JPH06138842A - Common electrode voltage regulator - Google Patents

Abstract

PURPOSE:To regulate the voltage of the common electrode of liquid crystal panels constituting a matrix type liquid crystal display device automatically. CONSTITUTION:The flickers of the videos by a liquid crystal projector 1 are detected by a photodiode 3 provided on a screen 2. After these flickers are converted to quantitative electric signals, these electric signals are outputted to a microcomputer 9. The signals meeting the electric signals are supplied to an electronic volume control 10 of the liquid crystal projector 1 in the microcomputer 9. A counter electrode signal generating circuit 11 is then controlled to adjust the flicker level to the min. state, i.e., the DC level of the common electrode voltage to an optimum value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a matrix type liquid crystal display device, and more particularly to a device for adjusting a voltage of a common electrode of a liquid crystal panel constituting a liquid crystal display device driven by an alternating current in synchronization with a horizontal synchronizing signal. .

[0002]

2. Description of the Related Art Conventionally, when an image is displayed using a liquid crystal, it is generally driven by an alternating current in order to improve the reliability and life of the liquid crystal. That is, in the case of displaying a video signal of the interlaced scanning method, in the case of displaying the image of one frame by alternately outputting the signals of the odd field and the even field to the liquid crystal panel having 240 scanning lines, as shown in FIG. As described above, there is known a technique of inverting the polarity of the video signal in synchronization with the display of the television image in synchronization with the horizontal scanning period.

Then, in the state where no direct current component is applied to the liquid crystal panel, the potential of the common electrode has an optimum predetermined value (potential at which the absolute levels of the potentials of the video signal and the inverted video signal are substantially equal). When the electric potential deviates from the predetermined value, D
The C component is added and the AC drive cannot be completed.

That is, when the electric potential of the common electrode deviates from a predetermined value, the absolute value of the voltage with respect to the electric potential of the common electrode applied to the liquid crystal changes, a DC component is added to the liquid crystal, and the liquid crystal is deteriorated or the life is remarkably reduced. Becomes Therefore, it is necessary to adjust the potential of the common electrode to a predetermined value.

Further, since the polarities are inverted in synchronization with the field period, when the potential of the common electrode deviates from a predetermined value, the brightness of the screen also changes for each field, resulting in a flicker of 30 Hz. appear.

However, the frequency of flicker is usually 30 Hz.
That is, it is difficult to visually recognize, and as a practical phenomenon, a decrease in contrast or a rise in black level is recognized.

Therefore, generally, the adjustment of the potential of the common electrode is
The adjuster adjusts the contrast while looking at the gray scale so that the contrast becomes the best, and
As already proposed in Japanese Patent No. 86308, an image is projected so that flicker can be easily recognized, and an adjuster makes an adjustment so that the image has no unevenness on the screen.

[0008]

However, since all of the above adjustments are made by the operator's eyes, it is difficult to make adjustments, and there is a possibility that adjustments will be made to values that deviate from the predetermined values. There is a risk that the reliability and life of the liquid crystal may be impaired.

Further, since the common electrode is adjusted by the operator's eyes, automation cannot be expected and there is a drawback that the adjustment cannot be speeded up.

[0010]

DISCLOSURE OF THE INVENTION The present invention is directed to a light detecting means for detecting a fluctuation in illuminance from a liquid crystal display device, a current / voltage converting means for converting an output current from the light detecting means into a voltage, and the current. / Rectifying means for full-wave rectifying the output from the voltage converting means, peak holding means for holding the peak value of the output from the rectifying means, and A / D for converting the analog voltage from the peak holding means into a digital voltage. Inputting the conversion means and the digital output from the A / D conversion means,
It is a common electrode voltage adjusting device including a microcomputer that controls a common electrode voltage of a liquid crystal panel that constitutes a liquid crystal display device.

Further, according to the present invention, a plurality of light detecting means for detecting a fluctuation in illuminance from the liquid crystal display device, a plurality of current / voltage converting means for converting an output current from the light detecting means into a voltage, and the current. / Switching means for switching the output from the voltage converting means, rectifying means for full-wave rectifying the output that has passed through the switching means, peak holding means for holding the peak value of the output from the rectifying means, and the peak holding means And an A / D conversion means for converting an analog voltage from the A / D converter into a digital voltage and a microcomputer for controlling a common electrode voltage of a liquid crystal panel constituting a liquid crystal display device by using a digital output from the A / D conversion means as an input. It is a common electrode voltage adjusting device.

