
the beam current.  The bleed current is present irrespective of beam
current.  ...