JPS6219835A - Lighting device for liquid crystal display device - Google Patents

Abstract

PURPOSE:To increase the contrast of a display image and to reduce the power consumption of a lighting system by varying the brightness of a light source by controlling a driving circuit according to a white peak detection signal. CONSTITUTION:The white peak signal detecting circuit 67 is provided between terminals 21 and 26 to detect whether or not a video signal applied to a liquid crystal panel contains a white peak signal larger than a specific value. Its white peak detection signal is applied to the connection point of resistances R1 and R2 through a voltage polarity inverting circuit 68 and then the white peak detection signal is held for at least one field period. Consequently, a control circuit A' controls a driving circuit B' according to the white peak detection signal to vary the brightness of an electroluminescence panel 60.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a matrix type liquid crystal image display device, and particularly to an illumination device thereof.

[Background of the invention]

It has multiple light sources, each light source being red and green.

A means for causing a liquid crystal display to perform color display by emitting light of a specific hue such as blue and sequentially switching the light sources or controlling the emission intensity of the light sources is disclosed in, for example, Japanese Patent Laid-Open No. 5 It is known from Japanese Patent Laid-Open No. 58-IE16791, etc.

However, in order to faithfully reproduce white color with a conventional liquid crystal display device, the brightness of the light source must be set high, which increases power consumption.

Another drawback is that as the brightness of the light source increases, the black level also increases, making it impossible to obtain sufficient contrast.

[Purpose of the invention]

The present invention provides lighting that can ensure good image quality such as high brightness, high contrast, and sufficient extension of the white peak in matrix type image display devices such as LCD color televisions, while simultaneously reducing the power consumption of the entire set. The purpose is to provide equipment.

[Summary of the Invention] The present invention adjusts the brightness of a light source according to the brightness level of a video signal, reduces power consumption in the case of a low-brightness video signal, and reduces power consumption in the case of a high-brightness video signal. The brightness of the light source is such that a white peak can be obtained. Specifically, we focused on the fact that the frequency of occurrence of signals that can be considered white peaks in television signals, etc. is relatively low compared to the overall frequency, and we usually lower the brightness of the light source, thereby reducing power consumption by the light source. Small.

The brightness of the light source is controlled by the state of the input signal so that only the light source brightness at the time when the white peak signal is present is increased.

Furthermore, as a light source device for illuminating one liquid crystal panel, it has a plurality of light sources such as 1' that partially illuminate the liquid crystal panel, and each of the plurality of light sources should share the illumination. Detects the presence or absence of a white beak signal at a location,
The brightness of each light source is controlled according to the detected information.

[Embodiments of the invention]

A first embodiment of the present invention is shown in FIG. 1 and will be described in detail.

In FIG. 1, 1 is a liquid crystal display panel, and a liquid crystal layer 2
is narrowed by glass plates 5, 4 and polarizing plates 5, 6.

This liquid crystal panel 1 applies a voltage between a common electrode and a pixel electrode (not shown) formed between a liquid crystal layer 2 and glass plates 3 and 4, respectively, and transmits light in combination with the action of polarizing plates 5 and 60. It is well known that the rate can be controlled.

It is also well known that an image can be displayed by selecting the pixel electrodes in an X, Y matrix and applying a video signal such as a television signal.

7 is a light source for illuminating the liquid crystal panel 1, which is a cold cathode fluorescent lamp. The fluorescent lamp 7 is driven by a drive circuit BK.

The drive circuit B includes a transistor 15 constituting an oscillation circuit, a resistor 14 for applying a pace bias voltage to the oscillation transistor 15, a transformer 16, a capacitor 17, diodes 18 and 19, and a resistor 20.

Drive circuit B is controlled by control circuit A. - The control circuit consists of a power supply 9, resistors 10 and 11, a transistor 12, and an emitter resistor 13, which constitute a bias circuit consisting of an emitter follower.

The operation of the control circuit and drive circuit B is as follows.

When a DC voltage is applied from the emitter 7 lower transistor 12 to the pace of the transistor 15 via the resistor 14, a collector current begins to flow into the transistor 15 through the transformer 160 winding 16-α. With this current, coil 1
A voltage is generated at 6-b, a pace current flows through the transistor 15 via the capacitor 17, and the collector current further increases.

However, when the collector current begins to saturate, the pace voltage begins to shift from forward bias to reverse bias, and the collector current decreases. The decrease in collector current causes a decrease in the flux linkage of the transformer 16, further reverse biasing the pace voltage, so that no collector current flows.

