VGA signal transmission is the hotspot of recent video signal transmission. Various transmission methods have aroused the attention of many engineering companies. Various publicity has also blurred the normal judgment of engineering companies. As a manufacturer of twisted pair transmission, VGA A small discussion of the development and principles of transmission is hoped to clarify possible misjudgments!
The VGA signal includes five types of R / G / B / H / V, which are the three primary colors and the line and field synchronization signals. Although the VGA cable contains 15 wires, only 5 wires actually transmit image signals in the VGA cable, so first look at the quality of the VGA cable to see the quality of the five wires used to transmit RGBHV.
Although the VGA cable core is very small and the attenuation is relatively large, there is basically no problem when the VGA cable is transmitted over a short distance. In the early days, in order to solve the problem of long transmission distance, the diameter of the wire core was generally increased, and the copper core was made very thick. However, after the transmission distance is long, the problem of crosstalk between the five signals in the VGA cable becomes serious. At the same time, the wiring of the thick VGA cable is extremely difficult in a more complex environment. The VGA cable is easily broken when turning, and there are many other problems. (For example: external interference, bad solder joints, etc.).
In order to solve the problem of VGA video transmission in the project, the development of VGA video transmission in chronological order is: (VGA cable + VGA amplifier) ​​→ (RGB cable + RGB long-line driver) → (twisted pair + twisted pair transmission device)
A) VGA signal amplifier technology:
It uses a simple amplification principle, or amplifies the signal at the sending end, or amplifies the signal that has been attenuated at the receiving end. The method of amplification at the receiving end is discarded as soon as it comes out, because it amplifies the interference in the transmission together, including crosstalk between internal signals. After adopting the special VGA video cable as the transmission medium, the equipment adopting the amplification at the sending end can transmit the VGA video signal of the computer to several tens of meters. But then people made the VGA cable core thicker and thicker, and the shortcomings without changing the principle of VGA transmission technology became more and more obvious:
First, the long-distance VGA cable is thick and hard, and it is not easy to find. It needs to be customized at the factory. It is also prone to internal breakage due to sharp corners, which is extremely inconvenient for wiring.
Second, the VGA head is also very prone to various problems when welding (such as standing wave interference, virtual welding, etc.).
Third, it cannot resist interference and cannot eliminate crosstalk. The VGA cable itself determines that it can transmit internal crosstalk and common mode interference over a long distance, which is very large. With the increase of the distance, some originally not very strong interference also becomes strong in the long-distance wire, resulting in some test environments that can be successful but the actual project has been done but there is no way to apply it, resulting in huge rework or unacceptable acceptance. loss. Therefore, this method should not be adopted if the distance exceeds 30 meters.
B) RGB signal transmission technology:
According to the principle that the VGA signal is divided into five RGBHV signals, the VGA cable is split and transmitted with five coaxial cables. This transmission method is called RGB transmission. This method effectively solves the problem of attenuation (the core of the RGB cable is much thicker than the single core of the VGA cable). At the same time, the shielding layer of the coaxial cable also has a certain suppression effect on crosstalk, but due to the transmission There is no fundamental change in the technical principle, and the problem of crosstalk is not really solved! And after the transmission distance of the RGB method reaches a certain distance, due to the complexity of the construction site environment, there are inconsistencies in the length of the five RGB cables that transmit RGBHV signals when wiring, and the arrival time is different The phenomenon that caused the RGB three primary colors and line and field signals cannot arrive synchronously. On the screen, there is a phenomenon that the three colors cannot be overlapped or even displayed. This is the problem of out-of-sync in RGB transmission, which is difficult to get a cheap solution.
C) VGA signal twisted pair transmission technology
In order to solve the problems in these transmissions, a VGA video transmission technology using ordinary network cables (twisted pair) as the transmission medium has rapidly become a hotspot of the current VGA video transmission technology in the past two years.
This transmission method uses a transmitter and a receiver. The transmitter re-encodes the VGA video signal as a differential signal and transmits it on the network cable, and then decodes and restores the VGA video signal to the remote receiver. The video differential technology used in network cable transmission. When each pair of twisted pairs transmits a signal, they simultaneously send out signals with the same waveform and opposite polarities. In this way, the signals emitted by the pair of twisted pairs cancel each other out to zero. The problem of crosstalk inside the VGA signal is solved.
There are only 4 pairs of twisted pairs in the network cable. What should I do when the network cable is used to transmit the five signals of VGA? In the early days, we divided the RGB in the VGA signal into 3 pairs of twisted pairs, and the remaining pair transmitted HV signals.
The length of each twisted pair in the network cable is also different. Isn't there also a problem of unsynchronization? Solution: line compensation! What is different from the RGB cable transmission is that the length of each twisted pair in the network cable has international standards. , We can know the line length of each twisted pair is roughly different according to this standard, and compensate the line according to this length, so that the VGA signal cannot be solved by RGB method and the problem is also solved here
Advantages of VGA signal twisted pair transmission technology:
The adopted differential technology itself has strong anti-interference performance, plus some technical processing, the anti-interference performance is greatly improved. It can be used to transmit VGA video over long distances even in the harshest power plant strong magnetic environment. Anti-interference is an important indicator to ensure the quality of the project. How to ensure the anti-interference of the product shows the level of the R & D personnel who designed the circuit. Different products have different anti-interference effects. Conditional users are best to do the interference test when selecting products. Unconditional users are better tested on the site or under worse conditions than the site before finalizing the purchase. Only in this way can the quality of the project be recognized by Party A and the cooperation between both parties can be happy. It is cheaper to use the network cable as the wire material. The crystal head is easier to weld than the VGA or RGB cable. The wire is soft and the wiring is convenient. At present, the price difference between the VGA video extender and the VGA long-line driver is not large. The farther away, the lower the cost of the wire. The better the anti-interference effect and the higher the cost performance. It can be said that it is already a trend to replace VGA signal amplifiers with VGA video extenders over 30 meters.
The technology is constantly improving, and the twisted pair transmission technology is also constantly improving! When the VGA video transmission technology can be solved by the twisted pair transmission method, the technical staff is thinking about whether more signals can be transmitted with a pair of twisted pair: There are only 4 pairs of twisted pairs in the network cable, RGB in the VGA signal is transmitted in 3 pairs of twisted pairs, and the remaining pair transmits HV signals. This method will not work when we need other computer functions at the same time. The technician re-encodes the H / V signal, mixes it into the RGB signal and transmits it, then decodes it to the receiver. In this way, only three pairs are needed to complete the transmission of VGA R / G / B / H / V five signals. We can transmit audio or RS485 signals or mouse and keyboard signals through the saved pair of wires, and even USB signals. Of course, the three signals of mouse and keyboard VGA are transmitted at the same time, the technical difficulty is higher, and the required anti-interference is stronger. The company has the technology of transmitting various interface signals of computer peripherals through twisted pairs and has always maintained a leading position in this regard, which can provide engineering users with better performance and more comprehensive products.
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