JPH09307916A - Focus control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increasingly control a defocus level depending on a reduction in a horizontal deflection frequency by conducting just fouces control when the horizontal defection frequency is higher and conducting defocus control attended with a decreasing horizontal deflection frequency so as to increase a bulue emission chatacteiratic. SOLUTION: A low level side terminal of a foucus volume pack 3 connects to ground via a resistor 4. A constant current circuit is configured with a resistor 5, a constant current driving transistor(TR) 6, and a constant current driving operational amplifier 7 to control a current (i). A CPU 9 detects a horizontal frequency from a received horizontal synchronizing signal and when the horizontal frequency is, e.g. 20kHz or over, a D/A converter 8 provides an output of a constant output at all times to an amplifier 7 and when less than 20kHz, the D/A converter 8 given an output decreasing linearly to the amplifier 7. Thus, a constant current is response to the horizontal frequency is supplied to the constant current circuit to control a focus level. Thus, a blue electric focus level is controlled depadening on a received video signal thereby obtaining a best balance state between the brightness and the resolution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CRT three-tube video projector having a multi-scan function.

[0002]

2. Description of the Related Art CR having a conventional multi-scan function
In the T3 tube type video projector, the blue phosphor has a characteristic that its average luminance level decreases as the concentration of the electron beam increases. Conventionally, in the focus adjustment of the blue CRT using the characteristics, the adjustment has been made as the just focus state when the brightness level becomes the minimum.

For example, Japanese Patent Laid-Open No. 3-34681 has been proposed, and FIG. 3 shows its configuration. In FIG. 3, it is a focus adjustment device realized in a video projector. Reference numeral 10 is a CRT having a blue fluorescent screen as a display screen.
The image on the CRT is projected on the screen 12 by the lens system 11.

The brightness level of the image on the screen 12 is detected by the television camera 13 and is detected by the signal processing circuit 14 as 1
It is converted into a digital signal at 5 and transmitted to the CPU 16.

Further, the CPU controls the drive circuit 17 for adjusting the focus on the CRT phosphor screen so that the brightness level becomes minimum.

[0006]

Normally, in a CRT three-tube video projector, when trying to maintain a predetermined color temperature, the current density of blue becomes higher than that of red and green. Further, in a region where the horizontal scanning frequency is low, the number of scanning lines is also reduced, so that the degree of concentration of electron beams in the CRT phosphor screen light emitting portion is increased, and thus the emission characteristics of the blue phosphor are further deteriorated.

In order to secure a predetermined color temperature, red,
Since the emission ratio of green and blue must be kept constant, the overall brightness will be further reduced.

For this reason, a method of defocusing a certain amount of blue to obtain higher brightness than in the just-focused state has also been adopted.

However, in the case of a display for displaying a computer image, the defocusing method requires a high resolution in the case of an image source with a high horizontal deflection frequency, so the blue defocus has a high resolution display capability. There was a problem of lowering it.

Further, when the horizontal deflection frequency is high, the number of scanning lines also increases, so that the degree of concentration of electron beams on the CRT phosphor screen light emitting portion is reduced, and even if defocused, a large effect cannot be obtained for improving the brightness and the resolution is lowered. There was also a problem that only one was conspicuous. The present invention is intended to solve the above problems.

[0011]

In order to solve the above-mentioned problems, the present invention provides just focus when the horizontal deflection frequency is high with respect to the focus voltage or focus current supplied to the blue CRT, and low horizontal deflection frequency. In order to increase the defocus level and increase the blue light emission characteristics as it becomes, the control function for the focus drive circuit is connected.

With the above structure, in the case of the electrostatic focusing CRT, the focusing voltage is normally supplied to the CRT focus electrode from the focus volume pack connected to the focus voltage supply terminal of the flyback transformer. Therefore, the focus level is controlled by grounding the grounding terminal of the focus volume pack through a resistor, connecting a constant current circuit to the grounding resistor, and supplying a constant current according to the horizontal frequency to the constant current circuit.

