JPH0119794B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a black level correction circuit, and more particularly to a black level correction circuit that can suppress the black level correction operation to a constant position regardless of the manual contrast adjustment position.

A black level correction circuit is used in a video circuit of a television receiver to correct the black level of a receiving screen. In this case, the conventionally commonly used black level correction circuit detects the black level in periods other than the horizontal and vertical blanking periods of the video signal, extracts the black level detection voltage, and adjusts the voltage to the video circuit according to this voltage. By controlling the pedestal clamp circuit and the gain control circuit, respectively, when the black level detection voltage changes toward the white side of the video signal, the pedestal clamp circuit corrects the video signal by shifting it toward the black side. At the same time, the contrast is made constant by controlling the gain of the gain control circuit in the direction of increasing it.

However, in the later-generation black level correction circuits mentioned above, a gain control circuit similar to contrast control is provided after the pedestal clamp circuit provided in the video circuit, so contrast can be manually adjusted. This causes a problem in that the relationship between the black level correction amount and the contrast deviates, which necessitates manual adjustment of the black level.

Therefore, an object of the present invention has been made to solve the above-mentioned problems, and hereinafter, a black level correction circuit according to the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a black level correction circuit according to the present invention. In the figure, 1 is a buffer amplifier for amplifying the video signal A, which has the video signal A as its base input, and a transistor 1b connected between the power supply +V _cc and the ground via a resistor 1a, and the transistor 1b. A series body of a resistor 1c and a capacitor 1d that supplies an output signal to the pedestal clamp circuit 2, and a resistor 1 connected between the connection point of the resistor 1c and the capacitor 1d and the ground.
e, and a capacitor 1f that supplies the output signal of the transistor 1b to the horizontal/vertical reverse blanking circuit 3.
It is composed of: And this horizontal
The vertical reverse blanking circuit 3 receives the output signal of the buffer amplifier 1 as a base input via a resistor 3a, and has an inverting amplification transistor 3d connected between the power supply + _Vcc and ground via resistors 3b and 3c. Connected between the collector of transistor 3d and ground, and reverse blanking pulse B
The collector output of the transistor 3d is used as the reverse blanking output. 4 is a pedestal clamp circuit,
A clamping transistor 4b connected between the input terminal of the horizontal/vertical reverse blanking circuit 3 and the ground via a resistor 4a, a resistor 4c supplying power +V _cc to the emitter of the transistor 4b, and a resistor 4a.
The capacitor 4d is connected in parallel to the capacitor 4d. 5 is a black level detection circuit, which has a reverse blanking output as its base input, and a black level detection transistor 5b connected between the power supply voltage +V _cc and ground via a resistor 5a.
and a capacitor 5c connected in parallel to the resistor 5a. A black level output control circuit 6 controls the clamp level and gain of the pedestal clamp circuit 2 and the gain control circuit 11 connected to the subsequent stage in accordance with the output signal of the black level detection circuit 5. The black level output control circuit 6 includes a transistor 6b whose base input is the output signal of the black level detection circuit 5 via a resistor 6a, a resistor 6c connected between the emitter of this transistor 6b and ground, and a collector resistor. 6d and a diode 6 which adds an output signal of a contrast adjustment level detection circuit 10 (described later) to a black level detection signal and supplies the result to a transistor 6b.
It is composed of e. Reference numeral 7 denotes a manual brightness adjustment circuit, which is composed of a variable resistor 7a connected between the power supply + _Vcc and ground. 8 is a clamp voltage control circuit, which includes a resistor 8a that synthesizes the emitter output of the transistor 6b constituting the black level output control circuit 6 and the output signal of the manual brightness adjustment circuit 7, and supplies the synthesized signal to the pedestal clamp circuit 2;
8b. Reference numeral 9 denotes a passive contrast adjustment circuit which divides the power supply +V _cc into a part of the contrast control signal that is supplied from the collector of the transistor 6b constituting the black level output control circuit 6 to the gain control circuit 11 via the resistor 6d. It is constituted by a series body of a resistor 9a that applies pressure and a variable resistor 9b. Reference numeral 10 denotes a transistor 10b whose base input is a contrast control signal supplied from the manual contrast adjustment circuit 9 to the gain control circuit 11 via a resistor 10a, and a collector connected between the power supply +V _cc and ground via this transistor 10b. It is constituted by a resistor 10c, and the collector output from the transistor 10b is supplied to the black level output control circuit 6 as a contrast adjustment level detection signal.

