JPH03110520A - Sharpening circuit for image contour of image displayed on liquid crystal display element - Google Patents

Abstract

PURPOSE:To emphasize the image contour by applying phase modulation upon a sampled signal with a signal related to a second derivative signal. CONSTITUTION:A video signal is supplied to cosine filters 3, 4, 24, and 25 to generate the second derivative signal and information for discriminating between the leading edge and trailing edge of the video signal is generated; and pulses which have a reference repetitive period predetermined so that a specific number of pulses appear on a time base in one sequential horizontal scanning period are used as driving pulses to be supplied to the driving circuit 13 for a liquid crystal display element 14. Then the period of the driving pulses is shortened right before and right after the part of the contour of the image and made long at the part of the contour of the image. Consequently, picture quality does not deteriorate and no large dynamic range is required.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a circuit for sharpening the contour of an image displayed on a liquid crystal display element.

(Prior Art) Various devices display images based on video signals (image signals) on liquid crystal display elements, or use liquid crystal display elements as light valves to provide light sources using liquid crystal display elements controlled by video signals. An image projector that modulates the intensity of the light emitted from a liquid crystal display element and forms an image on a screen via a projection optical system to project an image corresponding to a video signal onto the screen. It has come into practical use.

By the way, liquid crystal display elements use thermotropic liquid crystals to perform display operations in specific display modes.
For example, the orientation state of liquid crystal molecules in the nematic phase can be changed by an electric field.
I try to change it depending on.

When the liquid crystal display mode is, for example, the twisted nematic field effect mode, the operation of the liquid crystal display element is such that the light propagation mode of the nematic liquid crystal molecules arranged in twisted nematic orientation between a pair of substrates is the so-called waveguide mode. This is done in such a way that the conditions are satisfied.

(Problem to be Solved by the Invention) In a liquid crystal display element, due to the viscosity of the liquid crystal used therein, there is a delay in the change in the orientation of liquid crystal molecules in response to a change in the electric field. In other words, the transient response characteristics are poor, and the capacitance between the electrodes that apply the electric field to the liquid crystal is large, and the electrical characteristics including the liquid crystal drive circuit have a slow response to high-frequency signal components. An image displayed on a display element tends to have a blurred image outline.

Therefore, conventionally, images displayed on a liquid crystal display element are made by adding a single differential signal of the video signal to the video signal in order to clarify the outline of the image, or as shown in FIG. Image contours were emphasized using a cosine filter type contour enhancement circuit.

In FIG. 4, 1 is an input terminal for a video signal, 3.4 is a delay circuit, 25 is a subtracter, 24 is a coefficient circuit that doubles the amplitude of the input signal, and 26 is an adder. The supplied video signal Sa ((a) in FIG. 5) is input to the delay circuit 3.

It is supplied to the subtracter 25 as a subtraction signal Sa.

The signal Sb ((b) in FIG. 5) passed through the delay circuit 3 described above
)) is supplied to the adder 26 as the main line signal sb, and is also supplied to the coefficient circuit 24 and the delay circuit 4.

The signal Sc output from the delay circuit 4 ((
C)) is supplied to the subtraction circuit 25 as a subtraction signal, and the output signal of the coefficient circuit 24 (a signal twice as large as signal sb) is supplied to the subtraction circuit 25 as a minuend signal.

Therefore, the signal Sd shown in FIG. 5(d) is outputted from the subtracting circuit 25 and supplied to the adder 26, so that the signal Sd shown in FIG.
A signal Ss as shown in is output. τ in Figure 5
is the delay time of the delay circuits 3 and 4.

The signal Sa as shown in FIG. 5(e) is.

However, it emphasizes the outline of the image.

As is clear from the waveform diagram, undershoots and overshoots occur at the edges of one image, and unnatural white or black bands appear on the outline of the displayed image, resulting in poor image quality.

Furthermore, due to the presence of undershoot and overshoot at the edges of the image mentioned above, a large dynamic range is required for the related circuit, but the dynamic range of the liquid crystal drive circuit is too large due to the characteristics of the constituent elements. Since a dynamic range cannot be expected, there is a limit to the edge enhancement even if the edge enhancement circuit having the configuration shown in FIG. 4 is used.