Diodes 18 and 19 are provided to prevent the base reverse bias from becoming too deep. The reverse bias voltage applied to the transistor is discharged by the current supplied from the resistor 14, and when it becomes forward biased again, the transistor 1
5, the collector current begins to flow. By repeating the above oscillation operation, a high frequency and high voltage is induced in the secondary boost coil 16-c of the transformer 160, and the cold cathode fluorescent lamp 7 is turned on.

In drive circuit B, when the base voltage of transistor 120 decreases, the base bias voltage of transistor 150 also decreases, and in the aforementioned oscillation operation, it takes a long time for transistor 15 to recover from reverse bias to forward bias. The period during which no collector current flows becomes longer, and therefore the power supplied to the fluorescent lamp 7 via the transformer 16 decreases, and the luminance of the fluorescent lamp 7 decreases.

Conversely, when the base bias voltage of the transistor 12 increases, the power supplied to the fluorescent lamp 7 increases, and the luminance of the fluorescent lamp 7 increases.

In the embodiment of FIG. 1, the state of the drive circuit B is controlled,
In order to control the brightness of the fluorescent lamp 7, the following circuit configuration is further added. A video signal is applied from the input terminal 21 to one input terminal of a voltage comparator 22, and the other input terminal of the comparator 22 is connected to a reference voltage power source 25.

The voltage of the power supply 23 is set to be slightly lower than the white peak voltage of the video signal applied to the terminal 21. Therefore, the output voltage of the voltage comparator 22 becomes high level only when the video signal applied to the terminal 21 corresponds to the white peak, and becomes low level when the video signal is a normal video signal other than the white peak.

Next, the relatively short pulse-like output voltage of the voltage comparator 22 is detected by a detection circuit consisting of a diode 24° capacitor 25, and if there is a white peak signal in the video signal, the voltage at the terminal 26 is maintained for at least one field period. It is maintained at a low level. The holding time is resistance 10. It can be set by a time constant composed of N and a capacitor 25.

Since the voltage at the terminal 26 is supplied to the base of the transistor 120, the circuit configuration shown in FIG. 1 makes the light source brightness for illuminating the liquid crystal panel 1 brighter than normal only when a white peak signal is present. Therefore, the brightness of the reproduced image on the liquid crystal panel 1 can be increased. In normal times when there is no white peak signal, the power supplied to the fluorescent lamp 7 is kept low, and the power consumption of the entire set can be kept low in normal times.

Furthermore, from the perspective of image quality, when a white peak signal is present, even if the black level is slightly off, it can be recognized as an image with sufficient contrast; however, when normal image content is displayed, sufficient contrast cannot be obtained. For this purpose, it is desirable that the black level be as low as possible.

According to the means described in the embodiments of the present invention, since the light source brightness is in a relatively low state during normal image content display,
Even if it is assumed that the light transmittance of the panel when displaying black on the LCD panel is constant at 5%, for example, the black level can be relatively lowered in proportion to the light source brightness, and sufficient contrast can be obtained. Therefore, a large image quality improvement effect can be obtained.

Next, a second embodiment of the white peak signal detection and voltage level holding circuit from terminal 21 to terminal 26 will be described in detail as shown in FIG.

In FIG. 2, the same numbers are used to indicate the same contents as in FIG. 1. 27 is a triggerable multivibrator, which is connected to an external resistor 28 and a capacitor 2.
When a trigger pulse is input again within the time determined by the value of 9, the output pulse width is further extended for a certain period of time from the input time of the trigger pulse, and one continuous pulse output is obtained. As a specific example of a triggerable vibrator, for example, RCA's general-purpose IC-
CD4098B or the like may be used. Therefore, if you set the output pulse width for one trigger input signal of the triggerable multivibrator to be slightly longer than one field or one frame period, the white peak signal that occurs at the same location on the screen can be set. , it is possible to reliably capture the output pulse within a predetermined output pulse width, and the voltage at the terminal 26 is always kept stably at a high level during the period when the white peak signal exists (
be able to. Note that 30 is a power supply terminal. The embodiment shown in FIG. 2 is characterized in that a more stable voltage with less fluctuation can be obtained as the voltage at the terminal 26 compared to the embodiment shown in FIG.

