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How to judge the cause of the failure of the closed-circuit monitoring system from the image

Author: Date:11/12/2019 8:53:55 PM
    The TV monitoring system, the performance of the system quality, must be reflected from the image on the monitor. Therefore, based on the image on the monitor, we can judge and detect the quality of the TV monitoring system equipment or system construction project, as well as the quality and the causes of interference and failure, and seek solutions.


    Based on the relevant literature and materials, combined with the actual work experience, this paper introduces the display of images and their interference from the monitor, analyzes and judges the causes of TV monitor system or equipment failure, and proposes some solutions.

First, there is no image on the monitor

1. Reasons and solutions for monitoring the host and other devices and their connections cause no image display

    The microcomputer switches the mains output to the monitor's coaxial cable connector for short circuit or open circuit;

    The corresponding output of the microcomputer switching host is damaged;

    The TV set for dual use is not in the TV state, or the monitor is bad;

    If the recorder is connected at the same time, turn on the power of the recorder and adjust it to the TV state accordingly.

2. The cause and solution of no image display caused by hard disk recorder (DVR)

    Did not cancel screen saver or improper power management settings

    The graphics card is not compatible. It can be tested by Direct Draw, which is not the case if the test passes.

    The PCI interface is not well connected. You can change the PCI slot test.

    The board may be damaged. Consider changing a card test.

3. The reason why the camera causes no image display and its solution

    First check if the power supply is connected. If it is connected, check whether the power supply voltage and supply current meet the requirements of the camera.

    Whether the lens aperture on the camera is turned on (usually caused by the aperture is closed). If it is an auto iris, does the video or DC drive correspond to the camera, is the lens control cable connected (including the pair of three variable lenses)? If the connection is correct, then Check the decoder problem;

    Whether the video coaxial cable and the BNC connector are in poor contact, open circuit or short circuit;

    The camera itself has problems.

4. The reason why the optical fiber transmission mode causes the monitor to display no image and its solution

    First check whether the connection between the electric and optical cables, the optical transceiver and the receiving machine is correct. After the error is correct, check whether the supply voltage and current meet the requirements;

    The output carrier of the optical transmitter has no video input signal. Check the video input process on the optical transmitter: disconnect the video signal from the optical transmitter, and directly input the video signal into the monitor with the video coaxial cable. If there is an image, the optical transmitter has a problem and can be replaced;

Light receiving terminal problem. If there is no problem with the optical transmitter, monitor and connection, but it is still a black screen, replace the optical receiver;

    Monitor problem. If there is no problem with the light receiving and sending machine, and the connection with the monitor is correct, and it is still a black screen, replace the monitor.

Second, the image quality is poor, there is a snowflake

On the image of the monitor, a relatively uniform snowflake disturbance occurs, and the image quality is poor. The main reasons for this phenomenon can be analyzed from the following two aspects:

1. The reason why the optical fiber transmission method causes the image on the monitor to have snowflakes and its solution method

    Light receiving terminal problem. First, the optical power meter entering the optical receiving end machine is checked by an optical power meter. If the optical power meets the requirements, the optical receiving end machine needs to be replaced;

    Light emitting terminal problem. If the optical power entering the optical receiving end machine is lower than the calibration value, the optical power meter and a fiber jumper are used to check the optical output of the optical transmitting end. If the optical output is low, the optical transmitting end machine needs to be replaced;

    Optical connector problem. If it is checked that the light output of the light-emitting terminal meets the requirements, it is an optical connector problem. Wipe the optical connector or replace it with a good quality.

    The transmission distance is too far, the fiber loss is too large, and an optical amplifier is required.

2, coaxial cable and other transmission lines cause the image on the monitor has snow and its solution

    The illumination level of the monitoring point is too low, so that the amplitude of the video signal is reduced;

    The camera has low sensitivity or the lens aperture is too small;

    The monitor itself has quality problems;

    The video transmission line is not good, causing the video signal to attenuate too much, resulting in a smaller amplitude of the video signal;

    Video equipment such as video amplifiers in video transmission lines are of poor quality;

    Poor soldering of plugs and sockets of video cables.

   The solution is to use good quality video equipment and video transmission lines to ensure that the amplitude of the video signal meets the requirements of the standard.

