One disadvantage of baseband transmission is its poor anti-interference ability. The shielding layer of Coaxial Cable has a poorer shielding effect on electromagnetic waves with lower frequencies, so it is susceptible to broadcast interference and low-frequency electromagnetic waves. So, what kind of interference is prone to Coaxial Cable transmission?
When the Coaxial Cable is installed overhead, the cable itself becomes a very long antenna. When it is induced by broadcast electromagnetic waves, a potential difference is induced. This potential difference is generated at both ends of the cable shielding layer (the core wire is also induced Potential difference, but very small), then the shielding layer, signal source internal resistance, core wire and core wire, 75 ohm load, and shielding layer form a loop. This potential difference forms an interference current through the loop and is formed on the load resistance of 75 ohms. The interference voltage drop is superimposed on the video signal. This kind of interference generally ranges from several hundred KHz to several MHz, and produces relatively stable moire interference on the image. The higher the interference frequency, the finer and denser the moire. The interference greater than 10MHz does not basically affect the viewing effect.
The way to suppress this interference is to lay the cable buried in the ground, or use a lead-clad cable, or use a symmetrical balanced cable with an outer shield as the transmission line. When only coaxial transmission is used, the shielding layer of the cable should be grounded at a single end, and a symmetrical input cable compensator should be installed at the receiving end. The high-level transmission method can also suppress broadcast interference well. The method is to amplify the 1Vp-p video signal to 5 to 8Vp-p and then transmit it. The interference level at the receiving end is reduced relative to the video signal. , The transmission distance can also be longer.
Low frequency interference mainly refers to 50Hz power frequency interference. This kind of interference causes the image to produce horizontal black scroll bars, and in severe cases, the image cannot be viewed and desynchronized. The main cause of 50Hz interference is the ground potential difference. In places where there are many electric equipment and high power equipment, due to the three-phase unbalance or different grounding methods, a large ground current will be formed. When this current passes through the ground with ground resistance, a voltage will be formed between the two grounds. If both ends of the cable are grounded, a current will be formed on the cable through the internal resistance of the signal source, causing interference.
The method to suppress this interference is to ground the cable at one end.
The characteristic impedance of the Coaxial Cable is 75 ohms. Due to the wide video bandwidth, the impedance of the Coaxial Cable at low frequency and high frequency is not the same, and it is impossible to match. But the details of the image are in the frequency domain above 1MHz, so ensuring high-frequency impedance matching can basically meet the transmission requirements. Even if there is a slight mismatch in the low-frequency band, it will not cause obvious ghosting distortion to the image.
Impedance mismatch often causes interference from several vertical bars with equal spacing, and the frequency is basically an integer multiple of the horizontal frequency. The solution is generally to improve the method of "start-end series connection resistance" or "terminal parallel connection resistance".
Other interferences using Coaxial Cable transmission are often caused by excessive transmission distance, excessive loss, low cable quality, unclean power supply caused by the use of high-power thyristor equipment, etc. These interferences are relatively easy to solve, such as Install cable compensator, use clean power supply, select high-quality cable, etc.
The above is the interference that Coaxial Cable transmission is prone to. Because the image signal has a large amount of information, a wide bandwidth, and strong intuition during monitoring, the focus of the transmission is the transmission of the video image signal.