JPH0738829A - Contrast control circuit and liquid crystal projector using same - Google Patents

Abstract

PURPOSE:To sufficiently remove black staining accompanying with contrast control. CONSTITUTION:A lst black peak detecting circuit 4 detects a black peak value in the luminance signal from an input terminal 1 and a DC level control circuit 2 controls the DC level of the luminance signal based on the black peak value. Then a white peak detecting circuit 10 detects the white peak value of the output luminance signal outputted from the DC level control circuit 2 and an amplitude control circuit 3 controls the amplitude of the luminance signal based on the white peak value. An amplitude control circuit 3 controls the amplitude of the luminance signal. Consequently, the excellent luminance signal which is free from black staining is outputted from an output terminal 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal projector using a liquid crystal panel as a light valve, and more particularly to a projection type liquid crystal projector.

[0002]

2. Description of the Related Art A projection type liquid crystal projector using a liquid crystal panel as a light valve is being actively developed today as an effective means for increasing the screen size. In this type of projector, it is essential to increase the brightness of the light source in order to reproduce a vivid image, but in reality, it is impossible to completely block the light from the light source by the liquid crystal panel. Therefore, as the brightness of the light source increases, a phenomenon of black floating due to light leakage occurs in an actual image, and higher brightness of the light source is not necessarily reflected in the improvement of contrast. In addition, increasing the brightness of the light source is also a negative factor in terms of heat resistance of the liquid crystal panel. Especially, in the case of a dark screen in general, the amount of light blocked by the liquid crystal panel is large. It also increases the heat absorption of the liquid crystal, leading to damage to the liquid crystal panel.

Therefore, the applicant of the present invention has proposed a Japanese Patent Application No. 4-317565 for the purpose of solving the black floating caused by the high brightness of the light source and the damage due to heat absorption in the liquid crystal panel, the polarizing plate and the like.
No. has been filed.

However, the amplitude control in the above application is configured to detect the white peak value and increase the amplitude of the video signal based on the detected output. For this reason,
When a video signal with a black peak value higher than the pedestal level is input, the black peak value is also raised to the white level side at the same time, and the black level of the video signal rises, so-called black float still occurs. .

The occurrence of black floating will be described below with reference to FIG.

FIG. 6 shows input / output waveforms of the amplitude control circuit.

As is clear from this figure, when the amplitude control circuit controls the white peak value to be equal to the reference voltage, the black peak value is also lifted to the white side, and the above-mentioned black floating Will occur.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to sufficiently remove black floating caused by amplitude control.

[0009]

SUMMARY OF THE INVENTION The present invention provides a DC level control circuit for controlling the DC level of an input signal, and a black peak for detecting a black peak value as a voltage from an output signal from the DC level control circuit. A detection circuit; a reference voltage generation circuit that generates a reference voltage based on an input signal; a first comparison circuit that compares the voltage of the reference voltage generation circuit with the voltage of the black peak detection circuit; An amplitude control circuit that controls the amplitude of the signal by inputting the output signal from the level control circuit, a white peak detection circuit that detects the white peak value as a voltage from the output signal from the amplitude control circuit, and the white peak detection circuit Second comparing the voltage from the reference voltage with the voltage from
And the DC level of the DC level detection circuit is controlled by the control signal from the first comparison circuit, and the amplitude of the amplitude control circuit is controlled by the control signal from the second comparison circuit. This is a contrast control circuit.

The reference voltage generation circuit of the present invention further comprises a second black peak detection circuit for detecting the black peak value of the input signal, and a resistor for dividing the second black peak detection circuit.
It is composed of a reference power source that generates a pedestal level voltage.

The present invention also includes a DC level control circuit for controlling the DC level of an input signal, and a black peak detection circuit for detecting the black peak value as a voltage from the output signal from the DC level control circuit. From a reference voltage generation circuit that generates a reference voltage based on the input signal, a first comparison circuit that compares the voltage of the reference voltage generation circuit with the voltage of the black peak detection circuit, and the DC level control circuit. An amplitude control circuit that controls the amplitude of the signal by using the output signal of the input signal, a white peak detection circuit that detects a white peak value as a voltage from the output signal from the amplitude control circuit, and a voltage from the white peak detection circuit. A second comparison circuit for comparing with a reference voltage, the direct current level of the direct current level detection circuit is controlled by a control signal from the first comparison circuit, Is a liquid crystal projector using the contrast control circuit controlled by a control signal from the second comparator circuit.

[0012]

According to the present invention, with the configuration as described above, the DC level of the input video signal is controlled to lower the DC level of the black peak value. Next, the white peak value of the video signal is detected and the amplitude is controlled to make the amplitude level of the input video signal constant. Therefore, even if the black peak value of the input video signal is sufficiently higher than the pedestal level, black floating does not occur due to the amplitude control, and an image without black level floating can be obtained.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A liquid crystal projector of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a contrast control circuit used in the liquid crystal projector of the present invention. With respect to the luminance signal input from the input terminal 1, the DC level of the luminance signal is first controlled by the DC level control circuit 2. It

The DC level control operation will be described below.

