JPH05191678A - Shading correction circuit - Google Patents

Abstract

PURPOSE:To relax the reduction of peak luminance by weakening shading correction and making the peak luminance preferential in the case of luminance display equal over an entire picture by operating an ABL while a user calculates a brighter image and increases contrast. CONSTITUTION:This circuit is provided with a clip circuit 14 to detect brightness adjusted by the user and to extract brightness higher than a prescribed level, selecting circuit 12 to select a signal higher than the prescribed level and a signal at a fixed level with an ABL voltage as a control signal for preventing a beam current average for a long time from increase rather than the fixed level, and multiplier circuit 11 to multiply the output signal of the selecting circuit and a bipolar waveform shading correction signal generated from H and V pulses.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shading correction circuit in a display device using a cathode ray tube.

[0002]

2. Description of the Related Art Generally, in a projection type display, the peripheral portion of the projection lens has a narrower angle of view for capturing light than the central portion, and the image displayed on the screen has a lower luminance in the peripheral portion. ing.

A conventional technique for correcting the shading of the projection display disclosed in Japanese Patent Laid-Open No. 1-123589 is shown below.

A circuit for extracting a signal of a predetermined level (black level) or more from the luminance signal, a multiplication circuit for multiplying the luminance signal and a shading correction signal (parabolic wave generated from H pulse, V pulse), and the multiplication It is configured by an adder circuit that adds the output signal of the circuit and the luminance signal.

By using the output signal of the adder circuit as the luminance signal instead of the luminance signal, the amplitude of the video signal in the peripheral portion of the screen can be made higher than the amplitude of the video signal in the central portion of the screen. The luminance ratio of the periphery to the center can be improved.

[0006]

In the above prior art, when the ABL is used in the display device,
The luminance of the peripheral portion of the screen can be improved as compared with the luminance of the central portion of the screen, but the absolute luminance at the center of the screen is lower than that of a display without shading correction. Even if the user raises the contrast adjustment volume to obtain a brighter image, when the ABL is operating and the entire screen displays uniform brightness, the contrast of the video signal is limited at the center of the screen, so shading is performed. When the correction is performed, the brightness is darker than when the shading correction is not performed.

It is an object of the present invention to output the original brightness even when the user makes a brighter image and adjusts the contrast.

[0008]

When the brightness adjusted by the user is equal to or higher than a predetermined level, absolute brightness is prioritized to weaken shading correction, and when the brightness is equal to or lower than the predetermined level, the entire screen has an absolutely uniform brightness. Thus, the shading correction amount is controlled.

In such an operation, means for detecting brightness is provided, and a detection signal of a predetermined level or higher is used.
By controlling the amplitude of the shading correction waveform and superimposing it on the contrast control terminal of the video signal processing circuit,
realizable.

[0010]

[Function] The amplitude of the shading correction waveform is controlled only when the user brightness setting is high and the ABL operates.
When the average luminance level of the video signal is low such that does not operate, the amplitude of the shading correction waveform can be made constant, so that it is possible to give priority to the wholeness of luminance with respect to absolute luminance.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a block diagram showing an example in which the present invention is applied to a display device using a CRT.

The configuration of the embodiment is such that the video signal input terminal 1,
Video signal processing circuit 2, video output circuit 3, CRT 4, high voltage circuit 5, ABL detection resistor 6, ABL detection reference voltage terminal 7, contrast control voltage generation circuit 8, voltage input terminal 9, brightness adjustment voltage source 109, shading It comprises a correction waveform input terminal 10, a multiplication circuit 11, a selection circuit 12, a comparison voltage source 13, a clipping circuit 14, and a contrast terminal 15. The inside of the dotted frame is a block diagram showing the feature of the present invention, and is shown as the shading correction circuit 16.

Next, the operation will be described.

The video signal input from the video signal input terminal 1 is adjusted in contrast, brightness, clamp, etc. in the video signal processing circuit 2 and input to the video output circuit 3. The video output circuit 3 amplifies the output amplitude of the video signal processing circuit 2 and adds it to the CRT 4.

