JPH06178151A - Black level correction circuit - Google Patents

Abstract

PURPOSE:To obtain a circuit in which a black level can be prevented from being fluctuated even when an APL is changed by controlling the source voltage of a video output circuit fitting in the cutoff characteristic of a CRT by providing a power source that changes corresponding to the APL. CONSTITUTION:When the relation of cathode voltage to screen voltage set in a noncorrected state is shown in d(e), a beam current can be increased by increasing the APL, and voltages at both terminals of a resistor 3 are increased. An APL detector 4 detects the voltages, and performs such control to decrease(increase) a video output circuit power source 5. The video output circuit power source 5 decreases(increases) the voltage so as to conform a screen power source 7 and the cathode voltage of a video output circuit 6 to the cutoff characteristic of the CRT 8, and supplies it to the video output circuit 6. Thereby, the relation of cathode voltage to screen voltage of the CRT can always coincide with (a) in figure, which prevents the black level from being changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to correction of black level fluctuation due to APL change in a color television receiver.

[0002]

2. Description of the Related Art Conventionally, in a screen having a high APL, black level fluctuation (black part floating, sinking) has been a problem. This problem occurs when the cutoff adjustment of the CRT deviates from the cutoff point. FIG. 2 shows the cutoff characteristic of the CRT. In FIG. 2, when the relationship between the cathode voltage at the black level and the screen voltage is a, black is proper (black), but in the cases of b and c, the black is sunk and floats, respectively. Generally, the power supply of the video output circuit and the screen power supply are supplied from the flyback transformer of the deflection circuit, so that they fluctuate due to APL changes. Since the CRT cutoff adjustment is performed in a dark screen (low APL), the CRT cutoff adjustment changes like d and e due to an increase in beam current on a high APL screen, and the black portion Each rises and falls. As a technique for solving this problem, there have been methods of adjusting the brightness and correcting the screen voltage according to the beam current. As a conventional technique, there is JP-A-58-118325. In this invention, attention is paid to the fluctuation of the screen voltage due to the APL change, and the black level fluctuation is corrected by keeping this voltage constant. However, since the power supply of the video output circuit also undergoes voltage fluctuations, black level fluctuations also occur. Further, since the screen voltage is corrected, the correction voltage becomes several tens of volts, and there are problems that the cost of parts used is high and the reliability is low.

[0003]

In the conventional black level correction circuit, only the screen voltage is corrected, and it is insufficient to consider that the power supply voltage of the video output circuit changes due to the APL change. As a result, the black level correction was insufficient. Also, it is disadvantageous in terms of cost and reliability because it handles high voltage.

An object of the present invention is to provide a circuit in which black level fluctuation does not occur even if the APL changes, and to reduce the cost and improve the reliability of the circuit.

[0005]

In the prior art, since only the screen voltage was corrected, the correction was insufficient, resulting in high cost and low reliability. In the present invention, the object can be achieved by correcting the power supply voltage of the video output circuit according to the variation of the screen voltage so as to match the cutoff characteristic.

[0006]

By controlling the power supply voltage of the video output circuit according to the APL change, the fluctuations of the cathode voltage and the screen voltage can be matched with the cutoff point of the CRT. As a result, the black level does not change even if the APL changes. Further, since the voltage to be controlled is about 1/10 of the screen voltage, the cost of parts can be reduced and the reliability can be improved.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to FIGS. Figure 2 shows the relationship between the cathode voltage and the screen voltage in the uncorrected state.
In the case of d (e), the beam current increases and the voltage across the resistor 3 increases as APL increases.
The APL detector 4 detects the voltage and detects the video output circuit power supply 5
Control to lower (increase). Video output circuit power supply 5
Supplies the video output circuit 6 with the voltage lowered (raised) so that the cathode voltage of the screen power supply 7 and the cathode voltage of the video output circuit 6 match the cutoff characteristic of the CRT 8. This gives C
The relationship between the cathode voltage of RT and the screen voltage is always the same as that in FIG. 2A, and the black level does not change, and the purpose can be achieved.

Next, an embodiment of the present invention will be described with reference to FIG. Under the same conditions as above, first, the screen power supply 7 is directly supplied from the FBT 2 to the CRT 8. Next, when the APL becomes higher, the beam current increases.
Since the resistance 3 is returned and flows to the CRT 8, a voltage inversely proportional to the APL of V ₂₃ = E−Ib × R is generated at the terminal 23. This voltage is smoothed by the resistors 15 and 10 and input to the base of the transistor 12. The transistor 12, the power supply 11, the resistor 14, and the capacitor 15 have a regulator circuit configuration, and a voltage inversely proportional to APL is output to the capacitor 15. Video output circuit power supply 5 is FBT
The pulse generated in the second coil 18 is smoothed by the diode 16 capacitor 17, added to the output of the APL detection 4, and supplied to the video output circuit 6. Since a constant current flows through the resistor 19 of the video output circuit 6, the cathode voltage changes in the same manner as the power supply voltage changes. This makes it possible to match the cathode voltage and the screen voltage with the cutoff characteristic of the CRT 8. Further, the bell to be corrected can be changed by adjusting the division ratio of the resistors 9 and 10 of the APL detection 4.

[0009]

According to the present invention, even if the APL changes, C
Since the cathode voltage and the screen voltage can be matched with the cutoff characteristic of RT, the black level fluctuation due to the cutoff fluctuation can be completely corrected, and the cost and reliability of the circuit can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a cutoff characteristic graph of a cathode ray tube.

FIG. 3 is a circuit diagram of an embodiment of the present invention.

[Explanation of symbols]

1 ... Power supply, 2 ... FBT (flyback transformer), 3,
9, 10, 14, 19, 21 ... Resistance, 4 ... APL detection,
5 ... Video output circuit power supply, 6 ... Video output circuit, 7 ... Screen power supply, 8 ... CRT (CRT), 11 ... DC power supply, 12, 20 ... Transistor, 13, 15, 17 ... Capacitor, 16 ... Diode, 18 ... Video output circuit FBT coil for power supply, 22, 23 ... Terminals.

Claims (1)

[Claims] 1. In a video output circuit of a color television receiver using a cathode ray tube (hereinafter referred to as CRT), by providing a power supply that changes according to APL, the power supply voltage of the video output circuit is adjusted according to the cutoff characteristic of the CRT. A black level correction circuit characterized by controlling.