JPH05122547A - Video amplifier circuit - Google Patents

Abstract

PURPOSE:To reproduce an image having high fidelity without black floating or black fullness by executing stable retrace line erase not depending on the change of a luminance adjustment variable resistor to fix a black level while defining the retrace line period of a video signal for driving the cathode of an image receiving tube as a fixed voltage at all times. CONSTITUTION:Horizontal and vertical composite retrace line pulses C are inputted to a constant current circuit 9 of differential amplifiers 11 and 14 to input DC regenerative video signals A and defined as a video signal D always fixing the black level fixing the retrace line period of the video signal A to a prescribed voltage Vcc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to blanking of a video amplifier circuit in video equipment such as a television receiver and a display monitor.

[0002]

2. Description of the Related Art When a television signal to be received is weak, or when the life of a picture tube is approaching and the screen is dark due to a reduction in emission of electrons, a blank line (diagonal white line) appears on the screen when the brightness of the receiver is adjusted to be bright. This is because the black level of the video signal which is adjusted to be equal to or lower than the cutoff of the picture tube changes and the scanning line in the blanking period appears, resulting in deterioration of the image quality of the display screen.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional example, and an image amplifier circuit for fixing a black level by always setting a constant voltage as a blanking period of an image signal for driving a cathode of a picture tube. It provides stable blanking that does not depend on changes in the brightness control volume.

[0004]

According to the present invention, in a video amplifier circuit of a television receiver, horizontal and vertical composite retrace pulses are input to a constant current circuit of a differential amplifier which receives a DC reproduced video signal as input. The video signal is characterized in that the blanking period of the video signal is fixed to a predetermined voltage so that the black level is always constant.

[0005]

The monostable multivibrator circuits 1 and 2 for inputting the horizontal synchronizing signal Hsync shown in FIG. 1 to generate a horizontal retrace pulse, and the monostable multivibrator circuit 3 for receiving a vertical synchronizing signal Vsync and producing a vertical retrace pulse. , 4, a mixer 5 of a horizontal retrace pulse 18 and a vertical retrace pulse 20, differential amplifiers 11 and 14 having a constant current circuit 9, a clamp circuit 17 for direct current reproduction of a video signal VIDEO, and the like. 2A
When a video signal A having a blanking period BW in which the black level BL is clamped is input to the differential amplifier 11 and the horizontal / vertical composite retrace pulse C shown in C of FIG. Below the conduction level L of the circuit 9, the differential amplifiers 11 and 14 are cut off during the blanking period BW to be fixed at a predetermined voltage Vcc, and
The video signal D shown by D in FIG. 2 in which the black level BL is always constant at the conduction level L is output from the load resistor 12 of the differential amplifier 11.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a video amplifier circuit having a blanking function that fixes a blanking period of a video signal to a predetermined voltage and always keeps a black level constant. Further, FIG. 2 shows an operation waveform diagram of each part of FIG. 1 in FIG. 1 is the time constant when the horizontal sync signal Hsync is input.
A monostable multivibrator (MM) circuit which outputs a phase width pulse B of a horizontal scanning (1H) period by R1 and C1 and 2 receives the output pulse B of the MM circuit 1. An MM circuit that outputs a blanking pulse 18 of width,
Reference numeral 3 is an MM circuit which receives the vertical synchronizing signal Vsync as an input and outputs a phase width pulse 19 of a vertical scanning period according to time constants R3 and C3.
Is an MM circuit that receives the output pulse 19 of the MM circuit 3 and outputs a retrace pulse 20 having a vertical retrace period width by time constants R4 and C4. Reference numeral 5 is a horizontal retrace pulse 18 of the MM circuit 2 output and an output of the MM circuit 4 The mixer which mixes with the vertical retrace pulse 20 of and outputs the composite retrace pulse C, 17 is the clamp circuit of the DC level black level BL for superimposing the DC component on the video signal VIDEO, 6 is the coupling capacitance, and 9 is Constant current circuit of differential amplifiers 11 and 14 composed of base bias resistors 7 and 8 and emitter bias resistor 10; 1
1, 14 are collector loads 12, 13 and base bias resistors 15, 16
It is a differential amplifier consisting of.

The video signal VIDEO is reproduced by the clamp circuit 17 as a direct current, and the video signal A having the blanking period BW width in which the black level BL shown in FIG. A five-output horizontal / vertical composite retrace pulse C shown in FIG. 2C (only the horizontal retrace pulse is shown in the figure) is input to the base of the constant current circuit 9. Since the collector current i1 + i2 = i3 shown in FIG. 1 flows in the differential amplifiers 11 and 14, the input retrace pulse C of the constant current circuit 9 is cut off during the blanking period BW,
The video signal D output from the load 12 of the differential amplifier 11 is fixed to a predetermined voltage Vcc level (black level rather than black) during the blanking period BW as shown in D of FIG. Conduction level L of pulse C (emitter voltage VE of constant current circuit 9
The black level BL is always kept constant at + base-emitter voltage VBE), and the video signal D shown in D of FIG. 2 is output from the load resistor 12 of the differential amplifier 11 to the cathode of the picture tube CRT. That is,
The blanking period BW of the video signal is always fixed to a predetermined voltage Vcc (black level higher than black), and the black level of the video signal is outside the blanking period BW.
By outputting the image part with a constant BL from the differential amplifier 11, it is possible to reproduce a faithful image without black floating or black crushing on the screen.

[0008]

As described above, according to the present invention, the horizontal and vertical composite retrace pulses are input to the constant current circuit of the differential amplifier which receives the DC reproduction video signal, and the retrace period of the video signal is set to the predetermined voltage. The black level fixed at is always constant to realize a stable blanking that does not depend on the change in the brightness adjustment volume of the video amplifier circuit, and reproduces a faithful image without black floating or black crushing. be able to.

[Brief description of drawings]

FIG. 1 is a block diagram of a video amplifier circuit having a blanking function.

FIG. 2 is an operation waveform diagram of each part of FIG.

[Explanation of symbols]

 1 monostable multivibrator circuit 2 monostable multivibrator circuit 3 monostable multivibrator circuit 4 monostable multivibrator circuit 5 mixer 9 constant current circuit of differential amplifier 11 differential amplifier with 14 17 DC regeneration clamp circuit

Claims (2)

[Claims] 1. In a television receiver, a horizontal / vertical composite retrace pulse is connected to a constant current circuit of a differential amplifier that receives a DC reproduced video signal output from a pedestal clamp circuit or the like, and the differential amplifier output video signal is connected. A video amplifier circuit which keeps the black level of the signal constant while keeping the blanking period at a predetermined voltage. 2. A horizontal synchronizing signal is connected to the first monostable multivibrator circuit in which a first monostable multivibrator circuit and a second monostable multivibrator circuit are connected in series, and a vertical synchronizing signal is connected to a third monostable multivibrator circuit. The monostable multivibrator circuit and the fourth monostable multivibrator circuit are connected in series to the third monostable multivibrator circuit, and the horizontal retrace pulse of the output of the second monostable multivibrator circuit and the A vertical retrace pulse of the fourth monostable multivibrator circuit is connected to the mixer, and a composite retrace pulse of the mixer output of a differential amplifier that receives the DC reproduction video signal of the output such as a pedestal clamp circuit is input. It is connected to a constant current circuit to keep the blanking period of the differential amplifier output video signal at a predetermined voltage and to keep the black level of the signal constant at all times. An image amplification circuit characterized in that