Generally, in a display, an ABL control circuit is provided for the purpose of high voltage circuit and CRT protection. As the average brightness level on the tube surface of the CRT 4 increases, the average beam current Ib flowing from the high voltage circuit 5 to the CRT 4 increases. Here, when the ABL control voltage (terminal 110) is Vabl, the voltage of the ABL detection reference voltage terminal 7 is Vref, and the resistance value of the ABL detection resistor 6 is R,
It can be represented by Vabl = Vref-Ib * R. Therefore,
When Ib increases, the ABL control voltage Vabl decreases. The output of the contrast control circuit 8 decreases substantially in proportion to the decrease in the voltage of the terminal 110. Therefore, the voltage of the contrast terminal 15 is decreased to control the amplitude of the signal driving the CRT 4 to be small.

This state is shown in FIG. FIG. 2 is a diagram showing the relationship between the average brightness of the screen and the ABL voltage input to the terminal 110. When the average brightness becomes high, the ABL voltage becomes low, the ABL operates, and the ABL voltage becomes a fixed value.

The operation of the shading correction circuit 16 will be described below with reference to FIGS.

FIG. 3 is a diagram showing a concrete configuration example of the clip circuit 14 in FIG.

The structure of this circuit is such that the input terminal 9 and the resistor 1
8, diode 19, clip level adjustment voltage source 20,
It includes a transistor 21, a resistor 22, and an output terminal 23.

The voltage value of the clip level adjusting voltage source 20 is VR, the forward operating voltage of the diode 19 is VD, the voltage value of the brightness adjusted by the user, that is, the voltage value of the terminal 9 is V.
U

The voltage VU is input to the input terminal 9. When VU is (VR-VD) or less, the diode 19 is always on, and the base potential of the transistor 21 is (VR-VD).
Is kept at. When VU is (VR-VD) or more, the diode 19 is off and the base potential of the transistor 21 is equal to the voltage of the terminal 9.

The transistor 21 and the resistor 22 form a buffer circuit. The signal input to the base of the transistor 21 is output to the emitter with the potential lowered by VBE (approximately equal to VD).

Therefore, the clipping circuit 14 can output a voltage whose potential is VR or higher from the voltage VU input to the terminal 9 to the terminal 23.

The selection circuit 12 outputs the output voltage of the clipping circuit 14 when the ABL is operating and the comparison voltage source 01 when the ABL is not operating.
It has a function of outputting the voltage determined by 3. The multiplication circuit 11 controls the amplitude of the shading correction waveform 10 according to the value of the output voltage of the selection circuit 12. Multiplier circuit 1
The output signal 1 is applied to the contrast terminal 15 of the video signal processing circuit 2.

A more detailed description will be given below with reference to FIGS. 4 and 5.

FIG. 4 is a diagram showing a specific configuration example of the selection circuit 12 in FIG.

The configuration of the selection circuit 12 is as follows: ABL voltage input terminal 110, voltage source A13, voltage comparator 28, terminal 23,
It includes a voltage source B30, a resistor 31, an analog switch 32, a transistor 35, a resistor 36, and an output terminal 37.

Next, the operation will be described.

The ABL voltage input to the input terminal 110 is inverted and amplified by the circuit composed of the resistor 25, the transistor 26 and the resistor 27. This inverted amplified output is VABL, the voltage of the voltage source A13 is VCOMP, and the voltage of the voltage source B30 is VRE.
F, the clip circuit output voltage is V CLIP.

Further, the relationship of each voltage is assumed to satisfy the following conditions. If VABL is VCOMP or higher, ABL
Works. When the brightness adjusted by the user is below a predetermined level, VCLIP and VREF are equal. VCOMP is approximately equal to VREF.

VABL is compared with VCOMP in voltage comparator 28. When the ABL is not operating, VABL has a lower potential than VCOMP, and the voltage comparator 28 outputs VCOMP. AB
During the L operation, VABL has a higher potential than VCOMP, and the voltage comparator 28 outputs VABL.

When VCOMP is output, the analog switch 32 outputs the voltage VREF determined by the voltage source B30, and VREF is input to the base of the transistor 35.

When VABL is output, the analog switch 32 outputs the clip circuit output voltage VCLIP, and VCLIP
Is input to the base of the transistor 35.

The transistor 35 and the resistor 36 form a buffer circuit. The potential of the signal input to the base of the transistor 35 is V at the emitter (output terminal 37).
BE goes down and is output.

As described above, the selection circuit 12 outputs the output voltage of the clipping circuit 14 during the ABL operation and the voltage determined by the reference voltage source 30 during the ABL non-operation.

