JPH0323735Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to an automatic brightness limiting device that combines an average value type automatic brightness limiting circuit with a white peak suppression circuit.

[Prior Art] An automatic brightness limiting device for a television receiver limits the beam current flowing through the picture tube to below a specified value when the brightness increases abnormally, and controls the horizontal output transistor based on the overload of the high voltage output circuit. Various types of circuits are known, including an average value type that detects the average brightness of the screen and lowers the contrast of the contrast circuit.

The conventional automatic brightness limiting device 1 shown in FIG. 2 is mainly configured with an average value type automatic brightness limiting circuit 2. In order to suppress the blooming phenomenon caused by the white peak when the average value brightness level is low, A peak suppression circuit 3 is combined. As is well known, the average value type automatic brightness limiting circuit 2 uses a clamping diode D to clamp the output of the power supply + _B1 of the beam current flowing through the secondary winding of the flyback transformer 4.
It is clamped to the cathode potential of diode D + _B2 . Then, when the anode potential of diode D drops below + _B2 due to excessive beam current flowing, the average brightness value with the opposite polarity to the average brightness stored in capacitor _C1 connected to the anode of diode D is detected. The contrast of the contrast circuit 5 is lowered by lowering the output, lowering the base voltage for the contrast control transistor _Q1 , and lowering the contrast control output obtained as the emitter voltage of the transistor _Q1 .

In the white peak suppression circuit 3, a capacitor _C2 that stores a negative polarity white peak value through peak rectification is connected to the emitter of a transistor _Q2 to which the output of the video amplifier circuit 6 is applied. The transistor Q ₃ functions to draw out a collector current from the contrast circuit 5 according to the terminal voltage of the capacitor C ₂ . Transistor _Q4 receives the brightness average value detection output stored in capacitor _C1 in automatic brightness limiting circuit 2 as its base, and its emitter voltage is connected to a resistor.
The voltage is divided by R ₁ and R ₂ and applied to the base of transistor Q ₃ .

Therefore, the white peak suppression circuit 3 is a transistor
For Q ₃ , compare the luminance average value detection output, which has the opposite polarity to the base-injected average luminance, and the white peak detection output, which has the opposite polarity to the emitter-injected white peak, and calculate the output according to the difference between the two outputs. The contrast of the contrast circuit 5 will be controlled.

[Problems to be Solved by the Invention] The conventional automatic brightness limiting device 1 has a contrast circuit 5, an automatic brightness limiting circuit 2 and a white peak suppression circuit 3, which are closely related to each other.
Since two circuits that achieve a common purpose are connected separately, there are problems such as a complicated device configuration.

[Means for solving the problem] This invention solves the above problem, and detects the average brightness from the beam current flowing through the picture tube, and when this average brightness exceeds a certain value, contrast control is performed. An average value type automatic brightness limiting circuit that lowers the contrast of the screen by output, detects the white peak of the video signal, and monitors the difference between the white peak detection output, which has the opposite polarity to the white peak, and the contrast control output. The present invention is characterized in that it includes a white peak suppression circuit that apparently increases the average brightness as the difference increases, lowers the contrast of the screen by the contrast control output, and suppresses the white peak. .

[Operation] In this device, a white peak suppression circuit connected to an average value type automatic brightness limiting circuit monitors the difference between the white peak detection output, which has the opposite polarity to the white peak extracted from the video signal, and the contrast control output. However, in particular, when the average luminance is low (that is, the contrast control output is high) and the white peak is large (that is, the white peak detection output is low), the larger the difference, the higher the average luminance apparently increases, and the contrast increases. The control output lowers the contrast of the screen and suppresses the white peak, thereby performing a combination of average value type luminance restriction and white peak suppression.

[Example] Hereinafter, an example of this invention will be described with reference to FIG. FIG. 1 is a circuit diagram showing an embodiment of the automatic brightness limiting device of this invention. In FIG. 1, the same components as in FIG. 2 are given the same reference numerals.

In FIG. 1, an automatic brightness limiting device 11 has a simplified configuration of a white peak suppression circuit 13 connected to an average value type automatic brightness limiting circuit 12. That is, the collector of the transistor Q ₃ , whose collector was conventionally connected to the contrast circuit 5, is connected to the positive terminal of the capacitor C ₁ that stores the detected output of the average brightness value, which has the opposite polarity to the average brightness, and the collector of the transistor Q Of the voltage dividing resistors R ₁ and R ₂ that determine the base voltage of transistor _{Q 3} , resistor R ₁ is connected to the emitter of transistor Q ₁ .

Therefore, the conventionally used transistor _Q4 becomes unnecessary. In this example, transistor Q ₃
According to the emitter-injected white peak and the base-injected contrast control output, which have opposite polarity to the transistor Q ₃
The base bias of is determined. If the base bias corresponding to the difference between the contrast control output and the white peak detection output increases, the base current of the transistor _Q1 , which is shunted by the collector current of the transistor _Q3 , decreases, and the average brightness value stored in the capacitor _C1 decreases. The detection output decreases, and the apparent average brightness detected by the automatic brightness limiting circuit 12 increases.

Therefore, if there is a spot-like high brightness area on a screen with a low average brightness level, the contrast control output will be sufficiently high, but the negative polarity white peak detection output applied to transistor _Q2 will be large, and the capacitor C ₂ terminal voltage decreases. As the terminal voltage of capacitor C ₂ decreases, the collector current of transistor Q ₃ increases, so the base current of transistor Q ₁ becomes small. the result,
With the decrease of the emitter current of transistor _Q1 ,
The contrast control output of the contrast control circuit 5 also decreases, and the white peak is suppressed by decreasing the contrast.

This effectively prevents the blooming phenomenon caused by white peaks when the average luminance level is low.

[Effects of the invention] As explained above, according to this invention, when the average brightness detected from the beam current flowing through the picture tube exceeds a certain value, an average value type that lowers the screen contrast by contrast control output is provided. The automatic brightness limiting circuit monitors the difference between the white peak detection output, which has the opposite polarity to the white peak obtained from the video signal, and the contrast control output, and as this difference increases, the average brightness apparently increases, increasing the contrast. Since we have connected a white peak suppression circuit that uses control output to lower the screen contrast and suppress white peaks, we have replaced the average value type automatic brightness limiting circuit and white peak suppression circuit, which were previously connected separately to the contrast circuit. This provides excellent effects such as being able to simplify the circuit configuration by combining the circuits into a single circuit.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the automatic brightness limiting device of this invention, and FIG. 2 is a circuit diagram showing an example of a conventional automatic brightness limiting device. 5...Contrast circuit, 11...Automatic brightness limiting device, 12...Automatic brightness limiting circuit, 13...White peak suppression circuit.

Claims (1)

[Scope of utility model registration request] An average value type automatic brightness limiting circuit that detects the average brightness from the beam current flowing through the picture tube and lowers the screen contrast using a contrast control output when this average brightness exceeds a certain value, and detects the white peak of the video signal. The difference between the white peak detection output, which has a polarity opposite to that of the white peak, and the contrast control output is monitored, and as this difference becomes larger, the average luminance is apparently increased, and the contrast of the screen is increased by the contrast control output. An automatic brightness limiting device comprising a white peak suppression circuit that lowers the brightness and suppresses the white peak.