The present invention also includes a first light detecting means for detecting a change in illuminance of a lighting device such as a fluorescent lamp, a second light detecting means for detecting a change in illuminance from a liquid crystal display device, and the first and the second. Current / voltage converting means for converting the output currents from the second light detecting means into voltages, subtracting means for comparing and subtracting the voltage output from the current / voltage converting means, and full-wave rectification of the output from the subtracting means. Rectifying means, a peak holding means for holding a peak value of an output from the rectifying means, an A / D converting means for converting an analog voltage from the peak holding means into a digital voltage, and an A / D converting means. Is a common electrode voltage adjusting device including a microcomputer for controlling the common electrode voltage of a liquid crystal panel constituting a liquid crystal display device, using the digital output of the above as an input.

[0013]

According to the present invention, the flicker generated in the image projected on the screen is detected by the photodiode, converted into a quantitative electric signal, and then outputted to the microcomputer. Then, the microcomputer supplies a signal corresponding to the electric signal to the electronic volume of the liquid crystal projector and controls the counter signal generation circuit to set the flicker level to the minimum state, that is, the counter DC level.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

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

In FIG. 1, a screen 2 is provided with a photodiode 3, and a flicker of an image (for example, a red primary color signal) projected on the screen 2 by the liquid crystal projector 1 is detected by the photodiode 3. . Then, in the photodiode 3, the flowing current fluctuates depending on the flicker of the image. This fluctuation is
After being converted into a voltage by the voltage conversion circuit 4, the amplification circuit 5
Output to and amplify.

Next, the voltage amplified by the amplifier circuit 5 is rectified by the rectifier circuit 6, and the peak value (DC voltage) is held by the peak hold circuit 7. Here, the magnitude of the fluctuation of the peak value represents the strength of flicker. Then, the peak value is converted into a digital value by the A / D conversion circuit 8, and the digital signal is output to the microcomputer 9. Further, the DC level of the common electrode signal obtained by the counter signal generation circuit 11 is controlled by the electronic volume 10. Therefore, the microcomputer 9 controls the electronic volume 10 to adjust the DC level of the common electrode so that the fluctuation of the detected peak value is minimized, that is, the flicker is minimized.

FIG. 3 shows a concrete circuit diagram from the photodiode 3 to the peak hold circuit 7.

The circuit 4 for converting the amount of current flowing through the photodiode D1 into a voltage is composed of an operational amplifier A1, a resistor R1 and a capacitor C1. Here, the voltage generated by the current flowing in the photodiode D1 and the internal resistance of the photodiode D1 is non-inverted and amplified by the operational amplifier A1,
The amount of change in current is converted into the amount of change in voltage. The capacitor C1 is provided to prevent the operational amplifier A1 from becoming unstable in a high frequency region when an input capacitance is generated in the operational amplifier A1. Here, operational amplifier A1
The amplification factor is determined by the ratio of the internal resistance of the photodiode D1 and the value of R1. When an image with flicker is detected by the photodiode D1, the waveform at the point A becomes a voltage waveform corresponding to the flicker as shown in FIG. 4a. The horizontal axis is time, the vertical axis is voltage, and Vpp is the pp value of the waveform.

The output from the current / voltage conversion circuit 4 passes through the operational amplifier A2 for the voltage follower and the first high-pass filter composed of the capacitor C2 and the resistor R2, and has a waveform shown in FIG. 4B at the point B. .

Next, the output from the first high-pass filter is amplified by the amplifier circuit 5 including the operational amplifier A3 and the resistors R3 and R4, and has a waveform shown in FIG. 4b. The amplification factor is determined by the ratio of R3 and R4. Then, the output from the amplifier circuit 5 is guided to the rectifier circuit 6 after unnecessary noise components are removed by the second high-pass filter including the capacitor C3 and the resistor R5. At point D, p
The waveform shown in FIG. 4c is obtained by amplifying the −p value by (R3) / (R4) times.

The rectifier circuit 6 includes operational amplifiers A4, A5, diodes D2, D3, resistors R6, R7, R8, R9, R.
It is composed of 10 and. Then, the output from the second high-pass filter is supplied to the inverting input terminal of the operational amplifier A4 via the resistor R6.

When a signal of GND or more is input to the inverting input terminal of the operational amplifier A4, the diode D2 of the operational amplifier A4 is non-conductive, the diode D3 is conductive, and the resistor R6,
Amplify at the amplification factor determined by R8.

