How does CRT work?
The CRT display (known as "cathode ray tube") is such a device, which is mainly composed of an electron gun, a deflection coils, a shadow mask, a high pressure graphite electrode and a phosphor coating. (phosphor) and glass shell consists of five parts. One of the most impressive of us is the glass case, also known as the screen, because its inner surface can display rich color images and clear text. How does a CRT display use the principle of three primary colors? Of course, instead of directly painting the three primary colors on the screen, the electron beam is used for control and performance.
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How does the electron gun work?
This depends firstly on the phosphor layer. The phosphor screen is coated with phosphor dots or phosphor stripes of red, green and blue in a certain way, called phosphor units, adjacent red, green and blue. Each of the phosphor units is a group, and the scientific name is called a pixel. Each pixel has three primary colors of red, green, and blue (R, G, B). According to the three-primary theory we just mentioned, this has the basis for forming ever-changing colors. However, how do you mix these three primary colors into rich colors?
We solve this problem with an electron gun. Yes, the electron gun can be launched like a pistol, but it is not a bullet, but a very high-speed electron beam. The working principle is that the filament is heated by the filament, the cathode emits electrons, and then, under the action of the accelerating electric field, the focused electrode is concentrated into a very fine electron beam, and under the action of the anode high voltage, a huge energy is obtained, and the electron is obtained at a very high speed. Bombard the phosphor layer. The target of these electron beam bombardments is the three primary colors on the screen. To this end, the electron beam emitted by the electron gun is not a bundle, but three bundles, which are respectively controlled by the voltages of the three primary color video signals of the computer graphics cards R, G, and B to bombard the respective phosphor units. Excited by high-speed electron beams, these phosphor units emit red, green, and blue light with different strengths. According to the spatial color mixing method (the method of mixing the three primary colors simultaneously at three points adjacent to each other on the same surface), the method produces a rich color, and the method utilizes the characteristics that the human eye does not have high resolution after exceeding a certain distance. , produces the same effect as the direct color mixing method. In this way, pixels of different colors can be produced, and a large number of pixels of different colors can form a beautiful picture, and the constantly changing picture becomes a movable picture. Obviously, the more pixels, the sharper and more detailed the image is, and the more realistic it is. However, how can an electron gun be used to simultaneously illuminate these tens of thousands of pixels and form a picture?
How is the picture formed?
Scientists have come up with a very clever way, the principle is to take advantage of the visual residual characteristics of people's eyes and the afterglow of phosphors, which means that even if we only have one electron gun, as long as our three electron beams can be fast enough to all The neatly arranged pixels are excited and we can still see a complete image. Don't doubt that the electron guns in our current CRT monitors can emit these three electron beams and then scan all the pixels at very, very fast speeds. To form a very high-speed scanning action, we also need the help of Deflection coils, through which we can cause the electron beams in the picture tube to periodically bombard each pixel in a certain order so that each pixel emits light. And as long as the period is short enough, that is, the electron beam bombardment frequency is high enough for a pixel, we will see a complete image. We call this regular periodic motion of the electron beam a scanning motion.
Display scanning method
The schematic diagram of the working principle of the CRT display electron gun understands the three primary colors. If you are smart, you will definitely think that you can use such a principle to make a color display. That's right, our colorful CRT monitors today are made from this three-primary principle. Just now we mentioned that the choice of the three primary colors is arbitrary in principle, but through experimental research, it is found that the human eye responds most sensitively to the three colors of red, green and blue (the various colors seen by the human eye are because the light has The different wavelengths cause the naked eye to feel particularly strong for these three wavelengths, and they have a wide range of color matching. These three colors can be used to match most of the colors in nature. Therefore, in CRT monitors, red is used. The three colors of green and blue are used as the three primary colors, and are also represented by three letters R, G, and B. Now the question is coming. How can we express the light of these three primary colors? We need an electromechanical device to complete this performance process.
That's right, because there are a large number of neatly arranged pixels that need to be excited, it is inevitable that regular electron gun scanning movements will be efficient. Usually, there are many ways to scan, such as linear scanning, circular scanning, spiral scanning and so on. Among them, linear scanning can be divided into progressive scanning and interlaced scanning. I believe that everyone often hears it. In fact, both are used in CRT display systems. Progressive scanning is a way of scanning the electron beam from left to right in a line on the screen. It is a more advanced method. In interlaced scanning, the scanning of an image is not done in one field period, but in two field periods. All odd lines are scanned in the previous field period, called odd field scans, and all even lines are scanned in the next field period, called even field scans. Whether it is progressive scan or interlaced scan, in order to complete the scan of the entire screen, the scan line is not completely horizontal, but slightly inclined, so that the electron beam must be both horizontal and vertical. . The former forms a line of scanning, called line scanning, which forms a scan of a picture called a field scan.
