JP2516105B2 - Black level correction circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a black level correction circuit for supplying an optimum image corresponding to the black level of an input luminance signal.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing an example of a conventional black level correction circuit. In FIG. 4, the input luminance signal supplied from the input terminal is input to the clamp circuit 1 and its pedestal level is clamped to a predetermined level. The luminance signal output from the clamp circuit 1 is input to the black level control circuit 2. The black level control circuit 2 inputs the black level control signal output from the black level detection circuit 3, and the clamp circuit 1 corresponds to the level of the black level control signal.
The black level of the output luminance signal is changed, adjusted, and output. Here, the black level detection circuit 3 inputs the output luminance signal which is the output of the black level control circuit 2 and the black level detection reference voltage (the voltage supplied from the voltage source V1) which is the reference for the black level detection. The two are compared, and the error voltages of the both are input to the black level control circuit 3 as a black level control signal.

The black level detection circuit 3 is constructed, for example, as shown in FIG. The output luminance signal which is the output of the black level control circuit 2 is input to the base of the NPN transistor Q3, and the emitter of the transistor Q3 is connected to the current source I3 whose one end is grounded and the resistor R5. And
The other end of the resistor R5 is connected to the current source I2 whose one end is grounded and the emitter of the NPN transistor Q2. Further, the collector of the transistor Q2 has a power supply voltage V
The base is connected to a voltage source V1 whose one end is grounded. The voltage source V1 is the black level detection reference voltage described above.

The transistors Q2 and Q3, the current sources I2 and I3, the resistor R5, and the voltage source V1 constitute a differential amplifier, and this differential amplifier has an output luminance signal input to the base of the transistor Q3 and the transistor Q2. Is compared with the black level detection reference voltage input to the base of, and the error current flows to the collector of the transistor Q3. The collector of the transistor Q3 has a resistor R6 having one end connected to the power supply voltage Vcc and a capacitor C2 having one end grounded.
It is smoothed by and converted into voltage. Therefore, the average value of the error voltages corresponding to the difference between the output luminance signal and the black level detection reference voltage is obtained as the black level control signal. If the output luminance signal input to the base of the transistor Q3 is a high black level voltage and a low white level voltage (negative polarity), if there are many black signals exceeding the black level reference voltage among the output luminance signals, the black level control signal Becomes a low voltage, and conversely, when there are few black signals exceeding the black level reference voltage in the output luminance signal, the black level control signal becomes a high voltage.

In FIG. 4, the black level detection circuit 3 is shown in FIG.
In the case of the circuit configuration shown in FIG. 3, the black level control circuit 2 inputs the luminance signal clamped by the clamp circuit 1 and the black level control signal output from the black level detection circuit 3 as described above, When the black level control signal has a high voltage (when there are few black signals exceeding the black level reference voltage in the output brightness signal), the level control circuit 2 changes and adjusts in the direction in which the black signal of the output brightness signal increases. On the contrary, when the black level control signal has a low voltage (when there are many black signals exceeding the black level reference voltage among the output brightness signals), the black signal of the output brightness signal is changed and adjusted so as to decrease. The black level control circuit 2 includes a method of changing the signal on the black side by a non-linear circuit, a method of changing the DC component of the entire luminance signal by DC shift, and a method of using both of them. This makes it possible to increase the correction amount of the black level when the black signal is small in the input luminance signal and decrease the correction amount when the black signal is large. Therefore, the dynamic range of the circuit can be effectively used and the contrast You can get a certain video.

[0006]

In the above-described conventional black level correction circuit, the black level detection reference voltage (V1) for correcting the black level and the gain of the black level detection circuit 3 are Since it is constant for the display period, the black level correction amount is determined only for the level of the black signal regardless of the display position of the black signal on the display screen. Generally, the most important information in the video signal is near the center of the display screen, whereas the vicinity of the edge of the display screen is often less important as information. Correcting the black level of the entire display screen in accordance with the black signal near the edges of the display screen is not limited to information near the center of the display screen, which is highly important as information, and most other parts of the display screen. However, there is a problem that the information of the item may be damaged.

For example, when a telop with a black frame appears at the edge of the screen, the black level detection circuit 3 detects the black frame of the telop, and the black level control circuit 2 reduces the correction amount of the black level. , The correction amount is not good when there is no telop. Originally, the telop of movie software or the like is displayed at the edge of the screen so as not to affect the display screen, and the telop should not affect the luminance signals of other parts. The present invention has been made in view of such a problem, and provides a black level correction circuit that can always adjust the black level to an appropriate level even when a telop or the like appears on the edge of the screen. The purpose is to do.

[0008]

In order to solve the above-mentioned problems of the prior art, the present invention clamps a pedestal level of an input luminance signal and outputs it, and a luminance signal output from the clamp circuit. The black level control circuit for controlling the black level according to the black level control signal to obtain the output luminance signal, the black level detection reference voltage and the output luminance signal are input, and the both are compared to perform the black level control. A black position correction circuit configured to include a black level detection circuit that supplies a signal to the black level control circuit, and a display position correction circuit that generates a display position correction signal corresponding to a display position of a video display screen, Addition in which the display position correction signal and the black level detection reference voltage are added to generate a display position corrected black level detection reference voltage, and the black level detection reference voltage is supplied to the black level detection circuit. There is provided a black level correction circuit, characterized in that it comprises and.

