JPH04238476A - White peak clip circuit - Google Patents

Abstract

PURPOSE:To make a picture displayed in a liquid crystal display panel to be high quality by remaining the high frequency component of a video signal and compressing the video signal from a level where a low frequency component exists to a peak level in compression means. CONSTITUTION:Low band compression circuits 2-4 compress the signals from the level where the low frequency component exists to the peak level by remaining the high frequency component of R, G and B signals generated by a video chroma signal processing circuit 1. High band clip circuits 11-13 follow the R, G and B signals which are level-adjusted by pedestal clamping circuits 8-10 through picture quality correction circuits 5-7 to a high frequency area and amplify them, and they clip the signals amplified in a gradation level which cannot be displayed by the liquid crystal display panel. Thus, an excessive signal is not given to the liquid crystal display panel and the outline of the video can made clear by the clipped high frequency component.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] This invention relates to a white peak clipping circuit that clips the white level peak of a video signal and supplies it to a liquid crystal display panel, and in particular improves the gradation expression near the white level. It is related to a circuit that can do this.

0002

2. Description of the Related Art With the recent development of liquid crystal display panels, panels capable of not only black and white display but also color display have come to be commercially available. However, the gradation of a liquid crystal display panel is lower than that of a CRT, and the range of input levels is also narrower. Furthermore, in order to protect the liquid crystal display panel, it is necessary to limit the input level. Therefore, in the past, the peak (white level) of the input signal was clipped at a constant level.

0003

[Problems to be Solved by the Invention] However, if the image is clipped at a certain level, high frequency components are also lost at the same time, making the outline of the image unclear. Furthermore, gradation is lost, resulting in a screen with low contrast.

[0004] As a result, there is a problem in that a high quality image cannot be obtained with a liquid crystal display panel.

[0005] Therefore, an object of the present invention is to provide a white peak clipping circuit that can improve the quality of images displayed on a liquid crystal display panel.

[0006]

[Means for Solving the Problems] A white peak clipping circuit according to the present invention to achieve the above object is a white peak clipping circuit that clips the peak of the white level of a video signal and supplies the clipped signal to a liquid crystal display panel. It has the following characteristics. In other words, the high frequency components of the video signal are left and only the low frequency components are changed.
The present invention includes a compression means for compressing a low frequency component from a certain level to a peak level, and a level clipping means for clipping the output of the compression means at a level capable of expressing gradation on a liquid crystal display panel.

[0007]

[Operation] According to the present invention, the compression means compresses only the low frequency components of the video signal from a certain level to the peak level while leaving the high frequency components. By doing this, unlike the conventional example in which the level of the video signal is clipped to protect the display panel, the gradation of the liquid crystal display panel will not be lost. The remaining high frequency components are clipped at a level that allows the liquid crystal display panel to express gradation, so that an excessive signal is not applied to the liquid crystal display panel. Moreover, the contours of the image can be made clearer by using the high-frequency components after clipping.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an embodiment of the present invention. Referring to the figure, this white peak clipping circuit includes a video chroma signal processing circuit 1 that generates an R signal, a G signal, and a B signal based on an input chroma signal C and a luminance signal Y; It has compression circuits 2 to 4, image quality correction circuits 5 to 7, pedestal clamp circuits 8 to 10, and high frequency clip circuits 11 to 13. Note that the low frequency compression circuits 2 to 4, the image quality correction circuits 5 to 7, the pedestal clamp circuits 8 to 10, and the high frequency clipping circuits 11 to 13 are configured with similar circuits, and the difference is that the input signals are These are the R signal, G signal, and B signal.

The low frequency compression circuits 2 to 4 are circuits that compress only the low frequency components of the R, G, and B signals generated by the video chroma signal processing circuit 11, leaving the high frequency components. The image quality correction circuits 5 to 7 are circuits for removing high frequency noise contained in the output signals of the low frequency compression circuits 2 to 4 to obtain high quality images. Pedestal clamp circuit 8~
Reference numeral 10 denotes a circuit that clamps the pedestal levels of the R, G, and B signals that have passed through the image quality correction circuits 5 to 7 to a certain voltage level. The high frequency clip circuits 11 to 13 have R and G levels adjusted by the pedestal clamp circuits 8 to 12.
, B signal up to the high frequency range and amplify it,
This circuit clips signals amplified at gradation levels that cannot be expressed by a liquid crystal display panel. The operation of the white clip circuit shown in FIG. 1 will be explained with reference to FIG. 2.

