75-3 and 75-5 video line parameters
Coaxial cable has the advantages of being cheaper and more convenient to lay (compared to optical fiber). Therefore, generally in a small-scale monitoring system, because the transmission distance is very short, the coaxial cable is used to directly transmit the monitoring image to the image. The damage to the quality is not large, and it can meet the actual requirements.
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 amplitude of the signal is attenuated, but also the attenuation of each frequency component varies greatly. , Especially the maximum attenuation of the color part. Therefore, the coaxial cable is 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 has a certain degree of amplification for the video signal, 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 coaxial cables in a monitoring system, in order to ensure a better image quality, the transmission distance range 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 relatively thick, which is not convenient for wiring in intensive 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.
2. Twisted pair
Twisted-pair cables have been used for a long time. Telephone transmission uses twisted-pair cables. Twisted-pair cables are used in many industrial control systems, places with high interference, and long-distance transmission. The local area network we use today is also used. Twisted pair. The reason why twisted pair is so widely used is that it has many advantages such as strong anti-interference ability, long transmission distance, easy wiring, and low price. Since the twisted pair also has a large attenuation of the signal, when the transmission distance is long, the frequency of the signal cannot be too high.
High-speed signals such as Ethernet can only be limited to within 100m. For video signals, the bandwidth is up to 6MHz. If it is transmitted directly in twisted pair, the attenuation will be great. Therefore, the video signal must be amplified and compensated for long-distance transmission on twisted pair. Twisted pair video transmission The equipment accomplishes this function. After adding a pair of twisted pair video transceiver equipment, the image can be transmitted to 1 to 2km. Twisted-pair and twisted-pair video transmission equipment are very cheap, not only did not increase the cost of the system, but when the distance increased, the cost was much lower than that of coaxial cable. Therefore, the use of twisted pair for transmission in the monitoring system has obvious advantages:
1) Long transmission distance and high transmission quality. Due to the advanced processing technology used in the twisted pair transceiver, it perfectly compensates for the attenuation of the twisted pair to the video signal amplitude and the attenuation difference between different frequencies, and maintains the brightness and color of the original image and the real-time performance. When the transmission distance reaches 1km or more, the image signal is basically without distortion. If the relay mode is adopted, the transmission distance will be longer.
2) Convenient wiring and high cable utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, any pair of Category 5 unshielded twisted pairs that are widely laid in buildings can transmit one video signal without additional wiring. Even if it is rewiring, Category 5 cables are easier than coaxial cables. In addition, a category 5 cable has 4 pairs of twisted pairs. If a pair of wires is used to transmit video signals, the other pairs can also be used to transmit audio signals, control signals, power supply or other signals, which improves the cable Utilization rate, while avoiding the trouble caused by separate wiring of various signals, and reducing the project cost.
3) Strong anti-interference ability. Twisted-pair cable can effectively suppress common-mode interference. Even in a strong interference environment, twisted-pair cable can transmit excellent image signals. Moreover, the use of several pairs of twisted pairs in a cable to transmit different signals respectively without interference with each other.
4) High reliability and easy to use. Utilize twisted pair cable to transmit video signals, and a dedicated transmitter must be connected at the front end, and a dedicated receiver must be connected at the control center. This kind of twisted-pair transmission equipment is cheap and easy to use. It does not require professional knowledge or much operation. It is installed once and works stably for a long time.
5) The price is cheap and it is convenient to obtain materials. Because it uses the common category 5 unshielded cable or common telephone line that is widely used at present, it is easy to purchase and the price is very cheap, which brings great convenience to engineering applications.
