JP2009092917A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize a backlight operation at a low temperature. <P>SOLUTION: The liquid crystal display device displaying an image by widening a dynamic range of an image signal and suppressing the luminance of the backlight includes an after-start time detection circuit measuring the time after the start of the liquid crystal display device and outputting an activating time signal indicating the measured time. After the start of the liquid crystal display device until a prescribed time passes, an amount of suppressing the luminance of the backlight is lowered so that the backlight operation at a low temperature is stabilized. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to backlight control for controlling an inverter for backlight of a liquid crystal television, a liquid crystal display or the like.

  In recent years, video equipment using liquid crystal such as liquid crystal televisions and liquid crystal displays detects the feature amount of the video signal, and controls the dynamic range and gamma correction of the signal and the backlight brightness of the liquid crystal according to the detection result. Efforts have been made to maximize this (for example, described in Patent Document 1).

In general, a backlight used in a liquid crystal television or the like uses a cold cathode tube, and it is known that luminance cannot be obtained due to the characteristics of the constituent components of the lamp at a low temperature (for example, described in Patent Document 2). .
JP 2002-108305 A JP 2004-220810 A

  However, in the conventional method for controlling the luminance of the backlight, the gain of the video signal is increased and the gain of the backlight is reduced. In general, the backlight has low startability and low luminance characteristics at low temperatures, and there has been a problem that when the backlight is narrowed down by conventional backlight control, the luminance characteristics deteriorate or the lighting is not turned on.

  In addition, even if non-lighting is prevented by limiting the lower limit of backlight control, which is the conventional method, the backlight does not fully squeeze to the brightness that is originally desired to decrease, and the video signal control gain and the backlight control gain are There was a problem that a mismatch occurred and the image quality was uncomfortable.

  In order to solve the above-described problems, a liquid crystal display device according to the present invention includes a feature amount detection circuit that detects a dynamic range of a video signal, a time after the liquid crystal display device is started, and a start time signal that indicates the measured time. A post-startup time detection circuit that outputs, a dynamic range detected by the feature amount detection circuit, a video signal control signal that controls the contrast and brightness of the video signal based on the start-up time signal, and an arithmetic circuit that outputs an inverter control signal A video signal control circuit that controls the video signal based on the video signal control signal, an inverter circuit that outputs a backlight control signal based on the inverter control signal, a backlight that lights up based on the backlight control signal, and a video A liquid crystal module for displaying a video signal controlled by a signal control circuit, and a startup time signal If the time indicated is smaller than a predetermined value, the arithmetic circuit so that the luminance of the backlight is not smaller than a predetermined value, it is characterized in that for controlling the inverter circuit.

  As described above, according to the present invention, it is possible to provide a stable video without a sense of incongruity in a liquid crystal television and a display that control the luminance of the backlight according to the characteristics of the video signal.

(Embodiment 1)
FIG. 1 is an example of a configuration diagram of a liquid crystal display device of the present invention, FIG. 2 is an image diagram of an image quality improvement method for controlling backlight luminance, and FIG. 3 is an example of temperature control.

  In FIG. 1, 1 is a feature amount detection circuit, 2 is an arithmetic circuit, 3 is a post-startup time detection circuit, 4 is a video signal control circuit, 5 is an inverter circuit, 6 is a liquid crystal module, and 7 is a backlight.

  A specific operation will be described below. The feature amount detection circuit 1 detects information on the dynamic range such as the maximum luminance level and the minimum luminance level of the video signal, the average luminance level (APL), and the like, and outputs the information to the arithmetic circuit 2.

  The arithmetic circuit 2 determines the contrast and brightness of the video signal from the maximum luminance level, minimum luminance level, average luminance level (APL) detected by the feature amount detection circuit 1, and the activation time signal from the activation time detection circuit 3 described later. A video signal control signal for controlling the inverter and an inverter control signal for controlling the inverter circuit 5 are calculated.

  The post-startup time detection circuit 3 measures the time after the start of the liquid crystal display device and outputs a startup time signal indicating the measured time to the arithmetic circuit 2.

  The video signal control circuit 4 performs predetermined video signal processing such as gamma correction on the video signal, converts the dynamic range of the video signal based on the video signal control signal, and supplies it to the liquid crystal module 6.

  The inverter circuit 5 outputs a backlight control signal for controlling the backlight 7 based on the inverter control signal.

  The liquid crystal module 6 is driven based on the video signal output from the video signal control circuit 4.

  The backlight 7 is turned on based on a backlight control signal.

  FIG. 2 shows an image diagram of the operation of the liquid crystal display device in this embodiment. FIG. 2A shows an input video signal. The vertical axis indicates the brightness direction. The portion painted in black indicates the luminance distribution of the video signal on the screen. The highest part of black is the maximum luminance level, the lowest part is the minimum luminance level, and the difference between the two is the dynamic range.

  FIG. 2B shows an output to the liquid crystal module 6 of the video signal control circuit 4 when the image quality improvement method is not used. In this case, the dynamic range of the input video signal is not changed.

  FIG. 2C shows the luminance value of the backlight. When the image quality improvement method is not used, the value is constant. In this example, it is set to 100 cd / m2.