(Means for Solving the Problems) The present invention provides means for obtaining a second-order differential signal by applying a video signal to a cosine filter, means for detecting a rising edge and a falling edge of a video signal, and a means for detecting a rising edge and a falling edge of a video signal. A period of negative slope in the output signal of the cosine filter corresponding to the edge portion and a period of positive slope of the output signal of the cosine filter corresponding to the falling edge of the video signal are defined as a first period.

The period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of negative slope of the output signal of the cosine filter corresponding to the falling edge of the video signal were defined as the second period. Sometimes 1M next 1
A period of a drive pulse that has a predetermined standard repetition period such that a predetermined number of pulses appear on the time axis during a horizontal scanning period and is prepared to be supplied to a drive circuit of a liquid crystal display element. In a state where the number of driving pulses is constant during the total period of the first period and the second period, the first period is shorter than the reference repetition period, and in the second period, a circuit for steepening the image contour of the image displayed on the liquid crystal display element, comprising means for changing the contour for a longer period than the reference repetition period; and a video signal is applied to the cosine filter. means for obtaining a second-order differential signal; means for detecting a rising edge and a falling edge of a video signal; A period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal is defined as a first period, and a period of positive slope of the output signal of the cosine filter corresponding to the rising edge of the video signal is defined as a first period. Sampling with a predetermined reference repetition period to be supplied to the analog-to-digital converter, where the falling edge and the corresponding period of negative slope in the output signal of the cosine filter are the second period. In a state where the number of sampling pulses during the sum of the first period and the second period is constant, the pulse period is set to be lower than the reference repetition period in the first period. means for causing the change to be short and longer than the reference repetition period in the second period; and means for supplying the video signal with a predetermined amount of time delay to the analog-to-digital converter.

means for storing digital image data output from the analog-to-digital converter in a digital memory;
A liquid crystal display comprising means for reading digital image data from the digital memory using read pulses having a constant repetition period, and then converting the data from digital to analog to obtain a video signal to be displayed on a liquid crystal display element. A circuit for steepening the image contour of an image displayed on an element is provided.

(Operation) A video signal is given to a cosine filter to generate a second-order differential signal. Furthermore, information that can distinguish between the rising edge and the falling edge of the video signal is generated. Further, a drive pulse to be supplied to the drive circuit of the liquid crystal display element is a pulse having a predetermined reference repetition period such that a predetermined number of pulses appear on the time axis during one sequential horizontal scanning period. Close and prepare.

A period of negative slope in the output signal of the cosine filter corresponding to the rising edge portion of the video signal and a period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal are defined as a first period, The period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of negative slope of the output signal of the cosine filter corresponding to the falling edge of the video signal were defined as the second period. Sometimes the first period and the second period mentioned above
In a state in which the number of drive pulses during the total period is constant, the first period is shorter than the standard repetition period, and the second period is shorter than the standard repetition period. 1. For example, the period of the driving pulse described above is changed by phase modulating the driving pulse with a signal related to the second-order differential signal of the video signal described above, and this is used to drive the liquid crystal display element. When supplied to the circuit, the image displayed by the pixels arranged spatially at regular intervals in the liquid crystal display element has a sharp outline.

The image outline steepening circuit of the image displayed on the liquid crystal display element described above phase-modulates the drive pulse supplied to the drive circuit of the liquid crystal display element by a signal related to the second-order differential signal of the video signal. The explanation was about the case where the drive pulses supplied to the drive circuit of the liquid crystal display element have a constant repetition period, and the phase of the video signal is changed by a signal related to the second-order differential signal. Even when modulated, the image displayed by the pixels that are spatially arranged at regular intervals in the liquid crystal display element is
The outline of the image is made steeper.