2. Technical performance analysis
1. Coaxial cable
In monitoring systems, it is common to use 75Ω, -5 coaxial cables. Generally, the distributed capacitance of this kind of coaxial cable is about 50-60pF/m, and the DC resistance of the cable will attenuate the transmitted signal. Tests show that a signal with a frequency of 5MHz will be attenuated by about 5dB when transmitted within a 75Ω, -5 coaxial cable for 100m. The higher the signal frequency, the greater the attenuation. The image signal is a high-frequency broadband signal. The color part of the image is located at the high end of the frequency. When the color image signal is transmitted by a coaxial cable, its brightness and color will be attenuated, especially as the transmission distance increases. It will be faded or even distorted. Tests in the laboratory found that when the color image signal is transmitted within a 75Ω, -5 coaxial cable for about 200m, its amplitude and color have been significantly attenuated. If you want to transmit longer distances, you can only add a coaxial video amplifier. The following figure shows the laboratory test waveform when a 75Ω, -5 coaxial cable transmits a distance of 200m. 2. Twisted pair
When using twisted-pair cables as the transmission medium, generally used Category 5 UTP cables are generally used. Its characteristic impedance is around 100Ω
On the right, the distributed capacitance is about 15pF/m. Unlike the coaxial cable, the signal is transmitted in a balanced manner in the twisted pair, but it will also be attenuated.
The attenuation curve of the video signal when the transmission distance is about 150m. When using twisted-pair cable to transmit image signals in the monitoring system, twisted-pair video transmission equipment should be used. Its main function is to complete balanced-unbalanced conversion , impedance matching, and With functions such as amplification and compensation, the use of twisted-pair video transmission equipment and twisted-pair cable can ensure high-quality transmission of video signals within a distance of 1.5Km. In the laboratory, a special instrument was used to test the performance indicators of the twisted pair transmission image signal, and the results are as follows:
(1) When the transmission distance is 300m, the index: DG≤1%; DP≤1°; SNR≥65dB
The 17th line test waveform:
(2) When the transmission distance is 1200m, the index: DG≤2%; DP≤2°, SNR≥60dB
The test waveform of line 17 is as follows:
1. Twisted pair cable
Twisted pair cable (hereinafter referred to as twisted pair) is a transmission medium formed by encapsulating one or more twisted pairs in an insulating jacket. It is currently the most commonly used wiring material for local area networks. In order to reduce the degree of signal interference, each pair of twisted pair in the cable is generally formed by twisting two insulated copper wires, and the twisted pair is also named. Twisted pair cables are generally used for the wiring connection of a star network. RJ-45 heads (crystal heads) are installed at both ends to connect the network card and the hub. The maximum network cable length is 100 meters. Repeaters can be installed between the twisted wires, and up to 4 repeaters can be installed. For example, if 4 repeaters are installed to connect 5 network segments, the maximum transmission range can reach 500 meters.
Twisted pair cables are divided into two categories: unshielded twisted pair (UTP) and shielded twisted pair (STP). In the local area network, unshielded twisted pair cables are divided into categories 3, 4, 5 and super 5. Twisted pair cables are divided into two types: Category 3 and Category 5.
At present, the twisted-pair cables commonly used in local area networks are generally unshielded, Category 5, 4-pair (ie, 8 wires) cables. The transmission rate of these twisted pairs can reach 100Mbps.
The outer protective rubber of Category 3 twisted pair on the market is thin, the rubber is marked with “CAT 3”, and the outer carton is marked with “Category 3”, the price is lower; the outer protective rubber of Category 5 twisted pair Thick, “CAT 5” is marked on the rubber, and “Category 5” is marked on the outer carton. The price is higher. When buying, do not buy low-quality Category 5 twisted-pair cables for cheap. These products can often only be used as Category 3 twisted-pair cables.
Category 5 twisted pair is an unshielded twisted pair. Compared with ordinary Category 5 twisted pair, Category 5 twisted pair has a smaller attenuation and stronger anti-interference ability when transmitting signals. In a 100M network, the user equipment The degree of interference is only 1/4 of that of ordinary Category 5 lines, which is a solution for future network applications, and there are few applications at present.
2. Coaxial cable
A coaxial cable is composed of a hollow outer cylindrical conductor and an inner wire located on the central axis. The inner wire and the cylindrical conductor and the outside are separated by insulating materials. According to the different transmission frequency bands, coaxial cables can be divided into two types: baseband coaxial cables and broadband coaxial cables. According to different diameters, coaxial cables can be divided into thick cables and thin cables.