FIG. 2 is a diagram showing the output waveform of the R signal system in FIG. (c)
is the output waveform to the high frequency clipping circuit 11. Note that the horizontal axis represents time and the vertical axis represents voltage level. Assume now that the video chroma signal processing circuit 1 generates a signal (a). This signal (a) is given to the low frequency compression circuit 2, and when it exceeds a certain level ref1, it becomes
Only low frequency components are clipped. In this way, the signal (b) in which only the low frequency components are compressed from the low frequency compression circuit 2
is generated. The signal generated by the local compression circuit 2 (
After the high-frequency noise signal is removed in the image quality correction circuit 5, the signal b) is applied to the pedestal clamp circuit 8, where the pedestal is set to a certain voltage. The signal whose pedestal level has been adjusted is applied to the high frequency clipping circuit 11, where it is amplified and clipped at the maximum level that can be expressed on the liquid crystal display panel. The image displayed based on the signal (c) output from the high-frequency clipping circuit 11 is an image that gradually becomes brighter from black to white toward one end or the other end of the screen.

Next, the case of displaying a crosshatch pattern on a liquid crystal display panel will be explained with reference to FIG. In FIG. 3, (d) is the output waveform of the video chroma processing circuit 1, and (e
) is the output waveform of the local compression circuit 2, and (f) is the output waveform of the high frequency clipping circuit 11. The signal (d) is a signal for expressing a photon pattern (crosshatch pattern),
To make the outline clearer, the white level at the border is increased. This signal (d) is given to the low frequency compression circuit 2, which leaves the high frequency component as it is.
A waveform (e) is obtained in which the low frequency component is clipped. Here, since the white level of the low range is constant, the waveform is similar to that obtained by clipping at a constant level. The signal (e) is applied to a high frequency clip circuit 11 through an image quality correction circuit 5 and a pedestal clamp circuit 8. The signal (e) applied to the high frequency clipping circuit is clipped at the maximum level ref2 that can be expressed in gradation on the liquid crystal display panel, as described in FIG. 2, and the output is (f).

FIG. 4 is a circuit diagram showing details of the low frequency compression circuit. Note that the low-frequency compression circuits 2 to 4 are constructed of similar circuits, and FIG. 4 shows one of them. Referring to FIG. 4, the constant-area compression circuit includes a first differential amplifier 21, a first emitter follower circuit 22, a low-pass filter 23,
It has a second emitter follower circuit 24 and a second differential amplifier 25. The 21st operational amplifier 21 includes a transistor Q1, a transistor Q2, and a variable resistor VR1.
and a variable resistor VR2. This operating amplifier 2
1 clips the moving chroma signal at the clamp level ref1 set by the variable resistor VR2, and adjusts the level of the input signal higher than the clip ref1 by the variable resistor VR1. The second differential amplifier 25 includes an operational amplifier IC1 and resistors R4 to R7. This operational amplifier IC1 takes the difference between the signals input to the - terminal and the + terminal.

Next, the operation of the local compression circuit shown in FIG. 4 will be explained with reference to the waveform diagram shown in FIG. In FIG. 5, (g) shows the output waveform of the video chroma signal processing circuit 1, (h) shows the output waveform of the first differential amplifier 21, (i) shows the output waveform of the second emitter follower circuit 24, and (j ) is the output waveform of the second differential amplifier 25.

