JPH0127632B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video processing device that can improve the gradation of a video signal, and aims to provide a device that can perform more accurate gradation correction. .

Embodiments of the present invention will be described below with reference to the drawings. A video signal is applied to input terminal A, the black side of the video signal is expanded by expansion circuit 1, and outputted to output terminal B as an output signal. The video signal after the output terminal B is applied to the detection circuit 2 from the terminal E, and the third signal is applied to the terminal D.
A voltage is applied to set a certain level X of the video signal shown in FIG. a, and a signal below level X of the video signal is detected. The output of this detection circuit 2 is amplified by a smoothing circuit 3.
Amplify and smooth. The output of the amplification and smoothing circuit 3 is applied to the expansion circuit 1 via the control circuit 4, and the black side of the video signal is expanded in accordance with the output of the amplification and smoothing circuit 3. A blanking pulse is applied to terminal C, the shaping circuit 5 generates the pulse signal shown in FIG. Control as follows.

By adding this blanking pulse, both circuits 2 and 4
The purpose of stopping is as follows. First, in the detection circuit 2 that detects the black side of the video signal, the third
The black side of the video signal in Figure A is detected below a certain level It becomes inconvenient. Therefore, the synchronization signal period is not detected. Furthermore, this black side video signal is detected, it is amplified and smoothed, and the black side of the video signal is extended using this voltage, but it is inconvenient to also extend the synchronization signal period. For the above reasons, it is necessary to detect and expand the black side of the video signal during the video signal period, excluding the synchronization signal period.

Next, explanation will be given using the specific circuit shown in FIG. Add a video signal with the polarity shown in Figure 3a from the input terminal E, and set the black side setting level X to be detected.
Apply a voltage to terminal D that indicates . As a result, the differential amplifier consisting of transistors Q ₁ , Q ₂ , and Q ₃ allows
When the input video signal is blacker than the set level X, the transistor _Q1 is turned off, the transistor _Q2 is turned on, and a low-level detection voltage is output to the resistor _R1 . When the input video signal is not blacker than the set level X, transistor _Q1 is turned on.
Transistor Q ₂ is turned off and the sense voltage output of its collector resistor R ₁ goes high. At this time, by applying a blanking pulse as shown in Figure 3b from terminal C during the synchronizing signal period, transistor _Q4 is turned on via transistor _Q12 , and transistor _Q3 is turned off, so that this period is black. Disable detection of level side signals.

That is, the detection voltage from the collector of transistor _Q2 is at a high level during this synchronization signal period. Therefore, as the period of the signal on the black level side is shorter than the set level X, the period in which the detection voltage is at a low level becomes shorter.

Next, this detected voltage is inverted and amplified by a transistor _Q5 , and smoothed by a capacitor C to become a DC voltage. Therefore, the smoothed DC voltage becomes lower as the period of the signal on the black side is shorter than the set level X. Next, this detected DC voltage is inverted again by transistor Q ₆ and applied to the base of transistor Q ₇ , and is output from its emitter as an output voltage for control. As a result, the output voltage of the emitter of the transistor _Q7 becomes higher as the number of signals on the black side of the set level X in the input video signal a decreases.

On the other hand, a video signal having a polarity opposite to that of the video signal shown in FIG. 3a is applied to the video signal input terminal A. Then, a video signal having a low level on the black side and a high level on the white side appears at the collector of the transistor _Q11 and the emitter of the transistor _Q9 , as in FIG. 3a. As mentioned above, when the period of the signal on the black _side is shorter than _the set level In the black part that is lower than the control output voltage, current flows from the emitter of transistor Q ₇ to transistor Q ₉ through resistor R, substantially increasing the gain of transistor Q ₉ , and passing through the emitter follower of transistor Q ₁₀ to terminal B. A video signal in which only the black side signal is expanded is output. At this time, if the base voltage of transistor _Q7 is high during the synchronization signal period, the synchronization signal will also be extended, which is inconvenient, so by applying a blanking pulse to terminal C, the transistor
By turning on _Q8 during the synchronization signal period and lowering the base voltage of transistor _Q7 , the synchronization signal period is prevented from being extended.

The video signal output to terminal B may be fed back to terminal E, but in the circuit example shown in FIG. 2, the polarity of the video signal is reversed, so polarity reversal is required. If the signal is fed back to terminal E through another video signal processing circuit after terminal B, the feedback loop will continue until the black side is extended so that the period when the black side is blacker than a certain setting level X reaches a predetermined period. works.

It should be noted that when there are many periods in which the signal is blacker than the set _level The current flowing into the emitter of ₉ decreases,
Furthermore, it stops flowing, reduces the degree of expansion on the black side, and furthermore stops expanding.

In this way, the less black signals there are in the video signal, the more the black signals can be expanded.
It is possible to project an easy-to-see image.

