JPH0229274B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention provides an automatic method for automatically limiting the brightness of an RGB image when a television receiver is receiving an RGB signal such as a character multiplex signal or a captain signal. This invention relates to a brightness limiting device.

[Prior Art] Currently, in many television receivers, the beam current of the picture tube is adjusted to a specified value by detecting changes in the current flowing through the flyback transformer, that is, the anode current of the picture tube, and feeding it negative feedback to the video amplification circuit. Bright, easy-to-see images are obtained using an automatic brightness limiting circuit that limits the brightness below.

[Problem to be solved by the invention]

By the way, personal computers and television receivers equipped with input terminals for RGB signals based on character multiplexed signals are currently being developed, but since this input terminal is provided just before each electrode of the cathode ray tube, the RGB signals mentioned above are The automatic brightness limiting circuit for TV signals did not work, making the images difficult to see.

Therefore, an object of the present invention is to provide an easy-to-see image when receiving the RGB signals of the character multiplex signal and the captain signal by inputting the RGB signals of the character multiplex signal and the captain signal from the RGB input terminal of a television receiver. An object of the present invention is to provide an automatic brightness limiting device that can

[Means to solve the problem]

The automatic brightness limiting device of the present invention includes an automatic brightness limiting circuit that changes the brightness of a TV image by changing the DC level of an automatic brightness limiting terminal, and a contrast of RGB images by changing the DC level of an RGB contrast variable terminal. and an RGB contrast variable circuit that reduces the brightness of the television image and increases the anode current of the picture tube by changing the direct current level of the automatic brightness limiting terminal in response to an increase in the anode current of the picture tube. Said in response to
a DC level variable circuit that reduces the contrast of the RGB image by changing the DC level of the RGB contrast variable terminal;
The apparatus further includes an automatic brightness limiting control circuit that prohibits the DC voltage applied from the DC level variable circuit to the contrast variable terminal from responding to an increase in the anode current of the picture tube only when an RGB image is received in superpause mode.

[Effect]

According to the configuration of the present invention, the average beam current when receiving an RGB signal is also automatically detected and fed back to the RGB contrast variable circuit, and the contrast of the RGB signal is controlled.
The average beam current during RGB input can be kept below a certain level.

As a result, the conventional automatic brightness limiting circuit does not work.
Even when receiving RGB signals, it prevents excessive average beam current from flowing in scenes with high contrast, making it possible to obtain images that are easy to view.

〔Example〕

The drawing shows a circuit diagram of an automatic brightness limiting device according to an embodiment of the present invention. In the drawing, 1 is a flyback transformer, and 2 is an automatic brightness limiting circuit for TV signals. Changes in the current flowing through the flyback transformer 1 are inputted via a capacitor 3 and resistors 4 and 5, and the circuit automatically limits the brightness of the TV signal. conduct. 6 is
It is a variable resistor that changes the RGB contrast.
By changing this, the DC level of the RGB contrast variable terminal _T1 changes via the diode 7, and the output level of the RGB signal of the switching circuit 8, which switches and outputs the RGB signal and TV signal, changes in proportion to this. do. Transistor 9 is for RGB contrast feedback, and the divided voltage of resistor 10 and variable resistor 6 is applied to its emitter. The base of the transistor 9 is connected to the input terminal of the automatic brightness limiting circuit 2 for TV signals via resistors 11 and 12 and a diode 13. The collector of transistor 9 is
It is connected to the collector of the transistor 14 for RGB automatic brightness limit control. The emitter of transistor 14 is grounded, and the base is connected to a terminal.
An RGB automatic brightness limit control signal is input from _T2 via a resistor 15. This RGB automatic brightness limit control signal is at a low level when the teletext broadcasting and the captain are receiving an image in super pause mode, that is, a mode in which the teletext screen is displayed statically superimposed on the normal TV screen. This is a signal that is high level at other times.

Next, the operation of this circuit will be explained. now,
When an image with a large RGB contrast is being received, the beam current increases and the voltage at the connection point TPD ₁ decreases, and when the beam current exceeds a certain level, it changes from positive to negative (normally, the voltage at the connection point TPD 1 decreases).
TPD ₁ is a positive voltage and diode 13 is cut off), diode 13 is conductive. If at this time, a high level is applied to the base of transistor 14,
When the RGB automatic brightness limit control signal is applied (when not in superpause mode), transistors 9 and 14 are turned on, the emitter potential of transistor 9 decreases, and the voltage at the RGB control variable terminal _T1 decreases. As a result, the RGB output level becomes smaller and the brightness of the image decreases.

Next, when receiving an image in superpause mode, the base of transistor 14 is at LOW level, transistors 9 and 14 are turned off,
RGB automatic brightness limit operation is not performed.

As described above, in this embodiment, when an RGB signal is received in a mode other than the superpause mode, the switching circuit 8 is switched in response to an increase in the anode current flowing through the cathode ray tube by the switching operation of the transistors 9 and 14.
Since the DC level of the RGB control variable terminal _T1 is lowered to reduce the contrast of the RGB image and to limit the brightness, it is possible to receive a bright and easy-to-see image, and when in super pause mode, transistor 14 is turned off. Since the brightness is not limited by this, the characters can be bright and easy to see even in such a case.

〔Effect of the invention〕

According to the automatic brightness limiting device of the present invention, even when receiving an RGB signal where the conventional automatic brightness limiting circuit does not work, it is possible to prevent an excessive average beam current from flowing in a high contrast scene and obtain an easy-to-see image. There is an effect.

[Brief explanation of drawings]

The drawing is a circuit diagram showing the configuration of an automatic brightness limiting device according to an embodiment of the present invention. 1... Flyback transformer, 6... Variable resistor, 7... Diode, 8... Switching circuit, 9...
Transistor, 10-12... Resistor, 13... Diode, 14... Transistor, 15... Resistor.

Claims (1)

[Scope of Claims] 1. An automatic brightness limiting circuit that changes the brightness of a television image by changing the DC level of an automatic brightness limiting terminal; and changing the contrast of an RGB image by changing the DC level of an RGB contrast variable terminal. an RGB contrast variable circuit that reduces the brightness of the television image by changing the direct current level of the automatic brightness limiting terminal in response to an increase in the anode current of the picture tube, and also responds to the increase in the anode current of the picture tube; and said above
a DC level variable circuit that reduces the contrast of the RGB image by changing the DC level of the RGB contrast variable terminal;
and an automatic brightness limit control circuit that prohibits the DC voltage applied from the DC level variable circuit to the contrast variable terminal from responding to an increase in the anode current of the picture tube only when an RGB image is received in superpause mode. Device.