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This paper analyses the closed-circuit television monitoring system of sources
of interference, jamming performance, on how to curb interference in engineering
a practical way.
1 CCTV monitoring system (CCTV) in the
construction of more and more, the building of the electrical environment more
complicated, easy to create all kinds of interference, if not in the
construction process to take appropriate preventive measures, various kinds of
interference on Through transmission cables into the closed-circuit television
monitoring system, resulting decline in the quality of video images and control
system malfunction, operational instability phenomenon. So on closed-circuit
television monitoring of the source of interference in nature, to understand the
impact of the CCTV monitoring system, in order to take measures to solve the
interference problem to improve the quality of closed-circuit monitoring system
to ensure the stable operation of the system very useful.
2 the sources
of interference and influence way
CCTV monitoring system in the
transmission of signals there are two main types: One is analog video signal
transmission path from the camera to the matrix, from the matrix and to monitor
or VCR, a digital signal, including the Matrix and Camera Between the control of
information transmission, computer part of the matrix in the digital signal.
General equipment into the possibility of a very small source of interference,
mainly through the interference of signal transmission path into the system.
CCTV monitoring system twist the signal transmission path, through cable and
video transmission control signals in the twisted pair coupled with the
interference into the system: all kinds of high-frequency noise such as the
inductive load Commitment, ranging from the potential introduction of the Public
- Frequency interference, balanced transmission line noise suppression so that
imbalance will drop a total of frequency interference to a differential mode
interference, transmission line impedance matching is not a reflection of the
signal quality of the signal transmission, electrostatic discharge along the
transmission line into the interface chip equipment caused injury or damage .
Specific performance is as follows:
Impedance matching is not due to the
impact of the video images on the performance of the ghost. In signal
transmission lines would be along the pulse sequence before and after the
formation of shocks. The existence of shocks to the high-low threshold
difference between the smaller, when the shocks to the amplitude or other
interference can not be introduced at the right level to differentiate between
pulse value, resulting in longer time communications or communications
interruption. Grounding and shielding will not result in transmission lines
external electromagnetic interference ability to curb the decline, the video
image is reflected in the snow noise, interference and cross striations Netted,
such as rolling in signal transmission lines formed peak interference, resulting
in communication error. Transmission line will balance the imbalance between the
signal transmission lines formed peak interference. In addition to electrostatic
discharge will cause damage to equipment, but also affect the data storage, so
that equipment in some inexplicable mistake.
3 anti-jamming approach
Analysis interference from the source and that no special source of
interference to eliminate or reduce the interference of the theory, there are
many, closed-circuit television monitoring project for how to resolve the
interference problem, with little documentation involved, the following types of
work on closed-circuit television surveillance in common Interference and more
of a solution.
3.1 digital signal transmission in the anti-jamming
measures
Weak systems engineering in the transmission of digital signal
transmission is usually refers to long-term, common ways: through the
modulation, demodulation method in the electricity transmission line or videos
online digital signal through the communications industry-standard network
transmission, such as RS422, RS845 , RS485; self-developed automatic
transmission. Compared with the three, common or RS422, RS485, therefore focus
on the anti-jamming RS485 digital communication methods.
RS485 bus is a
difference balance of electrical interfaces, has strong anti-electromagnetic
interference ability, but in practical engineering RS485 bus did not achieve the
desired effect. Problems often appear in the following areas: the first network
topology unreasonable and not in accordance with the bus network cabling
topology, a star of the topology; transmission lines and terminal equipment to
receive and send connection is not correct, weakened the balance of the line
Anti-jamming capabilities third common twisted pair, not to take further
anti-jamming measures, such as using shielded twisted pair. Although the
interference caused by the way is different from interference in the performance
of only two forms: a reflection of the increasing degree of signal distortion;
external interference is a balance due to the destruction of conditions, common
mode interference into a string Die signal into the transmission line.
Reflection on the signal. According to electromagnetic theory, reduce
the long-term reflection on the signal impedance matching is the only way, if
the communication style of topology for the bus, impedance matching relatively
easy to achieve, but the star network topology, in accordance with engineering
experience can be conducted according to Figure 1 match , Sent-string with the
transmission line impedance characteristics of the same resistance RO, in the
receiving end as shown by the connection, of which R1> R2, R = (R1 * R2) /
(R1 + R2) = R0. R0 is generally sent in the driver's door output internal
resistance more than five times, can be sent high-level, receiving the impedance
matching is the formation of 5 V power, impedance matching at the same time
reduce the absorption power, so both reduce The radio and would not match
because of an increase of resistance absorb excessive signal power, the signal
level threshold poor smaller.