With the scan, you can form a picture, but how can you ensure that each of the three electron beams hits each pixel accurately during the scanning process? This is done by means of a shadow mask, which is located approximately 10 mm behind the screen (as viewed from the front of the screen) and a thin metal baffle of approximately 0.15 mm with a number of small holes or slots. They correspond to the same set of phosphor units, ie pixels. After passing through the small holes or slots, the three electron beams can only hit the corresponding phosphor unit in the same pixel, thus ensuring the purity and correct convergence of the color, so that we can see a clear image.
As for the continuity of the picture, it is determined by the speed of the field scan. The faster the field scan, the more single images are formed, and the smoother the picture. How many field scans can be performed per second is usually a measure of picture quality. We usually use frame rate or field rate (in Hz, Hertz). The larger the frame rate, the more continuous the image. We know that the 24Hz field frequency is to ensure the continuous feeling of the moving content of the image. The 48Hz field frequency is the feeling that the image display does not flicker. These two conditions are satisfied at the same time, so that the image with good effect can be displayed. In fact, this is similar to the formation principle of cartoons. The pictures of one sheet quickly flash through the eyes of the person, forming a continuous picture and becoming an animation.
1. What wireless network
Wireless network, as the name suggests, is a way to build a network without relying on network cables, which is mainly achieved by using wireless communication technology. There are two main ways of wireless network (such as 4G, 3G or GPRS) and wireless local area network (WiFi) through mobile communication networks.
2, What wireless bridge
Wireless bridge, as the name suggests, is the bridge of wireless network, which uses wireless transmission to achieve a communication bridge between two or more networks. In addition to having the basic characteristics of a wired bridge, a wireless bridge works in the 2.4G or 5.8G wireless license-free frequency band, making it easier to deploy than other wired network equipment.
From the function to understand the wireless bridge, it can be used to connect two or more independent network segments, these independent network segments are usually located in different buildings, hundreds of meters to tens of kilometers apart, can be widely used in the interconnection between different buildings.
At the same time, according to different protocols, wireless Bridges can be divided into 802.11b, 802.11g and 802.11n in the 2.4GHz band and 802.11a and 802.11n wireless Bridges in the 5.8GHz band.
3, the meaning of wireless bridge 2.4G and 5.8G
2.4G Bridge:
The advantages are low frequency, large wavelength and strong diffraction ability. Simply put, the propagation performance is good, and the propagation path is slightly shielded. Then there is the relatively low cost.
The disadvantage is that there are many devices using the 2.4G band, and the electromagnetic wave signal emitted by the bridge is easily interfered with the signal emitted by other devices, resulting in a decrease in transmission quality. Moreover, it is limited by the transmission bandwidth of the 2.4GHz band itself, which generally does not exceed 300Mbps
5.8G Bridge:
The advantages are high frequency, relatively pure channel, and large transmission bandwidth. The transmission bandwidth starts at 433Mbps and can easily reach more than 1Gbps. It is suitable for scenarios that require high data transmission.
The disadvantage is that the frequency is high, the signal wavelength is short, the penetration is poor, and there can be no shelter during the propagation. The cost of 5.8G equipment is higher than 2.4G, and it is still in the popularization stage.
Wireless bridge transmission system is usually composed of two or more wireless devices, due to the two-way transmission of data, each device needs to have the ability to send and receive wireless signals.
4, the working principle of wireless bridge
The working principle of the wireless bridge is that the bridge uses the air as a medium to transmit signals, which is simply that one end of the bridge converts the signal in the network cable into a wireless electromagnetic wave signal and transmits it to the air in a directional manner. The bridge at the other end does the opposite, picking up radio electromagnetic signals in the air and converting them into wired signals.
The wireless electromagnetic wave signal can be transmitted by air as the transmission medium, which can solve many problems of difficult construction of wired deployment: such as highways, rivers, mountain streams, or road hardening, and wired deployment and construction difficulties.
Wireless bridge networking has obvious advantages, and can achieve point-to-point or point-to-multipoint network connections at distances of up to 50 kilometers, and data transmission speeds of up to 108Mbps. Effectively solve the problem of network connectivity in the interval, as long as the wireless signal coverage area, the client can easily access the network, the fusion system, without any wiring, the wireless terminal can achieve zero configuration access, so it is very easy to carry out network maintenance and expansion.
Bridges are generally used in the following scenarios: wireless data acquisition, monitoring data transmission (outdoor and elevator), outdoor wireless coverage, outdoor remote wireless bridging, private ISP wireless broadband, unmanned monitoring station data return, etc.
