Look at the VGA signal transmission technology to solve the transmission problem

VGA signal transmission is the hotspot of recent video signal transmission. Various transmission methods have attracted the attention of many engineering companies. Various publicity also blur the normal judgment of engineering vendors. As a manufacturer of twisted pair transmission, VGA The development and principle of transmission is a small discussion, hoping to clarify everyone's possible misjudgment!

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The VGA signal contains five kinds of R/G/B/H/V, which are three primary colors and a line sync signal. Although the VGA cable contains 15 wires, there are only 5 wires in the VGA cable that actually transmit image signals, so look at the VGA cable. First, look at the core quality of the five wires used to transmit the RGBHV.

Although the VGA core is very small, the attenuation is relatively large, and the VGA cable is basically not problematic when transmitted over a short distance. In the early stage, in order to solve the problem of long transmission distance, it is generally to increase the core diameter and make the copper core very thick. However, after the transmission distance is long, the problem of crosstalk between the five signals in the VGA line is serious. At the same time, the coarse VGA cable routing is extremely difficult in a relatively complicated environment. When the corner is turned, the VGA line is easily broken, and other problems are also very much. (eg external interference, poor solder joints, etc.).

In order to solve the VGA video transmission problem in the project, the development of VGA video transmission according to the time sequence is: (VGA cable + VGA amplifier) ​​→ (RGB cable + RGB long-line driver) → (twisted pair + twisted pair transmission equipment)

A) VGA signal amplifier technology:

It uses a simple amplification principle, or amplifies the signal at the transmitting end or amplifies the signal that has been attenuated at the receiving end. The method of amplification at the receiving end is discarded as it amplifies the interference in the transmission, including crosstalk between internal signals. The device that uses the transmitter to amplify the device can transmit the VGA video signal of the computer to several tens of meters after using the special VGA video cable as the transmission medium. But then people have made the VGA wire core thicker and thicker, and the disadvantages of not changing the principle of VGA transmission technology are becoming more and more obvious:

First, the long-distance VGA cable is thick and hard, not easy to find, and needs to be customized at the factory. It is also prone to internal breaks when the corner is sharp, and the wiring is extremely inconvenient.

Second, the VGA head is also very prone to various problems during soldering (such as standing wave interference, soldering, etc.).

Third, it cannot resist interference and cannot eliminate crosstalk. The VGA cable itself determines its long-distance transmission of internal crosstalk and common mode interference. As the distance increases, some of the interference that is not very strong is also strong in the long-distance wire, which leads to success in some test environments, but the actual engineering has no way to apply it, resulting in huge rework or unacceptable loss. Therefore, this method should not be used beyond 30 meters.

B) RGB signal transmission technology:

According to the principle that the VGA signal is divided into five kinds of signals of RGBHV, the VGA cable is separated and transmitted by 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), and the shielding layer of the coaxial cable also has a certain suppression effect on crosstalk, but due to transmission There is no fundamental change in the technical principle, and the crosstalk problem has not been solved! Moreover, after the RGB mode transmission distance reaches a certain distance, due to the complicated construction environment, when the wiring of the RGB five transmission RGBHV signals are inconsistent in cable length and different arrival times, the RGB three primary colors and the line signals cannot arrive synchronously. There are three colors on the screen that cannot overlap or even display. This is an easy-to-synchronize problem in RGB transmissions, 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 a common network cable (twisted pair) as a transmission medium has rapidly become a hot spot of the current VGA video transmission technology in the past two years.

The transmission mode adopts a transmitter and a receiver, and the transmitter re-encodes the VGA video signal to transmit the differential signal on the network cable, and decodes and restores the VGA video signal to the remote receiver. The video differential technology used in the transmission of the network cable, when each pair of twisted pairs transmits a signal, the signals with the same waveform and opposite polarity are simultaneously emitted, so that the signals radiated by the pair of twisted pairs cancel each other 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 I use the network cable to transmit the five signals of VGA? In the early days, we transmitted RGB in the VGA signal to three pairs of twisted pairs, leaving a pair of HV signals.

The length of each pair of twisted pairs in the network cable is also different. Is there a problem of not being synchronized? Solution: Line compensation! Different from RGB cable transmission: there is an international standard for the length difference of each pair of twisted pairs in the network cable. We can know the line length of each pair of twisted pairs according to this standard, and perform line compensation according to this length. VGA signal can not be solved by RGB method and is also solved here.

VGA signal twisted pair transmission technology advantages:

The differential technology used has strong anti-interference performance, and with some technical processing, the anti-interference is greatly improved. Even the harshest power plants can use it to transmit VGA video over long distances. 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 of the design circuit. Different products have different anti-interference effects. Conditional users should first do interference test when selecting products. Users without conditions are better to test and re-form on the spot or worse than the site. In this way, the quality of the project can be guaranteed by Party A and the cooperation between the two parties can be guaranteed. The wire is made of wire, and the crystal head is easier to solder 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 the distance is, the lower the wire cost is, and the better the anti-interference effect is, the higher the cost performance is. It can be said that the replacement of VGA signal amplifiers by VGA video extenders over 30 meters is a trend.

Technology is constantly improving, and twisted pair transmission technology is constantly improving! When VGA video transmission technology can be solved by twisted pair transmission, the technician is thinking about whether more signals can be transmitted by a pair of twisted pairs: there are only 4 pairs of twisted pairs in the network cable, and RGB in the VGA signal is 3 For twisted pair transmission, the remaining pair transmits HV signals. When we need other functions of the computer at the same time, this method will not work. The technician re-encodes the H/V signal, mixes it into the RGB signal, and decodes it to the receiver. In this way, only three pairs of VGA R/G/B/H/V signals can be transmitted. Save a pair of lines we can transmit audio or RS485 signals or mouse and keyboard signals, or even USB signals. Of course, the three signals of the 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 twisted pair transmission of various interface signals of computer peripherals and has always been leading in this aspect, which can provide engineering users with products with better performance and more comprehensive functions.

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