Browse Prior Art Database

Triple Diode Modulator

IP.com Disclosure Number: IPCOM000111451D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 6 page(s) / 225K

Publishing Venue

IBM

Related People

Gibbs, T: AUTHOR [+3]

Abstract

Disclosed is a circuit that reduces picture geometry distortion caused by EHT beam current variations in CRT-based displays. The circuit employs a combination of line flyback time control to remove asymmetric geometry distortion and picture width control to remove symmetric geometry distortion. A low cost circuit which adds a third diode modulator is described with circuit values and achieves high performance with less power components than alternative solutions.

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Triple Diode Modulator

      Disclosed is a circuit that reduces picture geometry distortion
caused by EHT beam current variations in CRT-based displays.  The
circuit employs a combination of line flyback time control to remove
asymmetric geometry distortion and picture width control to remove
symmetric geometry distortion.  A low cost circuit which adds a third
diode modulator is described with circuit values and achieves high
performance with less power components than alternative solutions.

      The electron beam within the CRT is deflected by
electromagnetic scanning coils.  The energy required to deflect the
beam is mathematically related to the EHT voltage.  Thus a change in
picture size will result from the change in EHT voltage.  Examination
of a CRT-based display will confirm this fact.  The raster is scanned
one line at a time from top to bottom.  Lines having low brightness
will have low load on the EHT supply and normal picture width will
result.  Lines having a high brightness and beam current will load
the EHT.  As the load increases on a poorly regulated supply, the
voltage falls and the picture width will increase, a very undesirable
effect.  The performance and quality of a display is judged under
these conditions.  It is also important to maintain the EHT when the
display or TV is interlaced when alternate fields may be heavily or
lightly loaded, leading to differing sizes in alternate scans.

      One way to ease the problem is to employ an independent EHT
supply and separate scan circuit.  This system is prohibitively
expensive in a competitive market place.  This alone does not fix the
problem because of the high impedance (typically 2M ohms) of a
typical 25,000-volt supply.  Feedback can be added to reduce the
impedance and improve performance; however, the time lag between the
line of high beam current, the drop in EHT voltage and the subsequent
corrections by pumping in more energy over the following cycles is
all too slow to give the performance needed in a modern display.  A
more widely used method of EHT generation is to use the line-scanning
transformer to produce the necessary voltage.  As the electron beam
scans across the screen, energy builds in the line output or flyback
transformer.  At the completion of the line, the scanning transistor
is turned off.  The abrupt fall in current causes the magnetic field
to collapse, inducing a large back emf in the transformer.  A
secondary winding, employing a diode split technique produces the
required voltage through transformer coupling, with the appropriate
turns ratio.  This circuit has no active EHT regulation and so
performance is poor, but in this second method of generating EHT, a
diode modulator circuit is often used to control the EHT during
application of the correction parabola.  This diode modulator circuit
is sometimes fed with an LF, near DC, correction signal in an effort
to control overall picture width.  Even this te...