JPS5925510B2 - gamma adjustment device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to provide a gamma adjustment device that can suppress the peak of a video signal when the average current of the video signal becomes large.

In television receivers, it is desirable for cathode ray tube images to have a small amount of gamma correction in terms of impact, but since the dynamic range of cathode ray tubes is narrow, the amount of gamma correction must be increased to some extent.

Therefore, in the case of a video signal with a bright average brightness, the gamma correction is increased to suppress the white peak part, and in the case of a video signal with a dark average brightness, the gamma correction is decreased to extend the white peak part. The purpose is to make images more powerful, and embodiments of the present invention will be described below with reference to the drawings.

The figure shows one embodiment, in which a gamma correction resistor 3 of about 1 to 5 ohms is connected between the collector of the final stage transistor of the video amplifier circuit and the cathode of the cathode ray tube 2. A series circuit of resistors 4 and 5 having a resistance value about five times as large as the resistance value of the resistor 3 is connected in parallel to the resistor 3.
A capacitor 6 is connected between the connection point between the resistors 4 and 5 and the ground. A series circuit of a transistor T and a resistor 8 whose resistance value is about 10 times the resistance value of the resistor 3 is connected in parallel to the resistor 3, and a series circuit of a transistor T and a resistor 8 whose resistance value is about 10 times the resistance value of the resistor 3 is connected in parallel.
A series circuit with a resistor 10 of about 0Ω is connected. The base of the transistor T is connected to one end of the capacitor 6 via a resistor 12 of about 100Ω, and the base of the transistor 9 is connected to the collector of the transistor T via a resistor 11 of about 100Ω or more. Next, the operation of this circuit will be explained.

The beam current of the cathode ray tube flows from the cathode to resistor 3, resistor 4,
5 towards transistor 1. The current passing through the resistor also flows to the capacitor 6, and is smoothed.
A voltage corresponding to the average brightness level of the video signal appears at both ends of the signal. Therefore, the capacitor 6 and the resistor 5 constitute a detection circuit that detects a voltage corresponding to the average brightness level. Now, when the average brightness level is small, the voltage of the capacitor 6 is small, and the connection point between the resistor 3 and the collector of the transistor 1 is A, and the connection point between the cathode of the cathode ray tube 2 and the resistor 3 is B, the point B is the highest. The potential is high and one end C of the capacitor 6 has a lower potential than point B, and the potential of point A is lower than point C.

When the potential difference between point C and point A is kept below 0.7 volts, transistor T is cut off, the collector potential of transistor T becomes high, and transistor 9 becomes conductive. As a result, resistor 10 is connected in parallel to resistor 3,
Since the parallel resistance becomes smaller, the gamma correction amount becomes smaller.
The white peak of the cathode ray tube image is extended, producing an impressive image. When the average brightness level of the video signal increases, transistor 7 enters the active region, and as a result, transistor 9 also enters the active region, and transistor 9 has a certain resistance value.

Since this resistor is connected in series with the resistor 10, the combined resistance with the resistor 3 becomes larger than in the above case, and the gamma correction amount becomes larger than in the above case. When the average brightness level is very high, transistor 7 conducts and transistor 9 turns off, so that resistor 10 is disconnected from resistor 3. As a result, the gamma correction amount becomes maximum, and the white peak of the image is suppressed. In this way, the gamma correction amount can be automatically adjusted according to the average brightness level of the video signal. In the above description, the transistor 9 is connected in parallel with the resistor 3, but the same configuration can be achieved by connecting the transistor 9 and the resistor 3 in series.

As described above, according to the present invention, when the average brightness level of a video signal is small, the gamma correction amount is reduced to extend the white peak of the image, and when the average brightness level is high, the gamma correction amount is increased to extend the white peak of the image. Since it is pressed, the best image can be obtained depending on the state of the video signal.

[Brief explanation of the drawing]

The figure is a circuit diagram of a gamma adjustment device in one embodiment of the present invention. 1...Video amplification transistor, 2...
Cathode ray tube, 3... Resistor, 4, 5... Resistor, 6... Capacitor, 7, 9... Transistor, 10... resistance.

Claims (1)

[Claims] 1 Connect the output terminal of the video output circuit to the cathode of the cathode ray tube via a resistor, connect a variable impedance element having a control electrode in series or parallel to the resistor, and detect the voltage corresponding to the average brightness level of the video signal. A detection circuit is provided to change the impedance of the variable impedance element according to the output voltage of the detection circuit, and when the output voltage of the detection circuit is large, the output terminal of the video output circuit and the cathode of the cathode ray tube are connected. A gamma adjustment device characterized by suppressing the peak of a video signal by increasing the resistance value between the two.