JPH0769528B2 - Image sharpening circuit for the image displayed on the liquid crystal display device - Google Patents

Description

The present invention relates to a circuit for sharpening an image contour of an image displayed on a liquid crystal display device.

(Prior Art) Various devices adapted to display an image based on a video signal (image signal) on a liquid crystal display element, or a liquid crystal display element used as a light valve and a light source driven by the liquid crystal display element controlled by the video signal. An image projector or the like that intensity-modulates the light of, and forms the light emitted from the liquid crystal display element on the screen through the projection optical system and projects the image corresponding to the video signal on the screen. It came into practical use.

By the way, the liquid crystal display element uses a thermotropic liquid crystal so that a display operation in a specific display mode is performed.
For example, the alignment state of liquid crystal molecules in the nematic phase is changed by an electric field.

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 in the twisted nematic alignment between the pair of substrates is the so-called waveguide mode. It is performed so as to satisfy the conditions.

(Problems to be Solved by the Invention) In a liquid crystal display element, the rise time and the fall time are delayed because the change in the orientation of the liquid crystal molecules with respect to the change in the electric field is delayed due to the viscosity of the liquid crystal used therein. Is large, that is, the transient response characteristics are poor, the capacitance between the electrodes that apply an electric field to the liquid crystal is large, and the electrical characteristics including the liquid crystal drive circuit also have a slow response to high-frequency signal components. The image displayed on the display element is likely to have a blurred image contour.

Therefore, in order to clarify the image contour of the image conventionally displayed on the liquid crystal display element, for example, the primary differential signal of the video signal is added to the video signal, or as illustrated in FIG. The image contour is emphasized by using such a cosine filter type contour enhancement circuit.

In FIG. 4, 1 is an input terminal of a video signal, 3 and 4 are delay circuits, 25 is a subtractor, 24 is a coefficient circuit for doubling 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 and is also supplied to the subtractor 25 as the subtraction signal Sa.

The signal Sb that has passed through the delay circuit 3 ((b) in FIG. 5) is given 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) in FIG. 5) is supplied to the subtraction circuit 25 as a subtraction signal, and the output signal of the coefficient circuit 24 {double the signal Sb in magnitude. Signal is supplied to the subtraction circuit 25 as a minuend signal.

Then, the subtraction circuit 25 outputs the signal Sd as shown in FIG. 5 (d) and supplies it to the adder 26, so that the adder 26 outputs the signal Sd shown in FIG. 5 (e). A signal Se as shown in is output. Τ in FIG. 5 is the delay time of the delay circuits 3 and 4.

The signal Se as shown in (e) of FIG. 5 emphasizes the contour of the image, but as is clear from the waveform diagram, undershoot and overshoot occur at the edge of the image. The image quality is deteriorated because an unnatural white band or a black band appears in the outline of the displayed image due to the occurrence of a shoot.

Further, due to the existence of the undershoot and the overshoot at the edge portion of the image described above, a related circuit requires a large dynamic range, but the dynamic range of the liquid crystal drive circuit is too large due to the characteristics of the constituent elements. Since the dynamic range cannot be expected, there is a limit to the contour enhancement even if the contour enhancement circuit having the configuration shown in FIG. 4 is used.

(Means for Solving the Problem) According to the present invention, a means for obtaining a first-order differential signal of a video signal by first-order differentiation of the video signal, and a predetermined number of pulses appear on the time axis during one successive horizontal scanning period. The period of the drive pulse that is prepared to be supplied to the drive circuit of the liquid crystal display element having a predetermined reference repeating period such as
A period corresponding to the positive gradient period in the first-order differential signal of the video signal is referred to as a first period, and a period corresponding to the negative gradient period in the first-order differential signal of the video signal is referred to as a second period. In the state where the number of drive pulses is constant during the total period of the first period and the second period described above, corresponding to the gradient of the primary differential signal of the video signal. A display is provided on the liquid crystal display element, which has means for changing the period shorter than the reference repeating period in the first period and longer than the reference repeating period in the second period. An image sharpening circuit for an image, means for first-order differentiating a video signal to obtain a first-order differential signal of the video signal, and a predetermined reference repetition period to be supplied to the analog-digital converter. Specimen The period of the pulse corresponds to a period corresponding to the positive gradient period in the first-order differential signal of the video signal as a first period, and corresponds to the negative gradient period in the first-order differential signal of the video signal. When the period to be set is the second period, the first-order differential signal of the video signal in a state where the number of sampling pulses is constant during the total period of the first period and the second period described above. Corresponding to the slope of the reference period in the first period described above and longer than the reference period in the second period described above, and a predetermined amount of time delay. Means for supplying the given video signal to the analog / digital converter and digital image data output from the analog / digital converter.
Means for storing in a memory, and means for reading out digital image data from the digital memory with a read pulse having a constant repetition period, and then performing digital / analog conversion to obtain a video signal to be displayed on a liquid crystal display element. There is provided a circuit for sharpening an image contour of an image displayed on a liquid crystal display device comprising:

(Operation) The video signal is first-order differentiated to generate a first-order differential signal of the video signal, and a predetermined number of pulses appear on the time axis during successive first-order horizontal scanning periods. A pulse having a reference repetition period is prepared as a drive pulse to be supplied to the drive circuit of the liquid crystal display element.

A period corresponding to the positive gradient period in the first-order differential signal of the video signal is referred to as a first period, and a period corresponding to the negative gradient period in the first-order differential signal of the video signal is referred to as a second period. In the state where the number of drive pulses is constant during the total period of the first period and the second period described above, corresponding to the gradient of the primary differential signal of the video signal. For example, the drive pulse described above may be changed to be shorter than the reference repetition cycle in the first period and longer than the reference repetition cycle in the second period. When the period of the above-mentioned drive pulse is changed by phase-modulating with a differential signal and is supplied to the drive circuit of the liquid crystal display element, it is possible to obtain a state in which the liquid crystal display element is spatially arranged at a constant interval. Image displayed by the element is made to those conditions in which the contour of the image is steep.

In the case of the circuit for sharpening the image contour of the image displayed on the liquid crystal display element, the drive pulse supplied to the drive circuit of the liquid crystal display element is phase-modulated by the primary differential signal of the video signal. Was explained,
The drive pulse supplied to the drive circuit of the liquid crystal display element has a constant repetition period, and the video signal is set to 1
Even if the phase is modulated by the secondary differential signal, the image displayed by the pixels in the state where they are spatially arranged at constant intervals in the liquid crystal display element is in a state in which the contour of the image is sharpened.

That is, the cycle of the sampling pulse having a predetermined reference repetition cycle to be supplied to the analog-digital converter is generated by first-order differentiating the video signal to generate a first-order differential signal of the video signal. The period corresponding to the positive gradient period in the first-order differential signal of the video signal is the first period, and the period corresponding to the negative gradient period in the first-order differential signal of the video signal is the second period. In the state where the number of sampling pulses is constant during the total period of the first period and the second period described above, the period corresponds to the gradient of the first derivative signal of the video signal. In the first period described above, it is shorter than the reference repetition period,
In the above-mentioned second period, the sampling signal is phase-modulated by the primary differential signal so as to be changed longer than the reference repetition period.

A video signal given a predetermined amount of time delay is supplied to an analog-to-digital converter to which a sampling signal whose period is variable as described above is supplied, sampling and quantizing to generate digital image data. Image stored in a digital memory, read out digital image data from the digital memory with a read pulse having a constant repetition period, and then digital-analog converted to display an image on a liquid crystal display device. It is a signal.

(Embodiment) Hereinafter, specific contents of the image sharpening circuit of the image displayed on the liquid crystal display device of the present invention will be described in detail with reference to the accompanying drawings.

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

In FIGS. 1 and 3, 1 is an input terminal for a video signal,
2 is a signal of a horizontal scanning period, for example, an input terminal for a horizontal synchronizing signal, 3 and 4 are delay circuits, 5 is a subtractor, 6 is a differentiation circuit, and 7 and 16
Is a sample and hold circuit, 8 and 17 are low-pass filters, and 9
Is an adder, 10 and 18 are voltage controlled oscillators, 11 and 19 are frequency dividers, 1
2, 20 are trapezoidal wave generators, 13 are drive circuits for liquid crystal display elements, 14
Is a liquid crystal display element, 15 is a drive pulse input terminal, 21 is an AD converter, 22 is a memory, and 23 is a DA converter.

First, the video signal Sa ((a) in FIG. 2) supplied to the input terminal 1 in the embodiment shown in FIG. 1 is input to the delay circuit 3 and is also supplied to the subtractor 5 as the minuend signal Sa.

The signal Sb that has passed through the delay circuit 3 ((b) in FIG. 2) is used as the main line signal Sb to drive the drive circuit 13 of the liquid crystal display element.
Is also supplied to the delay circuit 4.