5. How to choose the right bridge
Wireless bridge is a wireless monitoring and transmission equipment commonly used in the field of wireless monitoring, although it is a wireless device like wireless AP and Wireless Router, but it does not need to build wifi coverage, but is used to wireless transmission of video data. Like the switch in wired monitoring, the wireless bridge is an important transmission equipment in wireless monitoring, which is widely used in the two major fields of outdoor surveillance video transmission and elevator surveillance video transmission.
There are many kinds of wireless Bridges on the market, so how to choose a bridge that suits your needs? We will answer your questions from the following aspects.
â‘ Transmission distance
Wireless monitoring projects need to transmit the distance is not the same, wireless bridge transmission distance there are many kinds, some transmission 1~3 km, some transmission 3~5 km, some transmission 5~10 km, some transmission more than 20 km, must be determined according to the monitoring of the transmission distance, as far as possible the maximum transmission distance of the bridge is greater than the monitoring transmission distance, Because rain, fog, snow and other weather in the actual application environment will cause the transmission performance of the bridge to decline, the project construction should reserve sufficient performance margin.
â‘¡ Transmission bandwidth
There are many kinds of transmission rates of wireless bridge, such as 150Mbps, 3000Mbps, 450Mbps, 600Mbps, 900Mbps, etc., which rate to choose can be determined according to wireless monitoring requirements. What users actually need to consider is the transmission performance of the bridge over a specific distance, rather than the theoretical bandwidth data. For example, the theoretical transmission bandwidth of the IP-COM AP625 bridge is 433Mbps, and the actual point-to-point transmission bandwidth can reach 200Mbps at a distance of 2 kilometers, with 25 200W cameras without pressure.
â‘¢ Operating frequency
There are two main operating frequencies of wireless Bridges, 2.4G and 5.8G, and the characteristics of the two Bridges are different. In general, 2.4G wireless bridge is the current mainstream frequency band, good compatibility, good diffraction ability, but the anti-interference is relatively poor, especially in the urban area is vulnerable to wireless signal interference transmitted by other WiFi devices. The 5.8G channel is relatively pure, the anti-interference ability is better, the transmission distance is long, but the diffraction ability is poor.
Urban, downtown, long-distance transmission, camera stream is large, 2.4G interference is more, choose 5.8G wireless bridge. Others such as the transmission distance is relatively close, relatively remote, and the same frequency interference is less, etc. 2.4G wireless bridge is used
â‘£ antenna
Antenna is an important part of wireless bridge, used to transmit and receive wireless signals, no antenna, wireless bridge can not achieve communication. There are many types of antennas, including omnidirectional antennas and directional antennas. Omnidirectional antenna is used for short-range coverage and transmission, and directional antenna should be selected for long-distance bridge. The larger the gain of antenna, the better the performance of wireless bridge.
⑤ Power supply mode
The working environment of the wireless bridge usually involves some complex outdoor environments, such as forests, ports, tunnels, reservoirs and other places, so the power supply is a more troublesome problem. Choosing a wireless bridge that supports POE cable power supply can solve this problem well. IP-COM bridge all support PoE injector power supply, power supply distance up to 60 meters.
â‘¥ Protection level
The wireless bridge works outdoors, the environment is changeable, such as rain, rain and snow, high temperature, etc., the first requirement is waterproof, dustproof, heat resistance, and condensation resistance. For a qualified wireless bridge, these "qualities" are a must.
⑦ Pairing method
At present, there are three mainstream bridge pairing methods: key pairing, dip code pairing, automatic pairing. In terms of the simplicity of engineering applications, especially for projects that install dozens or hundreds of network Bridges at a time, automatic pairing is undoubtedly the best choice. IP-COM network bridge supports the automatic pairing of two network Bridges when they are powered on in the factory state, greatly reducing the workload.
6, wireless bridge installation requirements
Wireless bridge is an important transmission equipment in wireless monitoring, which is widely used in outdoor surveillance video transmission and elevator surveillance video transmission. In addition to selecting the right wireless bridge, we also need the bridge equipment to give full play to its performance advantages after installation to assist users in the project. Therefore, for some installation requirements and precautions of wireless bridge, we should pay more attention to avoid various problems in the later stage.
â‘ Installation height
In the process of wireless transmission, obstacles such as trees, buildings and large reinforced buildings will weaken and block wireless signals. In order to improve the wireless transmission performance and prevent signal damage and weak signal, the user should try to ensure that there are no obstacles in the transmission route of the wireless bridge during installation, and meet the transmission conditions that the two ends can see each other.
Both ends can not be simply understood as point-to-point visibility, which means that there can be no obstacles or potential obstacles in the Fresnel zone (radio wave terminology) in which the antenna is propagated. The main RF energy between the antennas is transmitted in this area, so the transmitting antenna must be high enough above the obstacle to maintain a line of sight path between it and the receiving antenna in order for the communication link to work properly.