That is, the video signal is fed to a cosine filter to generate a second-order differential signal, and information that can distinguish between the rising edge and the falling edge of the video signal is generated and supplied to the analog-to-digital converter. The period of the sampling pulse having a predetermined reference repetition period is calculated by dividing the period of the negative slope in the output signal of the cosine filter corresponding to the rising edge part of the video signal and the cosine corresponding to the falling edge of the video signal. The period of positive slope in the output signal of the filter is defined as a first period, and the period of positive slope in the output signal of the cosine filter corresponds to the rising edge of the video signal, and the cosine corresponds to the falling edge of the video signal. A state in which the number of sampling pulses is constant during the sum of the first period and the second period, when the second period is the period of negative slope in the output signal of the filter. , the phase of the sampled signal is changed by a signal related to the second-order differential signal so that the first period is shorter than the reference repetition period, and the second period is changed longer than the reference repetition period. Modulate.

A video signal given a predetermined amount of time delay is supplied to an analog-to-digital converter that is supplied with a sampling signal with a variable period as described above, and is sampled and quantized to generate digital image data. The digital image data is read out from the digital memory using readout pulses having a constant repetition period, and then digital-to-analog converted to produce an image to be displayed on a liquid crystal display element. It is used as a signal.

(Example) Hereinafter, specific contents of a circuit for steepening the image contour of an image displayed on a liquid crystal display element of the present invention will be described in detail with reference to the accompanying drawings.

1 and 3 are block diagrams of embodiments of a circuit for steepening the image contour of an image displayed on a liquid crystal display element according to the present invention, and FIG. 2 is a waveform diagram for explaining the operation. be.

In FIGS. 1 and 3, 1 is a video signal input terminal;
2 is an input terminal for a horizontal scanning period signal 1, for example, a horizontal synchronization signal; 3 and 4 are delay circuits; 5 is a subtracter; 6 is a differential circuit; 7
, 16 is a sample and hold circuit, 8.17 is a low-pass filter, 9 is an adder, 10.18 is a voltage controlled oscillator, 1
1.19 is a frequency divider, 12.20 is a trapezoidal wave generator, 13 is a drive circuit for a liquid crystal display element, 14 is a liquid crystal display element, 15 is a drive pulse input terminal, 21 is an AD converter, 22 is a memory, 23 is a DA converter, 24 is a coefficient circuit, 25 is a subtracter, 27 is a changeover switch, 28 is a polarity inverter,
1 and 3, delay circuits 3, 4 . A component consisting of the coefficient circuit 24 and the subtracter 25 constitutes a cosine filter.

First, in the embodiment shown in the first diagram, the video signal Sa ((a) in FIG. 2) supplied to the input terminal 1 is input to the delay circuit 3, and is also input to the subtracter 5 and the subtracter 25 as a subtraction signal. Supplied.

The signal sb ((b) in FIG. 2) passed through the delay circuit 3 described above
)) is used as the main line signal sb by the drive circuit 13 of the liquid crystal display element.
It is also supplied to the coefficient circuit 24 and the delay circuit 4.

The signal Sc output from the delay circuit 4 ((
C)) is supplied to the subtraction circuit 25 as a subtraction signal, and the output signal of the coefficient circuit 24 (a signal twice as large as signal sb) is supplied to the subtraction circuit 25 as a minuend signal.

The subtracter 5 outputs the output signal Sc from the delay circuit 4.
A signal Ss as shown in FIG. 2(e) by subtracting the input video signal Sa from , that is, a signal Sa with the polarity inverted from the first-order differential signal of the single-line signal sb is output.
This signal Se becomes a negative polarity signal at the rising edge of the video signal.

Further, at the falling edge of the video signal, the signal becomes a positive polarity signal.

Further, the above-described subtraction circuit 25 outputs a second-order differential signal Sd as shown in FIG. and will be supplied. The output signal of the polarity inversion circuit 28 described above is transmitted to the changeover switch 27 described above.
is supplied to the fixed contact m.

The movable contact V of the changeover switch 27 is configured to be switched between a fixed contact m and a fixed contact p using the output signal Ss of the subtractor 5 as a switching control signal.

