Computer graphics processing and selective visual display system – Display driving control circuitry – Controlling the condition of display elements
Reexamination Certificate
2000-07-14
2003-05-13
Miller, John (Department: 2614)
Computer graphics processing and selective visual display system
Display driving control circuitry
Controlling the condition of display elements
C348S377000, C315S380000, C315S382100
Reexamination Certificate
active
06563496
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cut-off adjusting apparatus for a display using CRT (Cathode Ray Tube), and particularly to a circuit for detecting and correcting cathode current.
2. Description of the Related Art
The cut-off adjustment is generally defined as a critical bias voltage adjustment of a cathode voltage to a first grid voltage of CRT which is a boundary at which a raster is deleted or displayed from a screen. In other words, the cut-off adjustment is defined as such a black level adjustment that the dispersion of a cut-off voltage is absorbed and the black level is kept constant.
The display using CRT has such a problem as variation of cathode current due to time lapse, specifically brightness variation.
As a countermeasure to the above problem has been known a cut-off adjusting apparatus for receiving a specific signal for detecting cathode current to detect the cathode current corresponding to the signal and adjusting the cathode current, thereby performing the cut-off adjustment.
FIG. 4
is a circuit diagram showing the construction of a conventional cut-off adjusting apparatus.
As shown in
FIG. 4
, the cut-off adjusting circuit
10
comprises a video amplifier
11
, a cathode voltage clamp capacitor
12
, a cathode voltage clamp resistor
13
, a diode
14
, a cut-off control amplifier
15
for setting a cut-off voltage of CRT
20
, a cathode current detecting signal source
16
, a cathode current detecting resistor
17
, an analog/digital converter (ADC)
18
and a system controller
19
such as a microcomputer.
In the cut-off adjusting apparatus
10
, a video signal SIM is input to the video amplifier
11
to be amplified with a predetermined gain, the DC component thereof is cut in the capacitor
12
and only the alternating component is transmitted to the cathode
21
of CRT
20
.
At this time, the DC component for driving the cathode
21
of CRT
20
is reproduced in a DC reproducing circuit formed of the diode
14
, etc. and supplied to the cathode.
When the cathode current detection is carried out to correct the cathode current and the cut-off adjustment is carried out, a cathode current detecting reference signal SREF as shown in
FIG. 5A
is supplied to the video amplifier
11
by the signal source
16
whose driving operation is controlled by the system controller
19
. The potential of the cathode
21
at this time is shown in FIG.
5
B.
As shown in
FIG. 5
, the detection of the cathode current is carried out within a video signal section of a horizontal period, and it is shown in
FIG. 6
if it is observed at a vertical period.
The cathode current thus detected is converted to a voltage value by the cathode current detecting resistor
17
, and then converted to digital data by ADC
18
.
In the system controller
19
for controlling the display set, the digital value is compared with the digital value by ADC
18
to detect the error therebetween, and the base potential of a pnp type transistor P
15
connected between the cathode of the diode
14
and the ground line is controlled on the basis of a control signal S
19
so that the error is nullified.
FIG. 5
representatively shows one of three primary colors R(red), G(green), B(blue).
FIG. 5C
shows the voltage waveform in the current detecting resistor
17
.
FIG. 6
shows a state in which the video signal is omitted. As shown in
FIG. 6
, only a part of the detection signal appears as shown in
FIG. 6B
when observed at the vertical period.
However, the cut-off adjusting apparatus described above has the following disadvantages.
1) A cathode current detecting signal source is needed, and thus hardware therefor is needed, resulting in rise-up of the cost.
2) It is practical in a region having a relatively low horizontal deflection period, specifically below 30 kHz and at a specific frequency, however, in a region having a high horizontal deflection period, that is, above 30 kHz and in a system to which the frequency is not specified, an adverse effect such as dynamic focus remarkably appears as shown in
FIG. 5C
, so that it is difficult to detect the cathode current.
3) For the reason of 2), the error of the current value detected is large and as a result a large error occurs in the A/D conversion result, so that it is difficult to perform high-precision control. Therefore, it is difficult to perform correction control corresponding to the time variation even when this circuit is used.
4) The current detection is carried out by measuring a specific one point of brightness level. Therefore, when leakage of current occurs in the cathode, an error occurs in the measurement value and it may serve as a control error to induce variation of brightness.
The present invention has been implemented, and an object thereof is to provide a cut-off adjusting apparatus which needs no signal source for detecting cathode current, and can aim to reduce the cost, detect the cathode current with high precision and perform the correction corresponding to the time variation with high precision.
SUMMARY OF THE INVENTION
In order to attain the above object, according to the present invention, a cut-off adjusting apparatus for adjusting the cut-off of a display using a cathode ray tube, comprises: video signal processing means which can adjust at least contrast on the basis of a first control signal, and adjust the pedestal level of a video signal to the black level irrespective of input of the video signal and output the signal; a clamp capacitor having a first electrode connected to the output terminal of the video signal processing means and a second electrode connected to a cathode of the cathode ray tube; a rectifying element provided so that a forward direction is set from the connection point between the second electrode of the clamp capacitor and said cathode toward the reference potential; cut-off control means for adjusting the amount of current discharging from the clamp capacitor to the reference potential in accordance with a second control signal to keep the cathode voltage to a predetermined voltage; and current measuring means for outputting the first control signal to the video signal processing means to output a signal of black level when receiving an enforced adjustment instruction, and measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the black level signal.
According to the present invention, a cut-off adjusting apparatus for adjusting the cut-off of a display using a cathode ray tube, comprises: video signal processing means which can adjust at least contrast on the basis of a first control signal, and adjust the pedestal level of a video signal to the black level irrespective of input of the video signal and output the signal; a clamp capacitor having a first electrode connected to the output terminal of the video signal processing means and a second electrode connected to a cathode of the cathode ray tube; a rectifying element provided so that a forward direction is set from the connection point between the second electrode of the clamp capacitor and the cathode toward the reference potential; cut-off control means for adjusting the amount of current discharging from the clamp capacitor to the reference potential in accordance with a second control signal to keep the cathode voltage to a predetermined voltage; and current measuring means for outputting the first control signal to the video signal processing means to output a signal of black level when receiving an enforced adjustment instruction, measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the black level signal, comparing the current value thus measured with a predetermined set value, and outputting the second control signal to said cut-off control means so that the measured value is equal to the set value.
According to the present invention, a cut-off adjusting apparatus for adjusting the cu
Maioli Jay H.
Miller John
Sony Corporation
Yenke Brian
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