JPH07201292A - Sharpness improving circuit of crt display - Google Patents

Abstract

PURPOSE:To prevent the deterioration of the resolution by changing the spot shape according to the inputted signal level in a CRT display. CONSTITUTION:A detecting circuit 10 detects the average brightness level in one field by the inputted signal, and outputs the detected results to a ROM 11. The output value where the voltage value at a terminal 14 is the voltage value in the just-focus condition corresponding to the detected results is preliminarily stored in the ROM 11. This voltage is applied to a third grid 5, and the electron beam is converged by an electron lens to be constituted by this voltage, and a fluorescent surface is irradiated with the electron beam. This constitution converges the electron beam at the convergence voltage corresponding to the inputted signal even when the signal of any brightness level may be inputted, and leads to the constantly just-focus condition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for improving the sharpness of a display such as a television receiver or a computer using a CRT.

[0002]

2. Description of the Related Art CRTs are widely used as displays for realizing high-luminance and high-definition images, and in recent years, they have been used not only for televisions but also for data display. In addition, as demand for large screens has increased, projectors using a three-tube CRT have been commercialized.

In a CRT, as shown in FIG. 3, the distance from the deflection center differs depending on the location on the phosphor screen. Therefore, if the focus is adjusted to the center of the phosphor screen, for example, a focus shift occurs in the peripheral portion. However, this causes deterioration of resolution. In FIG. 3, 31 is a cathode ray tube, 32 is a deflection center, and 33 is an equidistant curve from the deflection center 32. In order to correct this focus shift, a dynamic focus method is known in which the focusing voltage is modulated by an addition signal of parabola waves in the horizontal and vertical directions. This is shown in FIG. In FIG. 4, 41 is a DC power source, 42
Is a modulation circuit, 43 is a horizontal parabolic wave generation circuit, 44 is a vertical parabolic wave generation circuit, and 45 is an addition circuit.

The operation of FIG. 4 will be described below. The horizontal parabola wave generation circuit 43 and the vertical parabola wave generation circuit 44 output signals corresponding to the distance from the center of the phosphor screen in synchronization with the input video signal. These signals are added by the adder circuit 4
5, the output voltage of the DC power supply 41 is modulated into a focusing voltage in the modulation circuit 42 using this as a modulation signal, so that the entire fluorescent screen is just focused.

In a projector, a method has been proposed in which an image on the projection tube is distorted because the projection directions of the projection tubes of three colors are different, and a peripheral focus shift caused by the distortion is eliminated (for example, Japanese Patent Laid-Open Publication No. 2000-242242). 60-1177
No. 66).

[0006]

However, in the conventional CRT display, not only the location of the fluorescent screen,
There is a problem that the electron beam is not properly focused even by the input signal level, and the resolution is deteriorated due to the increase in the diameter of the beam spot and the change in shape.

FIG. 2 is a diagram showing a configuration of an example of a general conventional CRT display. In FIG. 2, 1 is a cathode, 3 is a first grid, 4 is a second grid, 5 is a third grid, 6 is a fourth grid, and 7 is an anode.

The cathode voltage application circuit 2 applies a voltage according to the level of the input signal to the cathode 1, and electrons are generated on the cathode 1. The electrons fly out toward the anode 7 by the second grid 4 to which a positive voltage is applied. On the other hand, an electron lens corresponding to the magnitude of the voltage applied to each grid is formed between the third grid 5 and the fourth grid 6, and the electrons are focused by this electron lens and then irradiate the fluorescent screen. To form an image. The focusing characteristic of the electron beam is determined by the third grid 5 and the fourth grid 6.
Generally, the voltage value applied to the grid forming the electron lens is set to a fixed value that is an image of a predetermined level, although it is determined by the electron lens formed between them.

FIG. 5 is a diagram showing the relationship between the focusing voltage and the spot diameter when the signal levels V1 and V2 are input. As is apparent from FIG. 5, the focusing voltage at which the beam spot has the minimum diameter, that is, the just focus state changes with the input signal level. For example, the signal level V
In the case of 1, the optimum focusing voltage is Vf1 and the signal level V
In the case of 2, the optimum focusing voltage is Vf2. Therefore,
When the focusing voltage is fixed to a constant value, for example, when the input signal level is V1 in the case of Vf1, the just focus state is obtained, but when the signal level is V2, the spot diameter becomes large and the resolution due to the defocus state is increased. Will be deteriorated. Especially in the projector,
Since a large current is required to increase the brightness, the sharpness changes greatly and the image quality deteriorates depending on the signal level.

The present invention solves such a problem and provides a CRT.
It is an object of the present invention to improve the resolution of a spot, which is changed depending on the level of an input signal in a display.

[0011]

In order to solve the above problems, a sharpness improving circuit for a CRT display according to the present invention comprises:
A detection means for detecting the signal level, and a focusing voltage applying means for applying a voltage to the electrode for focusing the electron beam,
The focusing voltage applying means applies a focusing voltage to the electrodes so as to form an electron beam having substantially the same diameter or substantially the same shape according to the detection result of the detecting means.

[0012]

According to the present invention, the focusing voltage is applied so that the focus is always just in accordance with the level of the input signal.

