JPH07105901B2 - Black level correction device - Google Patents

Description

The present invention relates to a black level correction device used in a television receiver or the like.

Conventional Technology FIG. 3 shows a conventional example. A positive video signal is applied to the lead wire 1. In the clamp circuit 4, the pedestal level of the video signal 1 is compared with the reference voltage applied to the terminal 5 by the variable resistor 6, the difference voltage is charged in the capacitor C1, and the output voltage 3 controls the control circuit 10. , And serves to equalize the pedestal potential of the video signal of the conductor 1 with the reference voltage. A clamp pulse for removing the pedestal is applied from the lead wire 2.

Q1 is a PNP transistor that inverts and amplifies the video signal. The output is taken out from the collector and output to the terminal 7 through the emitter follower Q4. The resistor R2 and the NPN transistor Q2 are circuits for black level correction. When the emitter potential of the transistor Q1 becomes lower than that of the transistor Q2, the emitter resistance of the transistor Q1 becomes R1 and R2.
Parallel resistance, the emitter resistance value decreases, the amplification degree increases, and only the signal component near black is expanded. The power supply voltage is resistance-divided by the resistors R7 and R8 and becomes the base voltage of the transistor Q3.

When there are many black signals in the video signal of the lead wire 1, the switch 1 is provided to turn off the black correction circuit in order to eliminate the black correction effect. When the switch 1 is turned on, the base voltage of the transistor Q3 and the base voltage of the transistor Q2 drop, and the emitter voltage of the transistor Q2 becomes
Is lower than the black level of the video signal appearing at the emitter of, and the effect of black level correction is lost. The transistor Q3 compensates the Vbe voltage and temperature characteristics of the transistor Q2.

However, in such a conventional example, when the pedestal level of the output of the terminal 7 is set to a constant voltage (for example, 6V), the CRT is not solved.
Set to the cut-off voltage of (cathode ray tube). The amplitude of the output voltage at terminal 7 changes from black to white, that is, from 0IRE
When 100IRE is set to 3V _PP , the amplification factor in the transistor Q ₁ is tripled, and black expansion is performed only for a signal of 50IRE or less, the emitter potential V _E of the transistor Q 1 becomes (V _cc −2
V), the amplitude at the emitter of the transistor Q1 is 1V _PP , so the point of 50IRE of that signal is V _START = (V _cc -2V) + 1 / 2V = V _E + 0.5V Therefore, the black level start point (SW The values of the resistors R7 and R8 are selected so that the emitter voltage of Q2 when ₁ is OFF) becomes V _E + 0.5V. Figure 4 shows the situation. It is shown in the diagram of a.
The line indicated by the broken line a is the output of 50
The amplification on the black side has increased for signals below IRE.

However, in this circuit, since the black level start point is fixed with respect to the pedestal potential of the terminal 7, when the voltage of the terminal 7 is 6 V, black correction is applied to the signal of 50 IRE or less. When the voltage of is other than 6V, the starting point of black correction is shifted and accurate black correction cannot be performed. For example, the fourth
When the pedestal level of the output is set to 5.4 V as shown in b in the figure, there is a problem that the starting point of black correction becomes 30 IRE.

In order to solve this problem, in the present invention, the pedestal voltage of the emitter of the inversion transistor is higher than the first reference voltage.
By utilizing the fact that it is 1Vbe higher, it is possible to further
The voltage corresponding to 50IRE is obtained by the reference voltage of and the black level start voltage is generated. Therefore, even if the first reference voltage changes, the starting point of black correction depends only on the second reference voltage and not on the first reference voltage.
Kept in.

Action With such a configuration, even if the output pedestal voltage is changed by the first reference voltage, it is possible to prevent the black correction start point for the pedestal of the output signal from changing, and for that reason, it is always appropriate. A good black correction effect can be obtained.

Embodiment FIG. 1 shows an embodiment of the present invention. The same parts as those in FIG. 3 are denoted by the same numbers. The first reference voltage applied to the terminal 5 by the variable resistor 6 is applied to the base of the emitter follower of the PNP transistor Q5. The emitter voltage of transistor Q5 is
Equal to the pedestal potential of the signal appearing at the emitter of Q1.

If the amplitude of the signal at the emitter of the transistor Q1 is 1 V _pp and the black correction start voltage is the point of 50 IRE of the signal, the second
The reference voltage of 0.5 may be 0.5V. As the second reference voltage
Vbe of diode D1 (about 0.7V) is set to R9, R1 to obtain 0.5V.
It is divided by 0. This voltage is applied to the base of the second PNP transistor Q3 and its emitter voltage is applied to the base of the first NPN transistor Q2. Since the emitter voltage of the transistor Q2 is set to 0.5V higher than the pedestal voltage of the emitter of the transistor Q1, the black correction start point can be determined with respect to the pedestal level of the signal regardless of the reference voltage in FIG. . As shown in FIG. 2, black correction can be performed from the point of 50 IRE for a and b two signals having different output pedestals.

Effect of the Invention In the black correction circuit, it is desirable to perform black expansion for a black signal that is equal to or less than a certain portion of the signal. In the conventional circuit, when the pedestal level of the output is changed by changing the first reference voltage, the black correction start voltage is changed and a problem occurs. By using the black correction circuit of the present invention, proper black correction can always be performed.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a black level correction device according to an embodiment of the present invention, FIG. 2 is a characteristic diagram for explaining the black correction effect of the present invention, FIG. 3 is a circuit diagram of a conventional example, and FIG. FIG. 6 is a characteristic diagram for explaining a black correction effect of a conventional example. 1 ... Positive polarity video signal input terminal, 4 ... Clamp circuit, 5 ... First reference voltage, Q1 ... First PNP transistor, Q2 ... First NPN transistor, Q3 ... Second PNP
Transistor, Q5 ... third NPN transistor, D1, R9, R
10 …… Second reference voltage, C1 …… Clamp capacitor, 10
...... Control circuit, SW1 ...... Black correction NO / OFF switch.

Claims (1)

[Claims] 1. A pedestal clamp device for producing a positive polarity video signal having a pedestal level equal to a first reference voltage, the output of which is applied to the base of a first pnp transistor, and a resistor from its emitter. Through
It is connected to the emitter of an npn transistor, the first reference voltage is applied to the base of a third pnp transistor, and a voltage higher than the emitter potential by a second reference voltage is applied to the second pnp transistor.
In addition to the base of the p-transistor, its emitter voltage is added to the base of the npn-transistor so that the black level correction start point of the video output of the negative polarity from the collector of the first pnp-transistor does not depend on the first reference voltage. A black level correction device characterized in that the black level is taken out.