The output brightness signal from the DC level control circuit 2 is input to the first black peak detection circuit 4, and the black peak value of the brightness signal is detected by the first black peak detection circuit 4. The detected black peak value is compared with the reference voltage from the reference voltage generation circuit 7 in the first comparison circuit 6, and the comparison output is output to the first LPF 8. Next, the LPF 8 integrates the comparison output to prevent a rapid change in the DC level, and then outputs it as a control signal to the DC level control circuit 2.

Here, the reference voltage generation circuit 7 is a second black peak detection circuit 1 for detecting the black peak value of the input luminance signal.
3 and Ra and Rb for resistance-dividing the detection value from the second black peak detection circuit 13 and a reference voltage source, and the detection from the second black peak detection circuit 13 to the reference voltage of the reference voltage source. The output is superimposed. Therefore, the reference voltage from the reference voltage generating circuit 7 changes depending on the input luminance signal.

The amount of change of the reference voltage in the input luminance signal can be changed by changing the resistance ratio of the resistors Ra and Rb.

The DC level control operation will be described below with reference to FIG.
A description will be given with reference to FIGS. 2a and 3.

Incidentally, FIG. 2a shows the case where the black peak value of the input luminance signal is higher than the reference voltage.

The output brightness signal from the DC level control circuit 2 is input to the first black peak detection circuit 4, and the black peak value of the brightness signal is detected by the first black peak detection circuit 4. The detected black peak value is compared with the reference voltage from the reference voltage generation circuit 7 in the first comparison circuit 6, and the comparison output is output to the first LPF 8. Next, in the first LPF 8, after the comparison output is integrated to prevent a sudden change in the DC level, the DC level control circuit 2 is used as a control signal.
Output to.

Next, a specific circuit of the DC level control block will be described with reference to FIG.

The luminance signal input from the input terminal 1 of FIG. 3 has its direct current component cut by the capacitor C1.
The transistor TR is biased by the resistors R1 and R2.
1, a resistor R3, a transistor TR2, and a resistor R
4 are branched to the emitter followers.

The luminance signal from the transistor TR1 is pedestal clamped to the black level by the capacitor C2 and the switch SW1. And the switch SW2
L composed of resistor R7 and capacitor C4 via
It is output to the PF and its high frequency component is attenuated.

Next, a black peak value of the luminance signal is detected by a first black peak detecting circuit composed of a transistor TR5, a resistor R9, a diode D1 and a capacitor C6 via a switch SW5.

Here, the switch SW1 is controlled by the clamp pulse, and is turned on only during the horizontal blanking period, and the switch SW2 is turned on only during the video signal period.

The LPF is for preventing the edge of the contour of the luminance signal from being detected in the first black peak detection circuit.

The switch SW5 is turned on during the horizontal blanking period of the luminance signal to prevent the black peak detection circuit in the subsequent stage from operating during the blanking period.

After that, the black peak value detected by the first black peak detection circuit is output to the negative polarity side of the operational amplifier A1. Here, the resistor R11 plays a role of preventing the peak value charged in the capacitor C6 from being charged for a long time and discharging appropriately so as to follow the change of the input signal.

On the other hand, the luminance signal from the transistor TR1 is input to the second black peak detection circuit via the emitter follower composed of the transistor TR4 and the resistor R6, and the LPF composed of the resistor R8 and the capacitor C5. . The second black peak detection circuit is a transistor TR
6, a diode D2, and a capacitor C7,
Detect the black peak value in the luminance signal. Then, the detected black peak value and the black level determined by the reference power supply are
The operational amplifier A is synthesized by the resistors R16 and R15.
The reference voltage of 1 is input to the positive electrode side.

As a result, the reference voltage input to the operational amplifier A1 can be changed according to the input signal.

Next, the operational output of the operational amplifier A1 is output to the LPF composed of the resistor R19 and the capacitor C8.

The output from the LPF is the switch S.
The potential of the output luminance signal output via W4 and the switch SW3 is pedestal clamped to the potential of the operational amplifier, and the DC level thereof is controlled.

With such a circuit configuration, the control of the DC level is performed by the transistor TR when the black peak value detected by the transistor TR7 is higher than the reference voltage.
2 works to lower the DC level of the luminance signal.

On the contrary, when the black peak value detected by the transistor TR7 is lower than the reference voltage, it works to raise the direct current level of the luminance signal from the transistor TR2.

By repeating the above operation, the black peak value of the output luminance signal is controlled to be equal to the reference potential.

