Circuit Design of Video VGA Signal Distribution and Transmission Transmitter

The VGA distributor distributes the video signal from one signal source into two or more signals. A common application of high-resolution video distribution amplifier is to amplify the signal from a computer video port and distribute it to two or more high-resolution data display devices while maintaining the original signal quality. The distribution amplifier provides amplification and enhancement functions such as peak and level adjustment and equalization of signals at the same time. Each output of the distribution amplifier is buffered, so the clarity and strength of the original signal can still be maintained during signal distribution to fully achieve the desktop effect. The distributor can be made into 1:2, 1:4, etc. VGA twisted pair transmission has the ability of long-distance driving. It can balance the signals in the form of rgbhv output by the computer host through unshielded category 5 and above twisted pairs (CAT5 / 5E / 6), and transmit 30.0 m with high fidelity. After transmission, the video effect has no ghosting, no tail, the picture is as clear as before, and the wire can reach 500 m in good time. It can be widely used in LCD Plasma flat panel multimedia advertising project.

1 system composition

The system design includes two parts. One part is the local distribution of video signal, which realizes 1:2, 1:4, etc; The other part is the remote transmission system of video signal, which includes local transmitter and remote receiver. The system composition block diagram is shown in Figure 1.

2 VGA signal distribution

VGA signal distribution refers to local distribution, that is, one VGA signal with 2 and 4 displays. VGA signal distribution includes two parts: one is the distribution of video signals R, G and B; The second is the allocation of synchronization signals Hsync and Vsync.

2.1 shunt of synchronous Hsync and Vsync signals

After the horizontal and vertical synchronization signals Hsync and Vsync pass through the 74hc04 high-speed inverter, the output waveform is shaped. After improving the capacity with load, they are connected to the horizontal and vertical synchronization signal lines of output splitter 1 and 2 respectively to realize the 1:2 function of VGA horizontal and vertical synchronization signals. The shunt of synchronization signal is shown in Figure 2.

2.2 distribution of video signals R, G and B

The video signals R, G and B are shunted as shown in Fig. 3. C101 and R107 form a high pass network, and the lower cut-off frequency is 1 / (2 π RC). If C101 is 33 NF and R107 is 150 Ω, the lower cut-off frequency FL = 1 / (2 π RC) = 1 / (2 π & Themes; 150 & Themes; 33 & Themes; 10-9) = 3 215 Hz. The lower cut-off frequency can be changed by changing the values of resistance R107 and capacitance C101. Q100, Q101 and Q102 are high-frequency triodes, and the cut-off frequency shall be greater than the maximum frequency of video signal. The frequency signal is coupled to the base of Q100 through capacitor C101. Q100 is saturated and on. When on, the Vces of Q100 is about 0.3 V, the current flowing through R102 resistor is IC, and the current flowing through R101 resistor is ie. Because IC ≈ ie, the voltage of Q100 collector is. If VCC is set to 5V, VC = 1.76v. Q101 and Q102 constitute two-stage emitter followers. VGA out1 red and VGA out2 red are respectively connected to the red signal input terminals of 1 and 2 VGAs of the output splitter to drive the VGA red signal. The output voltage is

Vcc-vces = (5-0.3) v = 4.7 V, which improves the load capacity of the system. The branching principle of blue signal and green signal of VGA signal is consistent with that of red signal, so as to realize the 1-to-2 function of VGA signal.

3 VGA signal remote transmission system

The VGA signal remote transmission system includes two groups of transmitter and receiver. After differential amplification of the signal at the transmitting end, it is connected to the RJ-45 network port and transmitted to the remote end through Cat-5 twisted pair. The remote receiving end receives the signal from RJ-45 port, restores it into a clear video signal after processing, and accesses the local VGA interface.

3.1 VGA transmitter

In the remote transmission of VGA signal, the impedance of the transmission wire will cause the attenuation of the signal. The longer the transmission distance, the greater the attenuation, resulting in the incorrect reading of the signal. Therefore, the VGA signal needs to be differentially amplified at the transmitting end of remote transmission to improve the signal transmission capacity. El4543 is selected as the differential amplifier at the transmitter of the system. El4543 is a three-way differential twisted pair driver with synchronous coding produced by INTERSIL company. El4543 is a three-way differential amplifier with high bandwidth (350 MHz), which can completely encode the video synchronization signal. Its input is suitable for processing high-speed video or other communication signals in single ended or differential form. High bandwidth is the differential signal on standard twisted pair or coaxial cable, which has very low harmonic distortion. At the same time, internal feedback ensures that the output has stable gain and phase, so as to reduce radiated electromagnetic interference and harmonics. The embedded logic encodes the standard video horizontal and vertical synchronization signals into the common mode signal of the twisted pair. The VGA with synchronization information inputs the RGB signal and is connected with the 75 Ω terminal resistance on the input end of el4543. The single end RGB signal is converted into a differential mode signal. Hsync and Vsync encode the common mode signals of the three differential signals. The 50 Ω terminal output of el4543 drives differential R, G and B, and the synchronization signal is encoded in the common mode of Cat-5 twisted pair cable. The system without signal frequency equalization can transmit well on 200 foot Cat-5 twisted pair. For longer cables, frequency and gain equalization technology (el9111) and delay line technology (el9115) are used at the receiving end to compensate the attenuation of the signal, so as to adjust the phase mismatch of the signal at the receiving end. The transmitter el4543 is driven as shown in Figure 4.

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