【 Weak current College 】 videoconference system video Alto, and suppression of interference analysis---Power By 【 China power house network 】

<br> 1.1 videoconference system <br> Videoconferencing is the use of television technology and equipment through the transport channel in two or more locations for a meeting of a means of communication, it enables audio, video, text, data, text data, and .other multimedia integrated management and information network in the same way, unified real-time transmission. <br> Transmission of such signals by cable if strong electromagnetic field area, there will be superimposed on the signal interference. Cable length, interference. <br> .1.2 interference source types and disturbances <br> In the video conference system, the impact of video and audio system noise disturbance in addition to the equipment and transmission line itself of thermal noise and superimposed on the continuity of the "white noise" interference, depending .on the type of main sources of interference can be divided into two major categories, pulse interference and AC sound interference. Pulse interference due to pulse device produces a strong electromagnetic field coupling caused by entering the channel: motor, car engine spark plug ignition, switching .power supply will produce 60Hz-2MHz interference, such interference harmonic components fall into the sound, video, band; lightning, cosmic noise also produces 2KHz-100MHz impulse noise. AC noise interference is mainly due to the Earth system design, different bonding between potential .difference, allowing to complete the circuit; caused by high-voltage transmission lines and AC electric railway can also cause interference, such as the hum of AC electric railway interference except 50Hz base frequency, there is (2N + 1) × 50Hz, odd harmonics .. <br> In addition, according to outside interference sources of transmission of electromagnetic energy and the audio and video equipment coupled mode can be divided into radiation pattern interference and conduction mode interference. Conduction style interference is the result of the circuit (including stray capacitance .and mutual inductance, etc. can be lumped parameter represents the circuit component) to the affected devices, such as pulse interference, hum noise, mainly through transmission mode on a disturbed equipment; radiation pattern interference is through an antenna, the space to the affected .devices, such as high-voltage transmission lines on the interference interference device. <br> Video conference system of audio signals in the frequency range of the video signal bandwidth 300Hz-3.4KHz as 6MHz. Audio signal is usually adopted balance transfer, because of .the double line induction noise canceling function, interference to much, and even measure it. The video signal is usually uneven manner transfer, interference will be much more serious. So audio interference analysis and video interference analysis differ: for video, the main departure from .disturbances to explore; audio signal due to the wavelength, the communication building shielding effect of the more obvious, in contrast, radiation pattern interference is negligible. Through the power rectifier generates pulse interference on audio and video signal mechanism is the same, the solution is .also consistent, as a result, the audio interference discussions focused on the exchange of acoustic interference. <br> 2 video interference analysis and solution <br> In a real project, video cables are usually laid in the communication building metal tube. Although the .metal pipe jacketing and building construction sites, sometimes there may be interference, the monitor will see irregular thin line top scroll, the disturbance is generated when the SCR rectifier ignition pulse interference. UPS power pulse jamming waveforms far more complex, it is in addition to .rectifier ignition interference are inverter pulse interference and harmonic. Power Rectifiers and UPS power generated by pulsed interference radiation is everywhere, but mainly through distribution in the Conference Room of the AC power line transmission, the transmission mode interference. <br> 2.1 conduction mode interference .suppression <br> 2.1.1 pulse interference suppression <br> For pulsed interference, the solution adopted was to install filtering network. As shown in Figure 3, the three root FireWire input and output of the rectifier power respectively to access high pressure, large .-capacity capacitor to form low-pass filter circuit. <br> Due to the high power UPS power interference with more serious, with the exception of interference pulses range is relatively large, interfering with pulse waveform complex, frequency is also very rich. To .suppress interference in UPS power supplies into the line-side and line-end respectively equipped capacitor. Increase the capacity, although it can further lower pulse interference levels, but increases to a certain amount, the effect is not evident. You can use the " .Γ-type "or" π "low pass filter circuit, cable route around in ferrite magnetic ring, effective frequency range, equivalent to an inductance in series. <br> Its exciting inductance calculation formula is as follows: LP <br> Type: .N-transformer primary winding coils; <br> Μ-Toroid relative permeability rate; <br> The type is not difficult to see the amount you want to increase the size of the inductor, you can use the following several ways: <br> .