Here, the transistors TR3 and TR5
The signal input to the emitter follower configured by the horizontal input signal is the output luminance signal from the transistor TR1 during the horizontal blanking period and is the horizontal luminance in order to be the output luminance signal from the transistor TR2 during the video signal period. The switch SW2 is turned on and the switch SW3 is turned off during the blanking period, and the switch SW3 is turned on and the switch SW2 is turned off during the video signal period.

As a result, at the output terminal, an output signal whose DC level during the horizontal blanking period is equal to that of the input signal and whose DC level is controlled only during the video signal period is obtained.

With the above configuration, the DC level of the luminance signal is controlled in accordance with the black peak value of the screen, and the black peak value of the input luminance signal becomes the reference voltage from the reference voltage generating circuit 7. The DC level is controlled so as to be equal.

The luminance signal output from the DC level control circuit 2 is output to the amplitude control circuit 3.

The amplitude control circuit 3 controls the amplitude of the brightness signal from the DC level control circuit described above, and then outputs the brightness signal whose amplitude is controlled to the output terminal 9.

Here, the output luminance signal is also input to the white peak detection circuit 10, and the white peak detection circuit detects the white peak value as shown in FIG. 2b. The detected white peak value is compared with the reference voltage from the reference power supply 5 in the second comparison circuit 11, and this comparison output is output to the second LPF 12. Next, the second LPF 12 integrates the comparison output to prevent a sudden change in the amplitude, and then outputs it as a control signal to the amplitude control circuit 3.

As a result, the amplitude of the luminance signal is controlled according to the white peak value of the screen, and the amplitude is controlled so that the white peak value of the input luminance signal becomes equal to the reference voltage of the reference power supply 5. .

Note that FIG. 2b shows the case where the white peak value of the input luminance signal is lower than the reference voltage.

The amplitude control operation will be described in detail below with reference to FIG.

FIG. 4 is a concrete circuit diagram of the amplitude control block.

The output brightness signal from the DC level detection circuit 2 is input to the input terminal of the contrast control circuit shown in FIG.

Here, the circuit shown by the dotted line is the modulation circuit 17 which modulates the amplitude of the input signal according to the voltage applied to the CTL terminal. The description of the operation of the modulation circuit 17 is omitted. The modulation circuit 17 changes the contrast of the input signal by applying a black level to the base of the transistor TR1 and changing the voltage applied to the CTL terminal.

Then, an inverted signal of the input signal is output to the output terminal. The DC component of this output signal is cut by the capacitor C1, and the resistance R1 and the resistance R
It is biased to the potential divided by 2 and output to the inverting amplifier circuit composed of the transistor TR2, the resistor R3, and the resistor R4.

Here, this output signal is inverted and amplified,
It is pedestal clamped to the black level by the capacitor C2 and the switch SW1. After that, the output signal is the resistance R
It is input to the LPF constituted by 6 and the capacitor C3, and the LPF attenuates the high frequency component so that the edge of the contour of the input signal is not detected by the white peak detection circuit in the subsequent stage.

The high-frequency attenuated signal has its peak value detected by the white peak detection circuit composed of the transistor TR4, the resistor R7, the diode D1, and the capacitor C4, and is input to the positive electrode side of the operational amplifier A2. .

Here, the resistor R8 plays a role of preventing the peak value charged in the capacitor C4 from being charged for a long time and discharging appropriately so as to follow the change of the input signal.

A reference voltage is applied to the negative side of the operational amplifier A2, and its operation output is the resistance R12,
And CT via LPF composed of capacitor C5
It is output to the L terminal. This LPF is for preventing a sharp change in contrast.

With such a circuit configuration, the contrast is lowered when the peak value output from the transistor TR5 is higher than the reference voltage, and the contrast raising voltage is raised when the peak value is lower than the reference voltage. Will be applied to the terminals.

By repeating the above operation, the contrast of the input signal is controlled so that the white peak value becomes equal to the reference voltage.

With the above-described structure, a brightness signal without black level floating can be obtained even if the amplitude of the brightness signal is increased by using the amplitude control circuit 3.

However, although the amplitude of the luminance signal is changed in the above-mentioned embodiment, this also causes a problem that the color gain also changes according to the amplitude of the luminance signal.

To solve this, a second embodiment shown in FIG. 4 can be considered.

In FIG. 5, the luminance signal input from the input terminal 1A and the color difference signal input from the input terminals 1B and 1C are processed by the first matrix circuit 14 into R, G and B signals.
Are converted into the respective primary color signals, and then the respective primary color signals are output to the DC level control circuits 2A, 2B, 2C.

In the DC level control circuits 2A, 2B and 2C, the DC level of each primary color signal is controlled by the following operation.

A luminance signal is generated by the second matrix circuit 15 from each of the primary color signals from the DC level control circuits 2A, 2B and 2C. The luminance signal outputs the black peak value to the first comparator 6 after the black peak value is detected by the first black peak detection circuit 4.

