Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2001-10-31
2004-04-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S364000
Reexamination Certificate
active
06717377
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to raster display systems and, more particularly, to a circuit and method for reducing east-west geometry mismatch between the top and bottom of a raster display.
2. Related Art
Raster display systems are used in a variety of applications such as televisions and computer displays.
FIG. 1A
shows a cross-sectional side view of a conventional raster display system
100
. Raster display system
100
includes an electron gun
110
, a deflection system
120
, and a screen
130
. Electron gun
110
generates and accelerates an electron beam
115
toward deflection system
120
. Deflection system
120
deflects electron beam
115
horizontally and/or vertically at screen
130
. Screen
130
includes a phosphor-coated faceplate that glows or phosphoresces when struck by electron beam
115
.
Deflection system
120
includes a horizontal deflection generator
122
, a horizontal deflection coil
124
, a vertical deflection generator
126
, and a vertical deflection coil
128
. Horizontal deflection coil
124
and vertical deflection coil
128
are collectively referred to as the yoke. Although not shown, horizontal deflection coil
124
and vertical deflection coil
128
are wound at a ninety-degree angle relative to one another.
Horizontal deflection generator
122
generates a horizontal deflection current signal I
H
. When horizontal deflection current signal I
H
passes through horizontal deflection coil
124
, a magnetic field is created that deflects electron beam
115
horizontally. The horizontal angle of deflection (not shown) is proportional to the direction and the magnitude of horizontal deflection current signal I
H
. Similarly, vertical deflection generator
126
generates a vertical deflection current signal I
V
. When vertical deflection current signal I
V
passes through vertical deflection coil
128
, a magnetic field is created that deflects electron beam
115
vertically. The vertical angle of deflection &thgr; is proportional to the direction and the magnitude of vertical deflection current signal I
V
.
FIG. 1B
is a front view of raster display system
100
. Deflection system
120
deflects electron beam
115
from a left edge LE of screen
130
to a right edge RE of screen
130
to draw a first line L
1
. Electron beam
115
is then briefly turned off, moved downward, and brought back to left edge LE of screen
130
by deflection system
120
. Electron beam
115
is then turned on and deflection system
120
deflects electron beam
115
from left edge LE of screen
130
to right edge RE of screen
130
to draw a second line L
2
. This process continues very rapidly so that lines L
3
through L
N
(where N=1, 2, 3, . . . , N) are drawn thereby creating an raster on screen
130
. Note that the raster drawn on the screen
130
shown in
FIG. 1B
is ideal since lines L
N
extend to, but not beyond, left edge LE and right edge RE.
FIG. 2A
shows a conventional horizontal deflection generator
122
. Horizontal deflection generator
122
includes a sawtooth generator
210
and an amplifier
220
. Sawtooth generator
210
generates a horizontal sawtooth signal CS
1
having a constant amplitude.
FIG. 2B
shows a waveform for horizontal sawtooth signal CS
1
. Horizontal sawtooth signal CS
1
is amplified by amplifier
220
, which outputs horizontal deflection current signal I
H
.
In
FIG. 2B
, horizontal active time t
HA
corresponds to the time when electron beam
115
starts drawing a line L
N
at left edge LE of screen
130
to the time when electron beam
115
stops drawing a line L
N
at right edge RE of screen
130
. The horizontal retrace time t
HR
corresponds to the time when electron beam
115
stops drawing a line L
N
at right edge RE of screen
130
, moves from right edge RE of screen
130
back to left edge LE of screen
130
, and starts drawing another line L
N
.
One problem with using horizontal sawtooth signal CS
1
for horizontal deflection current signal I
H
is that it produces a distorted raster.
FIG. 2C
shows a distorted raster. Lines L
N
at the top portion of screen
130
and the lines L
N
at the bottom portion of screen
130
do not extend all the way to the left edge LE of screen
130
or all the way to the right edge RE of screen
130
which produces a distorted raster. Note that the distortions may have opposite phase, depending on the particular physical implementation. In this case, the top and bottom of the raster will be wider than the middle of the raster.
FIG. 3A
shows a conventional horizontal deflection generator
122
′ that can be used to address the problems shown in FIG.
2
C. Horizontal deflection generator
122
′ includes a horizontal geometry correction circuit
310
and a horizontal deflection signal generator
320
. Horizontal deflection signal generator
320
includes a horizontal sawtooth generator
322
, a modulator
324
, and an amplifier
326
. Horizontal geometry correction circuit
310
generates a correction signal CS
2
.
FIG. 3B
shows a waveform for correction signal CS
2
. Horizontal sawtooth generator
322
generates a horizontal sawtooth signal CS
1
having a constant amplitude (as shown in FIG.
2
B). Correction signal CS
2
modulates the amplitude of horizontal sawtooth signal CS
1
via modulator
324
. Modulator
324
outputs a horizontal deflection signal CS
3
(not shown), which is amplified by amplifier
326
. Amplifier
326
then outputs horizontal deflection current signal I
H
.
In
FIG. 3B
, the vertical active time t
VA
corresponds to the time when electron beam
115
starts drawing the first line L
1
at the top left of screen
130
to the time when electron beam
115
stops drawing the last line L
N
at the bottom right of screen
130
. The vertical retrace time t
VR
corresponds to the time when electron beam
115
stops drawing the last line L
N
at the bottom right of screen
130
to the time when electron beam
115
starts drawing the first line L
1
at the top left of screen
130
.
A problem with using horizontal deflection signal CS
3
for horizontal deflection current signal I
H
is that it produces asymmetry between the top and bottom of the raster on screen
130
.
FIG. 3C
shows such a raster. Note that lines L
N
at the top portion of screen
130
extend beyond left edge LE and right edge RE. This results in an east-west (or left-right) geometry mismatch between the top and bottom of screen
130
.
The reason that lines L
N
at the top portion of screen
130
extend beyond left edge LE and right edge RE is explained below. Horizontal deflection signal CS
3
includes two components: horizontal sawtooth signal CS
1
and correction signal CS
2
. When horizontal deflection signal CS
3
is amplified by amplifier
326
, amplifier
326
distorts the correction signal component CS
2
of horizontal deflection signal CS
3
. In particular, amplifier
326
distorts a discontinuous portion
330
of correction signal CS
2
, creating a distorted signal portion
331
.
FIG. 3B
shows a waveform for an amplifier output signal corresponding to correction signal component CS
2
. The reason amplifier
326
distorts discontinuous portion
330
is because amplifier
326
has a limited frequency bandwidth. Note that amplifier
326
eventually outputs a non-distorted signal portion
333
.
Distorted signal
331
causes the amplitude of horizontal deflection current signal I
H
to increase at the time the lines L
N
are being drawn at the top portion of screen
130
. As a result, the lines L
N
that are drawn at the top portion of screen
130
extend beyond left edge LE and right edge RE. Once amplifier
326
begins to output non-distorted signal portion
333
, the amplitude of horizontal deflection current signal I
H
is not distorted and thus the lines L
N
that are drawn extend to, but not beyond, left edge LE and right edge RE of screen
130
. Since the lines L
N
drawn at the top portion of screen
130
extend beyond left edge LE and right edge RE, while the lines L
N
drawn at the middle and bottom portions
Parsons Hsue & de Runtz LLP
Tran Chuc D
Wong Don
ZiLOG, Inc.
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