Coaxial cables are cables with a central copper conductor wrapped in layers of insulated cables. It is characterized by good anti-interference ability, stable data transmission, and low price. It is also widely used, such as closed-circuit television lines. Coaxial thin cables are generally sold at a few yuan per meter in the market, which is not too expensive. The coaxial cable is used to connect with the BNC head. The coaxial cable sold on the market is generally the finished product that has been connected with the BNC head. You can directly choose it.
However, according to the analysis of the characteristics of the coaxial cable itself, the attenuation of the signal when it is transmitted in the coaxial cable is related to the transmission distance and the frequency of the signal itself. Generally speaking, the higher the signal frequency, the greater the attenuation. The bandwidth of the video signal is very large, reaching 6MHz, and the color part of the image is modulated at the high end of the frequency. In this way, when the video signal is transmitted in the coaxial cable, not only the overall signal amplitude is attenuated, but also the attenuation of each frequency component varies greatly. , Especially the color attenuation is the largest. Therefore, coaxial cables are only suitable for short-distance transmission of image signals. When the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the color becomes dim and there is a sense of distortion.
In engineering practice, in order to extend the transmission distance, a coaxial amplifier is used. The coaxial amplifier can amplify the video signal to a certain extent, and can also compensate different frequency components in different sizes through equalization adjustment, so that the distortion of the video signal output by the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded indefinitely. Generally, only 2 to 3 coaxial amplifiers can be cascaded in a point-to-point system. Otherwise, the video transmission quality cannot be guaranteed and adjustment is difficult. Therefore, when using a coaxial cable in a monitoring system, in order to ensure a better image quality, the transmission distance is generally limited to about four or five hundred meters.
In addition, coaxial cables still have some shortcomings in the transmission of image signals in the monitoring system:
1. The coaxial cable itself is greatly affected by climate change, and the image quality is affected to a certain extent;
2. The coaxial cable is thicker, and wiring is not convenient for dense monitoring applications;
3. Coaxial cables generally can only transmit video signals. If the system needs to transmit control data, audio and other signals at the same time, additional wiring is required;
4. Coaxial cable has limited anti-interference ability and cannot be used in strong interference environment;
5. The coaxial amplifier also has the disadvantage of difficulty in adjustment.
Commonly used twisted pair cables are CAT5 Cables and CAT5E Cables, CAT6 Cables, and CAT7 Cables. The former has a thin cable diameter and the latter has a thick cable diameter. The models are as follows:
1. CAT5E cable: This type of cable has increased winding density, a high-quality insulation material, and a transmission rate of 100MHz. It is used for voice transmission and data transmission with a maximum transmission rate of 100Mbps. It is mainly used for 100BASE-T and 10BASE-T network. This is the most commonly used Ethernet cables.
CAT5E has low attenuation, less crosstalk, higher attenuation-to-crosstalk ratio (ACR), signal-to-noise ratio (Structural Return Loss), smaller delay error, and performance Greatly improved. The maximum transmission rate of Super CAT5 cable is 250Mbps.
3. CAT6 Cable: The transmission frequency of this type of cable is 1MHz～250MHz. The comprehensive attenuation crosstalk ratio (PS-ACR) of CAT6 wiring system should have a large margin at 200MHz, which provides 2 times that of CAT5 bandwidth. The transmission performance of CAT6 cabling is much higher than that of the Super CAT5 standard, and is most suitable for applications with a transmission rate higher than 1Gbps.
An important difference between CAT6 and CAT5 is that it improves the crosstalk and return loss performance. For the new generation of full-duplex high-speed network applications, excellent return loss performance is extremely important. The basic link model is cancelled in the six types of standards. The wiring standard adopts a star-shaped topology. The required wiring distance is: the length of the permanent link cannot exceed 90m, and the channel length cannot exceed 100m.
4. CAT6E cable: The CAT6E cable is an improved version of the CAT6E cable, which is also an unshielded twisted pair cable specified in ANSI/EIA/TIA-568B.2 and ISO CAT6/Class E standards. Mainly used in gigabit networks. In terms of transmission frequency, it is also 200-250 MHz like CAT6 lines, and the maximum transmission speed can also reach 1 000 Mbps, but it is greatly improved in terms of crosstalk, attenuation and signal-to-noise ratio.
CAT 5 and VII cable: This cable is the latest twisted pair in the ISO 7/F standard. It is mainly used to adapt to the application and development of 10 Gigabit Ethernet technology. But it is no longer an unshielded twisted pair, but a shielded twisted pair, so its transmission frequency can reach at least 500 MHz, which is more than twice that of CAT6 and CAT6E cables. Up to 10 Gbps.