FIG. 5 is a diagram showing a specific configuration example of the multiplication circuit 11 in FIG.

The configuration of this circuit is such that the amplitude control voltage input terminal 38, the shading correction waveform input terminal 39, and the resistor 4 are used.
0, 41, transistors 42, 43, 44, 45, resistors 46, 47, 48, 49, transistors 50, 51, resistors 52, 53, constant current source 54, bias voltage sources 55, 5
6, a capacitor 57, and an output terminal 58.

Next, the operation will be described.

Transistors 42, 43, 44, 45, 5
The collector currents of 0 and 51 are I42, I43, I44, and
I45, I50, and I51.

Transistors 50 and 51 are resistors 52 and 53.
Is connected to the constant current source 54 via. Therefore, when the level of the shading correction signal (parabola wave generated from the H pulse and V pulse) input to the input terminal 39 rises, I51 increases but I50 decreases. Since I51 increases, I44 and I45 increase. Since I50 decreases, I42 and I43 decrease.

When the bias voltage source 55 and the input terminal 38 have the same voltage value, the changes in I44 and I45 and the changes in I42 and I43 are equal in opposite phase, so that the collector voltage (output terminal 58) of the transistor 45 is applied. Does not change and remains at a constant potential.

When the signal at the input terminal 38 (amplitude control signal of the correction waveform) rises, the base potentials of the transistors 43 and 44 rise. Therefore, I43 and I44 increase,
I42 and I45 decrease.

I43 is a change in phase opposite to the voltage change at the input terminal 39, and I45 is a change in phase. Therefore, load resistance 4
The change in the current flowing through 1 is a reverse phase change to the voltage change at the input terminal 39, and the voltage change at the output terminal 57 is
It becomes in phase with the voltage change of. As a result, according to the magnitude of the amplitude control signal input to the input terminal 38, the input terminal 39
The amplitude of the signal (parabolic waveform) input to is controlled,
A shading correction signal is generated.

By the above operation, the shading correction corresponding to the ABL voltage and the brightness adjusted by the user can be performed. Even if the user seeks a brighter image and increases the contrast, when the ABL operates and the entire screen displays uniform brightness, the contrast of the video signal is limited at the center of the screen. Compared to the case without correction, the problem that the peak brightness is dark is alleviated.

Note that the clip circuit, the adder circuit, and the multiplier circuit may have configurations other than those of this embodiment as long as they have similar functions.

[0046]

When the user seeks a brighter image to increase the contrast and the ABL is operating and the entire screen displays uniform brightness, the shading correction is weakened and the peak brightness is prioritized. As a result, the decrease in peak brightness is alleviated.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship of voltages of respective parts.

FIG. 3 is a diagram illustrating a specific configuration example of a clip circuit.

FIG. 4 is a diagram showing a specific configuration example of a selection circuit.

FIG. 5 is a diagram illustrating a specific configuration example of a multiplication circuit.

[Explanation of symbols]

1 ... Video signal input terminal, 2 ... Video signal processing circuit, 3 ... Video output circuit, 4 ... CRT, 5 ... High voltage circuit, 6 ... ABL
Detection resistance, 7 ... ABL detection reference voltage terminal, 8 ... Contrast control voltage generation circuit, 9 ... Input terminal (brightness), 1
09 ... Brightness adjustment volume, 10 ... Shading correction waveform input terminal, 11 ... Multiplication circuit, 12 ... Selection circuit, 1
3 ... Comparison voltage source, 14 ... Clip circuit, 15 ... Contrast terminal, 16 ... Block diagram showing characteristics of the present invention,

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Eiji Takagi 292 Yoshida-cho, Totsuka-ku, Yokohama-shi Hitachi Ltd. Yokohama factory

Claims (1)

[Claims] 1. ABL (Automatic Brightness Limitter)
In a display device of a control system, a clip circuit that detects a brightness adjusted by a user and extracts a predetermined level or more, and a control of an ABL control circuit that limits a long-time average beam current of the display device to a constant value. A selection circuit that selects a signal having a predetermined level or higher and a fixed level signal by using a voltage as a control signal, and a multiplication circuit that multiplies the selection circuit output signal by a parabolic waveform shading correction signal generated from H pulse and V pulse. And a shading correction circuit characterized by being provided.