When the polarity of the signal Ss is positive, that is, at the falling edge of the video signal, the movable contact V is switched to the fixed contact P side, and when the polarity of the signal Ss is positive, that is, at the falling edge of the video signal. , at the rising edge of the video signal, the movable contact V is switched to the fixed contact m side.

Accordingly, the changeover switch 27 outputs the signal Sf shown in FIG. 2(f), that is, the second-order differential signal shown in FIG. 2(d) for the rising edge of the video signal. A signal Sf in which the polarity of Sd is inverted is output and supplied to the differentiating circuit 6.

Therefore, the above-mentioned differentiating circuit 6 outputs a signal Sg as shown in FIG.

The adder 9 described above consists of a sample hold circuit 7 to which a horizontal scanning period signal sh is supplied from an input terminal 2 → a low pass filter 8 → a voltage controlled oscillator 10 → a frequency divider 11 → a trapezoidal wave generation circuit 12 → a sample hold circuit Since it is provided in a well-known phase-locked loop (hereinafter sometimes referred to as PLL) consisting of a circuit consisting of 7→, the signal is supplied from the differentiating circuit 6 to the adder 9 as described above. The signal Sg as shown in FIG. 2(g) changes the oscillation frequency of the voltage controlled oscillator 10 in the PLL described above.

By the way, the signal Sg shown in FIG. 2(g), which is supplied via the adder 9 to the voltage-controlled oscillator 10 that operates as a one-frequency modulator, is the input video signal Sa as described above. As is well known, the signal Sf in which the polarity of the signal portion corresponding to the rising edge of the video signal in the second-order differentiated signal Sd is inverted is the signal differentiated by the differentiating circuit 6. When the input modulated wave of the frequency modulator is differentiated and then supplied to the frequency modulator, the first frequency modulator outputs a signal whose phase is modulated by the input modulated wave.
In the figure, from the voltage controlled oscillator 10 operating as a frequency modulator, a signal (pulse with a reference repetition period) generated from the voltage controlled oscillator 10 by the operation of one loop of the PLL is phase modulated by the signal Sf. A signal in the same state will be output.

Therefore, the drive pulse supplied from the voltage-controlled oscillator 10 in the PLL to the drive circuit 13 of the liquid crystal display element is the signal (of the reference repetition period) generated from the voltage-controlled oscillator 10 by the operation of one loop of the PLL. A pulse) is a pulse having a predetermined standard repetition period that causes a predetermined number of pulses to occur on the time axis during one sequential horizontal scanning period, and is a rising edge portion of a video signal. A period of negative slope in the output signal of the corresponding cosine filter, a period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal, and a period of positive slope in the output signal of the cosine filter corresponding to
The period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of negative slope of the output signal of the cosine filter corresponding to the falling edge of the video signal are defined as a second period. When the period is set to , in a state where the individuality of the drive pulse is constant during the total period of the first period and the second period, the reference repetition period in the first period is In the second period described above, the pulse changes for a longer period than the reference repetition period.

The drive circuit 13 of the liquid crystal display element receives the main line signal sb output from the delay circuit 3 and the voltage controlled oscillator 10 in the PLL.
(h) in FIG. 2 is a diagram illustrating the relationship on the time axis between the main line signal Sb and the drive pulses, and (h) in FIG. The small black circles in ) indicate the time positions of sequential drive pulses.

In (i) of FIG. 2, the period of the drive pulse output from the voltage controlled oscillator 10 in the PLL is the period of the negative slope of the output signal of the cosine filter corresponding to the rising edge of the video signal, and the period of the negative slope of the output signal of the video signal. During the first period between the falling edge of and the positive slope period of the corresponding cosine filter output signal, the repetition period is made shorter than the reference repetition period, and also corresponds to the rising edge portion of the video signal. During the second period of the positive slope period in the output signal of the cosine filter that corresponds to the falling edge of the video signal and the period of negative slope in the output signal of the cosine filter, the repetition period is lower than the reference repetition period. It is shown that this has been done for a long time.

That is, the period of the drive pulse is shortened immediately before and after the contour portion of the image, and the period of the drive pulse is lengthened at the contour portion of the image.