Next, the horizontal axis is the effective value of the applied voltage applied between the common electrode and the pixel electrode of the liquid crystal panel, and the vertical axis is the transmittance of the liquid crystal panel (or the brightness of the liquid crystal panel when the brightness of the light source is set). As an example of the characteristic, the curve shown by the solid line α in FIG. 3 is obtained. The curve α in Figure 3 is
The voltage applied to the liquid crystal panel is from O to ■. Up to ■, the light transmittance is almost zero. When the value exceeds 1, the transmittance increases almost linearly up to V, and when the value exceeds 2, the transmittance is saturated. If the brightness of the panel when the voltage is V on the curve α is Ll, the brightness from black to white on the liquid crystal panel is represented by the brightness from zero to up to. On the other hand, when a daybeak signal such as a superimposed white character is reproduced on a CRT, the brightness of the reproduced white peak is emphasized due to the effect of the CRT's own r correction. Such a correction effect cannot be expected with the brightness characteristics of the liquid crystal panel shown. Therefore, when displaying on a liquid crystal panel, when a white peak signal is received, the brightness of the light source itself may be increased to obtain a characteristic curve such that the maximum brightness increases. Examples of automatic light source brightness adjustment means for this purpose are shown in the embodiments shown in FIGS. 11 and 2.

However, as is clear from the difference between the characteristic curves α and b in FIG.
However, when the brightness of the light source is increased, the brightness increases to LI.
Excluding the white peak, unfavorable phenomena occur from the viewpoint of reproduced image quality.

This phenomenon occurs even in image reproducing devices using cathode ray tubes, as the DC transmission rate is not 100%, resulting in the AFL (Average Picture L) of the video signal.
This is the same phenomenon as the phenomenon in which the reproduced image brightness fluctuates as the brightness changes. Although the phenomenon in which the reproduced image brightness fluctuates due to the APL change is allowed to some extent, it is desirable to take measures to prevent the fluctuation. It is desirable that the panel applied voltage V1 is converted to V in the case of the characteristic curve. Since the voltages v0 and vl in FIG. 5 represent the black level and white level of the video signal, respectively, if the black level potential of the video signal is fixed and its amplitude is expressed as a characteristic curve, the characteristic curve α It is only necessary to control it so that it becomes small according to the ratio of Shikaku in the case. As shown in FIG. 4, specific means for this purpose include an amplifier 52 (for example, composed of a differential transistor pair) whose amplification degree can be varied according to the DC voltage applied to the terminal 31, and a DC voltage applied to the terminal 63. The video signal may be applied from the terminal 21 to a circuit configuration consisting of a clamp circuit 34 that fixes the black level potential of the video signal using a clamp pulse generated by the video signal, and the video signal may be taken out from the terminal 35. After the output signal from the terminal 35 is processed to be suitable for display on the liquid crystal panel, it is applied to the pixel electrode of the liquid crystal panel. If the amplification control signal applied to the terminal 31 is formed using the voltage at the terminal 26 shown in FIG. 1 or 2, correct control can be performed in synchronization with changes in the luminance of the light source.

Note that since the brightness of the light source is controlled using a threshold for white peak signals of a predetermined level or higher, even in the signal processing shown in FIG. It is desirable to perform amplitude control (r correction) so that the signal is relatively expanded compared to the amplitude of the signal.

Amplifier 32 of FIG. 4 also includes this function.

The specific circuit configuration means necessary for these video signal processing can be easily designed with knowledge of ordinary electronic circuits.

Next, a fourth embodiment of the present invention is shown in FIG. 5 and will be described in detail. Like FIG. 2, FIG. 5 shows an improved embodiment of the circuit configuration from terminal 21 to terminal 26 in FIG. That is, in the embodiments shown in FIGS. 1 and 2, the threshold is not set because the change from the characteristic curve α shown in FIG. The embodiment shown in FIG. 5 is configured so that it can change in an analog manner within the range from the characteristic curve α to the characteristic curve α. That is, in FIG. 5, the reference power supply 23
．． A clip circuit 47 for detecting white peak signals is constructed of a diode 66 and a resistor 37, and the output voltage level of the clip circuit is connected to the terminal 26 via an analog sample/hold circuit composed of FETs 38, 39, a capacitor 40, and a resistor 41. Output to. The circuit configuration in Figure 5 is F.
The input signal of ET3B or the output voltage of FET39 may be appropriately amplified.

The sampling pulse of the analog sample and hold circuit 48 is basically a white peak signal which is the output signal of the clipping circuit, which is subjected to switching processing by the switching circuit 42, and this pulse is used as the sampling pulse of the OR gate 4.
6, and the output terminal of the OR gate is connected to the gate electrode of the FET 38. Furthermore, in order to quickly reduce the light source brightness when the white peak signal disappears, the vertical synchronization signal or the vertical start signal for driving the matrix of the liquid crystal panel is set as a sampling pulse at least one field or more after the last white peak signal. The switching circuit 42 can be used as
The output pulse width is set slightly longer than one field period, and the output positive pulse of the triggerable multivibrator 27 (external C-R time constant circuit not shown) is connected to the inverter. A vertical synchronizing signal or a vertical start signal is applied from a terminal 46 to the other input terminal of the Φ gate 45 through a terminal 44. With the above configuration, in the normal case where there is no white peak signal, an output voltage corresponding to the voltage of the reference power supply 23 sampled by the vertical synchronization signal etc. is obtained at the terminal 26, and when there is a white peak signal, the output voltage is obtained at the terminal 26. An output voltage corresponding to the white peak voltage level is obtained.