On the other hand, GND is connected to the inverting input of the operational amplifier A4.
When the following signals are input, the diode D2 is turned on and the diode D3 is turned off to form a negative feedback loop. At point E, a signal is directly obtained from the inverting input of the operational amplifier A4. Here, by setting R8 = R6, at point E, a half-wave rectified waveform as shown in FIG. 4d is obtained.

Further, the anode of the diode D3 is connected to an operational amplifier A5 and an inverting adder circuit composed of resistors R7, R9 and R10. Here, R10 = R7 =
By setting to 2 × R9, the signal input from the resistor R7, that is, the signal at the point D and the signal twice the signal from the resistor R9 (the signal at the point E) are inverted and added, and eventually,
At point F, a full-wave rectified waveform as shown in FIG. 4e can be obtained.

Then, this signal is applied to the transistor TR1.
And a resistor R11 are passed through a buffer, and the peak hold circuit 7 is constituted by a diode D4, a resistor R12, and a capacitor C4. Therefore, at point G, the peak voltage is held and converted to a DC signal, as shown in FIG. 4f. The DC level at this time is {R3 / (2
× R4)} × Vpp, which corresponds to the strength of flicker.

Further, this signal is transferred to the transistors TR2 and R.
It is output to the A / D conversion circuit 8 through the buffer constituted by 13. The A / D conversion circuit 8 converts this DC level into a digital signal and outputs it to the microcomputer 9.

With the above circuit operation, when the flicker of the image is large, the amplitude of the signal detected from the photodiode 3 also becomes large, and the DC level at the point G also becomes high. vice versa,
In a flicker-free image, the DC level at point G is low.
Therefore, if the microcomputer 9 controls the electronic volume 10 to minimize the flicker and adjusts the common electrode voltage, the DC level can be set to the optimum value.

In this embodiment, the common electrode voltage is adjusted only for the liquid crystal panel corresponding to the red primary color signal, but the common electrode voltage is similarly adjusted for liquid crystal panels corresponding to the other primary color signals. Need to do.

However, in the above-mentioned embodiment, since there is only one photodiode, flicker cannot be detected when the light receiving surface of the photodiode is located exactly between pixels of the projected image. However, depending on the size of the diode, an efficient flicker cannot be detected when it straddles two or more pixels, and it may be difficult to adjust the DC level of the common electrode.

In order to solve the above problem, a second embodiment in which a plurality of photodiodes are provided on the screen will be described.

The second embodiment of the present invention will be described below with reference to FIG.

This embodiment is different from the first embodiment in that
This is a configuration in which three photodiodes are provided at different positions on the screen and the maximum one of the detection outputs from these diodes is selected.

Since the other points are similar to those of the first embodiment, the description of the same parts will be omitted and only the diode selection circuit will be described.

First, the switch 9 is switched by the microcomputer 9.
Is switched to the A side, and the flicker is detected by the photodiode 3a. Here, the method of deriving the detected flicker to the microcomputer 9 is the same as in the above-described embodiment, the amplifier circuit 5, the rectifier circuit 6, the peak hold circuit 7, and the A /
It is led out to the microcomputer 9 via the D conversion circuit 8 and the content thereof is stored in the memory a.

Next, the switch SW1 is on the B side and the switch S is on.
W2 is switched to the C side, the flicker is similarly detected by the photodiode 3b, and stored in the memory b of the microcomputer 9. Further, switch SW2 to the D side,
The flicker detected by the photodiode 3c is also stored in the memory c of the microcomputer 9.

Here, the contents of the memories a, b, and c are compared by the microcomputer 9, and the one having the highest flicker level is discriminated and the switches SW1 and SW2 are determined.
And keep that state. With this configuration, it can be specified whether the photodiode detects flicker most efficiently.

However, also in the second embodiment,
Since flicker of external light such as a fluorescent light is also detected at the same time, it is difficult to accurately adjust the DC level of the common electrode. Therefore, it is necessary to eliminate the influence of the flicker component due to the external light so that accurate adjustment can be performed.

The third embodiment of the present invention will be described below with reference to FIG.

This embodiment is different from the first embodiment in that
In addition to the diode for detecting the flicker of the image from the liquid crystal projector, a diode for detecting the flicker from the outside light such as a fluorescent lamp is additionally provided.

The description of the same parts as those in the first embodiment will be omitted.

In FIG. 6, the photodiode 3d is
The photodiode 3a is a dedicated photodiode for detecting flicker of external light, and the diode 3a is a photodiode for detecting flicker of an image from the liquid crystal projector.

