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Abstract: There are two main types of transmission signals in CCTV monitoring systems: one is analog video signals, the transmission path is from the camera to the matrix, from the matrix to the display or video recorder; the other is the digital signal including the matrix and the camera. Control information transmission, digital signal of computer part in matrix. General equipment is unlikely to be a source of interference, so interference is mainly transmitted through signals
There are two main types of transmission signals in CCTV monitoring systems: one is analog video signals, the transmission path is from the camera to the matrix, from the matrix to the display or video recorder; the other is the digital signal including the control information between the matrix and the camera Transmission, the digital signal of the computer part of the matrix. Generally, the possibility of equipment becoming a source of interference is small, so interference mainly enters the system through the signal transmission path.
The signal transmission path of the closed-circuit television monitoring system is that the interference that can be coupled into the system through the video cable and the twisted pair of control signals are: various high-frequency noises such as large inductive loads starting and stopping, and power frequency interference caused by unequal ground potentials. The imbalance of the transmission line reduces the noise suppression ability and changes the common frequency interference into differential mode interference. The impedance mismatch on the transmission line causes the reflection of the signal to reduce the signal transmission quality, and the electrostatic discharge enters the device along the transmission line to cause damage or damage to the interface chip. The specific performance is as follows:
The effects due to impedance mismatch appear as ghosting on the video image. Oscillations form on the front and back of the pulse sequence on the signal transmission line. The existence of the oscillation makes the threshold difference between high and low levels small. When the amplitude of the oscillation is large or other interference is introduced, the pulse level value cannot be correctly distinguished, resulting in longer communication time or communication interruption. Poor grounding and shielding will cause the transmission line to reduce the ability to suppress external electromagnetic interference, which is reflected in video images such as snowflake noise, moire interference, and horizontal scrolling; peak interference is formed on the signal transmission line, causing communication errors. Unbalanced transmission lines can also cause spike interference on signal transmission lines. In addition to the damage caused by the electrostatic discharge, it will also affect the data in the memory and cause some inexplicable errors.
From the analysis of interference sources, I know that there are no special interference sources. There are many theoretical discussions on eliminating or reducing the above interference. There is little literature on how to solve the interference problem for CCTV monitoring projects. The following are common in CCTV monitoring work. Discuss the interference and solutions.
1. Anti-interference measures in digital signal transmission
Digital signal transmission in weak current system engineering usually refers to long-line transmission. Common methods include: transmitting digital signals on power lines or video lines through modulation and demodulation methods; and transmitting through industry-standard communication networks, such as RS422, RS845, and RS485. ; Self-developed automatic transmission. Compared with the three, RS422 and RS485 are common, so we focus on the anti-interference methods of RS485 digital communication.
RS485 bus is a differential balanced electrical interface, which has a strong ability to resist electromagnetic interference. However, RS485 bus has not achieved the desired effect in actual engineering. The problems often appear in the following areas: The first network topology is unreasonable, and it is not wired in accordance with the bus network topology, becoming a star topology in the matter; the transmission line and the receiving and transmitting end equipment are not connected properly, which weakens the anti-interference of the balance line Capacity; the third public twisted pair, no further anti-interference measures have been taken, such as shielded twisted pair. Although there are differences in the methods of causing interference, there are only two forms of interference: one is that reflection increases the degree of signal distortion; and the other is that external interference is destroyed due to equilibrium conditions, and common-mode interference becomes a string. Mode signal enters the transmission line.
2. Interference of video signals
The interference of the video signal appears on the image as ground flower dots and 50Hz horizontal stripes scrolling. For snowflake dot interference, it is caused by signal attenuation on the transmission line and coupled with high-frequency interference. This interference is relatively easy to eliminate. Add a video amplifier at a reasonable location to increase the signal to noise ratio or change the path of the video cable to avoid high-frequency interference sources. The problem of high-frequency interference can be basically solved. More difficult to solve is the case of 50Hz horizontal stripes rolling and further adding high frequency interference, such as the output image of the camera in the elevator car. In order to suppress the above interference, first analyze the cause of the above problem.
There are generally three types of power supply required by the camera: DC 12V, AC 24V or 220V. In most engineering applications, it is not taken from the power supply of the elevator car. Instead, a power supply is provided to power the camera. The output image of the camera passes a soft The video cable is sent from above or below the hoistway. The video cable and power supply cable are bundled with the power line of the car. When the elevator runs, the electromagnetic field generated by the traction motor runs along the lighting power line, which obviously affects the camera power cable and video. Cable, when the shielding layer of the video cable is not tight enough, high frequency interference is transmitted back to the monitor via the video cable. For the 50Hz horizontal stripe rolling, according to electromagnetic theory, it is known that the shielding layer of the video cable can completely eliminate the 50Hz power frequency interference. It can be inferred that this part of the interference is not coupled through the video cable, but from the power line and the unreasonable video line connection.
3. Power supply of monitoring system
There are only two power supply modes for the monitoring system: one is centralized power supply, that is, the power is sourced from one place, and the other is distributed power supply. The camera takes power near the installation location. From the perspective of anti-interference effect, centralized power supply Better, can basically eliminate the situation where the reference potential varies. There are two main types of transmission signals in CCTV monitoring systems: one is analog video signals, the transmission path is from the camera to the matrix, from the matrix to the display or video recorder; the other is the digital signal including the control information between the matrix and the camera Transmission, the digital signal of the computer part of the matrix. Generally, the possibility of equipment becoming a source of interference is small, so interference mainly enters the system through the signal transmission path.