What is a Cable Transfer?

The transmission system connects the front-end equipment of the monitoring system with the terminal equipment. The image signal, sound signal and various alarm signals generated by the front-end equipment are transmitted to the control center through the transmission system, and the control instructions of the control center are transmitted to the front-end equipment. Depending on the size of the TV monitoring system, coverage area, signal transmission distance, information capacity, different requirements for system functions and quality indicators, and the type of transmission signal, different transmission methods can be used. Because the image signal has a large amount of information, a wide bandwidth, and strong intuitiveness during monitoring, the focus of transmission is the transmission of video image signals.

Chinese name: Coaxial cable transmission

Foreign name: coaxial-cable transmission
Applied discipline: Communication

Coaxial cable transmission

TV monitoring systems are generally small and medium-sized systems of short and medium distances, and almost all use coaxial cables to transmit video image signals. Video baseband refers to the bandwidth of the video signal itself (0 to 6MHz). The video signal is modulated onto a high-frequency carrier using amplitude modulation or frequency modulation, and then transmitted through a cable. The video signal is demodulated after receiving at the terminal. This method is called a modulation transmission method, which can better suppress the baseband transmission method All kinds of interference often exist, and can realize a single cable to transmit multiple video signals. However, in the actual monitoring system, the locations of the cameras are scattered, and the advantages of frequency division multiplexing cannot be exerted, and the addition of modulation and demodulation equipment will also increase the system cost. Baseband transmission is dominant. And most of the high-frequency transmission methods appear in cable television systems. ^[1]

Coaxial cable transmission

Coaxial cable is composed of two coaxially arranged inverted conductors, and the transmitted signal is completely enclosed inside the outer conductor, so it has significant features such as low high frequency loss, strong shielding and anti-interference ability, and frequency bandwidth. From outside to inside, the coaxial cable consists of a copper single wire, multiple copper wires twisted inner conductor, insulation medium, flexible copper wire or tinned wire braid, and a PVC sheath. ^[1]

Coaxial cables have characteristic resistances of 50 ohms and 75 ohms. The main models are SYV, whose insulation is solid polyethylene; SBYFV, whose insulation is foamed polyethylene; SYK. Its insulating layer is a polyethylene core. Commonly used in TV monitoring systems are SYV and SBYFV 75 ohm impedance coaxial cables.

Foam polyethylene material is less susceptible to loss of video signals than polyethylene. It also increases the flexibility of the cable and facilitates installation, but it is easy to absorb moisture and change electrical performance. Because of its rigidity, solid polyethylene has better shape retention than foam and can withstand external extrusion pressure.

The coaxial cable shielding copper network can shield electromagnetic interference or unwanted external signal interference from EMI. The number of twisted wires and copper content in the braid determine its ability to resist interference. Commercial cables with loose braids can shield 80% of interference signals, which is suitable for occasions with low electrical interference. It is better to use metal pipes. Use high-shielded or braided cables for high-noise applications. Cables made of aluminum foil shield or foil are not suitable for TV surveillance systems, but can be used to transmit radio frequency signals.

The thinner and longer the coaxial cable, the greater the loss, and the higher the signal frequency, the greater the loss. Take SYV cable as an example, domestic coaxial cables have specifications such as SYV75-3, SYV75-5, SYV75-7, SYV75-9.

When using coaxial cable to transmit images, the distance of less than 300 meters can generally not consider signal attenuation. When the transmission distance increases, you can consider using low-loss coaxial cables, such as SYV75-9, SYV75-18, or Add cable compensator.

Coaxial cable transmission cable compensator

The cable compensator is also called a cable equalizer. The high-frequency characteristics are compensated by a cable correction circuit, so that the overall frequency characteristics of the signal transmission channel are substantially flat. The circuit is mainly composed of RC circuits. Each group of RC series circuits has a center frequency f. The cable attenuation curve is divided into several sections, and a group of RC circuits is used to compensate for each section.

Generally, adding a first-level compensator can extend the transmission line by 500 meters, and appropriately adding a cable compensator for cables of different specifications can increase the effective transmission distance to about 2km.

Coaxial cable transmission Prone interference in baseband transmission of coaxial cable

One disadvantage of baseband transmission is poor anti-interference ability. The shielding effect of the coaxial cable's shielding layer on electromagnetic waves with lower frequencies is worse, so it is susceptible to broadcast interference and low-frequency electromagnetic waves.

Coaxial cable transmits a broadcast interference

When the coaxial cable is installed overhead, the cable itself becomes a very long antenna. When it is induced by the broadcast electromagnetic wave, a potential difference is induced. This potential difference is generated at both ends of the cable shield (the core wire also has a sense of Potential difference, but very small), then, the shield layer, the internal resistance of the signal source, the core and core wires, the 75 ohm load, and the shield layer form a loop. This potential difference forms an interference current through the loop, and the load resistance is 75 ohm The resulting interference voltage drop is superimposed on the video signal. This interference generally ranges from a few hundred KHz to several MHz, which produces relatively stable moire interference to the image. The higher the interference frequency, the finer and denser the moire, and the interference greater than 10MHz basically does not affect the viewing effect.

The best way to suppress this kind of interference is to lay the cables underground, or use lead-clad cables, or symmetrically balanced cables with an outer shield as the transmission line. When coaxial transmission is only available, the single end of the cable shield should be grounded, and a symmetrical input cable compensator should be set at the receiving end. The high-level transmission method can also effectively suppress broadcast interference. The method is to amplify a 1Vp-p video signal to 5 to 8Vp-p before transmitting, and the interference level at the receiving end is reduced relative to the video signal. The transmission distance can also be farther. ^[1]

Coaxial cable transmission

Low frequency interference mainly refers to 50Hz power frequency interference. This interference causes the image to have horizontal black scroll bars, which in serious cases make the image unviewable and out of sync. The main cause of 50Hz interference is the ground potential difference. When there are many electric equipment and high power equipment, a large ground current will be formed when the three phases are unbalanced or the grounding method is different. When this current passes through the ground with a ground resistance, a voltage will be formed between the two grounds. If the two ends of the cable are grounded, a current will be formed on the cable through the internal resistance of the signal source, which will cause interference.

The best way to suppress this interference is to ground the single end of the cable.

Coaxial cable transmission three characteristic impedance mismatch

The characteristic impedance of the coaxial cable is 75 ohms. Due to the wide bandwidth of the video, the impedance of the coaxial cable at the low and high frequencies is not exactly the same, and it cannot be completely matched. However, the details of the image are all in the frequency domain above 1MHz, so the 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.

Impedance mismatch will often have a number of vertical bar interference with equal spacing, the frequency is basically an integer multiple of the horizontal frequency. The solution is generally improved by "starting series resistance" or "terminal parallel resistance".

Coaxial cable transmission four other interference

Coaxial cable transmission often causes interference caused by long transmission distances, excessive losses, low cable quality, and unclean power sources caused by the use of high-power thyristor equipment. These interferences are relatively easy to resolve. Install cable compensators, use purified power, and select high-quality cables.