In the above explanation, the case where one fluorescent lamp is used as a light source for illuminating the liquid crystal panel 1 has been explained.
Images such as superimposed white characters, which should be reproduced with high brightness and extended white peaks, are often concentrated in a portion of the entire screen. In such a case, illuminating the entire liquid crystal panel 1 with a single fluorescent lamp is undesirable in terms of power consumption because a high-intensity light source is placed even in areas where high brightness is not required.

In order to solve this problem, as shown in Figure 6,
Each part of the LCD panel is configured to be illuminated with multiple fluorescent lamps so that the area where the white peak signal exists can be illuminated by multiple fluorescent lamps. It may be configured to increase the temperature by 1 degree. FIG. 6 shows a case where the illumination is divided into three parts parallel to the scanning line. The fluorescent lamps 7α, 7b, and 7c are each structurally or optically separated by a dashed line on the panel 1, or are configured to illuminate portions E and F independently. The drive circuits for the fluorescent lamps 7α, 7b, and 7c are 8α, 8b, and 8c, respectively. Signal output terminals 26αl 26bl 26C are used to control the states of the drive circuits 8α, 8b, 8c and change the brightness of the fluorescent lamps 7α, 7b, 7c. Each of the circuit blocks 50α, 50b, and 50c detects a white peak signal from the terminal 21 to the terminal 26 in FIG. 1, FIG. 2, or FIG. It includes a circuit configuration for forming a signal for changing the state of 8.

In Fig. 6, fluorescent lamps 7α, 7b, 7c are connected to panel 1.
Since each section above is illuminated in a shared manner, it is necessary to determine whether there is a white peak signal within the area assigned to each fluorescent lamp.

This is possible by using the video signal gate circuit 1 of 51α, 51b, and 51C shown in FIG.

As an example of the video signal gate circuit, a switch 52α consisting of an FET is used, and its gate terminal 53α is connected to the pixel electrode of the portion D on the liquid crystal panel 1.

(Only during the period when the video signal is written to) ・Apply the voltage of 1 level and turn on the video signal gate circuit 51α.

During the rest of the period, a low level voltage is applied to the gate terminal 53α to open the input and output terminals. Since the liquid crystal panel according to the present invention is matrix-driven, the above-mentioned high-level and low-level voltages can be easily formed by ordinary circuit design techniques by using the drive signals.

In Figure 6, the screen of one liquid crystal panel is divided into three parts by boundaries parallel to the scanning line, but the method of dividing the screen and the number of divisions are not limited to this. It is also possible to divide the signal and obtain a luminance control signal for a fluorescent lamp as in the case of FIG. Furthermore, it is also possible to simultaneously perform division in the direction parallel to the scanning line and in the direction perpendicular to the scanning line (*, and to arrange the light sources in a matrix).

Next, a sixth embodiment of the present invention is shown in FIG. 7 and will be described in detail. In FIG. 7(α), 60 is an electroluminescence (EL) panel for illuminating the liquid crystal panel 1. Specifically, as shown in FIG. 7(b), 60 is a protective glass plate and a reflector. 61, transparent conductive film electrode 62,
65 and a phosphor layer 64. transparent electrode 63
The side protection is also used as the glass plate of the liquid crystal panel 1. - As an example, assume that the EL panel 60 in FIG. 7 is a ■-shaped EI. B' is a DC power supply circuit for driving the hood panel 60, and has the configuration of a switching regulator (detailed explanation of the internal circuit configuration, operation, etc. is omitted). 66 is a DC power supply. This switching regulator divides its output voltage by resistors R and tRt to obtain a reference voltage E. Compared to this, the conduction period of the switching transistor is controlled to stabilize the output voltage. In other words, if the output voltage becomes slightly higher, the voltage after voltage division by R and R6 will also increase, forming a negative feedback loop in which the conduction period of the switching transistor becomes shorter and the output voltage decreases. There is. Therefore, if, on the contrary, the voltage at the connection point between R2 and R7 is forcibly controlled to be high from the outside, the output voltage of the switching regulator B' will decrease, and the brightness of the hole panel will also decrease. Furthermore, if the voltage at the connection point between R and R1 is controlled to be low, the output voltage of the switching regulator B' will increase, and the brightness of the EL hole panel will also increase. Therefore, as shown in FIG.
For example, a white peak signal detection circuit 67 shown between the terminal 21 and the terminal 26 in FIGS. 1, 2, and 5 is provided between the terminal 26 and the voltage polarity inversion circuit (transistor emitter, (grounded amplifier, etc.) 68, resistors R1 and R3.
If a light source brightness control signal is applied to the connection point with
By increasing the brightness of the light source, it is possible to improve the elongation of the white peak of the raw image. Note that the voltage polarity reversing circuit 68 shown in FIG. 7 is used because the specific configurations of the lighting power supply circuit B in FIG. This is because it is necessary to obtain display characteristics.

