JPH06160811A - Liquid crystal projector - Google Patents

Abstract

PURPOSE:To solve black floating according to the high brightness of a light source and breakdown due to heat absorption at a polarizing plate, etc. CONSTITUTION:The peak value of a brightness signal is detected by a peak value detection circuit 12 which detects the peak value of a video signal, and a detection signal is outputted to an adder 14 and a contrast control circuit 18 via an LPF 13. The brightness of the power source 6 can changed by adding a reference voltage from an auxiliary power source 8 on the detection signal by the adder 14, and applying PWM control to a regulator 3. Meanwhile, the contrast of the video signal can be controlled corresponding to the detection signal at the contrast control circuit 18.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal projector using a liquid crystal panel as a light valve, and more particularly to a projection type liquid crystal projector.

[0002]

2. Description of the Related Art A projection type liquid crystal projector using a liquid crystal panel as a light valve is being actively developed today as an effective means for increasing the screen size. In this type of projector, it is essential to increase the brightness of the light source in order to reproduce a vivid image, but in reality, it is impossible to completely block the light from the light source by the liquid crystal panel. Therefore, as the brightness of the light source increases, a phenomenon of black floating due to light leakage occurs in an actual image, and higher brightness of the light source is not necessarily reflected in the improvement of contrast. In addition, increasing the brightness of the light source is also a negative factor in terms of heat resistance of the liquid crystal panel. Especially, in the case of a dark screen in general, the amount of light blocked by the liquid crystal panel is large. It also increases the heat absorption of the liquid crystal, leading to damage to the liquid crystal panel.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and an object thereof is to solve the problem of black floating due to higher brightness of a light source and damage due to heat absorption in a liquid crystal panel, a polarizing plate or the like. To do.

[0004]

The present invention relates to a liquid crystal projector using a liquid crystal panel as a light valve,
A peak value detection circuit for detecting the peak value of the video signal, a brightness control means for changing the brightness of the light source, and a contrast control means for changing the contrast of the video signal,
The liquid crystal projector is characterized in that the brightness control unit controls the brightness of the light source according to the peak value from the peak value detection circuit, and the contrast control unit controls the contrast of the video signal.

[0005]

According to the present invention, the peak value of the video signal is detected, and thereby the brightness of the light source and the contrast of the video are changed. That is, the brightness of the light source is suppressed on a dark screen having a low peak value, and conversely, the brightness of the light source is increased on a bright screen having a high peak value. As a result, the contrast of the video signal also changes according to the change in the brightness of the light source, and the contrast seen by the viewer is constant.

[0006]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the liquid crystal projector of the present invention.

In FIG. 1, reference numeral 1 is an AC power supply, which is a normal commercial AC100V power supply. 2 is a double voltage rectifier circuit, and DC280V is obtained here. 3 is a chopper regulator, which is a PWM control circuit 9
A DC voltage is generated according to the pulse width from. This P
The WM control circuit 9 modulates the pulse width of the pulse train according to the difference voltage between the output of the chopper regulator 3 obtained from the error calculation circuit 10 and the reference voltage. Therefore, the output of the chopper regulator 3 can be changed by changing the reference voltage.

Reference numeral 6 is a light source such as a metal halide lamp which is widely used in liquid crystal projectors. Immediately after the power is turned on, the electrodes of the light source 6 are discharged by the igniter 5, and after the elapse of a certain time by the timer 11, the oscillation of the inverter 4 starts and the discharge by the igniter 5 ends, and the light source 6 lights up. Incidentally, 7 is an inverter pulse oscillation circuit, and 8 is an auxiliary power supply for operating each circuit.

Now, the luminance signal of the image is input to the luminance input terminal 17, the luminance signal is branched, and the peak value detection circuit 12 detects a white peak. Reference numeral 13 is an LPF for preventing a sudden change, and the output of the LPF 13 is output to the contrast control circuit 18. The contrast control circuit 18 controls the amplitude of the brightness signal according to the peak detection signal, and outputs the controlled brightness signal to the brightness output terminal 15. An adder 14 synthesizes the peak detection signal and the reference voltage of the error calculation circuit 10. Therefore,
The brightness of the light source 6 is also modulated according to the brightness of the brightest part of the screen.