â‘¡ Angle and signal debugging
Since the quality of the wireless bridge signal is directly related to the bandwidth and stability of the link, it is necessary to further debug the wireless bridge signal after the installation is completed (the signal strength can be adjusted by adjusting the direction of the antenna on both sides, the pitch Angle, etc.). The signal strength can be checked according to the signal status indicator of the bridge device (three lights, three signals are best, two are average, and one is poor) or the software. Detailed installation procedure for outdoor wireless aps
â‘¢ Requirements for lightning rod
When the wireless bridge is installed in the field, if there are no tall buildings or lightning rod protection nearby, lightning protection measures need to be considered, usually using lightning rod, generally installed in the urban area or surrounded by lightning rod protection, can not be set separately.
It can be seen from the characteristics of the lightning rod that the lightning rod is a lightning source, and the lightning rod will discharge in the grounding path when it is hit by lightning. Therefore, the lightning rod and the protected equipment need to be insulated, otherwise the lightning rod will have a lightning effect on other equipment when discharging. Due to the tip discharge characteristics of the lightning rod, it is easier to cause lightning discharge than the general equipment, so if the lightning rod cannot be isolated from the protected equipment, it will increase the probability of lightning strikes of other equipment. Therefore, the lightning rod grounding must be separate from the device grounding and cannot share the same grounding.
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â‘£ Power supply requirements
The normal input voltage of the PoE power module of the bridge ranges from 100V to 240V and the output voltage ranges from 24V to 48V. If the voltage is lower than or higher than this voltage, the normal operation of the device is affected or the PoE power module is damaged. In view of the unstable and high voltage field power supply, it is necessary to design a PoE power supply solution (such as regulated power supply and UPS power supply) that ADAPTS to the working voltage.
Because the bridge is a precision electronic equipment, it has high requirements for power supply, and is vulnerable to the impact and influence of other power supply equipment, so the power supply of the bridge should be separated from other high-power equipment such as pumping units, oil pumps, etc. When taking power at the same location, UPS, regulated power supply or isolation transformer should be installed to filter out the influence and interference on the power supply when the high-power motor is working.
When the PoE module is used to power the bridge through the network cable, the recommended distance is not more than 60 meters. When this distance is exceeded, it is recommended to move the transmission line near the bridge to meet the recommended power supply distance.
⑤ Device grounding requirements
The wireless bridge should be grounded. If the device is not grounded, the device may run abnormally or be damaged. The grounding resistance of the device should be less than 4 ohms, and the grounding cannot be shared with the lightning rod or strong current line. If PoE power is used, the ground cable also needs to be grounded. Users can ground a Class 5 (or higher) shielded network cable with a ground cable and a PoE adapter to effectively prevent static electricity and lightning strikes. The ground cable and ground point should be waterproof using waterproof tape and waterproof cement according to the waterproof requirements to prevent the ground point from being exposed to the air for a long time, which may cause oxidation or rust.
7, wireless bridge erection method
There are four common wireless bridge transmission modes, the distribution is point-to-point, point-to-multipoint, relay, reflection, here the first monitoring combined with pictures for you to briefly introduce the characteristics of these four wireless transmission modes.
â‘ point-to-point transmission
Point-to-point transmission mode is the simplest transmission mode, that is, we often say PTP, to a single device transmission, and then received by a single device, one-to-one transmission and reception is simple and direct. Point-to-point transmission mode of wireless bridge is often used in the case that the transmission distance is long, or the monitoring point is widely distributed, and point-to-multipoint transmission cannot be achieved.
â‘¡ Point-to-multipoint transmission
The point-to-multipoint transmission mode is developed based on the point-to-point transmission mode, which is often represented as a receiving end to multiple transmitting ends, and is often used for the situation where the transmission distance is relatively close, there are more monitoring points and the distribution is dense.
â‘¢ Relay transmission
The relay transmission mode is because the transmitting end and the receiving end can not avoid the barrier blocking the microwave signal, so the transfer equipment is added in the middle, so that the microwave signal is smoothly transmitted to the receiving end through the transfer equipment, this mode is due to increase the transfer equipment and increase the equipment cost investment, so it is generally not used when unnecessary.
â‘£ Reflection transmission
Reflection transmission is microwave transmission with the help of objects outside the transmission device, such as the transmitting end and the receiving end can not be seen, but there is a large building or a mountain with smooth rock walls in the middle, so that you can adjust the Angle of the equipment, through the building or mountain to reflect microwave signals for wireless communication.
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Shenzhen MovingComm Technology Co., Ltd. , https://www.movingcommiot.com