By the way, the pixels in the liquid crystal display element are shown in (i) in Figure 2.
They are spatially arranged at regular intervals on a straight line, as shown in the horizontal spacing between the small black circles (the spacing between each perpendicular line when a perpendicular line is drawn at the position of each black circle mark in the figure). .

The successive drive pulses indicated by the small black circles in (h) of FIG. 2 and the successive pixels of the liquid crystal display element spatially arranged in a straight line at regular intervals are respectively There is a one-to-one correspondence (second
The arrow shown between (h) in the figure and (i) in FIG. As shown in FIG. 2(i), the rising and falling edges of the signal are steeper than the rising and falling edges of the original video signal.

Next, the embodiment shown in FIG. 3 will be described.

In the embodiment shown in FIG. 3, the video signal Sa ((a) in FIG. 2) supplied to the input terminal 1 is input to the delay circuit 3, and is also supplied to the subtracter 5 and the subtracter 25 as a subtraction signal. Ru.

The signal sb ((b) in FIG. 2) passed through the delay circuit 3 described above
)) is given to the AD converter 21 as the main line signal sb, and is also supplied to the coefficient circuit 24 and the delay circuit 4.

The signal Sc output from the delay circuit 4 ((
C)) is supplied to the subtraction circuit 25 as a subtraction signal, and the output signal of the coefficient circuit 24 (a signal twice as large as signal sb) is supplied to the subtraction circuit 25 as a minuend signal.

The subtracter 5 outputs the output signal Sc from the delay circuit 4.
A signal Se as shown in FIG. 2(e) by subtracting the input video signal Sa from the main line signal sb, that is, a signal Ss with the polarity inverted from the first differential signal of the main line signal sb is output.
This signal Se becomes a signal of negative polarity at the rising edge of the video signal, and becomes a signal of positive polarity at the falling edge of the video signal.

Further, the above-mentioned subtraction circuit 25 outputs a second-order differential signal Sd as shown in FIG. and will be supplied. The output signal of the polarity reversing circuit 28 described above is supplied to the fixed contact m of the changeover switch 27 described above.

The movable contact V of the changeover switch 27 is configured to be switched between a fixed contact m and a fixed contact p using the output signal Ss of the subtractor 5 as a switching control signal, and when the polarity of the signal Se is positive, that is, , at the falling edge of the video signal, the movable contact V is switched to the fixed contact P side, and when the polarity of the signal Ss is positive,
At the rising edge of the video signal, the movable contact V
is switched to the fixed contact m side.

As a result, the changeover switch 27 outputs the signal Sf shown in FIG. The signal Sf with its polarity inverted is output and supplied to the differentiating circuit 6.

Therefore, a signal Sg as shown in FIG.
is provided in a well-known PLL consisting of a circuit consisting of a sample-hold circuit 7 to which is supplied → low-pass filter 8 → voltage-controlled oscillator 10 → frequency divider 11 → trapezoidal wave generating circuit 12 → sample-hold circuit 7 → From the voltage controlled oscillation 1910, the signal (sampled signal of the reference repetition period) generated from the voltage controlled oscillator 10 by the operation of one loop of the PLL is phase modulated by the signal Sf as described above. A signal is output and supplied to the AD converter 21 as a sampling signal (sampling pulse).

The sampling signal supplied from the voltage controlled oscillator 10 to the AD converter 21 in the PLL described above, that is, the sampling pulse (of the reference repetition period) generated from the voltage controlled oscillator 10 by the operation of one loop of the PLL. (sampling pulse) is a sampling pulse that has a predetermined standard repetition period that causes a predetermined number of pulses to occur on the time axis during one sequential horizontal scanning period. a period of negative slope in the output signal of the cosine filter corresponding to the part; a period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal; and a period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal; sampling during the sum of a period of positive slope in the output signal of the cosine filter corresponding to the part and a second period of negative slope in the output signal of the cosine filter corresponding to the falling edge of the video signal; In a state where the number of pulses is constant, the repetition period of the sampling pulse during the first period described above is shorter than the reference repetition period, and the repetition period of the sampling pulse during the second period described above is shorter than the reference repetition period. It is a pulse whose state changes for a longer time than the repetition period of .

