JPS61187472A - Circuit for limiting automatically brightness - Google Patents

Abstract

PURPOSE:To minimize a size change in a transient screen at the time of the sudden change of brightness connecting a resistance/capacitor serial circuit to a resistance for deciding a change in a signal pedestal clamp level in parallel. CONSTITUTION:The serial circuit composed of the capacitor 20 and the resis tance 21 is connected to the resistance 9 deciding the time constant of an auto matic brightness limiter ABL in parallel. The time constant satisfies the resis tance 9 > resistance 21 and the capacitor 5 > capacitor 20. In such a transient moment when a screen is abruptly brightened, the AC time constant of the ABL is approximately decided by the values of the resistance 21 and the capaci tor 20. When the time constant of an automatic contrast control ACC is made smaller, a transient ripple is transmitted to a video amplitude variable circuit. However the ACC time constant is set equal to or larger than the AC ABL time constant, whereby the AC ABL time constant suppresses the fluctuation of brightness, which is made smaller.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to automatic brightness limiting, and more particularly to preventing brightness oscillations that occur when beam current limiting is accelerated.

[Background of the invention]

In the figure, the part excluding the dotted line is a conventional beam current limiting circuit. Although this is a well-known fact, it will be briefly explained.

When the beam current of the cathode ray tube 12 increases, the voltage drop across the resistor 13 increases and the base voltage of the transistor 18 decreases. The emitter voltage also decreases, and as a result, the amplification degree of the video amplifier circuit decreases, the amplitude of the video signal decreases, and the brightness of the screen begins to be suppressed. Here, it is called ACC (Automatic Foot Trust Control). Also, when the brightness increases and the voltage of the capacitor 14 becomes lower than the DC potential determined by the pedestal clamp (the base voltage of the transistor 6), the diode 8 turns on, and the voltage increases through the resistor 9.
It is connected to capacitor 14 and lowers the base voltage of transistor 6. Here, it is called ABL (automatic brightness limiter).

The signal appearing at the emitter of the transistor 6 is connected to the cathode ray tube 12 by DC coupling, and the brightness is suppressed by the above two operations.

The beam current limiting effect starts from ACC and finally from A.
BL is activated and a predetermined beam current value is determined.

Here, in order to accelerate the limiting effect, it is necessary to accelerate ACC. If the capacitance of capacitor 17 is reduced to accelerate ACC, the time constant of capacitor 17 x resistor 16 (ACC time constant'
I&) becomes small, and the ripple voltage of the capacitor 14 that detects the beam current cannot be reached due to the ACC time constant, and is applied to the video amplitude variable circuit.

When the screen brightness suddenly becomes brighter, the transient ripple voltage appearing on the capacitor 14 is transmitted to the variable amplitude circuit, and as a result, it is modulated by the phase lag, resulting in an oscillation phenomenon when the brightness suddenly changes.

For this reason, conventionally, the time constant (
ACC time constant), resistor 9 x capacitor 5 time constant (AB
L time constant)> resistor 13 x capacitor 14 time constant,
The vibration component generated in the capacitor 14 that detects the beam current is not transmitted to the variable amplitude circuit, so that vibration does not occur when the brightness suddenly changes.

Therefore, due to the above-mentioned time constant, the beam current limiting action is delayed, and when the brightness changes transiently, the horizontal output circuit becomes overloaded, the high voltage decreases, and the screen size increases. Then, after the beam current limit takes effect, it returns to its normal size.

The above phenomenon has been a problem for display televisions and the like that require high image quality.

Incidentally, such a conventional circuit is shown on pages 44, 45, and 176 of "Television Technology '769 September Special Issue".

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a circuit that does not cause transient brightness oscillation even if the beam current limiting action is accelerated.

[Summary of the invention]

In order to achieve the above purpose, in parallel to the conventional ABL circuit,
Connect circuits in parallel that are effective only when the screen brightness changes.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

The part surrounded by the dotted line is the proposed part.

A capacitor 2 is connected in parallel to a resistor 9 that determines the time constant of ABL.
A series circuit of 0 and 21 resistors is connected.

Here, the constant is resistance 9>resistance 21. The value of capacitor 5>capacitor 20 is selected.

When the screen suddenly becomes bright, the AC time constant of ABL is
The value is determined by the values of the resistor 21 and the capacitor 20. When the ACC time constant is made small, as explained above, the transient ripple is transmitted to the video amplitude variable circuit, but by making the ACC time constant ≧ the AC ABL time constant,
Brightness oscillations are suppressed and reduced by the AC ABL time constant.

〔Effect of the invention〕

According to the present invention, since a circuit that is effective only when the brightness changes is added, it is not related to the ABL when the brightness is steady. After such a configuration, even if the ACC is accelerated, the brightness fluctuation becomes slight, and the transient screen size change when the brightness suddenly changes can be suppressed to a small level.

[Brief explanation of drawings]

The figure is a circuit diagram showing one embodiment of a beam current limiting circuit according to the present invention. 1...Resistor, 2...Transistor, 3...Resistor,
4... Capacitor, 5... Capacitor, 6... Transistor, 7... Resistor, 8... Diode, 9...
...Resistor, 10...Ara 1' Back transformer, 11.
...High voltage diode, 12...Cathode ray tube, 13...
・Beam current detection resistor, 14... Capacitor, 15.
...Diode, 16...Resistor, 17...Capacitor, 18...Transistor, 19...Resistor, 20...
... Capacitor, 21... Resistor. 7,・′

Claims (1)

[Claims] In a limiting circuit that limits the cathode ray tube beam current mainly based on changes in the video signal amplitude and changes in the signal pedestal clamp level, a series circuit of a resistor and a capacitor is connected in parallel with the resistor that determines the changes in the signal pedestal clamp level. An automatic brightness limiting circuit characterized by: