JPS6387881A - Bright correction circuit for liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent white collapse in a bright screen and black collapse in a dark screen by providing a smoothing circuit smoothing a picture signal and converting said signal into DC and a brightness setting circuit applying a control voltage to a brightness control terminal of a pedestal clamp circuit in response to the output of this smoothing circuit to correct the brightness automatically. CONSTITUTION:The emitter output of a 1st transistor (TR) Q1 is fed to the smoothing filter 9, where it is smoothed to DC. Since the smoothing filter includes an inverting circuit when the mean brightness level of the video signal is high, the output voltage is low, and when the mean brightness level is low, the output voltage is increased. Then the voltage of a moving contact of a user control variable resistor VR2 of the brightness setting circuit 10 is lower when the output of the smoothing filter 9 is low to lower the pedestal clamp level, and increases the voltage VB when the output of the smoothing filter 9 is higher. Thus, the white level is lowered at the time of bright screen and the black level is floated at the time of dark screen by correcting the brightness automatically.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a brightness correction circuit for a liquid crystal display device such as a color liquid crystal television receiver, which displays images by driving a liquid crystal display panel with alternating current.

(B) Prior Art Conventionally, color and black-and-white liquid crystal display panels are AC driven at a cycle twice as long as the vertical synchronization of an image signal such as a video signal in order to prevent deterioration of their lifespan.

A color liquid crystal television receiver, which is an example of this kind of liquid crystal display device, uses a liquid crystal display panel, for example, "Sanyo F Kunica/L/
L/H: L-1 Vot, 17 & IFEB.

1985J, pages 6 to 10 - In order to include a liquid crystal display panel and drive the display panel with alternating current signals using six primary color signals, for example, published on January 25, 1985 (7)
rNEC Technical Report Vot, 38.4 (2/ 1985)
Model number μ listed on pages 14 to 19 of Nippon Electric Culture Center)
It is equipped with a signal processing circuit including a PCl472G chroma integrated circuit.

When a color video signal such as a received television signal is input to the signal processing circuit, the pedestal level of the input color video signal No. 8 is clamped by the pedestal clamp circuit of the processing circuit, and the pedestal clamp circuit The signal is converted into three primary color signals based on matrix processing of the luminance component.

Furthermore, the color video signal converted to the 6yA color signal is
The polarity inversion circuit of the processing circuit inverts the polarity for each vertical synchronization period, that is, for each field, by using, for example, the intermediate potential of the power supply voltage as the inversion reference potential EO, and converts the three primary colors with a period twice the vertical synchronization period. is converted into an AC drive signal.

Then, AC drive signals of the three primary colors are supplied to the liquid crystal display panel through a panel drive circuit formed by traps such as registers, and the display panel is AC driven.

By the way, color liquid crystal display panels have 6II pixels per pixel.
A transparent electrode is provided which serves as an electrode for displaying color A '#1 and forming a counter electrode to the display electrode.

The polarities of the AC drive signals for the three primary colors are as shown in Figure 3 (-).
, (b), when the reference potential EO of 6V is used as a reference and is inverted every field Tv, the transparent electrode is held at the reference potential EO, and based on the operation of the panel drive circuit, the AC drive signals of primary colors are sequentially supplied to the display electrodes of the three primary colors of each pixel, and at this time, the light of the liquid crystal layer between the display electrode and the transparent electrode is changed depending on the absolute value of the potential difference between the transparent electrode and the display electrode. As the transmittance changes, the brightness of the three colors changes, and a second screen is displayed on the display panel.

In FIGS. 6(a) and 6(b), the broken line W+ and solid line B1 indicate the white level and black level when the polarity is not reversed, and the broken line W2 and solid line B2 indicate the white level when the polarity is reversed. Level, indicates black level, black level B +, F32
Become Kahedestal level.

In addition, in FIG. 6(b), the level of the bright control terminal (described later) is set higher than in the case of FIG. 6(b), and the pedestal is
・Indicates when the level has been raised.

Furthermore, since the light transmittance of the display panel changes depending on the absolute value of the potential difference between the transparent electrode and the display electrode, the display panel displays 11! If the potential of the pole is, for example, 4
V and 8V, the light transmittance becomes the same and the display brightness becomes the same.

