CRT common video circuit analysis

The video circuit is one of the important components of the display circuit. The performance of electrical appliances is directly related to the quality of the display image, and the quality of the image is a reflection of the overall performance of the display. At present, the display video loop generally includes the following components: a video signal input interface circuit, a video signal processing circuit, a video amplification output circuit, a brightness control circuit, a contrast control circuit, an ABL circuit, and a blanking and highlight circuit. The following sections will be introduced separately .

The video signal is sent from the display card to MONITOR, and its signal processing process is shown in the block diagram 1 above. The video signal of the graphics card was a composite synchronous video signal in the early days. Now it is changed to separate synchronous R, G, and B video signals. First, go to the connector of the VIDEO board, and send the R, G, and B signals to the interface circuit of the VIDEO board on the connector. ; Then the video signal is preprocessed, that is, the VPP is 0.7V to amplify the video signal to about 4V; then the video amplification output circuit, the VPP is amplified from about 4V to about 60V to drive the CRT cathode, and reproduce the image we want to display. The following is a brief introduction of each circuit:

1. Input interface circuit

After years of development, the interface circuit has gradually improved from simple to complex. The S790N model uses a strong function interface circuit, and its circuit is shown in the circuit diagram. Among them, R801 is the matched impedance of 75 ohm coaxial cable, D801 and D802 are overvoltage protection (including too high or too low protection); capacitor C801 plays the role of coupling the signal to IC801.

2. Video signal processing

There are two kinds of ICs used by AOC for founder's machine video signal processing, that is, S790N's video signal processing IC uses Mitsubishi's MM1381, D551V model LM1269NA. The following first analyzes the video loop of the S790N model.

The functions of each pin of MM1381 are as follows:

Pin1 OSD Rin

Pin2 OSD Gin

Pin3 OSD Bin

Pin4 OSD input

Pin5 Video Rin

Pin6 power supply

Pin7 GND

Pin8 Video Gin

Pin9 power supply

Pin10 GND

Pin11 Video Bin

Pin12 internal reference voltage

Pin13 contrast control

Pin14 OSD contrast control

Pin15 clamp

Pin16 blanking

Pin17 G clamp capacitor

Pin18 R output

Pin19 G clamp capacitor

Pin20 G output

Pin21 GND

Pin22 power supply

Pin23 B output

Pin24 B clamp capacitor

Pin25 Cutoff point adjustment

Pin26 R driver

Pin27 G driver

Pin28 B driver

MM1381 working principle and main features:

There are three same channel amplifiers inside MM1381, each channel has contrast control circuit, which can synchronously change the gain of the three amplifiers to achieve contrast adjustment; light balance adjustment circuit, which can adjust the light balance through the CPU; in addition there are clamp control, elimination Implicit control.

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