    Third, the image has a grainy shape

    The image on the monitor has a grainy pattern, which often does not overwhelm the normal image when it is slight, but even destroys the synchronization when it is severe, making the image unobservable. There are many reasons for this kind of failure phenomenon, and there are several reasons for this:

1, the quality of the video transmission line is not good

    The quality of the video transmission line is not good, mainly in the following aspects:

    The shielding performance of the wire is poor. For example, the shielding mesh is not a good quality copper wire mesh, or the screen mesh is too thin to be shielded. Generally, a copper wire with a shield of 96 braids and a coaxial cable with multiple copper wires should be used.

    The line resistance of such video lines is too large, resulting in a large attenuation of the signal. The core wire of the coaxial cable should be a copper wire with a relatively low resistivity. If the core wire resistance is too large, the signal attenuation is too large, so that the mesh interference is aggravated.

    The characteristic impedance of this type of video line is not 75Ω, and the distribution parameters are out of specification.

It should be pointed out that the wood grain pattern on the screen is not necessarily the fault caused by the bad video line (for example, there are two reasons behind), so it is accurate and prudent in judging. Only after the other possibilities are excluded, the remaining video cable (if there is no surplus, a segment of such a cable needs to be intercepted in the system) is sent to the inspection department for testing. If the test result is unsatisfactory, it can be determined that the cable quality problem. If it is judged that the quality of the video transmission line is not good, but it has been completed after the construction is completed, it is difficult to solve the problem by changing the line. Therefore, the selection of video cables that meet the standards and requirements before construction must be guaranteed in advance. Never buy a poor quality video cable for consideration of saving money.     If it is definitely a cable quality problem, the best way is to replace all of this cable, which is the best way to completely solve the problem.

    It is worth mentioning that if the interference is not very serious, you can try to reduce the power or reduce the interference by cleaning the power supply and connecting the UPS to the entire system. However, this method sometimes has different effects due to different spatial signals around the system, or it may be useful or sometimes useless.

2. The power supply of the power supply system is not "clean".

    The power supply referred to here is not "clean", that is, the interference signal that is compared to the comparison, specifically, the interference signal is superimposed on the 50 Hz sine wave. The interference signals on such power sources are mostly derived from the thyristor devices used in the power grid. In particular, high-current, high-voltage thyristor equipment, which is very polluted by the power grid, causes the power supply in the same power grid to be "clean". If there are high-power thyristor frequency modulation speed regulation devices, thyristor rectifier devices, thyristor AC/DC converters, etc. in this power grid, it will pollute the power supply.

    The solution to this situation is relatively simple, as long as the entire system is powered by a clean power source or an online UPS, this interference can basically be eliminated.

3. There is a strong electromagnetic interference source near the system.

    There is a strong source of electromagnetic interference near the TV monitoring system, and there is also a malfunction caused by such interference. In this case, it can be judged by investigating and understanding the environmental conditions in the vicinity of the TV monitoring system. If it is for this reason, the solution is to strengthen the shielding of video equipment such as cameras and video amplifiers, as well as the grounding, plug and pipe ports of the video cable, and good grounding treatment.

Fourth, the image has vertical bars with equal spacing

    On the screen of the monitor, a number of vertical bars of equal spacing are sometimes generated, and the frequency of such an interference signal is substantially an integer multiple of the horizontal frequency. If the waveform of the disturbed image is viewed with an oscilloscope, it will be found that a higher amplitude line-frequency harmonic oscillation waveform is superimposed on the back shoulder of the line sync head, and the vertical strip interference is caused thereby.

1. Reasons for this interference

    A fault phenomenon caused by a mismatch in the characteristic impedance of the transmission line. It is caused by the impedance mismatch caused by the characteristic impedance of the video transmission line is not 75Ω;

    Through the analysis of the waveform and the quantitative measurement of the video cable, it is found that the impedance of the video cable does not meet the requirements, and the distribution parameters are not satisfactory, which is one of the reasons for the impedance mismatch.

Therefore, this interference phenomenon is caused by the inconsistency of the characteristic impedance and distribution parameters of the video cable. If the "starting end" or "terminal" impedance of the inferior coaxial cable or the signal transmission of more than 250m is seriously mismatched, or even the open end, it may cause the above interference.