Then, the AD conversion operation performed on the main line signal sb output from the delay circuit 3 in the AD converter 21 is P
Since the sampling pulse outputted from the voltage controlled oscillator 10 in the LL is used, the sampling of the main line signal sb in the AD converter 21 is performed during the total period of the first period and the second period. In a state in which the number of sampling pulses is kept constant during the above-mentioned first period, sampling pulses whose repetition period is shorter than the repetition period of the reference sampling pulse are used during the above-mentioned first period, and during the above-mentioned second period In some cases, sampling pulses are used whose repetition period is longer than that of the standard sampling pulse.

Digital image data output from the AD converter 21 described above is stored in the memory 22. The digital image data stored in the memory 22 described above is generated by a sample hold circuit 16 to which a horizontal scanning period signal sh is supplied from the input terminal 2 → a low pass filter 17 → a voltage controlled oscillator 18 → a frequency divider 19 → a trapezoidal waveform. A well-known P circuit consisting of a circuit consisting of a generation circuit 20→sample hold circuit 16→
Pulses with a constant repetition period outputted from the voltage controlled oscillator 18 provided in the LL are read out as readout pulses, but the video signal read out from the memory 22 is read out immediately before the outline of the image. Immediately after, the time axis is expanded, and at the outline of one image, the time axis is compressed.

The digital image data read out from the memory 22 by the readout pulse with a constant repetition period is
The A converter 23 converts the video signal into an analog signal and supplies it to the liquid crystal display element drive circuit 13. A drive pulse having a predetermined constant repetition period is supplied from a terminal 15 to a drive circuit 13 for the liquid crystal display element.

As a result, even in the case of the embodiment shown in the third figure, the first
As in the illustrated embodiment, the outline of the image displayed by successive pixels in the liquid crystal display element is shown in FIG.
), the rises and falls become steeper than the rises and falls of the original video signal.

(Effects of the Invention) As is clear from the above detailed explanation, the circuit for sharpening the image contour of the image displayed on the liquid crystal display element of the present invention provides the video signal to the cosine filter and performs the quadratic differentiation. A predetermined method that generates a signal and generates information that can distinguish between a rising edge and a falling edge of a video signal so that a predetermined number of pulses appear on the time axis during one sequential horizontal scanning period. A pulse having a standard repetition period is prepared as a drive pulse to be supplied to a drive circuit of a liquid crystal display element, and a period of negative slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the video are determined. a falling edge of the signal and a corresponding period of positive slope in the output signal of the cosine filter as a first period;
The period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of negative slope of the output signal of the cosine filter corresponding to the falling edge of the video signal were defined as the second period. Sometimes, in a state where the number of driving pulses during the sum of the first period and the second period is constant, the first period is shorter than the reference repetition period, and the number of driving pulses is shorter than the reference repetition period. For example, the above-mentioned driving pulse is phase-modulated by a signal related to the second-order differential signal of the above-mentioned video signal so as to change the period longer than the reference repetition period in the second period. By changing the
By supplying it to the drive circuit of the liquid crystal display element,
An image displayed by pixels arranged at regular intervals in a liquid crystal display element can be made into a state in which the outline of one image is steepened, or a video signal can be fed to a cosine filter. Sampling with a predetermined reference repetition period to generate a second-order differential signal and to generate information capable of distinguishing the rising edge and the falling edge of the video signal, respectively, and to be supplied to the analog-to-digital converter. The period of the pulse is determined by dividing the period of negative slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal. 1, and a period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal, and a period of negative slope in the output signal of the cosine filter corresponding to the falling edge of the video signal. 2, when the number of sampling pulses during the sum of the first period and the second period is constant, the first period is longer than the reference repetition period. The sampling signal is phase-modulated by a signal related to the second-order differential signal so that the second period is changed longer than the reference repetition period, and the video image is given a predetermined time delay. The signal is supplied to an analog-to-digital converter supplied with a sampling signal having a variable period as described above, and is sampled and quantized to generate digital image data, which is stored in a digital memory. ,
The digital image data is read out from the digital memory using readout pulses having a constant repetition period, and then converted into a digital-to-analog image signal to be displayed on the liquid crystal display element. An image displayed by pixels arranged at regular intervals is defined as a state σ in which the outline of the image is steepened.
-,/''14 However, in the present invention, in order to prevent undershoots and overshoots from occurring in the outline of the image, unnatural outlines of the displayed image, which had been a problem when applying conventional means, are avoided. There are no white bands or black bands that degrade the image quality, and there are no undershoots or overshoots at the edges of the image, so a large dynamic range is required for the associated circuitry. According to the present invention, the above-mentioned problems can be satisfactorily solved.