By the way, the light transmittance of the liquid crystal display panel is determined by the peak-to-peak value (P-P value) of the AC section 1 signal, that is, the maximum amplitude level Vpp shown in FIGS. 6(a) and (b) K. y with the characteristics shown in Figure 4, vt-vh in the figure becomes the dynamic range of the display panel, and vt
The contrast ratio of the display panel is determined by the light transmittance of -vh.

Note that when the reference potential EO mentioned above is 6v, vt-v
h falls within a narrow range of about 4 to 8 volts in the negative role.

Moreover, compared to cathode ray tubes, etc., crystal display panels are
Since the dynamic range is narrow and the contrast ratio is low, if the kukunpu level of the pedestal clamp circuit described above is fixed, the black level will be distorted on the display screen of the display panel for a dark screen where the overall level vpp is low. On the other hand, bright images with a high overall level VpI (VpI) tend to have jumps in the white level. A control terminal is provided, and a bright adjustment volume is provided to variably adjust the level of the control terminal, and by operating the volume to vary the level of the bright control terminal, the level of the pedestal level can be adjusted to brightness. It is designed to be varied in proportion to the level of the control terminal.

Then, by operating the brightness adjustment volume while looking at the screen on the display panel, raising the pedestal level when the screen is dark, and conversely lowering the pedestal level when the screen is bright, you can eliminate the black level collapse and white Level jumps are prevented.

By the way, when a color liquid crystal display panel is AC driven based on a color video signal as in the above-mentioned color liquid crystal television receiver, the color image 4s signal whose pedestal level is clamped by a pedestal clamp circuit is converted into three primary color signals. After conversion, it is converted into AC driving signals of three primary colors. However, when driving a black and white LCD panel with AC based on a black and white video signal, such as in a black and white LCD television receiver, for example, A black and white video signal whose pedestal level has been clamped by a pedestal clamp circuit is directly input to a polarity inversion circuit and converted into a black and white AC drive signal.

Also, in the case of black and white LCD panels, the dynamic range is narrower and the contrast ratio is lower than that of cathode ray tubes, etc., so the pedestal clamp circuit uses the aforementioned bright adjustment polychrome to variably adjust the clamp level. It is composed of

In other words, in conventional liquid crystal display devices of this type, the dynamic range of the liquid crystal display panel is narrow and the contrast ratio is small, so the black level tends to collapse when the screen is dark, and the white level jumps when the screen is bright. easy.

Therefore, a brightness adjustment volume for varying the clamp level of the pedestal clamp circuit is provided, and by operating the volume, the pedestal level is varied depending on whether the screen is 1 or dark, thereby solving the above-mentioned inconvenience.

(c) Problems to be Solved by the Invention However, it is difficult to operate the brightness adjustment volume each time the brightness or darkness of the screen changes.

In -e usage conditions, the brightness adjustment volume is rarely operated, and therefore,
The clamp level of the pedestal clamp circuit is fixed,
There is a problem in that the display screen becomes very unsightly due to the black level collapse and white level jump.

2) Means for solving the problem of intermediate points The present invention provides a smoothing circuit that smoothes an image signal and converts it into a direct current;
The brightness setting circuit applies a control voltage to the brightness control terminal of the pedestal clamp circuit in accordance with the smoothing circuit output.

(e) Operation By means of the above-described means, the present invention automatically controls the clamp level of the pedestal clamp circuit according to the average level of the image signal, and lowers the pedestal level for bright screens, and lowers the pedestal level for dark screens. On the contrary, it acts as if it were lifted up.

(f) Example An embodiment of the present invention will be described below with reference to the drawings.

Figure 1 shows the brightness correction circuit in the tree embodiment (1
) is an input terminal into which a positive polarity video signal is input, (2) is an M combination capacitor (C1), and a bias resistor (Rz) (R
2), an amplification circuit consisting of a first transistor (Ql), a load resistor (R3), and a resistor (R4).

(3) is a delay circuit that delays the negative polarity video signal that is the collector output of the first transistor (Q10) and includes a subcarrier trap. (4) is a chroma and video signal Ji!r processing IC. , for example μpc1472G (
(manufactured by NEC Corporation) is allowed. This IC 14) includes a pedestal clamp circuit (5), a color demodulation circuit (6) equipped with a polarity inversion circuit, and the like.