2, the solution

    By "starting series resistor" or "terminal parallel resistor". That is, the "starting end" string 75Ω resistor, "terminal" and 75Ω resistor to avoid impedance mismatch and excessive distribution parameters;

    Use high quality coaxial cable. That is, the outer shielding net is made of copper, not less than 96 pieces, and the core wire is also multi-strand copper;

    It is worth noting that when the video transmission distance is less than 150m, the above-mentioned interference phenomenon may not occur when the video cable with the above impedance mismatch and excessive distribution parameters is used. Therefore, in a system where the transmission distance is very different, when analyzing such a fault phenomenon, do not be confused by short distance without interference.

    The fundamental solution to the above problem is to ensure quality when purchasing video cables. The cable should be sampled if necessary.

Fifth, the image has black and white bars and scrolls up and down

1. The cause and judgment

    On the monitor's screen, a black bar or white bar appears and slowly scrolls up or down. The reason for this phenomenon is mostly caused by the system's ground loop and the introduction of 50Hz AC interference. However, sometimes the malfunction occurs due to poor power supply performance or partial damage of the control host such as a camera or matrix switcher, or system grounding, equipment grounding, and the like. Therefore, when analyzing such fault phenomena, it is first necessary to distinguish between two different causes of faults.

    To distinguish between the problem of the power supply or the problem of the ground loop, generally on the control panel, the output signal of the camera with no power supply connected to the nearest power supply. If the above phenomenon does not appear on the monitor, the control host has no problem. . Next, a portable monitor can be used to connect to the video output of the front camera, and one camera can be viewed one by one to find out if there is any interference caused by power problems. If so, the processing is performed; if not, the interference is caused by other reasons such as the ground loop.

2. Causes of power problems or system and equipment grounding problems

Video equipment, especially the camera power supply voltage is too low, or the power supply is not "clean", the ripple factor is relatively large, or a relatively large interference is involved;

    Poor grounding of the system or equipment;

    Video equipment such as video cameras or video amplifiers or video matings have quality problems;

    There are serious electromagnetic interferences in the surrounding areas of the system, such as electric welding, radio transmission, interference from large motors and large relays.

Sixth, the image has a large area of mesh

1, the cause

    The large area of the web generated on the monitor will cause a serious degradation of the image quality. In severe cases, the image is usually destroyed, that is, the image and the sync signal are not formed. The main causes of this failure are:

    This kind of interference often occurs in BNC connectors, or other types of video connectors are not well connected;

    The short circuit of the video cable and the shielded network are short-circuited or broken, such as cable laying, the outer shield is damaged, and the shielded cable is broken;

    After the cable is laid, it will encounter corrosive liquid, gas or rat damage, which will damage the outer shield of the cable;

    The cable plug is not well connected to the video device.

2, the solution

    Obviously, such fault phenomena are easier to judge, because when such a fault phenomenon occurs, it is often not a problem with each signal of the entire system, but only for the number of bad joints. This problem can be solved by carefully checking these connectors one by one.

Seven, the image has a trailing, glitch or distortion, or even the line is not synchronized

1, the cause

    On the monitor, the image has smearing, glitch or distortion, and even the line is out of sync, which is mainly caused by the following reasons:

    The video transmission line uses a poor quality coaxial cable, or the cable shielding line is only connected by a few small network cables, or there is a break in the middle;

    In the video transmission line, the BNC plug is poorly soldered;

    The transmission distance is long, no video amplifier is added (if the device is added, the quality of the device is poor), and the 75Ω impedance is seriously mismatched;

    The monitor or the camera itself is not synchronized, or the camera power supply voltage is too low, such as 12VDC voltage is less than or equal to 10V;

    The power supply is not "clean" and contains interference signals;

    If high-frequency wired, wireless or optical fiber transmission is used, most of them are quality problems of high-frequency transmission equipment or optical transmission equipment. For example, the quality of a radio frequency equipment is poor, so that the synchronization head is cut and cut.

