JP2515869B2 - Video signal processing circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal circuit, and more particularly to saturation prevention of a cathode ray tube cathode drive transistor.

[Conventional technology]

It is known that long-term use of television receivers deteriorates the electron emission from the prawn tube cathode. Further, the television video signal circuit is generally provided with a beam current limiting circuit for detecting the cathode ray tube beam current and limiting the beam current.

In a television receiver designed in this way, if the emission luminance signal level is large and the beam current limiting circuit normally works and the beam current is limited, if the cathode emission degradation occurs, the emission degradation will occur. The beam current reduction circuit causes the beam current limiting circuit to become inoperative.

Therefore, the video signal circuit operates to increase the cathode amplitude until the beam current limiting circuit operates.

This state progresses with the deterioration of the cathode emission. Finally, the collector voltage of the cathode drive transistor falls below the base voltage and becomes saturated. When saturation occurs, the Ts (storage time) of the transistor becomes long and the image quality deteriorates significantly.

Similarly, when the power switch is turned on when the transmission luminance signal level is high, the cathode ray tube heater is turned on,
Since the beam current is decreasing during the transition period until it is sufficiently warmed up, the cathode amplitude may operate with a large amplitude and may be saturated according to the same principle as the cathode emission deterioration.

In order to solve this problem, the conventional device is provided with a clip circuit so that the collector voltage of the driving transistor does not decrease until it reaches the saturation region, as in Japanese Patent Laid-Open No. 62-199194.

[Issues to be solved]

In the above conventional technique, the high frequency component of the video signal is reduced by the junction capacitance of the clipping diode. In addition, during the clip operation, the waveform is flattened, and there is a risk that the image quality is degraded.

It is an object of the present invention to reduce video high frequency components and prevent image quality deterioration due to clip operation.

[Means for solving the problem]

In order to achieve the above object, in the device of the present invention, the peak voltage between the color difference signals of the three colors, which is the input signal of the base electrode of the cathode drive transistor, is detected, and this amplitude voltage and the negative voltage of the input signal are also detected. When the level of the potential difference from the luminance signal becomes equal to the magnitude of the input voltage corresponding to the saturation of the cathode drive transistor, the color saturation control voltage and the contrast control voltage are controlled so as to reduce the color saturation and the contrast.

[Action]

When the voltage exceeds the drive transistor saturation voltage, the input amplitude is reduced, so that the image quality is not deteriorated due to the frequency characteristic deterioration in the saturated portion. Further, since the peak voltage between the three color signals and the color difference signals is detected and the saturation is monitored by this voltage and the brightness signal amplitude, the saturation in the single color can be prevented without malfunction.

〔Example〕

One embodiment of the present invention is shown in FIG. The part surrounded by the dotted line is the present invention part.

FIG. 2 is a diagram showing a state in which a signal for displaying vertical stripe color bars is received on the screen.

FIGS. 3 and 4 are diagrams showing voltage waveforms of respective portions of FIG. 1 when receiving the color bar of FIG.

In FIG. 1, 1 is a color band pass block, 2 is a color demodulation block, 3 is a luminance contrast block, 4 is a variable color saturation VR, 5 is a variable contrast VR, 6, 7 and 8 are cathode drive transistors, 9, 10, 11 is collector resistance, 12, 13, 14 is emitter resistance, 15 is luminance signal transistor, 16 is cathode ray tube, 17,
18,19 is the bias setting resistance of the drive transistor, 20,21,22
Is a diode, 23 is a zener diode, 24 is a resistor, and 25 is N.
PN transistor, 26 is an emitter resistance, 27 is a collector resistance, 2
8,30,32,33 are resistors and 29,31 are diodes. The color components from the composite video signal are separated by the color band pass block 1 and then converted by the color demodulation block 2 into color difference signals of RY, GY, BY, and cathode drive transistors 6, 7 respectively. , Is given to the base of 8.

The luminance component is led to the base of the luminance output transistor 15 after its amplitude is adjusted by the luminance contrast block 3.

The amplitude of each color difference signal guided to the bases of the transistors 6, 7 and 8 and the amplitude of the luminance signal guided to the base of the transistor 15 determine the magnitude of the current flowing through the resistors 12, 13 and 14, The gain is almost collector resistance / emitter resistance. The gain is generally about 40 times.

Transistor saturation is a phenomenon that occurs when the collector voltage drops below the base voltage, and generally begins to occur when the collector voltage falls below about 10V. The input amplitude at which saturation begins is determined to be (200-10) v / 40 times = 4.75V.

The resistors 17, 18 and 19 absorb variations in the cathode-ray cathode cutoff voltage and match the low-brightness cutoff characteristic.

When the voltage of the color saturation VR4 supplied to the block 1 becomes higher, the amplitude of each color difference signal becomes larger and the color becomes darker.

Further, when the voltage of the 5 contrast control VR supplied to the block 3 becomes higher, the amplitude of the brightness signal becomes larger and the brightness becomes brighter.

The case where the color bar shown in FIG. 2 is received will be described in detail. 3, each color-difference signal is a negative-polarity luminance signal.