Then, the relationship between the peak detection signal and the brightness and contrast of the light source 6 in this embodiment is set as shown in FIG.

That is, when the peak luminance of the screen is high, the luminance of the light source 6 is increased, and conversely, when it is low, the luminance is decreased. At the same time, the amplitude of the brightness signal when the peak brightness is low is made relatively smaller than the amplitude of the brightness signal when the peak brightness is high so that the contrast of the screen does not change with the brightness change.

Although the amplitude of the luminance signal is changed in the embodiment shown in FIG. 1, this also causes a problem that the color gain changes according to the amplitude of the luminance signal.

In order to solve this, for example, as shown in FIG. 3, a second embodiment is conceivable in which peak detection is performed by a luminance signal and amplitude control is performed by R, G, B primary color signals. .

In this figure, 17 is a luminance input terminal, 1
9 and 20 are color difference input terminals for inputting color difference signals,
Reference numeral 21 is a matrix circuit, 22, 23, 24 are contrast control circuits, 25, 26, 27 are primary color output terminals for outputting R, G, B primary color signals, 12 is a peak detection value circuit,
Reference numeral 14 is an LPF.

Although the operation is almost the same as that of the embodiment shown in FIG. 1, the feature of this embodiment is that, instead of changing the amplitude of the luminance signal, R, G, B after the matrix circuit 21 is output.
That is, the contrast is changed by each of the primary color signals.

Further, in the embodiment of FIG. 1, the control system for changing the amplitude of the luminance signal is the feedforward system by peak detection, but the control by the third embodiment by the feedback system as shown in FIG. 4 is also possible. is there.

In the operation shown in the figure, the brightness signal input from the brightness input terminal 17 has its amplitude changed by the contrast control circuit 18, and is output to the brightness output terminal 15. on the other hand,
The luminance output signal is also input to the peak value detection circuit 12, the peak value detection circuit 12 detects the peak value, and the LPF 1
After passing through 3, it is compared with the 29 reference voltage in the error detection circuit 28. By using this comparison output as a control signal for contrast and light source brightness, the same effect as that of the embodiment of FIG. 1 can be obtained.

Further, in the feedback system, R. G. B
As an embodiment in which the contrast of each primary color signal is changed, the method shown in FIG. 5 can be considered. In this figure, R,
The G and B primary color inputs are input to the contrast control circuits 22 and 2 respectively.
The contrast is controlled by 3 and 24, respectively, and output to the output terminals 25, 26 and 27.

On the other hand, each contrast control circuit 22, 2
The outputs from 3 and 24 are also output to the matrix circuit 21 that generates a luminance signal from the R, G, and B signals. here,
A luminance signal is created, and after the peak value is detected by the peak value detection circuit 12, the luminance signal is output to the error detection circuit 28 via the LPF 13. Then, in the error detection circuit 28, it is compared with the reference voltage 29, and this comparison output is used as a control signal for the contrast control circuits 22, 23, 24 and also as a modulation signal for the brightness of the light source.

[0021]

According to the present invention having the above-mentioned structure,
The brightness of the light source in the projection type liquid crystal projector is controlled to the minimum required level according to the screen, and it is possible to prevent black floating due to light leakage. It is also advantageous in terms of heat resistance, and can save power consumption.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the relationship between peak luminance, contrast, and light source luminance in the present invention.

FIG. 3 is a diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram showing a third embodiment of the present invention.

FIG. 5 is a diagram showing a fourth embodiment of the present invention.

[Explanation of symbols]

 3 Regulator 6 Light Source 8 Auxiliary Power Supply 9 PWM Control Circuit 10 Error Calculation Circuit 12 Peak Value Detection Circuit 13 LPF 14 Adder 18 Contrast Control Circuit

Claims (1)

[Claims] 1. A liquid crystal projector using a liquid crystal panel as a light valve, a peak value detection circuit for detecting a peak value of a video signal, a brightness control means for changing the brightness of a light source, and a contrast for changing the contrast of the video signal. A liquid crystal comprising a control means, wherein the brightness control means controls the brightness of the light source according to the peak value from the peak value detection circuit, and the contrast control means controls the contrast of the video signal. projector.