TV conference system alto, video analysis and interference suppression-- MCMYK
2011-08-30 21:40:01 click： 441
1.1 Videoconferencing system components http://www.mcmyk.com
Video conferencing is the use of television technology and equipment through the transmission channel in two or more locations to meet a means of communication, which implements the audio, video, text, data, graphics and other multimedia data in an integrated process, and at the same information network Road running, unified real time transmission.
If the transmission of these signals by the strong electromagnetic field cables area, there will be interference superimposed on the signal. The longer the cable, the more obvious interference.
1.2 Interference source types and disturbances http://www.mcmyk.com
In the video conference system, the impact of video and audio system noise interference in addition to their own devices and transmission lines superimposed on the thermal noise and the continuity of its "white noise" outside interference, according to the main types of interference sources can be divided into two categories, pulse interference and hum interference. Pulse interference is due to strong electromagnetic pulse generated by the device coupled into the channel due to: motor car engine spark plug ignition, switching power supply will produce 60Hz-2MHz of interferences, such as harmonic components fall into the audio and video band; Lightning , cosmic noise will produce 2KHz-100MHz impulse noise. Hum interference was mainly due to ground system design is unreasonable, different ground potential difference exists between such ground currents caused by the formation of loops; high-voltage transmission lines and ac electrified railway will cause hum interference, such as an AC electrified railway generated 50Hz interference baseband addition, there are (2N +1) × 50Hz other odd harmonics.
In addition, according to outside sources of interference of electromagnetic energy transmission and right audio and video equipment can be divided into radiation mode coupling interference and conduction disturbances. Conduction disturbances is through the circuit (including stray capacitance and mutual inductance, etc. can be represented using lumped circuit elements) reached the affected device, such as pulse interference, hum interference, primarily through conduction disturbances acting on the device; radiation otherwise interfere through the antenna, by the spatial spread of the affected devices, such as high voltage power line interference on the victim device.
TV conference system of the audio signal frequency range of 300Hz-3.4KHz, the video signal bandwidth of 6MHz. Usually a balanced audio signal transmission, because the lane there is a mutual offset by induced noise, interference is much lighter, and even test it out. The video signal is usually unbalanced transmission, interference can be more serious. Therefore, the audio and video interference analysis interference analysis differ: For video interference, mainly from the disturbances starting to explore; audio signal due wavelengths greater communication building shielding effect is more obvious, compared with negligible interference radiation pattern do not count. Through the power supply rectifier pulse interference generated by the audio and video signals the same mechanism, the solution also agreed, therefore, the discussion focused on audio interference hum interference on.
2 Video interference analysis and solutions http://www.mcmyk.com
In practical engineering, the video cables are usually laid in the communications building metal tube. Although the metal tube construction with the building skin linked together, and sometimes may still be susceptible to interference, will see on the monitor irregular thin scroll from top to bottom, the interference is generated by the thyristor firing pulses interference caused. UPS power supply glitches resulting waveform is more complex in addition to its Rectifier Ignition outside interference generated pulses generated by the inverter there is interference and its harmonics. UPS power supply rectifier and the resulting radiation pulse interference, although everywhere, but mainly distributed in the meeting room through the AC power line communication for conduction disturbances.
2.1 Conduction interference suppression http://www.mcmyk.com
2.1.1 Pulse interference suppression http://www.mcmyk.com
For pulse interference, the solution adopted is to install filter network. Figure 3 shows, in three FireWire input and output ends of the rectifier power supply connected to the ground high voltage, large-capacity capacitor, forming a low-pass filter circuit.
Because high-power UPS power interference is more serious, apart from interfering pulse amplitude is relatively large, the interference pulse waveform complex frequency components is also very rich. To suppress interference in UPS power cord into the outlet ends of the side and install the capacitor. To increase the capacity, although the interference pulses can be further reduced level but increases to a certain amount, the effect is not obvious. Then you can use "Γ-type" or "π-type" low-pass filter circuit, ie cable through a ferrite ring, in the effective frequency range is equivalent to a series inductance.
