JPH07274092A - Beam current control circuit - Google Patents

Abstract

PURPOSE:To realize a beam current control circuit where beam current per unit area in a CRT fluorescent screen becomes fixed so that luminance is not changed when a fixed level of video input signal is inputted. CONSTITUTION:The contrast value of a contrast adjusting circuit 11 is controlled by following a control voltage VsS14 which is proportionated to a raster area on a CRT surface so as to permit beam current per unit area in the fluorescent screen to be fixed as against the fixed level of video input signal S11 even if a raster size on the CRT surface is changed. By the above configuration, the beam current control circuit where fixed luminance is obtained even if the raster size is changed as against the fixed level of video input signal S11 is realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a cathode ray tube (hereinafter referred to as CRT).
Beam current control circuit of a video display device using the above).

[0002]

2. Description of the Related Art Generally, in a CRT of an image display device, a phosphor is saturated, so that the cathode current Ik per unit area of the phosphor screen of the CRT cannot flow beyond a certain value. Therefore, the beam current flowing in the CRT is detected, and the beam current is limited (hereinafter referred to as ABL) with a constant current value according to the raster size.

A conventional beam current control circuit will be described below. FIG. 2 shows a configuration diagram of a conventional beam current control circuit. In FIG. 2, as components, 1 is a contrast adjustment circuit, 2 is a video output circuit, 3 is a cathode current detection circuit, 4 is a non-linear circuit, and 5 is a CRT.

The operation of the beam current control circuit having the above components will be described below. CRT5
In the image display device using the video input circuit S1, the video input signal S1 passes through the contrast adjusting circuit 1 which controls the gain according to the contrast control voltage Vc S5, and then the video output circuit 2
It is amplified by and drives CRT5. The cathode current flowing in the CRT 5 is converted into a voltage by the cathode current detection circuit 3 and becomes the cathode voltage Vk S3. The nonlinear circuit 4 compares the cathode voltage Vk S3 with the control voltage Vs S4. When the cathode voltage Vk S3 is smaller than the control voltage Vs S4, its output Vc S5 outputs zero, and the gain control of the contrast adjusting circuit 1 is performed. Is not done. When the cathode voltage Vk S3 exceeds the control voltage Vs S4, the cathode voltage Vk S3
The contrast control voltage Vc S5 is output in proportion to the difference between the control voltage Vs S4 and the control voltage Vs S4, and the contrast adjusting circuit 1 is controlled to reduce the contrast to control the beam current.

The above operation prevents the beam current density on the phosphor screen from exceeding a certain value.

[0006]

However, in the above conventional structure, the cathode voltage Vk S3 is equal to the control voltage Vs S.
In a region smaller than 4, even if the raster size is changed, a constant cathode current is applied to a constant level image input signal, so that the current per unit area of the phosphor screen changes and Had a problem that was changed.

In consideration of the above-mentioned conventional problems, the present invention makes the beam current per unit area on the phosphor screen of the CRT constant when the image input signal of a constant level is input, even if the raster size of the CRT changes. An object of the present invention is to provide a beam current control circuit whose brightness does not change.

[0008]

In order to achieve the above object, the present invention provides a contrast adjusting circuit for performing contrast control of a video signal with a control voltage Vs and a contrast control voltage Vc proportional to the area of a raster on a tube surface. A video output circuit that amplifies the output of the contrast adjustment circuit and drives the CRT, a cathode current detection circuit that detects the cathode current of the CRT and converts it into a voltage, and cathode voltages Vk and Vs that are the outputs of the cathode current detection circuit. And compare with Vk
Is smaller than Vs, the output is zero, and when Vk exceeds Vs, the beam current control circuit is provided with a non-linear circuit that outputs a contrast control voltage proportional to the difference between Vk and Vs.

[0009]

In the above structure, the operating point of the ABL is changed according to the raster size on the surface of the CRT, and the ABL does not operate. The contrast value is controlled so that the beam current per unit area on the surface is constant, and even if the raster size on the CRT tube surface is changed, a constant brightness can be obtained for a constant level image input signal. Become.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. As shown in FIG.
Is a contrast adjustment circuit, 12 is a video output circuit, 13
Is a cathode current detection circuit, 14 is a non-linear circuit, and 15 is C
It is RT.

The operation of the beam current control circuit having the above components will be described with reference to FIGS. 1 and 3.
The video input signal S11 is the contrast control voltage Vc S
15 and a control voltage V proportional to the raster area on the CRT tube surface
s After passing through the contrast adjustment circuit 11 that performs gain control with S14, the video output circuit 12 amplifies the CRT.
Drive 15 The cathode current flowing through the CRT 15 is
The voltage is converted by the cathode current detection circuit 13 to become the cathode voltage Vk S13. The non-linear circuit 14 compares the cathode voltage VkS13 with the control voltage Vs S14. When the cathode voltage Vk S13 is smaller than the control voltage Vs S14, the contrast control voltage Vc S15 outputs zero, and the contrast adjusting circuit 11 Is not controlled. When the cathode voltage Vk S13 exceeds the control voltage Vs S14, a contrast control voltage Vc S15 proportional to the difference between the cathode voltage Vk S13 and the control voltage Vs S14 is output, and the contrast adjusting circuit 11 is controlled to lower the contrast. Limit the beam current. In addition to the above operation, by controlling the contrast of the contrast adjusting circuit 11 to be proportional to the control voltage Vs S14 which is proportional to the raster area on the CRT tube surface, a fluorescent screen can be displayed for a constant level image input signal. By controlling the contrast value so that the beam current per unit area in C is constant, even if the raster size on the CRT tube surface is changed, a constant brightness can be obtained for a constant level image input signal. A beam current control circuit is provided.

The characteristics of the contrast control voltage and the beam current per unit area according to the present embodiment and the characteristics of the conventional contrast control voltage and the beam current per unit area are shown in comparison with FIGS. 4 and 5.

[0013]

As is apparent from the above description of the embodiments, the present invention realizes a beam current control circuit for obtaining a constant brightness for a video input signal of a constant level even when the raster size is changed. be able to.

[Brief description of drawings]

FIG. 1 is a block diagram of a beam current control circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional beam current control circuit.

FIG. 3 is a characteristic diagram of an output obtained by converting the cathode current into a voltage and a contrast control voltage in the present invention.

FIG. 4 is a characteristic diagram of contrast control voltage and beam current per unit area in the present invention.

FIG. 5 is a characteristic diagram of a conventional contrast control voltage and a beam current per unit area.

[Explanation of symbols]

 11 Contrast adjustment circuit 12 Video output circuit 13 Cathode current detection circuit 14 Non-linear circuit 15 CRT

Claims (1)

[Claims] 1. A video display device using a cathode ray tube, wherein the contrast control of a video signal is controlled by a control voltage Vs and a contrast control voltage Vc proportional to the area of a raster on the tube surface.
, A video output circuit that amplifies the output of the contrast adjustment circuit and drives a cathode ray tube, a cathode current detection circuit that detects the cathode current of the cathode ray tube and converts it into a voltage, and a cathode current detection circuit When the cathode voltage Vk is smaller than the control voltage Vs, the output is zero, and when the cathode voltage Vk exceeds the control voltage Vs, the cathode voltage Vk and the control voltage Vs are compared. A beam current control circuit equipped with a non-linear circuit that outputs a contrast control voltage proportional to the difference.