  The display of the liquid crystal module 6 can be considered as the product of this video signal and the backlight. FIG. 2D represents an image displayed on the liquid crystal module 6.

  Consider the case where a black level is floating as shown in FIG. 2A and a white level is not high, that is, a signal having a wide dynamic range is input. The feature amount detection circuit 1 detects information about the dynamic range of the video signal such as the maximum luminance value and the minimum luminance value of the video signal and inputs the detected information to the arithmetic circuit 2. The arithmetic circuit 2 determines from the maximum luminance value, the minimum luminance value, etc. that the dynamic range can still be expanded as a video signal.

  The video signal control circuit 4 controls the contrast and brightness of the video signal to widen the dynamic range of the video signal as shown in FIG. On the other hand, since the overall brightness becomes too bright as it is, the inverter control signal for controlling the inverter circuit 5 is reduced. In this embodiment, it is set to 60 cd / m 2 as shown in FIG. In this case, the display of the liquid crystal module 6 is as shown in FIG.

  As is clear from FIG. 2G, the maximum luminance does not change and the minimum luminance value becomes small. As a result, the dynamic range of display is widened on the liquid crystal module 6, and a high-definition image in which black is tightened can be provided. At the same time, since the brightness of the backlight 7 is suppressed, there is an advantage that power consumption is reduced. Generally, the above control changes the frame control in accordance with the feature amount of the input video signal, and a higher effect can be obtained by changing the inverter control signal of the inverter circuit 5 as much as possible.

  On the other hand, if there is a mismatch between the video signal control and the backlight 7 control, the brightness changes and the display image feels uncomfortable. Since the display of the liquid crystal module 6 is the product of the video signal and the backlight 7, even if the same input video signal is used, if the dynamic range of the video signal is expanded and the control signal for controlling the inverter circuit 5 is lowered, the fluctuation of the inverter is changed. The degree of contribution can be controlled.

  In addition, it is known that when a liquid crystal display is viewed in an environment where the surroundings are dark, black appears due to its principle. This is due to leakage of light from the backlight. When the ambient illuminance is detected and the surroundings are dark, the control signal for controlling the inverter circuit 5 is reduced to control the dynamic range of the video signal. Thus, it is possible to suppress the black float as described above.

  On the other hand, if the inverter control signal for controlling the inverter circuit 5 is low at a low temperature, the cold cathode tube used in the backlight 7 may not be lit or the luminance may be significantly reduced due to the characteristics of the cold cathode tube. Generally, the temperature of a video display device is low when the power is turned on, but the temperature of the device rises due to self-heating when a certain time elapses. Therefore, by estimating the elapsed time after startup without using expensive temperature detection means such as a temperature sensor, it is possible to estimate at least how many times the ambient temperature of the inverter circuit 5 is at least more according to the time after startup. Can do.

  Therefore, the elapsed time after startup is measured by the post-startup time detection circuit 3, and the contribution of the backlight control signal (hereinafter referred to as backlight control gain) is changed according to the elapsed time after startup. That is, after a certain period of time has elapsed since startup, as described above, the inverter control signal of the inverter circuit 5 is varied as much as possible to obtain the maximum image quality improvement effect. On the other hand, the inverter control signal of the inverter circuit 5 does not fluctuate so much immediately after start-up, so that the control signal does not become unstable between the cold cathodes.

  FIG. 3 is a diagram showing an example of a time after activation and an elapsed time characteristic after activation of the backlight control gain. Assuming that the backlight control gain is 1 after the elapse of a certain time, the gain is gradually reduced, that is, the fluctuation is reduced as the elapsed time after startup becomes shorter. By controlling in this way, the image quality improvement effect at normal temperature and high temperature and a stable image at low temperature can be provided without using temperature detection means.

  The liquid crystal display device according to the present invention can provide a stable image without a sense of incongruity. It is useful in backlights for controlling backlight inverters such as liquid crystal televisions and liquid crystal displays.

The figure which showed the block diagram of the liquid crystal display device of this invention The image figure which showed the mode of image quality improvement of this invention The figure which showed the elapsed time characteristic example after starting of the backlight control signal of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Feature detection circuit 2 Arithmetic circuit 3 Time detection circuit after starting 4 Video signal control circuit 5 Inverter circuit 6 Liquid crystal module 7 Backlight

Claims (2)

A liquid crystal display device,
A feature amount detection circuit for detecting the dynamic range of the video signal;
The time is measured after the liquid crystal display device is started, and a start time signal indicating the measured time is output, a dynamic range detected by the feature amount detection circuit, and an image based on the start time signal A video signal control signal for controlling the contrast and brightness of the signal, and an arithmetic circuit for outputting an inverter control signal;
A video signal control circuit for controlling the video signal based on the video signal control signal;
An inverter circuit that outputs a backlight control signal based on the inverter control signal;
A backlight that is turned on based on the backlight control signal;
A liquid crystal module for displaying a video signal controlled by the video signal control circuit;
With
If the time indicated by the activation time signal is smaller than a predetermined value,
The liquid crystal display device, wherein the arithmetic circuit controls the inverter circuit so that the luminance of the backlight does not become lower than a predetermined value. 2. The liquid crystal display device according to claim 1, wherein the amount of suppression of the luminance of the backlight is gradually reduced according to the time immediately after startup.

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