RS485 twisted pair transmission as a
electromagnetic induction noise suppression have a stronger ability, but the
electrostatic induction noise from the less inhibited, the RS485 transmission
lines should use unshielded twisted pair. Twisted pair of shield to the right
ground, we speak of here "and" should be the driver of the bus logic gate "to"
rather than "cases", "protection", but in many practical devices are often not
given A grounding connectors, so in such circumstances need to be shielded with
a line drive and the logic gate connected to the integrated circuit.
3.2
video signal interference
The video signal interference in the image on
the performance and point to spend 50 HZ cross striations rolling, snow,
interference is due to transmission lines and signal attenuation caused by
coupling the high-frequency interference, relatively easy to eliminate such
interference, and the camera Matrix control between a reasonable position to
increase video amplifier, the signal to noise ratio of sales to improve or alter
the path to video cables to avoid high-frequency interference, high-frequency
interference problem can be basically resolved. Is more difficult to resolve the
cross striations 50 HZ rolling and further increase high-frequency interference,
such as elevator car, the output of camera images. In order to curb the
interference, the first analysis of the cause of the problem.
Power
supply requirements of the camera are generally three types: DC 12 V, exchange
of 24 V or 220 V, the majority of engineering applications in the elevator car
not from the power supply on the admission, but another set electricity power
supply to the camera , The camera output images through a soft video cable from
the wells, the only side or bottom out, video cables and power supply cables and
Cabin bundled with the power line, when the elevator operation traction motor
running along the electromagnetic field Lighting power transmission line, the
camera obviously will affect the power supply cables and video cables, video
cables when the shielding of the lack of tight, high-frequency interference on
the video monitor cable to return. For 50 HZ cross striations rolling under the
theory that electromagnetic shielding layer video cables can be completely
eliminated 50 HZ frequency interference. From this we can infer this
interference is not part via video cables from coupling, but from the power cord
and unreasonable line video link.
The image of high-frequency
interference, the band because it is still less than 8 MHZ, a porosity of 50% of
the net to shield basic fire high-frequency interference, but to achieve 50% of
the porosity shielding the numbers required for each network At length with more
than 60, such a high density and flexibility of the cable will drop, the better
way is to use a double-shielding the video cables.
Video cable shield is
grounded, if the video signal "to" monitor "and" relative "grid" the potential
of different, as shown in Figure 3 2 location relative to grid "to" the
different voltages, Power then passed between the camera and display a power
circuit, so the 50 HZ frequency interference into the display, from the map of
the electrical connection can see that the elimination of 50 HZ frequency
interference There are two ways, first, to find ways to make the The ""
potential and "grid" the same as the potential difference, or cut off
circulation to form the path. As the project environment is relatively complex,
so that the entire "" potential, and so completely is difficult to increase the
camera only through the power cable diameter, as far as possible to reduce the
loop resistance. Or to use to cut off the circulation loop method, in-camera
monitor or do not have one end grounded, usually in the end not to display the
power supply, although this can not completely eliminate the interference but
can be as large as 50 HZ reduce the interference.
From the above
analysis to see if the power line coupling on the high-frequency noise, even
video cables shielded cable shielding even the best, the noise will be sent to
the monitor, video camera so the power supply line to the best Shielding the
measures needed in engineering design and construction should be fully taken
into account can be achieved, if the system to debug found that there may be
interference modulation and demodulation method will filter out noise, in-camera
set up a video signal modulator To move tens of MHz frequency of the paragraph,
in a display-based low-pass filter will be lower than 8 NHZ signal to all
filtered, and then after demodulation video image reduction.
3.3
monitoring system means the power supply
Monitoring the power supply
system there are only two ways: one is to concentrate all the power supply means
that since a primer, and the other is distributed power supply, install cameras
in the near take power from the perspective of anti-jamming effect Stresses,
concentrated way better power supply, can basically eliminate the entire range
of potential reference.
4 Interference in the main control system of
communication in the digital communications and video images of the
interference, is the key to resolve the interference during the construction
works will start considering the overall anti-jamming measures, in order to
fundamentally solve the interference problem, and not wait until the project The
latter part of the measures to take remedial measures.
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