Moreover, when the EL hole panel electrode structure in FIG. 7 is viewed from a plan view, if it is divided into nine parts, for example, as shown by the broken line in FIG. 7 (α), the white peak signal detection For the circuit 67, for example, nine circuits may be arranged in parallel instead of the three circuits shown in FIG. 6, and nine switching regulators B' may also be arranged in parallel. In that case, it goes without saying that the gate signal to be applied to the gate terminal 53α for the video signal gate in FIG. 6 must be optimized depending on the actual method of dividing the light source.

〔Effect of the invention〕

As described above, according to the present invention, the brightness of the light source for illuminating the liquid crystal panel during the time or portion where the white peak signal does not exist is set to low, and the brightness of the light source for illuminating the liquid crystal panel during the time or portion where the white peak signal exists is set to low. As a result, during normal image display, the display brightness of the black level signal can be displayed blacker, the contrast can be improved, and the white peak can be sufficiently extended. , the overall image quality can be greatly improved.In addition, dividing the illumination light source and narrowing the illumination range that each light source is responsible for is especially effective when using fluorescent lamps as the light source. (Equivalent to placing the light source farther away from the LCD panel, the viewing angle to the LCD panel for the portion of the shared illumination from the light source becomes smaller), making it easier to illuminate the LCD panel with parallel light as illumination light. This K makes it possible to improve the contrast of the displayed image and reduce the viewing angle dependence of the contrast, and the overall effect of improving the reproduced image quality is extremely large. Additionally, the power consumption of the lighting system as a whole is greatly reduced.

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of the present invention. The road configuration diagram, Figure 2, is a portion of Figure 1. A circuit configuration diagram showing a modified second embodiment, FIG. 3 is a characteristic diagram showing an example of applied voltage versus brightness characteristics of a liquid crystal panel, and FIG. FIG. 5 is a block diagram of a third embodiment of the present invention showing means for controlling the video signal level, and FIG. 5 is a circuit configuration showing a fourth embodiment, which is a portion of FIG. 1 taken out and modified in the same way as FIG. 2. 6 is a circuit diagram showing a fifth embodiment of the present invention, and FIG. 7 is a circuit diagram showing a sixth embodiment of the present invention. 1...Liquid crystal display panel, 7...Light source for illumination, 8
...drive circuit, 27... triggerable multivibrator, 32
...variable amplifier, 64...clamp circuit, 4
7... Clip circuit for white peak signal detection, 48...
Analog sample hold circuit, 60... Electroluminescence panel, h, A'... Control circuit,
B, B'...drive circuit. Private Patent Attorney Katsuo Koyo Figure 1? Figure 5 Figure 6 Figure 7 Figure 8

Claims (3)

[Claims] (1) A liquid crystal display consisting of a liquid crystal layer and a polarizing plate, which has a light source that illuminates the liquid crystal panel and whose light transmittance changes depending on the voltage of the video signal applied to each pixel, and a drive circuit that drives the light source. In the lighting device of the display device, the white peak signal detection means detects whether or not the video signal applied to the liquid crystal panel includes a white peak signal of a predetermined value or more, and outputs a white peak detection signal; holding means connected to the white peak signal detection means and holding the daily peak detection signal for at least one field period; and holding means connected to the holding means and the driving means and controlling the driving means according to the white peak detection signal. An illumination device for a liquid crystal display device, comprising control means for changing the brightness of the light source. (2) In claim 1, the control means controls the drive means to increase the brightness of the light source when the white peak signal detection means detects the presence of a white peak signal. A lighting device for a liquid crystal display device characterized by: (3) In claims 1 and 2, the light source is comprised of a plurality of light sources that illuminate each part of the liquid crystal panel, and the driving means detects the white peak signal for each light source. 1. An illumination device for a liquid crystal display device, comprising: a holding means; and a controlling means.