(1) reduction in magnetic ring inner diameter of the cable slot; <br> (2) select high permeability rate of magnetic ring; <br> (3) uses the high frequency magnetic materials; <br> (4) an increase in .the number of ring magnet. <br> 2.1.2 AC acoustic interference suppression <br> As mentioned, the exchange of acoustic disturbance is mainly due to complete the circuit, through the transmission mode of the video the receiving device. To do this can .be done by transmission line transformer isolation video source and the receiver. <br> 2.1.3 design of transmission line transformer <br> Design before clear parameters are: <br> (1) the maximum operating frequency Fmax and minimum working frequency Fmin; .<br> (2) determine the output load resistor R1 and the signal source resistance Rs; <br> (3) there must be clear lines of transmission line transformer isolated in this only plays a role. <br> At design time, the .formulas (2-1), (2-2), (2-3) is not difficult to find the minimum length of transmission lines Lmin, Lmax and maximum length of the characteristic impedance of Zc. <br> Practice has proved, Φ .= (0.27 - 0.77) mm high strength wire-wound, and around, Zc $ (60-80) Ω; frequency lower than 50MHz, selectedMN-Zn ferrite oxygen (MXO). Ferrite permeability by (2-4)-confirmed. .<br> Transmission line transformer design requires amplitude-frequency characteristic of low consumption, plug in, as well as the return loss is high. As a result of the transmission line transformer is a passive device, no distortion, no differential gain and phase distortion ., so would not introduce new noise. Access transmission line transformer is discovered pulse interference suppression by a large degree, on the monitor screen is completely fail to see any interference phenomena. Thus in the video signal long-distance transmission, at the end of the .installation of the transmission line transformer can obviously stray electromagnetic interference and sharing of acoustic interference. <br> 2.2 radiation pattern interference suppression <br> Modernization of the power system itself is a strong electromagnetic interference sources, mainly through radiation pattern interference within the frequency band .of communication equipment. To suppress external high-voltage transmission lines of interference, the measures adopted-grounding, the common ground approach are discussed separately below: <br> 2.2.1 dispersed grounding <br> Decentralized grounding is the communications building, lightning, .power system grounding, communications equipment of various types of ground, as well as other equipment grounding respectively access isolated grounding system, because the ground system is increasing, ground effects of coupling between potential are often difficult to avoid, but dispersed grounding can easily lead to .interference. At the height of the main building has increased its grounding with insecurities. When a facility was struck by lightning, easily formed underground counterattack, damage to other equipment. <br> 2.2.2 joint grounding <br> Joint ground also known as .single-point grounding ground system that all share a common "and". Joint ground has the following features: <br> (1) the entire building, grounding system form a cage equalizing body, lightning, building the same tier points comparison evenly; .for lightning, cage equalizing body and building of frame structure of external electromagnetic interference also provides 10-40dB of shielding effects; <br> (2) General joint grounding resistance is very small, there is a variety of grounding effect of coupling between, to .reduce the interference; <br> (3) can save metal materials, covering less. <br> From the top is not difficult to see, using joint grounding can inhibit an external high-voltage transmission lines. <br> 3 audio interference analysis .and solution <br> On the audio device signal line input and output interface forms, there is a balance and do not balance in two ways: balance - dual differential pressure, with strong common mode interference suppression capability, hum noise small, often used for .long distance or strong interference signal transfer. Not balance - single-ended type, used for close range or internal signal transfer between devices. To suppress hum noise, note the following: <br> (1) avoid 2 ground potential may be different in .the signal ground between devices directly connected or forming ground loops. <br> (2) try to avoid or decrease two devices the direct link between electricity. <br> (3) the electrical connections of the section shielding in a system, the signal .ground wire or screen in the system side grounding. <br> (4) long distance transmission signal transmission with balancing transformers. Both sides should have balanced transformer, the shield to ground on one side, may also be vacated No. Grounding can play a .role, can also be shielded to prevent electric shock accident occurred when prescribed lap. Not grounded, the ends of balancing transformer can play a role in balancing the insulation isolation transformer grounding, Centre for venting static electricity. <br>.