The signal Sc {(c) in FIG. 2} output from the delay circuit 4 is supplied to the subtraction circuit 5 as a subtraction signal. Then, the subtraction circuit 5 outputs the primary differential signal Sd as shown in FIG. 2 (d), which is supplied to the differential circuit 6. As a result, the signal Se as shown in (e) of FIG. 2 is output from the differentiating circuit 6 and supplied to the adder 9.

The adder 9 is a sample hold circuit 7 to which a signal Sh of the horizontal scanning cycle is supplied from the input terminal 2 → low pass filter 8 → voltage controlled oscillator 10 → frequency divider 11 → trapezoidal wave generation circuit 12 → sample hold circuit The surrounding phase-locked loop (hereinafter, PLL
The signal Se as shown in (e) of FIG. 2 supplied from the differentiating circuit 6 to the adder 9 as described above is PL
The oscillation frequency of the voltage controlled oscillator 10 in L is changed.

By the way, a voltage controlled oscillator that operates as a frequency modulator
In contrast to 10, the signal Se shown in (e) of FIG. 2 supplied through the adder 9 is a signal obtained by first-order differentiating the input video signal Sa, that is, the video One of the signals
As is well known, when the input modulation wave of the frequency modulator is differentiated and then supplied to the frequency modulator, it is the state that the phase modulation is performed by the input modulation wave from the frequency modulator. Signal is output from the voltage controlled oscillator 10 operating as the frequency modulator in FIG. 1 described above, the signal generated from the voltage controlled oscillator 10 by the operation of one loop of the PLL (reference signal A signal in a state in which a pulse having a repeating period) is phase-modulated by the primary differential signal Sd of the video signal Sa is 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 PL
The signal generated from the voltage controlled oscillator 10 by the operation of one loop of L (pulses of the reference repetition period) is preliminarily generated so as to generate a predetermined number of pulses on the time axis during one successive horizontal scanning period. A pulse having a predetermined reference repetition period is a primary differential signal of the video signal Sa described above.
When the period corresponding to the positive gradient period in Sd is the first period and the period corresponding to the negative gradient period is the second period, the above-mentioned first period and second period In the state where the number of drive pulses is constant during the total period, the period is shorter than the reference repetition period in the first period described above corresponding to the gradient of the primary differential signal Sd of the video signal Sa, and In the second period, the pulse is changed in a state longer than the reference repetition period.

The drive circuit 13 of the liquid crystal display element is supplied with the main line signal Sb output from the delay circuit 3 and the drive pulse output from the voltage controlled oscillator 10 in the PLL. It is a figure which illustrated the relationship on the time axis of the above-mentioned main line signal Sb and a drive pulse, and the small black circle mark in (f) of Drawing 2 shows the time position of a sequential drive pulse.

In FIG. 2 (f), the period of the drive pulse output from the voltage controlled oscillator 10 in the PLL is the first period corresponding to the positive slope period in the first-order differential signal Sd of the input video signal Sa. In some cases, the repetition period is shorter than the reference repetition period, and during the second period corresponding to the period of the negative slope in the first-order differential signal Sd of the input video signal Sa, the repetition period is shorter than the reference repetition period. It is shown that the repetition period is long.

That is, the black level in the contour portion of the image is densely sampled in the gray level, and the gray level in the contour portion of the image is sparsely sampled in the white level.

By the way, the pixel in the liquid crystal display element is (g) in FIG.
Are arranged at regular intervals on one straight line, such as the space between the small black circles in the horizontal direction (the space between the vertical lines when the vertical lines are drawn at the positions of the black circles in the figure). .

The sequential drive pulses indicated by the small black circles in (f) of FIG. 2 and the sequential pixels in the liquid crystal display element spatially arranged at regular intervals are One-to-one correspondence (second
The arrow shown between (f) in FIG. 2 and (g) in FIG. 2 indicates the above-mentioned correspondence), and the outline of the image displayed by the successive pixels in the liquid crystal display element. As shown in (g) of FIG. 2, the rising and falling edges are steeper than the rising and falling edges of the original video signal.

That is, the portion from the black level to the gray level in the video signal is expanded on the time axis, the slope of the edge of the image corresponding to that portion becomes gentle, and the portion from the gray level to the white level in the video signal is expanded on the time axis. When compressed, the edge of the image corresponding to that portion becomes steep, and the outline of the image displayed by the successive pixels in the liquid crystal display element is as shown in (g) of FIG. The rise and fall of the original video signal becomes sharper than that 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 supplied to the input terminal 1 ((a) in FIG. 2) is supplied to the delay circuit 3 and is also supplied to the subtractor 5 as the minuend signal Sa.