First, the signal (e) is applied to the base of the transistor Q1, and is also applied to the + terminal of the operational amplifier IC1 through the capacitor C1 and further through the resistor R5. A variable resistor VR is connected to the base of the transistor Q2.
A DC voltage variable by 2 is provided, and a level ref1 for clipping the primary color signals R, G, B signals (g) is determined by this DC voltage. In this way, a clipped signal (h) is output from the first operational amplifier 21. The gain for signals with a level higher than the clip level ref1 is determined by the variable resistor VR.
1 and the first emitter follower circuit 2
2 and is input to a low-pass filter 23 composed of a resistor R3 and a capacitor C3. Low pal filter 2
The signal (h) that has passed through the second emitter follower circuit 24 is current amplified by the second emitter follower circuit 24. Current amplified signal (i
) is a low frequency component with a level higher than the clip level ref1. The output of the second emitter follower circuit 24 is
Operational amplifier IC1 through capacitor C4 and resistor R4
is given to the - terminal of The operational amplifier IC1 receives the signal (
A signal (j) obtained by taking the difference between signal (g) and signal (i) is output. This signal (j) has a waveform in which the level higher than the clip level ref1 is compressed, and the gradation is not lost.

FIG. 6 is a circuit diagram showing details of the high frequency clipping circuit. Referring to the figure, this high-frequency clipping circuit includes an amplifier 26 that tracks and amplifies a signal at a higher level than the clamp level Vref to a high frequency region, and the output of the amplifier 26 at a level that can be expressed on a liquid crystal display panel. and a clip circuit 27 for clipping. Amplifier 26 includes an operational amplifier IC2, resistors R11 to R13, and a reference voltage Vref set to a pedestal clamp level. Clip circuit 27 includes a transistor Q11, a transistor Q12, a variable resistor VR11, a resistor R14, and a capacitor C11.

Next, the operation of the high frequency clipping circuit shown in FIG. 6 will be explained with reference to FIG. In FIG. 7, (k) is
Output waveform of pedestal clamp circuit, (l) is amplifier 2
In the output waveform of 6, (m) is the output waveform of the clip circuit 27. Signal input from the pedestal clamp circuit (
k) is applied to the + terminal of the operational amplifier IC2 through the resistor R11, and is amplified here. Amplified signal (l
) is applied to transistor Q11. Furthermore, the emitter of the transistor Q12 is connected to the emitter of the transistor Q11, and the clip level ref2 can be varied by the setting level of the variable resistor VR11 connected to the base of the transistor Q12. transistor Q
A signal (m) is output from the 11 emitters.

In the above embodiment, the low frequency compression circuit and the high frequency clipping circuit are connected after the chroma signal processing circuit, and the R
, G, and B signals, it is also possible to adjust the level of the luminance signal Y by connecting a low frequency compression circuit and a high frequency clipping circuit before the video chroma signal processing circuit.

[0018]

According to the present invention, the compression means compresses the low frequency components from a certain level to the peak level while leaving the high frequency components of the video signal, so that there is no loss of gradation as in the conventional example. do not have. Furthermore, the contrast can be increased and the outline of the image becomes clearer. therefore,
Even with a crystal panel with a narrow dynamic range, it is possible to obtain sharp images.

[Brief explanation of the drawing]

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

FIG. 2 is a diagram of output waveforms of each circuit in FIG. 1;

FIG. 3 shows output waveforms of each circuit for displaying a crosshatch pattern.

FIG. 4 is a circuit diagram showing details of a low frequency compression circuit.

FIG. 5 is a diagram showing output waveforms of each circuit of the low frequency compression circuit shown in FIG. 4;

FIG. 6 is a circuit diagram showing details of a high frequency clipping circuit.

FIG. 7 is a diagram showing output waveforms of each circuit in FIG. 6;

[Explanation of symbols]

1 Video chroma signal processing circuit 2-4 Low frequency compression circuit 5-7 Image quality correction circuit 8-10 Pedestal clamp circuit 11-13 High frequency clip circuit

Claims (1)

[Claims] 1. A white peak clipping circuit for clipping the white level peak of a video signal and supplying the clipped signal to a liquid crystal display panel, wherein the high frequency component of the video signal is left and only the level of the low frequency component is clipped. A white peak clip characterized by comprising a compression means for compressing a low frequency component from a certain level to a peak level, and a level clipping means for clipping the output of the compression means at a level capable of expressing gradation on a liquid crystal display panel. circuit.