JPS61192170A - Black level correcting circuit - Google Patents

Abstract

PURPOSE:To keep a black level setting level always at the level of 20% of a black to white peak regardless of a contrast control by shifting the pedestal setting level of a pedestal clamping circuit corresponding to a contrast control voltage. CONSTITUTION:The gain of a video signal is controlled by a contrast control circuit 2 according to the contrast control voltage of a terminal 5. At this stage, when a contrast control is at maximum, a level conversion circuit 4 sets up a pedestal setting voltage that is converted from the control voltage of the contrast control at a pedestal clamping circuit 3 so that a black level setting level which has a fixed electric potential against a pedestal level is kept at the level of 20% of the black to white peak of the video signal. When the contrast control is at minimum, the level conversion circuit 4 converts the contrast control voltage corresponding to the video signal level outputted from the control circuit 2 and shifts the pedestal level to the side of the black level setting level so that it is kept at the level of 20% of the black to white peak of the video signal outputted from the control circuit 2.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a television receiver having a constant black level;
In particular, the black level is constant regardless of the adjustment position of the contrast control, and the set level is kept constant.

[Conventional technology]

The constant black level is used in the video signal processing circuit of the TV II/receiver to keep the black level of the video signal constant and prevent the black level from rising or sinking on the screen.

The concept of black level correction operation will be explained using FIG. The video signal, as shown by the solid line in FIG. 3(a),
The distance from the black level to the white level of the video signal is 100 yen,
20% from now on based on pedestal level (0chi)
In some cases, there is no black level within this range, and in other cases, there is a black level within 20 degrees from 01, as shown by the dotted line in FIG. 3(a).

When the black level is constant, a black level within 0 to 20% of the horizontal and vertical scanning periods of the video signal is detected, and correction is applied so that this black level becomes a constant level. Third
K as shown by the dotted line in Figure (a).

If the black level is within 0% to 20%, the darkest level is the black level setting level (as shown in Figure 3(b)).
In FIG. 3, the level of the video signal is shifted to 20% (K) of the video signal. On the other hand, if there is no black level within 0 to 2 degrees, as shown by the solid line in FIG. 2A, no level shift is performed. Therefore, 0-20%
Since the darkest level in the image is always kept at a constant level, black floating and black sinking are reduced.

FIG. 2 shows a conventional technique in which the black level is constant. A video signal is input to an input terminal 1, and the contrast control circuit 2 receiving this signal is gain controlled by DC control from a contrast control terminal 5. The output of the circuit 2 is input to a pedestal clamp circuit 3 and a level shift circuit 6. The pedestal clamp circuit 3 needs to fix the pedestal level in order to detect the black level of the video signal, and for this purpose, the pedestal level of the output of the contrast control circuit 2 is fed back so that it becomes the same as the pedestal setting potential 10. multiply. The level shift circuit 6 changes the amount of level shift based on the output from the black level detection circuit 7, and the output is inputted to the black level detection circuit 7.

The black level detection circuit 7 compares the black level in the output signal of the level shift circuit 6 with the black level setting potential 8, extracts a black level detection signal, and outputs it to the level shift circuit 6. Due to the feedback loop in the level shift circuit 6 and the black level detection circuit 7, when there is a black level between the pedestal setting potential 10 and the black level setting potential 8, the dark level becomes the same potential as the black level setting potential 8. When feedback is applied and there is no black level, the level shift circuit 6 does not operate, and the video signal is output to the output terminal 9 as it is.

As described above, the video signal is gain controlled, its pedestal level is clamped, and then the black level is corrected and output.

[Problem that the invention seeks to solve]

When the above-mentioned conventional black level is constant, the pedestal setting potential 10 is a fixed potential, so the pedestal potential of the output of the contrast control circuit 2 is kept constant by the clamp circuit 3 regardless of the contrast control. Further, since the black level setting potential 8 is also a fixed potential, the black level setting level (the 20th level from the black to the white peak of the video signal) is constant regardless of the contrast control. However, the output level after contrast control changes depending on the contrast control voltage from terminal 5. In this way, while the pedestal potential and the black level setting value are constant, the output signal level from circuit 2 is changing, and as a result, the black level setting level shifts, and the black level is fixed. There is a drawback that the amount may deviate.