The respective color difference signals are guided by the diodes 20, 21, 22 and the peak value between the signals appears. This signal is shown in the waveform diagram. It is a voltage dropped by the diode VF (0.7V).

The voltage setting value of the Zener diode 23 is determined so that the transistor 25 is turned on when the voltage difference between this voltage and the luminance signal shown by is 4.05 V under the saturation condition. For explanation, FIG.
It is assumed that a potential difference of 4.05 V or more occurs in the A and B portions of the waveform.

(A−B> 4.05V.) At this portion, the transistor 25 is turned on and the collector voltage is lowered.

As shown in FIG. 3B, since the collector voltage is lowered, the diodes 29 and 31 are turned on, and the color saturation control voltage and the contrast control voltage are also lowered as shown by. In this part, the color becomes lighter and the brightness also decreases. This can prevent saturation of the cathode drive transistor.

In this circuit, when an excessive input with an amplitude difference of 4.05V or more is applied, the ON voltage of the transistor 25 rises by the voltage drop of the emitter resistor 26, and the potential difference voltage that turns ON is 4.05V. If it increases more and there is a danger of saturation, in this case increase the value of the resistor 21 and increase the sensitivity, prevent it by lowering the set value of the Zener diode in anticipation of an excessive signal. Can be done.

Further, in the configuration of the present invention, if the G-Y signal is small and there is no possibility that the drive transistor will be saturated, the diode 21
Of course it does not matter if you delete it.

Further, from the above description, it is of course considered that the collector voltage drop of the transistor 25 is peak-held for several tens of ms, and the peak-held DC voltage controls the voltage in DC. To be

In addition, as a second embodiment, due to the difference in the color signal processing circuit,
The color difference signal and the luminance signal are combined in an integrated circuit, etc.
Even in a drive configuration in which a primary color signal is obtained, this primary color signal is input to the base of an inverting amplification transistor, an inverted output is obtained from the collector, and the cathode is driven by this output, between each color primary color signal input to the base. Beam value is detected,
When the peak input amplitude reaches the magnitude of the input amplitude commensurate with the saturation of the cathode drive transistor, the color saturation and the contrast are reduced.

As described above, in the description of the embodiment, the signal of the transistor 25 superimposed on the control voltage from the control voltage is delayed in phase as compared with the composite video signal input to the color band pass block 1 and the brightness contrast block 3. Naturally conceivable.

In the actual measurement, the delay in the color band pass block 1 and the color demodulation block 2 is 200 ns, and the delay in the luminance contrast block 3 is about 60 ns. Naturally, the smaller the phase shift delay, the better the performance.

In the case of standard speed TV (15.734KHz), it was confirmed that the above values can be used practically.

A means for further reducing the phase delay is to shorten the loop from the saturation detection section to the feedback control section, and the following means is naturally conceivable.

In the cathode drive stage configuration, a demodulation signal of the primary color is obtained from the color signal processing circuit, this primary color signal is input to each base of the driving inverting amplification transistor, a common bias DC voltage is applied to the emitter, and the collector is supplied from the collector. Get the inverted output,
This output is used to drive each cathode ray tube cathode, and the increase in the amplitude of the primary color base input voltage is detected. When the amplitude exceeds the voltage and saturation, the bias DC potential common to the emitter of this transistor is raised. To do so. The phase delay is small and the collector amplitude is suppressed for all three colors, and the white balance does not change.

〔The invention's effect〕

The present invention has an effect that it can be constructed inexpensively without causing image quality deterioration.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is an explanatory view of a television receiving screen for explaining the present invention, and FIG.
A and FIG. 3B are waveform charts showing voltage waveforms of respective portions of the circuit of FIG. 1 ... Color band pass block 2 ... Color demodulation circuit 3 ... Luminance contrast block 6,7,8 ... Cathode drive transistor 16 ... CRT 23 ... Zener diode 25 ... NPN transistor

Claims (2)

(57) [Claims] 1. A plurality of NPN transistors to which the same power supply voltage is applied, and R-on a base electrode of each NPN transistor.
A means for giving Y, G-Y, and BY color difference signals and means for giving a negative luminance signal from the emitter electrode of each transistor via a resistor are provided to the collector electrode to obtain a primary color signal. In the drive circuit for driving the cathode ray tube cathode, the peak value between the color difference signals of the three colors is detected, and the potential difference level between this voltage and the negative polarity luminance signal is the input amplitude of the cathode drive transistor saturation. A video signal processing circuit comprising means for controlling the color saturation control circuit and the contrast control circuit so as to reduce the color saturation or the contrast when the size is reached. 2. A plurality of NPN transistors to which the same power supply voltage is applied are provided, and R, G, B are provided at the bases of the respective NPN transistors.
The primary color signal is given, the inverted amplified output is obtained from the collector, the input peak value in the three color primary color signals is detected in the circuit that drives the cathode with this output, and this peak value causes the saturation of the driving transistor. A video signal processing circuit characterized by controlling a color saturation control circuit and a contrast control circuit so as to reduce the color saturation or ton trust when the input amplitude becomes suitable.