[0009]

Embodiments of the black level correction circuit of the present invention will be described below.
Description will be given with reference to the accompanying drawings. 1 is a block diagram showing an embodiment of a black level correction circuit of the present invention, FIG. 2 is a circuit diagram showing an embodiment of a display position correction circuit 4 in FIG. 1, and FIG. 3 is a black level correction circuit of the present invention. 5 is a waveform diagram for explaining FIG. In FIG. 1, the same parts as those in FIG. 4 are designated by the same reference numerals and the description thereof will be omitted.

In the black level correction circuit of the present embodiment shown in FIG. 1, the black level detection reference voltage output from the voltage source V1 and the display position correction signal output from the display position correction circuit 4 are added by the adder 5 And the black level detection circuit 3
Is input to As will be described later, this display position correction signal changes the detection level of the black level detection circuit 3 in accordance with the display position of the video display screen, whereby the black level output from the black level detection circuit 3 is changed. The control signal changes according to the display position on the video display screen. Then, the brightness signal is controlled by the black level control circuit 2.

Next, a specific configuration of the display position correction circuit 4 shown in FIG. 1 will be described with reference to FIG. In FIG.
One end of the inductor L1 is connected to the capacitor C1 whose one end is grounded and the base of the NPN transistor Q1, and a blanking pulse having a horizontal cycle is input to the inductor L1. The collector of the transistor Q1 is connected to the power supply voltage Vcc, and the emitter is connected to the current source I1 whose one end is grounded and the resistor R1. The other end of the resistor R1 is connected via a resistor R2 to a voltage source V1 whose one end is grounded. The voltage source V1 is the black level detection reference voltage described above.

A blanking pulse having a horizontal period as shown in FIG. 3A is waveform-shaped by a low-pass filter composed of an inductor L1 and a capacitor C1, and a horizontal video image as shown in FIG. 3B is obtained. During the period, the central portion of the display position is low and the end portion of the display screen has a high voltage waveform. The waveform shown in FIG. 3B becomes the display position correction signal. Here, since the blanking pulse of the horizontal cycle shown in FIG. 3A is obtained from a known circuit, the description of its generation method and the like will be omitted. Then, the display position correction signal is transmitted to the emitter of the transistor Q1 by the emitter follower composed of the transistor Q1 and the current source I1. The display position correction signal and the black level detection reference voltage V1 are
1 and R2 are added by a ratio determined by
The black level detection reference voltage whose display position has been corrected is output to the contact point between 1 and the resistor R2.

This display position-corrected black level detection reference voltage is connected in place of the black level detection reference voltage V1 of the black level detection circuit of FIG. At the end of the position, the black level cannot be detected unless the black signal is lower, and the higher the black signal can be detected, the closer to the center of the display screen. Therefore, the detection level of the black level detection circuit 3 can be changed according to the display position on the video display screen, and even if a black telop appears at the edge of the screen, more optimal black level correction is possible. Becomes

The display position correction circuit shown in FIG. 2 is a case where the display position correction is performed in the horizontal period, and it is also possible to perform it in the vertical period or in both the horizontal period and the vertical period. Furthermore, this display position correction is a method in which the detection level of the black level detection circuit 3 is continuously changed. However, by changing the waveform shaping circuit of the display position correction, the black level of only a specific part of the screen is changed. It is also possible to carry out detection. As described above, the present invention is not limited to the embodiments shown in FIGS. 1 and 2, and various modifications can be made without departing from the scope of the present invention.

[0015]

As described in detail above, since the black level correction circuit of the present invention is configured as described above, the detection level of the black level detection circuit is changed in accordance with the display position of the video display screen. Therefore, an optimum black level control signal is always obtained corresponding to the display position of the video display screen, and an appropriate black level correction is possible, which is a very excellent effect in practical use.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a black level correction circuit of the present invention.

2 is a circuit diagram showing an embodiment of a display position correction circuit 4 in FIG.

FIG. 3 is a waveform diagram for explaining a black level correction circuit of the present invention.

FIG. 4 is a block diagram showing a conventional black level correction circuit.

5 is a specific circuit diagram of the black level detection circuit 3 in FIGS. 1 and 4. FIG.

[Explanation of symbols]

 1 Clamp circuit 2 Black level control circuit 3 Black level detection circuit 4 Display position correction circuit

Claims (1)

(57) [Claims] 1. A clamp circuit which clamps and outputs a pedestal level of an input luminance signal, and a black which obtains an output luminance signal by controlling a black level of the luminance signal output from the clamp circuit according to a black level control signal. A level control circuit,
Input the black level detection reference voltage and the output luminance signal,
In a black level correction circuit configured by comparing the two and a black level detection circuit for supplying the black level control signal to the black level control circuit, a display position correction corresponding to a display position of a video display screen. A display position correction circuit that generates a signal, and the display position correction signal and the black level detection reference voltage are added to generate a display position corrected black level detection reference voltage, which is supplied to the black level detection circuit. And a black level correction circuit.