Analysis Of Closed-circuit Television Monitoring System Application In The Interference Problem -

CCTV transmission systems, there are two main types of signals: one is the analog

video signal transmission path from the Video camera To the matrix, from the matrix and then to monitor or VCR; One is digital signal, including matrix and camera control information between the transmission matrix in the computer part of the digital signal. General equipment into the possibility of interference sources is small, so the signal transmission path, mainly through interference into the system.

CCTV system signal transmission path is through the video cable and twisted pair transmission control signal interference coupled into the system: a variety of high-frequency noise such as start and stop a large inductive loads, ranging from the introduction of ground potential frequency interference, balanced transmission line noise suppression capability of balance to decrease the frequency of interference were transferred into differential mode interference, transmission line impedance mismatch caused by signal reflection so that the signal transmission quality decreased, electrostatic discharge into the device along the transmission line interface chip caused by injury or damage. Specific performance is as follows:

As the impact of impedance mismatch in the video image showed ghosting. In the signal transmission line will be along in the pulse sequence before and after the formation of shocks. The existence of shock between high and low thresholds for smaller differences, when the oscillation amplitude of another major or any other interference introduced when not properly distinguish the pulse-level value, resulting in Communicate Or communication interruption time longer. Will lead to poor grounding and shielding transmission line capacity to inhibit the decline in external electromagnetic interference, reflected in the video image is snow noise, textured stripes and rolling interference and so on; the formation of spikes in the signal transmission line interference caused by communication error. Balanced transmission line of balance will form a peak in the signal transmission line interference. ESD will not only damage the equipment, it will also affect the data in memory, so that device appears some inexplicable error.

Interference method Learned from the analysis of interference and no particular source of interference, to eliminate or reduce the interference theory of too many, how CCTV works for resolve the interference problem, there is little literature related to, the following types of work on CCTV common method to resolve the interference of some of.

(1) digital signal transmission in the anti-interference measures

In weak digital signal transmission system engineering usually refers to long-term transport, common methods are: through the modulation, demodulation or video online in the power lines to transmit digital signals; by Industry Standard Communications network for transmission, such as RS422, RS845, RS485; self-developed automatic type transmission. Three compared to the common or RS422, RS485, RS485 digital communications so focused on anti-interference method.

RS485 bus is a differential balance Electric Interface, has strong anti-electromagnetic interference, but in practical engineering RS485 bus did not meet expected results. Problems often appear in the following aspects: first the network topology is unreasonable, not in accordance with the bus network topology wiring, as matters of the star topology; transmission line and receiver and transmitter devices are not correct, balanced line weakened immunity capacity; third common twisted pair, not to take further anti-interference measures, such as shielded twisted-pair. Although the interference caused by different modes of expression in the form of interference is only two: one is to increase the signal distortion of the reflection; one is the interference of external balance condition is due Damage , Common mode interference signal into a series mode into the transmission line.

On the signal reflection. According to electromagnetic theory, to reduce signal reflections on long-term the only way to impedance matching, if the communication style of topology for the bus type, impedance matching is easier to achieve, but is a star network topology, according to engineering experience may be at the sending end transmission line with series the same characteristic impedance resistor R0, as shown in the receiver by connecting, in which R1> R2, R0 = (R1 * R2) / (R1 + R2). Send R0 is generally in the gate drive output of more than 5 times the internal resistance, can be a higher level to send, receive matching impedance by 5V Power supply Formed in the impedance matching while reducing the absorption of power, this will reduce the fire, but also increased the match will not absorb too much resistance signal power, signal level threshold difference becomes smaller.

RS485 twisted pair transmission as a pair of strong electromagnetic noise suppression, but the electrostatic induction noise caused by poor immunity, so RS485 shielded twisted pair transmission lines should be used. Twisted pair shield grounded to the right, where talk of "land" should be driving the bus gates "to" rather than "case", the "protected", but in many practical devices are often not given grounding connectors, so in this case a line will need to lead shielding and drive logic integrated circuits to connect the door.

by:gaga

POST COMMENT

Servo Motor Repair Usa, Canada, Industrial Electronics Repair Usa, Servo Drive Repair Usa, Canada Pay Attention to LPG Valve to Ensure Full Safety All American Pressure Canner - Safety First Safety Issues Can Not Be Neglected When Buying World Of Warcraft Gold Online The Question Of Safety And The Choice Of A Security Company Radio And Wireless Nurse Call Safety Assurance By Electrical Contractors Digital Multimeters and User Manual for Proper Safety Safety Begins to Improve on Irish Roads Learning to Repair Electronics The DIY Way – Is it Possible? How New Laws Improved Road Safety in Ireland Constant current / constant voltage - Low voltage LEDs and their driving circuits Benefits Of Buying Electronic And Furniture Online