2, the solution

    If you suspect a device, you can use the replacement method to determine. Other corrections for the cause, so that the transmission line can meet the requirements.
Eight, the image has a ghost

1, the cause

    On the monitor, the displayed image overlaps with the shadow of another image. When the image is brighter, the shadow does not look serious; when the image is darker, the ghost is more obvious. The reason for this failure is:

    The video transmission line uses a poor quality coaxial cable, or the outer shield of the cable is damaged seriously, or the cable or system is poorly grounded;

    The wiring in the system is seriously unreasonable, causing mutual crosstalk, such as the unreasonable routing between the transmission coaxial cable and the other cable, resulting in serious signal sensing and crosstalk

    The isolation of the video matrix is too small to meet the technical requirements;

    The load impedance is not 75Ω, but a serious mismatch;

    If you use high-frequency, wireless transmission, it may be that the system's cross-talk modulation and intermodulation is too large.

2, the solution

    The solution is that if it is a device, it can be determined by the replacement method, and the other is corrected for the cause.

Nine, the image is light and the contrast is too small

1. The image of the monitor is light and the contrast is too small.

    Mainly due to insufficient amplitude of the video signal. The specific reasons are mainly:

    Improper adjustment of the image contrast of the monitor or quality of the monitor itself;

    The transmission distance is too far or the video transmission line is attenuated too much, and the transmission line plug and socket are poorly soldered;

    The illumination level of the monitoring point is too low, or the camera sensitivity is low or the lens aperture is too small.

2, the solution

    The solution is to add a line amplification and compensation device to make the video signal amplitude meet the specified requirements.

Ten, the image is not clear, the edge is not clear or the color is lost

1, the cause

    The image clarity is not high, the edges are unclear, and the details are partially lost. In severe cases, color signals are lost or the color saturation is too small. The specific reasons are mainly:

    The high frequency end loss of the image signal is too large, so that the signal at a frequency above 3 MHz is substantially lost;

    The transmission distance is too far, and there is no amplification compensation device in the middle, and the amplitude of the video signal is excessively attenuated;

    The transmission capacitance of the video transmission cable is too large, so that the high frequency component is attenuated too much;

    The clarity of the camera and monitor is not high;

    The bandwidth of the video signal path is too narrow, such as the bandwidth of the video amplifier is too narrow, or the transmission cable quality is low;

    In the transmission link, a concentrated capacitance equivalently appears between the core line and the shield line of the transmission line.

2, the solution

    It can be solved for the above reasons, such as high frequency end compensation, amplification compensation, and the like.
Eleventh, image tonal distortion

    This is a phenomenon that is prone to occur in long-distance video baseband transmission.

1, the cause of color distortion

    It is mainly caused by the excessive phase shift of the signal in the high frequency band caused by the long-distance transmission line. When the transmission distance is not too far, the image tones are not easily recognized by the human eye.

2, the solution

    A phase compensator should be added. The precautionary measure is to use high-quality coaxial cable, and the transmission line should be short and not suitable.

Twelve, the image picture and video file playback have mosaic phenomenon

1. Because the host signal is not good, the network is not smooth, which causes some video and audio signals to be lost;

2. During the video recording of the mosaic, there are other reasons for the client to cause serious shortage of resources. If the client previews the data transmitted by the host while     while performing the multi-channel playback operation beyond the system capability, the CPU resources are exhausted. Therefore, once the host resources are exhausted, the        recorded data cannot be written to the hard disk normally, so mosaics may occur.

Thirteen, the image has an animation

1. In the recording setting of the corresponding channel of the server host, the frame rate is adjusted too low;

2. The bandwidth of network communication is not enough. At this time, the video quality of the host should be appropriately lowered.

3. Since the server host sets this channel to the LAN transmission mode and is in the same LAN as the first-level client and the extended client, the extended client may not preview properly at this time.

    Usually, when there is a problem in the system, the system recovery disk that will be brought along is put into the floppy disk drive to restart the system. At this time, the system will automatically recover, that is, return to the factory settings. After the system recovery is complete, remove the floppy disk and restart the machine. When the system is started, reset it according to your needs.

Fourteen, the image screen has a sense of jitter

    The reason is: because the display refresh rate setting is too low.

    The solution is: enter the "display properties", click "Settings", select "Advanced", then select "Monitor", adjust the new frequency to 75Hz, and confirm the exit to solve this problem.