The principle and classification of video anti-jamming device

　　Video anti-interference device is the best auxiliary tool to solve the problem of video interference in video surveillance. At present, our anti-jamming device mainly has two methods, one is the frequency shift anti-jamming method, which can solve the interference relatively completely; the other is this kind of amplification anti-jamming method. In principle, this anti-jamming device is parallel Can not eliminate interference, but it can effectively reduce the amplitude of the interference signal. Assuming that the output amplitude of the video signal is U1 and the amplitude of the interference signal is U2, the amplitude of the video signal reaching the terminal is the sum of the source signal and the interference signal, namely U1+U2 (without considering the attenuation of the signal itself), where the amplitude is The interference signal of U2 causes serious interference to the image displayed on the monitor screen. If the original video signal of the front end is transmitted by an amplifier to amplify its amplitude by n times to Nu1 before transmission, after the interference signal of U2 is also mixed, the signal arriving at the terminal becomes NU1+U2, which obviously exceeds Display the allowable input level requirements of the device, so an attenuator is needed to attenuate the synthesized signal mixed with the interference signal received at the terminal by n times, to obtain (nU1+U2)/n=U1+U2/n, and the result is the terminal The signal amplitude at is restored to U1, and the interference component becomes U2/n. It can be seen that after the signal is first amplified and then compressed, the video signal output to the display device can still maintain the original amplitude, while the interference signal amplitude is reduced by n times. This means that from the monitor screen alone, the original strong interference will become weaker, while the original not too strong interference will not be obvious.

　　species

　　Dynamic zoom

　　Dynamically amplified anti-jamming device, which works on some long distances or poor lines, and some relatively slight strong electricity, elevator interference

　　Frequency shift

　　Frequency shift anti-jamming device, it works against some strong current interference, radio, power plant, elevator interference

　　How the interference occurs:

　　Coaxial cables, whether they have one, two or four shielding layers, are electrically connected to each other as a coaxial outer conductor shielding layer, but the specific structure and thickness are different. In practical engineering applications, interference sources mainly include fluorescent lamp interference, elevator interference, strong current interference, power generation equipment, frequency conversion equipment and radio transmission equipment interference, etc.

　　The camera output video amplitude Vo=2Vp-p, the output impedance is 75Ω, the equivalent impedance of the inner conductor of the coaxial cable is Rc, the equivalent impedance of the outer conductor is Rd, Vi is the induced electromotive force formed by the interference on the longitudinal impedance of the coaxial outer conductor ( The size is proportional to Rd, strictly proportional to the longitudinal reactance Zd), the end device is a Rh=75Ω matched load to the transmission line. Obviously, the signal voltage obtained by the terminal load Rh from the transmission loop is the result of the combined effect of the video signal Vo and the interference electromotive force Vi.

　　Vab=(Vo ×75)÷[75×2+Rc+Rd)] + (Vi×75) ÷[75×2+Rc+Rd)]

　　Among them, the first term is the effective video signal obtained by the load, Voh=(Vo ×75)÷[75×2+Rc+Rd)],

　　The second item is the effective interference signal obtained by the load, Vih=(Vi×75) ÷[75×2+Rc+Rd)], when the cable is short, the resistance of the inner and outer conductors can be ignored, Rc+Rd=0, then , The effective video signal Voh=(Vo ×75)÷75×2+0)= Vo ×75÷75×2= Vo/2=1Vp-p; because the interference induced electromotive force Vi is proportional to (Rc+Rd), at this time Vi=0, Vih =0; it is worth noting that the interference signal Vi is determined by the longitudinal distribution parameters of the cable (impedance or resistance), not a concentrated

　　The point signal source, the important thing is that it is connected in series in the video signal transmission loop. When the load obtains the camera video signal, it must also obtain the interference signal. The nature of interference is "additive interference", and it always exists regardless of whether the video signal is present or not.

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