The signal Sb that has passed through the delay circuit 3 ((b) in FIG. 2) is supplied to the AD converter 21 as the main line signal Sb and also to the delay circuit 4. The signal Sc output from the delay circuit 4 ((c) in FIG. 2) is supplied to the subtraction circuit 5 as a subtraction signal, and the subtraction circuit 5 outputs a linear signal as shown in (d) in FIG. The differential signal Sd is output.

A sample hold circuit in which a signal Se as shown in FIG. 2 (e) output from the differentiating circuit 6 is supplied to the adder 9 and thereby a signal Sh of the horizontal scanning period is supplied from the input terminal 2. 7 → low-pass filter 8 → voltage-controlled oscillator 10 → frequency divider 11 → trapezoidal wave generation circuit 12 → sample hold circuit 7 → from the voltage-controlled oscillator 10 provided in the well-known PLL consisting of a circuit The signal generated from the voltage controlled oscillator 10 by the operation of one loop of the PLL (the sampling signal of the reference repetition period) is 1 of the video signal Sa.
A signal in a state of being phase-modulated by the secondary differential signal Sd is output, which is supplied to the AD converter 21 as a sampling signal (sampling pulse).

The sampling signal supplied from the voltage-controlled oscillator 10 in the PLL to the AD converter 21 is a sampling pulse generated from the voltage-controlled oscillator 10 by the operation of a loop of the PLL (a sampling pulse having a reference repetition period). Is a first-order derivative of the video signal Sa described above, in which a sampling pulse having a predetermined reference repetition period that causes a predetermined number of pulses on a time axis during one successive horizontal scanning period. When the period corresponding to the positive slope period in the signal Sd is the first period and the period corresponding to the negative slope period is the second period, the first period and the second period described above are In the state where the number of sampling pulses is constant during the total period of, the period is shorter than the reference repetition period in the first period described above in correspondence with the gradient of the first-order differential signal Sd of the video signal Sa. In the above-described second period, the sampling pulse is in a state of changing longer than the reference repetition period.

Therefore, the AD conversion operation performed on the main line signal Sb output from the delay circuit 3 in the AD converter 21 is performed by the sampling pulse output from the voltage controlled oscillator 10 in the PLL. The sampling of the main line signal Sb is performed by sampling with a repetition period shorter than the repetition period of the reference sampling pulse during the first period corresponding to the period of the positive gradient in the first derivative signal Sd of the input video signal Sa. The input video signal also made in pulses
During the second period corresponding to the period of the negative gradient in the first-order differential signal Sd of Sa, the sampling pulse has a repetition period longer than that of the reference sampling pulse.

That is, the black level in the contour portion of the image is densely sampled in the gray level, and the gray level in the contour portion of the image is sparsely sampled in the white level.

The digital image data output from the AD converter 21 is stored in the memory 22. The digital image data stored in the memory 22 is the sample hold circuit 16 to which the signal Sh of the horizontal scanning period is supplied from the input terminal 2 → low pass filter 17 → voltage controlled oscillator 18 → frequency divider 19 → trapezoidal wave generation. A pulse having a constant repetition period output from the voltage controlled oscillator 18 provided in a well-known PLL including a circuit including a circuit 20 → a sample hold circuit 16 → is read out as a read pulse.

Therefore, the video signal read from the memory 22 is in a state where the black level to gray level portion is expanded in the time axis, and the gray level to white level portion in the video signal is compressed in the time axis. It has been put into a closed state.

Then, the digital image data read from the memory 22 by the read pulse having a constant repetition period is converted into a video signal in the form of an analog signal by the DA converter 23 and supplied to the drive circuit 13 of the liquid crystal display element. The drive circuit 13 of the liquid crystal display element is supplied with a drive pulse having a predetermined constant repetition period from the terminal 15.

Thereby, in the case of the embodiment shown in FIG.
As in the case of the illustrated embodiment, the contour of the image displayed by the successive pixels in the liquid crystal display element is, as shown in FIG. 2 (g), compared with the rising and falling edges of the original video signal. A sharp rise and fall.

That is, the portion from the black level to the gray level in the video signal is expanded on the time axis, the inclination of the contour of the image corresponding to that portion becomes gentle, and the portion from the gray level to the white level in the video signal is compressed on the time axis. As a result, the inclination of the edge portion of the image corresponding to that portion becomes steep, and the contour of the image displayed by the successive pixels in the liquid crystal display element is as shown in (g) of FIG. The rising and falling edges are sharper than the rising and falling edges of the original video signal.