[0013]

The operation of a sharpness improving circuit for a CRT display according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a sharpness improving circuit of an electrostatic focusing CRT display according to an embodiment of the present invention. In FIG. 1, 1 is a cathode, 2 is a cathode voltage application circuit that applies a voltage according to an input signal level to the cathode 1, 10 is a detection circuit, 3 is a first grid, 4 is a second grid, and 8a is a second grid. A first DC power supply for applying a voltage to 4, 5 is a third grid, 6 is a fourth grid, 7 is an anode, 8c is a third DC power supply for applying an anode voltage to the anode 7, and 11 is an input value in advance. ROM for storing corresponding output value, 8b second DC power supply of voltage V, 12 fixed resistor with resistance value R1, 13 variable resistor with resistance value R2, 14 fixed resistance Is a terminal located between the device 12 and the variable resistor 13.

In this case, the potential Vf of the terminal 14 is

[0016]

[Equation 1]

[0017] The first grid 3 is grounded, and the second grid 4 and the anode 7 are respectively
A voltage is applied by the first DC power supply 8a and the third DC power supply 8c.

The CRT of this embodiment constructed as described above
The operation of the display sharpness improving circuit will be described below with reference to FIG.

The detection circuit 10 detects the average luminance level based on the input signal and outputs the detection result to the ROM 11.
The ROM 11 stores output values corresponding to the detection results input in advance. For example, when a signal whose average luminance level in one field is V1 in FIG. 5 is input, the detection circuit 10 outputs the V1 value to the ROM 11. In the ROM 11, the voltage Vf at the terminal 14 is Vf1 in FIG.
The output resistance value R2 of the variable resistor 13 is stored as follows. Therefore, the output voltage Vf = V1 of the terminal 14 is applied to the third grid 5, and the electron beam is focused on by the electron lens constituted by this voltage value and then is irradiated on the phosphor screen, so that the just focus state is achieved.

When a signal having an average brightness level of V2 is input, the detection circuit 10 similarly outputs the V2 value to the ROM 11, and the ROM 11 outputs the voltage Vf at the terminal 14 as Vf in FIG.
The output resistance value R2 of the variable resistor 13 which becomes 2 is output. Therefore, the output voltage Vf = V2 of the terminal 14 is applied to the third grid 5, and the electron beam is focused on by the electron lens constituted by this voltage value and then is irradiated on the phosphor screen, so that the just focus state is achieved.

By the above operation, the electron beam is focused by the focusing voltage corresponding to the input brightness level, so that the just focus state is always maintained regardless of the signal level.

In this embodiment, the focusing voltage is controlled by the average brightness level of one field period, but the period is not limited to this, and may be a predetermined dot period, a predetermined line period or a predetermined field period. Further, the detection of the average brightness level is not limited to the input signal, but may be detected by, for example, the anode current or the cathode voltage.

Further, although the present embodiment describes the electrostatic focusing CRT display, in the electromagnetic focusing CRT display in which the electron beam is focused by the focusing coil, the current value supplied to the focusing coil is controlled by the average brightness level. Even in this case, the same effect as that of this embodiment can be obtained.

[0024]

As is apparent from the above description, the sharpness improving circuit of the CRT display according to the present invention controls the focusing voltage in accordance with the level of the input signal, so that the electronic device is always in the just focus state. It becomes possible to illuminate the fluorescent screen with a beam, and a sharp image can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a sharpness improving circuit of a CRT display according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of a conventional CRT display.

FIG. 3 is a diagram showing the relationship between the distance from the deflection center of the cathode ray tube and the fluorescent screen.

FIG. 4 is a diagram of a conventional example in which a focusing voltage is modulated by an addition signal of horizontal and vertical parabolic waves.

FIG. 5 is a diagram showing the relationship between the focusing voltage and the spot diameter when the signal level changes.

[Description of Reference Signs] 1 cathode 3 first grid 4 second grid 5 fourth grid 6 fifth grid 7 anode 10 detection circuit 11 ROM

Claims (7)

[Claims] 1. A control means comprising: detection means for detecting a signal level of an input signal; focusing means for focusing an electron beam; and control means for supplying a predetermined voltage or current to the focusing means. Is a sharpness improving circuit for a CRT display, wherein a voltage or current according to the detection result of the detecting means is applied to the focusing means. 2. The sharpness improving circuit for a CRT display according to claim 1, wherein the control means supplies a voltage or current to the focusing means so as to form an electron beam having a substantially equal diameter or a substantially equal shape. 3. The sharpness improving circuit for a CRT display according to claim 1, wherein the control means supplies a voltage or current to the focusing means so as to form an electron beam having a predetermined diameter or a predetermined shape. 4. The sharpness improving circuit for a CRT display according to claim 1, wherein the focusing means is an electrostatic focusing type that focuses the electron beam in accordance with the magnitude of the voltage supplied to the electrodes. . 5. The sharpness improving circuit for a CRT display according to claim 1, wherein the focusing means is an electromagnetic focusing type which focuses the electron beam in accordance with the magnitude of the current supplied to the coil. 6. The sharpness improving circuit for a CRT display according to claim 1, wherein the detecting means detects a signal level or an average value of signal levels in a predetermined period. 7. The sharpness improving circuit for a CRT display according to claim 6, wherein the predetermined period is in field units.