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By PURPLELEC | 15 December 2023 | 0 Comments

Transmission technology of VGA signal extender

  VGA signals include R/G/B/H/V, which are three primary colors and horizontal and vertical synchronization signals. Although the VGA cable contains 15 wires, there are only 5 wires in the VGA cable that actually transmit image signals. Therefore, to see if the VGA cable is good, first look at the core quality of the five wires used to transmit RGBHV.
  Although the VGA wire core is very small and has relatively large attenuation, there is basically no problem with the VGA wire during short-range transmission. 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, when the transmission interval 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 cables in a relatively complex environment. The VGA cable is easy to break when turning, and there are many other problems. .
  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 equipment)
VGA signal extender
  1. VGA signal amplifier
  It adopts a simple amplification principle, either amplifying the signal at the sending end or amplifying the attenuated signal at the receiving end. The method of amplification at the receiving end was abandoned as soon as it came out, because it will amplify the interference in the transmission, including crosstalk between internal signals. Equipment that uses transmitter amplification can transmit the computer's VGA video signal up to tens of meters after using a special VGA video cable as the transmission medium. However, people then 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:
  1. The VGA head is also very prone to various problems when welding.
  2. It cannot resist interference and cannot eliminate crosstalk. The VGA cable itself determines that its long-distance transmission internal crosstalk and common-mode interference are very large. As the distance increases, some interference that was not very strong in the past also becomes stronger in long-distance wires. As a result, some test environments can be successful but there is no way to apply them in actual projects, resulting in serious rework or failure to accept acceptance. loss. Therefore, the method of exceeding 30 meters distance should not be adopted.
  3. Long-distance VGA cables are thick and hard and difficult to find. They need to be customized at the factory. They bend sharply and are prone to internal fractures, making wiring extremely inconvenient.
  2. RGB transmission technology
  According to the principle that VGA signals are divided into five types of RGBHV signals, people separate the VGA cable and use five coaxial cables for transmission. This transmission method is called RGB transmission. This method effectively solves the problem of attenuation. At the same time, the shielding layer of the coaxial cable also has a certain inhibitory effect on crosstalk. However, because the principle of transmission technology has not fundamentally changed, the crosstalk problem has not been truly solved.
  Due to the complex environment of the construction site, the five RGB cables transmitting RGBHV signals have inconsistent lengths and different arrival times during wiring, causing the RGB three primary colors and horizontal and vertical signals to not arrive simultaneously. There is a scene on the screen where three colors cannot overlap or even be displayed. This is the out-of-sync problem that easily occurs in RGB transmission.
  3. VGA twisted pair transmission technology
  In order to solve these transmission problems, in the past two years, a VGA video transmission technology that uses ordinary network cables (twisted pairs) as the transmission medium has quickly become a hot spot in the current VGA video transmission technology.
  This transmission method uses a transmitter and a receiver. The transmitter re-encodes the VGA video signal and transmits it as a differential signal on the network cable, and the remote receiver decodes and restores the VGA video signal. The video differential technology used in network cable transmission means that when each pair of twisted pairs transmits a signal, they simultaneously send out signals with the same waveform and opposite polarity. In this way, the external signals of a pair of twisted pairs cancel each other to zero. Technically, it solves the crosstalk problem within the VGA signal.

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