[Brief explanation of drawings]

1 and 3 are block diagrams of embodiments of a circuit for steepening the image contour of an image displayed on a liquid crystal display element of the present invention, respectively, FIGS. 2yA and 5 are waveform diagrams for explaining the operation, and FIGS. FIG. 4 is a circuit diagram of a conventional example. DESCRIPTION OF SYMBOLS 1... Input terminal for a video signal, 2... Input terminal for a horizontal scanning period signal, 3, 4... Delay circuit, 5... Subtractor. 6... Differential circuit, 7, 16... Sample hold circuit. 8.17...Low pass filter, 9...Adder, 1
0°18... Voltage controlled oscillator, 11.19... Frequency divider, 12.20... Trapezoidal wave generator, 13... Liquid crystal display element drive circuit, 14... Liquid crystal display element, 15...
- Drive pulse input terminal, 21...AD converter, 22
...Memory, 23...DA converter, 24...Coefficient circuit, 25...Subtractor, 26...Adder, 27...
・Selector switch, 28...Polarity reversal switch, stone 3

Claims (1)

[Claims] 1. Means for obtaining a second-order differential signal by applying a video signal to a cosine filter, means for detecting rising edges and falling edges of the video signal, and means corresponding to the rising edge portion of the video signal. A period of negative slope in the output signal of the cosine filter corresponding to the falling edge of the video signal and a period of positive slope in the output signal of the cosine filter corresponding to the falling edge of the video signal are defined as a first period,
The period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of negative slope of the output signal of the cosine filter corresponding to the falling edge of the video signal were defined as the second period. sometimes sequential 1
A period of a drive pulse that has a predetermined standard repetition period such that a predetermined number of pulses appear on the time axis during a horizontal scanning period and is prepared to be supplied to a drive circuit of a liquid crystal display element. In a state where the number of driving pulses is constant during the total period of the first period and the second period, the first period is shorter than the reference repetition period, and In the second period, the image contour steepening circuit 2 of the image displayed on the liquid crystal display element is provided with means for making the change longer than the reference repetition period, and the video signal is applied to the cosine filter. means for obtaining a second-order differential signal; means for detecting a rising edge and a falling edge of a video signal; a period of positive slope in the output signal of the cosine filter corresponding to the falling edge is a first period;
The period of positive slope in the output signal of the cosine filter corresponding to the rising edge of the video signal and the period of negative slope of the output signal of the cosine filter corresponding to the falling edge of the video signal were defined as the second period. Sometimes, the period of the sampling pulse having a predetermined reference repetition period to be supplied to the analog-to-digital converter is sampled during the sum of the first period and the second period. In a state where the number of pulses is constant, the first period described above is shorter than the reference repetition period,
means for making the change longer than the reference repetition period in the second period; means for supplying the video signal with a predetermined amount of time delay to the analog-to-digital converter; A means for storing digital image data output from a digital converter in a digital memory, and a means for reading out the digital image data from the digital memory using a read pulse having a constant repetition period, converting the digital image data from digital to analog, and displaying the digital image data on a liquid crystal display. A circuit for sharpening the image contour of an image displayed on a liquid crystal display element, comprising means for generating a video signal to be displayed on the element.