In the case of an electromagnetic focusing CRT, a just focus level can be realized by passing a current through a focus coil attached to the CRT. However, since this current can be controlled by low voltage driving, it is usually DAC controlled by the CPU. It can be adjusted and this adjustment data is stored in memory. Therefore, the CPU detects the horizontal scanning frequency, multiplies the adjustment data at the time of just focusing by a coefficient according to the horizontal scanning frequency, and outputs the adjusted data to the DAC, thereby increasing the defocus level according to the decrease in the horizontal frequency. It will be possible.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is, when the horizontal deflection frequency is high with respect to the focus voltage or the focus current for controlling the focus level of the blue CRT, just focus is set, and the horizontal deflection frequency is set. The focus control circuit has a control function to increase the blue emission characteristic by controlling the state to be shifted from the just focus level (hereinafter referred to as defocus) as the value becomes lower. Has the effect of increasing control.

The invention according to claim 2 is a blue CRT.
When the number of scanning lines is high with respect to the focus voltage or the focus current for controlling the focus level of, the focus has a control function of increasing the defocus level and increasing the blue emission characteristic as the number of scanning lines decreases. This is a control circuit, and has the effect of increasing the defocus level according to the decrease in horizontal frequency.

According to a third aspect of the invention, in a display equipped with both RGB input and composite video input as video input signals, the focus level of the blue CRT is set to just focus when RGB input is made. The focus control circuit has a control function of increasing the brightness by defocusing at the time of video input, and has the effect of increasing the defocus level according to the decrease in the horizontal frequency.

An embodiment of the present invention will be described below with reference to the drawings. (Embodiment) FIG. 1 shows a block diagram of a focus control circuit in an embodiment of the present invention. In FIG. 1, reference numeral 1 is an electrostatic focusing CRT having a blue fluorescent screen.
It is. Reference numeral 2 is a flyback transformer for high voltage generation, which has a voltage output terminal for focusing, and its voltage is supplied to the focus pack 3 and is variably adjusted by a built-in volume CRT1.
Is transmitted to the focus electrode of.

The low potential side terminal of the focus pack 3 is grounded through the resistor 4. 5, 6, and 7 form a constant current circuit, and control the current i. The CPU 9 detects the horizontal frequency from the input horizontal synchronizing signal, and supplies the voltage having the frequency characteristic shown in FIG. 2 from the DAC 8 to the constant current circuit. The DAC 8 outputs the position when the horizontal frequency fH is fd or more, and fd
The characteristic becomes lower in the following, and fd is usually set near 20 KHz.

[0019]

As is apparent from the above description, by adopting the circuit configuration according to the present invention, it is possible to defocus the blue light emission characteristic which is lowered when receiving the image signal of the low horizontal frequency in the CRT three tube type video projector. It is possible to prevent the brightness from lowering while lifting and maintaining a predetermined color temperature. When a video signal with a low horizontal frequency is received, a high-resolution image signal source is generally not required, so blue defocus does not impair the image quality. In this way, it is possible to realize the best balance between brightness and resolution by controlling the blue electric focus level according to the image signal that receives the image.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a focus control circuit according to an embodiment of the present invention.

FIG. 2 is a DA output characteristic diagram of FIG.

FIG. 3 is a block diagram of a conventional focus control circuit.

[Explanation of symbols]

 1 Electrostatic Focusing Blue CRT 2 Flyback Transformer 3 Focus Volume Pack 4, 5 Resistor 6 Constant Current Driving Transistor 7 Constant Current Driving Operational Amplifier 8 DA Converter 9 CPU

Claims (3)

[Claims] 1. A just focus when the horizontal deflection frequency is high with respect to a focus voltage or a focus current for controlling the focus level of the blue CRT, and a state where the horizontal focus frequency is shifted from the just focus level as the horizontal deflection frequency becomes lower (hereinafter referred to as "defocus"). Focus control circuit that has a control function to increase the blue emission characteristics by controlling the focus). 2. The focus voltage or focus current for controlling the focus level of the blue CRT is just focus when the number of scanning lines is high, and the defocus level is increased and the blue emission characteristic is increased as the number of scanning lines is decreased. A focus control circuit that has a control function that makes it possible. 3. In a display equipped with both RGB input and composite video input as video input signals, the focus level of the blue CRT is set to just focus for RGB input, and defocus for video input. Focus control circuit that has a control function to improve brightness.