Television – Cathode-ray tube display beam current control
Reexamination Certificate
1999-06-22
2001-03-20
Peng, John K. (Department: 2714)
Television
Cathode-ray tube display beam current control
C348S380000, C348S673000, C348S678000, C348S687000, C348S694000, C348S696000, C315S387000, C315S383000
Reexamination Certificate
active
06204882
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to cutoff and gain control for a device for displaying video signals. It is based on a colour tube in a colour television set, and on projection tubes for displaying video signals.
One precondition for correct colour-video reproduction is that monochrome pictures are reproduced with neutral colours. This requires adaptation of the tube cutoff points—also called cutoff trimming—as well as a specific ratio of the cathode currents at a so-called white point, which can be achieved by means of an adjustment process which is known as gain or drive trimming.
Control circuits are used in order, in particular, to keep the cutoff points of a colour tube stable, despite ageing. To this end, measurement lines are injected in a sequential manner for the three colour channels, also called RGB colour channels, during the frame blanking period, and these measurement lines are normally equivalent to a relatively small signal amplitude so that the correction process is carried out to a value which comes as close as possible to the actual cutoff points of tube systems. The cathode currents during the measurement lines are measured, and are compared with a reference value. A DC voltage offset, in practice equivalent to a brightness offset, is used to raise or lower the measurement lines for each colour channel in the event of a control error, in such a manner that the measurement currents correspond to the reference value. However, the control process cannot ensure that the actual black level of the signal corresponds to the cutoff points of the tube. Furthermore, there is no correction for any changes in the gain.
Recent circuit concepts thus inject a further measurement line, whose amplitude is greater, into each colour channel for gain control, which is also known as drive control (for example so-called cutoff measurement lines in one field and so-called drive measurement lines in the other field, alternately). Such a control system by means of two different reference values represents a two-point control system which, in practice, is formed by two control loops. Two different cathode control voltages V
K1
and V
K2
, to which two different voltage levels of the respective RGB signal correspond, and two corresponding cathode currents I
K1
and I
K2
are defined as reference values—corresponding to the circuit concept used. Values that may be used in practice are, I
K1
(as I
cutoff
)≈7 . . . 15 &mgr;A and I
K2
(as I
Drive
)≈15 . . . 30 &mgr;A.
Owing to the physically dependent beam-current characteristic profile I
K
=f (V
K
) as a function of the control voltage V
K
at the respective cathode of a colour tube, the following mathematical relationship may be used for the ratio of two cathode currents I
K
, based on the two-point control system:
I
K2
I
K1
=
(
V
K2
V
K1
)
γ
The described two-point control system is thus based on an “assumed” exponent &ggr; since, from the definition of the reference voltages and currents, the “theoretical” characteristic exponent is also defined uniquely from the above equation. For colour tubes that are known at present in the consumer area, the value of the exponent &ggr; is about 2.0 to 2.4. This is subject to only relatively minor scatter for tubes with the same system design.
Assuming a constant value for the characteristic exponent &ggr;, it is known, for example from the “One-chip TV processor” TDA 884X/885X-N1 from Philips, for a cutoff and drive control system designed as a two-point control system to define reference currents and reference voltages which respectively correspond to two different cathode currents, which are called the first and second reference cathode currents, I
K1
and I
K2
, respectively, in the following text. Since, in practice, an IC concept can be used with widely differing tube types, it has been found, however, that, if the value for the characteristic exponent &ggr; or for the voltage ratio V
K1
/V
K2
is assumed or defined, it is not always possible to achieve an optimum result for displaying monochrome pictures.
SUMMARY OF THE INVENTION
The object of the invention is thus to specify a method for cutoff and drive control, using which an optimum result can be achieved for all colour tubes.
This object is achieved by the invention specified below.
A further object of the invention is to provide a video processor having a control device for cutoff and drive control, by means of which an optimum result can be achieved for all colour tubes.
This object is achieved by the invention specified below.
The invention is based on the recognition that a cutoff control system which is based on an assumed exponent &ggr; which does not correspond to the actual tube exponent &ggr; is unable to control the black levels of the RGB signals such that they are coincident with the cutoff points of a colour tube. Thus, for example, an actual tube exponent &ggr;, which differs from the assumed exponent &ggr;, is compensated for by a DC offset (that is to say corresponding to a cutoff error) and a drive offset (that is to say corresponding to a gain error).
The invention is based on the idea, in a video processor having a control device of the type mentioned above, of automatically matching the value (which until now has been assumed in the control algorithm) for the exponent &ggr; to the actual exponent &ggr; of the respective colour tube. Specifically, if the actual value of the exponent &ggr; in an RGB colour channel in the colour tube used differs from the assumed value on which the cutoff and drive control is based, then both control loops will compensate for this discrepancy, which is interpreted as an error, in accordance with the mathematical relationship
I
K2
I
K1
=
[
(
v
1
f
×
u
2
+
u
0
)
×
v
2
-
C
00
(
v
1
f
×
u
1
+
u
0
)
×
v
2
-
C
00
]
γ
via a DC voltage offset u
0f
and a gain matching factor &Dgr;v, where &ggr; is the actual value of the tube exponent and v
1
f
is the gain adapted by &Dgr;v (called the erroneous gain in the following text), while u
1
and u
2
are measurement signal voltages, generated in the respective RGB colour channel of the video processor, for the two measurement lines (cutoff and drive) which,—in accordance with the description below—amplified by the erroneous gain v
1
f
and shifted by a DC voltage offset called the total DC voltage offset u
0
in the following text, occur at an RGB colour signal output of the video processor. As a consequence of further amplification, called the gain v
2
in the following text, by the output amplifier stage (which is known per se) in the respective RGB colour channel, the measurement signal voltages u
1
and u
2
and the total DC voltage offset u
0
are amplified further before they reach the correspondingly associated tube cathode (
FIG. 1
b
). Coo denotes an operating-point-dependent voltage offset of the actual cutoff point C
0
of a tube system in the colour tube. In principle, this forms a difference (
FIG. 2
b
), between the black level of the RGB signal at the output of an output amplifier stage, and the actual cutoff point C
0
of the corresponding tube system, which difference is compensated for, by means of the cutoff control system, with a corresponding DC voltage offset u
00
. This DC voltage offset u
00
and the &ggr;-dependent DC voltage offset u
0f
are taken into account—as corresponding elements—in the total DC voltage offset u
0
mentioned above.
To explain the control algorithm further, this relationship is rewritten in the form
I
K2
I
K1
=
[
(
v
1
f
×
u
2
+
u
0
)
×
v
2
-
C
00
(
v
1
f
×
u
1
+
u
0
)
×
v
2
-
C
00
]
γ
=
[
(
v
1
f
×
n
f
×
u
1
+
u
0
)
×
v
2
-
C
00
(
v
1
f
×
u
1
+
u
0
)
×
v
2
-
C
00
]
γ
where n
f
is the erroneous ratio of the amplitudes of the measurement signal voltages u
1
and u
2
of the two measurement lines of an RGB colour signal. According to the prior art, the ratio n (and n
f
if &ggr; is not correct) of the measurement signal voltages u
1
an
Deutsche Thomson-Brandt GmbH
Kiel Paul P.
Natnael Paulos
Peng John K.
Tripoli Joseph S.
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