Working principle of VGA video transmission technology

VGA Signal Amplifier

VGA signal transmission is a hot topic in recent video signal transmission. Various transmission methods have attracted the attention of many engineers, and various publicity have also blurred the normal judgment of engineers. As a manufacturer of twisted pair transmission, we would like to make a brief discussion on the development and principle of VGA transmission, hoping to clarify possible misjudgments!

VGA signals include R/G/B/H/V, which are the three primary colors and horizontal and vertical synchronization signals. Although VGA cables contain 15 wires, only 5 of them actually transmit image signals. Therefore, to see whether a VGA cable is good or not, we must first look at the quality of the cores of the five wires used to transmit RGBHV.

Although the VGA wire core is very small and has a relatively large attenuation, there is basically no problem with VGA wires when transmitting over short distances. In the early days, in order to solve the problem of long transmission distances, the wire core diameter 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, it is extremely difficult to route thick VGA wires in a relatively complex environment. The VGA cable is easy to break when turning, and there are many other problems (such as external interference, poor welding points, etc.).

In order to solve the VGA video transmission problem 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 equipment)

A) VGA signal amplifier

It uses a simple amplification principle, either to amplify the signal at the sending end or to amplify the attenuated signal at the receiving end. The amplification method at the receiving end was abandoned as soon as it came out, because it would amplify the interference in the transmission, including the crosstalk between internal signals. The equipment using the sending end amplification can transmit the computer's VGA video signal for dozens of meters after using a special VGA video cable as the transmission medium. But then people made the VGA cable core thicker and thicker, and the shortcomings of not changing the principle of VGA transmission technology became more and more obvious:

First, long-distance VGA cables are thick and hard, not easy to find, and need to be custom-made at the factory. They are also prone to internal breakage due to sharp turns, making wiring extremely inconvenient.

Second, VGA headers are prone to various problems when soldering (such as standing wave interference, cold soldering, etc.).

Third, it cannot resist interference and cannot eliminate crosstalk. The VGA cable itself determines that its internal crosstalk and common mode interference are very large during long-distance transmission. As the distance increases, some interference that was not very strong originally also becomes strong in the long-distance cable, resulting in some successful test environments but no way to apply them in actual projects, resulting in huge losses such as rework or failure to accept. Therefore, this method should not be used for distances exceeding 30 meters.

B) RGB transmission technology

According to the principle that VGA signals are divided into five signals, RGB, HV, people split the VGA cable and use five coaxial cables for transmission. This transmission method is called RGB transmission. This method effectively solves the attenuation problem (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 inhibitory effect on crosstalk. However, since the transmission technology principle has not changed fundamentally, the crosstalk problem has not been really solved! Moreover, after the RGB transmission distance reaches a certain distance, due to the complex construction site environment, the lengths of the five RGB cables transmitting RGBHV signals are inconsistent and the arrival time is different during wiring, resulting in the failure of the RGB three primary colors and the line field signals to arrive synchronously. The three colors on the screen cannot overlap or even cannot be displayed. This is the problem of asynchrony that is easy to occur in RGB transmission, and it is difficult to find a cheap solution to this problem.

C) VGA twisted pair transmission technology

In order to solve these transmission problems, a VGA video transmission technology that uses ordinary network cables (twisted pair cables) as the transmission medium has rapidly become a hot spot in 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. The remote receiver decodes and restores it to a VGA video signal. The network cable transmission uses video differential technology. When each twisted pair transmits a signal, it simultaneously sends out signals with the same waveform and opposite polarity. In this way, the signals emitted by a pair of twisted pairs will cancel each other out to zero, which technically solves the crosstalk problem within the VGA signal.

There are only 4 pairs of twisted pairs in the network cable. What should we do when using the network cable to transmit the five VGA signals? In the early days, we divided the RGB in the VGA signal into 3 pairs of twisted pairs for transmission, and the remaining pair was used to transmit the HV signal.

The length of each twisted pair in the network cable is also different. Isn't there also a problem of asynchrony? Solution: line compensation! Unlike RGB cable transmission: there are international standards for the length difference of each twisted pair in the network cable. We can know the approximate difference in the line length of each twisted pair according to this standard, and perform line compensation based on this length. In this way, the problem that VGA signal cannot solve using RGB method can also be solved here!

Other advantages of VGA video twisted pair transmission technology

The differential technology used has strong anti-interference performance. With some technical processing, the anti-interference performance is greatly improved. Even in the worst power plant strong electric and magnetic environment, it can be used 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 performance of the product shows the level of the R&D personnel who designed the circuit. Different products have different anti-interference effects. Users with conditions should do interference tests before choosing products. Users without conditions should test them on site or in worse conditions than on site before finalizing the purchase. In this way, the quality of the project can be recognized by Party A and the cooperation between the two parties can be guaranteed. It is cheaper to use network cables as wire materials. The crystal head is easier to make than the welding of VGA or RGB cables. The wire is soft and the wiring is convenient. At present, the price difference between VGA video extenders and VGA long-line drivers is not large. The farther the distance, the lower the wire cost, the better the anti-interference effect, and the higher the cost performance. It can be said that it is a trend for VGA video extenders to replace VGA signal amplifiers at a distance of more than 30 meters.

Technology is constantly improving, and twisted pair transmission technology is also constantly improving! When VGA video transmission technology can be solved by twisted pair transmission, technicians are thinking about whether a pair of twisted pairs can be used to transmit more signals: there are only 4 pairs of twisted pairs in the network cable, and the RGB in the VGA signal is transmitted by 3 pairs of twisted pairs, and the remaining pair transmits the HV signal. When we need other functions of the computer at the same time, this method will not work. The technicians re-encode the H/V signal, mix it into the RGB signal for transmission, and then decode it at the receiver. In this way, only 3 pairs are needed to complete the transmission of the five signals of R/G/B/H/V of VGA. We can use the saved pair of wires to transmit audio or RS485 signals or mouse and keyboard signals, or even USB signals. Of course, the simultaneous transmission of the three signals of mouse, keyboard and VGA is more technically difficult and requires stronger anti-interference. Our 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 products with better performance and more comprehensive functions.