In the black level correction circuit configured in this way, the video signal A shown in FIG.
After being amplified by the transistor 1b constituting the circuit, the signal is supplied to the pedestal clamp circuits 2 and 4, respectively. In the pedestal clamp circuit 4, the video signal A shown in FIG. 4a supplied from the buffer amplifier 1 is horizontally and
The pedestal level of the video signal A supplied to the vertical reverse blanking circuit 3 is clamped. The video signal whose pedestal level has been clamped is inverted and amplified by the transistor 3b, so that the output signal waveform of the transistor 3b becomes as shown in FIG. 4b. If the black level is simply detected, it will be held by the synchronizing signal portion shown in FIG. 4b, making it impossible to accurately detect the black level. For this purpose, a reverse blanking pulse B is supplied to the base of the transistor 3e and becomes low level during the blanking period including the synchronizing signal portion of the video signal A shown in FIG. 4c, which is supplied from a gate pulse generation circuit (not shown). As a result, blanking is applied to the signal shown in FIG. 4b outputted from the transistor 3d. Then, in the signal waveform supplied from the horizontal/vertical reverse blanking circuit 3 to the transistor 5b constituting the black level detection circuit 5, as shown in FIG. Only. Next, the black level detection circuit 5 detects the black level using the signal output from the horizontal/vertical reverse blanking circuit 3, thereby making it possible to accurately detect the black level.

Therefore, the horizontal/vertical reverse blanking circuit 3, the pedestal clamp circuit 4, and the black level detection circuit 5 constitute a black level detection section that detects the black level in periods other than the horizontal and vertical blanking periods of the video signal. This means that The black level signal outputted from this black level detection circuit 5 is transmitted to a transistor 6 which constitutes a black level output control signal 6.
b, the emitter output is amplified in the clamp voltage control circuit 8, and the manual brightness adjustment circuit 7
After being combined with the output of the video signal, the signal is supplied to the pedestal clamp circuit 2 as a clamp control signal, and the black level of the video signal is corrected. Further, the collector output of the transistor 6b is combined with the output signal of the variable resistor 9b in the manual contrast adjustment circuit 9, and then supplied as a contrast control signal to the gain control circuit 11 to vary the gain. Contrast control of the video signal is performed. In this case, the gain control circuit 11 for contrast adjustment
Since this corresponds to the downstream side of the pedestal clamp circuit 2 provided for black level adjustment, manual adjustment of the contrast will cause the black level to fluctuate. However, in this circuit, the transistor 10b constituting the contrast adjustment level detection circuit 10 detects the contrast adjustment level and feeds the contrast adjustment level detection signal to the input side of the black level output control circuit 6 via the diode 6e. are doing. As a result, the brightness variation due to manual contrast adjustment is corrected, and the corrected black level output control circuit 6
The pedestal clamp circuit 2 is controlled by the output signal of , and the brightness is controlled.

Figure 2 shows the reference level of each color signal relative to the pedestal level. Assuming that there is a setup level of +5% (0 to 10%) in addition to the black level, there will be a maximum difference of 10% depending on the setup level. become.

In contrast, in the present application, the setup level is detected and reverse blanking is applied to this period, thereby eliminating fluctuations in the black level due to the setup level.
In this way, it becomes possible to reproduce black that is closer to black, and it becomes possible to reproduce a screen with a three-dimensional effect.

Figure 3 shows the relationship between the black level and the contrast with respect to the blackest level of the video signal. In a conventional circuit that does not perform black level correction, as shown by dotted line A, While the black level signal fluctuates greatly as the level fluctuates,
When the circuit shown in Figure 1 is used, as shown by the solid line B in Figure 3, within the range where the maximum black level of the video signal fluctuates up to approximately 15% IRE, the black level will change. By keeping the correction value approximately constant and further raising the white level that has decreased due to the amount of correction, it is possible to make the contrast constant as shown by the solid line C in FIG.

As explained above, the black level correction circuit according to the present invention has a simple configuration even when the gain control circuit for contrast control is provided at the downstream side of the pedestal clamp circuit for brightness control. However, it has an excellent effect of making the black level correction amount constant regardless of contrast adjustment.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the black level correction circuit according to the present invention, FIG. 2 is a diagram showing the relationship between the pedestal level and each color signal, and FIG. 3 is the black level correction circuit shown in FIG. 1. FIGS. 4a to 4d are characteristic diagrams showing the relationship between the conventional black level correction circuit and the conventional black level correction circuit, and FIGS. 4a to 4d are operation waveform diagrams of each part of the circuit shown in FIG. 1 is a buffer amplifier, 2 and 4 are pedestal clamp circuits, 3 is a horizontal/vertical reverse blanking circuit,
5 is a black level detection circuit, 6 is a black level output control circuit, 7 is a manual brightness adjustment circuit, 8 is a clamp voltage control circuit, 9 is a manual contrast adjustment circuit, 10
11 is a contrast adjustment level detection circuit, and 11 is a gain control circuit.

Claims (1)

[Claims] 1. A black level detection section that detects the black level of the video signal other than the horizontal and vertical blanking periods, and a black level detection section provided in the video signal system that detects the black level in accordance with the black level detection signal supplied from this black level detection section. a black level output control circuit that performs gain control for a clamp level of a pedestal clamp circuit for adjustment and a gain control circuit for contrast adjustment provided at a stage subsequent to the pedestal clamp circuit;
In a black level correction circuit including a manual contrast adjustment circuit that varies gain control for the gain control circuit by the black level output control circuit,
A black level correction circuit comprising a contrast adjustment level detection circuit that detects the output level of the manual contrast adjustment circuit and supplies the detected output level to the input side of the black level output control circuit.