[Means for solving problems]

The constant black level according to the present invention is achieved by a contrast control circuit that performs gain control through DC control from a contrast control terminal, and a pedestal clamp circuit that sets the pedestal level of its output to a certain level in order to prevent such drawbacks. , a level conversion circuit that converts the level of DC control from the contrast control terminal and supplies it to the pedestal clamp circuit, a black level detection circuit that detects the black level in the horizontal and vertical scanning periods of the video signal, and detection from the black level detection circuit. The present invention is characterized by comprising a level shift circuit whose level shift amount changes depending on a signal.

That is, the present invention creates the pedestal set potential based on the control voltage of the contrast control terminal,
This is supplied to the pedestal clamp circuit. Therefore, the disadvantage that the black level correction amount deviates is prevented.

[Embodiment] Next, an embodiment of the present invention will be described in detail using FIG. 1, but the same parts as those in FIG. 2 are given the same reference numerals and the explanation will be omitted.

In FIG. 1, the contrast control terminal 5 is connected to the contrast control circuit 2 and the level conversion circuit 4, and the level conversion circuit 4 is connected to the pedestal clamp circuit 2.

The operation will be explained with reference to FIG.
a) and (C) are output signal waveforms when there is no black level, and FIG. 4tb) and (d) are output signal waveforms when there is a black level. To simplify the explanation, FIGS. 4(a) and 4(b) are for cases where the contrast control is at its maximum, and FIGS. 4(C) and (d) are for cases where the contrast control is at its minimum.

A video signal is input to the input terminal IK, and the gain is controlled by the contrast control circuit 2 according to the contrast control voltage at the terminal 5. This output is input to a level shift circuit 6 and a pedestal clamp circuit 3. Here, when the contrast control is at its maximum, the level conversion circuit 4 uses the contrast control control voltage so that the black level setting level, which is a fixed potential with respect to the pedestal level, is 20 degrees from the black to white peak of the video signal. is converted to set the pedestal setting potential in the pedestal clamp circuit 3. Therefore, as shown in FIG. 4(a), the pedestal clamp circuit 3 sets the pedestal level to 0 to 20%.
If there is a black level within the range, a negative feedback loop formed by the level shift circuit 6 and the black level detection circuit 7 causes the darkest level to be fixed at the focus level setting level and output as shown in FIG. 4 tbl.

Next, when the contrast control is at the minimum, the level conversion circuit 4 converts the contrast control voltage from the terminal 5 in accordance with the video signal level of the output of the contrast control circuit 2, and shifts the pedestal set potential compared to when the contrast control is at its maximum. . That is, as shown in FIG. 4C, the pedestal level is shifted toward the swallow level setting level so that the black level setting level becomes 20 points from the black to white peak of the video signal output from the contrast control circuit 2. Therefore, even when the contrast control is at its minimum, the black level setting level is correctly set to the level of 20qb of the video signal, and if there is a black level within O~20t4, the darkest level is set as shown in Figure 4(d). The constant black level operates so that the black level is fixed at the black level setting level. With the above operation,
This prevents the drawback that the black level correction amount deviates.

Next, a specific example circuit configuration will be explained using FIG. 1. Circuit blocks corresponding to those in the block diagram of FIG. 1 are surrounded by dotted lines and given the same numbers.

In Figure 5, 1 is an input terminal, 5 is a contrast control terminal, 13, 25.41 is a bias supply terminal, 3
0.33 is a constant current source, 44 is a black level detection pulse input terminal, and 9 is an output terminal.

The contrast control circuit 2 includes transistors 14~
18 and 21 and resistors 19, 22, 23, and 24. Gain control is performed by DC control from the contrast control terminal 5, and the polarity of the input signal is reversed, and the transistor 34 Output from the emitter. The pedestal clamp circuit 3 applies feedback to the transistors 18 and 21 so that the pedestal level of the video signal output to the emitter of the transistor 34 becomes equal to the collector potential of the transistor 29. The level shift circuit 6 includes resistors 48, 50 . ) transistor 49, and the detection output of the black level detection circuit 7 (the amount of level shift changes). Capacitor 46 and resistor 47 for
It operates only during the horizontal and vertical scanning periods by the pulse input to the black level detection pulse input terminal 44.