(7) is a synchronous peak value type clamp circuit into which the emitter output of the %1 transistor [Q10] is input, and includes a capacitor (C2), resistors (R5) (R6) (R7), and a second transistor (C2). ). (8) is an impedance conversion circuit which takes the emitter output of the emitter of the second transistor as input and is composed of a third transistor (CQS) and a resistor (R8), and (9) smoothes the emitter output of the third transistor (CQS). It is a smoothing filter that converts the current into direct current.
Rlo) (Rz) (Rz2) and a fourth transistor for inversion (C4).

Further, the output of the third transistor is also supplied to a driving circuit of the liquid crystal panel. C0 is a bright setting circuit that determines the clamp level I of the pedestal clamp circuit (5), and basically there is a sub-bright volume (VR+), a user control volume ( VR2) and a resistor (R15) in series, the volume (VR2) and the resistor (Rz3
) is connected to the collector of the fourth transistor (C4) via a resistor (R14), and the movable contact of the resistor (VR2) is connected between the series resistor (R15) and ground. The pedestal clamp circuit 151 is connected to the pedestal clamp circuit 151 via a capacitor (C4).

Further, qυ is an input terminal to which a switching pulse for inverting the demodulated outputs of R%G and B is input for each field.

Next, the operation of the bright correction circuit described above will be explained.

1. First, the positive polarity video signal shown in FIG. The signal) passes through the delay circuit (3) and is input to the pin of the IC (4).

After the pedestal of the video signal is clamped by the pedestal clamp circuit (5), this input is demodulated by the color demodulation circuit (6).
) The polarity is reversed every field from K, and the polarity is reversed from 0 in Fig. 2.
) The demodulated outputs of R and GB as shown in the figure are obtained from the bins ■■■.

On the other hand, the emitter output of the first transistor (Ql) is clamped at the synchronization tip by the clamp circuit (7), then impedance-converted by the impedance conversion circuit (8), and outputted as a positive polarity signal. Ru. This output is supplied to a drive circuit that drives a liquid crystal panel (not shown), and is also supplied to a smoothing filter (9)K where it is smoothed into direct current. This smoothing filter includes an inverting circuit, so
When the average brightness level of the video signal is high, the output voltage is low, and when the average brightness level is low, the output voltage is high.

Therefore, the voltage of the movable contact of the user control volume (VR2) of 11 constant circuit (1) becomes lower when the smoothing filter output becomes lower (when the average brightness level is high).
lower and lower the pedestal clamp level, i.e. Figure 2? It operates so that one voltage VB becomes smaller. Further, 13 steps are performed so that as the smoothing filter output increases, the voltage VB increases.

Therefore, the brightness is self-help corrected so that the white level is depressed when the screen is bright, and the black level is raised by t when the screen is dark.

Note that the volume (VR2) can be varied according to the operator's preference.

(g) Effects of the Invention As described above, according to the present invention, brightness correction is automatically performed so that the white level sinks when the screen is bright and the black level rises when the screen is dark. Can prevent black spots.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a bright correction circuit of a liquid crystal display device according to an embodiment of the present invention, FIG. 2 is a waveform diagram of the main part, FIG. 3 (aXb) is a waveform diagram of an AC drive signal, The figure is a characteristic diagram of light transmittance of a color liquid crystal display panel. (5)...Pedestal clamp circuit, (6)...Color demodulation circuit, +71...Clamp circuit, (8)...
- Impedance conversion circuit, (9)...Smoothing filter, +1(fl...Bright i constant circuit.

Claims (1)

[Claims] (1) A pedestal clamp circuit that controls the pedestal level of the input image signal to a clamp level proportional to the level of the bright control terminal, and a vertical cycle of the polarity of the image signal whose pedestal level is clamped by the pedestal clamp circuit. In the liquid crystal display device, the liquid crystal display device includes a polarity reversing circuit that inverts the image signal to generate an AC drive signal, and a drive circuit that AC drives a liquid crystal display panel using the AC drive signal. and,
A brightness correction circuit for a liquid crystal display device, comprising a brightness setting circuit that applies a control voltage to the brightness control terminal according to the output of the smoothing circuit.