First, before projecting a video signal from the liquid crystal projector, external light flicker components from the photodiodes 3a and 3d are detected. here,
By subtracting the external light flicker component of the photodiode 3d from the external light flicker detected by the photodiode 3a, the influence of external light can be almost eliminated. However, normally, the external light flicker components detected from the photodiodes 3a and 3d are not always at the same level, and the level slightly changes depending on the position where the photodiode is installed. Therefore, the photodiodes 3a and 3
In order to keep the external light flicker component from d constant, the ratio is controlled in advance by the microcomputer 9 so that the external light flicker component can be sufficiently canceled.

Then, the flicker is adjusted in the following order.

First, the external light flicker component detected by the photodiodes 3a and 3d is output to the current / voltage conversion circuit 4. Then, the subtraction circuit 12 subtracts the flicker component of the photodiode 3d from the flicker component of the photodiode 3a. Here, the rate of subtraction can be controlled by the electronic volume 10.

Next, the differential output from the subtraction circuit 12 is amplified by the amplification circuit 5, rectified by the rectification circuit 6, and then the peak value is held by the peak hold circuit 7. And A
After being converted to a digital value by the / D conversion circuit 8, it is output to the microcomputer 9.

Therefore, the difference between the flicker components of the photodiodes 3a and 3d is subtracted from the input to the microcomputer 9, and the flicker of external light can be canceled well. That is, the influence of external light can be almost eliminated by controlling the electronic volume 10 by the microcomputer 9 and changing the subtraction rate so as to minimize the DC level.

[0048]

Since the present invention is configured as described above, it is possible to perform more accurate adjustment than visual adjustment of the DC level of the common electrode voltage. This not only improves the image quality of the liquid crystal projector and the reliability of the liquid crystal panel, but by adopting the present invention in a mass production line, there is no variation among products, and quick adjustment and automation can be achieved. .

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a common electrode voltage adjusting device according to the present invention.

FIG. 2 is a diagram illustrating AC driving of a liquid crystal panel.

FIG. 3 is a circuit diagram of a main part of a first embodiment of a common electrode voltage adjusting device according to the present invention.

FIG. 4 is a waveform diagram in the main part circuit diagram of FIG. 3.

FIG. 5 is a block diagram showing a second embodiment of the common electrode voltage adjusting device according to the present invention.

FIG. 6 is a block diagram showing a third embodiment of the common electrode voltage adjusting device according to the present invention.

[Explanation of symbols]

 1 Liquid crystal projector 2 Screen 3 Photodiode 4 Current / voltage conversion circuit 5 Amplification circuit 6 Rectifier circuit 7 Peak hold circuit 8 A / D converter 9 Microcomputer 10 Electronic volume 11 Counter electrode signal generation circuit

Claims (3)

[Claims] 1. A light detecting means for detecting a fluctuation in illuminance from a liquid crystal display device, a current / voltage converting means for converting an output current from the light detecting means into a voltage, and an output from the current / voltage converting means. Rectification means for full-wave rectification, peak hold means for holding the peak value of the output from the rectification means, A / D conversion means for converting an analog voltage from the peak hold means into a digital voltage, and the A / D A common electrode voltage adjusting device comprising a digital output from a converting means as an input and a microcomputer for controlling a common electrode voltage of a liquid crystal panel constituting a liquid crystal display device. 2. A plurality of photo-detecting means for detecting illuminance fluctuations from the liquid crystal display device, a plurality of current / voltage converting means for converting an output current from the photo-detecting means into a voltage, and the current / voltage converting means. Means for switching the output from the switch means, a rectifying means for full-wave rectifying the output passing through the switch means, a peak holding means for holding the peak value of the output from the rectifying means, and an analog voltage from the peak holding means. And A / D conversion means for converting the
A common electrode voltage adjusting device comprising a microcomputer for controlling a common electrode voltage of a liquid crystal panel which constitutes a liquid crystal display device, using a digital output from the / D conversion means as an input. 3. A first light detecting means for detecting a change in illuminance of a lighting device such as a fluorescent lamp, a second light detecting means for detecting a change in illuminance from a liquid crystal display device, and the first and second light detecting means. Current / voltage converting means for converting output currents from the respective means into voltages, subtracting means for comparing and subtracting voltage outputs from the current / voltage converting means, and rectifying means for full-wave rectifying the output from the subtracting means. A peak hold means for holding the peak value of the output from the rectification means, an A / D conversion means for converting an analog voltage from the peak hold means into a digital voltage, and a digital output from the A / D conversion means. A common electrode voltage adjusting device including a microcomputer as an input, which controls a common electrode voltage of a liquid crystal panel constituting a liquid crystal display device.