To simplify the explanation, we will explain the operation of the maximum and minimum contrast controls. First, when the contrast control is at the maximum, transistors 15 and 16 are conductive and the video signal is (Rゎ+Rz4)/Rt.

The gain is controlled so as to double, and an output with inverted polarity is obtained from the emitter of the transistor 34. In the level conversion circuit 4, since the transistor 28 is conductive and the transistor 27 is non-conductive, the pedestal potential of the signal output to the transistor 34 is clamped to the emitter level of the transistor 26, and is input to the level shift circuit 6. . Since the collector potential of the transistor 32 that sets the black level setting level is set by the voltage drop across the resistor 31, the current value of the constant current source 33 and the resistance of the resistor 31 are adjusted so that the voltage is 20 times greater from the black to the white peak of the video signal. Set the value.

Next, when the base potential of the transistor 49 is Ov, the level shift circuit 6 does not operate, and the video signal is input to the black level detection circuit 7, and if there is a black level within 0 to 20 pixels, the transistor 49 only operates during that period. 40 and 38 conduct and charge capacitor 46. The time constant of the resistor 47 and capacitor 46 is set to be long enough for vertical synchronization, and the black level is detected and smoothed to a DC voltage. The transistor 49 is turned on by the black level detection voltage, and due to the voltage drop across the resistor 48, the level of the trap video signal is shifted so that the darkest level of the video signal level becomes equal to the black level setting level, and is output from the output terminal 9.

When contrast control is minimum, transistor 1
4.17 becomes conductive, the gain of the video signal is controlled to be 23/Rtt times, the polarity is reversed, and the transistor 34
From the emitter of
The pedestal clamp level is shifted from the emitter potential by the voltage drop across the resistor 27. At this time, the current value of the constant current source and the resistance value of the resistor 27 are set such that the collector potential (black level setting level) of the transistor 32, which is a fixed potential, is 20% of the black to white peak of the video signal at the minimum contrast control. is set, and the pedestal is clamped so that the output black level setting level of the contrast control circuit 2 and the pedestal level are in a correct positional relationship. Therefore, the black peak of 0 to 20% is fixed at the black level setting level and output as in the case of the maximum contrast control. If there is no black level within 0 to 204, the level shift circuit does not operate and the signal is output as is.

〔Effect of the invention〕

As explained above, the black level according to the present invention is constant.By shifting the pedestal setting level of the pedestal clamp circuit in accordance with the contrast control voltage, the black level setting level can be changed from black to white peak regardless of contrast control. It can be maintained at a level of 204. Therefore, regardless of the contrast control, the amount of correction for a constant black level remains constant, and stable black level correction is performed.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing a conventional example, FIG. 3 is a waveform diagram for explaining the black level correction operation, and FIG. 4 is a block diagram of the conventional example. The waveform diagram used in the explanation, FIG. 5, is a circuit diagram showing a specific example of FIG. 1. 1 is the input terminal, 2 is the control 2nd control circuit, 4
5 is a level conversion circuit, 5 is a contrast control terminal, 6 is a level shift circuit, 7 is a black level detection circuit, 8 is a black level setting potential, 9 is an output terminal, and 10 is a pedestal setting potential. ('/1) (α) $ 3 (mu) Area

Claims (1)

[Claims] A first circuit to which a video signal is input and changes the signal level by a control voltage, a second circuit that clamps the pedestal level of the output of this first circuit, and a second circuit that receives the video signal with the pedestal level clamped. a third circuit that level-shifts and outputs a black level; and a third circuit that detects the black level of the output of the third circuit according to a DC voltage that sets a black level detection level, and sets a level shift amount of the third circuit. and a fourth circuit, the clamp level of the pedestal clamp circuit being changed in response to the control voltage so that the black level is constant with respect to the output signal level of the first circuit. black level correction circuit.