(Effects of the Invention) As is clear from the above description, the circuit for sharpening the image contour of the image displayed on the liquid crystal display element of the present invention is the drive pulse supplied to the drive circuit of the liquid crystal display element. As a first period corresponding to a positive gradient period in the first-order differential signal of the video signal obtained by first-order differentiating the video signal, and a negative gradient period in the first-order differential signal of the video signal described above. In a state where the number of drive pulses is constant during the total period of the corresponding second period, the drive pulses are repeated in the first period described above corresponding to the gradient of the primary differential signal of the video signal. The cycle is shorter than the reference repetition cycle, and the repetition cycle of the drive pulse is changed to be longer than the reference repetition cycle in the second period, or the predetermined reference repetition supplied to the AD converter is repeated. The period of the sampling pulse having a period is the total period of the first period corresponding to the period of the positive gradient in the first-order differential signal of the video signal and the second period corresponding to the period of the negative gradient. In the state where the number of sampling pulses in is constant, the period is shorter than the reference repetition period in the first period described above corresponding to the gradient of the first-order differential signal of the video signal, and in the second period described above in the second period. The output from the AD converter is read at a constant repetition period by changing the value longer than the reference repetition period so that the liquid crystal display elements are spatially arranged at regular intervals. The contour of the image displayed by the successive pixels in the liquid crystal display element in the state where the black level to the gray level in the video signal is time-axis expanded, The slope of the edge of the image corresponding to the part becomes gentle, and the part from the gray level to the white level in the video signal is compressed on the time axis, and the slope of the edge of the image corresponding to that part is steep. And the second
As shown in (g) of the figure, the rising and falling edges of the video signal are steeper than the rising and falling edges of the original video signal. In order to prevent shoots, unnatural white or black bands appear on the outline of the displayed image, which has been a problem in applying conventional means, and the image quality is not deteriorated. Since undershoot and overshoot do not occur in a part, a circuit having a large dynamic range is not required as a related circuit, and the present invention can satisfactorily solve the above-mentioned problems.

[Brief description of drawings]

1 and 3 are block diagrams of an embodiment of an image sharpening circuit for an image displayed on a liquid crystal display device according to the present invention, and FIGS. 2 and 5 are waveform diagrams for explaining the operation. Fourth
The figure is a circuit diagram of a conventional example. 1 ... Video signal input terminal, 2 ... Horizontal scanning cycle signal input terminal, 3, 4 ... Delay circuit, 5 ... Subtractor, 6 ...
Differentiator circuit, 7,16 …… Sample and hold circuit, 8,17 …… Low pass filter, 9 …… Adder, 10,18 …… Voltage controlled oscillator, 11,19 …… Frequency divider, 12,20 …… Trapezoid 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,

Claims (2)

[Claims] 1. A means for first-order differentiating a video signal to obtain a first-order differential signal of the video signal, and a predetermined number of pulses appearing on a time axis during one successive horizontal scanning period. The period of the drive pulse having a reference repetition period and prepared to be supplied to the drive circuit of the liquid crystal display element is set to a period corresponding to the period of the positive gradient in the first-order differential signal of the video signal described above. When the first period is set and the period corresponding to the negative gradient period in the first-order differential signal of the video signal is set as the second period, the first period and the second period are In a state in which the number of drive pulses is constant during the total period, the period is shorter than the reference repetition period in the first period described above corresponding to the gradient of the primary differential signal of the video signal, and the second period described above is used. Repeat the standard in the period of Steepening circuit of the image contour of the image displayed on the liquid crystal display device comprising a means for so varying longer than the period 2. Means for obtaining a first-order differential signal of a video signal by first-order differentiating the video signal, and a period of a sampling pulse having a predetermined reference repetition period to be supplied to an analog-digital converter. Is a period corresponding to the positive gradient period of the first-order differential signal of the video signal as a first period, and a period corresponding to the negative gradient period of the first-order differential signal of the video signal is When the second period is set, the gradient of the first-order differential signal of the video signal is set in a state where the number of sampling pulses is constant during the total period of the first period and the second period described above. Correspondingly, in the first period described above, it is shorter than the reference repetition period,
In the above-mentioned second period, means for changing the repetition period longer than the reference repetition period, means for supplying a video signal given a predetermined time delay to the analog-digital converter, and the analog Means for storing digital image data output from the digital converter in a digital memory, and liquid crystal display by digital-analog conversion after reading the digital image data from the digital memory with a read pulse having a constant repetition period. A circuit for sharpening an image contour of an image displayed on a liquid crystal display device, comprising a means for setting a video signal to be displayed by the device