In the first comparison circuit 6, the reference voltage output from the reference voltage generation circuit 7 is compared, and this comparison output is passed through the first LPF 8 to the DC level control circuits 2A, 2
It is output to B and 2C and controls the DC level of each primary color signal. Here, since the reference voltage generating circuit 7 and the first LPF 8 have the same functions as those in the first embodiment, their description will be omitted.

The primary color signals of which the DC level is controlled are output to the output terminals 7A, 7B and 7C after their amplitudes are controlled by the amplitude control circuits 3A, 3B and 3C.

The amplitude control operation will be described below.

A luminance signal is generated from the output signals from the DC level control circuits 3A, 3B and 3C by the third matrix circuit 16. The white peak value of the luminance signal is detected by the white peak detection circuit 10, and then the white peak value is output to the second comparison circuit 11.

In the second comparison circuit 11, the white peak value and the reference voltage are compared, and the comparison output is the second LPF 12
Is output to the amplitude control circuits 3A, 3B, 3C via
It controls the amplitude of each primary color signal. Here, since the second LPF 12 has the same function as that of the first embodiment, its explanation is omitted.

As a result, the DC level and the amplitude of each of the R, G, and B primary color signals are controlled.

In this embodiment, the luminance signal for controlling the direct current level and the luminance signal for controlling the amplitude are generated by two independent matrix circuits, but the luminance signal is generated by one matrix circuit. Alternatively, the brightness signal may be generated and used as a brightness signal for controlling the direct current level and as a brightness signal for controlling the amplitude.

[0069]

Since the present invention is configured as described above, in a display having a narrow dynamic range such as a liquid crystal panel, a contrast control circuit or the like is used to make a white peak value constant and display a bright screen. However, it is possible to realize an image with good contrast without black level floating.

[Brief description of drawings]

FIG. 1 is an embodiment of a contrast control circuit of the present invention.

FIG. 2 is an input / output waveform diagram of the contrast control circuit of the present invention.

FIG. 3 is a specific circuit diagram of a DC level control block.

FIG. 4 is a specific circuit diagram of an amplitude control block.

FIG. 5 is another embodiment of the contrast control circuit of the present invention.

FIG. 6 is an input / output waveform diagram of a conventional contrast control circuit.

[Explanation of symbols]

 1 Input Terminal 2 DC Level Control Circuit 3 Amplitude Control Circuit 4 First Black Peak Detection Circuit 5 Reference Power Supply 6 First Comparison Circuit 7 Reference Voltage Generation Circuit 8 First LPF 9 Output Terminal 10 White Peak Detection Circuit 11 Second Comparing circuit 12 Second LPF 13 Second black peak detection circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04N 5/57 9/73 B 8626-5C

Claims (3)

[Claims] 1. A DC level control circuit for controlling a DC level of an input signal, a black peak detection circuit for detecting a black peak value as a voltage from an output signal from the DC level control circuit, and an input signal. A reference voltage generation circuit that generates a reference voltage based on the above, a first comparison circuit that compares the voltage of the reference voltage generation circuit with the voltage of the black peak detection circuit, and an output signal from the DC level control circuit. An amplitude control circuit that controls the amplitude of a signal as an input, a white peak detection circuit that detects a white peak value as a voltage from an output signal from the amplitude control circuit, and a voltage from the white peak detection circuit and a reference voltage are compared. And a second comparison circuit for controlling the direct current level of the direct current level detection circuit by a control signal from the first comparison circuit, and the amplitude of the amplitude control circuit is controlled by the second comparison circuit. A contrast control circuit characterized by being controlled by a control signal from a road. 2. The second reference voltage generation circuit includes a second black peak detection circuit for detecting a black peak value of an input signal,
2. The contrast control circuit according to claim 1, comprising a resistor for dividing the second black peak detecting circuit and a reference power source for generating a pedestal level voltage. 3. A DC level control circuit for controlling a DC level of an input signal, a black peak detection circuit for detecting a black peak value as a voltage from an output signal from the DC level control circuit, and an input signal A reference voltage generation circuit that generates a reference voltage based on the above, a first comparison circuit that compares the voltage of the reference voltage generation circuit with the voltage of the black peak detection circuit, and an output signal from the DC level control circuit. An amplitude control circuit that controls the amplitude of a signal as an input, a white peak detection circuit that detects a white peak value as a voltage from an output signal from the amplitude control circuit, and a voltage from the white peak detection circuit and a reference voltage are compared. And a second comparison circuit for controlling the direct current level of the direct current level detection circuit by a control signal from the first comparison circuit, and the amplitude of the amplitude control circuit is controlled by the second comparison circuit. A liquid crystal projector using a contrast control circuit controlled by a control signal from the road.