Thin cables have developed rapidly in recent years, so computer local area networks generally use thin cables for networking unless there are special requirements. Thin cables are generally used for bus-type network wiring connections. Use the T-type BNC interface connector to connect the BNC interface network card, and terminal resistors need to be installed at both ends. The maximum length of each trunk line of the thin cable network is 185 meters, and each trunk line can access up to 30 users. If you want to widen the network range, you need to use a repeater, such as using 4 repeaters to connect 5 network segments, so that the maximum distance of the network can reach 925 meters.
The installation of thin cables is easier and the cost is lower. However, due to the limitation of the network wiring structure, its daily maintenance is not very convenient. Once a user fails, it will affect the normal work of other users.
Thick cables are suitable for the network trunks of larger LANs, with longer wiring distances and better reliability. Users usually use external transceivers to connect to the network trunk. The length of each section of the thick cable LAN can reach 500 meters, and the maximum length can reach 2500 meters after connecting 5 network segments with 4 repeaters. If you use a thick cable to connect directly to a network card, the network card must have an AUI interface (15-pin D-type interface). Although using thick cables to build a local area network has higher performance and larger transmission distance, it is difficult to install and maintain the network, and the cost is higher.
Impedance 75 ohms What does Level 3 mean? What does 64 conductors mean? +AL refers to aluminum foil. 1) SYV75-3, 5, 7, 9…, 75 ohm, polyethylene insulated solid coaxial cable. In recent years, some people call it “video cable”;
2) SYWV75-3, 5, 7, 9…75 ohm, physical foamed polyethylene insulated coaxial cable. Some people call it “RF cable”
3) Basic performance:
l SYV physical structure is 100% polyethylene insulation; SYWV is a physical foamed polyethylene insulated cable with a foaming rate of 70-80%;
l Due to dielectric loss, the attenuation of SYV solid cable is obviously greater than that of SYWV physical foamed cable; among commonly used engineering cables, physical foamed cables are still the cables with the best transmission performance and the lowest price, in video, radio frequency, and microwave bands. It’s all like this. The test data given by the manufacturer also illustrates this point;
l Coaxial cables can be used in DC, RF, and microwave bands. Differentiating cables according to “RF”/”video” is not only insufficient, but also easy to mislead: it seems that video transmission must or can only choose solid cables (choose the ones with large attenuation and high prices?); from the perspective of engineering applications, or It is more practical to distinguish the types according to “solid core” and “foamed” cables;
l The difference between the characteristics of high-stranded (128) and low-stranded (64) cables: experimental research by eie laboratory shows that in the frequency band below 200KHz, the “low resistance” of the shielding layer of high-stranded cables plays a major role, so low-frequency transmission attenuation is less than that of low-stranded cables. cable. However, in the video, radio frequency, and microwave bands above 200-300KHz, because the “high frequency skin effect” plays a major role, the high braided cable has lost the advantage of “low resistance”, so the high frequency attenuation of the two cables is basically the same.
1. The thinner the cable, the greater the attenuation: For example, the attenuation of the 75-7 cable at 1,000 meters is roughly equivalent to the attenuation of the 75-5 cable over 600 meters, or the transmission effect of the 1000-meter 75-7 cable is the same as that of the 75-5 cable over 600 meters. The transmission effect is roughly equivalent;
2. The longer the cable, the greater the attenuation: For example, the “decibel” of the 6M frequency attenuation of the 75-5 cable is 750 meters, which is 75% of the “decibel” of the attenuation at 1000 meters, which is 15db; 2000 meters (1000+1000) attenuation It is 20+20=40db, the calculation method of other frequency points is the same. According to the above 1000-meter cable test data, when calculating the attenuation of cables of different lengths, please remember that “decibels are based on the relationship of adding alkali” or “decibels of attenuation can be calculated according to the percentage relationship of length change”, and you can use it flexibly;
3. Frequency distortion characteristics: low-frequency attenuation is reduced, and high-frequency attenuation is large. The difference between the high/low side frequency attenuation can be called the “side frequency difference”, which is a very important parameter. The longer the cable, the greater the “side frequency difference”; fully understanding and mastering the “frequency distortion characteristics” of the coaxial cable is of great significance in engineering; this is the most critical characteristic that affects the image quality, and it is also The most overlooked problem in the project;