Next, the drawbacks of this circuit will be explained. When a signal for blanking the synchronization signal period is applied from terminal C, transistor _Q8 is turned on during that period.
The base voltage of transistor _Q7 drops. Therefore, during this period, the video signal appearing at the emitter of transistor _Q9 passes through resistor R to transistor _Q7.
When applied to the emitter of _Q7 , a reverse bias state is created between the emitter and the base of Q7 during that period. Therefore, by inserting a diode between the base and emitter _of transistor _Q7 ,
It is possible to prevent the emitter reverse breakdown voltage from being exceeded, but inserting a diode would be inconvenient because it would have temperature characteristics. Also, if you insert a diode, the voltage that operates the black correction will drop by about 0.7 volts, which is inconvenient.

Therefore, in the present invention, as shown in FIG. 4, the transistor _Q13 is connected between the DC power supply V _CC line and the base of the transistor _Q7 , and a constant DC voltage is applied to the terminal G connected to the base of the transistor _Q13 . In addition, during the blanking period when transistor _Q8 is on, the voltage at the base of transistor _Q7 is connected to terminal G.
DC voltage of 0.7 volts (transistor Q ₁₃
V _BE ) is fixed at a low voltage, and this voltage is used to prevent the voltage _at the base of transistor Q ₇ from becoming lower than a certain level than the voltage of the video signal appearing at the emitter of transistor Q ₉ during this period.・It is designed to prevent the reverse breakdown voltage of the emitter from exceeding. Of course, the DC voltage applied to terminal G is lower than the lowest value (+0.7 volts) of the detected DC voltage supplied from transistor Q ₆ , except during the blanking period when transistor Q ₈ is turned on. Transistor _Q13 is turned off except during this period, so as not to affect the expansion control of the black side signal.

The circuit in Figure 4 is the same as the right half of the circuit in Figure 2 except for G and Q ₁₃ , the emitter of transistor Q ₆ is grounded through resistor R ₇ , and the collector is connected to V through resistor R ₆ . _CC is also directly connected to the base of transistor _Q7 . Further transistor Q ₆
The collector of is grounded through resistor _R8 and transistor _Q8 . The base of this transistor Q ₈ is connected to the emitter of transistor Q ₁₂ via a resistor R ₉ . This emitter is grounded via resistor _R16 and the collector is connected to V _CC . The base of transistor _Q12 is connected to terminal C. The emitter of transistor Q ₉ is resistor R ₁₁
The _collector is connected to the base of the transistor Q ₁₀ and to ground via the resistor R ₁₂ . The emitter of transistor _Q10 is also grounded to output terminal B via resistor _R13 . Terminal A is connected to the base of transistor Q ₁₁ , the emitter of transistor Q ₁₁ is grounded via resistor R ₁₅ , and the collector is connected to the base of transistor Q ₉ and to V _CC via resistor R ₁₄ .

As explained above, in the present invention, in a circuit that processes a signal that does not have a black side signal of a video signal, creates a black side signal, and improves gradation, processing of such a black side signal is performed using a pedestal clamp circuit. The circuitry required when the process is performed earlier can be provided in a simple circuit configuration.

In addition, the first
The collector of the transistor is connected to the base of the second transistor constituting the control circuit.
The collector of a third transistor whose on/off is controlled by a blanking pulse is connected to the base of the transistor, and the emitter-collector circuit of a fourth transistor is connected between the base of the second transistor and the power supply voltage line. By connecting , and applying a constant DC voltage to the base of the fourth transistor, the temperature characteristics can be adjusted to ensure that the base-emitter reverse breakdown voltage of the second transistor is exceeded when a blanking pulse is applied. It can be prevented without causing problems.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a video signal processing device according to an embodiment of the present invention, FIG. 2 is a specific circuit diagram of the device, and FIGS. 3 a and b are waveform diagrams for explaining the device.
FIG. 4 is a circuit diagram of a circuit that is a further improvement of the circuit shown in FIG. DESCRIPTION OF SYMBOLS 1...Extension circuit, 2...Detection circuit, 3...Amplification and smoothing circuit, 4...Control circuit, A, E...Video signal input terminal, D...Level X voltage application terminal, B...
...Output terminal, C...Granking pulse application terminal.

Claims (1)

[Claims] 1. A detection circuit that detects a video signal on the black side from a set level, a smoothing circuit that converts the output of this detection circuit into a DC voltage, an expansion circuit that expands the black side of the video signal, and a video signal of this expansion circuit. A control circuit that controls the extent to which the black side of the signal is extended by the output voltage of the smoothing circuit, and a blanking pulse that controls the detection circuit and the control circuit so that they do not operate during the synchronization signal period. The control circuit includes a first transistor whose base receives the output voltage of the smoothing circuit, and a base of which is connected to the collector of the first transistor and whose emitter is connected to the control terminal of the expansion circuit. a second transistor connected thereto, and a control period limiting transistor inserted between the base and ground of the second transistor, whose on/off is controlled by the blanking pulse during the synchronization signal period inputted to the base. 3 and the second transistor
An emitter-collector circuit is connected between the base of the second transistor and the power supply line, and a constant DC voltage is applied to the base of the emitter-collector circuit, thereby increasing the base voltage of the second transistor to a predetermined voltage or higher during the period of the blanking pulse. A video signal processing device comprising: a fixed fourth transistor.