Its magnetizing inductance LP calculated as follows:
Where: N-transformer primary coil turns;
μ-relative permeability of the toroidal core;
It is obvious from the above equation to increase the inductance, you can use the following ways:
(1) to reduce the inner diameter of the cable ring slit;
(2) selection of high magnetic permeability;
(3) using high-frequency magnetic materials;
(4) increasing the number of ring.
2.1.2 Hum interference suppression
As mentioned above, the hum interference is formed mainly due to a current loop, by conduction effect to the video receiving device. You can do this through a transmission line transformer isolation video source and the receiver.
2.1.3 Transmission line transformer design
Required before definitive design parameters:
(A) the maximum operating frequency and the lowest operating frequency Fmax Fmin;
(2) determine the output load resistor R1 and the source resistance Rs;
(3) to clear the transmission line transformer isolation only play this role.
At design time, by the formula (2-1), (2-2), (2-3) of the transmission line is not difficult to determine the minimum length Lmin, the maximum length Lmax and the characteristic impedance Zc.
Practice has proved that, with Φ = (0.27-0.77) mm high-strength wire wound and around the time, Zc is about (60-80) Ω; Operating at frequencies below 50MHz, use MZ oxygen (MXO) ferrite. The magnetic ferrite rate (2-4) is determined.
Transmission line transformer is designed for amplitude-frequency characteristics, low insertion loss and return loss is higher. As the transmission line transformer is a passive device with no gain distortion, no differential phase distortion, it will not introduce new noise. Access transmission line transformer, and found a large degree pulse interference suppression on the monitor screen has been completely fail to see any interference. Thus explained in the video signal transmission over long distances, in the end of the installation of the transmission line transformer can inhibit stray electromagnetic interference and hum interference.
2.2 Radiation mode interference suppression
Modern power system which itself is a strong source of electromagnetic interference, mainly through radiation way interfere with the band communications equipment. To suppress the interference of external high voltage transmission line effects, the use of grounding, common ground in two ways, respectively, are discussed below:
2.2.1 Distributed grounding
Ground is to disperse traffic building lightning protection and grounding, power system grounding, communications equipment, and other types of grounding equipment grounding respectively connected to the grounding system separated from each other due to the growing ground systems, ground potential coupling between effects often difficult to avoid dispersing ground but easily lead to interference. While increasing the height of the main building, the grounding brought insecurity is also growing. When a facility is struck by lightning, it is easy to form underground counterattack, damage to other equipment.
2.2.2 Joint grounding
Joint grounding method, also known as single-point ground that all grounding systems share a common "ground." Joint ground has the following characteristics:
(A) the entire building grounding system to form a cage equalizing body, for direct lightning, the building of the points with a layer of relatively uniform; For lightning, cage equalizing body and the building frame structure to external electromagnetic interference to provide 10-40dB shielding effect;
(2) General Joint grounding grounding resistance is very small, there is no coupling between various grounding effect, helps to reduce interference;
(3) can save metal materials, small footprint.
It is obvious from the above, a joint grounding method can effectively suppress the interference of external high voltage transmission lines.
3 Audio interference analysis and solutions
Between the signal line on the audio device input and output interface in the form of speaking, there are two kinds of balanced and unbalanced: Balanced - two-wire differential, with a strong common mode interference rejection capability, hum interference, commonly for long distance or under strong interference signals. Unbalanced - Single Single-ended, used for close-up or an internal signal transmission between devices. To suppress interference hum, you should note the following:
(1) 2 to avoid ground potential may be different between the devices directly connected to signal ground or ground loop is formed.
(2) two devices try to avoid or reduce the direct link between electricity.
(3) the electrical connection portion of the shield in a system, the signal ground or shielding layer on one side of the system ground.
(4) long-distance transmission signal using balanced transformer transmission. Ends should have a balanced transformer shield grounded at one end, can also be unsettled to connect. Grounding can play a shielding effect, also prevents overlap occurs when electricity for electric shock. Not grounded, can play both ends of the balun insulate role in balancing transformer center grounded electrostatic discharge.